84672-EH Riser System Field of the invention The present invention regards freely hanging riser systems for taking up large amounts of cold seawater from large depth to an intake room or similar in a vessel that lays in a fixed position. The invention is in particular relevant where a huge demand for cooling is present, such as with LNG-production from natural gas on a vessel of the type FPSO (Floating Production Storage and Offloading). More specific the invention regards freely hanging riser systems for taking up large amounts of cold seawater from large depth to an intake room on board a vessel that by spread anchoring or dynamical positioning lays fixed positioned and in principle without rotation on the sea surface.

Prior art Several arrangements are known for taking up cold seawater from large depth.

Some examples on such and other relevant art, are as follows: In patent publication NO 172681 an arrangement is described that prevents rotation of a flexible pipe, illustrating some fundamental problems with respect to riser systems.

In patent publication JP 57008369 an arrangement is described for reducing movements and thereby strain while taking in a seawater pipe, by use of a special suspension arranged outside the hull of the vessel.

In patent publication US 4350014 a platform is described for making use of the thermal energy present in the sea, by use of Canot's principle for transforming thermal energy of the sea to useable kinetic energy. The invention with respect to said publication regards the platform per se and a support structure in the platform. But it is also described that cold seawater is taken up with a pipe that extends into the sea. The pipe to take up seawater to the platform, which in practice is a semi-submersible structure, is suspended in cables in a support structure in the semi-submerged platform. The platform is placed where the water depth is more than 600 m, preferably in tropical areas where the temperature difference between the surface water and deep water reaches at least 20 °C (column 6, lines 39-42). The pipe to take up the cold seawater is not a specific topic, however, it is described that the pipe is flexible and preferably fabricated from reinforced synthetic material, comprising cylindrical components whose actual length is substantially equal to the radius, stiffened by hoops which are preferably fabricated from glass fiber reinforced plastic material and connected to one another by woven lanyards of a nylon-type synthetic material. (Column 7, lines 40-47). Above the pipe for intake of cold seawater are arranged pumps and other equipment which are used in connection

with the power production on the platform. It is not known that a platform according to the above publication has been fabricated.

In patent publication GB 2324120A a transfer vessel is described for collecting cold water from the depth of the sea, in connection with transformation of thermal energy in a natural source of seawater to useable energy. On page 1, second paragraph of said patent publication, it is described that the surface temperature of oceans in tropical regions is typically about 25 °C, but at greater depths, e. g. 1000 meters below the surface, the temperature is typically 5 °C. In the next paragraph it is described that it is necessary to supply large volumes of cold water from the ocean depth to the condenser to ensure effective operation of the cycle (for energy production by transforming thermal energy from the ocean). It is described that the proposed systems use a very large cold water pipe, typically about 30 m diameter and 1000 m long, suspended in a surface vessel. The engineering demands to provide a cold water pipe of such a size and with structural integrity to withstand not only its own weight but also the mechanical loading from ocean currents and surface waves are formidable. As far as it is known it has never been attempted to fabricate a pipe for taking up cold seawater having dimensions near the above mentioned. The objective of GB 2324120 is to avoid the disadvantages with such a pipe, which disadvantage in practice is that such a pipe so far neither can be fabricated, installed or operated according to the aim.

In the published international patent application PCT/NO00/00447 a system is described for providing cooling water to a cooling system on a floating vessel for production of hydrocarbons. On page 1, line 34, to page 2, line 6, it is described that a LNG-plant on board a FPSO-vessel can require about 30000 m 3/h cooling water.

However, for most FPSO-vessels it is used pumps taking up seawater from a seawater intake that is hanging freely as flexible pipes or similar to a depth of maximum 40 m. It is described that the length of the seawater intake pipes is limited to avoid collisions with the anchor lines. Further the advantage of taking up seawater from large depths is known (page 2, lines 7-11). With the invention according to said publication it is provided seawater intake pipes without limited length to avoid collisions with the anchor lines, at the same time as cold seawater can be taken up from large depths. This is achieved by having the cooling water pipes located within the anchoring system and geostationary positioned with respect to the seabed, whereby they will not be in conflict with the anchoring system and the production risers when the vessel is weathervaning. The system for supplying cooling water to the production processes on the vessel includes one or more seawater risers which are shown to extend between a turning unit and the seabed, and which are connected at a lower end to an anchoring means to the seabed, for instance a seawater lifting pump (page 4, lines 14-17). A seawater pump is also shown to be arranged on the buoyancy unit (page 4, lines 21-22). Further it is apparent that the seawater risers generally may consist of one large or several smaller risers extending

