GB2458109A - Low level pumped OTEC cold water pipe - Google Patents

Abstract

A cold water pipe for Ocean Thermal Energy Conversion (OTEC) applications has pumps located at the low level base of the pipe, so that the pipe contains pressurized water. The pipe is therefore free from the danger of collapse under negative pressure, and can be made with thinner walls, or flexible materials.

Description

DESCRI PTION

In normal Ocean Thermal Energy Conversion (OTEC) plants, the essential Cold Water Pipe (CWP) that goes down into the depths of the seas and oceans to collect large quantities of cold water is generally constructed of rigid materials, usually steel or concrete, but also of composite laminates, generally solidly fixed to the ship or platform from which it emanates, its hull holding the pumps that are bringing up the cold water necessary for the condensation cycle of the OTEC system. In heavy seas, these rigid structures are exposed to high stresses, and have often broken away and sunk.

According to the present invention, there íç provided a!ow leve! pumped cold water pipe, (CWP) of varying configurations and materials, fitted with pumps located at the deep end of the pipe, such that the pumped water induces pressure in the pipe and contributes to give it its shape, allowing for the use, not only of rigid pipe wall materials, but also flexible or semi-flexible materials, which present many advantages over rigid materials conventionally used for CWPs, these advantages being made evident as the CWP leaves the craft, platform or land base on which the OTEC systems are located, in a zone affected by heavy seas, and where craft or platforms are used, movements being induced that cause stress in the CWPjunction to its carrier, often causing its rupture and subsequent sinking of the pipe under its heavy weight, these stresses being avoided where flexible materials are used, the direct junction of the CWP to its carrier being preferably avoided, the CWP becoming a separately moored and buoyed entity, with the pumping station at is bottom end, its upper end located below any significant wave motion, which is generally considered to be at a depth of one sixth of the longest probable wavelength, this distance doubled where there are risks of rogue waves, this separately moored and buoyed upper end supplied with an upward opening cone to receive a larger diameter flexible junction pipe, or for additional safety in storm conditions, hydraulically retractable telescopic units that retract into the carrier to insure that no wave motion can affect them, being lowered into the receiving cone as sea conditions improve, these telescopic elements designed to have a certain flexibility to avoid stresses as the carrier moves differentially compared to the receiving end of the CWP, to which they lock in position, insuring normal flow from the separate CWP to the carrier, which can either run vertically to waters typically 1000 meters down, to its end where the pumps are located, the said CWP moored also along its height to give it stability against sea currents and flows, the pumping station being lowered on a platform eccentric to the pipe base, by a cable and crane, and moved into position by robotics, or, when supplied with its own buoyancy unit, becomes weightless and can readily be moved and locked into position by subsea specialist craft, the pumping station being fitted with a protective cage to prevent animal intervention, protecting the pumps in the event of any unforeseen collision, their tubing, centrifuges, aerators, and other necessary equipment and the protective cage forming the low level pumping station fitted with anchoring cables when near land, or single or multiple moorings to the seabed where vertical CWPs are used, and with a packed balloon, 94, structure that can be released and inflated by compressed air to provide a lifting force bringing the pumping station to the surface for maintenance, in some instances the CWP being gradually filled with air so that it rises to float on the surface, the pumps fed by electric cabling protected in a sheath running down the length of the CWP, obtaining the power required to circulate the cold water to the surface at the rate required by the OTEC plant, the pipe or pipes' diameters dictated by the flow, and by the nature of the pipe wall surface which has to be clean of any marine growth and as smooth as possible, these features dictating the nature of the material used for the inside skin, robotic polishing equipment able to circulate freely within the CWP, the CWP, whether comprising one or more pipes, being brought to site on a specialised vessel that is capable of installing the CWP in the manner chosen, either unrolled from the vessel in its simplest application, or with thicker material composites, stored in interlinking layers, and gradually let down by motorised gripping tubes, or formed on board from a single or composite set of materials pulled over a former where, once formed, it is welded, glued, heated or otherwise jointed, and prepared to be lowered into the seas by gripping tubes, or, in a more complex configuration, whilst the tube is being prepared, a number of circumferentially disposed reels add matter and structure to the tube, these structures being typically made of stainless steel or carbon fibre rods or tubes formed around the CWP to create three dimensional structures around it, either left open, or closed with an outer skin to provide a sandwich structural wall of greater strength and rigidity, the said structures protecting the pipe from shark attack or their habit of rubbing away parasites on immersed structures and any other accidental laceration that could damage the internal CWP, the structures being also useful when the CWP no longer descends vertically but is taken out to sea and, making use of its relative flexibility, is lowered until it rests on the sea shelves, as it descends in steps to the required depth, generally to collect cold water, but in some instances, to collect hot volcanically heated water to bring up to the OTEC station to power a different OTEC cycle able to use the great difference of temperature more efficiently, such a hot water pipe needing different pumps and tubing, that can be readily made up by the tailor made material composites and structures used for the pipe construction, this capacity being used to make a variety of CWPs ranging from a single tube made from a material such as insulated neoprene internally lined with teflon or another low friction material weighted down when necessary by an external wrapping of welded spirally wound metal plates, to twin skin pipes with an insulated core between them forming a relatively rigid structure if the core is stiff, or remaining flexible if the core itself is supple, the inner skin lined to create the least resistance to flow as possible, with the possibility also of a CWP made up of a sheet, spirally wound, welded to make up the main tube, but then continued round the tube with space left free for water to circulate, this process of welding allowing numbers of surrounding sleeves to be created in which cold water can be circulated upwards accompanying the main CWP cold water upward flow, forming a layer or layers of active insulation, and preventing the inner water mass from being in touch with the external water which is becoming increasingly hot as the column rises, and ranging to, in its more complex version, the CWP surrounded by wound structures welded glued or otherwise restrained fixed at regular intervals to the CWP, supporting and protecting it, the structure either left open or surrounded by an outer tube, giving it exceptional sandwich structure strength and high insulation values, the CWP aerated at its base by compressed air nozzles and given a spiral motion as necessary, to reduce resistance to flow.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: - Figure 1 shows a conventional OTEC Cold Water Pipe (CWP) next to a low-level pumped CWP Figure 2 shows the various ways in which the low level pumped CWP are constructed and deployed, with diverse typical CWP sections achieved Figure 3 shows a low level pumped CWP feeding a land based OTEC station, and the method by which it can be lowered over a fast falling underwater wall surface, and how it can be raised to the surface.

