System and method for assembling an offshore structure

Abstract

An assembly system 100 for assembling an offshore structure (such as a wind turbine 110), comprising a floating base 112 of the offshore structure, a lifting structure 120 configured to land one or mo

Description

FIELD The present disclosure relates to a system and method for assembling an offshore structure such as a floating wind turbine or jacket. In particular, the disclosure relates to heave reduction for a floating base of an offshore structure during assembly of said structure. BACKGROUND In the field of wind turbines, in particular offshore wind turbines, there is a general trend towards installing increasingly high power-output wind turbines, for example, turbine capable of outputting 10 Mega Watts or more. However, along with increased power capabilities, the physical dimensions of such wind turbines are also increasing. Historically, wind turbines, or at least assembled or semi-assembled components of wind turbines, may have been manufactured and / or assembled at a first location, then transported by road to a further location for deployment and / or further assembly. However, due to their increased physical dimensions, transport by road of modern turbines, or assembled portions of modern turbines, is no longer practically possible. At present, wind turbines and their associated structures may be separately constructed onshore and transported in sections to an offshore assembly area site. The sections may then be moved into position and assembled using various slings and cranes. Offshore assembly of floating structures, such as wind turbines, presents significant challenges as, for example, due to water conditions such as waves or swell at a sea, so called “static-to-static” lift operations are difficult to achieve. This may result in potential damage to the floating structure and / or assembly system, particularly at the point at which elements of the floating structure are mated together during assembly. A safe, simple and efficient method of assembling a floating structure is thus desirable. It is therefore an aim of at least one embodiment of at least one aspect of the present disclosure to obviate or at least mitigate at least one of the above identified shortcomings of the prior art. SUMMARY Various aspects of the present invention are defined in the independent claims. Some preferred features are defined in the dependent claims. According to a first aspect of the present disclosure there is provided an assembly system for assembling an offshore structure, the assembly system comprising: a floating base of the offshore structure; a lifting structure configured to perform a landing operation, the landing operation comprising landing one or more components of the offshore structure onto the floating base; and a heave reduction system configured to selectively adjust a draft of the floating base during the landing operation. Advantageously, by selectively adjusting the draft of the floating base during the landing operation, the stability of the floating base relative to the bed of the body of water in which the base is floating may be improved. The heave reduction may additionally simplify assembly operations and / or mitigate the impacts of weather, tides, currents and the like upon assembly of the offshore structure. This improvement in stability may enable so called static-to-static lift operations (i.e. from one effectively static body (the lifting structure) to another effectively static body (the stabilised floating base)) when landing the one or more components of the offshore structure onto the floating base, thereby minimising a risk of damage to said components, and / or the floating base during assembly. Furthermore, such a static-to-static lift arrangement may advantageously allow assembly of offshore structures in relatively shallow bodies of water (e.g. relatively shallow assembly ports) further reducing the cost, risk and complexity of assembling the offshore structure. Landing the one or more components onto the floating base may comprise aligning one or more bolts with one or more corresponding bolt holes, e.g. aligning bolts on the floating base and bolt holes on the component or vice versa. Tapered pins e.g. so called podger pins or the like may be used to assist the alignment of the bolts and bolt holes as the components of the offshore structure are brought together. For example, selectively increasing the draft of the floating base prior to landing the one or more components onto the floating base may essentially ‘preload’ the floating base. This preloading may reduce the heave experienced by the floating base. In other words the vertical motion of the floating base relative to the bed of the body of water due to water conditions such as waves or swell at a sea based assembly area may be reduced. This reduction in vertical motion may provide for a reduced contact force when mating the components of the offshore structure to the floating base and / or each other, reducing or entirely eliminating mechanical contact ‘bounce’ i.e. the number of subsequent separations and contacts between components after an initial contact when brought together, attached or connected. In other cases the draft of the floating base may be reduced during the landing of the one or more components of the offshore structure on the floating base. This reduction in draft may counteract and / or be related or based on (e.g. using real time feedback) the increase in draft which may occur whilst landing the one or more components onto the floating base. Thereby the contact forces experienced when assembling the offshore structure may be further reduced. In other cases the draft of the floating base may be reduced prior to the landing of the one or more components of the offshore structure on the floating base and then maintained during the landing of the one or more components of the offshore structure on the floating base. The draft and / or heave may be adapted e.g reduced or increased by more than 1m, preferably by more than 2m (for example 2.5m) and more preferably by more than 3m. The heave may be reduced to less than 3m (for example 2.5m), preferably to less than 2m and more preferably to less than 1m. It will be appreciated that the expression “draft of the floating base” used in the present disclosure refers to the depth of the floating base below the surface of the body of water at the floating base (otherwise referred to in the art as the “waterline”), measured vertically to a lower reference point. The lower reference point may be the lowest point of the floating base, or another reference point on the floating base, below the waterline. As will be understood by those skilled in the art the stability of a floating body may be related e.g. may be proportional to the draft of said floating body, whereby an increase in the draft may result in an increase in stability, all other factors being equal. The term “offshore structure” used in the present disclosure may encompass, but is not limited to, a plurality of parts or components which may be assembled together. The offshore structure may comprise or include at least one of a segmented offshore structure; a floating / buoyant foundation; a wind turbine; a monopile; or a jacket e.g., a three-leg jacket, a four-leg jacket or a twisted jacket. In the case of the structure comprising a wind turbine, the wind turbine may comprise a plurality of parts or components comprising one or more of: a column or tower section; a nacelle; and a plurality of turbine blades. The term “landing operation” used in the present disclosure may encompass, but is not limited to, the landing of each of the one or more components of the offshore structure onto the floating base and / or onto each of the other components of the offshore structure i.e. to assemble components of the offshore structure on the floating base. This may include a time before, at, during and / or after the landing of each of the components of the offshore structure onto the floating base and / or onto each of the other components of the offshore structure. The floating base may comprise a floater; floating spar; a tension-leg platform; a barge; a semi-submersible platform and / or a combination thereof. The heave reduction system may be configured to