WO2015197875A1

WO2015197875A1 - Water intake riser assembly

Info

Publication number: WO2015197875A1
Application number: PCT/EP2015/064954
Authority: WO
Inventors: Michalakis Efthymiou; Guido Leon Kuiper
Original assignee: Shell Internationale Research Maatschappij BV; Shell Oil Co
Current assignee: Shell Internationale Research Maatschappij BV; Shell USA Inc
Priority date: 2014-06-26
Filing date: 2015-07-01
Publication date: 2015-12-30
Anticipated expiration: 2016-12-26
Also published as: AU2015279061B2; BR112016029224B1; BR112016029224A2; WO2015197875A8; AP2016009584A0; AU2015279061A1; BR112016029224A8; AU2018202624B2; AU2018202624A1

Abstract

A water intake riser assembly (105) that is suspendable from an off-shore structure. (102) The assembly comprises at least one water intake riser (106A-B) stretching along a length direction. The at least one water intake riser comprises, seen in the length direction, a proximal end comprising suspension means and a distal end comprising a water-intake section (111). The water intake section is in fluid communication with the proximal end. The at least one water intake riser comprises an injection system for injecting an anti-fouling agent into the water intake riser. The anti-fouling agent prevents marine growth in the water intake riser.

Description

WATER INTAKE RISER ASSEMBLY

TECHNICAL FIELD

The present invention relates to water intake riser assembly. The invention further relates to an off-shore structure from such a water riser assembly is suspended, a method of setting up such a water intake riser assembly, a method of producing a liquefied hydrocarbon stream and a method of producing a vaporous hydrocarbon stream.

STATE OF THE ART

Water riser assemblies are known from the prior art, for instance from WO2012/066040 and WO2010/085302.

WO2010/085302 discloses a marine system including a Floating Liquefied Natural Gas (FLNG) plant on/in a surface of the ocean. The FLNG plant may cool and liquefy natural gas to form LNG, or alternatively heat and gasify LNG . A water riser assembly is suspended from the FLNG plant to take in cold water at depth and convey the cold water upward to the FLNG plant. The water riser assembly comprises tubular structures projecting downwardly into the ocean and connected together with a plurality of spacers. The spacers have openings through which respective ones of the tubular structures are disposed. One of the tubular structures may serve as a structural support or structural riser for the spacers. One or more tubular structures of an array or grouping connected with FLNG plant may be used to bring water from the ocean to the plant. In one example nine tubular structures are arranged in a three-by-three rectangular array .

The tubular structures used for bringing water from the ocean to the plant may suffer from marine growth on the inside and may clogg over time. Filters are provided on each of the bottoms of the tubular structures. However, marine growth cannot be prevented by these filters. Clogging and blockage of the water intake should preferably be avoided at all times as it results in less effective operation of the FLNG plant and expensive cleaning operations.

SHORT DESCRIPTION

It is an object to provide an improved water intake riser assembly that is more effective in operation and requires less or none cleaning.

The term 'comprising' is used in this text to indicate that all the enlisted elements are encompassed without excluding the presence of additional non-named elements.

According to a first aspect there is provided a water intake riser assembly that is suspendable from an off-shore structure, comprising at least one water intake riser stretching along a length direction, the at least one water intake riser comprising, seen in the length direction, a proximal end comprising suspension means and a distal end comprising a water-intake section, the water intake section being in fluid communication with the proximal end, wherein the at least one water intake riser comprises an injection system for injecting an anti-fouling agent into the water intake riser.

The off-shore structure may be a floating structure, such as a floating LNG-plant . The LNG plant may be a plant for liquefying a vaporous hydrocarbon containing feed stream and/or for gasifying a liquefied hydrocarbon stream. In other words, the LNG plant may cool and liquefy natural gas to form LNG and/or heat and gasify LNG.

The injection system is provided to add anti-fouling agent to the water taken in by the water-intake section. The anti-fouling agent prevents marine growth development inside the water intake riser and the cooling water system connected thereto. The anti-fouling agent may be sodium hypochlorite (NaCIO) or hypochlorite.

Other anti-fouling agents may be used as well.

The water intake riser can be made by a plurality of riser segments which are connected to each other to form a conduit, forming the fluid communication between the water intake section and the proximal end. The water intake risers and possibly the structural riser may be formed as a tubular conduit. The water intake riser may have any suitable length, for instance a length of more than 100 meters, more than 200 metres or more than 300 meters.

