Deep sea mining system and riser thereof

Abstract

A riser (20) for a deep sea mining system (1). The riser (20) comprises a lower end (20l) and an upper end (20u). The riser (20) further comprises at least one up pipe (21) for containing a slurry mixture of mined material and carrier water pumped up from the lower end (20l) of the riser (20) to a water separation mechanism (14) disposed on a surface vessel (15) and being configured to separate the mined material and the carrier water; at least one top-up pipe (22) for containing top-up water pumped up from the lower end (20l) of the riser (20) to the surface vessel (15); and at least one down pipe (23) for containing the carrier water and the top-up water pumped down from the surface vessel (15) to the lower end (20l) of the riser (20).

Description

[0001] DEEP SEA MINING SYSTEM AND RISER THEREOF

[0002] Background to the Invention

[0003] The present invention relates to a deep sea mining system and a riser thereof.

[0004] Deep sea mining systems are typically configured to collect mined materials (such as polymetallic nodules and seafloor massive sulphides) from a seabed located up to 6000m below sea level. These materials are typically mixed together with carrier water from the seabed environment to form a slurry. A lower pumping station then typically pumps the slurry up through an up pipe of a riser and towards a surface vessel of the deep sea mining system, on which the mined material and the carrier water are separated. While the mined material is collected on the surface vessel, an upper pumping station pumps the carrier water down through a down pipe of the riser in order to drive the lower pumping station. In order to equalise flow between the up pipe and down pipe, the volume lost from collecting the mined material must be replaced.

[0005] US9243497B2 discloses a method of replacing this lost volume with surface seawater, in that a matching volume of surface seawater is also pumped down through the down pipe. However, this method incurs the risk of contaminating the seabed environment with surface seawater.

[0006] It is an object of the present invention to avoid or minimise the foregoing disadvantage.

[0007] Summary

[0008] A riser for a deep sea mining system, the riser comprising: a lower end; an upper end; at least one up pipe for containing a slurry mixture of mined material and carrier water pumped up from the lower end of the riser to a water separation mechanism disposed on a surface vessel and being configured to separate the mined material and the carrier water; at least one top-up pipe for containing top-up water pumped up from the lower end of the riser to the surface vessel; and at least one down pipe for containing the carrier water and the top-up water pumped down from the surface vessel to the lower end of the riser.

[0009] Preferably, the riser is a deep sea mining riser.

[0010] Optionally the lower end of the riser is arranged to be disposed in a seabed environment, (i.e. proximately close to a seabed, typically up to / around / greater than 6000m below sea level). Optionally the surface vessel and optionally the upper end of the riser are arranged to be disposed in a surface environment.

[0011] Optionally each of the at least one up pipe, the at least one top-up pipe and the at least one down pipe extend along a respective longitudinal axis and optionally a respective throughbore optionally extending along the respective longitudinal axis. Optionally each of the respective longitudinal axes are mutually parallel to each other. Optionally each throughbore has a cross section transverse to the respective longitudinal axis, optionally having a cross sectional area which is typically substantially uniform along its respective longitudinal axis.

[0012] Optionally each of the at least one up pipe, the at least one top-up pipe and the at least one down pipe have substantially equal lengths. Alternatively, the at least one top-up pipe may be shorter or longer than the at least one up pipe and the at least one down pipe.

[0013] Optionally the riser comprises a total up pipe cross sectional area consisting of the cross sectional area (i.e. of the throughbore) of the or each up pipe. Optionally the riser comprises a total top-up pipe cross sectional area consisting of the cross sectional area (i.e. of the throughbore) of the or each top-up pipe. Optionally the riser comprises a total down pipe cross sectional area consisting of the cross sectional area (i.e. of the throughbore) of the or each down pipe.

[0014] Optionally where the riser consists of one up pipe, the total up pipe cross sectional area consists of the cross sectional area (i.e. of the throughbore) of the said one up pipe. Optionally where the riser consists of one top-up pipe, the total top-up pipe cross sectional area consists of the cross sectional area (i.e. of the throughbore) of the said one top-up pipe.

[0015] Optionally, the riser consists of two or more down pipes, and where there are two or more down pipes, the total down pipe cross sectional area consists of the sum of the cross sectional area (i.e. of the throughbore) of each of the two or more down pipes.

[0016] Optionally the total up pipe cross sectional area is substantially equal to the total down pipe cross sectional area.

[0017] Optionally the total top-up pipe cross sectional area is arranged to be substantially equal to a percentage of the total up pipe cross sectional area. Optionally said percentage is substantially equal to a volumetric concentration of the mined material in the slurry mixture. Optionally said volumetric concentration is between 10%-20%.

[0018] Optionally the riser comprises a total up pipe volume consisting of a volume (i.e. of the throughbore) of the or each up pipe. Optionally the riser comprises a total top-up pipe volume consisting of a volume (i.e. of the throughbore) of the or each top-up pipe. Optionally the riser comprises a total down pipe volume consisting of a volume (i.e. of the throughbore) of the or each down pipe.

[0019] Optionally the total up pipe volume is substantially equal to the total down pipe volume.

[0020] Optionally the total top-up pipe volume is arranged to be substantially equal to a percentage of the total up pipe volume. Optionally said percentage is substantially equal to a volumetric concentration of mined material in the slurry mixture. Optionally said volumetric concentration is between 10%-20%.

[0021] Optionally the total top-up pipe volume is arranged to be equal to the total volume of mined material in the slurry mixture.

