This invention relates to the construction of an offshore power storage facility and the construction of a reservoir wall.

Background

Pumped storage is a well known technology for storing energy, specifically electric energy. The principle is using electricity to power a pump, pumping water from a lower reservoir to an upper reservoir. When electricity is needed, water from the upper reservoir is led back to the lower reservoir while spinning a hydro turbine connected to a generator and thereby producing electricity. The height difference between the upper reservoir and the lower reservoir is necessary to drive the hydro turbine. But this required height difference makes it difficult to find suitable locations on shore, which does not collide with the interests of the local population.

Moving the energy storage to an offshore location makes finding a suitable location much easier, but in the same time makes the construction process more complicated and expensive.

Offshore pumped storage projects have been proposed earlier, where a large embankment forms an enclosure which is the lower reservoir and the sea is the upper reservoir. The energy is stored by pumping water from the lower reservoir (inside the embankment) to the sea using a pump or a combined pump-turbine and the electricity can be reproduced by letting water flow from the sea to the inner reservoir through the pump-turbine or a separate turbine, spinning a generator to produce electricity. The construction of such an embankment is known, but can prove difficult in offshore conditions where weather conditions have a very large impact on the construction process. Such an embankment will take several years to complete and as cost of offshore work is several times higher than similar work on shore or at the coast, the cost of such an offshore embankment will be much higher than a comparable structure on shore.

Related prior art

WO 2009/123465 discloses the working principle of an offshore power plant. However, this document fails to disclose how to build such an offshore power plant in an efficient manner.

Pumped storage projects of the coast of the Nederland and Belgium has been proposed earlier where an Energy Island is constructed offshore. These Energy Islands is just like a normal island, but with a large lake in the middle of the island, where the lake in the middle serves the purpose of lower reservoir and the surrounding sea is the upper reservoir. The island is constructed from excavated seabed material positioned to form the island. This is a process similar to construction of a very large embankment reaching from the seabed to above the sea. Embankments are known technology to protect low lying areas from being flooded. However, embankment energy islands have yet to be constructed due to high costs and long construction time. WO 2013/044976, WO 2013/044977 and WO 2013/044978 disclose pumped storage facilities constructed from prefabricated wall elements of reinforced concrete. Reinforced concrete is well suited for constructing offshore structures as the concrete protects the steel from the offshore environment. Offshore walls for damming up water are large elements due to water depth and the forces from damming up water. Concrete is a low cost construction material which makes it suitable for construction of large elements. Reinforced concrete is well suited for the construction of the disclosed wall elements. Immersed tunnels are another example of offshore structures constructed from large elements of reinforced concrete. Summary

There is provided a pumped storage facility located at an offshore location. The pumped storage facility comprises a reservoir wall positioned on the seabed, configured to dam up water and separating an inner reservoir from a surrounding sea. The pumped storage facility also comprises at least one pump-turbine system, configured for pumping water from the inner reservoir to the surrounding sea and for letting water from the surrounding sea drive turbines when running from the surrounding sea into the inner reservoir. The pumped storage facility is characterized in that the reservoir wall comprises multiple prefabricated large diameter steel piles, where the prefabricated large diameter steel piles are positioned adjacently and are embedded into the seabed. The pumped storage facility is also characterized in that a first steel sheet extending between two adjacent large diameter steel piles, such that a wall segment is established between the piles; and the large diameter steel piles are at least partially filled with seabed material.

The large diameter steel piles provide enclosures for the fill material. The fill material provides the necessary weight for the wall structure to withstand the enormous forces from damming up the surrounding sea, thus the fill material can also be regarded as ballasting material. By providing such an enclosure, capable of withholding the fill material, the need for fill material is dramatically reduced, compared to the embankment reservoir walls suggested by the prior art. Further, by confining the fill material, the requirements to the quality of the fill material are reduced, as it does not have to provide at stable structure by itself as it is the case for the embankment solution suggested by the prior art. This makes it very probable that local seabed material can be used as fill material, instead of having to import the fill material. This reduces risks on both cost and construction time of the facility. Further, the large diameter steel piles comprised in the reservoir wall means that the sloping sides of an embankment solution are avoided. Such sloping sides often requires an inclination of 1 :2 or less, in order to be stable, thus making the reservoir wall very wide and dramatically increasing in width with increasing water depth. The reduced material consumption of this invention means that the requirements for the local seabed material are lower (both quality and amount) and the total construction time and costs of the reservoir are reduced. Further, as the pumped storage reservoir wall is narrower when constructed from the large diameter steel piles, the amount of piping required to connect the pump-turbine system both to the inner reservoir and the surrounding sea is reduced. A reduction of piping reduces cost, but also reduces losses within the pump-turbine system during operation, which increases the profitability of the pumped storage reservoir.