According to an embodiment the injection system has one or more injection outlets inside the water intake riser.

The injection system comprises one or more injection inlets for receiving the anti-fouling agent. The one or more injection inlets are in fluid communication with the one or more injection outlets which are positioned inside the water intake riser to inject the anti-fouling agent into the water intake riser. In between the one or more injection inlets and the one or more injection outlets, a distributor may be provided to distribute the anti-fouling agent over the injection outlets.

According to an embodiment the injection system comprises a ring-shaped nozzle, wherein the injection outlets are positioned on an inner perimeter of the ring-shaped nozzle.

The water intake riser may be formed as a tubular conduit having a diameter D_tUbuiar conduit · The ring-shaped nozzle has a diameter that is in the range of 0.8 - 1.0 times the diameter of the water intake riser: 0.8D_water intake riser ≤ Dnozzie ≤ 1.0

Dwater intake riser · The ring-shaped nozzle comprises a ring-shaped channel or distributor forming the fluid communication between the one or more injection inlets and the one or more injections outlets. The injection outlets may be uniformly distributed along the inner perimeter of the ring-shaped nozzle. The number of injection outlets may be greater than 12, for instance in the range of 12 - 24 injection outlets. The injection outlets and/or the ring-shaped channel may also have non-uniform dimensions to ensure a uniform inflow of anti-fouling agent along the perimeter.

The use of a ring-shaped nozzle ensures a uniform distribution of the fluid throughout the flow area of the water intake riser.

According to an embodiment the injection system has one or more injection outlets positioned at a distal portion of the water intake riser.

The injection outlets, e.g. the ring-shaped nozzle is thus positioned at the distal portion of the water intake riser, i.e. nearby the distal end, e.g. less than 10 or 5 times the diameter of the water intake riser D_water intake riser away from the distal end. The injection system is for instance positioned somewhere along the water intake section or directly downstream thereof.

This ensures that the agent is effective along the major part of the water intake riser.

According to an embodiment the injection system has one or more injection outlets positioned along the upper edge of the water intake section or downstream of the water-intake section .

The water intake section may be formed by a perforated sleeve or filter having a length along the central axis of the water intake riser of approximately 1 - 10 times the diameter of the water intake riser D_water intake riser. The injection outlets, e.g. the ring-shaped nozzle, may thus be positioned downstream of the water-intake section of the water intake riser. This prevents the agent from

accumulating upstream of the injection outlets and resulting in an accumulation of chlorine with negative impact on the water intake riser assembly.

The injection outlets are preferably positioned along the upper edge of the water intake section to ensure that the anti-fouling agent is effective in the major part, e.g. more than 90%, of the length of the water intake riser.

The injection outlets are preferably positioned less than one diameter downstream of the water intake riser D_water intake riser downstream or above the water-intake section.

According to an embodiment the injection system is fluidly connected to an anti-fouling supply unit.

The anti-fouling supply unit may be a vessel comprising the anti-fouling agent. The anti-fouling supply unit may further comprise a mixing device for mixing the anti-fouling agent with water to prepare a required concentration. The anti-fouling supply unit may further comprise a pump to transport the anti-fouling agent to the injection system.

The anti-fouling supply unit is preferably located on the off-shore structure where the water intake riser is suspended from.

According to an embodiment the at least one injection system is in fluid connection with an associated anti-fouling supply line wherein the respective anti-fouling supply lines extend in the length direction along the water intake riser assembly .

In use, the anti-fouling supply lines run along the length of the water intake riser assembly between the proximal end and the distal end of the water intake riser assembly, connecting the anti-fouling supply unit to the injection systems. The anti-fouling supply unit is typically positioned on the off-shore structure the water intake riser assembly is suspended from.

According to an embodiment, the water riser assembly comprises a plurality of water intake risers projecting downwardly into the water and being connected together to form a bundle by a plurality of spacers. The spacers have openings, formed as guiding sleeves, through which respective ones of the water intake riser are disposed. The guiding sleeves guide the respective water intake risers during building and lowering. During use, the spacers keep the risers together to form a bundle but prevent the risers from colliding. The spacers may be positioned on predetermined positions along the length direction of the water intake riser assembly.

