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

Some examples and other relevant art, are as follows: In patent publication NO 172681 an arrangement is described that prevents rotation of a flexible pipe, and some fundamental problems with respect to riser systems are discussed. On Figure 3 a riser 6 is illustrated, which by use of a base 7 is anchored to the seabed 5. The riser 6 can be used to transfer the production from one or more oil wells 8.

In patent publication JP 57008369 an arrangement is described to reduce movements and thereby strain while taking in a subsea line, by use of a special arrangement outside the hull of the vessel. In patent application JP 09310651 a seawater intake is described having a fixed installation, with which seawater can be brought up from large depth. Since the arrangement is an installation fixed to the seabed, without a vessel, the described art is less preferable with respect to seawater intake from depths from several hundred meters and deeper.

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 m3/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 depth to a turret or similar in a vessel that in principle lays in a fixed position on the sea surface by spread anchoring from the turret or by dynamical positioning, which vessel can rotate freely around the turret while the turret in principle is without rotation with respect to the sea surface. The riser system is distinguished in that it comprises : a single riser having a large flow cross-section, which riser in operation extends from a lower bottom anchored 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 a little below or in the sea surface, a buoyant body arranged in the upper end of the riser, which buoyant body holds the riser in upright position, one or more flexible pipes arranged from the upper end of the riser to the turret, to pass the cold seawater from the riser to the vessel, with fluid communication through or by-passing the buoyant body.

With in principle fixed position it 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 a turret in principle without rotation it is meant a maximum rotation which is given of the stiffness of the anchoring system and the riser system, more specific 270° to 360°, dependent on the design 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.

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

Preferably the riser system comprises one single round pipe having a length of about 1000 m and diameter of from 1.5 m to 5 m, whereby the riser can take up a flow rate from 11000 to over 50000 m3/h cold seawater from about 1000 m depth. More preferable the riser system comprises a riser having 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 is preferably built from joined pipe sections of reinforced polymer material having external frames for fixation of a number of external symmetrically arranged risers for natural gas, with a buoyancy body in the upper end of the riser and four flexible pipes of 42"from the buoyancy body to the vessel, with bottom anchoring of the riser from its lower end, upper end and optionally intermediate positions.

Preferably the riser is constructed of one single or several joined sections of corrosion protected steel pipes with external buoyancy material, or polymer pipes reinforced with steel armor and/or synthetic fiber armor, for example kevlar, aramide fiber, boron fiber, carbon fiber or glass fiber, which sections of polymer pipes optionally are provided with buoyancy material, so that each section with connected further risers and equipment is having neutral or a slight negative buoyancy.

Preferably the riser system has a slight negative buoyancy so that the weight of the bottom anchoring and the lower part ensures the negative buoyancy and positioning

with respect to the seabed, while the remaining parts of the riser system have positive buoyancy so that by means of own buoyancy it takes an in substance upright position, which upright position is secured by anchoring from the upper end of the riser and optionally from positions along the riser.

The riser is preferably manufactured from curved, long, joined plates, which riser around the cross-section comprises two wide convex plates joined in one (side) end, and one narrower concave plate, placed in between and joined to the wider 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 form of the cross-section will become torpedo like or like a drop.

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

The riser system is preferably connected to a FPSO-vessel having a LNG-plant, wherein the riser system supply cooling water to the LNG-plant, and wherein the riser system further comprises one or more risers for taking up natural gas to the LNG-plant, which risers for natural gas are arranged external to the riser for taking up cooling water.

Another embodiment of the present invention comprises a riser system of the above-mentioned type, distinguished in that it comprises one single riser having a large flow cross-section, which riser in operation extends from a lower bottom anchored end at large depth wherefrom large amounts of cold seawater are taken up through at least one opening and is passed through the riser to an upper end that is located within the turret, in a suspension that is elastic around a nominal suspension position, and optionally one or more further risers, pipes, cables or buoyancy material arranged in substance parallel with and fastened to said riser, so that the riser together with connected equipment acts as one unit.

The riser system having the upper end of the riser placed directly into the turret preferably comprises one or more of the above mentioned features regarding the riser.