Further, the wave impacts and the flow of water around such an offshore facility increases the risk of the material for an embankment reservoir wall slowly washes away. By locating the fill material inside the steel piles, this risk is dramatically reduced.

Daily operation of the pumped storage facility involves draining and refilling the inner reservoir over and over again. This means that the water level in the inner reservoir will have large variations. During daily operation the water level inside the reservoir will be lower than the water level of the surrounding sea. The height difference results in a pressure difference that tries to push water from the surrounding sea into the inner reservoir. This and the flow of water from draining and filling the inner reservoir, thereby storing and reproducing energy, can cause problems with erosion which might lead to structural problems if the fill material is subjected to this possible erosion. But the fill material providing the strength of the reservoir wall is well protected inside the large diameter steel piles. By protecting the fill material within the large diameter steel piles, the risk of erosion and so called piping is reduced, which reduces the risk of reservoir failure when damming up water. These risk reductions has a positive impact of the lifetime of the facility.

The cyclic nature of the load on the reservoir wall from draining and filling the inner reservoir means that the reservoir wall must be inspected and monitored routinely during its lifetime. The hard surface of the large diameter steel piles means that they can more easily be inspected compared to the embankment reservoir suggested by the prior art, as the water by design is penetrating deep into the embankment structure and thus can cause problems that are difficult to detect. The easier inspection of this invention means that problems are more easily detected and can be dealt with in time.

By constructing the reservoir wall from prefabricated wall elements, the offshore construction work is reduced and the overall construction time of the reservoir structure is lowered. As the fill material provides the necessary weight and strength for the reservoir wall structure to be stable, the filled large diameter steel piles can be regarded as gravity structures. The fill material exerts an outgoing force on the large diameter steel piles from the inside, causing tensile stress within the pile material. Steel has excellent tension strength and a good strength to weight ratio, making the prefabricated large diameter steel piles low in weight compared to their size and their ability to withhold fill material. The high strength of the steel results in a low material use, which reduces material costs and installation costs, as the weight of the steel piles are kept to a minimum. The material use and overall costs for the prefabricated large diameter steel piles are much lower than for the reinforced concrete elements suggested by the prior art.

Further, the process of installing the piles is much faster as the handling is easier and less costly when the weight is kept to a minimum. The proposed pumped storage facility located offshore can thus be erected faster than both the embankment reservoir structures and the reinforced concrete structures proposed in the prior art. A reduction in construction time also reduces the sensitivity towards weather conditions which further reduces construction costs. The prefabricated steel piles are manufactured on shore or in a dry dock facility, where the weather has a much smaller impact on the work being performed than at an offshore site. This controlled manufacturing environment results in lower prices of the steel piles and ensures a high and uniform quality. The diameter of the steel piles is large in order for the fill material to be utilized to add to the strength of the reservoir wall. Long and slender piles will take more of the forces acting on the wall as bending by the steel piles and less by the fill material inside the steel piles. This would require more strength in the steel piles; if a larger share of the forces is to be absorbed by bending within the steel piles, a larger wall thickness of the piles is required. A larger wall thickness of the piles would significantly add to the steel material use, and thus the overall costs of the wall.

The large diameter of the prefabricated steel piles for the reservoir makes handling and thus accuracy of installation challenging. By positioning the piles adjacently and establishing a wall segment with a steel sheet extending between the adjacent piles, a coherent wall structure is formed and the required accuracy of the steel pile installation is reduced. The smaller steel sheet element can be customized to fit a specific distance between piles measured after installation, or the steel sheet can be constructed as a structure with inherent flexibility capable of adjusting to differences in distance between the steel piles, e.g. sheet piling or an arched steel sheet. This reduction in required accuracy of steel pile installation increases the installation speed and thus reduces the weather sensitivity and the overall construction costs.

The large diameter steel piles reaches from the seabed and up above the sea surface. The ballasted large diameter steel piles provide the majority of the strength of the wall, whereas the steel sheet extending between adjacent steel piles forms a wall segment and ensures a coherent wall structure capable of damming up water. The impermeability of the steel piles and the steel sheet enables the reservoir wall to dam up water and thus be used for energy storage purposes. The combination of steel pile, ballasting material and steel sheet is necessary in order for the reservoir wall to be able to dam up water. The large diameter piles can have a round or substantially circular cross section or a rectangular cross section shape or a polygon cross section shape. For a pile with a rectangular or polygon cross section, the diameter refers to the diameter of the circumscribed circle.