One riser may be provided which serves as structural support for the spacers. This riser, which may or may not be arranged to take in water, is referred to as the structural riser. The structural riser is typically positioned in the middle of the bundle. The bundle may for instance comprise eight water intake risers and one structural riser, arranged in a three by three matrix with the structural riser

positioned in the middle. Alternatively, a plurality of water intake risers may be arranged in a circular pattern, the structural riser being positioned in the middle.

According to an embodiment the respective anti-fouling supply lines run through the structural riser.

The structural riser may comprise one or more anti- fouling supply lines for the respective injection systems of the respective water intake risers. The supply lines are at the proximal end connectable to the anti-fouling supply unit on the off-shore structure and at the distal end connected or connectable to the respective injection systems. The water intake riser assembly may be suspended from the off-shore structure in any suitable manner. Reference is made to US Patent 7,318,387 which describes a particularly suitable riser hanger construction involving a flexible load transfer element, such as a chain, which is surrounded by a hose to convey the water. A similar construction may be used for suspending the structural riser. The anti-fouling supply lines may be provided inside the hose.

The distal end of the structural riser may comprise a bottom interface, formed as a bottom plate with one or more openings through which the anti-fouling supply lines are connected or connectable to the respective injection systems. The connection may be established by means of (flexible) conduits. The (flexible) conduits may be connected using divers or preferably by means of a remotely operated

underwater vehicle (ROV) .

According to an embodiment the respective anti-fouling supply lines running through the structural riser and the respective injection systems are in fluid communication by (flexible) conduits, such as flying leads.

The conduits, which may be flexible conduits are arranged to transport anti-fouling agent from the anti-fouling supply lines to the injection systems. The (flexible) conduits are with one end connected to the respective anti-fouling supply line and with the other end to the respective injection inlet .

According to an embodiment the water intake riser assembly comprises a first water intake riser and a second water intake riser generally stretching side by side along the length direction, wherein the water intake section of the first water intake riser is positioned a first distance Di away in the length direction from the proximal end of the first water intake riser and wherein the water intake section of the second water intake riser is positioned a second distance D₂ away in the length direction from the proximal end of the second water intake riser, wherein D i > D₂ .

The distal ends of the water intake risers are positioned at different depths. The water intake risers are thus provided in a staggered configuration. In case more than two water intake risers are provided, a first sub-set may have water intake sections at the first distance and the second sub-set may have water intake sections at the second

distance .

The first distance is thus greater than the second distance. If the length of the water intake section is L_Wi_S , D i is preferably at least equal to the second distance plus the length of the water intake section: D i ≥ D₂ +L_Wi_S , such that the water intake sections do not overlap in the length direction .

The water intake risers in the water intake riser assembly serve to convey water taken in via the water intake section at the distal portion to the proximal portion. By providing a bundle of at least a first water intake riser and a second water intake riser, the first water intake riser being longer than the second water intake riser such that the water intake sections are at different, preferably non- overlapping, heights, the risk of full interruption of water conveyed to the proximal portion due to clogging of the water intake section is reduced.

Firstly, by providing at least two water intake risers it is achieved that water supply is still possible if one of the two water intake risers is blocked from taking in water.

Secondly, by operating the water intake riser assembly with the water intake sections at different heights, the risk of both of the two water intake sections being blocked at the same time (for instance by a single cause) is reduced. Moreover, by staggering the water intake sections in the way described, the inflow in each water-intake section of each tubular conduit behaves much more independently since the intake of the neighbouring riser (at the same water depth) is further away.

Clearly the water intake riser assembly may be based on a bundle of more than two water intake risers, for instance eight or nine water intake risers arranged in a rectangular cross sectional pattern at least having one water intake riser at each of the four corners and one water intake riser between sets of two of the corners. Alternatively, the water intake risers may be arranged in a concentric and/or circular pattern. By increasing the number of water intake risers, the operational risk of blockage may be further reduced. The water intake risers may have water intake sections at two staggered heights.

According to an embodiment the water intake riser assembly further comprises a structural riser, wherein the distal end of the structural riser is a distance D_structurai away in the length direction from the proximal end of the structural riser and the distal end of the longest water intake riser is a second distance D_{longest wir} away in the length direction from the proximal end of the water intake riser, wherein D_structurai > Di_ongest „_ir.