down to the seabed or to a chosen depth at which the seawater temperature is sufficiently low (page 4, lines 24-26). Further it is described that the seawater pipes between the buoyancy unit and the seabed may have the same course as the production risers or they may in general extend vertically from the buoyancy unit to the seabed. In both cases they will be kept in position at the seabed by anchoring means (page 4, lines 26-29). There is no focus on the number of risers for seawater intake, and on the drawings only one embodiment with several risers for seawater intake is illustrated. The invention according to PCT/NO00/00447 in reality regards provisions with respect to the turning unit in the vessel, and there is no guidance with respect to particular choices regarding number and dimensions of the seawater risers in particular or the seawater risers in general. There is no guidance with respect to specific methods to arrange further risers, for example production risers, in particular ways with respect to the seawater risers. The above mentioned pumps appear to be obligatory in the system for seawater intake. It is mentioned that a process plant on a FPSO vessel can require intake of a cooling water amount up to 30000 m3/h, which will require use of a seawater riser with a flow area corresponding to a pipe having a diameter up to about 2000 mm.

Now it has surprisingly been found possible to design riser systems for taking up large amounts of cold seawater from large depths, which riser system by specific constructive arrangements provide unexpected technical and economical advantages of far more significance than found indicated in the description of the prior art.

Summary of the invention With the present invention a riser system is provided for taking up large amounts of cold seawater from large dept to an intake room in a vessel than in principle lays in a fixed position on the sea surface by spread anchoring or dynamical positioning, which vessel in principle is without rotation with respect to the sea surface. The riser system is distinguished in that it is comprising one single riser with large flow cross section, which riser in operation extends from a lower freely hanging end at large depth from where large amounts of cold seawater are taken in through at least one opening and is passed through the riser to an upper end that is placed within said intake room, in a suspension that is elastic around a nominal suspension position.

With in principle fixed position is meant that the vessel has a drift of maximum 7 % of the ocean depth from the nominal position on the sea surface. The drift will typically be far less.

With in principle without rotation it is meant a maximum rotation as given by the stiffness of the anchoring or stiffness of additional connected equipment that can extend down to the seabed, more particular 270° to 360°, dependent on the embodiment of the riser system.

With large depth it is meant several hundred meters depth, preferably at least 600 m, typically 1000 m depth, in principle to a depth where the water temperature is stable and low in all conditions and seasons. In cold waters sufficient depth may accordingly be shallower than in warm waters.

The riser system preferably comprises additional risers, pipes, cables and/or buoyancy material which in principle extend parallel with the riser system and are fastened to the riser, such that the riser with connected equipment acts as one single unit.

The riser system preferably comprises one single round pipe having a length of about 1000 m and a diameter of 1.5 m to 5 m, whereby the riser can take up a flow rate of 11000 to more than 50000 m3/h cold seawater from about 1000 m depth. The riser system more preferably comprises a riser having a diameter 2.5-3. 2 m, whereby the riser can take up a flow rate of 30000 to 50000 m3/h cold seawater from large depth.

The riser has in its lower end preferably a clump weight that keeps the riser in substance vertically oriented in the ocean. The clump weight has preferably several openings for intake of cold seawater to the riser, such that the clump weight also functions as a sieve.

The suspension of the riser in the intake room is preferably with a short flexible element between the riser and the intake room. The suspension in the intake room preferably comprises rotationally symmetrical arranged elastic rubber-steel lamellae, more specific flex joints, for example of the type manufactured by Murdoch Oil States Company. The suspension in the intake room is preferably suspended in a system for heave compensation, for example with a constant tension winch.

The riser is preferably fabricated from one single or several joined sections of corrosion protected steel pipes with external buoyancy material, or sections of thermo polymer or other polymer reinforced with steel armouring and/or synthetic fiber armouring, for example kevlar, aramide fiber, boron fiber, carbon fiber or glass fiber, which sections of polymer pipe optionally are equipped with buoyancy material and optional fixation frames for fixing a number of external symmetrically arranged risers for natural gas, with optionally additional anchoring of the riser to the vessel from close to its upper end.

The riser system as water-filled preferably has buoyancy so that the clump weight and the lower end have negative buoyancy while the upper end has positive buoyancy, so that the riser naturally will take an in substance vertical position in the sea.

The riser is preferable fabricated from curved, long joined plates, which riser around the cross-section comprises two wide convex plates joined in one (side) end, and a smaller concave plate placed between and joined to the widest plates in their other (side) end, with a number of additional risers, cables, buoyancy material and fixation elements in substance arranged within the concave part of the smaller plate, so that the cross-section becomes torpedo like or drop formed.

The riser consists in principle of a load-bearing part and a part for cooling water transport, which parts are integrated or separate.

The riser system according to the invention is preferably connected to a FPSO- vessel with a LNG-plant, in that the riser system supplies cooling water to the LNG-plant, and wherein the riser system further comprises one or more risers for taking in natural gas to the LNG-plant, which risers for natural gas are arranged external to the riser for cooling water intake.