Figure 4 shows a plan and section of the low level pumped station illustrating options for plankton filtration, and active pumped water insulation Figure 5 shows how the OTEC platform or craft can be moored by a single cable holding the low level pumped CWP, and how it's upper part is designed to allow the platform or craft to move with waves, wind and currents, linked together by a larger diameter flexible pipe Referring to the drawings, there is shown in Figure 1 a conventional CWP next to a low level pumped CWP. The conventional CWP, 1, has its pumps, 2, mounted at the bottom of the craft, platform, or land based station, 5, feeding the OTEC condenser, 3, linked to its evaporator, 4, the pipe generally fixed rigidly to the craft, and subjected to important stresses occurring in heavy seas. Figure 1 also shows the low level pumped CWP with its support, 5, the CWP, 10, now detached from its support, and held in position by a buoyant collar, 7, independently buoyed and moored, multiple moorings possibly joining main craft moorings lower down, or by a single mooring system, running down the length of the CWP, and beyond, such that the CWP is held at a distance from the OTEC plant, provided at its top end with a receiving cone, 8, allowing rotation of the craft or platform above, into which locks a removable flexible pipe or a series of telescopically retractable pipe sections having some inherent flexibility, stored either in the top end of the CWP or in the base of the OTEC plant, so that, in heavy weather, the OTEC plant structure can be fully detached from the GPS located CWP to avoid breakage, also thereby allowing an OTEC plant to be towed away for maintenance. The low level pumps, 18, can be fixed to a mobile platform, 18, with cables forming a cradle, 15, such that it can be initially raised from the OTEC platform deck by a pivoting crane, 6, holding a superstructure, 11, comprising a motor, 13 and two way winch, 12, which are able, once the crane has pivoted the base pumps alongside the CWP axis, to lower the pumps in their cradle, 5, to be level with the weighted bottom of the pipe, 17, and pivot them back on axis so that they can lock in position automatically, or with the help of subsea robotics. An alternative system is one in which the low level pumps, 18, are mounted on a buoyed platform, 17, being virtually weightless, and easy to manoeuver by subsea robotic vessels, to be locked in position, and at any later stage, removed for servicing, another set of low level pumps available to replace those being serviced to avoid any lengthy closure of the plant.

