NO315953B1 - Riser systems for recording large amounts of cold fun from deep depths - Google Patents

Description

Field of the invention

The present invention relates to bottom-anchored riser systems for taking large quantities of cold seawater from great depths to a stationary turret or equivalent in a fixed-positioned vessel which can rotate about said turret. The invention is particularly relevant where there are very significant cooling needs, such as in the case of LNG production of natural gas on a vessel of the FPSO (Floating Production Storage and Offloading) type. More specifically, the invention relates to bottom-anchored riser systems for the absorption of large quantities of cold seawater from great depths to a turret or an equivalent device on board a vessel which, by scattered anchoring from the turret or by dynamic positioning, is firmly positioned on the sea surface, so that the turret is firmly positioned and in principle without rotation in relation to the sea surface, while the vessel can turn freely about the turret according to the weather conditions.

Known technique

Several devices are known for taking up cold seawater from great depths. Some examples of such as well as other related technology are as follows: In patent publication NO 172681, a device is described to prevent twisting of a flexible cable, which illustrates some basic issues in connection with riser systems. Figure 1 illustrates a riser 6 which is anchored to the seabed 5 by means of a foundation 7. The riser 6 can serve to transfer the production from one or more oil wells 8.

In patent publication JP 57008369, a device is described to reduce movements and thus strain when taking in a seawater line, using a special suspension placed outside the vessel's hull. Patent application JP 09310651 describes a seawater intake with a fixed installation, with which seawater can be retrieved from great depths. As it is a fixed installation on the seabed, without a vessel, the described technique is less advantageous for seawater intake at depths of several hundred meters and deeper.

In patent publication US 4350014, a platform is described for making use of the thermal energy found in the sea, by using Carnot's principle to convert thermal energy in the sea into usable kinetic energy. The invention according to the aforementioned publication relates to the platform itself 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 for taking up the seawater in the platform, which is in practice a semi-submersible structure, is suspended by cables in a support structure in the semi-submersible platform. The platform is placed where the water depth is greater than 600 m, preferably in tropical areas where the temperature difference between surface water and deep water reaches at least 20 °C (column 6, lines 39-42). The pipe itself for receiving cold seawater is not a specific subject, but it is described that the pipe is flexible and preferably fabricated from reinforced synthetic material, comprising cylindrical components whose axial length is substantially equal to the radius, stiffened by compression stiffeners preferably fabricated from glass fiber reinforced plastic material, and the components are connected to each other with woven connections of nylon-type synthetic material. (Column 7, lines 40-47). Above the pipe for the intake of cold seawater, there are pumps and other equipment used in connection with power production on the platform. It is not known that a platform has been built according to the aforementioned patent.

In patent publication GB 2324120A, a collection container is described for collecting cold water from the ocean depths, in connection with the conversion of thermal energy in a natural seawater source into usable power. On page 1, second paragraph of the aforementioned patent publication, it is described that the surface temperature of the sea in tropical areas is usually approx. 25 °C, but at greater depths, e.g. 1000 m below the surface, the temperature is typically 5 °C. In the next section, it is described that it is necessary to supply large volumes of cold water from the ocean depths to the condenser to ensure efficient operation of the cycle (for energy production by converting thermal energy from the sea). It is described that the proposed systems make use of a very large pipe for the intake of cold water, typically with a diameter of approx. 30 m and with a length of 1000 m, suspended in a surface vessel. The engineering challenges of producing a cold water pipe of such size and with structural integrity to withstand not only its own weight, but also the mechanical stress from ocean currents and surface waves, are formidable. As far as is known, no attempt has ever been made to produce a pipe for the intake of cold sea water with dimensions close to the above. The purpose of GB 2324120 is to avoid the disadvantages associated with such a pipe, which disadvantage in practice is that such a pipe cannot yet be manufactured, installed or operated according to its intended purpose.

