Field of the Invention
The present invention generally concerns a subsea tank system, in particular a marine system with structural elements made of concrete. Primary applications are storage tanks and subsea barges for the offshore energy industry.
Prior and Related Art
Between 1975 and 1995, a series of Gravity Based Structures (GBS) of the Condeep (concrete deep water structure) platform type was built close to land and towed to their destinations in the North Sea. A Condeep structure rests on a cluster of large storage tanks for hydrocarbons, and comprises one, three or four shafts extending from the tanks at the sea bed to about 30 m above sea level. The storage tanks and shafts were made by vertical slip forming, a continuous casting process in which concrete is poured into formwork that is jacked slowly upwards as the concrete below hardens to sufficient strength.
In august 1991, a design flaw in the interconnected tanks of the Sleipner A platform caused the cluster of storage tanks to rupture, and the entire structure sank. [source: The Sleipner Platform Accident, B. Jakobsen, F. Rosendhal, Structural Engineering International 3/94] The rupture occurred at the tricell walls between three adjacent tanks. Differential water pressure acted on the tricell walls and resulting shear forces exceeded the load bearing capacity of the reinforcing steel in the tricell walls. In later designs, more reinforcing steel was added to the tricell walls and their supports towards the cell joints of the storage tanks. The largest Condeep platform, Troll A, was deployed in 1995 at over 300 m water depth, and has a total height of about 470 m. The Condeep platforms have provided valuable knowledge on structural design, materials and construction methods for marine concrete structures that can withstand the harsh and salty conditions in the North Sea.
A Condeep-platform is relatively expensive, mainly due to the long construction time of up to four years, the amount of steel in the reinforcement and the tall cranes and other special equipment required during its construction. After 1995, less expensive floating rigs and subsea production facilities have replaced the Condeep type platforms.
Marine concrete structures are still cost effective, e.g. as GBS for offshore oil and gas developments in arctic environments, liquefied natural gas (LNG) terminals, subsea tanks and, in particular, for immersed tunnels in relatively shallow waters. Immersed tunnels are assembled from tunnel elements pre-constructed on land. Tunnel element structures may, for example, comprise sections with a cross section 10×40 m2 and length 120 m. The sections are placed end to end with a flexible gasket between them to create a watertight tunnel. The tunnel elements are designed such that one element is forced against the next by the water pressure.
Also, concrete technology has advanced. For example, ultra-high performance concrete (UHPC) is a high strength material lending itself to be utilized in deep sea constructions. More particularly, UHPC is a mixture of Portland cement, silica fume, quartz flour, fine silica sand, super-plasticiser, water, and steel or organic fibres. UHPC is characterised by compressive strengths above 150 to 200 MPa, high flexural strengths up to 45 MPa and creep coefficients of 0.2 to 1.0 which are much lower than creep coefficients of normal strength concrete. Other important UHPC characteristics are a high modulus of elasticity (above 45-55 GPa), low capillary porosity, resulting in very low water and gas permeability, and low diffusion of chloride ions, e.g. occurring in seawater.
Today, offshore production of oil and gas from the fields closest to land and in the most shallow waters, e.g. in the North Sea or Gulf of Mexico, diminishes as the fields are depleted. Thus, exploration and production of hydrocarbons move toward fields in deeper waters further from land, possibly in arctic regions such as the Barents Sea. These factors, i.e. longer distances, deeper waters and colder environments, favour autonomous subsea installations which take over some or all of the functions typically performed by surface based production installations on platforms or ships. A typical subsea installation comprises a production unit and storage tanks, both of which should be brought to the field at the lowest possible cost. At the field, the subsea installation may rest on a bed of gravel, which typically is prepared by a vessel with specialized equipment to deposit the gravel at the desired spot. This involves additional installation costs.
Barges for transport, subsea production platforms and storage tanks provide new opportunities for marine concrete structures, as reinforced concrete is generally less expensive than steel.
