WO2013044976A1

WO2013044976A1 - A set of building elements for an offshore power storage facility

Info

Publication number: WO2013044976A1
Application number: PCT/EP2011/067121
Authority: WO
Inventors: Lars Stig Nielsen; Janus MÜNSTER-SWENDSEN
Original assignee: Seahorn Energy Holding ApS
Priority date: 2011-09-30
Filing date: 2011-09-30
Publication date: 2013-04-04

Abstract

A set of elements (101) configured to be assembled at an offshore location as a portion of a reservoir for damming up water, said set of elements comprising: a first and a second wall supporting element (102, 103) each with a first contact surface (104, 105); a wall panel having at each of its respective ends a second contact surface (106); wherein the wall supporting elements and the wall panel are configured for an installation of the wall supporting elements at a nominal distance; wherein the first and second contact surfaces are complementary in the sense that they form a contiguous contact over a substantial portion of the height of the wall panel when positioned adjacently; and wherein the contact surfaces of either the wall panel or the wall supporting elements have a horizontally, uniform shape such that said contiguous contact between the contact surfaces of the wall supporting element and the wall panel is formed also if the distance between the wall supporting elements deviates from the nominal distance. Further, there is provided an offshore reservoir and a method of building an offshore reservoir.

Description

A set of building elements for an offshore power storage facility

This invention relates to the construction of an offshore power storage facility, a set of elements for assembling an offshore wall, a reservoir and a method of installing a set of elements for an offshore power storage facility.

Background

Renewable energy is receiving much focus due to the dependency on fossil fuels of the current power supply and the mitigation of man-made climate changes. Renewable energy output from wind turbines, wave energy converters, solar panels and other renewable energy converters is intermittent or irregular depending on the presence and strength of wind, waves and sunlight. Conventional fossil fuel operated power plants generate power regulated to meet an estimated demand by the consumers. Power is supplied to the consumers via existing distribution systems. However, in frequently occurring situations the demand from the consumers does not match the presence and strength of wind, waves and sunlight.

This causes difficulties since it is not possible to store significant amounts of power in conventional distribution systems.

Conventional fossil fuel operated power plants are regulated to constantly balance consumption and production of power. Further, a stable power supply is a necessity for a modern society, and the power consumption is highly inflexible and follows, to a large extent, a daily cycle. Therefore, it is challenging to meet the power demand with energy from intermittent renewable resources.

Power storage is a viable solution to this challenge. Power can be stored when renewable energy production is high and demand is low and be supplied to the power system when renewable energy production is low and demand is high. Thus power storage enables a higher share of intermittent renewable energy in a power system and reduces the need for backup capacity of conventional fossil fuel power production. The storage of power as hydro potential is a well known and proven technology. But the conventional hydro power solutions including storage require an area to be dammed up, causing severe changes to the local environment. In many places the conventional hydropower has limited resources as it is competing for the land with for instance buildings and recreational areas. Further, there is an increasing pressure for renewable energy converters such as wind turbines to be located away from human residential and recreational areas.

But offshore constructions are often difficult and costly to construct, which can lead to increased prices of the energy delivered by the offshore facility. An offshore environment is less controllable than an on shore environment, and weather is an important factor in offshore construction work. Often bad weather causes so-called downtime, where construction work has to be discontinued. Costly measures have to be taken to secure accurate handling and placement during construction work. Thus, conventionally, many forces work against cost-efficient production, conversion and storage of renewable energy offshore.

Related prior art

WO2009123465 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. Summary

There is provided a set of elements configured to be assembled at an offshore location as a portion of a reservoir for damming up water, said set of elements comprising a first and a second wall supporting element each with a first contact surface and a wall panel having at each of its respective ends a second contact surface. The wall supporting elements and the wall panel are configured for an installation of the wall supporting elements at a nominal distance. The first and second contact surfaces are complementary in the sense that they form a contiguous contact over a substantial portion of the height of the wail panel when positioned adjacently. The contact surfaces of either the wall panel or the wall supporting elements have a horizontally uniform shape, such that said contiguous contact between the contact surfaces of the wall supporting element and the wall panel is formed also if the distance between the wall supporting elements deviates from the nominal distance.

