An apparatus for supplying air

Abstract

An apparatus 400 for supplying air from an air supply 106 to two air release units 200 disposed on a hull (102, Fig 1A) of a vessel (100, Fig 1A) comprises a piping network 402 comprising two branches

Description

TECHNICAL FIELD This disclosure relates to an apparatus for supplying air. More particularly, it relates to an apparatus for supplying air from an air supply to at least two air release units, a system for supplying air to two air release units, a vessel, a method of modifying pressure loss across a piping network of an air lubrication system of a vessel, a method of calibrating a flow restrictor position of a variable flow restrictor, and a method of installing a system for supplying air to two air release units. BACKGROUND Much of the world’s international trade relies on high-capacity cargo ships such as large oceangoing tankers, freighters, bulk carriers, container ships or other cargo vessels. Such vessels ply oceans and seas to transport goods between countries and continents. Similarly, oceangoing passenger ships such as cruise ships and ocean liners carry passengers across seas or oceans for the purposes of tourism or transportation. Oceangoing vessels such as these have high shipping capacities and high weights. For instance, the water displacement of these vessels may be 50,000 tons or more. Smaller vessels may be suitable for carrying cargo or passengers for inland use. The maritime shipping industry is key for maintaining international supply chains. However, the shipping industry is also responsible for significant carbon emission volume. It would be advantageous to reduce carbon emissions from shipping, for instance by increasing vessel efficiency. In recent years, increases in vessel fuel efficiency - and therefore reductions in vessel emission - can be realised by air lubrication techniques. Air lubrication mitigates frictional resistance between the hull of a ship and the water by generating a layer of air between the hull and the water to lubricate the hull as the ship travels through the water. An example air lubrication system is described in EP3969358, filed in the name of the present applicant. For instance, air lubrication can be achieved by injecting air into at least one open cavity disposed in the surface of the hull of the vessel at a hydrostatic pressure such that a substantially flat water-air interface is formed at the cavity opening. As a result, Kelvin-Helmholtz mixing at the water-air interface causes a stream of air bubbles to be released from the rear of the cavity. This stream of air bubbles from all the cavities forms a lubricating carpet of bubbles which reduces frictional resistance between the hull and the water. Such cavities were found to be stable and efficient and to reduce frictional drag sufficiently to outweigh the energy requirement of injecting pressurised air into the cavity. It has been found that these air lubrication systems can reduce vessel fuel consumption by up to 10%. To continuously release the stream of air bubbles, the hydrostatic pressure in each cavity is such that a flat water-air interface is formed at substantially the cavity opening. However, the cavities are distributed across the width of the vessel. Therefore, vessel motion - particularly vessel pitch, trim, roll and heel - can introduce pressure imbalances between the cavities. Pressure imbalances may cause uneven airflow distribution between the cavities, which can affect the consistency and characteristics of air bubbles released therefrom. It would be advantageous to provide systems or methods which address these shortcomings and which can compensate for pressure imbalances between cavities across the width of the vessel. SUMMARY One or more aspects of the invention of the present application are set out in the claims. According to a first aspect, there is provided an apparatus for supplying air from an air supply to two air release units disposed on a hull of a vessel. The apparatus comprises: a piping network comprising two branches; a variable flow restrictor disposed in each branch; and a controller configured to adjust a flow restriction of the variable flow restrictors based on an airflow parameter of the apparatus. Each branch is suitable for fluidically coupling the air supply to a respective one of the two air release units. The airflow parameter may be a parameter indicating an airflow rate. The airflow parameter may be indicative of airflow in the piping network. In some implementations, the apparatus may include more than two air release units (such as at least four air release units, or any other number of air release units described herein), in which case the piping network may include a corresponding number of branches in which variable flow restrictors are disposed. Each of the corresponding number of branches may be for fluidically coupling the air supply to a respective one of the more than two air release units. In some implementations, the controller may be configured to adjust the flow restriction to provide greater flow restriction at lower values of the airflow parameter and lesser flow restriction at higher values of the airflow parameter. Accordingly, at lower airflow rates, the restriction may be higher which can reduce airflow redistribution between the branches. Inversely, at higher airflow rates, the restriction may be lower which can increase efficiency by reducing average pressure loss across each branch, while also maintaining lower airflow redistribution due to the high airflow rate. In some implementations, the controller may be configured to maintain an equal flow restriction for each variable flow restrictor. Accordingly, variable flow restrictor lifespan can be increased. In some implementations, the airflow parameter may comprise a measured airflow rate in the piping network. In some implementations, a flowmeter may be disposed to measure the airflow rate in the piping network. For instance, the flowmeter may be provided in a common pipe suitable for (e.g., disposed to) fluidically couple the air supply to each of the two branches. When the airflow parameter comprises the measured airflow rate, delivered airflow rates can be monitored and therefore variable flow restrictor accuracy can be increased. In some implementations, the airflow parameter may comprise a target airflow rate. The target airflow rate may be a requested airflow rate from the air supply. The controller may be configured to communicate the target airflow rate to the air supply. The controller may be configured to receive the target airflow rate from the air supply. The target airflow rate may be positively correlated with vessel speed and / or a draft of the vessel. The target airflow rate may be determined based on a condition of the air release unit. In some implementations, the controller may be configured to receive one or more of vessel speed data, vessel draft data, and / or air release unit condition data and setting the target airflow rate based on the one or more of vessel speed data, vessel draft data, and / or air release unit condition data. When the airflow parameter comprises the target airflow rate, feedback in flow restrictor positions due to undesirable transient effects can be avoided. In some implementations, the controller may be configured to adjust the flow restriction in each variable flow restrictor between a plurality of discrete flow restriction levels based on the airflow parameter. Accordingly, variable flow restrictor lifespan can be increased. Each discrete flow restriction level may correspond to a range of airflow parameter values. In some implementations, the discrete flow restriction levels may correspond to respective flow restrictor positions. The controller may therefore be configured to calibrate a first flow restrictor position for a first flow restriction level of the discrete flow restriction levels for a first one of the two variable flow restrictors by: positioning the first variable flow restrictor at an initial flow restrictor position; measuring a first pressure loss across the first variable flow restrictor; if the first pressure loss is outside a threshold range of a target pressure loss for the first flow restriction level, positioning the first variable flow restrictor at a tuned flow restrictor position; measuring a second pressure loss across the first variable flow restrictor; and if the second pressure loss is within the threshold range, calibrating the first flow restrictor position based on the tuned flow restrictor position. A differential pressure gauge may be disposed in a first branch of the two branches and arranged to measure pressure loss across a first variable flow restrictor disposed in the first branch. In some implementations, the variable flow restrictor may comprise one or more of an actuatable valve, a butterfly valve, an airflow damper, and / or an adjustable orifice plate. According to a second aspect, there is provided a system for supplying air to two air release units disposed on a hull of a vessel. The system comprises the apparatus of the first aspect and an air supply fluidically coupled to the piping network of the apparatus. The air supply may comprise an engine of the vessel. The piping network may be disposed to scavenge air from the engine. The air supply may comprise a compressor configured to discharge air into the piping network. According to a third aspect, there is provided a vessel. The vessel comprises: a hull; two air release units disposed on the hull; and the system for supplying air of the second aspect. According to a fourth aspect, there is provided a method of modifying pressure loss across a piping network of an air lubrication system of a vessel. The air lubrication system comprises the piping network, an air supply, and two air release units disposed on a hull of the vessel. The piping network fluidically couples the air supply to the two air release units and comprises two branches, each branch fluidically coupling the air supply to a respective one of the two air release units. A variable flow restrictor is disposed in each branch. The method comprises: receiving an airflow parameter of the air lubrication system; and adjusting a flow restriction of the variable flow restrictors based on the airflow parameter. In some implementations, the method may be carried out by a controller of the air lubrication system. In some implementations, adjusting the flow restriction of the variable flow restrictors may comprise providing greater flow restriction at lower values of the airflow parameter and lesser flow restriction at higher values of the airflow parameter. In some implementations, adjusting the flow restriction of the variable flow restrictors may comprise maintaining an equal flow restriction for each variable flow restrictor. In some implementations, the method may further comprise measuring the airflow rate in the piping network. The airflow parameter may comprise the measured airflow rate. Measuring the airflow rate may comprise measuring the airflow rate with a flowmeter. The flowmeter may be provided (e.g., disposed) in a common pipe. The common pipe may be suitable for fluidically coupling the air supply to each of the two branches. In some implementations, the airflow parameter may comprise a target airflow rate. The target airflow rate is a requested airflow rate from the air supply. The method may further comprise requesting the air supply to supply air to the piping network at the target airflow rate or receiving the target airflow rate from the air supply. The target airflow rate may be positively correlated with vessel speed and / or a draft of the vessel. The target airflow rate may be determined based on a condition of the air release unit. In some implementations, the method may further comprise receiving one or more of vessel speed data, vessel draft data, and / or air release unit condition data and setting the target airflow rate based on the one or more of vessel speed data, vessel draft data, and / or air release unit condition data. In some implementations, adjusting the flow restriction of the variable flow restrictors may comprise adjusting the flow restriction of the variable flow restrictors between a plurality of discrete flow restriction levels based on the airflow parameter. Each discrete flow restriction level may correspond to a range of airflow parameter values. According to a fifth aspect, there is provided a method of calibrating a first flow restrictor position of a first variable flow restrictor disposed in a first branch of a piping network of an air lubrication system of a vessel. The air lubrication system comprises the piping network, an air supply, and two air release units disposed on a hull of the vessel. The piping network fluidically couples the air supply to the two air release units and comprises two branches, each branch fluidically coupling the air supply to a respective one of the two air release units. A variable flow restrictor is disposed in each branch. The first flow restrictor position is for a first flow restriction level of the first variable flow restrictor. The method comprises: positioning the first variable flow restrictor at an initial flow restrictor position; measuring a first pressure loss across the