More particularly, the basic idea is to utilize advances in design and composition of marine concrete structures to solve technical challenges including cost, size and availability of vessels, e.g. for heavy lifting or depositing gravel; weather dependency; wave loads on large structures; lifting operations (air, wave zone, deepwater lowering); static and dynamic forces on cables and objects; heave compensation; landing on the seabed, retrieval; subsea assembly and testing; access and cost for inspection, maintenance and repair; spill response (ultra deep, arctic); long step-out power supply and storage of energy subsea.
In short, the purpose of the present invention is to solve at least one of the problems or challenges presented above while retaining the benefits of prior art.
This is achieved by a subsea tank element according to claim 1, a tank system to according to claim 15 and a method for manufacturing according to claim 20.
In a first aspect, the invention concerns a subsea tank element comprising at least one tank section, the tank section(s) forming a cylindrical concrete-tank closed at its opposite ends by two end caps. The subsea tank element is distinguished by a rectangular structure surrounding the cylindrical tank and a connection between the rectangular structure and the cylindrical tank permitting a motion of the wall of the cylindrical tank within predetermined limits for deflection of the rectangular structure.
The concrete may be UHPC or any other known quality suitable for the application at hand. The rectangular structure facilitates connecting several tank elements into an array of buoyancy tanks or storage tanks. With a fixed distance and orientation of the tank elements relative to each other, fitting connections, pipes, pumps, valves etc. in an array of tank elements is trivial. If desired, several tanks, e.g. two or three tanks besides each other, can be surrounded by one rectangular structure.
The connection element permitting a limited relative motion of the cylinder walls relative to the rectangular structure ensures integrity of the tank element. In particular, the inner volume of the cylindrical tank may contain air at atmospheric pressure, such that the cylindrical tank compresses or expands radially in accordance with the ambient pressure or depth. For a tank element with a diameter of 10-20 m in deep waters, this radial deflection could be, for example, 50-100 mm. Thus, the radial deflection can cause severe strains, and must be accounted for in the design of a tank element according to the invention, as further discussed below.
If the cylindrical tank is not affixed to the rectangular structure, the cylinder may contract or expand depending on pressure without deflecting the rectangular walls. If the tank is affixed to the rectangular structure at some point, the point cannot move so far that the rectangular structure breaks or stress concentrations occur in the cylinder wall with possibly unfavourable shear or tensile stress.
In a preferred embodiment, the subsea tank element comprises longitudinal tensioning cables forcing the end caps together with sufficient force to ensure that the cylindrical tank is fluid tight under operational conditions. The required force is relatively small in applications where an external pressure forces the tank sections and end caps together during operation. As a concrete structure typically withstand large compressive stress, but is considerably more vulnerable to tensile or shear stresses, longitudinal tensioning cables, possibly combined with flexible elements between the sections and end caps, provide a desired flexibility. Tensioning cables can also carry tensile forces in the tank structure which may occur during construction, transport and installation. Tensioning cables may be led in ducts inside the cylinder wall or outside the cylinder wall, the latter allowing easy inspection and replacement during the structure's lifetime. If elastic deformations or plastic deformations from creep cause loss of initial tension, the tensioning cables can be tightened during the lifetime of the cylinder.
The rectangular structure preferably comprises at least one pre-stressed concrete slab forming any or all of a plane top plate, a bottom slab and a sidewall. Such concrete slabs are commercially available, and relatively inexpensive. Thus, using them for top, bottom and both sidewalls is a cost-effective alternative. However, the invention does not exclude using alternatives to such pre-stressed slabs at any or all sides of the rectangular structure. Thus, in principle, the rectangular structure can be made of, e.g. steel, non-pre-stressed walls of concrete or a monolithic structure of reinforces concrete.
If concrete slabs are used, each slab is preferably attached to the rectangular structure by a tensioning cable. This ensures flexibility for the reasons explained above.
In a second embodiment, the rectangular structure surrounding the cylindrical tank comprises a continuous floor cast on the site of assembly. The floor transfer horizontal forces in each tank element and reduces the need for tensioning cables. In embodiments with several tanks side by side in the rectangular structure, the floor also transfer horizontal forces between tank elements, further reducing the need for tensioning cables in the rectangular structure.