In the international patent publication WO 01/47768 Al, a system for supplying cooling water to a cooling system on a floating vessel for the production of hydrocarbons is described. On page 1, line 34 to page 2, line 6, it is described that an LNG plant on board an FPSO vessel may require approx. 30,000 m/h cooling water. However, for most FPSO vessels, pumps are used that draw up seawater from a seawater intake that hangs freely as flexible hoses or the like to a maximum depth of 40 m. It is described that the length of the seawater intake pipes is limited to avoid collisions with the moorings . Furthermore, the advantage of taking in seawater from greater depths is known (page 2, lines 7-11). With the invention according to the aforementioned publication, seawater intake pipes are provided without limited length to be able to avoid collisions with the moorings, while at the same time cold seawater can be taken in from greater depths. This is achieved by the fact that the cooling water pipes are placed inside the anchoring system and are geostationary in relation to the seabed, whereby they will not be in conflict with the anchoring system and the production risers when the vessel turns under the influence of weather and wind. The system for supplying cooling water to the production processes on the vessel includes one or more seawater risers shown extending between a turning unit and the seabed, and which are connected at their lower end to an anchoring device on the seabed, e.g. a seawater lift pump (page 4, lines 14-17). A seawater lift pump is also shown arranged on a floating unit (page 4, lines 21-22). It is also clear that the seawater risers can generally consist of one large or several small risers that extend down to the seabed or to a selected depth where the seawater temperature is sufficiently low (page 4, lines 24-26). It is further described that the seawater pipes between the floating unit and the seabed can have the same path as the production risers, or they can extend generally vertically from the floating unit to the seabed. In both cases, they are held in position on the seabed by anchoring (page 4, lines 26-29). No point has been made of the number of risers for seawater intake, and only embodiments with several risers for seawater intake are illustrated in the drawings. The invention according to WO 01/47768 Al in reality relates to measures in connection with the turning unit itself in the vessel, and there is no guidance with regard to special choices regarding the number and dimensions of the seawater seat pipes in particular or the riser pipes in general. There is no guidance on specific ways to arrange additional risers, e.g. production risers, in certain ways in relation to the seawater risers. The mentioned pumps appear as mandatory in the system for seawater intake. It has been mentioned that a process plant on an FPSO vessel may require intake of a cooling water quantity of up to 30,000 m<3>/h, whereby use can be made of seawater risers with a flow area that corresponds to a pipe with a diameter of up to approx. 2000 mm.

It has now surprisingly been found that it is possible to design riser systems for the absorption of large quantities of cold seawater from great depths, where certain structural designs can achieve completely unexpected technical and economic advantages, far beyond what has been found indicated in the description of previous known technique.

Summary of the invention

With the present invention, a riser system is provided for the absorption of large quantities of cold seawater from a great depth to a turret or equivalent in a vessel which is in principle fixedly positioned on the sea surface by scattered anchoring from the turret or by dynamic positioning, which vessel can rotate freely about the turret while the turret is in principle without rotation in relation to the sea surface. The riser system is distinctive in that it comprises: a single riser with a large flow cross-section, which riser in operation extends from a lower bottom-anchored end at great depth from which large quantities of cold seawater are taken in through at least one opening and carried through the riser to an upper end slightly below or in the sea surface,

a floating body arranged at the upper end of the riser, which floating body keeps the riser in an upright position,

one or more flexible pipes arranged from the upper end of the riser to the turret to carry the cold seawater from the riser to the vessel, with fluid communication through or around the floating body, and

additional risers, lines, cables and/or floating material which essentially extend parallel to the riser system and are attached to the riser, so that the riser with connected equipment acts as one unit.

In principle, fixed position means that the vessel has a drift of a maximum of 7% of the sea depth from the nominal position on the sea surface. The drift will typically be much smaller.

In principle, a turret without rotation means a maximum rotation given by the stiffness of the anchoring or the stiffness of the riser system, more specifically ± 270 ° to 360 °, depending on the design of the riser system.

By great depth is meant a depth of several hundred metres, preferably at least 600 m, typically 1000 m deep, in principle to a depth where the water temperature is stably low in all seasons and weather conditions. Consequently, in cold waters sufficient depth may be shallower than in warm waters.

The riser pipe system advantageously comprises a single round pipe with a length of approx. 1,000 m and diameter of 1.5 m to 5 m, whereby a flow rate of 11,000 to over 50,000 m7h of cold seawater from approx. 1000 m deep. The riser system preferably includes the riser having a diameter of 2.5-3.2 m, whereby a flow rate of 30,000 to 50,000 m<3>/h of cold seawater from great depths can be taken up with the riser.