first variable flow restrictor; if the first pressure loss is outside a threshold range of a target pressure loss for the first flow restriction level, positioning the first variable flow restrictor at a tuned flow restrictor position; measuring a second pressure loss across the first variable flow restrictor; and if the second pressure loss is within the threshold range, calibrating the first flow restrictor position based on the tuned flow restrictor position. In some implementations, the first flow restriction level is of a plurality of discrete flow restriction levels. In some implementations, the first pressure loss and second pressure loss may be measured using a differential pressure gauge disposed in the first branch to measure pressure loss across the first variable flow restrictor. According to a sixth aspect, there is provided a method of installing the system of the second aspect. The method comprising fitting a variable flow restrictor into each branch of the piping network. The method may comprise routing the piping network between the air supply and the two air release units such that each branch of the piping network fluidically couples the air supply to a respective one of the two air release units. The method may further comprise disposing the two air release units on the hull of the vessel. BRIEF DESCRIPTION OF THE DRAWINGS Specific exemplary embodiments of the present disclosure are described below in the detailed description by way of example only and with reference to the accompanying drawings, in which: Fig. 1a shows an example vessel which includes an air lubrication system; Fig. 1b shows an example distribution of air release units on the underside of a vessel’s hull; Fig. 2 shows an example cavity-type air release unit; Fig. 3 shows an example apparatus for supplying air from an air supply to air release units; Fig. 4 shows an improved example apparatus for supplying air from an air supply to air release units; Fig. 5 shows a method of controlling pressure loss across a piping network; Fig. 6 shows a method of calibrating a first flow restrictor position; Fig. 7 shows a method of installing an air lubrication system onto a vessel; and Fig. 8 shows one implementation of a computing device. Like reference numerals are used for like components throughout the drawings. DETAILED DESCRIPTION In overview, and without limitation, the disclosure relates to an apparatus for supplying air from an air supply to at least two air release units of a vessel’s air lubrication system. The apparatus includes a piping network, with branches fluidically coupling the air supply to each respective air release unit. Each branch includes a variable flow restrictor which is selectively adjustable between varying degrees of restriction. The respective variable flow restrictors can therefore change the level of restriction and the resistance to flow in each branch of the piping network. As examples, the variable flow restrictors may be actuatable valves, butterfly valves, airflow dampers, and / or adjustable orifice plates. The apparatus also includes a controller configured to adjust a flow restriction of the variable flow restrictors based on an airflow parameter of the apparatus. The airflow parameter is a parameter indicating an airflow rate. The airflow parameter is indicative of airflow in the piping network. For instance, the airflow parameter may be a measured airflow rate in the piping network. Alternatively, the airflow parameter may be a target airflow rate requested of an air supply supplying air to the piping network. In summary, the airflow parameter is a parameter indicative of, or approximating, the airflow rate or expected airflow rate throughout the piping network. As used herein, airflow rates may be mass airflow rates (i.e., airflow rates by mass). By adjusting flow restriction of the variable flow restrictors based on the airflow parameter, the amount by which airflow redistributes between branches of the piping network to compensate for dynamic pressure imbalances between the branches induced as a result of vessel motion - particularly vessel pitch, trim, roll and heel- can be minimised. Accordingly, by controlling the flow restriction in each branch, airflow distribution between branches can be substantially balanced, even during vessel pitch, trim, roll and heel. For instance, at lower values of the airflow parameter - which are indicative of lower airflow rates - the variable flow restrictor can be set to a more restrictive position (i.e., a position providing a greater flow restriction), thereby minimising airflow redistribution from variations in hydrostatic pressure at the air release units resulting from vessel motion. On the other hand, at higher values of the airflow parameter - which are indicative of higher airflow rates - the variable flow restrictor can be set to a less restrictive position (i.e., a position providing lesser flow restriction), thereby achieving low airflow redistribution due to hydrostatic pressure variation resulting from vessel motion at the air release units, but preventing excessive average pressure losses resulting from high restriction. For instance, as airflow rate increases, the same restriction in the piping network induces a greater amount of pressure loss. Accordingly, at higher values of the airflow parameter, when the variable flow restrictor is set to a less restrictive position, the pressure differential induced by flow restrictors in the piping network is increased. Therefore airflow redistribution from variations in hydrostatic pressure at the air release units resulting from vessel motion can still be minimised (e.g., without requiring a more restrictive position of the variable flow restrictor). In some examples, each variable flow restrictor is adjustable between a plurality of discrete flow restriction levels. Accordingly, the controller may be configured to adjust the flow restriction in each variable flow restrictor between a plurality of discrete flow restriction levels. Each discrete flow restriction level may correspond to a particular range of values of the airflow parameter. For instance, the correspondence of the discrete flow restriction levels and the airflow parameters may be stored in a lookup table at the controller. Alternatively, the controller may dynamically calculate or calibrate the correspondence during or prior to operation. The life of (e.g., the time between servicing or replacing) the variable flow restrictor is determined by the number of flow restrictor cycles carried out between flow restriction levels. When the flow restriction levels are discrete, the flow restrictor only cycles between flow restriction levels when transitioning from one discrete level to another. Therefore, adjusting the flow restriction in each variable flow restrictor between a plurality of discrete flow restriction levels corresponding to the airflow parameter increases the variable flow restrictor lifespan. Accordingly, the amount of time before the variable flow restrictor need be serviced or replaced - for instance at dock - is reduced. In addition, the controller may be configured to maintain an equal flow restriction for each variable flow restrictor. As such, the flow restrictor in each branch can be adjusted equally, without requiring additional complexity and flow restrictor cycling because the flow restrictors are not adjusted dynamically or as part of a complex feedback control loop. Vessel and air lubrication system overview Fig. 1a shows an example vessel 100 in which the systems and apparatuses of the present disclosure may be implemented. The vessel 100 is any suitable vessel, such as a displacement vessel. For instance, the vessel 100 may have a displacement of 10,000 tons or more or 50,000 tons or more. The vessel has a hull 102 which may be a substantially flat-bottomed hull, or a v-profile hull. In some examples, the hull 102 has a length of, for instance, at least 50m or at least 100m. In some examples, the hull may have a length of less than 400m. The vessel includes a plurality of air release units 200 and a system 104 for supplying air to the air release units 200. The air release units may be disposed on (e.g., in or at) the hull 102 of the vessel. The system 104 may be referred to as an air lubrication system. The system 104 for supplying air includes a corresponding apparatus 120 and an air supply 106. Example implementations of the apparatus 120 are described in more detail herein, particularly with respect to Figs. 3 and 4. In overview, the apparatus 120 includes a piping network 122 suitable for fluidically coupling the air supply 106 to each air release unit 200. To fluidically couple the air supply 106 to each air release unit 200, the piping network 122 includes a plurality of branches, each branch coupling the air supply to a respective one or more of the plurality of air release units. Each branch is connected throughout the piping network 122 in parallel. As such, the number of branches may equal the number of air release units or may be less than the number of air release units. The apparatus 120 may additionally include a common pipe 124 for fluidically coupling the air supply to each of the two branches. For instance, the common pipe 124 may be connected in series to fluidically couple the air supply to each branch. The common pipe 124 may therefore be connected between the air supply and at least some, or all, of the branches, in series. The apparatus 120 also includes a controller 126. The controller 126 is configured to control the apparatuses and systems described herein. For instance, as described in more detail herein, particularly with respect to Figs. 3 and 4, the controller 126 may be configured to control and / or communicate with the engine of the vessel 100, the air supply 106, and / or flowmeters and / or variable flow restrictors disposed in the piping network 122. The air supply 106 may take any form suitable for supplying air to the piping network 122 at any pressure suitable for outputting air from the air release units 200. For example, the air supply 106 may comprise a compressor configured to discharge air into the piping network 122. The compressor is configured to intake air, such as atmospheric air, compress the air and discharge compressed air into the piping network 122. Alternatively, the air supply 106 may be provided within an engine of the vessel 100. For instance, the air supply 106 may be configured to scavenge air from an engine (such as the main engine). In these examples, compressed, and pressurised air may be held in a holding chamber of the engine. Accordingly, the air supply may be configured to scavenge air from the engine by selectively drawing pressurised air from the holding chamber and discharging the pressurised air into the piping network 122. Fig. 1b shows an example distribution of air release units 200 on the underside of the hull 102 of the vessel 100. As shown, the plurality of air release units 200 may be distributed across the width of the hull. For instance, the plurality of air release units 200 may be distributed in a V-formation in the forward half of the vessel 100, about the centreline 108 of the hull 102. When distributed in a V-formation, the air release units 200 are disposed gradually further from the center line from the bow 110 of the hull in a rearward direction along the hull. In this formation, the front air release units 200 are those closest to, or substantially at, the centreline 108. However, in other examples, the plurality of air release units 200 may be distributed in any other suitable distribution, such as a W-formation, U-formation, or any other regular or irregular formation. 