adjust the draft by varying a pulling force and / or a pushing force acting upon the floating base. The pulling and / or pushing force, or at least a part thereof, may be applied in the direction of a bed of a body of water. The heave reduction system may be configured to selectively adjust the draft in response to the landing of one of the one or more components of the offshore structure onto the floating base. In some case the draft may be adjusted, e.g. the pulling or pushing force may be adjusted, at substantially the same time as one of the one or more components is landed onto the floating base. Advantageously, by reducing the pulling and / or pushing force (e.g. a portion of or all of the pulling and / or pushing force) acting upon the floating base when a component is landed onto the floating base, the reduction in pulling and / or pushing force from the heave reduction system may substantially oppose the force acting on the floating base due to the landing of the component. The opposing forces may act to increase the initial contact force when the component and the floating base and / or another component are initially brought together, which may further reduce or entirely eliminate contact ‘bounce’ as detailed above. In other cases the draft may be adjusted after or over a predetermined period of time after one of the one or more components is landed onto the floating base. In some cases, the draft may be adjusted, e.g. the pulling or pushing force may be adjusted, at substantially the same rate as (or at a rate based on) the rate the force acting upon the floating base from a component increases, as the component is landed onto the floating base. The rate of adjustment may be predetermined and / or may be adjusted in real time e.g. using a real time visual or other sensory feedback means. This beneficially may further reduce the ongoing contact force during the landing of the component on the floating base whilst reducing or entirely eliminating the potential for mechanical contact ‘bounce’ as detailed above. In some examples, the heave compensation system may, in combination, adjust the draft before the landing of a component on the floating base or another component and / or on initial contact of the component on the floating base or another component and / or during the landing of the component on the floating base or another component and / or after the landing of the component on the floating base or another component. Advantageously, in this way each ‘phase’ of the landing operation may be individually designed to reduce and / or increase contact forces where possible whilst reducing or entirely eliminating the potential for mechanical contact ‘bounce’ between a component and the floating base or other component during the landing operation. Varying the pulling force may comprise adjusting a tension in a coupling between the floating base and an anchor arrangement which may be coupled to the bed of the body of water. Advantageously, by coupling the floating base directly to the bed of the body of water no additional grounded structures (i.e. structures fixedly attached to the bed of the body of water) are required, further reducing the cost and complexity of assembling the offshore structure. Adjusting the tension may comprise increasing or decreasing the tension in response to a landing of one of the one or more components on the floating base e.g. completely removing the tension in response to a first contact of one of the one or more components and the floating base. Adjusting the tension may comprise increasing or decreasing the tension at a rate based on the rate of landing one of the one or more components on the floating base. The heave reduction system may comprises an actuatable device configured to adjust the tension in the coupling. The actuatable device may comprise at least one of: a winch, a jack, a strand jack, a hydraulic ram, and / or a tug. The winch may be a linear winch. Beneficially, strand jacks in particular are widely available to purchase or hire, and can withstand the extreme loads and conditions associated with offshore assembly activities. The use of a jack, ram or winch advantageously allows the tension in the coupling to be adjusted remotely, reducing the risk and / or potential for injury or damage to equipment and personnel operating the assembly system. The actuatable device may be coupled to the floating base. The actuatable device may be coupled to the anchor arrangement. The actuatable device may be coupled to the lifting structure. The actuatable device may be coupled by at least one of: an articulated joint; a rotatable and / or flexible joint; a universal joint; a multi-axis joint; and a gimbal. A coupling whose orientation is not permanently fixed may allow the force to be applied when base is not directly above the coupling to the bed of the body of water (e.g. the sea bed or the like) without applying any lateral forces (i.e. stress) to the actuatable device or its coupling to the floating base. Such a coupling also allows for some lateral movement of the floating base due to sea conditions further reducing stress on the actuatable device and / or its coupling to the floating base. The coupling may comprise at least one of: a line; a chain; a wire; a strand; a coupling plate and / or a combination thereof. Coupling plate may comprise one or more mooring plates (e.g. a so called monkey face plate) which beneficially allow a force to be applied to one or more lines with a single actuatable device further reducing the cost and complexity of assembling the offshore structure. The coupling may comprise a plurality of different element connected in series or parallel e.g. a plurality of wires or a wire and chain. The combinations may be adapted to the expected water conditions and / or the requirements of the actuatable devices. A plurality of lines or the like in parallel may beneficially provide redundancy in the case of failure or degradation in one of the lines reducing the potential for complete failure of the coupling. The anchor arrangement may be selectively releasable. The anchor arrangement may comprise one or more pulley mechanisms. Advantageously, using a selectively releasable anchor arrangement may provide a repeatable coupling to the bed of the body of water which may be operated remotely e.g. using ROV and / or remote wired / wireless actuators. A selectively releasable anchor arrangement may also allow reuse of the system in several locations and may additionally allow quick and / or easy repair or replacement of elements of the system as and when required. The lifting structure may comprise a plurality of legs and / or an apex member. The legs may be connected to or connectable to the apex member. The lifting structure may comprise a lifting arrangement coupled to one or more of the plurality of legs and / or the apex member. The lifting structure may comprise a tripod or tripod arrangement or comprise three legs. The tripod may comprise three legs. In other examples, the lifting structure may comprise a tetrapod. The lifting arrangement may, in use, be provided below, e.g. directly below, the apex member. The lifting arrangement may depend from the apex. An assembly area for assembling the offshore structure may be provided below, e.g., directly below, the apex member. The lifting arrangement may be configured for landing the one or more components of the offshore structure onto the floating base. Each leg may be fixedly coupled to the bed of the body of water. Varying the pushing force may comprise adjusting a weight which may be coupled to the floating base. The weight may comprise a chamber. Adjusting the weight may comprise at least partially filling or draining the chamber. Adjusting the weight may comprise at least partially filling or draining the chamber in response to a landing of one of the one or more components on the floating base e.g. completely draining the chamber by fully opening a valve or hatch or opening or the like in a surface of the chamber in response to a first contact of one of the one or more components and the floating base. Adjusting the weight may comprise at least partially filling or draining the chamber at a rate based on the rate of landing one of the one or more components on the floating base e.g. by a selectively adjustable