This configuration reduces the costs of building a wail in offshore conditions, as the requirements to the offshore installation machinery and the weather sensitivity of the offshore installation process are reduced. The offshore wail is assembled from multiple wall elements, which can be manufactured individually with dimensions that give advantages of precast construction while being manageable by machinery designated to the installation process. Compared to in situ casting techniques, sensitivity to bad weather is significantly reduced.

Further, the contiguous contact that is formed between the wall supporting elements and the wall panel also if the distance between the wall supporting elements deviates from the nominal distance, reduces the accuracy required of the positioning of the wall supporting elements for assembling the offshore wall. This in turn reduces the required positioning accuracy of the installation machinery. The nominal distance is the length between the installed wall supporting elements, when the set of elements is placed as it is designed for. But the offshore conditions mean that the placing of the wall supporting element at the nominal distance is difficult and the actual distance between the wall supporting elements often differs from the nominal distance. The horizontal uniformity of the contact surfaces ensures that a contiguous contact is established also if the actual distance deviates from the nominal distance, such that the set of elements can be assembled. At the contiguous contact, the two contact surfaces are abutting within a tolerance area acceptable for constructions of this size. Due to the texture of the contact surfaces and manufacturing tolerances, there may be small gaps between the contact surfaces along the contiguous contact, but these gaps are very small compared to the height of the contact surfaces. The complementary contact surfaces have corresponding profiles, such that when placed adjacently they form a contiguous contact over a substantial portion of the height of the contact surfaces.

The wall supporting element is an element that can provide at least some foundation and/or support for a wall. The wall supporting element is installed on the seabed as a single unit or multiple units, but it is configured for connection with a contact surface of a wall panel. Typically, the longest dimension of the wall supporting element is the height, when the wail supporting element is in an installed position. The wail pane! is an element that comprises a wall face or a wall surface area with a larger horizontal expanse than the wall supporting elements. A 'wall panel' or 'panel' is also denoted a 'wail face element'. A wall panel is configured to connect to/be assembled with at least two wall supporting elements.

The set of elements is assembled by interconnecting the wall panel to the wall supporting elements. Interconnection is also referred to by the term joining within this technical field. The contact surfaces of the wall panel are abutting respective contact surfaces on the wall supporting elements and are joined together by means of a wet or dry interconnection to establish a joint. Generally, joints can be classified as wet and dry. Wet joints are constructed with cast-in-place concrete poured between the precast waif elements. To ensure structural continuity, protruding reinforcing bars from the wall elements (dowels) are welded, looped, or otherwise connected in the joint region before the concrete is placed. Dry joints are constructed e.g. by bolting or welding together steel plates or other steel inserts cast into the ends of the precast wall elements for this purpose. Wet joints more closely approximate cast-in-place construction, whereas the force transfer in structures with dry joints is accomplished at discrete points. Combinations of wet and dry joints are also viable for interconnecting wall elements.

The set of elements is assembled into an offshore wall for damming up water. A single offshore wall element of the same size is difficult to position accurately, especially considering that offshore conditions can mean significant wind and wave loading, unless construction is restricted to limited time windows with sufficiently calm weather conditions. Constructing the offshore wall as a set of elements with reduced size makes the handling and positioning of each element easier. The reduced installation accuracy requirements of the wall supporting elements further reduce the needed accuracy and thus enable the offshore installation to take place in conditions with significant wave and wind loading and thus reduce the weather sensitivity of the offshore installation process. Further, the reduced accuracy requirements enable the use of a wider range of installation vessels which typically lowers the overall installation costs. Said installation vessels can be e.g. a tow boat, a jack-up barge, a boat with a crane, a catamaran, a barge or external floating elements. Typically, the wall panels are larger and thus more difficult to handle than the wall supporting elements. By installing the wall supporting elements on the seabed first, they can provide fixed structures to be used to advantage for facilitating the positioning of the wall panel. Since the set of elements can be handled easier and positioned with less accuracy, the assembling of the offshore wall from the set of elements becomes simpler and faster compared to conventional offshore building methods. The fast and simple assembly process of the set of elements enables the cost-saving measure of prefabrication by manufacture of the elements individually on-shore, where weather-protected series production with conventional manufacturing techniques is possible. The elements are then transported to an offshore erection site for onsite assembly and installation.