12 air release units 200 are depicted in Fig. 1b. However, in other examples, substantially any number of air release units 200 may be disposed on the hull of the vessel 100. For instance, up to 24 air release units 200 may be disposed on the hull 102 of the vessel 100. Equally, any even number of air release units 200 - e.g., any even number of air release units up to and including 24 air release units 200 - may be disposed on the hull of the vessel 100. For instance, at least 2, at least 4, at least 8, at least 12, at least 14, at least 16, at least 18, at least 20, at least 22, or at least 24 air release units 200 may be disposed on the hull. Preferably 24 or up to 48 air release units 200 are disposed on the hull 102. Alternatively, any odd number of air release units 200 may be disposed on the hull 102, in which case the foremost of the air release units 200 may be disposed on the centreline 108. In some examples, air release units may be disposed along up to 40% of the length of the hull, from the bow 110. Independently of the number of air release units 200 disposed on the hull of the vessel 100, some or all of the air release units 200 may be supplied by the same air supply 106 and / or via the apparatus 120. For instance, the apparatus 120 may be suitable for supplying air from the air supply 106 to two air release units 200 disposed on the hull 102 of the vessel 100. In this case, the piping network 122 of the apparatus 120 may comprise two branches, each for fluidically coupling the air supply to a respective air release unit 200. Alternatively, the apparatus 120 may be suitable for supplying air from the air supply 106 to any number of air release units 200 disposed on the hull 102, such as any even number of air release units up to and including 24 air release units 200, as otherwise described herein. In these examples, the piping network 122 of the apparatus 120 may comprise a number of branches corresponding to (e.g., equal) the number of air release units supplied by the apparatus 120. In some examples, the air supply 106 may supply multiple apparatuses, each apparatus supplying air from the air supply 106 to a subset of air release units 200 disposed on the hull 102. As the vessel 100 travels through the water, air is released from the air release units 200, forming a layer of air 112 between the hull 102 and the water, lubricating the hull 102 against the water and thereby reducing frictional resistance between the hull and the water. The air release units 200 may therefore comprise any means suitable for releasing or expelling air, for instance for releasing or expelling air between the hull 102 of the vessel and the water, such as nozzles, outlets, diffusers, apertures, or cavities. Air inlets or outlets of the respective nozzles, outlets, diffusers, apertures, or cavities may therefore be fluidically coupled to the respective branches of the piping network 122 described herein. By way of example, Fig. 2 shows an example air release unit 200. The air release unit 200 of Fig. 2 is also referred to herein as a cavity 200. As such, the term “cavity” as used herein, when not referring specifically and solely to the example of Fig. 2, may be used interchangeably with the general term “air release unit”. The cavity 200 of Fig. 2 can be constructed as an integral module that can be fitted into the bottom of the hull 102 of the vessel 100. For instance, the cavity may be substantially as described in EP3969358, filed in the name of the present applicant, which is hereby incorporated herein by reference in its entirety. The cavity is defined by sidewalls 202a, 202b, and top wall 204. As shown, the sidewalls 202a, 202b may meet at a point at the front end 206 of the cavity 200. For instance, the front end 206 of the cavity may comprise a dagger-shaped nose section. At least a portion of the sidewalls 202a, 202b may diverge away from each other as they extend from the front end 206 to the rear end 208. Towards the rear end 208 of the cavity, downwardly sloping rear wall part 210 extends from the top wall 204 to the rear end 208. As shown, the sidewalls 202a, 202b can, in some examples, be supported with their lower edges on a flange 214. The flange 214 may be weldable into the hull 102 of the vessel 100. Accordingly, when welded into the hull of the vessel, the sidewalls 202a, 202b delimit an opening that is substantially level with the surface of the vessel. Therefore, in operation, the opening is substantially level with the water to create an air-water interface thereat. As shown, an air inlet 212 may be disposed in the top wall 204 of the cavity 200. The air inlet 212 is arranged to discharge pressurised air into the cavity. Accordingly, the air inlet 212 may be fluidically coupled to the piping network 122 otherwise described herein. In particular, the air inlet 212 may be fluidically coupled to the respective branches of the piping network 122. As such, via the piping network 122, the air inlet 212 may provide the outlet of the fluidic coupling between the air supply 106 to the cavity 200. In use, the opening of the cavity 200 forms a substantially smooth air-water interface plane in which Kelvin-Helmholtz mixing occurs at the water-air interface. As a result of Kelvin-Helmholtz mixing, air bubbles are generated at the air-water interface, which then pass from the cavity 200 onto the downstream portion of the hull 102 of the vessel 100. In particular, the downwardly sloping rear wall part 210 is arranged to guide the air and water inside the cavity in a smooth flow pattern to the downstream portion of the hull 102. As the air bubbles pass from the cavity 200 onto the downstream portion of the hull 102, a continuous carpet of air bubbles is formed on the hull 102, between the hull 102 and the water, thereby reducing the frictional resistance between the hull of a ship and the water. In some examples it has been found that reducing these frictional resistances can have an effect of reducing vessel fuel consumption by up to at least 10%. Apparatuses for supplying air to air release units Fig. 3 shows a schematic diagram of an example apparatus 300 for supplying air from an air supply 106 to a plurality of air release units 200a, 200b disposed on a hull of the vessel, such as the vessel 100 of Fig. 1a. Apparatus 300 may be an example implementation of the apparatus 120 described herein with respect to Fig. 1. Accordingly, apparatus 300 may be suitable for supplying air from an air supply 106 to two air release units 200a, 200b and may comprise a piping network 302 with two branches 304a, 304b, each branch fluidically coupling the air supply 106 to a respective air release unit 200a, 200b. The piping network 302 of apparatus 300 may be designed to compensate for a static airflow differential between the respective fluid paths from the air supply 106 and each air release unit 200a, 200b. An airflow differential refers to the difference in airflow distribution across, for instance, the first branch 304a compared to the second branch 304b. Static airflow differentials are independent of vessel motion and may arise due to, for instance, differences between the respective pressure loss coefficients across each branch. Each branch may have a different loss coefficient due to differences in the: length of, number of bends in, sharpness of bends in, and / or number of piping components in each branch. As each branch couples the same air supply to the air release units, the pressure loss across each branch is substantially equal. Accordingly, if the pressure loss coefficient across the first branch 304a is substantively higher than the pressure loss coefficient across a second branch 304b, airflow is unevenly distributed between the branches 304a, 304b such that air is discharged from the first branch 304a at a lower rate than from the second branch 304b. Consequently, the properties and stability of the lubricating air layer differ between each air release unit 200a, 200b. The piping network 302 shown in Fig. 3 is designed to compensate for static airflow differentials between the air supply 106 and the air release units 200a, 200b, and therefore minimise the differences between the properties and stability of the lubricating layer of air between different air release units 200a, 200b. For instance, each branch 304a, 304b of the piping network may comprise fixed restrictors 306a, 306b suitable for and configured to introduce restrictions within each branch 304a, 304b. Such restrictions are designed to minimise the proportional difference in pressure loss coefficient between the air supply 106 and the respective air release unit 200a, 200b of each branch 304a, 304b. For instance, as shown in Fig. 3, each branch 304a, 304b may comprise any suitable number or type of fixed restrictors 306a, 306b for introducing restrictions such that the pressure loss coefficient differential across each branch 304a, 304b is within a threshold. As shown in Fig. 3 the fixed restrictors 306a, 306b may comprise any suitable combination of piping components, such as reducers, restrictors, non-return valves and / or two-way valves (e.g., gate valves). In addition, the distance and / or number of pipe bends in each branch 306a, 306b may be varied to introduce the desired restrictions. Furthermore, the apparatus 300 shown in Fig. 3 may include additional fixed restriction means - such as orifice plates 308a, 308b - in certain branches 304a, 304b to increase the pressure loss coefficient differential in less restricted branches 304a, 304b. The orifice plates 308a, 308b can therefore help counteract static airflow differentials and balance airflow distribution between air release units 200a, 200b. While the arrangement of Fig. 3 enables some airflow imbalances to be corrected, the present inventors have identified that this arrangement is not suitable in all cases and suffers from certain shortcomings. In particular, the apparatus 300 of Fig. 3 may not be suitable for compensating for dynamic pressure imbalances between air release units 200a, 200b. Dynamic pressure imbalances relate to imbalances in hydrostatic pressure between branches arising due to vessel motion, such as vessel pitch, trim, roll and heel. For instance, as described with respect to Fig. 1b, the air release units 200 may be spread across the width and / or length of the hull 102. The longitudinal distance between any two air release units is related to the amount of dynamic pressure imbalance induced by vessel pitch or trim between the respective branches coupled thereto. Similarly, the transverse distance between any two air release units is related to the dynamic pressure imbalance induced by vessel roll or heel between the respective branches coupled thereto. As used herein, the longitudinal distance is along the length of the hull and the transverse distance is perpendicular to the longitudinal distance, and along the width of the hull. As a result of dynamic pressure imbalances, vessel motion can cause airflow to be unevenly distributed between the air release units distributed about the hull. As such, the apparatus 300 of Fig. 3 may not be suitable for supplying air from an air supply 106 to many air release units 200 disposed distally across the length and / or width of the hull when dynamic pressure imbalances are significant. The apparatus 300 of Fig. 3 may therefore not be suitable for supplying air from an air supply 106 to more than two or more than four air release units 200. Instead, the apparatus 300 of Fig. 3 may only be suitable for supplying air from an air supply 106 to air release units 200 disposed substantially proximally to each other, such as only to neighbouring air release units 200. Fig. 4 shows a schematic diagram of an improved apparatus 400 for supplying air from an air supply 106 to a plurality of air release units 200a, 200b, 200c, 200d. The air release units 200a, 200b, 200c, 200d may be disposed on the vessel, such as the vessel 100 of Fig. 1a. As shown, the apparatus 400 of Fig. 4 includes a piping network 402 comprising a plurality of branches 404a, 404b, 404c, 404d, ... 404n. As shown, the piping network 402 may include at least 4 branches. However, as described with respect to Figs. 1a and 1b, the piping network may include a number of branches corresponding to the number of air release units supplied air by the apparatus 400. For instance, the number of branches may be equal to the number of air release units. Alternatively, the number of branches may be lower than the number of air release units, in which case one or more of the branches may respectively be arranged to fluidically couple the air supply 106 to multiple air release units. In either instance, the piping network may include any number of branches up to and including 24 branches, such as at least 2, at least 4, at least 8, at least 12, at least 14, at least 16, at least 18, at least 20, at least 22, or at least 24 branches. As shown in Fig. 4, each branch of the piping network 402 may comprise any fixed restrictors 406a, 406b, 406c, 406d suitable for minimising the static airflow differential between respective air release unit 200a, 200b, 200c, 200d of each branch 404a, 404b, 404c, 404d. Each of the fixed restrictors 406a, 406b, 406c, 406d may be as described with respect to fixed restrictors 306a, 306b of Fig. 3. In the apparatus 400 according to the present disclosure, fixed restrictors 406a, 406b, 406c, 406d are optional and need not be provided. In some examples, fixed restriction may be provided by the piping network 402 itself. The apparatus 400 of Fig. 4 differs from the apparatus 300 of Fig. 3 in that the apparatus of Fig. 4 includes a variable flow restrictor 408a, 408b, 408c, 408d disposed in at least a subset of the branches 404a, 404b, 404c, 404d. In some examples, a variable flow restrictor 408a, 408b, 408c, 408d is disposed in every branch 404a, 404b, 404c, 404d of the apparatus 400. Each variable flow restrictor is selectively adjustable to vary the amount of restriction in the respective branch. For instance, the variable flow restrictors may be selectively adjusted or actuated to adjust the resistance to airflow in each respective branch of the piping network 402. As examples, the variable flow restrictors may be actuatable valves, butterfly valves, airflow dampers, and / or adjustable orifice plates. Actuatable valves are actuatable between varying positions providing different amounts of restriction. Butterfly valves are examples of actuatable valves. Butterfly valves can regulate the flow of fluid therethrough by, for instance, selectively rotating a disk in an opening thereof. The relative orientation of the disk and the opening determines the amount of restriction provided by the butterfly valve. Airflow dampers control the rate of flow therethrough by adjusting the amount of free space in the opening thereof. Adjustable orifice plates may be examples of airflow dampers. For