The distal end of the structural riser is thus at the same depth or below the distal end of the longest water intake riser when suspended from the floating structure.

This makes connecting the injection systems provided on the respective water intake risers with the anti-fouling supply lines running through the structural riser easier.

This connection is typically made when the water intake riser assembly is in its operating position, i.e. suspended in a vertical and at least largely submerged position. The distal end of the structural riser is now easily accessible by divers or ROV s . This makes connecting the (flexible) conduits, e.g. flying leads, to fluidly connect the anti- fouling supply lines running through the structural water and the respective injection systems provided on the respective water intake risers relatively easy.

According to an embodiment the injection system of the first water intake riser is connected to the anti-fouling supply line by means of a (flexible) conduit and the

injection system of the second water intake riser is

connected to the anti-fouling supply line via the injection system of the first water intake riser.

By connecting the injection system of the second, shorter, water intake riser, via the injection system of the first, longer, water intake riser a simpler and more reliable system is achieved. The need of relatively long (flexible) conduits (covering the distance from the structural riser to the shorter second water intake riser) is omitted.

A further (flexible) conduit, e.g. flying lead, may be provided to fluidly connect the injection system of the first and second water intake risers.

According to an aspect there is provided an off-shore structure from which a water riser assembly according to any one of the preceding claims is suspended. The off-shore structure may be a floating structure, such as a floating LNG plant. As stated above, the LNG plant may be a plant for liquefying a vaporous hydrocarbon containing feed stream and/or for gasifying a liquefied hydrocarbon stream.

According to an aspect there is provided a method of setting up a water intake riser assembly, wherein the water intake riser assembly comprises at least one water intake riser stretching along a length direction comprising an injection system for injecting an anti-fouling agent into the water intake riser and wherein the water intake riser assembly further comprises a structural riser with at least one anti-fouling supply line running through the structural riser, wherein the method comprises:

- suspending the water intake riser assembly from an offshore structure,

- providing a conduit, e.g. flexible conduits, to provide a fluid connection between one of the anti-fouling supply lines and the injection systems.

The water intake riser assembly may be as described above and may comprise a plurality of water intake risers with associated water intake sections and injection systems. The method may comprise providing a plurality of (flexible) conduits to provide fluid connections between each injection system and a respective anti-fouling supply line.

According to an embodiment the water intake riser assembly comprises a first water intake riser with a first injection system and a second water intake riser with a second injection system, wherein the first and second water intake risers are generally stretching side by side along the length direction, wherein the method further comprises

- providing a further conduit, e.g. a flexible conduit to provide a fluid connection between the second injection system and the first injection system.

As described above, the first water intake riser may be longer than the second water intake riser, the first water intake section being at a greater depth than the second water intake section.

According to an aspect there is provided a method of producing a liquefied hydrocarbon stream, comprising:

- feeding a vaporous hydrocarbon containing feed stream to an off-shore structure; - forming a liquefied hydrocarbon stream from at least a part of the vaporous hydrocarbon containing feed stream comprising at least extracting heat from at least said part of the vaporous hydrocarbon containing feed stream;

- supplying water to the off-shore structure via a water intake riser assembly comprising an injection system for injecting an anti-fouling agent into the water intake riser;

- adding at least part of the heat removed from said at least a part of the hydrocarbon containing feed stream to at least part of the water supplied via the water intake riser assembly;

- subsequently disposing of the at least part of the water.

The water intake riser assembly may be as defined above. The method may comprise supplying an anti-fouling agent to the injection system.

According to an aspect there is provided a method of producing a vaporous hydrocarbon stream, comprising:

- providing a liquefied hydrocarbon stream on an off-shore structure ;

- forming a vaporous hydrocarbon stream from at least a part of the liquefied hydrocarbon stream comprising adding heat to the said part of the liquefied hydrocarbon stream;

- drawing at least part of the heat for adding to the said part of the liquefied hydrocarbon stream from at least part of the water supplied via the water intake riser assembly;

- subsequently disposing of the at least part of the water.

The water intake riser assembly may be as defined above.

The method may comprise supplying an anti-fouling agent, to the injection system. A variety of suitable installations and line ups are available in the art for regasification or vaporisation of previously liquefied hydrocarbons streams and adding heat to such a liquefied hydrocarbon stream, and need not be further explained herein.