The riser is advantageously made up of interconnected pipe sections of reinforced polymer material with external fixing frames for fixing a number of external symmetrically arranged risers for natural gas, with a buoyancy body at the upper end of the riser and 4 pcs. 42" flexible pipes from the buoyancy body of the vessel, with bottom anchoring of the riser from its lower end, upper end and any intermediate positions.

The riser is advantageously made from a single or several interconnected sections of corrosion-protected steel pipes with external buoyancy material, or polymer pipes reinforced with steel reinforcement and/or synthetic fiber reinforcement, for example Kevlar, aramid fibre, boron fibre, carbon fiber or glass fibre, which sections of polymer pipe are optionally equipped with buoyancy material, thus that each section with connected additional risers and equipment has neutral or slightly negative buoyancy.

The riser system advantageously has weak negative buoyancy, so that the weight of the bottom anchorage and the lower end ensures negative buoyancy and positioning in relation to the seabed, while otherwise the riser system has positive buoyancy so that by virtue of its own buoyancy it obtains an essentially upright position, which upright position secured by anchoring from the upper end of the riser and possibly from positions along the riser.

The riser is advantageously made of curved, long jointed plates, which riser around the cross-section comprises two wide convex plates joined at one end, and a narrower concave plate, placed between and joined to the wider plates at their other end, with a number of further risers, cables, buoyancy material and fastening elements mainly arranged inside the concave part of the narrower plate, so that the cross-sectional shape becomes torpedo-like or drop-shaped.

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

A riser system is advantageously connected to an FPSO vessel with an LNG plant, as the riser system supplies cooling water to the LNG plant, and where the riser system further comprises one or more risers for intake of natural gas to the LNG plant, which risers for natural gas are arranged on the outside of the cooling water intake riser.

Another embodiment of the present invention comprises a riser system of the above type, characterized in that it comprises

a single riser with a large flow cross-section, which riser in operation extends from a lower bottom-anchored end at great depth from which large quantities of cold seawater are taken in through at least one opening and passed through the riser to an upper end located inside said turret, in a spring suspension about a nominal suspension position,

and where further risers, wires, cables or floating material are arranged essentially parallel to and fixed in said riser, so that the riser together with connected equipment acts as a unit.

The riser system with the upper end of the riser placed directly into the turret advantageously includes one or more of the above features that deal with the riser.

With the riser system according to the present invention, depending on the design, from 11,000 to more than 50,000 m<3>/h of cold seawater can be taken up for cooling in connection with an LNG plant on an FPSO vessel. For a typical LNG plant with production of 5 million tonnes of LNG per year and with a cooling water intake of typically 1,000 m deep versus 50 m deep, in a typically selected warm water approx. 30% less cooling water requirement. This reduces the power requirement by approx. 50 MW in a facility for LNG production. This means that two gas turbines do not need to be installed, each of which costs approx. NOK 250-300 million (for example LM 6000 Nouvo Preone). Normally, around 5% of the amount of energy in the gas that is taken up (the feed for the LNG plant) is used to cool the rest of the gas to LNG conditions. With the present invention, approx. 3.8% of the feed used for cooling to LNG conditions. This not only results in reduced C02 emissions, but also very significant reductions in terms of weight and equipment on the vessel, with formidable technical and economic effects. A saving of 15% of dry-placed LNG process equipment is assumed on an FPSO with a length of approx. 400 m and width approx. 70 m, with a complete LNG plant installed.

With the riser systems according to the present invention, significant advantages are achieved during installation, reduced risk of collisions and easier manipulation of the components. The riser systems can mainly be handled as a unit.

Furthermore, the flow loss through the riser system is reduced, particularly for embodiments with a round cross-section of the riser. Thereby, a so-called "draw down effect" of only approx. 7-8 m pressure head that must be exceeded in order for cold seawater to be able to flow by itself into the turret from great depth. Said pressure head corresponds to loss of pressure measured as a column of water, due to effects of friction, temperature, pressure and salinity. The water level in the tower must therefore be at least 7-8 m lower than the seawater level in order to avoid the use of pumps in the riser system, but in practice the water level in the tower will be kept lower, more specifically at a level that ensures sufficient pressure head for lifting pumps placed in the tower to bring the cold cooling water further. It is mandatory that the riser system does not contain a pump, which is made possible by the submerged water table in the turret.