Embodiments with a continuous floor may comprise a load bearing structure on the floor and/or a ceiling in the rectangular structure, wherein the load bearing structure is configured to carry a vertical force imposed by the cylindrical tank and to permit a relative motion caused by pressure variations. Load bearing structures on the continuous floor might be useful in applications, e.g. on the seafloor, where the weight of the tank provides a net downward force. Similar load bearing structures on a ceiling of the rectangular structure might be useful in applications, e.g. subsea barges, where the buoyancy of the tank impose a net load on the ceiling of the rectangular structure. These load bearing structures are not mandatory as alternative means, e.g. suspension, are generally known. If present, the load bearing structure may be regarded as a special case of the connection between the rectangular structure and the cylindrical tank. Specifically, as a continuous floor does not expand or contract due to pressure, one or more cylindrical tank(s) resting on the floor must be able to move relative to the floor. The same conditions apply to a structure on the ceiling taking a load in the opposite direction.
The second embodiment preferably also comprises a wall cast on the site of assembly, i.e. any side or end wall. The purpose is to create a structure capable of transferring vertical forces in addition to the horizontal forces in and between tank elements, thereby further reducing or eliminating the need for tensioning cables in the rectangular structure.
The connection between the rectangular structure and the cylindrical tank may comprise a longitudinal protrusion on the outer surface of the cylindrical tank and ribs mounted above and/or below the protrusions on the surfaces facing the tank. The protrusions and ribs transfer net vertical forces up and/or down from the cylindrical tank to the rectangular structure. In one example, cast walls facing the tank comprise ribs above and below the protrusions along the tank replace the load bearing structure on the floor and the ceiling. In another example, a load bearing structure on the floor is provided to carry a heavy load most of the time and a rib above the protrusion prevents the tank from floating to the top of the rectangular structure. In a subsea barge, the large force would be imposed on the ceiling. Thus, the actual configuration is a design issue depending on the intended application for the tank element.
Regardless of construction, the tank element may comprise a skirt for settling in seafloor sediments. In use, the skirt transfers horizontal forces to the seafloor and thereby stabilises the tank element. The tank element may further comprise openings for injecting cement into the skirt during installation on a seafloor. The purpose is to fill cement into any space between the sediments and the bottom of the tank element, thereby providing a firm and stable fundament for a tank element permanently deployed on the seafloor.
In a preferred embodiment, a space between the lower half of the outer surface of the cylindrical tank and the rectangular structure contains a first permanent ballast. Furthermore, a similar space between the upper half of the outer surface of the cylindrical tank and the rectangular structure may contain a second permanent ballast.
The main objectives of the permanent ballast is firstly to provide a near neutral buoyancy for the entire tank element, and secondly to place the centre of mass below the centre of buoyancy for static stability. Both of these objectives can be met by using sand or gravel of, for example and in increasing order of cost and density: granite, eclogite, olivine or magnetite. Additionally or alternatively, the thickness of the bottom slab may be increased for ballasting purposes. The choice of ballast material depends on numerous factors, and must be left to the skilled person knowing the application at hand. The filling of the ballast space with the chosen ballast material may be performed in a cost-effective way on the construction site, e.g. in a dry dock, with commercially available equipment such as cranes or conveyor belts. Installing the majority of the permanent ballast on-shore or near shore is much more cost effective than, for example, placing ballast by a subsea rock installation from a special purpose flexible fall pipe vessel commonly used in pipeline rock installations.
In a particularly preferred embodiment, the second permanent ballast has less density than the first permanent ballast. This includes an embodiment wherein the second permanent ballast is water, e.g. an embodiment wherein water is permitted to flow through the rectangular structure in order to equalize pressure, permanent ballast is only provided in the lower half of the tank element.
As noted, the permanent ballast preferably provides near neutral buoyancy. Preferably, the buoyancy is slightly positive to facilitate towing as described below, and the subsea tank element accordingly comprises a ballast tank for water in order to adjust the buoyancy from slightly positive to slightly negative as known in the art. The ballast tank can comprise several lengths of commercially available pipes for additional reduction of cost. The ballast tank provides an important function in control of buoyancy at greater water depths to compensate for change of cylinder buoyancy due to hydrostatic wall compression and change in cylinder displacement.