In an embodiment, the piles have a round or substantially circular cross section shape. Thereby the steel usage is kept at a minimum as a circular shape is the most efficient regarding resisting an outgoing pressure from inside the pile. The prefabricated piles and the steel sheet are constructed from steel, but other materials may be added, e.g. to reduce corrosion. This could for instance be Zinc anodes or a protective concrete layer in the splash zone. Various types of steel alloy can be used for the large diameter steel piles. Embedding the prefabricated steel piles into the seabed increases the ability of the reservoir wall to withstand the forces from damming up water as a larger portion of the seabed is utilized for resisting the forces from damming up water. This enables the reservoir wall structure to be narrower than a pure gravity based reservoir wall, thus saving both steel and fill material and thereby reducing the overall construction costs. The relatively thin walled prefabricated large diameter steel piles are well suited for penetrating into the seabed, due to the low cross section area and the high strength of the steel, compared to e.g. concrete. Further, by embedding the impermeable steel piles into the seabed, the flow of water underneath the reservoir wall when damming up water is reduced, as the steel piles increases the length that the water needs to flow to get underneath the reservoir wall and into the reservoir. This is a great advantage as the purpose of damming up the water is to store energy and any flow of water into the reservoir, not coming through the turbines, results in a loss of stored energy. By embedded is meant that a portion of the large diameter steel pile penetrates significantly into the seabed, e.g. 2 m or 5 m or 10 m or 15 m. This makes the large diameter steel pile better fixated to the seabed.

Further, having the large diameter steel piles embedded into the seabed and the fill material positioned inside the hollow piles, results in that no foundation is needed for the large diameter steel piles. This reduces the installation time and costs.

The effects from directly embedding the large diameter steel pile into the seabed reduces number of offshore installation operation, increases installation speed and thus reduces cost, compared to the reinforced concrete reservoirs suggested by the prior art.

In an embodiment, the steel sheets extending between the steel piles also penetrate into the seabed, thereby further reducing the flow of water underneath the reservoir wall. In an embodiment, the large diameter steel piles embedded within the seabed penetrates into a clay layer within the seabed, thereby reducing the flow of water through the seabed underneath the reservoir wall, as the clay layer has a lower permeability than other layers of seabed, e.g. sand layers. In an embodiment, the large diameter steel piles comprise steel reinforcement inside the piles, e.g. l-profiles or T-profiles. The steel sheet can also comprise such reinforcement.

The large diameter piles are hollow. By hollow is meant that the large diameter steel piles can be filled with a fill material. The large diameter piles can be considered as shell structures. The large diameter steel piles have a small cross section area compared to the cross section area of the hollow space within the large diameter steel piles. The large diameter steel piles can therefore relatively easily penetrate into the seabed.

By pump-turbine system is meant a system capable of pumping water from the inner reservoir to the surrounding sea and produce energy by letting water from the surrounding sea flow into the inner reservoir. This can e.g. be a separate pump powered by a motor and a hydro turbine connected to a generator or it can e.g. be a reversible pump-turbine connected to a motor- generator. The pump-turbine system also comprises valves, guide vanes etc. for controlling the flow of water through the pump-turbine system.

By wall segment is meant a part of a wall. The large diameter steel piles and the steel sheets form a coherent wall structure capable of damming up water, i.e. a reservoir wall. The wall segment reaches at least from the seabed to above the sea surface. This is necessary in order to dam up water.

The large diameter steel piles must be protected against corrosion in the offshore environment, e.g. by means of cathodic protection, anti-corrosive painting etc. This adds to the total construction costs. But a pumped storage reservoir comprising multiple large diameter steel piles still proves a cost efficient alternative to the embankment structures and the reinforced concrete elements suggested by the prior art.