[0022] Optionally the riser comprises a plurality of riser segments interconnected at junctions. Optionally each riser segment comprises an up pipe segment for the or each up pipe, a top-up pipe segment for the or each top-up pipe and a down pipe segment for the or each down pipe. Optionally the or each up pipe segment, the or each top-up pipe segment and the or each down pipe segment comprises a respective pin end and a respective opposite box end. Optionally the pin end of the or each up pipe segment is configured to be received by and optionally secured within the box end of an up pipe segment of an adjoining riser segment. Optionally the pin end of the or each top-up pipe segment is configured to be received by and optionally secured within the box end of a top-up pipe segment of an adjoining riser segment. Optionally the pin end of the or each down pipe segment is configured to be received by and optionally secured within the box end of a down pipe segment of an adjoining riser segment.

[0023] Optionally each of the at least one up pipe, the at one least top-up pipe and the at least one down pipe are arranged such that the weight is evenly distributed across each of the at least one up pipe, the at least one top-up pipe and the at least one down pipe.

[0024] Optionally the riser comprises one up pipe, optionally one top-up pipe and optionally two down pipes. Optionally the cross sectional areas (i.e. of the respective throughbores) of each down pipe are equal to each other.

[0025] Optionally, the riser is arranged whereby the said one up pipe is located on a central longitudinal axis of the riser such that the longitudinal axis of the said one up pipe is coincident with the central longitudinal axis of the riser.

[0026] Optionally the longitudinal axis of the said one up pipe represents an origin in a transverse plane of the riser.

[0027] Optionally, the longitudinal axis of the said one top up pipe is radially offset from the longitudinal axis of the said one up pipe by a radial distance R1 , wherein R1 is greater than the radius (i.e. of the throughbore) of the said one up pipe.

[0028] Typically, the central longitudinal axis of the said one top up pipe lies on the said transverse plane of the riser but is radially offset from the central longitudinal axis of the said one up pipe by a radial distance R1. Optionally, each longitudinal axis of the said two down pipes is radially offset from the central longitudinal axis of the said one up pipe by a radial distance and further optionally both longitudinal axes of the said two down pipes are radially offset from the longitudinal axis of the said one up pipe by the same radial distance R2, wherein R2 is greater than a radius (i.e. of the throughbore) of the said one up pipe.

[0029] Optionally each longitudinal axis of the said two down pipes is angularly offset from the longitudinal axis of the top-up pipe (optionally in opposite directions to each other), with respect to the origin (that is the central longitudinal axis of the up pipe) by an angular distance and optionally both longitudinal axes of the said two down pipes are angularly offset from the top-up pipe by the same angular distance 01. Typically, 01 is between 90° and 180°.

[0030] Optionally, the said two down pipes and the said one top up pipe are arranged within the riser such that they are circumferentially spaced apart around the said one up pipe, such that each of the said two down pipes and the said one top up pipe are arranged within the riser such that they are in a geo-stationary orbit around the said one up pipe.

[0031] Optionally an acute angle, with respect to the origin, between the longitudinal axes of the two down pipes is equal to an angle a. Optionally an obtuse angle, with respect to the origin, between the longitudinal axes of the two down pipes is equal to an angle p, where a + p = 360°. Optionally an angle between the longitudinal axis of the one top up pipe and the longitudinal axis of each of the two down pipes (i.e. 01) is substantially equal to half of p.

[0032] This arrangement has the advantage that, with the ratio of R1 and R2, the angular positions (i.e. the values of 01 , a and P) and the respective cross-sectional areas of the said one up pipe, said two down pipes and said one top up pipes being taken into account, the riser as a whole can be load balanced, wherein the centre of gravity of the riser is co-incident with the longitudinal axis of the up pipe, such that any off- central longitudinal axis load can be minimised or mitigated, therein providing increased stability to the riser, for handling. Optionally a lower end of the or each top-up pipe is arranged to be open ended and optionally configured to provide an inlet for top-up water to be drawn in through, Optionally the inlet of the or each top-up pipe is arranged to be disposed within a seabed environment (i.e. at or proximate to the lower end of the riser). Optionally the inlet of the or each top-up pipe is arranged to be disposed on a lowermost top-up pipe segment.

[0033] A deep sea mining system comprising: a water separation mechanism disposed on a surface vessel and being configured to separate mined material and carrier water of a slurry mixture; a riser comprising: a lower end; an upper end; at least one up pipe; at least one top-up pipe; at least one down pipe; wherein the deep sea mining system is arranged to pump the slurry mixture up from the lower end of the riser through the or each up pipe to the water separation mechanism; wherein the deep sea mining system is arranged to pump top-up water up from the lower end of the riser through the or each top-up pipe to the surface vessel; and wherein the deep sea mining system is arranged to pump the carrier water and the top-up water down from the surface vessel through the or each down pipe to the lower end of the riser.

[0034] Optionally the lower end of the riser is disposed in the seabed environment, (i.e. proximately close to a seabed, typically up to / around / greater than 6000m below sea level). Optionally the surface vessel and optionally the upper end of the riser are disposed in the surface environment. Optionally the surface vessel is positioned at sea level.

[0035] Typically the water separation mechanism is configured to separate the mined material and the carrier water such that mined material can be extracted from the slurry mixture. Optionally the deep sea mining system comprises a lower pumping station.

[0036] Optionally the lower pumping station is disposed in the seabed environment.

[0037] Optionally the lower pumping station is connected to the lower end of the riser. Optionally the lower pumping station is connected to a lower end of the or each up pipe and optionally a lower end of the or each down pipe.

[0038] Optionally the lower pumping station is arranged to pump the slurry mixture up from the lower end of the riser (optionally from the lower pumping station) through the or each up pipe to the water separation mechanism.

[0039] Optionally the lower pumping station comprises a pressure exchange chamber.

[0040] Optionally the deep sea mining system comprises a top-up pump. Optionally the top- up pump is disposed in the seabed environment.

[0041] Optionally the top-up pump is arranged to pump the top-up water from the lower end of the riser (optionally from the top-up pump) through the or each top-up pipe to the surface vessel).