instance, adjustable orifice plates may comprise an actuatable ring closable to adjust the amount of free space in the opening thereof. For instance, the adjustable orifice plate may comprise an iris damper comprising an iris-like shutter mechanism. As shown in Fig. 4, the apparatus 400 may further comprise a controller 126. The controller 126 may be as described with respect to Fig. 1a. In addition, the controller 126 may be configured to adjust a flow restriction of the variable flow restrictors 408a, 408b, 408c, 408d. Accordingly, the controller 126 is arranged to communicate with the variable flow restrictors 408a, 408b, 408c, 408d. For instance, the controller 126 may include a transmitter arranged to wirelessly emit a signal and each variable flow restrictor 408a, 408b, 408c, 408d may include a cooperating receiver arranged to receive the signal from the transmitter. Alternatively, the controller 126 may communicate with the variable flow restrictors 408a, 408b, 408c, 408d through wired communication channels or fibre optic cables local to the apparatus 400 and / or the vessel. Example wired communication channels may form a Local Area Network (LAN), e.g., via Ethernet cables or copper wire. The controller may be configured to instruct one or more actuators coupled to one or more variable flow restrictors to adjust the flow restriction thereof. In these examples, the controller may be configured to adjust a flow restriction of the variable flow restrictors based on an airflow parameter of the apparatus. The airflow parameter may be a measured airflow rate in the piping network 412, or a target airflow rate requested of an air supply 106. In either implementation, the airflow parameter is indicative of (e.g., approximating) an airflow rate (e.g., a mass airflow rate) in the piping network 402. When the airflow parameter is a measured airflow rate, the airflow parameter directly indicates airflow rate in the piping network 402. When the airflow parameter is a target airflow rate requested of an air supply 106, the airflow parameter indicates the expected airflow rate in the piping network 402. Accordingly, the apparatus 400 of the present disclosure can compensate for dynamic pressure imbalances by selectively adjusting the flow restriction in each branch 404a, 404b, 404c, 404d of the apparatus 400 such that the selected flow restriction in each branch induces a larger pressure differential across all branches 404a, 404b, 404c, 404d (i.e., the pressure differential from air supply 106 to air release units 200a, 200b, 200c, 200d) than the dynamic pressure imbalance between branches introduced by vessel motion. For instance, the flow restriction in each variable flow restrictor may be selected to induce a pressure differential across all branches of a higher magnitude (e.g., a magnitude above a set threshold or of a higher order) than the dynamic pressure imbalance between any two branches. As such, the dynamic pressure imbalance can have a negligible impact (e.g., an impact below a set tolerance or threshold) on airflow redistribution. Here, the flow restriction in each branch may comprise the sum of each pipe’s fixed restrictors 406a, 406b, 406c, 406d and the selected flow restriction of the respective variable flow restrictor 408a, 408b, 408c, 408d. In some examples, the airflow rate in the piping network 402 may vary according to the operation of the vessel on which the apparatus 400 is installed. At higher vessel speeds, the airflow parameter (e.g., target or measured airflow rate) may increase. Similarly, at higher vessel drafts, the airflow parameter (e.g., target or measured airflow rate) may also increase. However, the pressure differential across a restrictor at a particular level of restriction varies according to airflow rate in the piping network. Accordingly, by adjusting a flow restriction of the variable flow restrictors based on an airflow parameter of the apparatus, the controller 126 of the present disclosure can mitigate dynamic pressure imbalances under multiple vessel operating conditions, such as vessel speeds and / or drafts, while preventing excessive average pressure loss across the branches which would result from high fixed restriction at high airflow rates. For instance, it has been found that as airflow rate in the piping network 402 increases, the pressure differential induced by restrictions in the piping network 402 - such as fixed restrictors 406a, 406b, 406c, 406d or the piping network 402 itself - also increases. Accordingly, the airflow distribution between each branch 404a, 404b, 404c, 404d, 404n of the apparatus 400 can be kept within a range (e.g., a range indicating an acceptable or threshold airflow distribution balance) by adjusting the flow restriction of the variable flow restrictors 408a, 408b, 408c, 408d to provide greater flow restriction at lower values of the airflow parameter and, in contrast, lesser flow restriction at higher values of the airflow parameter. As used herein, the greater flow restriction is greater with respect to the lesser flow restriction. In some examples, the controller may be configured to maintain an equal flow restriction in each variable flow restrictor 408a, 408b, 408c, 408d. That is, each variable flow restrictor 408a, 408b, 408c, 408d in each branch 404a, 404b, 404c, 404d is adjusted evenly such that the same restriction adjustments are made to each branch simultaneously. As a result, localised airflow differentials and flow rates need not be measured directly. Instead, flow rate measurements may be taken at, for instance, a common pipe 424, before splitting within the branches. Accordingly, the controller 126 of the apparatus 400 of Fig. 4 need not implement a complex feedback control loop (such as from a PID controller) to dynamically and individually control the variable flow restrictor 408a, 408b, 408c, 408d in each branch 404a, 404b, 404c, 404d, as the flow restriction in each branch varies. Instead, the controller can more simply adjust the flow restrictions across the apparatus 400 based on an airflow parameter of the entire piping network 402, such as an airflow parameter indicative of the total flow rate through all branches 404a, 404b, 404c, 404d. Furthermore, by avoiding a complex feedback control loop, the number of variable flow restrictor cycles undergone in normal operation can be reduced, thereby increasing the variable flow restrictor lifetime, and reducing the interval between docking and servicing of the vessel at which the apparatus 400 is installed. In some examples, the controller may be configured to adjust the flow restriction in each variable flow restrictor between a plurality of discrete flow restriction levels based on the airflow parameter. Each discrete flow restriction level represents a step change in flow restriction. The flow restriction may be adjusted equally for each sequential flow restriction level. For instance, the flow restriction may change linearly for each sequential flow restriction level. Alternatively, the flow restriction may change non-linearly. The plurality of discrete flow restriction levels may be set by discrete positions able to be taken by the variable flow restrictors. Alternatively, the plurality of discrete flow restriction levels may be discrete positions on a continuous spectrum of flow restriction levels at the variable flow restrictor. Accordingly, the controller may be configured to instruct the flow restriction to a discrete position by either instructing the variable flow restrictors to take a particular discrete position, or to take a discrete value on the continuous spectrum of flow restriction levels. Each discrete flow restriction level may correspond to a predetermined range of airflow parameter values. Therefore, each discrete flow restriction level may correspond to a (indicated) range of airflow rates in the piping network 402. Corresponding the discrete flow restriction levels and the predetermined range of airflow parameters involves setting a discrete position of the variable flow restrictor 408a, 408b, 408c, 408d for substantially every value of the airflow parameter. For instance, a first range of the airflow parameter may correspond to a first flow restriction level, a second range of the airflow parameter may correspond to a second flow restriction level, a third range of the airflow parameter may correspond to a third flow restriction level, and so on. Each possible airflow parameter value may correspond to no more than one (e.g., only one) discrete flow restriction level. The correspondence of the discrete flow restriction levels and the airflow parameters may be predetermined. For instance, the correspondence of the discrete flow restriction levels and the airflow parameters may be set when installing the air lubrication system onto a vessel. Similarly, the correspondence of the discrete flow restriction levels and the airflow parameters may be set or updated during a commissioning or trial process, such as the process described herein, particularly with respect to Fig. 6. However the correspondences of the discrete flow restriction levels and the range of airflow parameters are determined, the correspondences may be stored in a lookup table or an index accessible by or at the controller 126. Accordingly, when adjusting the flow restriction in each variable flow restrictor between a plurality of discrete flow restriction levels based on the airflow parameter, the controller 126 may access the lookup table or index and, from the lookup table or index, identify the discrete flow restriction level corresponding to the current airflow parameter. The controller 126 may subsequently instruct each variable flow restrictor 408a, 408b, 408c, 408d to adjust to a flow restriction position which provides the corresponding discrete flow restriction level. After a set number of flow restrictor cycles, the variable flow restrictor may need servicing or replacing. As such, the life of the variable flow restrictor, and the capability of the apparatus 400 described herein to continuously operate when the vessel is at sea is determined by the undergone number of flow restrictor cycles. Adjusting the flow restriction in each variable flow restrictor between a plurality of discrete flow restriction levels can minimise the number of cycles undergone by the variable flow restrictors 408a, 408b, 408c, 408d during operation. In these examples, the flow restrictors only cycle between flow restriction levels when transitioning from one discrete level to another, such as when the airflow parameter transitions from one predetermined range to another. When the variable flow restrictors are continuously operating in the same flow restriction level the variable flow restrictors 408a, 408b, 408c, 408d do not undergo any cycles. Therefore, adjusting the flow restriction in each variable flow restrictor between a plurality of discrete flow restriction levels can increase the variable flow restrictor lifetime and reduce the interval between docking and servicing of the vessel at which the apparatus 400 is installed. For instance, when the airflow parameter substantially remains within a predetermined range (which would often be the case during normal operation, e.g., while the vessel is travelling at a steady speed), the variable flow restrictors 408a, 408b, 408c, 408d do not cycle away from the discrete value set by the predetermined range. As described herein, the airflow parameter may comprise a measured airflow rate. A measured airflow rate is an airflow rate measured in the piping network 402. For example, the measured airflow rate in the piping network 402 may be the overall mass airflow rate passing through the piping network 402. As such, the measured airflow rate may be measured at any point in the piping network 402 suitable for measuring the overall mass airflow rate passing through the piping network 402. The measured airflow rate may be measured at any upstream portion of the piping network 402 relative to the branches 404a, 404b, 404c, 404d, 404n. Alternatively, the measured airflow rate may be determined by summing respective airflow rates measured in each branch 404a, 404b, 404c, 404d, 404n. The portions of the piping network 402 upstream of the branches 404a, 404b, 404c, 404d, 404n are the portions of the piping network 402 through which total airflow can pass before being split. When the airflow parameter is a measured airflow rate, the apparatus 400 of the present disclosure can monitor the airflow rate delivered to the piping network 402 (and therefore to the air release units 200a, 200b, 200c, 200d), For instance, the controller’s adjustment of flow restriction of the variable flow restrictors 408a, 408b, 408c, 408d can be based on the delivered airflow and therefore can account for variations therein. The airflow parameter being a measured airflow rate can therefore, in effect, increase the accuracy of the restrictor position in operation, and therefore increase the consistency of operation of the apparatus 400. When the air supply 106 comprises a compressor, the restrictor position accuracy can be improved to account for faults and / or inaccuracies in airflow delivery from the compressor. Similarly, when the air supply 106 comprises an engine of the vessel, the apparatus 400 described herein