valve or the like. The chamber may be at least partially filled with at least one of a fluid (e.g. a liquid or a gas) and / or a granular material and / or other material with a density greater than the body of water e.g. sand or sediment which may be extracted from the bed of the body of water. The chamber may be at least partially filled with at least one of a fluid (e.g. a liquid or a gas) and / or a granular material and / or other material with a density less than the body of water. The chamber may be at least partially filled at a pressure at, or greater than, atmospheric pressure. Adjusting the weight may comprise tensioning a line extending from the lifting structure to a lever arm extending from the lifting structure and coupled to the floating base. Adjusting the tension may comprise increasing or decreasing the tension in response to a landing of one of the one or more components on the floating base e.g. completely removing the tension in response to a first contact of one of the one or more components and the floating base. Adjusting the tension may comprise increasing or decreasing the tension at a rate based on the rate of landing one of the one or more components on the floating base. According to a second aspect of the present disclosure there is provided the assembly system of the first aspect in combination with the offshore structure. The offshore structure may comprise one of a wind turbine or component thereof, a jacket or the like. According to a third aspect of the present disclosure there is provided a method of assembling an offshore structure, the method comprising: performing, using a lifting structure, a landing operation, the landing operation comprising, landing one or more components of the offshore structure onto a floating base, wherein a draft of the floating base is adjusted, using a heave reduction system, during the landing operation. Selectively adjusting the draft of the floating base may comprise selectively adjust the draft in response to the landing of one of the one or more components of the offshore structure onto the floating base. The method may comprise one or more of the preceding steps of: transporting the one or more components of the offshore structure to an assembly area below the lifting structure; suspending the one or more components from the lifting structure; and transporting the floating base to the assembly area. Adjusting the draft may comprise varying a pulling and / or pushing force acting upon the floating base in the direction of a bed of a body of water. The method may comprise one or more of the subsequent steps of: removing the pulling or pushing force acting upon the floating base; transporting the floating base out of the assembly area; transporting one or more further components of the offshore structure to the assembly area; suspending the one or more further components from the lifting structure; transporting the floating base to the assembly area; performing, using the lifting structure, a further landing operation, the further landing operation comprising, landing the one or more further components of the offshore structure onto the floating base and / or onto the one or more components. The draft of the floating base may be adjusted, using the heave reduction system, during the further landing operation. Transporting the floating base to and / or out of the assembly area may comprise coupling the floating base to a vessel and / or transporting the floating base to and / or out of the assembly area using the vessel. Varying the pulling force may comprise adjusting a tension in one or more couplings (e.g. one or two or three or four or more) between the floating base and one or more anchor arrangements (e.g. one or two or three or four or more) coupled to the bed of the body of water. The tension on each of a plurality of couplings may be adjusted selectively e.g. individually or in groups or simultaneously at a predetermined rate and / or time. Adjusting the tension may comprise increasing or decreasing the tension in response to a landing of one of the one or more components on the floating base e.g. completely removing the tension in response to a first contact of one of the one or more components and the floating base. Adjusting the tension may comprise increasing or decreasing the tension at a rate based on the rate of landing one of the one or more components on the floating base. The heave reduction system may comprise an actuatable device configured to adjust the tension in the coupling. The actuatable device may comprise at least one of: a winch, a jack, a strand jack, a hydraulic ram; and a tug. The anchor arrangement may be selectively releasable. The method may comprise a step of engaging and / or releasing the selectively releasable anchor arrangement before and / or after transporting the floating base to and / or out of the assembly area. Varying the pushing force may comprise adjusting a weight coupled to the floating base. The weight may comprise a chamber. Adjusting the weight may comprise at least partially filling or draining the chamber. Adjusting the weight may comprise at least partially filling or draining the chamber in response to a landing of one of the one or more components on the floating base e.g. completely draining the chamber by fully opening a valve or hatch or opening or the like in a surface of the chamber in response to a first contact of one of the one or more components and the floating base. Adjusting the weight may comprise at least partially filling or draining the chamber at a rate based on the rate of landing one of the one or more components on the floating base e.g. by a selectively adjustable valve or the like. The chamber may be at least partially filled with at least one of a fluid (e.g. a liquid or a gas) and / or a granular material and / or other material with a density greater than the body of water e.g. sand or sediment which may be extracted from the bed of the body of water. The chamber may be at least partially filled with at least one of a fluid (e.g. a liquid or a gas) and / or a granular material and / or other material with a density less than the body of water. The chamber may be at least partially filled at a pressure at, or greater than, atmospheric pressure. Adjusting the weight may comprise tensioning a line extending from the lifting structure to a lever arm extending from the lifting structure and coupled to the floating base. Adjusting the tension may comprise increasing or decreasing the tension in response to a landing of one of the one or more components on the floating base e.g. completely removing the tension in response to a first contact of one of the one or more components and the floating base. Adjusting the tension may comprise increasing or decreasing the tension at a rate based on the rate of landing one of the one or more components on the floating base. The lifting structure may comprise a plurality of legs. The lifting structure may comprise a lifting arrangement coupled to one or more of the plurality of legs. The method may comprises using the lifting arrangement to land the one or more components of the offshore structure onto the floating base. Each leg may be fixedly coupled to the bed of the body of water. The method may be performed using the assembly system of the first aspect. While the steps of the method are shown and described in a particular order, it should be understood that this does not imply that these steps must be carried out in that order. While some steps are required to be performed sequentially (for example, the components of the offshore structure must be transported to the assembly area before being suspended from the lifting structure), other steps may be performed in any order. In particular, the step of adjusting the draft of the floating base may be performed at any time before, during and / or after landing the components of the offshore structure onto the floating base in order to minimise the heave during assembly of the offshore structure. The above summary is intended to be merely exemplary and nonlimiting. The disclosure includes one or more corresponding aspects, embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. It should be understood that features defined above in accordance with any aspect of the present disclosure or below