[0042] Optionally the top-up pump is configured to draw in the top-up water, optionally from the seabed environment. Optionally the top-up pump is configured to draw in the top- up water via the inlet of the or each top-up pipe.

[0043] Optionally the top-up pump comprises a centrifugal pump.

[0044] Optionally the deep sea mining system comprises an upper pumping station. Optionally the upper pumping station is disposed on the surface vessel. Optionally the upper pumping station is connected to the upper end of the riser. Optionally the upper pumping station is connected to an upper end of the or each top-up pipe, optionally via a flexible pipe (optionally a relatively low-pressure flexible pipe). Optionally the upper pumping station is connected to an upper end of the or each down pipe, optionally via a flexible pipe (optionally a relatively high-pressure flexible Pipe). Optionally the deep sea mining system comprises a water storage tank, optionally disposed on the surface vessel. Optionally the water storage tank is connected between an upper end of the or each top-up pipe. Optionally the water storage tank is connected to the upper end of the or each top-up pipe via a flexible pipe (optionally a relatively low-pressure flexible pipe).

[0045] Optionally the upper end of the or each up pipe is connected to the water separation mechanism.

[0046] Optionally the water separation mechanism is connected to the upper pumping station. Optionally the water storage tank is connected between the water separation mechanism and the upper pumping station.

[0047] Optionally the top-up pump is connected between a lower end of the or each top-up pipe (i.e. the inlet of the or each top-up pipe) and the upper pumping station and optionally between a lower end of the or each top-up pipe and the water storage tank.

[0048] Optionally the top-up pump is arranged to pump the top-up water from the lower end of the riser (optionally from the top-up pump) through the or each top-up pipe, typically in an upwards direction, to the upper pumping station.

[0049] Optionally the upper pumping station is arranged to pump the carrier water and the top-up water down from the surface vessel (optionally the upper pumping station) through the or each down pipe to the lower end of the riser (optionally the lower pumping station).

[0050] Optionally the water separation mechanism is configured to transport (i.e. pump) the carrier water to the water storage tank. Optionally the carrier water and top-up water are gathered together within the water storage tank.

[0051] Optionally the upper pumping station is arranged to draw in the carrier water and the top-up water from the water storage tank. Optionally the upper pumping station comprises a pumping mechanism and typically, the pumping mechanism comprises a positive displacement pump.

[0052] Optionally the lower pumping station is configured to set the volumetric concentration of the mined material in the slurry mixture (i.e. pumped up through the or each up pipe) to a substantially consistent value (i.e. between 10%-20%).

[0053] Optionally the lower pumping station is configured to be driven by a driving portion of the carrier water and the top-up water pumped down from the surface vessel (optionally the upper pumping station) through the or each down pipe to the lower end of the riser (optionally the lower pumping station). Optionally the driving portion of the carrier water and the top-up water is used as a driving fluid in a pressure exchange chamber.

[0054] Optionally the lower pumping station is configured to dispose a disposable portion of the carrier water and the top-up water pumped down from the surface vessel (optionally the upper pumping station) through the or each down pipe to the lower end of the riser (optionally the lower pumping station) into the seabed environment.

[0055] Optionally, the deep sea mining system further comprises a subsea harvester or miner. Optionally the subsea harvester / miner is connected to the lower end of the riser, optionally to the lower pumping station. Optionally the subsea harvester is disposed in the seabed environment. Optionally the subsea harvester is disposed on and optionally traversable along the seabed. Optionally the subsea harvester is configured to draw in the mined material (i.e. from the seabed) and the carrier water (i.e. from the seabed environment) to form the slurry mixture. Optionally the subsea harvester is connected to the lower pumping station, optionally via a slurry flexible Pipe.

[0056] Optionally the subsea harvester is configured to transport (i.e. pump) the slurry mixture to the lower end of the riser (i.e. the lower pumping station), optionally through the slurry flexible pipe.

[0057] Optionally the mined material comprises polymetallic nodules, seafloor massive sulphides or any other suitably mineable deep sea material. A method of operating a deep sea mining system, the deep sea mining system comprising: a water separation mechanism disposed on a surface vessel and being configured to separate mined material and carrier water of a slurry mixture; a riser comprising: at least one up pipe; at least one top-up pipe; at least one down pipe; a lower end; an upper end; wherein the deep sea mining system is arranged to pump the slurry mixture up from the lower end of the riser through the or each up pipe to the water separation mechanism; wherein the deep sea mining system is arranged to pump top-up water up from the lower end of the riser through the or each top-up pipe to the surface vessel; and wherein the deep sea mining system is arranged to pump the carrier water and the top-up water down from the surface vessel through the or each down pipe to the lower end of the riser; and the method comprising: pumping the slurry mixture up from the lower end of the riser through the or each up pipe to the water separation mechanism; operating the water separation mechanism to separate the mined material and carrier water of the slurry mixture; pumping the top-up water up from the lower end of the riser through the top- up pipe to the surface vessel; and pumping the carrier water and the top-up water down from the surface vessel through the or each down pipe to the lower end of the riser.

[0058] Optionally the method comprises the step of operating the subsea harvester to draw in the mined material and the carrier water to form the slurry mixture. Optionally the method comprises the step of transporting (i.e. pumping) the slurry mixture from the subsea harvester, optionally through the slurry flexible pipe, to the lower end of the riser (i.e. the lower pumping station).

[0059] Optionally the method comprises the step of operating the lower pumping station to pump the slurry mixture up from the lower end of the riser (i.e. the lower pumping station) through the or each up pipe to the water separation mechanism.

[0060] Optionally the method comprises the step of operating the top-up pump to draw in the top-up water, optionally from the seabed environment and optionally via the inlet of the or each top-up pipe.

