NO315909B1 - Riser system for recording large amounts of cold fun from a deep depth - Google Patents

Description

Field of the invention

The present invention relates to free-hanging riser systems for

intake of large quantities of cold seawater from a great depth into an intake room or equivalent in a fixed-positioned vessel. 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 free-hanging riser systems for the intake of large quantities of cold seawater from great depths to an intake space on board a vessel which, by scattered anchoring or dynamic positioning, is fixedly positioned and in principle without rotation on the sea surface.

Known technique

Several devices are known for taking up cold seawater from great depths. Some examples of such as well as other related techniques are as follows:

In patent publication NO 172681, a device is described to prevent twisting

of a flexible line, which illustrates some basic issues in connection with riser systems.

In patent publication JP 57008369, a device for reducing

movements and thus stress when taking in a seawater line, using a special suspension placed outside the vessel's hull.

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 the 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<3>/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 risers in particular or the risers 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 taking in 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 prior art.

Summary of the invention

With the present invention, a riser system is provided for

intake of large quantities of cold seawater from a great depth to an intake space in a vessel which is in principle firmly positioned on the sea surface by means of scattered anchoring or dynamic positioning, which vessel is in principle without rotation in relation to the sea surface. The riser system is characterized by the fact that it comprises a single riser with a large flow cross-section, which riser in operation extends from a lower free-hanging end at great depth from which large quantities of cold seawater are taken in through at least one opening and led through the riser to an upper end that is placed inside said intake space, in a suspension with suspension about a nominal suspension position, 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 on it

maximum 7% of the sea depth from the nominal position on the sea surface. The drift will typically be much smaller.

By the no-rotation principle is meant a maximum rotation given by the rigidity of the anchorage or the rigidity of further connected equipment that can extend down to the bottom, 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 the riser can take up a flow rate of 11,000 to over 50,000 m<3>fh 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.

At the lower end, the riser advantageously has a lump weight which keeps the riser essentially vertically oriented through the sea. The lump weight advantageously has many openings for the intake of cold seawater to the riser, so that the lump weight functions as a strainer.

The suspension of the riser in the intake space is advantageous with a short flexible element between the riser and the intake space. The suspension in the intake space can advantageously comprise rotationally symmetrically arranged springy rubber-steel lamellae. more specifically flexjoints, for example of the type manufactured by the Murdoch Oil States Company. The suspension in the intake room is advantageously suspended in a system for heave compensation, for example with a constant tension winch.

The riser is advantageously made from a single or several interconnected sections of corrosion-protected steel pipes with external buoyancy material, or sections of thermoplastic or other polymer reinforced with steel reinforcement and/or artificial fiber reinforcement, for example Kevlar, aramid fiber, boron fiber, carbon fiber or glass fiber, which sections of polymer pipe are equipped with buoyancy material and possibly fixing frames for fixing a number of external symmetrically arranged risers for natural gas, with possible further anchoring of the riser to the vessel from near its upper end.

The riser system, which is filled with water, has advantageous buoyancy so that the lump weight and the lower end have negative buoyancy while the upper end has positive buoyancy, so that the riser will naturally assume an essentially vertical position in the sea.

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.

The riser system according to the invention 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 lines for natural gas are arranged on the outside of the cooling water intake 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 an entire LNG plant installed.

With the riser system 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 intake space 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 intake space 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 will be kept lower, more specifically at a level that ensures sufficient pressure head for lift pumps that are placed in the intake space 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 intake space.

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

Drawings

Figure 1 shows the riser system according to the present invention.

Figures 2, 3 and 4 show cross-sections of the fastening of the riser system, for three different embodiments according to the invention.

Figure 5 shows the riser system according to the invention in an embodiment with additional risers for hydrocarbons, cables and wires.

Detailed description

Figure 1 shows a free-hanging riser system suspended in an intake space in an FPSO with a section of flexible pipe. By the term intake room is meant primarily non-rotating sea boxes or similar, or alternatively rotatable intake rooms such as a turret, which intake rooms can take in the riser to a room with a submerged water mirror for the cold sea water, which room has space for equipment to lead the cold seawater further. From its upper end, the riser extends down to a lower end 1000 m into the sea, where there are a number of inlets (not illustrated) for cold sea water. The inner diameter of the riser is greater than or equal to 1.5 m. A short flexible element, with greater flexibility than the riser itself, is arranged between the riser and the intake space.

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

Figure 2 illustrates suspension of the riser with flexible elements rotationally symmetrically arranged to a suspension collar with an integrated trumpet-shaped bending limiter located below. Figure 3 illustrates an embodiment with additional axial and rotational flexibility in that the suspension collar is fixed in a double spring suspension. Figure 4 illustrates an embodiment with additional flexibility in that the suspension collar is additionally fixed in a system for heave compensation. Increased suspension in the suspension provides a beneficial reduction of the stresses on the riser and suspension.

Figure 5 shows the riser system according to the invention with additional risers, specifically for hydrocarbons, as well as cables and wires, arranged so that the riser with connected equipment acts as a unit. Any floating bodies are not shown.

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 dead weight to lower 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 intake space, 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 draw in the riser to the intake space.

There is the lowest flow resistance for the cold seawater with a round cross-section of the riser, 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 produced 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, with simplified production and reduced drift as a result.

Claims (13)

1. Riser pipe system for the intake of large quantities of cold seawater from a great depth to an intake space in a vessel which is in principle firmly positioned on the sea surface by means of scattered anchoring or dynamic positioning, which vessel is in principle without rotation in relation 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 free-hanging 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 which is placed inside said intake space, in a suspension with suspension about a nominal suspension position, 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 large deep. 4. Riser system according to any preceding claim, characterized in that the riser at the lower end has a lump weight which keeps the riser essentially vertically oriented through the sea. 5. Riser system according to claim 4, characterized in that the lump weight has many openings for the intake of cold seawater to the riser, so that the lump weight functions as a strainer. 6. Riser system according to any preceding claim, characterized in that the suspension in the intake space is with a short flexible element between the riser and the intake space. 7. Riser system according to any preceding claim, characterized in that the suspension in the intake space comprises rotationally symmetrically arranged springy rubber-steel lamellae. 8. Riser system according to any preceding claim, characterized in that the suspension in the intake space is suspended in a system for heave compensation. 9. 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 sections of thermoplastic or other polymer reinforced with steel reinforcement and/or artificial fiber reinforcement, for example Kevlar, aramid fiber, boron fiber, coal fiber or glass fiber, which sections of polymer pipe are possibly equipped with buoyancy material and possibly fixing frames for fixing a number of external symmetrically arranged risers for natural gas, with possible further anchoring of the riser to the vessel from near its upper end 10. Riser system according to any preceding claim, characterized in that when filled with water, it has buoyancy such that the lump weight and the lower end have negative buoyancy while the upper end has positive buoyancy, so that the riser will naturally assume an essentially vertical position in the sea. 11. Riser system according to any preceding claim, characterized in that the riser is made of curved, long joined 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 with the wider plates at their other end, with a number of further risers, cables, buoyancy material and fastening elements arranged essentially inside the concave part of the narrower plate, so that the cross-sectional shape becomes torpedo-like or drop-shaped. 12. 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. 13. Riser system according to any preceding claim, characterized in that an FPSO vessel is connected to an LNG plant, the riser system supplying 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 the plant, which lines for natural gas are arranged on the outside of the riser for cooling water intake.