relating to any specific embodiment of the disclosure may be utilized, either alone or in combination with any other defined feature, in any other aspect or embodiment or to form a further aspect or embodiment of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the present disclosure will now be described, by way of example only, with reference to the accompanying Figures, in which: Figure 1 is a perspective view of a known assembly system for assembling an offshore structure at an assembly area; Figure 2 is a side view of an assembly system for assembling an offshore structure according to an example embodiment of the present disclosure; Figure 3 is a side view of an assembly system for assembling an offshore structure according to an example embodiment of the present disclosure; Figure 4 is a side view of an example anchor arrangement for use in the example assembly systems of Figures 2 and 3; Figure 5 is a side view of an assembly system for assembling an offshore structure according to an example embodiment of the present disclosure; Figure 6 is a perspective view of an assembly system for assembling an offshore structure according to an example embodiment of the present disclosure; Figure 7 is a perspective view of an assembly system for assembling an offshore structure according to an example embodiment of the present disclosure; Figure 8 is a flowchart of a method of assembling an offshore structure according to an example embodiment of the present disclosure; Figure 9 is a plan view of an assembly system for a assembling an offshore structure according to an example embodiment of the present disclosure; Figure 10 is a plan view of the assembly system of Figure 9 with the legs of the lifting structure omitted; Figure 11 is a plan view of the assembly system of Figure 10 including a feeder barge; Figure 12 is a plan view of the assembly system of Figure 11 including a boat landing; Figure 13 is a plan view of the assembly system of Figure 11 including a nacelle and turbine blades on the feeder barge; Figure 14 is a plan view of an assembly system for a assembling an offshore structure according to an example embodiment of the present disclosure; Figure 15 is a plan view of the assembly system of Figure 14 with the legs of the lifting structure omitted; Figure 16 is a plan view of the assembly system of Figure 10 including additional stabilization lines and an illustration of an additional floating base configuration. In the Figures, like parts are denoted by like reference numerals. It will be appreciated that the drawings are for illustration purposes only and are not drawn to scale. DETAILED DESCRIPTION OF THE DRAWINGS Referring to Figure 1 there is shown a perspective view of an example of an assembly system 1 for assembling an offshore structure 10, such as a floating wind turbine or jacket. In this example the offshore structure 10 comprises a floating base 12 and components 14, 16 to be assembled onto the floating base 12. The system 1 comprises a lifting structure 20 configured to land the components 14, 16 of the offshore structure 10 onto the floating base 12 of the offshore structure 10. In this example, the lifting structure 20 comprises three elongate legs 22, each leg 22 fixedly coupled at a first end to a bed of a body of water 50 via grounded structures 26. Each of the legs 22 is further coupled to each of the other legs 22 at a second end, opposite the first end, by an apex member 24 to form a tripod. An assembly area 54 in the body of water 50 is defined below the lifting structure 20, within which the offshore structure 10 is assembled. The grounded structures 26 may comprise one or more of a fixed offshore platform, a jacket platforms, a spar platforms, a gravity base structure and / or the like. In other examples the lifting structure 20 may comprise more or less or no legs 22 e.g. the lifting structure 20 may comprise four legs 22 to form a quadpod, or may alternatively or additionally comprise a floating and / or grounded vessel or other grounded structure and thus have no legs. At least one lifting arrangement (not shown) may be coupled to the lifting structure 20 e.g. to one or more legs 22 of the lifting structure 20 and / or to the apex member 24 and / or to one of the grounded structures 26. The lifting arrangement may comprise a winch, a crane, a sling, one or more lines and / or the like, the lifting arrangement being adapted to lift components 14, 16 of the offshore structure 10, e.g. from a vessel 30, which may be referred to in the art as a feeder barge, used to transport the components 14, 16 to the assembly area 54. The lifting arrangement may be further adapted to suspend the components 14, 16 from the lifting structure 20, and land the components 14, 16 onto the floating base 12, once the floating base 12 is located under the lifting structure 20 within the assembly area 54. As described in detail below, the floating base 12 may be maintained in, and / or manoeuvred into and out of, the assembly area 54, by one or more vessels (not shown) e.g. one or more tugs or the like using lines 40 attached to the floating base 12. Additionally or alternatively one or more lines may be coupled between the floating base 12 and one or more of the legs 22 or grounded structures 26 of the lifting structure 20 to change or maintain the location of the floating structure 12 or the floating base 12 may comprise one or more permanent and / or temporary propulsion means configured to manoeuver the floating base 12 on the body of water 50. For purposes of example only, the components 14, 16 of the offshore structure 10 comprise a pre-assembled tower section 14 and a plurality of turbine blades 16. In Figure 1 the tower section 14 is depicted after being landed onto the floating base 12, whilst the plurality of turbine blades 16 is depicted being transported to the assembly area 54 on the vessel 30. It will be appreciated that in other embodiments, fewer or greater number of components 14, 16 may be required and the components may differ from the examples shown, depending on the offshore structure 10 to be assembled. Referring now to Figure 2, there is shown a side view of an example of an assembly system 100 for assembling an offshore structure 110, such as a floating wind turbine or jacket in accordance with an example embodiment of the present disclosure. In this example the offshore structure 110 comprises a floating base 112 and tower section 114, to be assembled onto the floating base 112. Similar to the assembly system 1 of Figure 1, the assembly system 100 comprises a lifting structure 120 configured to land the tower section 114 of the offshore structure 110 onto the floating base 112. Two of three elongate legs 122 of the lifting structure 120 are shown (the third leg 122 is omitted for ease of description). Each of the legs 122 are fixedly coupled at a first end to a bed 152 of a body of water 150 via grounded structures 126. Each of the legs 122 is further coupled to each of the other legs 122 at a second end, opposite the first end, by an apex member 124. A lifting arrangement 128, coupled to the apex member 124 is configured to suspend components (such as the tower section 114) of the offshore structure 110 above an assembly area defined between the legs 122 of the lifting structure. The lifting arrangement 128 is further configured to land the components onto a floating base 112 of the offshore structure 110 during assembly of the offshore structure 110. The lifting arrangement 128 may comprise means for connecting or supporting one or more of the components of the offshore structure 110 to be lifted. The connection may comprise one or more hooks, shackles or the like. The lifting arrangement 128, therefore, provides a single point lifting arrangement, e.g., comprising a single lifting point for supporting a load such as a component of the offshore structure 110. The assembly system 100 further comprises lines 129 provided between each of the legs 122 and the floating base 112 of the offshore structure 110. In other examples the lines 129 may alternatively or additionally extend to or from each of the grounded structures 126. The lines 129 may comprise control, mooring, tagging lines or the like and are configured to provide lateral stability of the floating base 112 before, during, and / or