In a further alternative method, the pumping station can be attached to the CWP before it is lowered, its weight giving form to the deployed CWP.

Referring to Figure 2, various low level pumped CWPs fabrication and deployment systems are illustrated; The simplest shows a CWP stored on a craft, 20, rolled flat on a number of replaceable reels, 21, such that the sheet of folded material is drawn through a set of rotating gripping tubes, 22, until above the water and the CWP section shaped weight, 24, which is attached to the sheet, the weight giving it its shape as the material, 23, is drawn through, forming the CWP as the weight descends, a material joint being foreseen between material sheets as one reel is brought in place to replace an empty one.

A different deployment system is employed where a thicker material composite is used, and where it cannot readily be stored on reels, in which case the CWP material is laid out in superimposed layers, 25, and is run through motorised, 27, gripping tubes, 26, to a further reel, 28, located over the water such that the material, 29, is given its shape by the CWP section shaped weight, 30, and drawn down by the weight to form the CWP.

In a further fabrication and deployment method, reels of unshaped material, 31, are drawn by a motor, 38 and gripping collar, 34, over a forming shape, 32, the material, 37, being forced into grips, 33, where the material is joined together by heating, welding, gluing, sewing, or any other suitable jointing systems, so that it is already a shaped CWP as it leaves the forming shape, 32, and carries on down into the sea following the shaped weighted section as it takes up its position.

In a further embodiment of the invention, shown in Figure 2, the CWP forming equipment as presented is located at the centre of a number of circumferencially located reels, 50, of different structural materials, such as stainless steel, titanium, carbon fibre, of tubular or solid rod or flat sheet sections, pulled down, by forming grip wheels, 51, and heated or otherwise treated in an enclosure, 57, so as to be given the necessary shape to be brought down to a jointing ring, 52, where they are welded, glued, sleeved or otherwise jointed, to form an exo-structure around the internal lining of the CWP, that can be a simple metallic sheet layer, or a three dimensional helically wound structure of some depth and stiffness, which, for further stiffness can be given an outer skin, forming, with the inner lining, a sandwich structure which is strong enough to be used, not only with low level pumped CWPs, but also conventional CWPs with pumps located at craft or platform level. Where used with low level pumped CWPs, the weighted collar, 58, can already have attached to it the pumps, 54, as well as the buoyant element, 55, and the protective cage, 56, so that the low level pumped CWP is ready to operate as soon as it has reached the required depth, is moored independently, and is linked by flexible or telescopic tubing to the OTEC carrying craft or platform, either floating or land based.

Referring to Figure 2, a number of low level pumped CWPs are shown as an illustration of the great variety of sections that can be achieved using the CWP fabrication and deployment methods referred to above. In its simplest embodiment, the tube section is made up of two flexible material tubes, 61, with a flexible insulation layer between them, such that it can be easily rolled onto and off a reel.

In another configuration shown in the section, 57, two tubes are shown, the inner one suited to the least impeded movement of water, 63, the outer skin made of a resistant material, such as spiral wound and welded stainless steel strips or kevlar, 62, with a rigidised insulating core made of honeycomb or foam, between them, 64, forming a sandwich shell structure of great strength, weighted to neutralise any air or gas pockets in the insulating material, rivalling in strength a steel or concrete tube, and therefore suitable for all CWP construction requirements.

In another embodiment of the invention shown in section, 58, a single sheet of material, after forming an initial circle, is further wrapped, the unjointed end continuing to form a second circle, and so on, two being generally sufficient to provide voids, 65, through which small quantities of water can be pumped alongside the cold water being raised in the CWP, to provide it with active insulation, the outer skin, 66, composed of a more resistant skin, such as metal or kevlar sheeting.