Furthermore, the need for reinforcement per unit flow rate reduced. Likewise, there is a lower weight per unit flow rate.

In one embodiment of the invention, there is flexible attachment to the turret, arranged as chain lines, whereby it is particularly achieved to make the riser system easier to handle. In another embodiment of the invention, the riser is attached directly to the turret.

Drawings

Figure 1 shows the riser system according to the present invention, in an embodiment with flexible pipes towards the vessel.

Figures 2a, 2b and 2c show a cross-section of the riser system illustrated in Figure 1, in three positions.

Detailed description

Figure 1 shows a riser system with flexible pipes hanging like chain lines from the top of the riser to a turret or equivalent in an FPSO vessel, for example 170 m horizontally from the upper end of the riser. The term turret or equivalent also means rotatable sea boxes, turning bodies, and other rotatable units that can take in seawater lines and other equipment in a manner similar to a turret, and with a submerged water table for the cold seawater, and with space for equipment to lead it cold sea water further. The chain lines hang in three layers, which from above are made up of cables and umbilical cords; seawater intake pipes, for example 4 pieces of 42" flexible pipes; and at the bottom a layer of production pipes, which means flexible pipes for the transfer of natural gas. The commercially available flexible pipes which today have the largest diameter are 42". Flexible pipes with an even larger diameter would be preferable, provided they can be handled and can be provided. The flexible pipes are connected to the upper end of a riser, for example at a depth of 50 m, where a floating body is arranged to keep the riser upright. The floating body has buoyancy adapted to keep the riser appropriately prestressed and vertically positioned under the prevailing stresses, such as current. From its upper end, the riser extends down to a lower end 1,000 m into the sea, where a number of inlets for sea water are indicated on a separate detailed section in the figure. Below the riser's lower end, there is bottom anchoring, parallel to which the riser for natural gas is arranged. In the figure, the anchoring is in the form of tension rods or wires. On the seabed there is a basic structure for the anchoring, illustrated as a basic structure for point anchoring, and it is also indicated how pipelines for natural gas can be arranged.

Figures 2a, 2b and 2c show cross-sections of the riser system illustrated in figure 1, respectively at the floating body, along the riser, and below the lower end of the riser. A typical example of design is shown, with typical dimensions indicated in the figures. The floating body has an outer diameter of, for example, 9 m and a length of, for example, 60 m. The floating material in the floating body and in the riser system is chosen appropriately with a density and lifetime adapted to the design life of the riser system. The risers for natural gas, specifically 3 pieces, as well as cables and a further 5 risers for future field connection, are arranged around the riser for seawater intake along its entire length, and further down to the seabed anchorage. Along the riser, the outer diameter is, for example, 3.3 m and below the riser, for example, 2.4 m.

Fabrication of the riser preferably takes place at a facility with a port, so that the entire riser or sections of the riser can be filled with air and towed to an FPSO that can be anchored or unanchored in the desired position. Installation can be done using a crane vessel by lowering the lower end of the riser like a pendulum. External equipment, such as mounting frames, risers for natural gas and floating material, are preferably prepared and connected in advance, to the greatest extent possible. Alternatively, manageable sections of the riser can be connected in position, fully equipped and lowered successively through the turret, with lifting and handling equipment on the vessel and possibly on a crane vessel. Rail devices or the like can be arranged on the outside of the vessel's hull to maneuver in and pull in flexible pipes or possibly the riser itself to the turret. The flexible pipes between the riser and the vessel, for the embodiments with such, are advantageously installed last.

There is the lowest flow resistance for the cold seawater with a round cross-section of the riser and a single flexible pipe for seawater intake from the riser to the vessel, due to the lowest ratio pipe surface/flow cross-section. For reasons of fabrication and the influence of ocean currents, it may be advantageous to use a different cross-section than around the riser. Thereby, the riser can be made from bent plates that are joined together, and the riser as fully equipped can be given a torpedo-like or drop-like cross-sectional shape.