after assembly of the offshore structure 110. The lines 129 provide a means of counteracting lateral forces experienced by, for example, the floating structure 112 due to conditions in the body of water 150. The assembly system 100 further comprises a heave reduction system configured to selectively adjust a draft of the floating base 112 before, during and / or after landing of components of the offshore structure 110, such as the tower section 114, onto the floating base 112. In the example of Figure 2, the heave reduction system comprises a plurality of actuatable devices in the form of two strand jacks 160 coupled in a fixed orientation to opposing sides of the floating base 112. Each strand jack 160 is configured to tension a line 164 coupled to the bed 152 of the body of water 150 in which the offshore structure 110 is to be assembled. In this example the line 164 is coupled to the bed 152 using a selectively releasable anchor arrangement in the form of a socket and latch arrangement 166, described in more detail below with reference to Figure 4. In use, prior to landing the component 114 of the offshore structure 110 onto the floating base 112, each of the socket and latch arrangements 166 are engaged so as to couple each of the lines 164 to the bed 152 of the body of water 150. The socket and latch arrangements 166 may be engaged remotely e.g. using one or more remotely operated vehicles (ROVs), one or more lines extending above the surface of the body of water 150, an automated wired or wirelessly activated engagement system and / or the like. The tension of each the lines 164 can then be individually or collectively adjusted by operating each of the strand jacks 160 thus selectively adjusting the draft of the floating base 112. By, for example, increasing the draft of the floating base 112 before or during the landing of the component 114 onto the floating base 112, the movement of the floating base 112 relative to the bed 152 of the body of water 150, i.e. the heave, can be reduced or eliminated. By reducing the heave during such a landing operation it may be possible to prevent any mechanical ‘bounce’ when component 114 is brought into contact with the floating base 112 thereby reducing the risk of damage to components 114, floating base 112 and / or lifting structure 120 during assembly. This may result in a significant reduction in the cost, risk and complexity of assembling an offshore structure such as a wind turbine in an offshore location. In other examples, there may be more or less strand jacks 160 (for example one or three or four strand jacks 160) coupled to the floating base 112, or the heave reduction system may alternatively or additionally comprise a winch, hydraulic ram, tug or other suitable means of selectively adjusting the tension of the lines 164. In other examples, the anchoring arrangement may alternatively or additionally comprise a so called ball grip arrangement, one or more clump weights, one or more pulleys and / or the like. In further examples, the grounded structures 126 may be replaced by one or more mud-mats or mud-mat barges. The mud-mat barges may be ballasted such that said mudmad barges provide lateral stability from forces, e.g. wind and / or tidal forces, during assembly of the offshore structure 110. In some example, such ballast may be achieved by at least one of: pumping in water or other higher density material to provide a gravity anchor, e.g. mud or haematite; sheet piles around the mud-mat barge; piled anchors; suction anchors; anchors attached by chain; wire or rope; and / or securing lines to the shore. Referring now to Figure 3, there is shown a side view of an example of an assembly system 100 for assembling an offshore structure 110, such as a floating wind turbine or jacket in accordance with an example embodiment of the present disclosure. This example differs from that shown in Figure 2, only in that one of the strand jacks 160a of the heave reduction system is coupled to a side of the floating base 112 using a gimballed coupling 168. The gimballed coupling 168 allows the orientation of the strand jack 160a to change relative to the floating base 112, reducing any potential stress or strain experienced by the line 164a if, for example, the position on the floating base 112 to which the strand jack 160a is coupled does not initially align with the anchoring arrangement 166 e.g. the line 164a is orientated at an angle a to the side of the floating base 112. This may provide further flexibility in the positioning of the floating base 112 thereby further reducing the cost, risk and complexity of assembling an offshore structure such as a wind turbine in an offshore location. Figure 4 illustrated a side view of an example selectively releasable anchoring arrangement 166 which may be comprised in a heave reduction system for an assembly system such as those described above with reference to Figures 2 and 3. The selectively releasable anchoring arrangement 166 comprises a socket element 166a coupled, e.g. embedded in, a bed 152 of a body of water 150, and a complimentary latch element 166b coupled to an end of a line 164. The latch element 166b comprises a plurality of opposing wings 164c coupled within the latch element 166b using a spring mechanism 166d. In use, the latch element 166b is inserted into the socket element 166a such that the wings 164c are latched by protrusions within the socket element 166a. The spring mechanism 166d is configured to ensure that the wings 164c remain in the latched position holding the latch element 166b within the socket element 166a. When the latch element 166b is to be removed from the socket element 166a a release mechanism (not shown) is activated which opposes the spring mechanism 166d retracting the latches 164c thereby allowing the latch element 166b to be removed from the socket element 166a. Referring now to Figure 5, there is shown a side view of an example of an assembly system 200 for assembling an offshore structure 210, such as a floating wind turbine or jacket in accordance with an example embodiment of the present disclosure. In this example the offshore structure 210 comprises a floating base 212 and tower section 214, to be assembled onto the floating base 212. Similar to the assembly system 100 of Figure 2, the assembly system 200 comprises a lifting structure 220 configured to land the tower section 214 of the offshore structure 210 onto the floating base 212. Two of three elongate legs 222 of the lifting structure 220 are shown (the third leg 222 is omitted for ease of description). Each of the legs 222 are fixedly coupled at a first end to a bed 252 of a body of water 250 via grounded structures 226. The assembly system 200 further comprises a heave reduction system configured to selectively adjust a draft of the floating base 212 before, during and / or after landing of components of the offshore structure 210, such as the tower section 214, onto the floating base 212. In the example of Figure 5, the heave reduction system comprises a plurality of actuatable devices in the form of two strand jacks 260, each strand jack 260 coupled to one of the grounded structures 226. Each strand jack 260 is configured to tension a line 264 coupled to the floating base 212 of the offshore structure 210 via pulleys 266a, 266b. Each of pulleys 266a are coupled to each of the grounded structures 226. Each of pulleys 266b are anchored to the bed 252 of the body of water 250 in which the offshore structure 210 is to be assembled by clump weights 268. The pulleys 266a, 266b are configured such that an adjustment of the tension of a line 264 results in an adjustment of the draft of the floating base 212. In use, prior to landing the component 214 of the offshore structure 210 onto the floating base 212, each of the lines 264 are fixedly coupled to the floating base 212. The tension of each the lines 264 can then be individually or collectively adjusted by operating each of the strand jacks 260 thus selectively adjusting the draft of the floating base 212. By, for example, increasing the draft of the floating base 212 before or during the landing of the component 214 onto the floating base 212, the movement of the floating base 212 relative to the bed 252 of the body of water 250, i.e. the heave, can be reduced or eliminated. By reducing