Referring to Figure 2, another section is shown, 59, in which the inner circles are like those described in section, 57, or others, but that additionally, a triangulated curved frame, 68, is "spun" and welded around the inner tubes, glued or otherwise joined at its intersections, as well as to vertical members, 67, the inner one of which is joined to the main tubes, the outer one locating an outer circular triangulated frame or a complete outer circular, ovoid, or any other sectional shaped skin required by the CWP, this skin being itself of single or of sandwich skin construction, giving great strength, permanence and resistance, within its marine environment, to this multilayered composite CWP comprising inner and outer skins and a rigidifying lightweight space frame between them, this structure being strong enough to be used with both low level pumped or sea level pumps.

In a further embodiment of the invention, the components making up these different sections can be further divided to be readily stacked on board an assembly craft, both portions of tubes and portions of space frames, so that they can be gradually assembled under a rig over the water, and, as each section is assembled, the element so formed can be lowered into the water, gradually forming the complete CWP.

Referring to Figure 3, a method of laying a low levelled pumped CWP, 42, is shown, originating from a land based or floating platform, 40, deployed on the sea surface if filled with air, or taking a catenary curve if filled with water, the CWP being pulled out by a craft, 43, carrying on board a winch, 44, such that it is able to lower the low level pumping station, 47, with a cable, 45, until it comes to rest on the sloping underwater land edge, 46, being then, if necessary, fixed in position by cables and ground anchors, 95, these being released when the CWP station needs to be serviced, either moved by subsea robots, or by activating a compressed air balloon, 94, fed by a high pressure tube, 97, which is packed in a container, 93, released by its cover, 94, mounted on the weight relieving annulus, 90, above the protection frame, 91. the balloon lifting the CWP station to the surface where it can be serviced on board a repair craft or taken to land, replaced by a spare station to minimise plant closure time.

Referring to Figure 4, a low level pumping assembly is shown, comprising a single or a number of pumps, 78, drawing in sea water through openings, 80, the pumps driven by electric motors, 84, the pumping tubes being fitted with finned deflectors, 77, giving the pumped water a spiralling motion, such that the denser plankton is centrifugally pulled to the outside walls, thin outlets allowing plankton rich water to be expelled into a central tube, 76, between separation plates, 74, and thence downwards to an opening, 81, away from the pump inlets, 80, the system, according to its design, filtering out all or part of the plankton, depending on possible fish farming requirements. The pumped water then flows under pressure into the main CWP, 70, some water entering the cavity, 71, between the two skins, the inner skin, 73, and the outer skin, 72, providing a degree of active insulation, the water column being aerated by a compressed air or liquified air converter, 87, the compressed or liquified air brought down by a flexible tube, 86, to directional nozzles, 88, or other means of aerating the column, such as a rotating arm supplied with nozzles, or side mounted air jet propellants, the beginning of the CWP possibly also fitted with finned deflectors to induce a spiral motion in the water column to reduce its upward flow resistance. The lower part of the low level pumping station is protected by a framed structure, 82, solidly fixed to the CWP by reinforced brackets, 79, against shock, animal attack or accidental damage, the frame being fitted with a mesh, 83, to prevent small fish or objects from being drawn into the pumping chambers.

A buoyant cushion, 90, surrounds the top part of the pumping unit, joined to its base by surrounding frames, 91, which offer further protection when the unit lies on uneven slopes, or is hit by any obstacle. Ajointing system, 92, is designed to allow the low level pumping unit to be removed from the bottom of the CWP and brought to the surface for servicing, a spare unit always ready so as not to cause the interruption of the OTEC's continued performance.

Referring to Figure 5, a low level pumped CWP is shown with a single mooring line, 105. fixed to a mooring block, 108, on the seabed. 109, below its pumping station, 107, the line running alongside the CWP, until it reaches the buoyed linking cone, 106, the line continuing to the prow of the craft or platform, such that these can move as required by seas and winds, the connection, 103, between the linking cone and the craft, having enough flexibility to adapt to these movements, the flexible linking device, as described above, either made of a larger diameter flexible material, or made from retractable telescopic units, both detachable to allow the craft or platform to be moved if necessary, the CWP location given by GPS monitors.

The low level pumping station is primarily used to pump up cold water, but can also be used to exploit warm geothermal or soft water sources.