can be used to monitor the accuracy of the airflow rate of the pressurised air drawn from the engine, into the piping network 402. Further, by measuring the overall mass airflow rate passing through the piping network 402, the number of flowmeters required in the apparatus 400 is reduced. That is, additional flowmeters need not be provided in each branch 404a, 404b, 404c, 404d, 404n. By obviating the need for additional flowmeters in each branch 404a, 404b, 404c, 404d, 404n, a complex feedback control loop for each variable flow restrictor 408a, 408b, 408c, 408d can also be avoided. As otherwise discussed herein, the controller can therefore more simply adjust the flow restrictions across the apparatus 400 based on an airflow parameter of the entire piping network 402 and the number of variable flow restrictor cycles undergone by normal operation can be reduced. When the airflow parameter is a measured airflow rate, the airflow rate may be measured by at least one flowmeter. For instance, the apparatus may further comprise at least one flowmeter disposed in the piping network 402. The flowmeter may be disposed in a common pipe 424, which may be as otherwise described herein with respect to common pipe 124 of Fig. 1a. The common pipe is disposed to fluidically couple the air supply to each of the branches 404a, 404b, 404c, 404d, 404n of the apparatus 400. In other examples, the airflow rate may be measured indirectly. For instance, the controller 126 may be arranged to infer or measure a pressure difference across the piping network 402. In these examples, the controller 126 may additionally be arranged to detect airflow temperature in the piping network 402, for instance to calculate air density in the piping network. Accordingly, the airflow rate may be measured indirectly from a measured or inferred pressure difference across the piping network and the airflow temperature and / or air density. As described herein, the airflow parameter may comprise a target airflow rate. The target airflow rate is the airflow rate requested from the air supply 106. That is, the target airflow rate is the airflow rate that is requested to be delivered by the air supply 106 into the piping network 402. To identify the target airflow rate, the controller 126 may be in communication with the air supply 106. For instance, the controller may be configured to communicate the target airflow rate to the air supply. In this case, the controller 126 may be configured to receive operating conditions of the vessel and / or of the air release units 200a, 200b, 200c, 200d and the controller 126 may be configured to calculate the target airflow rate from at least some of these operating conditions. In other implementations, the target airflow rate may be calculated separately from the controller 126 (such as at a separate processor in communication with the air supply 106), and the target airflow rate may be communicated from the air supply 106 (e.g., from a separate processor in communication therewith) to the controller 126. In examples where the air supply 106 comprises a compressor, the target airflow rate may be set as the airflow output of the compressor. In examples where the air supply 106 comprises the engine, the target airflow rate may be set as the airflow output selectively drawn from the engine, such as from a holding chamber of the engine. The target airflow rate may be related to one or more operating conditions of the vessel and / or of the air release units 200a, 200b, 200c, 200d. For instance, the airflow requirements of each air release unit 200a, 200b, 200c, 200d - e.g., the airflow required to achieve a consistent layer of air between the hull and the water - may be dependent on the vessel speed and / or vessel draft. Higher airflow rates at the air release units 200a, 200b, 200c, 200d may be required for faster vessel speeds and / or deeper vessel drafts. Accordingly, the target airflow rate may be positively correlated with vessel speed and / or vessel draft. Therefore, the controller 126 or separate processor in communication with the air supply 106 may be arranged to receive indications of vessel speed and / or vessel draft and, from such indications, calculate the required target airflow rate to meet the current airflow requirements of the air release units 200a, 200b, 200c, 200d. Indications of vessel speed and / or vessel draft may comprise vessel speed data and vessel draft data respectively. Alternatively, the required target airflow need not be calculated from vessel speed, but from one or more conditions of the air release units 200a, 200b, 200c, 200d. Such air release unit conditions indicate current operation of the air release units 200a, 200b, 200c, 200d and therefore can indicate whether greater or lesser airflows are required at the air release units 200a, 200b, 200c, 200d to achieve an at least substantially consistent layer of air between the hull and the water. For instance, the air release unit conditions may include a thickness or uniformity of the layer of air between the hull and the water. Similarly, the air release unit conditions may include the size of the of air bubbles released from the air release units 200a, 200b, 200c, 200d, or the rate at which the air bubbles are released. Therefore, the controller 126 or separate processor in communication with the air supply 106 may be arranged to receive indications of one or more air release unit conditions and, from these indications, calculate the required target airflow rate to meet the current airflow requirements of the air release unit 200. Indications of one or more air release unit conditions may comprise air release unit condition data. When the airflow parameter is a target airflow rate - instead of, for instance, a measured airflow rate - the airflow parameter used to determine adjustment of the variable flow restrictors 408a, 408b, 408c, 408d can be constant. Accordingly, transient, and temporary fluctuations in the airflow delivered from the air supply 106 to the piping network 402 do not result in adjustments to the variable flow restrictors 408a, 408b, 408c, 408d. Since temporary fluctuations do not result in adjustments to the variable flow restrictors, flow restrictor cycles can be further minimised, increasing the lifespan of the apparatus 402. Additionally, adjusting the variable flow restrictors 408a, 408b, 408c, 408d due to transient and temporary fluctuations may generate undesirable feedback loops, further increasing the number of flow restrictor cycles and reducing the effectiveness of the apparatus 400. The apparatus 400, particularly the controller 126 thereof, may be arranged to calibrate the flow restrictor positions, such as by carrying out the method described herein with respect to Fig. 6. In these examples, the apparatus 400 may further comprise a differential pressure gauge disposed in at least one of the branches 404a, 404b, 404c, 404d, 404n. The differential pressure gauge is arranged to measure pressure loss across the variable flow restrictor(s) disposed in the respective branch. In some examples the differential pressure gauge may only be installed in one branch, in which case the flow restrictor positions may be calibrated based on measurements only taken in that branch. Alternatively, differential pressure gauges may be disposed in each branch to assess the relative performance of variable flow restrictor(s) in each branch and / or to monitor variable flow restrictor health. Methods of modifying pressure loss Fig. 5 shows a flowchart of an example method of controlling (e.g., modifying) pressure loss across a piping network of an air lubrication system of a vessel. The piping network and air lubrication system are as otherwise described herein, particularly with respect to any of Figs. 1a-4. For instance, the piping network may be comprised in an apparatus, such as apparatus 400 described with respect to Fig. 4. At least a portion of the method shown in Fig. 5 may be carried out by a controller, such as the controller 126 of the apparatus 400 described with respect to Fig. 4. Any steps of the method may be as otherwise described in more detail with respect to any of Figs. 1a-4. The method may optionally begin at step S100 at which a target airflow rate is received. The target airflow rate may be received by the controller of the apparatus. The target airflow rate is the airflow rate that is requested to be delivered by the air supply into the piping network. Receiving the target airflow rate may comprise calculating the target airflow rate based on the airflow requirements of the air release unit(s). The airflow requirements of the air release units may be calculated from operating conditions of the vessel and / or of the air release units. In alternative implementations, the target airflow rate may be received by the controller from a separate processor in communication with the air supply. The method may optionally include step S105 at which the airflow rate is measured in the piping network. The airflow rate may comprise a total flow rate through all branches of the piping network. For instance, the airflow rate may be measured before being split between branches. The airflow rate may be measured by at least one flowmeter disposed at any point in the piping network, such as in a common pipe of the piping network. Alternatively, the airflow rate may be measured indirectly from a measured pressure difference across the piping network and the airflow temperature. Fig. 5 shows step S105 carried out sequentially after step S100. However, steps S100 and S105 may alternatively be carried out in parallel, or step S100 may be carried out sequentially before, step S105. Alternatively, only one or neither of steps S100 and S105 may be provided. For instance, the airflow rate may be measured before, in parallel to, or after the target airflow rate is received. The method may start or otherwise continue at step S110 at which an airflow parameter of the air lubrication system is received. The airflow parameter may be received at the controller of the system. As otherwise described herein, the airflow parameter indicates or approximates an airflow rate (e.g., a mass airflow rate) in the piping network. The airflow parameter may be the target airflow rate received at step S100. The airflow parameter may be the measured airflow rate at step S105. Responsive to receiving the airflow parameter, the method may continue to step S115 at which flow restrictions of the variable flow restrictors of the system are adjusted based on the airflow parameter. Adjusting the flow restriction of each variable flow restrictor may include selectively adjusting each respective variable flow restrictor to vary the amount of restriction in the respective branches within which the variable flow restrictor is disposed. As otherwise described herein, the variable flow restrictors may be actuatable valves, butterfly valves, airflow dampers, and / or adjustable orifice plates. In some implementations, adjusting the flow restriction of the variable flow restrictors comprises providing greater flow restriction at lower values of the airflow parameter and lesser flow restriction at higher values of the airflow parameter. Adjusting the flow restriction of the variable flow restrictors may comprise maintaining an equal flow restriction for each variable flow restrictor. Similarly, adjusting the flow restriction of the variable flow restrictors may comprise adjusting the flow restriction of the variable flow restrictors between a plurality of discrete flow restriction levels based on the airflow parameter. In this case, each discrete flow restriction level may correspond to a range of airflow parameter values. Methods of calibrating flow restrictor positions Fig. 6 shows a flowchart of an example method of calibrating a first flow restrictor position of a first variable flow restrictor disposed in a branch of a piping network of an air lubrication system of a vessel. The first flow restrictor position is for a first flow restriction level of the first variable flow restrictor. For instance, the first flow restriction level may be a known restriction level of, or a restriction level corresponding to a known pressure differential at a set airflow rate through, the first flow restrictor. In the method of Fig. 6, the piping network, air lubrication systems, and vessel are as otherwise described herein, particularly with respect to any of Figs. 1a-4. For instance, the air lubrication system may include an apparatus including a piping network, such as apparatus 400 described with respect to Fig. 4. At least a portion of the method shown in Fig. 6 may be carried out by a controller, such as the controller 126 of the apparatus 400 described with respect to Fig. 4. The method shown in Fig. 6 may be carried out in parallel with, or sequentially before or after, the method of modifying pressure loss across a piping network discussed herein with respect to Fig. 5. The method of calibrating a flow restrictor position may start at step S200 at which a first variable flow restrictor in a branch of the piping network is positioned at an initial flow restrictor position. The initial flow restrictor position may be a starting position of the variable flow restrictor. For instance, the initial flow restrictor