In an embodiment, the length to diameter ratio of the large diameter steel piles is less than two. With a length to diameter ratio of less than two, the piles for the reservoir wall are at least half as wide as they are long. This feature enables a relatively thin walled pile, where the fill material adds significantly to the strength of the reservoir wall. The higher the length to width ratio, the more of the forces acting on the wall would be taken up as bending by the steel piles and less by the fill material. If a large share of the forces is to be absorbed by bending within the steel piles, a high wall thickness of the piles is required. Thus, the length to width ratio of less than two of the steel piles results in lower steel material consumption per unit length reservoir wall and thus a lower cost. In an embodiment, the diameter of the large diameter steel piles is at least 15 m or 20 m. The larger the diameter of the large diameter steel piles, the more fill material can be placed inside them and the larger forces can be resisted. The more forces that can be resisted by the reservoir wall, the more energy can be stored inside the reservoir, thereby reducing the unit costs of the energy storage.

In an embodiment, the height difference between the water level of the inner reservoir and the water level of the surrounding sea is at least 15 m when the water level of the inner reservoir is at the minimum level. The higher the height difference between the water level of the inner reservoir and the water level of the surrounding sea, the more energy is stored in the pumped storage reservoir. This means that the circumference of the reservoir can be smaller for the same amount of energy storage, meaning that fewer steel piles must be installed. This reduces the number of offshore operations and therefore the erection time of the reservoir.

In an embodiment, the minimum material thickness of a large diameter steel pile is in the range 10-20 mm or in the range 10-30 mm. Thereby reducing steel material use as still enabling the large diameter steel pile to contain the fill material. A reduced material consumption reduces material costs, but also costs for transporting and installing as the weight is reduced. The large diameter steel pile can be reinforced with steel profiles.

In an embodiment, the steel material thickness varies along the height of the large diameter steel pile. Thereby material consumption can be minimized as the loads on the pile are different in different heights. Load from fill material inside the pile, wave loading etc. varies with height level. Further, the large diameter steel pile can have different material thickness due to different corrosion levels corresponding to different height levels, e.g. the splash zone experiences high corrosion. In an embodiment, a large diameter steel pile comprises a substantially vertical flange welded to the outer side of the large diameter pile. The flange provides a joint between the large diameter steel pile and the steel sheet extending between the large diameter steel pile and an adjacent large diameter steel pile. In order for the reservoir wall to be able to dam up water, a joint must be established between the large diameter steel pile and the steel sheet. The joint is formed by physical contact between the large diameter steel pile and the steel sheet, wherein the flange and the steel sheet are overlapping. The flange welded to the outer side of the large diameter steel sheet makes the process of establishing a joint between the large diameter steel pile and the steel sheet simple and fast which reduced installation costs.

In an embodiment, the seabed material used as fill material for the large diameter steel piles is excavated from a seabed area inside the inner reservoir. By excavating seabed material from the inner reservoir for fill material, the seabed level in at least a portion of the inner reservoir is lowered. This means that the height difference, and thereby the pressure head, between the water level in the inner reservoir and the water level of the surrounding sea can be larger, resulting in a increased energy storage capacity of the reservoir. This means that the unit costs of the reservoir storage capacity are reduced. Further, it is a better solution environmentally to excavate the seabed material from the inner reservoir compared to outside the reservoir, as this portion of the seabed is already allocated for the pumped storage facility and any spill from the excavation is more contained inside the reservoir. This also means that the needed permits are likely to be issued faster, resulting in a lower total project period. By seabed area inside the inner reservoir is meant a portion of the seabed enclosed by the reservoir wall when the pumped storage facility is constructed.

In an embodiment, the seabed material is excavated from the seabed area inside the inner reservoir before the reservoir wall for the pumped storage facility is fully constructed and thus before the reservoir wall encloses inner reservoir. Thereby reducing the overall construction time of the pumped storage facility, as the process of excavating the seabed material and filling it into the large diameter steel piles is performed in parallel with the construction of the reservoir wall.

In an embodiment, a large diameter steel pile is a single unit comprising multiple steel plates welded together. As a single unit, the installation of the large diameter steel pile is faster as the number of installation operations are kept at a minimum. Further, the welded steel plates ensure a high strength and thus ability to withstand the outgoing load from the fill material inside the large diameter steel pile. Further, steel plates welded together have a high degree of impermeability and the large diameter steel piles are therefore well suited for damming up water.

In an embodiment, the pumped storage facility comprises a berm located on the reservoir side of the reservoir wall. Thereby a downwards load on the seabed just inside the reservoir is established by the weight of the berm. This downward load increases the stability of the seabed and thus the stability of the reservoir wall, enabling a narrower reservoir wall or less penetration of the large diameter steel piles into the seabed. By berm is meant a barrier or a mound or a substantially level shelf raised above the original level of the seabed. In an embodiment, at least a portion of the berm located on the reservoir side of the reservoir wall is constructed from seabed material excavated from a seabed area inside the inner reservoir.