SHORT DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate

corresponding parts, and in which:

Figures la-lc schematically show a floating liquefied natural gas plant provided with a water intake riser

assembly;

Figures 2a-2c schematically show partial views of the water intake riser assembly according to different

embodiments .

DETAILED DESCRIPTION

For the purpose of this description same reference numbers refer to similar components. For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line.

The person skilled in the art will readily understand that, while the invention is illustrated making reference to one or more a specific combinations of features and measures, many of those features and measures are functionally

independent from other features and measures such that they can be equally or similarly applied independently in other embodiments or combinations.

Figures la - lc illustrate an example of a marine system 100 in which embodiments of the present invention may be implemented. The marine system 100 in this example includes an off-shore structure 102 on/in a surface of the ocean 104, here represented in the form of a floating structure. The off-shore structure 102 may comprise a Floating Liquefied Natural Gas (FLNG) plant as one example. The FLNG plant may cool and liquefy natural gas, or alternatively heat and vaporize LNG . The hardware needed for cooling and/or heating are not shown and will not be discussed in more detail here.

A water intake riser assembly 105 is suspended from the off-shore structure 102. The water intake riser assembly 105 may be used to bring water from the ocean to the off-shore structure 102. The water intake riser assembly 105 as shown in Fig. la comprises a bundle 106 of water risers 106A, 106B. All but one of the risers may be water intake risers 106A and one may be a structural riser 106B. The structural riser 106B carries the spacers 110 through which the water intake risers 106A are guided to prevent the risers 106A, 106B from colliding, while keeping the risers 106A, 106B bundled.

The water intake risers 106A take in cold water at depth, and convey the cold water upward to the off-shore structure 102. The cold water may be input to heat exchangers (not shown) to add or remove heat to/from a process performed on the off-shore structure 102. Heated or cooled ocean water from the outlet of the heat exchangers may be discharged back into the ocean at the surface, or alternatively conveyed back to depth with a discharge system (not shown) .

The risers 106A, 106B generally stretch side by side along a length direction. Seen in the length direction, each of the risers have a proximal portion, followed by a

connecting portion, followed by a distal portion. The distal portions of the tubular conduits together, when fully suspended, form the distal part of the water intake riser assembly. The lowest part of the distal portion is referred to as the distal end. The part of the proximal portion connected to the floating structure 102 is referred to as the proximal end. Preferably, the distal end of the water intake riser assembly hangs free from the ocean floor 103.

The proximal portion comprises suspension means by which the tubular conduit is suspended from the off shore structure 102. Due to the ocean current, the risers 106A, 106B may deflect from vertical, up to around 20 degrees or so (not shown) . To accommodate for such deflection, the risers 106A, 106B may be suspended from the off-shore structure through a swivel joint, a ball joint, a riser hanger, or other

pivotable or hingeable coupling. Particular reference is made to US Patent 7,318,387 which describes a particularly suitable riser hanger construction involving a flexible load transfer element and a hose to convey the water.

The distal portions of the water intake risers 106A comprise a water intake section 111. The water intake section 111 may be formed as a perforated sleeve or filter and is shown and discussed in more detail in Fig.'s 2a - 2c further below .

As shown in Fig.'s la - lc, the water-intake sections 111 of the different water intake risers 106A may be positioned in a staggered orientation. This can best be seen in Fig. lb, showing the structural riser 106B, surrounded by water intake risers 106A. In total eight water intake risers 106A are present. The risers 106A, 106B are arranged in a three by three matrix with the structural riser 106B positioned in the middle and eight water intake risers 106A arranged in a square around the structural riser 106B, of which five are visible in Fig. lb.

Fig. la shows an anti-fouling supply unit positioned on the off-shore structure 102 comprising an anti-fouling storage or vessel 201 and a pump 202. The pump 202 is in fluid connection with the anti-fouling storage or vessel 201 and is arranged to pump the anti-fouling agent to the respective injection systems via associated anti-fouling supply lines 203 and (flexible) conduits 204. The anti- fouling supply lines 203 extend in the length direction L along the water intake riser assembly 105, preferably through the structural riser 106B. At the distal end of the

structural riser 106B the anti-fouling supply lines 203 are connected to the injection system via (flexible) conduits 204.