position may be a position previously calibrated to correspond to the first flow restriction level. Next, the method continues to step S205 at which a first pressure loss is measured across the first variable flow restrictor, while the first flow restrictor is at the initial flow restrictor position. The first pressure loss may be measured using a differential pressure gauge disposed in the branch containing the first variable flow restrictor. If the first pressure loss is outside a threshold range of a target pressure loss for the first flow restriction level, the method continues to step S210 at which the first variable flow restrictor is repositioned to a tuned flow restrictor position. The threshold range comprises a range (e.g., an acceptable range, within a set tolerance) of pressure losses about the target pressure loss for the first flow restriction level at which the first flow restrictor is to be calibrated. For instance, the threshold range may correspond to a specific range of pressure loss coefficient values. The tuned variable flow restrictor position is a position different to the initial flow restriction position. For instance, the tuned variable flow restrictor position may be a more restricted position if the first pressure loss is lower than a threshold range of a target pressure loss. On the other hand, the tuned variable flow restrictor position may be a less restricted position if the first pressure loss is higher than a threshold range of a target pressure loss. Alternatively, if the first pressure loss is outside a threshold range of a target pressure loss for the first flow restriction level, the first flow restrictor position is calibrated based on (e.g., to correspond to), the initial flow restrictor position. Next, the method continues to step S215 at which a second pressure loss is measured across the first variable flow restrictor, while the first flow restrictor is at the tuned flow restrictor position. Like the first pressure loss, the second pressure loss may be measured using a differential pressure gauge disposed in the branch containing the first variable flow restrictor. If the second pressure loss is outside a threshold range of a target pressure loss for the first flow restriction level, the method may repeat steps S210 and S215 for a further tuned flow restrictor position. Otherwise, if the second pressure loss is within the threshold range, the method continues to step S220 at which the first flow restrictor position is calibrated based on the tuned flow restrictor position. Calibrating the first flow restrictor position based on the tuned flow restrictor position may comprise setting the tuned flow restrictor position as the first flow restrictor position. Alternatively, calibrating the first flow restrictor position based on the tuned flow restrictor position may comprise setting the first flow restrictor position to the discrete flow restrictor position nearest to the tuned flow restrictor position. After calibrating the first flow restrictor position at step S220, the method may proceed to optional step S225 at which other flow restrictor positions may be calibrated. For instance, one or more of the other flow restrictor positions may be calibrated using steps that correspond to steps S200-S220 described herein with respect to the first variable flow restrictor. In particular, another flow restrictor position of the first variable flow restrictor may be calibrated by positioning the first variable flow restrictor at another initial flow restrictor position. Then, a pressure loss may be measured across the other variable flow restrictor. Then, the other variable flow restrictor may be positioned to a corresponding tuned flow restrictor position. Then, another pressure loss may be measured across the other variable flow restrictor at the corresponding tuned flow restrictor position and, if the other pressure loss is within a threshold range, the other flow restrictor position may be calibrated based on the corresponding tuned flow restrictor position. Alternatively, the other flow restrictor positions may be calibrated based on the difference between the tuned flow restrictor position and the initial flow restrictor position for the first flow restrictor position. In these examples, the other flow restrictor positions may comprise the other flow restrictor positions of the first variable flow restrictor and / or the flow restrictor position of the other variable flow restrictors in the piping network. In some examples, the method shown in Fig. 6 may be repeated for the other variable flow restrictors in the piping network. For instance, a differential pressure gauge may be disposed in multiple branches of the piping network and may measure pressure losses individually across the respective variable flow restrictors. Alternatively, a pressure gauge may be temporarily disposed in a branch of the piping network to, for instance, investigate a fault. Methods of installing air lubrication systems Fig. 7 shows a flowchart of an example method of installing an air lubrication system onto a vessel. The air lubrication system may be as otherwise described herein, particularly with respect to any of Figs. 1a-4. The method optionally starts at step S300 at which a plurality of air release units are disposed in (e.g., installed into) the hull of the vessel. The air release units may be disposed on the hull by any suitable means, such as by welding a flange of the air release unit into the hull. The method then optionally proceeds to step S305 at which a piping network is routed between an air supply and the plurality of air release units. The piping network may be as described with respect to the apparatus of Fig. 4. Routing the piping network between an air supply and the plurality of air release units may fluidically couple each branch of the piping network to a respective one of the plurality of air release units. The method then optionally proceeds to, or starts at, step S310 at which respective variable flow resistors may be fit into each branch of the piping network. In some examples, particularly when the method starts at step S310, fitting a variable flow restrictor into each branch of the piping network may comprise retrofitting a variable flow restrictor into the piping network of an air lubrication system already provided in the vessel. For instance, the already provided air lubrication system may be provided without variable flow restrictors. Computing devices and computer readable media The approaches and methods described herein may be computer-implemented, in other words may be embodied on a computer-readable medium, which may be a non-transitory computer-readable medium. The computer-readable medium may carry computer-readable instructions arranged for execution upon one or more processors so as to make the one or more processors carry out any or all of the methods described herein. The term “computer-readable medium” as used herein refers to any medium that stores data and / or instructions for causing a processor to operate in a specific manner. Such storage medium may comprise non-volatile media and / or volatile media. Non-volatile media may include, for example, optical or magnetic disks. Volatile media may include dynamic memory. Exemplary forms of storage medium include, a floppy disk, a flexible disk, a hard disk, a solid state drive, a magnetic tape, or any other magnetic data storage medium, a CD-ROM, any other optical data storage medium, any physical medium with one or more patterns of holes, a RAM, a PROM, an EPROM, a FLASH-EPROM, NVRAM, and any other memory chip or cartridge. Fig. 8 illustrates a block diagram of one implementation of a computing device 1000 within which a set of instructions, for causing the computing device to perform any one or more of the methodologies discussed herein, may be executed. For instance, computing device 1000 may be an example of the controller 126 described herein. In alternative implementations, the computing device may be connected (e.g., networked) to other machines in a Local Area Network (LAN), an intranet, an extranet, or the Internet. The computing device may operate in the capacity of a server or a client machine in a client-server network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. The computing device may be a personal computer (PC), a tablet computer, a set-top box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a server, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single computing device is illustrated, the term “computing device” shall also be taken to include any collection of machines (e.g., computers) that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. The example computing device 1000 includes a processing device 1002, a main memory 1004 (e.g., read-only memory (ROM), flash memory, dynamic random-access memory (DRAM) such as synchronous DRAM (SDRAM) or Rambus DRAM (RDRAM), etc.), a static memory 1006 (e.g., flash memory, static random-access memory (SRAM), etc.), and a secondary memory (e.g., a data storage device 1018), which communicate with each other via a bus 1030. Processing device 1002 represents one or more general-purpose processors such as a microprocessor, central processing unit, or the like. More particularly, the processing device 1002 may be a complex instruction set computing (CISC) microprocessor, reduced instruction set computing (RISC) microprocessor, very long instruction word (VLIW) microprocessor, processor implementing other instruction sets, or processors implementing a combination of instruction sets. Processing device 1002 may also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. Processing device 1002 is configured to execute the processing logic (instructions 1022) for performing the operations, methods and steps discussed herein. The computing device 1000 may further include a network interface device 1008. The computing device 1000 also may include a video display unit 1010 (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)), an alphanumeric input device 1012 (e.g., a keyboard or touchscreen), a cursor control device 1014 (e.g., a mouse or touchscreen), and an audio device 1016 (e.g., a speaker). The alphanumeric input device 1012 and the cursor control device 1014 may be considered together as a single input mechanism. The data storage device 1018 may include one or more machine-readable storage media (or more specifically one or more non-transitory computer-readable storage media) 1028 on which is stored one or more sets of instructions 1022 embodying any one or more of the methodologies or functions described herein. The instructions 1022 may also reside, completely or at least partially, within the main memory 1004 and / or within the processing device 1002 during execution thereof by the computer system 1000, the main memory 1004 and the processing device 1002 also constituting computer-readable storage media. The various methods described above may be implemented by a computer program. The computer program may include computer code arranged to instruct a computer to perform the functions of one or more of the various methods described above. The computer program and / or the code for performing such methods may be provided to an apparatus, such as a computer, on one or more computer readable media or, more generally, a computer program product. The computer readable media may be transitory or non-transitory. The one or more computer readable media could be, for example, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, or a propagation medium for data transmission, for example for downloading the code over the Internet. Alternatively, the one or more computer readable media could take the form of one or more physical computer readable media such as semiconductor or solid-state memory, magnetic tape, a removable computer diskette, a random-access memory (RAM), a read-only memory (ROM), a rigid magnetic disc, and an optical disk, such as a CD-ROM, CD-R / Wor DVD. In an implementation, the modules, components and other features described herein can be implemented as discrete components or integrated in the functionality of hardware components such as ASICS, FPGAs, DSPs or similar devices. A “hardware component” is a tangible (e.g., non-transitory) physical component (e.g., a set of one or more processors) capable of performing certain operations and may be configured or arranged in a certain physical manner. A hardware component may include dedicated circuitry or logic that is permanently configured to perform certain operations. A hardware component may be or include a special-purpose processor, such as a field programmable gate array (FPGA) or an ASIC. A hardware component may also include programmable logic or circuitry that is temporarily configured by software to perform certain operations. Accordingly, the phrase “hardware component” should be understood to encompass a tangible entity that may be physically constructed, permanently configured (e.g., hardwired), or temporarily configured (e.g., programmed) to operate in a certain manner or to perform certain operations described herein. In addition, the modules and components can be implemented as firmware or functional circuitry within hardware devices. Further, the modules and components can be implemented