In a second aspect, the invention concerns a subsea tank system comprising at least two subsea tank elements according to any preceding claim connected to form a platform.
Thus, several tank elements can be connected into a platform or barge for heavy loads. Such a platform can be provided with slightly positive buoyancy in order to float at the surface. Alternatively, the platform can be provided with slightly negative buoyancy and the buoyancy required to keep it floating can be provided through buoys, e.g. vertical cylindrical elements, at the surface. In the latter case, the subsea barge may keep a load below the wave zone, i.e. such that waves and surface conditions affect the buoys only. This eliminates, for example, wave loads and icing on the platform and/or its cargo. In either case, tensile forces are taken up by the tensioning cables or an integrated structure, and the platform provides a flat and stable support for the cargo.
The tank system may further comprise a network of pipes, pumps and valves for interconnecting the tank elements.
Some embodiments with a network interconnecting the tank elements comprise equipment for using the tank elements as storage tanks, e.g. for oil or gas. Such equipment is well known in the art, and may include a line to a loading buoy on the surface.
Other embodiments comprise equipment for using the tank elements as low-pressure tanks in a deepwater pumped storage hydroelectric plant. Again, the equipment is well known in the art, and may include a ventilation line to the surface. In particular, the head at 300-1200 meters corresponds to the head from a water reservoir in a mountain, so that single or multistage turbines for this range are well proven and commercially available from several vendors.
Still other embodiments further comprise equipment for using the tank system as a platform for a marine installation. The term “marine installation” implies any application at sea wherein the tank elements are used as housing or platform for equipment. The tank elements can, for example, be used as housing for a long step-out power supply, a transformer or converter station, subsea hydrocarbon separation and processing equipment, living quarters for a crew, etc. In another example, the system can be used as a platform for large equipment, e.g. a gas compression unit or fluid pump, at the seafloor. These embodiments do not necessarily need interconnected tanks. However, a controllable network might be useful for balancing ballast water.
In a third aspect, the invention concerns a method for manufacturing a subsea tank system, comprising the steps of: casting a cylinder for a tank section; casting end caps for closing a tank element; assembling a fluid tight tank from at least one cylinder for a tank section and exactly two end caps; arranging a rectangular structure around the fluid tight tank; and connecting the rectangular structure with the cylindrical tank such that the wall of the cylindrical tank is permitted to move within predetermined limits for deflection of the rectangular structure.
This method allows several concrete cylinders and end caps to be cast and cured simultaneously, whereby the production time is reduced significantly compared to the traditional slip forming method of monolithic structures used for e.g. the Condeep platforms discussed in the introduction. Furthermore, the casting can be performed indoors under favourable conditions for the chosen quality of concrete, e.g. UHPC. The tank element is conveniently assembled in a dry dock or on a slip, or alternatively in water near a casting hall.
In one embodiment, the method further comprises the step of towing the subsea tank element to a system assembly site.
In an alternative embodiment, arranging a rectangular structure around the fluid tight tank involves casting a continuous floor for at least one tank element in a drydock and assembling the cylindrical tank on the casted floor.
In both embodiments, the method preferably further comprises the step of providing the tank element with equipment according to any embodiment in the second aspect of the invention.
The system assembly site may, of course, be close to the production site and/or the site of deployment depending on the size of the system and on the application.
Other features and advantages will become clear from the accompanying claims and the following detailed description.
The invention will be explained in greater detail by way of exemplary embodiments with reference to the accompanying drawings, in which:
The drawings are schematic, and merely intended to illustrate the principles of the invention. Thus, they are not necessarily to scale, and numerous details obvious to one skilled in the art are omitted for clarity.
The reader should keep in mind that a typical tank element described below has cross-sectional dimensions that may exceed 20 m and lengths that may exceed 100 m. Hence, replacing expensive steel with less expensive concrete wherever possible has a significant effect on manufacturing costs. The savings multiply as several tank elements are combined into a tank system. Similarly, forces, deflections and other parameters acting on a large structure are not directly comparable to those acting on a smaller structure. Accordingly, effects that are trivial in a small structure can be significant in a larger structure. Furthermore, the articles “a”, “an” and “the” as used herein, in particular in the claims, mean “at least one”, whereas “one” means exactly one.