[0061] Optionally the method comprises the step of operating the top-up pump to pump the top-up water from the lower end of the riser (i.e. the top-up pump) through the or each top-up pipe to the surface vessel (i.e. the upper pumping station).

[0062] Optionally the method comprises the step of operating the water separation mechanism to separate the mined material and the carrier water of the slurry mixture. Optionally the method comprises the step of extracting the mined material from the slurry mixture.

[0063] Optionally the method comprises the step of transporting (i.e. pumping) the carrier water from the water separation mechanism to the water storage tank.

[0064] Optionally the method comprises the step of operating the upper pumping station to draw in the carrier water and the top-up water from the water storage tank.

[0065] Optionally the method comprises the step of operating the upper pumping station to pump the carrier water and the top-up water down from the surface vessel (preferably down from the upper pumping station) through the or each down pipe to the lower end of the riser (preferably down to the lower pumping station).

[0066] Optionally the method comprises using the driving portion of the carrier water and the top-up water pumped down from the surface vessel (optionally down from the upper pumping station) through the or each down pipe to the lower end of the riser (optionally down to the lower pumping station) to drive operation of the lower pumping station.

[0067] Optionally the method comprises the step of disposing the disposable portion of the carrier water and the top-up water pumped down from the surface vessel (optionally the upper pumping station) through the or each down pipe to the lower end of the riser (optionally the lower pumping station) into the seabed environment.

[0068] The various aspects of the present invention can be practiced alone or in combination with one or more of the other aspects, as will be appreciated by those skilled in the relevant arts. The various aspects of the invention can optionally be provided in combination with one or more of the optional features of the other aspects of the invention. Also, optional features described in relation to one aspect can typically be combined alone or together with other features in different aspects of the invention. Any subject matter described in this specification can be combined with any other subject matter in the specification to form a novel combination.

[0069] Various aspects of the invention will now be described in detail with reference to the accompanying figures. Still other aspects, features, and advantages of the present invention are readily apparent from the entire description thereof, including the figures, which illustrates a number of exemplary aspects and implementations. The invention is also capable of other and different examples and aspects, and its several details can be modified in various respects, all without departing from the spirit and scope of the present invention. Accordingly, each example herein should be understood to have broad application, and is meant to illustrate one possible way of carrying out the invention, without intending to suggest that the scope of this disclosure, including the claims, is limited to that example. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope. In particular, unless otherwise stated, dimensions and numerical values included herein are presented as examples illustrating one possible aspect of the claimed subject matter, without limiting the disclosure to the particular dimensions or values recited. All numerical values in this disclosure are understood as being modified by "about". All singular forms of elements, or any other components described herein are understood to include plural forms thereof and vice versa. Language such as "including", "comprising", "having", "containing" or "involving" and variations thereof, is intended to be broad and encompass the subject matter listed thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers or steps. Likewise, the term "comprising" is considered synonymous with the terms "including" or "containing" for applicable legal purposes. Thus, throughout the specification and claims unless the context requires otherwise, the word “comprise” or variations thereof such as “comprises” or “comprising” will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0070] Any discussion of documents, acts, materials, devices, articles and the like is included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention.

[0071] In this disclosure, whenever a composition, an element or a group of elements is preceded with the transitional phrase "comprising", it is understood that we also contemplate the same composition, element or group of elements with transitional phrases "consisting essentially of”, "consisting", "selected from the group of consisting of”, “including” or "is" preceding the recitation of the composition, element or group of elements and vice versa. In this disclosure, the words “typically” or “optionally” are to be understood as being intended to indicate optional or non- essential features of the invention which are present in certain examples but which can be omitted in others without departing from the scope of the invention.

[0072] References to directional and positional descriptions such as upper and lower and directions e.g. “up”, “down” etc. are to be interpreted by a skilled reader in the context of the examples described to refer to the orientation of features shown in the drawings, and are not to be interpreted as limiting the invention to the literal interpretation of the term, but instead should be as understood by the skilled addressee. Brief Description of the Drawings

[0073] Embodiments of the present invention will now be described, by way of example only and with reference to the accompanying drawings, in which:-

[0074] Fig. 1 shows a schematic view of an embodiment of a deep sea mining system in accordance with the present invention;

[0075] Fig. 2 shows a plan end view of a riser of the deep sea mining system of Fig. 1 ;

[0076] Fig. 3 shows a side view of a riser segment of the riser of Fig. 2;

[0077] Fig. 4 shows a cross sectional side view of an upper end and a lower end of the riser segment of Fig. 3 along the line A-A of Fig. 2;

[0078] Fig. 5 shows a cross sectional side view of the upper end and the lower end of the riser segment of Fig. 3 along the line B-B of Fig. 2;

[0079] Fig. 6 shows a cross sectional side view of the respective ends of two adjoining riser segments along the line of A-A of Fig. 2; and

[0080] Fig. 7 shows a cross sectional side view of the two adjoining riser segments of Fig. 6 along the line of B-B of Fig. 2.

[0081] Detailed Description

[0082] Referring now to the drawings, a schematic view of an embodiment of a deep sea mining system 1 in accordance with the present invention is shown in Fig. 1. The deep sea mining system 1 of this example comprises a subsea harvester / miner 2 disposed on a seabed 3 within a seabed environment 4, typically up to / around / greater than 6000m below sea level 5. The subsea harvester 2 of this example is connected to a lower pumping station 6 (also disposed in the seabed environment 4) via a slurry flexible pipe 7.

[0083] The lower pumping station 6 of this example is connected to a lower end 20I of an embodiment of a riser 20 in accordance with the present invention. The riser 20 of this example extends longitudinally up from the lower end 20I disposed in the seabed environment 4 to an upper end 20u disposed in a surface environment 8. For clarity purposes, a broken view of the riser 20 is shown in Fig. 1. Typically, the total length of the riser 20 would constitute a significant majority of the distance between sea level 5 and the seabed 3, and therefore could be many hundreds or thousands of metres in length.