in any combination of hardware devices and software components, or only in software (e.g., code stored or otherwise embodied in a machine-readable medium or in a transmission medium). Machine learning techniques may be employed to optimise any of the parameters of the present disclosure - such as any of the threshold values - through the training of a computational neural network on example training data, for example. As such, a database of past operations may be provided, either locally or at a remote content management system. Once the parameters have been trained by machine learning techniques for a given type or genre of audio track, further active machine learning need not be applied. Unless specifically stated otherwise, as apparent from the preceding discussion, it is appreciated that throughout the description, discussions utilizing terms such as "receiving”, “determining”, “comparing ”, “enabling”, “maintaining,” “identifying” or the like, refer to the actions and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. Penultimate comments It will be understood that certain terminology is used in the preceding description for convenience and is not limiting. The terms “a”, “an” and “the” should be read as meaning “at least one” unless otherwise specified. The term “comprising” will be understood to mean “including but not limited to” such that systems or method comprising a particular feature or step are not limited to only those features or steps listed but may also comprise features or steps not listed. Equally, terms such as “over”, “under”, “front”, “back”, “right”, “left”, “top”, “bottom”, “side”, “clockwise”, “anti-clockwise” and so on are used for convenience in interpreting the drawings and are not to be construed as limiting. Additionally, any method steps which are depicted in the figures as carried out sequentially may alternatively be carried out in series in any order unless otherwise specified. Further, any method steps which are depicted as dashed or dotted flowchart boxes are to be understood as being optional. The above description is intended to be illustrative, and not restrictive. Many other implementations will be apparent to those of skill in the art upon reading and understanding the above description. Although the present disclosure has been described with reference to specific example implementations, it will be recognized that the disclosure is not limited to the implementations described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. Further, the disclosure comprises the following clauses: Clause 1. An apparatus for supplying air from an air supply to two air release units disposed on a hull of a vessel, the apparatus comprising: a piping network comprising two branches, each branch for fluidically coupling the air supply to a respective one of the two air release units; a variable flow restrictor disposed in each branch; and a controller configured to adjust a flow restriction of the variable flow restrictors based on an airflow parameter of the apparatus. Clause 2. The apparatus of clause 1, wherein the controller is configured to adjust the flow restriction to provide greater flow restriction at lower values of the airflow parameter and lesser flow restriction at higher values of the airflow parameter. Clause 3. The apparatus of clause 1 or clause 2, wherein the controller is configured to maintain an equal flow restriction for each variable flow restrictor. Clause 4. The apparatus of any preceding clause, wherein the airflow parameter comprises a measured airflow rate in the piping network. Clause 5. The apparatus of any preceding clause, further comprising a flowmeter disposed to measure the airflow rate in the piping network. Clause 6. The apparatus of clause 5, wherein the flowmeter is provided in a common pipe, the common pipe for fluidically coupling the air supply to each of the two branches. Clause 7. The apparatus of any preceding clause, wherein the airflow parameter comprises a target airflow rate, wherein the target airflow rate is a requested airflow rate from the air supply. Clause 8. The apparatus of clause 7, wherein the controller is configured to communicate the target airflow rate to the air supply. Clause 9. The apparatus of clause 7 or clause 8, wherein the controller is configured to receive the target airflow rate from the air supply. Clause 10. The apparatus of any of clauses 7-9, wherein the target airflow rate is positively correlated with vessel speed and / or a draft of the vessel. Clause 11. The apparatus of any of clauses 7-10, wherein the target airflow rate is determined based on a condition of the air release unit. Clause 12. The apparatus of any of clauses 7-11, wherein the controller is configured to receive one or more of vessel speed data, vessel draft data, and / or air release unit condition data and set the target airflow rate based on the one or more of vessel speed data, vessel draft data, and / or air release unit condition data. Clause 13. The apparatus of any preceding clause, wherein the controller is configured to adjust the flow restriction in each variable flow restrictor between a plurality of discrete flow restriction levels based on the airflow parameter. Clause 14. The apparatus of clause 13, wherein each discrete flow restriction level corresponds to a range of airflow parameter values. Clause 15. The apparatus of clause 13 or clause 14, wherein the discrete flow restriction levels correspond to respective flow restrictor positions, and wherein the controller is configured to calibrate a first flow restrictor position for a first flow restriction level of the discrete flow restriction levels for a first one of the two variable flow restrictors by: positioning the first variable flow restrictor at an initial flow restrictor position; measuring a first pressure loss across the first variable flow restrictor; if the first pressure loss is outside a threshold range of a target pressure loss for the first flow restriction level, positioning the first variable flow restrictor at a tuned flow restrictor position; measuring a second pressure loss across the first variable flow restrictor; and if the second pressure loss is within the threshold range, calibrating the first flow restrictor position based on the tuned flow restrictor position. Clause 16. The apparatus of clause 15, further comprising a differential pressure gauge disposed in a first branch of the two branches and arranged to measure pressure loss across a first variable flow restrictor disposed in the first branch. Clause 17. The apparatus of any preceding clause, wherein the variable flow restrictor comprises one or more of an actuatable valve, a butterfly valve, an airflow damper, and / or an adjustable orifice plate. Clause 18. The apparatus of any preceding clause, wherein the apparatus is for supplying air from the air supply to four air release units disposed on a hull of the vessel, and wherein the piping network comprises four branches, each branch for fluidically coupling the air supply to a respective one of the four air release units. Clause 19. A system for supplying air to two air release units disposed on a hull of a vessel, the system comprising: the apparatus of any previous clause; and an air supply fluidically coupled to the piping network of the apparatus. Clause 20. The system of clause 19, wherein the air supply comprises an engine of the vessel, and wherein the piping network is disposed to scavenge air from the engine. Clause 21. The system of clause 19 or clause 20, wherein the air supply comprises a compressor configured to discharge air into the piping network. Clause 22. A vessel comprising: a hull; two air release units disposed on the hull; and the system for supplying air of any of clauses 19-21. Clause 23. A method of modifying pressure loss across a piping network of an air lubrication system of a vessel, the air lubrication system comprising the piping network, an air supply, and two air release units disposed on a hull of the vessel, wherein the piping network fluidically couples the air supply to the two air release units and comprises two branches, each branch fluidically coupling the air supply to a respective one of the two air release units, and wherein a variable flow restrictor is disposed in each branch, the method comprising: receiving an airflow parameter of the air lubrication system; and adjusting a flow restriction of the variable flow restrictors based on the airflow parameter. Clause 24. The method of clause 23, wherein the method is carried out by a controller of the air lubrication system. Clause 25. The method of clause 23 or clause 24, wherein adjusting the flow restriction of the variable flow restrictors comprises providing greater flow restriction at lower values of the airflow parameter and lesser flow restriction at higher values of the airflow parameter. Clause 26. The method of any of clauses 23-25, wherein adjusting the flow restriction of the variable flow restrictors comprises maintaining an equal flow restriction for each variable flow restrictor. Clause 27. The method of any of clauses 23-26, the method further comprising measuring the airflow rate in the piping network, wherein the airflow parameter comprises the measured airflow rate. Clause 28. The method of clause 27, wherein measuring the airflow rate comprises measuring the airflow rate with a flowmeter, wherein the flowmeter is optionally provided in a common pipe, the common pipe for fluidically coupling the air supply to each of the two branches. Clause 29. The method of any of clauses 23-28, wherein the airflow parameter comprises a target airflow rate, wherein the target airflow rate is a requested airflow rate from the air supply. Clause 30. The method of clause 29, the method further comprising requesting the air supply to supply air to the piping network at the target airflow rate or receiving the target airflow rate from the air supply. Clause 31. The method of clause 29 or clause 30, wherein the target airflow rate is positively correlated with vessel speed and / or a draft of the vessel. Clause 32. The method of any of clauses 29-31, wherein the target airflow rate is determined based on a condition of the air release unit. Clause 33. The method of any of clauses 29-32, the method further comprising receiving one or more of vessel speed data, vessel draft data, and / or air release unit condition data and setting the target airflow rate based on the one or more of vessel speed data, vessel draft data, and / or air release unit condition data. Clause 34. The method of any of clauses 23-33, wherein adjusting the flow restriction of the variable flow restrictors comprises adjusting the flow restriction of the variable flow restrictors between a plurality of discrete flow restriction levels based on the airflow parameter. Clause 35. The method of any of clauses 34, wherein each discrete flow restriction level corresponds to a range of airflow parameter values. Clause 36. A method of calibrating a first flow restrictor position of a first variable flow restrictor disposed in a first branch of a piping network of an air lubrication system of a vessel, the air lubrication system comprising the piping network, an air supply, and two air release units disposed on a hull of the vessel, wherein the piping network fluidically couples the air supply to the two air release units and comprises two branches, each branch fluidically coupling the air supply to a respective one of the two air release units, wherein a variable flow restrictor is disposed in each branch, and wherein the first flow restrictor position is for a first flow restriction level of the first variable flow restrictor, the method comprising: positioning the first variable flow restrictor at an initial flow restrictor position; measuring a first pressure loss across the first variable flow restrictor; if the first pressure loss is outside a threshold range of a target pressure loss for the first flow restriction level, positioning the first variable flow restrictor at a tuned flow restrictor position; measuring a second pressure loss across the first variable flow restrictor; and if the second pressure loss is within the threshold range, calibrating the first flow restrictor position based on the tuned flow restrictor position. Clause 37. The method of clause 36, wherein the first flow restriction level is of a plurality of discrete flow restriction levels. Clause 38. The method of clause 36 or clause 37, wherein the first pressure loss and second pressure loss are measured using a differential pressure gauge disposed in the first 5 branch to measure pressure loss across the first variable flow restrictor. Clause 39. A method of installing the system according to any of clauses 19-21 onto a vessel, the method comprising fitting a variable flow restrictor into each branch of the piping network. Clause 40. The method of clause 39, the method further comprising routing the piping 10 network between the air supply and the two air release units such that each branch of the piping network fluidically couples the air supply to a respective one of the two air release units. Clause 41. The method of clause 39 or clause 40, the method further comprising disposing the two air release units on the hull of the vessel. 15