In an embodiment, the berm is in contact with the reservoir wall. Thereby the material of the berm can add strength to the reservoir wall as the reservoir wall can utilize the passive soil pressure of the berm to withstand the forces from damming up water. Thereby, adding strength to the reservoir wall to withstand the main load on the reservoir wall from damming up water.

In an embodiment, the pumped storage comprises a second steel sheet extending between the two adjacent large diameter steel piles, such that a compartment is formed by the first steel sheet and the second steel sheet and the two adjacent large diameter steel piles; wherein the compartment is at least partially filled with seabed material. Thereby improving the weight and strength of the reservoir wall near the steel sheets and creating a robust wall. The second steel sheet and the fill material reduce the permeability of the reservoir wall and thus improve the ability of the wall to dam up water. Further, this will create a reservoir wall with a larger average width, making transport of equipment and personnel on the top of the reservoir wall, e.g. for operation & maintenance, easier.

In an embodiment, the compartment formed by the first steel sheet and the second steel sheet and the two adjacent large diameter steel piles comprises a grout column within the seabed material, where the grout columns is located near a joint between a large diameter steel pile and a steel sheet. The grout column has contact to the large diameter steel pile and the grout column has contact to the steel sheet. This reduces the permeability of the reservoir wall as grout has a low permeability. As the grout column is in physical contact with both the large diameter steel pile and the steel sheet, the flow of water through the joint between the large diameter steel pile and the steel sheet is impeded. This increases the efficiency of the pumped storage facility, as less water will flow into the reservoir unintentionally, i.e. not through the pump-turbine system. This reduces the losses when storing power within the reservoir.

Further the risk of piping and washing away of fill material is reduced. Piping and washing away of fill material can weaken the reservoir wall and ultimately result in reservoir wall failure if not counteracted. By grout column within the seabed material is meant a volume filled with a mixture of the seabed material and grout. The grout column has a larger vertical extend than horizontal extend.

In an embodiment, the grout column is created by means of jet-grouting. Thereby the grout column is constructed in a fast and simple manner after the seabed material has been filled into the compartment formed by the first steel sheet and the second steel sheet and the two adjacent large diameter steel piles. In an embodiment, the grout column reaches at least from the original seabed level and up to the average sea level. Thereby flow of water through the joint between a large diameter steel pile and a steel sheet is impeded in the majority of the length of the joint. In an embodiment, the reservoir wall forms a round or substantially circular reservoir with a diameter of at least 1 km. A round or substantially circular shape of the reservoir results in the largest storage capacity per reservoir wall length and thus the lowest unit costs of storage capacity for a given length of reservoir wall. By round or substantially circular is also meant regular polygon shapes with more than 12 sides. Considering a circular reservoir, the costs for the reservoir wall scales with the diameter, whereas the surface area inside the reservoir and thus the energy storage capacity scales with the square of the diameter. This means that the bigger the reservoir is the lower are the unit costs for the energy storage. Therefore the reservoir has to be more than 1 km in diameter in order to be feasible. By diameter of the reservoir is meant largest distance across the reservoir, passing through the center and measured from the inner edge of the reservoir wall. In an embodiment, the pump-turbine system is installed below the seabed level of the inner reservoir. By installing the pump-turbine system below the level of the seabed of the inner reservoir, it is possible to pump out more water of the reservoir without risking cavitation within the pump-turbine, compared to a pump-turbine installed at a higher level. This means more energy can be stored within the reservoir, thus maximizing the storage capacity of a reservoir of a given size.

By installed below the seabed level of the inner reservoir is meant that the distributor centerline of the pump-turbine system is below the average level of the seabed inside the inner reservoir. In an embodiment, the pump-turbine is installed 2 m or more below the average level of the seabed inside the inner reservoir.

In an embodiment, the pumped storage facility comprises at least one pump- turbine installed in a separate power house structure, where the power house structure is embedded within the seabed and located in the inner reservoir near the reservoir wall. The pump-turbine is connected to the sea via a pipe passing through the reservoir wall above seabed level. Thereby a simple integration of the pump-turbine unit is achieved, relatively independent of the reservoir wall construction progress. The power house structure is a separate construction and does not support the reservoir wall structurally. By near the reservoir wall is typically meant at a distance less than 50 m. However if the pumped storage facility comprises a berm located on the reservoir side of the reservoir wall, the distance might be increased due to the size of the berm. By connected to the sea via a pipe is meant that the pump-turbine can pump water from the inner reservoir to the surrounding sea through the pipe and that water from the surrounding sea can run through the pipe into the inner reservoir while driving the pump-turbine. As the pipe passes through the reservoir wall above seabed level the installation of the pipe is simpler compared to the pipe passing through the wall beneath seabed level.