[0084] The riser 20 of this example comprises an up pipe 21, a top-up pipe 22 and two down pipes 23 (see Fig. 2). However, other examples of the riser 20 may comprise more than one up pipe 21, more than one top-up pipe 22, a single down pipe 23 or more than two down pipes 23.

[0085] As seen in Fig. 1, a lowermost segment 20slm of the riser 20 in this example comprises an open lower end 24 (i.e. an inlet 24) of the top-up pipe 22, above which a top-up pump 6a is connected in-line with the top-up pipe 22.

[0086] In this example, an intermediate pipe 9 provides connections betweeman upper end of the up pipe 21 and a lower end of a support pipe 10; an upper end of the top-up pipe 22 and a lower end of a relatively low- pressure flexible pipe 11; and respective upper ends of each down pipe 23 and a lower end of a relatively high-pressure flexible pipe 12.

[0087] In this example, an upper end of the support pipe 10 is connected to a lower end of a slurry surface pipe 13. An upper end of the slurry surface pipe 13 of this example is connected to a water separation mechanism 14 disposed on a surface vessel 15. Additionally, the support pipe 10 of this example is connected to a load bearing mechanism 16 disposed on the surface vessel 15.

[0088] In this example, an upper end of the low-pressure flexible pipe 11 is connected to a water storage tank 19 disposed on the surface vessel 15. Further in this example, the water storage tank 19 and an upper pumping station 17 disposed on the surface vessel are connected via a combined water pipe 19a.

[0089] In this example, an upper end of the high-pressure flexible pipe 12 is connected to the upper pumping station 17. The water separation mechanism 14 and the water storage tank 19of this example are connected via a separated water pipe 18. In operation, the subsea harvester 2 typically traverses the seabed 3, from which it gathers mined material. The mined material may comprise polymetallic nodules, seafloor massive sulphides or any other suitably minable deep sea material. The subsea harvester 2 typically also gathers carrier water from the seabed environment 4. US9243497B2 describes a suitable subsea harvester 2 and method of preparing a seabed for deep sea mining activity. The gathered mined material and carrier water are mixed to form a slurry mixture, which the subsea harvester 2 may then pump through the slurry flexible pipe 7 to the lower pumping station 6.

[0090] The lower pumping station 6 of this example pumps the slurry mixture all the (relatively long) distance up from the lower pumping station 6 through the up pipe 21 of the riser 20, the intermediate pipe 9, the support pipe 10 and the surface slurry pipe 13 to the water separation mechanism 14 on the surface vessel 15.

[0091] Additionally, the lower pumping station 6 of this example is configured such that the volumetric concentration (Cv) of mined material in the slurry is substantially consistent across the slurry mixture pumped up to the water separation mechanism 14. In other words, the lower pumping station 6 may, if needs be, modify the slurry mixture pumped in from the subsea harvester 2 such that the mined material makes up a desired percentage of the total volume of the slurry mixture that is subsequently pumped up to the water separation mechanism 14. The lower pumping station 6 may either dilute the slurry mixture to reduce Cvor concentrate the slurry mixture to increase Cv. The lower pumping station 6 is typically arranged to set Cvbetween 10% - 20%. Typically for a given lower pumping station 6, the specific value of Cvis dependent on the material or materials comprised within the mined material.

[0092] The lower pumping station 6 typically comprises any suitable pumping system capable of handling solid laden and relatively abrasive fluids (i.e. the slurry mixture) and pumping fluids up the full length of the riser 20 (i.e. up to / around / greater than distances of 6000m).

[0093] Such pumping systems may comprise a pressure exchange chamber (not shown). Typically in operation, a pumped fluid inlet valve (not shown) is opened to fill the pressure exchange chamber with a pumped fluid (i.e. the slurry mixture). This displaces a driving fluid (typically water) out of the pressure exchange chamber through an open driving fluid outlet valve (not shown). After closing the pumped fluid inlet valve and the driving fluid outlet valve, a pumped fluid outlet valve (not shown) and driving fluid inlet valve (not shown) are opened. Highly pressured driving fluid entering through the driving fluid inlet valve (typically pressurised using a positive displacement pump) then displaces the pumped fluid out of the pressure exchange chamber, pumping it towards an intended destination (i.e. the surface vessel 15). This cycle is repeated to allow for the continuous upward pumping of pumped fluid.

[0094] Alternatively, the lower pumping station 6 may comprise a subsea slurry lift pump (SSLP) (not shown) such as that disclosed in US9243497 along with any suitable required modifications.

[0095] The water separation mechanism 14 of this example is configured to separate the mined material and the carrier water of the slurry mixture, allowing for the mined material to be extracted and stored on the surface vessel 15. In some examples, the water separation mechanism 14 may use a set of screens (not shown) configured to sieve out the mined material. Other examples of the water separation mechanism may comprise a hydrocyclone separator (not shown) configured to set the slurry mixture into a vortex, allowing the mined material and the carrier water to be separated using centrifugal force. Further examples of a water separation mechanism 14 may comprise a bunker (not shown), in which the slurry mixture is disposed in a bunker, in which the carrier water would be left to drain out of the slurry mixture via a suitable screen (or the like) arranged to retain the mined material within the bunker.

[0096] The separated carrier water is then typically transported from the water separation mechanism 14 to the water storage tank 19 via the separated water pipe 18.

[0097] The top-up pump 6a of this example is configured to draw in top-up water from the seabed environment 4 through the inlet 24 of the top-up pipe 22. The top-up pump 6a of this example may then pump the top-up water through the top-up pipe 22, the intermediate pipe 9 and the low-pressure flexible pipe 11 to the water storage tank 19 on the surface vessel 15. Although the top-up pump 6a of this example is disposed towards the lower end of the top-up pipe 22, in other examples it may be disposed anywhere in line with the top-up pipe 22 between the inlet 24 of the top-up pipe 22 and the water storage tank 19.