the heave during such a landing operation it may be possible to prevent any mechanical ‘bounce’ when component 214 is brought into contact with the floating base 212 thereby reducing the risk of damage to components 214, floating base 212 and / or lifting structure 220 during assembly. This may result in a significant reduction in the cost, risk and complexity of assembling an offshore structure such as a wind turbine in an offshore location. In other examples, one or more of the strand jacks 260 may be coupled to a side of the grounded structure 226 or otherwise orientated such that the pulley 266a coupled to the grounded structure 226 can be omitted. In other examples, one or more of the strand jacks 260 may be coupled to one of the legs 222 of the lifting structure 220 and / or coupled to the lifting structure 220 using a gimballed coupling (as illustrated in the example assembly system 100 of Figure 3). In other examples, the pulleys 266b may be anchored to the bed 252 of the body of water 250 by selectively releasable anchoring arrangements such as the socket and latch arrangement 166 described above with reference to Figure 4. Referring now to Figure 6, there is shown a perspective view of an example of an assembly system 300 for assembling an offshore structure 310, such as a floating wind turbine or jacket in accordance with an example embodiment of the present disclosure. In this example the offshore structure 310 comprises a floating base 312 and tower section 314, to be assembled onto the floating base 312. Similar to the assembly system 100 of Figure 2, the assembly system 300 comprises a lifting structure 320 configured to land the tower section 314 of the offshore structure 310 onto the floating base 312. Two of three elongate legs 322 of the lifting structure 320 are shown (the third leg 322 is omitted for ease of description). Each of the legs 322 are fixedly coupled at a first end to a bed 352 of a body of water 350 via grounded structures 326. The assembly system 300 further comprises a heave reduction system configured to selectively adjust a draft of the floating base 312 before, during and / or after landing of components of the offshore structure 310, such as the tower section 314, onto the floating base 312. In the example of Figure 6, the heave reduction system comprises a plurality of adjustable weights in the form of hollow chambers 360 (only one is shown in Figure 6) to opposing sides of the floating base 312. In use, prior to or whilst landing the component 314 of the offshore structure 310 onto the floating base 312 each of the hollow chambers 360 is partially filled with water from the body of water 350 thus selectively adjusting the draft of the floating base 312. In this example a vessel 330 is coupled to the floating base 312 by a line 340 to both transport the floating base 312 into and out of the assembly area beneath the lifting structure 320 and additionally to provide lateral stability of the floating base 312 relative to the lifting structure 320. By, for example, increasing the draft of the floating base 312 before or during the landing of the component 314 onto the floating base 312, the movement of the floating base 312 relative to the bed 352 of the body of water 350, i.e. the heave, can be reduced or eliminated. By reducing the heave during such a landing operation it may be possible to prevent any mechanical ‘bounce’ when component 314 is brought into contact with the floating base 312 thereby reducing the risk of damage to components 314, floating base 312 and / or lifting structure 320 during assembly. This may result in a significant reduction in the cost, risk and complexity of assembling an offshore structure such as a wind turbine in an offshore location. In other examples, one or more of the chambers 360 may be additionally or alternatively partially filled with a further fluid and / or a granular material and / or other material with a density greater than the body of water 350, such as sand or sediment which may be extracted from the bed 352 of the body of water 350 using suitable means which will be apparent to those skilled in the art. In other examples one or more of the chambers 360 may be replaced by a fixed weight e.g. a clump weight which may be coupled to the floating base 312 after it is transported to beneath the lifting structure 320 in order to increase the draft during assembly of the offshore structure 310. Referring now to Figure 7, there is shown a perspective view of an example of an assembly system 400 for assembling an offshore structure 410, such as a floating wind turbine or jacket in accordance with an example embodiment of the present disclosure. In this example the offshore structure 410 comprises a floating base 412 and tower section 414, to be assembled onto the floating base 412. Similar to the assembly system 300 of Figure 6, the assembly system 400 comprises a lifting structure 420 configured to land the tower section 414 of the offshore structure 410 onto the floating base 412. Two of three elongate legs 422 of the lifting structure 420 are shown (the third leg 422 is omitted for ease of description). Each of the legs 422 are fixedly coupled at a first end to a bed 452 of a body of water 450 via grounded structures 426. Each of the legs 422 is further coupled to each of the other legs 422 at a second end, opposite the first end, by an apex member 424. The assembly system 400 further comprises a heave reduction system configured to selectively adjust a draft of the floating base 412 before, during and / or after landing of components of the offshore structure 410, such as the tower section 414, onto the floating base 412. In the example of Figure 7, the heave reduction system comprises a fixed weight in the form of clump weight 466 suspended from a lever arm 468 extending from one of the grounded structures 426 and coupled to the floating base 412. The heave reduction system further comprises an actuatable device in the form a strand jack 460 coupled in a fixed orientation to the apex member 424. The strand jack 460 is configured to tension a line 464 coupled to the lever arm 468. In other examples the line may be coupled to the clump weight 466 and / or to the floating base 412. In use, prior to landing the component 414 of the offshore structure 410 onto the floating base 412, the tension of the line 464 can be increased using the strand jack 460 such that the ‘downward force’ i.e. the force in the direction of the bed 452 of the body of water 450 acting upon the floating base 412 due to the clump weight 466 is partially or fully counteracted. The tension of the line 464 can than then be decreased before or during the landing of the component 414 onto the floating base 412 thus selectively adjusting the draft of the floating base 412. By, for example, increasing the draft of the floating base 412 before or during the landing of the component 414 onto the floating base 412, the movement of the floating base 412 relative to the bed 452 of the body of water 450, i.e. the heave, can be reduced or eliminated. By reducing the heave during such a landing operation it may be possible to prevent any mechanical ‘bounce’ when component 414 is brought into contact with the floating base 412 thereby reducing the risk of damage to components 414, floating base 412 and / or lifting structure 420 during assembly. This may result in a significant reduction in the cost, risk and complexity of assembling an offshore structure such as a wind turbine in an offshore location. Although the example of Figure 7 shows only one lever arm 468, clump weight 466, and line 464, in other cases there may more of at least one of these features. For example, a lever arm 468 may extend from each of the grounded structures 426 to the floating base 412 and a clump weight 466 may be suspended from each of the lever arms 468. In some examples the weights may additionally or alternatively comprise chambers which may be selectively filled and or emptied to adapt the force acting upon the floating base 412 due to the weight. In other examples, there may be more or less strand jacks 460 (for example one or three or four strand jacks 460) coupled to the apex member 424 and / or to one or more of the legs 422. The heave reduction system may alternatively or additionally comprise a winch, hydraulic