Fig. 2a shows an exploded view of a water intake section 111 of a water intake riser 106A. The water intake section 111 comprises a (tubular) side wall circumferencing around the length axis stretching in the length direction of the water intake riser 106A. Herewith a flow passage is defined in the length direction L. Water intake openings 112 are provided as a plurality of through holes through the side wall. The water intake openings are distributed along the water intake section 11, in particular along the length and the circumference of the water intake sections 111.

Each through hole defines a transverse access port into the flow passage and during operation allows a transversely directed flow of cold water from the ocean into the flow passage .

The perforated wall of the water intake section 11 may be made of carbon steel with a steel grade of X70 or equivalent thereto.

Furthermore, the distal portion of the water intake riser 106 may comprise a shoe piece 113 at the distal end to provide a rounded tip, which facilitates guiding the water intake riser through the spacers when lowering the water intake riser into position.

In the example shown in the figures, the distal portion of some, e.g. four of the eight, water intake risers 106A extend further in the length direction L, than the remaining water intake risers 106A.

As shown in Fig. 2a, the injection system comprises an injection inlet 206 to receive anti-fouling agent and a plurality of injection outlets 205 to inject anti-fouling agent into the water intake riser 106A. the injection inlet

206 is in fluid communication with a (flexible) conduit 204.

In between the injection inlet and outlets a distributor

207 or the like is provided. In the example shown, the distributor comprises a ring-shaped nozzle, with the

injection outlets 205 positioned on the inner perimeter of the ring-shaped nozzle.

The embodiment shown in Fig. 2a shows an injection system with a distributor which comprises two ring-shaped nozzles 207 positioned at different heights along the water intake riser 106A. The two ring-shaped nozzles 207 are fluidly connected by a conduit 208, which also may comprise injection outlets 205.

The embodiment shown in Fig. 2b only comprises a single ring-shaped nozzle 207 positioned along the upper edge of the water intake section.

As shown in Fig. lb, the water intake risers 106A may be provided in a staggered configuration with water intake sections at different depths. A first and second water intake riser 106A are shown, the first water intake risers 106A having its water intake section at a lower position than the water intake section of the second water intake riser 106A. The distal end of the structural riser 106B is shown at the same depth as the longest water intake riser 106A, but it may also be located at a deeper position. As shown, the injection system of the first water intake riser 106A is connected to the anti-fouling supply line 118 running through the

structural riser 106B with a (flexible) conduit 204 and the injection system of the second water intake riser is

connected to the anti-fouling supply line 118 via a

(flexible) conduit 204 connecting the injection system of the second water intake riser 106A to the injection system of the first water intake riser 106A.

The water intake riser assembly is set up by first suspending the water intake assembly from the off-shore structure 102 and subsequently connecting the anti-fouling supply lines 118 to the anti-fouling vessel 201 and to the respective injection systems by (flexible) conduits 204.

In use, water may be pumped to the floating structure 102 via the respective water intake risers 106A while anti- fouling agent is transported from the anti-fouling vessel 201 via the anti-fouling supply line 118 and one or more

(flexible) conduits 204 to the injection system, where the anti-fouling agent flows into the water intake riser 106A and is conveyed to the floating structure 102 by the water flow through the water intake riser 106A.

The water intake riser assembly as described above may be used to supply process water to any process carried out on the off-shore structure.

The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims.

Claims

C L A I M S

1. A water intake riser assembly (105) that is

suspendable from an off-shore structure (102), comprising at least one water intake riser (106A-B) stretching along a length direction, the at least one water intake riser (106A- B) comprising, seen in the length direction, a proximal end comprising suspension means and a distal end comprising a water-intake section (111), the water intake section (111) being in fluid communication with the proximal end, wherein the at least one water intake riser (106A-B) comprises an injection system for injecting an anti-fouling agent into the water intake riser (106A-B) .

2. Water intake riser assembly (105) according to claim

1, wherein the injection system has one or more injection outlets (205) inside the water intake riser (106A-B) .

3. Water intake riser assembly (105) according to claim

2, wherein the injection system comprises a ring-shaped nozzle, wherein the injection outlets (205) are positioned on an inner perimeter of the ring-shaped nozzle.

4. Water intake riser assembly (105) according to any one of the preceding claims, wherein the injection system has one or more injection outlets (205) positioned at a distal portion of the water intake riser.