[0098] Typically, the top-up pump 6a comprises any suitable pumping system capable of pumping the top-up water from the seabed environment 4 to the water storage tank 19 (i.e. up to / around / greater than distances around distances of 6000m). Such pumping systems may comprise a suitable centrifugal pump, which operates by imparting the rotational kinetic energy of a rapidly rotating impeller (typically an engine or electric motor) (not shown) on to pumped fluid (i.e. the top-up water).

[0099] The load bearing mechanism 16 of this example is configured to support the load of the riser 20. Additionally, the load bearing mechanism 16 may be configured to act as a heave compensator in order keep the support pipe 10 and all the components connected below it (including the riser 20) substantially motionless with respect to the seabed 3 as the sea level 5 (and therefore the vertical height of the surface vessel 15 above the upper end of the riser 20) changes.

[0100] Within the water storage tank 19 of this example, the separated carrier water is topped up with at least some of the top-up water pumped to the water storage tank 19 on the surface vessel 15 via the top-up pipe 22. The upper pumping station 17 of this example then draws in this combination of carrier water and top-up water and pumps it down through the high-pressure flexible pipe 12 and each of the two down pipes 23 to the lower pumping station 6.

[0101] In operation, the separated carrier water is topped up with a volume of top-up water substantially equal to the volume of mined material extracted from the slurry mixture. This is to ensure that there is a flow equilibrium between the up pipe 21 and the two down pipes 23.

[0102] The upper pumping station 17 typically comprises any pumping system capable of pumping the carrier water and the top-up water down the full length of the riser 20 (i.e. up to / around / greater than distances around distances of 6000m). Such pumping systems may comprise a suitable positive displacement pump (not shown), which operates by expanding a cavity in order to draw in a fixed volume of pumped fluid (i.e. the carrier water and the top-up water) through an inlet valve (not shown) and shrinking said cavity in order to force the fixed volume of pumped fluid out an outlet valve (not shown) and towards a desired destination (i.e. the lower pumping station). This cycle is repeated in order to allow for the continuous downward pumping of pumped fluid.

[0103] Alternatively, the upper pumping station 17 may comprise multiple triplex or centrifugal pumps installed on the surface vessel 15 as disclosed in US9243497B2 along with any suitable required modifications.

[0104] The lower pumping station 6 is typically configured to be driven by at least some of the carrier water and top-up water pumped down from the upper pumping station 17, meaning that the deep sea mining system 1 operates as a closed loop system. Any remaining carrier water and top-up water may be released back into the seabed environment 4 via the lower pumping station 6. As both the carrier water and the top- up water were initially gathered from the seabed environment 4, there is a greatly reduced risk that the release of the carrier water and top-up water into the seabed environment 4 would contaminate it. This is in stark contrast to other deep sea mining systems in which top-up water is supplied from the surface environment 8, as releasing such surface top-up water (which could contain certain bacteria etc.) into the seabed environment 4 may severely damage said seabed environment 4.

[0105] Fig. 2 shows a plan view of the riser 20 of this example. In this example, each of the up pipe 21 , the top-up pipe 22 and the two down pipes 23 extend along a respective longitudinal axis, which extends along the whole vertical length of the riser 20. In this example, the up pipe 21, the top-up pipe 22 and two down pipes 23 have substantially equal lengths. In other examples, the top-up pipe 22 may have a different length compared to the other pipes.

[0106] Typically, each of the up pipe 21 , the top-up pipe 22 and the two down pipes 23 have a throughbore 21t, 22t, 23t extending along their respective longitudinal axis. Typically each throughbore 21t, 22t, 23t has a cross section transverse to their respective longitudinal axis. Each said cross section typically has a respective cross sectional area that in this example is substantially uniform along their respective longitudinal axis. The throughbore 211 of the up pipe 21 may have a diameter between 10 inches-20 inches (25.4cm-50.8cm), the throughbore 22t of the top-up pipe 22 may have a diameter between 4 inches-8 inches (10.2cm-20.3cm) and the throughbore 23t of each down pipe 23t may have a diameter between 7 inches-14 inches (17.8cm- 35.6cm). In this example, the throughbores 22t of the two down pipes 23 have equal diameters and therefore equal cross sectional areas.

[0107] In this example, the diameters of the throughbores 211, 23t of the up pipe 21 and the two down pipes 23 are determined such that the cross sectional area of the throughbore 211 of the up pipe 21 is substantially equal to the sum of cross sectional areas 23t of the throughbores of each down pipe 23. In particular, the throughbore 211 of the up pipe 21 of this example has a diameter of 10 inches (25.4cm) and therefore has a cross sectional area of 25TT inches2(161.3TT cm2) - approximately 78.5 inches2(506.5 cm2) (calculated using TT(D / 2)2, where D is the diameter of the throughbore). The sum of the cross sectional areas of the throughbores 23t of the two down pipes 23 in this example is also 25TT inches2(161.3TT cm2). Therefore, each throughbore 23t of the down pipes 23 has equal cross sectional areas of 12.5TT inches2(80.6TT cm2) and equal diameters of approximately 7.1 inches (18.0cm). As the up pipe 21 and the two down pipes 23 of this example have substantially equal lengths, the volume of the throughbore 211 of the up pipe 21 is therefore substantially equal to the sum of volumes of the throughbores 23t of the two down pipes 23.