ram, tug or other suitable means of selectively adjusting the tension of the line 464. Figure 8 illustrates a flowchart of a method 500 of assembling an offshore structure, such as a floating wind turbine or jacket in accordance with an example embodiment of the present disclosure. The method 500 comprises, at step 560, performing a landing operation of one or more components of the offshore structure onto a floating base of the offshore structure e.g. using a lifting structure under which the floating base is located 560 in an assembly area. During the lifting operation, at step 570, a draft of the floating base is selectively adjusted using a heave reduction system. By, for example, increasing the draft of the floating base before or during the landing of the components onto the floating base, the movement of the floating base relative to a bed of a body of water, i.e. the heave, can be reduced or eliminated. The method 500 may comprise the preceding steps of transporting a components of the offshore structure to the assembly area 530, suspending a component of the offshore structure from the lifting structure 540 and transporting the floating base of the offshore structure to the assembly area 550. The method may comprise the subsequent steps of disengaging the heave reduction system, transporting the floating base out of the assembly area, transporting a further component of the offshore structure to the assembly area, suspending the further component of the offshore structure from the lifting structure, transporting the floating base back to the assembly area, and performing a further landing operation, during which the draft of the floating base is selectively adjusted using the heave reduction system. The subsequent steps may be repeated until the offshore structure is assembled or until an alternative means of assembly is required to complete assembly of the offshore structure. Figures 9 and 10 are plan views of an assembly system 600 for a assembling an offshore structure, such as a floating wind turbine or jacket in accordance with an example embodiment of the present disclosure. In this example the offshore structure comprises a floating base 612, tower section (not visible), and turbine blades 616, to be assembled onto the tower section attached to the floating base 612. The assembly system 600 comprises a lifting structure configured to land the tower section and the turbine blades 616 of the offshore structure onto the floating base 612. The lifting structure comprises three elongate legs 622 and each leg 622 is fixedly coupled at a first end to a bed of a body of water 650 via grounded structures 626. Each of the legs 622 is further coupled to each of the other legs 622 at a second end, opposite the first end, by an apex member. The floating base 612 is coupled to a vessel 630 (such as a barge) using a line 640 attached at two points of the floating base 612. The vessel 630 may be used during assembly of the offshore structure, such as during the method of assembly described with reference to Figure 8, to transport the floating base into and out of an assembly area defined between the legs 622 of the lifting structure. Additionally or alternatively the vessel 630 may be used to provide lateral stabilisation of the floating base 612 during a landing operation of components of the offshore structure, such as turbine blades 616, onto the floating base 612. Figure 10 is a plan view of the assembly system 600 of Figure 9 having the legs 622 omitted, and prior to the landing of the components (e.g. the tower section and turbine blades 616) of the offshore structure onto the floating base 612. This shows the placement of the floating base 612 underneath the lifting structure, between each of the legs 622 in the assembly area. Figure 11 is a plan view of the assembly system 600 of Figure 9 having the legs 622 omitted, and prior to the landing of the turbine blades 616 of the offshore structure onto the tower section attached to the floating base 612. In this example, the vessel 630, coupled to the floating base 612 using lines 640, has transported the floating base 612 out of the assembly area following the landing of the tower section onto the floating base 612. A feeder barge 632, is shown transporting the turbine blades 614 to into the assembly area to allow them to be suspended from the lifting structure prior to further landing operations. Figure 12 is a plan view of the assembly system 600 of Figure 11 further comprising a boat landing 634 coupled to the floating base 612 and configured to allow further vessels e.g. vessels transporting additional equipment and / or personnel, to safely ‘land’ or ‘dock’ to the floating base 612 during the assembly process. The assembly system 600 of Figure 12 further comprises additional lines 642, 644 which may provide additional lateral stabilisation when the floating base 612 is located both inside and out of the assembly area. Lines 642, 644 may be selectively tensioned by an actuatable device as is described throughout this disclosure (e.g. using a strand jack coupled to each of the grounded structures 626). Figure 13 is a plan view of the assembly system 600 of Figure 11 further comprising a nacelle 636 and the turbine blades 614 of the offshore structure on the feeder barge 632 during transportation to the assembly area. Figures 14 and 15 are plan views of an assembly system 700 for a assembling an offshore structure, such as a floating wind turbine or jacket in accordance with an example embodiment of the present disclosure. Assembly system 700 differs from assembly system 600 of Figures 9 to 13 in that the floating base 712 is coupled to a vessel 730 (such as a barge) using a line 740 attached at a single point of the floating base 712. Similar to Figure 10, Figure 15 is a plan view of the assembly system 700 of Figure 14 having the legs of the lifting structure omitted, and prior to the landing of the components of the offshore structure onto the floating base 712. This shows the placement of the floating base 712 underneath the lifting structure, in the assembly area. Although the example assembly systems of Figures 9 to 15 show a single vessel coupled to the floating base of the offshore structure, it will be appreciated that during the assembly process e.g. during the manoeuvring of the floating base into and out of the assembly area, a plurality of vessels may be coupled to the floating base e.g. two or three or four vessels. It will be further appreciated that not all of the plurality of vessels may be required at every stage of the assembly process and / or one vessel may alter the point or points of the floating base to which it is coupled. For example, the assembly system may comprise two vessels coupled on opposite sides of the floating base; one to manoeuver the floating base into the assembly area and one to manoeuver the floating base out of the assembly area. Alternatively, a single vessel may be coupled and uncoupled from various points on the floating base to manoeuver the floating base into and out of the assembly area during the assembly process. Figures 16 is a plan view of an assembly system 800 for a assembling an offshore structure, showing two alternative floating bases 812a and 812b, each which may be referred to in the art as a “floater” or “floating foundation”, onto which components of the offshore structure may be assembled. Floating base 812a comprises an asymmetrical tri-column triangular platform, with the offshore structure assembled onto one of the three columns. Floating base 812b comprises a symmetrical triangular platform comprising a central column onto which the offshore structure is assembled. The symmetrical triangular platform floating base 812b beneficially provides a single central point to couple a heave reduction system to during assembly of the offshore structure, reducing the cost and complexity 5 of assembly. Although specific examples of systems and methods of assembling an offshore structure are described above in relation to the Figures, it will be appreciated that variations on the above examples are possible. Although the disclosure has been described in terms of preferred embodiments 10 as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the 15 disclosure, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