Brief description of the figures fig. 1 shows an offshore pumped storage facility;

fig. 2 shows two large diameter steel piles;

fig. 3 shows a top view of sheet walls extending between two adjacent piles; fig. 4 shows a power house structure and a cross section of a reservoir wall; and

fig. 5 shows a top view of a joint between a steel sheet and a steel pile. Detailed description

Figure 1 shows an offshore pumped storage facility (101 ) with a reservoir wall (102) comprising multiple prefabricated large diameter steel piles (103). The reservoir wall (102) is damming up water and separating an inner reservoir (104) from the surrounding sea (105). The inner reservoir (104) is shown emptied from water. Steel sheets (106) are extending between adjacent large diameter steel piles (103). The level of the seabed in a part of the inner reservoir is lowered (107) as a result of the seabed material used as fill material (109) for the large diameter steel piles (103) has been excavated from a seabed area inside the inner reservoir (104). A connection from the pump-turbine system (108) to the surrounding sea (105) is shown. Figure 2 shows two large diameter steel piles (103) positioned adjacently and embedded into the seabed (201 ). The seabed (201 ) is shown with a portion cut out to illustrate the penetration of the piles (103) into the seabed (201 ). The large diameter piles (103) are partially filled with seabed material (109). The large diameter steel piles (103) shown are a single unit with a length to diameter ratio of about 1 .3. A second steel sheet (202) is extending between the two adjacent large diameter steel piles (103), such that a compartment (203) is formed by the first steel sheet (106) and the second steel sheet (202) and the two adjacent large diameter steel piles (103). The compartment (203) is partially filled with seabed material. The steel sheets (106, 202) extending between the steel piles (103) also penetrate into the seabed (201 ), Figure 3a-c shows a top view of sheet walls (106, 202) extending between two adjacent large diameter steel piles (103).

Figure 3a shows two adjacent piles (103) with a steel sheet (106) extending between them.

Figure 3b shows two adjacent piles with a first steel sheet (106) and a second steel sheet (202) extending between them. The shown steel sheets (106, 202) are sheet piling steel sheets. A compartment (203) is formed by the first steel sheet (106) and the second steel sheet (202) and the two adjacent steel piles (103). The compartment (203) is at least partially filled with seabed material.

Figure 3c shows two adjacent piles with a first steel sheet (106) and a second steel sheet (202) extending between them. The shown steel sheets (106, 202) are arched steel sheets. A compartment (203) is formed by the first steel sheet (106) and the second steel sheet (202) and the two adjacent steel piles (103). The compartment (203) is at least partially filled with seabed material and comprises four grout columns (301 ).

Figure 4 shows a separate power house structure (401 ) and a cross section of a reservoir wall (102) damming up water and separating an inner reservoir (104) from the surrounding sea (105). The power house structure (401 ) is located in the inner reservoir (104) near the reservoir wall (102) and is embedded within the seabed (201 ). A pump-turbine is installed in the power house (401 ) and is connected to the sea (105) via a pipe (402) passing through the reservoir wall (102) above seabed level. The level of the seabed of a part of the inner reservoir is lowered (107). A berm (403) is located on the reservoir side of the reservoir wall (102). The water level of the inner reservoir (404) is lower than the water of the surrounding sea (105) and thus energy is stored in the pumped storage facility. Figure 5 shows a top view of joints (501 ) between a steel sheet (106) and two adjacent steel piles (103). The large diameter steel piles (103) comprises a substantially vertical flange (502) welded to the outer side of the pile, providing a joint (501 ) between the large diameter steel piles (103) and the steel sheet (106) extending between the large diameter steel piles (103). A compartment (203) formed by the first steel sheet (106) and a second steel sheet and the two adjacent large diameter steel piles (103) comprises a grout column (301 ) located near a joint (501 ) between a large diameter steel pile (103) and a steel sheet (106). The grout column (301 ) has contact to the large diameter steel pile (103). The grout column has contact to the steel sheet (106). Steel T-profiles for reinforcement (503) of the large diameter steel piles (103) are shown.