[0108] Further in this example, the diameter of the throughbore 22t of the top-up pipe 22 is configured such that the cross sectional area of the throughbore 22t of the top-up pipe 22 is equal to a percentage of the cross sectional area of the throughbore 211 of the up pipe 21. In this example, this said percentage is equal to the predetermined volumetric concentration of mined material in the slurry mixture (Cv). Assuming Cv is equal to 20%, the cross sectional area of the throughbore 22t of the top-up pipe 22 is arranged to be equal to 20% of 25TT inches2(161.3TT cm2), which is 5TT inches2(32.3TT cm2). This gives the throughbore 22t of the top-up pipe 22 of this example a diameter of 4.5 inches (11 ,4cm).

[0109] As best seen in Fig. 2, the up pipe 21 , the top-up pipe 22 and the two down pipes 23 in this embodiment may be arranged such that the weight of the up pipe 21 , the top- up pipe 22, the two down pipes 23 and the fluid contained therein is evenly distributed across the riser 20, thereby increasing its stability, when handling. In this example, the up pipe 21 is coincident with a central longitudinal axis of the riser 20. This central longitudinal axis may represent an origin of a transverse plane of the riser 20.

[0110] In this example, the longitudinal axis of the top-up pipe 22 is radially offset from the central longitudinal axis of the up pipe 21 by a radial distance R1 while the longitudinal axis of each of the two down pipes 23 are radially offset from the central longitudinal axis of the up pipe 21 by a radial distance R2.

[0111] Further in this example and as particularly shown in Fig. 2, each longitudinal axis of the two down pipes 23 is angularly offset from the longitudinal axis of the top-up pipe 22 with respect to the central longitudinal axis of the up pipe 21 , in opposite directions to each other and by an angular distance 01.

[0112] In other terms, there is an acute angle a and an obtuse angle p between each longitudinal axis of the two down pipes 23 and where the top-up pipe 22 is positioned half way between the two down pipes 23 on the obtuse angle side thereof. In this example, a + p is equal to 360° and 01 is equal to half of p.

[0113] Typically, the radial and angular positions of the top-up pipe 22 and the two down pipes 23 are chosen such that any off-central longitudinal axis load can be mitigated or minimised.

[0114] Whilst the riser 20 in use is relatively long, particularly for manufacturing, handling, transportation and / or installation purposes, the riser 20 may comprise a plurality of riser segments 20s arranged to be interconnected at junctions. As best seen in Fig. 3, each riser segment 20s may comprise a lower end 20sl and an upper end 20su. Additionally, each riser segment 20s may have a length between 40ft (12.2m) and 90ft (27.4m).

[0115] Additionally in this example, each riser segment 20s respectively comprises an up pipe segment 21s, a top-up pipe segment 22s and two down pipe segments 23s, each having respective box ends 21b, 22b, 23b and respective pin ends 21 p, 22p, 23p (see Figs 4-7). As best seen in Figs. 4 & 5, in this example the pin end 21 p of the up pipe segment 21s and the box end 22b of the top-up pipe segment 22s and the box ends 23b of the two down pipe segments 23 are disposed at the upper end 20su of the riser segment 20s. Furthermore, the box end 21 b of the up pipe segment 21s, the pin end 22p of the top-up pipe segment 22s and the pin ends 23p of the two down pipe segments 23s are disposed at the lower end 20sl of the riser segment 20s.

[0116] Figs. 6 and 7 show an example of two adjoining riser segments 20s, in which the each pin end 21 p, 22p, 23p is received by and secured within a corresponding box end 21b, 22b, 23b. In some examples, each connection between adjacent riser segments 20s may be secured using a suitable securing mechanism, where a preferred securing mechanism is the Merlin™ Connector provided by Oil State Industries of Houston, Texas, USA although any other suitable securing mechanism could be used instead.

[0117] Each riser segment 20s of this example comprises two flanges 25 extending radially outwards from the up pipe segment 22s, where one flange 25 is disposed at the lower end 20sl of the riser segment 20s and the other flange 25 is disposed at the upper end 20su of the riser segment 20s.

[0118] As best seen in Fig. 3, a plurality of clamps 26 are disposed along the length of the riser segment 20s. In this example, each of the up pipe segment 21s, the top-up pipe segment 22s and the two down pipe segments 23s pass through respective apertures of each clamp 26 in order to maintain a mutually parallel arrangement between the up pipe segment 21s, the top-up pipe segment 22s and the two down pipe segments 23s.

[0119] Modifications and improvements may be made to the embodiments described hereinbefore, without departing from the scope of the invention.

Claims

Claims1. A riser for a deep sea mining system, the riser comprising: a lower end; an upper end; at least one up pipe for containing a slurry mixture of mined material and carrier water pumped up from the lower end of the riser to a water separation mechanism disposed on a surface vessel and being configured to separate the mined material and the carrier water; at least one top-up pipe for containing top-up water pumped up from the lower end of the riser to the surface vessel; and at least one down pipe for containing the carrier water and the top-up water pumped down from the surface vessel to the lower end of the riser.

2. The riser according to claim 1, wherein the riser further comprises: a total up pipe cross sectional area consisting of the cross sectional area of the or each up pipe; a total top-up pipe cross sectional area consisting of the cross sectional area of the or each top-up pipe; and a total down pipe cross sectional area consisting of the cross sectional area of the or each down pipe.

3. The riser according to claim 2, wherein the total up pipe cross sectional area is substantially equal to the total down pipe cross sectional area.

4. The riser according to claim 2 or claim 3, wherein the total top-up pipe cross sectional area is arranged to be substantially equal to a percentage of the total up pipe cross sectional area; and wherein said percentage is substantially equal to a volumetric concentration of the mined material in the slurry mixture.

5. The riser according to any of claims 2-4,wherein each of the at least one up pipe and the at least one down pipe have substantially equal lengths.

6. The riser according to any preceding claim, wherein a lower end of the or each top-up pipe is arranged to be open ended and configured to provide an inlet for top-up water to be drawn through.