5. Water intake riser assembly (105) according to any one of the preceding claims, wherein the injection system has one or more injection outlets (205) positioned along the upper edge of the water intake section (111) or downstream of the water-intake section (111) .

6. Water intake riser assembly (105) according to any one of the preceding claims, wherein the injection system is fluidly connected to an anti-fouling supply unit (201, 202) .

7. Water intake riser assembly (105) according to anyone of the preceding claims, wherein the at least one injection system is in fluid connection with an associated anti-fouling supply line (118) wherein the respective anti-fouling supply lines (118) extend in the length direction along the water intake riser assembly (105) .

8. Water intake riser assembly (105) according to claim

7, wherein the respective anti-fouling supply lines (118) run through a structural riser (106B) .

9. Water intake riser assembly (105) according to claim

8, wherein the respective anti-fouling supply lines (118) running through the structural riser (106B) and the

respective injection systems are in fluid communication by conduits (204), such as flying leads.

10. Water intake riser assembly (105) according to any one of the preceding claims, wherein the water intake riser assembly (105) comprises a first water intake riser (106A) and a second water intake riser (106A) generally stretching side by side along the length direction, wherein the water intake section (111) of the first water intake riser (106A) is positioned a first distance D i away in the length

direction from the proximal end of the first water intake riser (106A) and wherein the water intake section (111) of the second water intake riser (106A) is positioned a second distance D₂ away in the length direction from the proximal end of the second water intake riser (106A), wherein Di > D₂.

11. Water intake riser assembly (105) according to any one of the preceding claims, wherein the water intake riser assembly (105) further comprises a structural riser (106B) , wherein the distal end of the structural riser is a distance D structurai away in the length direction from the proximal end of the structural riser (106B) and the distal end of the longest water intake riser (106A) is a second distance D i_{o nge s}t _Wi_r away in the length direction from the proximal end of the water intake riser (106A) , wherein D _{st ruc}turai > D iongest wn -

12. Water intake riser assembly (105) according to any one of claims 10 and 11, wherein the injection system of the first water intake riser (106A) is connected to the anti- fouling supply line (118) by means of a conduit (204) and the injection system of the second water intake riser (106A) is connected to the anti-fouling supply line (118) via the injection system of the first water intake riser (106A) .

13. An off-shore structure from which a water riser assembly (105) according to any one of the preceding claims is suspended.

14. Method of setting up a water intake riser assembly (105), wherein the water intake riser assembly (105)

comprises at least one water intake riser (106A) stretching along a length direction comprising an injection system for injecting an anti-fouling agent into the water intake riser (106A) and wherein the water intake riser assembly (105) further comprises a structural riser (106B) with at least one anti-fouling supply line running through the structural riser (106B), wherein the method comprises:

- suspending the water intake riser assembly (105) from an off-shore structure (102),

- providing a conduit (204) to provide a fluid connection between one of the anti-fouling supply lines (118) and the injection systems.

15. Method according to claim 14, wherein the water intake riser assembly (105) comprises a first water intake riser (106A) with a first injection system and a second water intake riser (106A) with a second injection system, wherein the first and second water intake risers (106A) are generally stretching side by side along the length direction, wherein the method further comprises

- providing a conduit (204) to provide a fluid connection between the second injection system and the first injection system.

16. Method of producing a liquefied hydrocarbon stream, comprising :

- feeding a vaporous hydrocarbon containing feed stream to an off-shore structure (102) ;

- forming a liquefied hydrocarbon stream from at least a part of the vaporous hydrocarbon containing feed stream comprising at least extracting heat from at least said part of the vaporous hydrocarbon containing feed stream;

- supplying water to the off-shore structure (102) via a water intake riser assembly (105) comprising an injection system for injecting an anti-fouling agent into the water intake riser (106A) ;

- adding at least part of the heat removed from said at least a part of the hydrocarbon containing feed stream to at least part of the water supplied via the water intake riser assembly (105 ) ;

- subsequently disposing of the at least part of the water.

17. Method of producing a vaporous hydrocarbon stream, comprising :

- providing a liquefied hydrocarbon stream on an off-shore structure (102) ;

- drawing at least part of the heat for adding to the said part of the liquefied hydrocarbon stream from at least part of the water supplied via the water intake riser assembly (105) ;

- subsequently disposing of the at least part of the water.