Claims

1. An apparatus for supplying air from an air supply to two air release units disposed on a hull of a vessel, the apparatus comprising:a piping network comprising two branches, each branch for fluidically coupling the air supply to a respective one of the two air release units;a variable flow restrictor disposed in each branch; anda controller configured to adjust a flow restriction of the variable flow restrictors based on an airflow parameter of the apparatus.

2. The apparatus of claim 1, wherein the controller is configured to adjust the flow restriction to provide greater flow restriction at lower values of the airflow parameter and lesser flow restriction at higher values of the airflow parameter.

3. The apparatus of claim 1 or claim 2, wherein the controller is configured to maintain an equal flow restriction for each variable flow restrictor.

4. The apparatus of any preceding claim, wherein the airflow parameter comprises a measured airflow rate in the piping network.

5. The apparatus of any preceding claim, further comprising a flowmeter disposed to measure the airflow rate in the piping network, wherein the flowmeter is provided in a common pipe, the common pipe for fluidically coupling the air supply to each of the two branches.

6. The apparatus of any preceding claim, wherein the airflow parameter comprises a target airflow rate, wherein the target airflow rate is a requested airflow rate from the air supply.

7. The apparatus of claim 6, wherein: the target airflow rate is positively correlated with vessel speed and / or a draft of the vessel; and / or the target airflow rate is determined based on a condition of the air release unit.

8. The apparatus of any preceding claim, wherein the controller is configured to adjust the flow restriction in each variable flow restrictor between a plurality of discrete flow restriction levels based on the airflow parameter, wherein each discrete flow restriction level corresponds to a range of airflow parameter values.

9. The apparatus of claim 8, wherein the discrete flow restriction levels correspond to respective flow restrictor positions, and wherein the controller is configured to calibrate a firstflow restrictor position for a first flow restriction level of the discrete flow restriction levels for a first one of the two variable flow restrictors by:positioning the first variable flow restrictor at an initial flow restrictor position;measuring a first pressure loss across the first variable flow restrictor;if the first pressure loss is outside a threshold range of a target pressure loss for the first flow restriction level, positioning the first variable flow restrictor at a tuned flow restrictor position;measuring a second pressure loss across the first variable flow restrictor; andif the second pressure loss is within the threshold range, calibrating the first flow restrictor position based on the tuned flow restrictor position.

10. The apparatus of claim 9, further comprising a differential pressure gauge disposed in a first branch of the two branches and arranged to measure pressure loss across a first variable flow restrictor disposed in the first branch.

11. The apparatus of any preceding claim, wherein the variable flow restrictor comprises one or more of an actuatable valve, a butterfly valve, an airflow damper, and / or an adjustable orifice plate.

12. A system for supplying air to two air release units disposed on a hull of a vessel, the system comprising:the apparatus of any previous claim; andan air supply fluidically coupled to the piping network of the apparatus.

13. The system of claim 12, wherein the air supply comprises: an engine of the vessel, wherein the piping network is disposed to scavenge air from the engine; and / or a compressor configured to discharge air into the piping network.

14. A vessel comprising:a hull;two air release units disposed on the hull; andthe system for supplying air of claim 12 or claim 13.

15. A method of modifying pressure loss across a piping network of an air lubrication system of a vessel, the air lubrication system comprising the piping network, an air supply, and two air release units disposed on a hull of the vessel, wherein the piping network fluidically couples the air supply to the two air release units and comprises two branches, each branch fluidically coupling the air supply to a respective one of the two air release units, and wherein a variable flow restrictor is disposed in each branch, the method comprising:receiving an airflow parameter of the air lubrication system; andadjusting a flow restriction of the variable flow restrictors based on the airflow parameter.

16. The method of claim 15, wherein adjusting the flow restriction of the variable flow restrictors comprises providing greater flow restriction at lower values of the airflow parameter and lesser flow restriction at higher values of the airflow parameter.

17. The method of claim 15 or claim 16, wherein adjusting the flow restriction of the variable flow restrictors comprises maintaining an equal flow restriction for each variable flow restrictor.

18. The method of any of claims 15-17, further comprising measuring the airflow rate in the piping network, wherein the airflow parameter comprises the measured airflow rate.

19. The method of claim 18, wherein measuring the airflow rate comprises measuring the airflow rate with a flowmeter, wherein the flowmeter is provided in a common pipe, the common pipe for fluidically coupling the air supply to each of the two branches.

20. The method of any of claims 15-19, wherein the airflow parameter comprises a target airflow rate, wherein the target airflow rate is a requested airflow rate from the air supply.

21. The method of claim 20, wherein: the target airflow rate is positively correlated with vessel speed and / or a draft of the vessel; and / or the target airflow rate is determined based on a condition of the air release unit.

22. The method of any of claims 15-21, the method further comprising receiving one or more of vessel speed data, vessel draft data, and / or air release unit condition data and setting the target airflow rate based on the one or more of vessel speed data, vessel draft data, and / or air release unit condition data.

23. The method of any of claims 15-22, wherein adjusting the flow restriction of the variable flow restrictors comprises adjusting the flow restriction of the variable flow restrictors between a plurality of discrete flow restriction levels based on the airflow parameter, wherein each discrete flow restriction level corresponds to a range of airflow parameter values.

24. A method of calibrating a first flow restrictor position of a first variable flow restrictor disposed in a first branch of a piping network of an air lubrication system of a vessel, the air lubrication system comprising the piping network, an air supply, and two air release units disposed on a hull of the vessel, wherein the piping network fluidically couples the air supply to the two air release units and comprises two branches, each branch fluidically coupling the air supply to a respective one of the two air release units, wherein a variable flow restrictor is disposed in each branch, and wherein the first flow restrictor position is for a first flow restriction level of the first variable flow restrictor, the method comprising:positioning the first variable flow restrictor at an initial flow restrictor position;measuring a first pressure loss across the first variable flow restrictor;if the first pressure loss is outside a threshold range of a target pressure loss for the first flow restriction level, positioning the first variable flow restrictor at a tuned flow restrictor position;measuring a second pressure loss across the first variable flow restrictor; andif the second pressure loss is within the threshold range, calibrating the first flow restrictor position based on the tuned flow restrictor position.

25. A method of installing the system according to claim 12 or claim 13 onto a vessel, the method comprising fitting a variable flow restrictor into each branch of the piping network.

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