7. The riser according to claim 6, wherein the inlet of the or each top-up pipe is arranged to be disposed in a seabed environment.

8. The riser according to any preceding claim, wherein the riser comprises a plurality of riser segments interconnected at junctions; wherein each riser segment comprises an up pipe segment for the or each up pipe, a top-up pipe segment for the or each top-up pipe and a down pipe segment for the or each down pipe9. The riser according to claim 8 when dependent on claim 7 , wherein the inlet of the or each top-up pipe is arranged to be disposed on a lowermost top-up pipe segment.

10. A deep sea mining system comprising: a water separation mechanism disposed on a surface vessel and being configured to separate mined material and carrier water of a slurry mixture; a riser comprising: a lower end; an upper end; at least one up pipe; at least one top-up pipe; at least one down pipe;wherein the deep sea mining system is arranged to pump the slurry mixture up from the lower end of the riser through the or each up pipe to the water separation mechanism; wherein the deep sea mining system is arranged to pump top-up water up from the lower end of the riser through the or each top-up pipe to the surface vessel; and wherein the deep sea mining system is arranged to pump the carrier water and the top-up water down from the surface vessel through the or each down pipe to the lower end of the riser.

11. The deep sea mining system according to claim 10, wherein the riser further comprises: a total up pipe cross sectional area consisting of the cross sectional area of the or each up pipe; a total top-up pipe cross sectional area consisting of the cross sectional area of the or each top-up pipe; and a total down pipe cross sectional area consisting of the cross sectional area of the or each down pipe.

12. The deep sea mining system according to claim 11 , wherein the total up pipe cross sectional area is substantially equal to the total down pipe cross sectional area.

13. The deep sea mining system according to claim 11 or claim 12, wherein the total up pipe cross sectional area is arranged to be substantially equal to a percentage of the total up pipe cross sectional area; and wherein said percentage is substantially equal to a volumetric concentration of the mined material in the slurry mixture.

14. The deep sea mining system according to any of claims 10-13, wherein the deep sea mining system comprises a top-up pump arranged to pump the top-up water from the lower end of the riser through the or each top-up pipe to the surface vessel.

15. The deep sea mining system according to claim 14, wherein a lower end of the or each top-up pipe is arranged to be open ended and configured to provide an inlet for top-up water to be drawn through; and wherein the top-up pump is configured to draw in the top-up water via the inlet of the or each top-up pipe.

16. The deep sea mining system according to claim 15, wherein the inlet of the or each top-up pipe is arranged to be disposed in a seabed environment.

17. The deep sea mining system according to any of claims 14-16, wherein the lower pumping system is configured to set the volumetric concentration of the mined material in the slurry mixture to a substantially consistent value.

18. The deep sea mining system of any of claim 14-17, wherein the lower pumping station is configured to dispose a disposable portion of the carrier water and the top-up water pumped down from the surface vessel though the or each down pipe to the lower end of the riser into a seabed environment.

19. A method of operating a deep sea mining system, the deep sea mining system comprising: a water separation mechanism disposed on a surface vessel and being configured to separate mined material and carrier water of a slurry mixture; a riser comprising: at least one up pipe; at least one top-up pipe; at least one down pipe; a lower end; an upper end; wherein the deep sea mining system is arranged to pump the slurry mixture up from the lower end of the riser through the or each up pipe to the water separation mechanism;wherein the deep sea mining system is arranged to pump top-up water up from the lower end of the riser through the or each top-up pipe to the surface vessel; and wherein the deep sea mining system is arranged to pump the carrier water and the top-up water down from the surface vessel through the or each down pipe to the lower end of the riser; and the method comprising: pumping the slurry mixture up from the lower end of the riser through the or each up pipe to the water separation mechanism; operating the water separation mechanism to separate the mined material and carrier water of the slurry mixture; pumping the top-up water up from the lower end of the riser through the top-up pipe to the surface vessel; and pumping the carrier water and the top-up water down from the surface vessel through the or each down pipe to the lower end of the riser.

20. The method according to claim 19, wherein the wherein the riser further comprises: a total up pipe cross sectional area consisting of the cross sectional area of the or each up pipe; a total top-up pipe cross sectional area consisting of the cross sectional area of the or each top-up pipe; and a total down pipe cross sectional area consisting of the cross sectional area of the or each down pipe.

21. The method according to claim 20, wherein the total up pipe cross sectional area is substantially equal to the total down pipe cross sectional area.

22. The method according to claim 21, wherein the total top-up pipe cross sectional area is arranged to be substantially equal to a percentage of the total up pipe cross sectional area; and wherein said percentage is substantially equal to a volumetric concentration of the mined material in the slurry mixture.

23. The method according to any of claims 19-22, wherein the deep sea mining system comprises a top-up pump arranged to pump the top-up water from the lower end of the riser through the or each top-up pipe to the surface vessel; and wherein the method comprises the further step of: operating the top-up pump to pump the top-up water from the lower end of the riser through the or each top-up pipe to the surface vessel.

24. The method of claim 23, wherein a lower end of the or each top-up pipe is arranged to be open ended and configured to provide an inlet for top-up water to be drawn in through; wherein the top-up pump is configured to draw in the top-up water via the inlet of the or each top-up pipe ; and wherein the method comprises the further step of: operating the top-up pump to draw in the top-up water via the inlet of the or each top-up pipe.

25. The method of claim 23 or claim 24, wherein the deep sea mining system comprises a lower pumping system disposed in the seabed environment; wherein the lower pumping station is configured to dispose a disposable portion of the carrier water and the top-up water pumped down from the surface vessel though the or each down pipe to the lower end of the riser into a seabed environment; and wherein the method comprises the further step of: disposing the disposable portion of the carrier water and the top-up water pumped down from the surface vessel through the or each down pipe to the lower end of the riser into the seabed environment.

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