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 a LNG-plant on a FPSO-vessel. For a typical LNG-plant with production of 5 mill tons LNG per year and with cooling water intake on typical 1000 m depth versus 50 m depth, it is achieved in typically chosen warm waters, 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 that is taken up (the feed to the LNG-plant) be used to cool 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 C02-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 with length of about 400 m and width of about 70 m, with a complete LNG-plant installed.

With the riser systems 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 systems can in principle be handled as one single 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 turret 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 turret 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 of the turret will be kept lower, more specific at a level that ensures sufficient pressure height for lifting pumps that is to be placed in the turret to bring the cold seawater further. It is obligatory that the riser system does not contain any pump, which is possible because of the lowered water surface in the turret.

Further, the amount of armoring per unit flow rate will be reduced. Likewise the weight per unit flow rate will be reduced.

In one embodiment of the invention there is flexible fastening to the turret, arranged as catenary lines, whereby it in particular is achieved to make the riser system easier to handle. In another embodiment of the invention the riser is fastened directly to the turret.

Drawings On Figure 1 the riser system according to the present invention is illustrated, in an embodiment with flexible pipes towards the vessel.

On the Figures 2a, 2b and 2c cross-sections of the riser system illustrated on Figure 1 are shown, in three positions.

Detailed description On Figure 1 is shown a riser system with flexible pipes hanging as catenary lines from the top of the riser to a turret or similar in a FPSO-vessel, for example 170 m horizontally from the upper end of the riser. With the term turret or similar it is meant also turning sea-chests, rotating bodies and other rotating units that can take in seawater

lines and other equipment in a similar way as a turret, and with lowered water surface for the cold seawater, and with space for equipment to transport the cold seawater further.

The catenary lines are hanging in three layers, which from the top is consisting of cables and umbilicals; seawater intake pipes, for example 4 flexible pipes of 42" ; and below a layer with production pipes, which means flexible pipes for transport of natural gas. The commercially available flexible pipes at present having largest diameter are 42". Flexible pipes having even larger diameter will be preferable, if they can be handled and can be provided. The flexible pipes are connected to the upper part of a riser, for example at 50 m depth, where a buoyant body is arranged that keeps the riser upright. The buoyant body has buoyancy adapted to keep the riser in a convenient pretensioning and vertical position under the prevailing conditions, such as by ocean current. From its upper end the riser extends down to a lower end 1000 m down into the sea, where it is indicated a number of inlets for seawater on a specific detail on the figure. Below the lower end of the riser is bottom anchoring, parallel to which riser for natural gas is arranged. On the figure the anchoring is in the form of tension legs or wires. On the seabed is provided a basis structure for the anchoring, illustrated as a basis structure for single point anchoring, and it is also indicated how the pipelines for natural gas can be arranged.

On the Figures 2a, 2b and 2c cross-sections of the riser system illustrated on Figure 1 are shown, at the buoyancy body, along the riser and below the lower end of the riser, respectively. It is shown a typical example of design. The buoyancy body has a diameter of for example 9 m and a length of for example 60 m. The buoyancy material of the buoyancy body and further in the riser system is chosen conveniently with density and design life adapted to the design life of the riser system. The risers for natural gas, more specific 3 risers, and cables and further 5 risers for future field connections, are arranged around the riser for seawater intake along its full length, and further down to the seabed anchoring. Along the riser the outer diameter is for example 3.3 m and the lower riser for example 2.4 m.

Fabrication of the riser preferably takes place at a plant with a harbor, such that the whole riser or sections thereof can be filled with air and towed to a FPSO that can be anchored or without anchor in desired position. The installation can take place by use of a crane vessel by deploying 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 beforehand, as far as possible. Alternatively handleable sections of the riser can be connected in position, completely fitted and deployed successively through the turret, with lifting and handling equipment on the vessel and optionally a crane vessel. It can be arranged rail means or similar outside of the hull of the vessel in order to maneuver in and tie in flexible pipes or optionally the riser to the turret. The flexible pipes between the riser and the vessel, for embodiments with such, are preferably installed at last.

The lowest flow resistance for the cold seawater is achieved with a round cross section of the riser and one single flexible riser for seawater intake from the riser to the vessel, because of the lowest ratio pipe surface/flow cross section. With respect to fabrication and impact of ocean current it can be preferable to use another cross-section than round for the riser. Thereby the riser can be manufactured of curved plates that are joined, and the riser as fully equipped can be given a torpedo like or drop like cross- section.