Claims (11)

1. Riser system for taking large quantities of cold seawater from great depths to a turret or similar in a vessel which is in principle fixedly positioned on the sea surface by scattered anchoring from the turret or by dynamic positioning, which vessel can rotate freely around the turret while the turret in principle is without rotation relative to the sea surface, characterized by the fact that it includes a single riser with a large flow cross-section, which riser in operation extends from a lower bottom-anchored end at great depth from which large quantities of cold seawater are taken in through at least one opening and passed through the riser to an upper end slightly below or at the sea surface, a floating body arranged at the upper end of the riser, which floating body keeps the riser in an upright position, one or more flexible pipes arranged from the upper end of the riser to the turret to carry the cold seawater from the riser to the vessel, with fluid communication through or around the floating body, and additional risers, lines, cables and/or floating material which essentially extend parallel to the riser system and are attached to the riser, so that the riser with connected equipment acts as one unit. 2. Riser system according to claim 1, characterized in that the riser for water intake is a single round pipe with a length of approx. 1,000 m and diameter of 1.5 m to 5 m, whereby a flow rate of 11,000 to over 50,000 m<3>/h of cold seawater from approx. 1000 m deep. 3. Riser system according to any preceding claim, characterized in that the riser has a diameter of 2.5-3.2 m, whereby a flow rate of 30,000 to 50,000 m<3>/h of cold seawater from great depths can be taken up with the riser. 4. A riser system according to any preceding claim, characterized in that the riser is made up of interconnected pipe sections of reinforced polymer material with external fixing frames for fixing a number of external symmetrically arranged risers for natural gas, with a buoyancy body at the upper end of the riser and 4 pcs. 42" flexible pipes from the buoyancy body of the vessel, with bottom anchoring of the riser from its lower end, upper end and any intermediate positions. 5. A riser system according to any preceding claim, characterized in that the riser is made of a single or several interconnected sections of corrosion-protected steel pipes with external buoyancy material, or polymer pipes reinforced with steel reinforcement and/or artificial fiber reinforcement, for example Kevlar, aramid fibre, boron fibre, carbon fiber or glass fibre, which sections of polymer pipe are optionally equipped with buoyancy material , so that each section with connected additional risers and equipment has neutral or slightly negative buoyancy. 6. A riser system according to any preceding claim, characterized in that it has weak negative buoyancy, so that the weight of the bottom anchorage and the lower end ensures negative buoyancy and positioning in relation to the seabed, while the riser system otherwise has positive buoyancy so that, by virtue of its own buoyancy, it obtains an essentially upright position, which an upright position is ensured by anchoring from the upper end of the riser and possibly from positions along the riser. 7. Riser system according to claim 1, characterized in that the riser is made of curved, long jointed plates, which riser around the cross-section comprises two wide convex plates joined at one end, and a narrower concave plate, placed between and joined to the wider plates at their other end, with a number of additional risers, cables, buoyancy material and fastening elements mainly arranged inside the concave part of the narrower plate, so that the cross-sectional shape becomes torpedo-like or drop-shaped. 8. A riser system according to any preceding claim, characterized in that the riser basically consists of a load-bearing part and a part for cooling water transport, which parts are integrated or separate. 9. A riser system according to any preceding claim, characterized in that an FPSO vessel is connected to an LNG plant, with the riser system supplying cooling water to the LNG plant, and where the riser system further comprises one or more risers for the intake of natural gas to the LNG plant, which risers for natural gas are arranged on the outside of the cooling water intake riser. 10. Riser pipe system for taking large quantities of cold seawater from great depths to a turret or equivalent in a vessel which is in principle firmly positioned on the sea surface by scattered anchoring from the turret or by dynamic positioning, which vessel can rotate freely around the turret while the turret in principle is without rotation relative to the sea surface, characterized by the fact that it includes a single riser with a large flow cross-section, which riser in operation extends from a lower bottom-anchored end at great depth from which large quantities of cold seawater are taken in through at least one opening and passed through the riser to an upper end located inside said turret, in a spring suspension about a nominal suspension position, and where further risers, wires, cables or floating material are arranged essentially parallel to and fixed in said riser, so that the riser together with connected equipment acts as a unit. 11. Riser system according to claim 10, characterized in that it includes one or more of the features according to claims 2,3, 5,7, 8 or 9.