1. An assembly system for assembling an offshore structure, the assembly system comprising:a floating base of the offshore structure;a lifting structure configured to perform a landing operation, the landing operation comprising landing one or more components of the offshore structure onto the floating base; anda heave reduction system configured to selectively adjust a draft of the floating base during the landing operation.

2. The assembly system of claim 1 wherein the heave reduction system is configured to selectively adjust the draft in response to the landing of one of the one or more components of the offshore structure onto the floating base.

3. The assembly system of claim 1 or claim 2 wherein the heave reduction system is configured to adjust the draft by varying a pulling or pushing force acting upon the floating base in the direction of a bed of a body of water.

4. The assembly system of claim 3 wherein varying the pulling force comprises adjusting a tension in a coupling between the floating base and an anchor arrangement coupled to the bed of the body of water.

5. The assembly system of claim 4 wherein the heave reduction system comprises an actuatable device configured to adjust the tension in the coupling, the actuatable device comprising at least one of: a winch, a jack, a strand jack, a hydraulic ram; and / or a tug.

6. The assembly system of claim 5 wherein the actuatable device is coupled to the floating base and / or anchor arrangement and / or lifting structure by at least one of:an articulated joint;a rotatable and / or flexible joint;a universal joint;a multi-axis joint; anda gimbal.

7. The assembly system of any one of claims 4 to 6 wherein the coupling comprises at least one of: a line; a chain; a wire; a strand; and a coupling plate.

8. The assembly system of any one of claims 4 to 7 wherein the anchor arrangement is selectively releasable and / or comprises one or more pulley mechanisms.

9. The assembly system of any preceding claim wherein the lifting structure comprises a plurality of legs and a lifting arrangement coupled to one or more of the plurality of legs, the lifting arrangement configured for landing the one or more components of the offshore structure onto the floating base.

10. The assembly system of claim 9 wherein each leg is fixedly coupled to the bed of the body of water.

11. The assembly system of any preceding claim in combination with the offshore structure, wherein the offshore structure is one of: a wind turbine or component thereof, and a jacket.

12. The assembly system of claim 3 wherein varying the pushing force comprises adjusting a weight coupled to the floating base.

13. The assembly system of claim 12 wherein the weight comprises a chamber and adjusting the weight comprises at least partially filling or draining the chamber.

14. The assembly system of claim 12 or claim 13 wherein adjusting the weight comprises tensioning a line extending from the lifting structure to a lever arm extending from the lifting structure and coupled to the floating base.

15. A method of assembling an offshore structure, the method comprising: performing, using a lifting structure, a landing operation, the landing operation comprising, landing one or more components of the offshore structure onto a floating base,wherein a draft of the floating base is selectively adjusted, using a heave reduction system, during the landing operation.

16. The method of claim 15 wherein selectively adjusting the draft of the floating base comprises selectively adjust the draft in response to the landing of one of the one or more components of the offshore structure onto the floating base.

17. The method of claim 15 or claim 16, comprising preceding steps of:transporting the one or more components of the offshore structure to an assembly area below the lifting structure;suspending the one or more components from the lifting structure; and transporting the floating base to the assembly area.

18. The method of any one of claims 15 to 17 wherein adjusting the draft comprises varying a pulling or pushing force acting upon the floating base in the direction of a bed of a body of water.

19. The method of claim 18 comprising subsequent steps of:removing the pulling or pushing force acting upon the floating base;transporting the floating base out of the assembly area;transporting one or more further components of the offshore structure to the assembly area;suspending the one or more further components from the lifting structure;transporting the floating base to the assembly area;performing, using the lifting structure, a further landing operation, the further landing operation comprising, landing the one or more further components of the offshore structure onto the floating base and / or onto the one or more components,wherein the draft of the floating base is adjusted, using the heave reduction system, during the further landing operation.

20. The method of any one of claims 17 to 19 wherein transporting the floating base to and / or out of the assembly area comprises coupling the floating base to avessel and transporting the floating base to and / or out of the assembly area using the vessel.

21. The method of any one of claims 18 to 20 wherein varying the pulling force comprises adjusting a tension in a coupling between the floating base and an anchor arrangement coupled to the bed of the body of water.

22. The method of claim 21 wherein the heave reduction system comprises an actuatable device configured to adjust the tension in the coupling, the actuatable device comprising at least one of: a winch, a jack, a strand jack, a hydraulic ram; and a tug.

23. The method of claim 21 or claim 22 wherein the anchor arrangement is selectively releasable and the method comprises a step of engaging and / or releasing the selectively releasable anchor arrangement before and / or after transporting the floating base to and / or out of the assembly area.

24. The method of any one of claims 18 to 20 wherein varying the pushing force comprises adjusting a weight coupled to the floating base.

25. The assembly system of claim 24 wherein the weight comprises a chamber and adjusting the weight comprises at least partially filling or draining the chamber.

26. The assembly system of claim 24 or claim 25 wherein adjusting the weight comprises tensioning a line extending from the lifting structure to a lever arm extending from the lifting structure and coupled to the floating base.

27. The method of any one of claims 16 to 26 wherein the lifting structure comprises a plurality of legs and a lifting arrangement coupled to one or more of the plurality of legs, and the method comprises using the lifting arrangement to land the one or more components of the offshore structure onto the floating base.

28. The method of claim 27 wherein each leg is fixedly coupled to the bed of the body of water.s

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