The outer shape of the cross section is essentially rectangular so that several tank sections 1 conveniently can be connected beside each other to form a larger platform with a plane top. In some applications, the inner volume of the cylinder 2 may contain air at atmospheric pressure for buoyancy. In other application, the inner volume may contain oil or gas at approximately ambient pressure. Accordingly, the wall thickness of the cylinder must be adapted to the operational pressure difference. In addition, there may be a desire to increase the wall thickness for safety reasons and/or to add weight in order to reduce the need for permanent ballast. As noted, the top plate 3, bottom slab 4 and sidewalls 5 are preferably made of reinforced concrete. In many applications, this concrete can be a less expensive quality than the concrete in the cylindrical concrete-tank 2. For example, the rectangular structure may permit water to enter the space between the cylinder 2 and the rectangular structure 3, 4, 5 such that the pressure will be equal on both sides of the rectangular walls. If the cylinder 2 is flexibly attached to the rectangular walls 3, 4 and 5, it may contract and expand without causing significant deflection on the rectangular walls. Hence, concrete with a broad range of tensile or flexural strength will withstand the pressure at any practical depth, and other design criteria determine the quality and reinforcement of the concrete in the rectangular walls. For example, it may be desirable to increase the thickness and/or density of the bottom slab 4 to save on permanent ballast 6, or to increase the thickness and/or ductility of the top plate 3 to accommodate heavy loads.
In claim 1, the connection between the rectangular structure 3, 4, 5 and the cylindrical tank 2 comprises any element transferring a force between the rectangular structure and the cylinder 2, including optional supports for adjacent slabs, beams for distributing loads etc. Such elements are not shown in the figures for clarity. However, the choice and configuration of elements is a design issue that must be left to the skilled person knowing the application at hand, and must of course be designed such that radial motion of the cylinder wall due to pressure does not harm or tear apart the cylinder wall or the rectangular structure. This connection also allows certain compression-induced radial strains (radial motions) to occur in the cylinder wall and avoids large tensile or shear stress in the cylinder walls, which would otherwise have to be to controlled by heavy steel reinforcement. Thus, the cylinder wall may be constructed in a simplified and cost effective method with little or no conventional steel reinforcement, beyond fibres.
For the cylindrical tank, ultra high performance concrete (UHPC) with fibres can be a cost effective alternative to traditional reinforced concrete with passive reinforcing steel, especially in deepwater applications with large pressure differences over the walls of the tank 2. In particular, during manufacture it is difficult and time consuming to ensure that concrete fills all spaces within a rebar cage, whereas UHPC essentially can be poured into a mould with little or no rebars. In deep water applications, compression of a tank may cause a significant loss of displacement and buoyancy, which must be carefully monitored and compensated. UHPC helps reducing this problem as it is stiffer, i.e. has a higher module of elasticity, than other qualities of concrete. On the other hand, UHPC with fibres is not likely to withstand other forces, e.g. longitudinal forces that occur during towing, as good as concrete with a large amount of steel reinforcement. However, this can be handled in a convenient manner, e.g. by adding tension cables within or outside the cylindrical tank to take up longitudinal forces. As above, the choice of concrete quality and reinforcement depends on the application at hand, and is left to the skilled person.
The top 3, bottom 4 and sidewalls 5 are preferably made of commercially available precast and pre-stressed hollow rectangular concrete slabs. If desired, the hollow spaces within such slabs can be sealed off to provide cells for extra ballast or buoyancy. In deep water applications, such cells might initially be filled with ballast water. When lowering such a cell in the sea, the ballast water could gradually be replaced with pressurised air as the pressure from the water column causes the cylindrical tank to contract, and hence decreases the displacement and buoyancy. In such an application, the air must have a sufficiently large pressure to avoid that the hollow slabs collapse from the external pressure. Supplying air from the surface at such pressures, attaching the required lines to a cell within a concrete slab and other problems may render it impractical to use the hollow spaces for ballast in deep water applications. However, extra cells for buoyancy can be useful during towing, completion, harbour work and other operations where the tank element is close to the surface.