With the riser system according to the present invention it can, dependent on the embodiment, be brought up from 11000 to more than 50000 m3/h cold seawater for cooling in connection with the LNG-plant on a FPSO-vessel. For a typical LNG-plant with a production of 5 mill tons LNG per year and with a cooling water intake on typically 1000 m depth versus 50 m depth, it is achieved in a typically chosen warm water about 30 % less cooling water demand. Thereby the power consumption is reduced with about 50 MW in a plant for LNG-production. This means that two gas turbines will not be required, which each costs about 250-300 millions Norwegian kroner (for example LM 6000 Nouvo Preone). Normally will around 5 % of the energy content of the gas taken up (the feed to the LNG-plant) be used to cool down the remaining gas to LNG- conditions. With the present invention about 3.8 % of the feed will be used for cooling to LNG-conditions. This results not only in reduced CO2-emissions but also very significant reductions with respect to weight and equipment on the vessel, with formidable technical and economical effects. It is considered a saving of 15 % of the LNG-process equipment placed dry on a FPSO having a length of about 400 m and width of about 70 m, with a complete LNG-plant installed.

With the riser system according to the present invention it is achieved significant advantages with respect to installation, reduced risk for collisions and easier manipulation of the components. The riser system can in principle be handled as one unit.

Further, the flow loss through the riser system will be reduced, in particular for embodiments with round cross section of the riser. Thereby it is achieved a so called "draw down effect"of only about 7-8 m pressure height, which has to be exceeded for cold seawater to flow by itself into the intake room from large depth. Said pressure height corresponds to loss of pressure measured as water column because of effects of friction, temperature, pressure and salinity. The water surface in the intake room accordingly has to be at least 7-8 m lower than the water surface of the sea to avoid use of pumps in the riser system, however, in practice the water surface will be kept lower, more specific at a level that ensures sufficient pressure height for the lifting pumps that are to be placed in the intake room to bring the cold cooling water further. It is obligatory that the riser system does not contain any pump, which is made possible because of the lowered water surface in the intake room.

Further, the demand of armouring per unit flow rate will be reduced.

Likewise a lower weight per unit flow rate will be achieved.

Drawings On Figure 1 the riser system according to the present invention is illustrated.

On Figures 2,3 and 4 cross-sections of the suspension of the riser system, for three different embodiments according to the invention, are shown.

On Figure 5 the riser system according to the present invention is shown, in an embodiment having additional risers for hydrocarbons, cables and pipes.

Detailed description On Figure 1 it is shown a freely hanging riser system suspended in an intake room in a FPSO with a section of a flexible pipe. With the term intake room it is meant primarily non-turnable sea-chests or similar, or subsidiary turnable intake rooms such as a turret, which intake room can take in the riser to a room with lowered water surface for the cold seawater, which room has space for equipment to bring the cold seawater further.

From its upper end the riser extends to a lower end 1000 m down into the sea, where there is a number of inlets (not illustrated) for cold sea water. The inner diameter of the riser is larger than or equal to 1.5 m. A short flexible element, having larger flexibility than the riser, is arranged between the riser and the intake room.

The Figures 2,3 and 4 show cross-sections of the suspension of the riser system.

Figure 2 illustrates suspension of the riser with flexible elements rotationally symmetrical arranged to a suspension collar with an integrated bell mouth formed bend restrictor below. Figure 3 illustrates an embodiment with additional axial and rotational flexibility in that the suspension collar is fastened in a double elastic suspension. Figure 4 illustrates an embodiment with additional flexibility in that the suspension collar in addition is fastened in a system for heave-compensation. Increased elasticity in the suspension results in an advantageous reduction of the strain on the riser and the suspension.

Figure 5 illustrates the riser system according to the invention with additional risers, more particular for hydrocarbons, and cables and pipes, arranged such that the riser with connected equipment acts as one single unit. Optional buoyancy bodies are not illustrated.

The fabrication of the riser preferably takes place at a workshop with harbour, so that the whole riser or sections thereof can be filled with air and be towed to a FPSO that can lie anchored or without anchoring in an appropriate position. The installation can take place by use of a dead weight to sink down the lower end of the riser as a pendulum.

External equipment, such as fixation frames, risers for natural gas and buoyancy material are preferably prepared and connected in advance, as far as possible. Alternatively

handleable sections of the riser can be joined in position, be fully equipped and submerged successively down through the intake room, with lifting and handling equipment on the vessel and optionally on a crane vessel. It can be arranged rail means or similar on the outside of the hull of the vessel to manoeuvre in and pull in the riser to the intake room.

The lowest flow resistance for the cold seawater is achieved with a round cross- section of the riser, because of lowest ratio pipe surface/cross section for flow. With regard to fabrication and impact of ocean current it can be preferable to make use of another cross section than round for the riser. Thereby the riser can be prepared from curved plates that are joined, and the riser as fully equipped can be given a torpedo like or drop like cross section, with simplified fabrication and reduced drift as a result.