Preferably, the centre of mass is below the centre of buoyancy to ensure static stability. This can be achieved, for example, by increasing the weight of the bottom slab 4, or by using permanent ballast 6 with high density in the lower space between the outer cylinder wall and the bottom slab 4 and sidewalls 5 as shown in
One or more tank sections 1 can be combined into a tank element 10 as further described in connection with
Longitudinal and lateral cable channels 9 are provided for post tensioning and connection to adjacent tank sections and elements as further described below. During assembly, steel cables are drawn through these channels and provided with a predetermined tension before the channels 9 can be filled with grout or grease for corrosion protection. Techniques for such post tensioning are well known, and thus not discussed in greater detail herein.
In the leftmost tank element 10, tensional cables 160, e.g. steel wire, run from top to bottom through the sidewalls. These cables tie the top plate 3 to the bottom slab 4, and also keep the sidewalls 5 in place. Similar cables 160 also connect the top and bottom slabs of the other tank elements and sections, but are not shown for clarity.
In addition to the compression forces discussed above, the loads applied during assembly, towing and lowering/lifting must also be accounted for. For example, the tank elements 10 and/or system 100 may be designed such that the rectangular structure 3, 4, 5 takes the load during assembly and towing. In this case, a tank made of fibre reinforced UHPC does not need to be designed for the relatively large tensile loads that may occur during towing.
The first example is a hydroelectric plant operating at a seafloor in 300-1200 m depth, which roughly corresponds to the head from a traditional water reservoir of a pumped storage plant in a mountain. Single or multistage reversible pump turbines for this range are well proven and commercially available from several vendors. In this example, the equipment 120 depicts a turbine unit, the tank elements 10, 10b are low pressure tanks, and the network 110 allows pressurised water into the tank elements through the pump turbine unit 120. The elements 121 are connecting beams. Such a hydroelectric power plant can advantageously comprise a ventilation line to the surface.
In the second example illustrated by
In some embodiments of a system 100 at the seafloor, e.g. as shown in
It should be understood that the examples presented above are just some of numerous applications.
As noted, several cylinder sections 2 and end caps 12 for the tank can be cast and cured simultaneously, preferably in a production hall with favourable conditions, and then assembled to the tank element 10 shown in
Referring back to
After assembly, the tank element 10 is typically towed to a system assembly site where it is connected into a system 100, and where the system 100 is fitted out according to its intended purpose.
Similar to
In contrast to the previous embodiment, the floor 40 and walls 50 form a continuous concrete structure or caisson along the entire length of the tank 2, so this embodiment does not need the tensioning cables 140, 150, 160 connecting the rectangular structure as described with reference to
The only other connection between the rectangular structure 40, 50 and the cylindrical tank 2 are longitudinal protrusions 22 on the outer surface of tank 2 and ribs 52 mounted on the surfaces facing the tank 2 and above the protrusions 22. The ribs 52 on the fixed walls prevent the tank from reaching the top or ceiling of the rectangular structure, and may be regarded as structures transferring buoyancy. By symmetry, structures on the ceiling bearing buoyancy and structures on the walls transferring gravity loads are anticipated.
The caisson 40, 50 is provided with a skirt 42. During towing, the skirt 42 may be filled with compressed air to provide additional buoyancy to the caisson. In use, the skirt 42 will settle in the seafloor sediments. In some embodiments, the skirt 42 may be allowed to settle for some time after deployment before cement is injected to fill any spaces left between the seafloor sediments and floor 40. The injection is similar to that used in cementing a casing to seafloor sediments in the oil and gas industry, so suitable cements or compositions are commercially available.
The invention has been described with reference to exemplary embodiments. However, the scope of the invention is defined by the accompanying claims.
Number | Date | Country | Kind |
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20141458 | Dec 2014 | NO | national |
Filing Document | Filing Date | Country | Kind |
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PCT/NO2015/050236 | 12/2/2015 | WO | 00 |