Thus, a reduction of the costs for erecting an offshore wall is achieved. Less costly machinery can be utilized, and installation processes can take place in a wider range of weather conditions. In an embodiment, the wall supporting elements are configured to provide foundation for the offshore wall and are positioned and installed at the seabed prior to the wall panel. Thereby the location of the offshore wall is effectively determined by the positioning of the wall supporting elements, and the wall panel can be positioned by establishing a contact between the contact surfaces of the wall panel and the contact surfaces of the installed wall supporting elements. Thereby the wall panel inherits the positioning accuracy of the wall supporting elements. This is a great advantage in conditions where the wall panel is more difficult to position than the wall supporting element, e.g. due to a large surface area being affected by wind, waves and ocean currents. By the term 'providing foundation' is intended 'transferring a least some load to the seabed and helping to fixate the offshore wall to the seabed'.

In an embodiment, the first contact surfaces of the wall supporting elements are placed on that half of the wall supporting element that is facing towards the highest water level when the offshore wail is damming up water. This configuration ensures that the hydrostatic pressure from the water dammed up is pushing the contact surfaces of the wail panel towards the contact surfaces of the wall supporting elements. Thus a good connection with good load transferring capabilities between wall panel and wall supporting element is ensured, as the hydrostatic loads on the wall panel are transferred to the wall supporting element as compressive load across the connection. This enables the wall supporting elements to act as foundation for the wall panel, which reduces the total offshore wall installation costs. In an embodiment, the contiguous contact between the contact surfaces of the wall supporting element and the wall panel is formed as long as the distance between the wall supporting elements does not deviate from the nominal distance with more than about 10%, 5%, 1 % or 0.1 % of wall panel length. Thereby the required positioning accuracy of the set of elements is affected by the size of the set of elements to the effect that larger, and thus less manageable, elements require less positioning accuracy. This reduces the costs of installing a set of elements.

In an embodiment, the contiguous contact between the contact surfaces of the wall supporting element and the wall panel is formed as long as the distance between the wall supporting elements does not deviate from the nominal distance with more than about 5 m, 3 m or 1 m.

In an embodiment, the contact surfaces of the wail supporting elements and the wall panel are plane and the contact surfaces of the wall supporting elements are either substantially vertical or inclined backwards by less than about 15 degrees or less than about 30 degrees. Thereby the contact surfaces of the wall panel are able to abut on the contact surfaces of the wall supporting elements also when the wall panel is placed in a position above its designated location on the seabed, and to maintain this contact also when the wall panel moves vertically relative to the wall supporting element, thus allowing wall supporting elements installed on the seabed to guide the wall panel as it is lowered to the designated location on the seabed. The wall panel thus inherits the accuracy of the placement of the wall supporting elements.

In an embodiment, the first contact surface of the wall supporting element and the second contact surface of the wall panel are complementary in such a way that they are abutting so ciosely that the contiguous contact formed between them establishes a seal tight enough to withhold grouting material, e.g. grouting material in a column with a height of about 30 m, 20 m, 10m or 5 m. Thereby the wall panel, the wall supporting element and the contiguous contact between them can form part of an enclosure filled with grouting material to provide a strong interconnection between wail panel and wall supporting element.

In an embodiment, the set of elements comprises bolts placed between the wall panel and the wall supporting element, thus connecting the wall panel to the wall supporting elements. Thereby the wall panel and the wall supporting elements are interconnected with a well proven and known technology.

In an embodiment, the contiguous contact, which is formed between the wall supporting elements and the wall panel, is formed in the entire height of the wall panel. Thereby a larger contact area between wall supporting element and wall panel is established and thus a better load transfer is ensured. This reduces the stress within the contact surface material and thus the requirements and thereby the costs of the contact surface material.

In an embodiment, the set of elements or a portion thereof is made of concrete or reinforced concrete. Thereby a relatively inexpensive set of elements with high strength and durability is ensured. Further, a set of elements made of concrete or reinforced concrete is suitable for the corrosive offshore environment. The reinforced concrete is to be reinforced with e.g. bars or fibres. The reinforcement material can e.g. be steel, glass fibre, polypropylene fibre or synthetic fibre, e.g. carbon fibre. In an embodiment, the wall panel or the wall supporting elements or both consists of multiple smaller segments assembled to the single wall panel or wall supporting element by post tensioning at the manufacturing site. Thereby, the manufacturing process is easier and thus less expensive as the shorter segments can be handled better during the manufacturing process and can more easily be cast in one continuous process, which ensures a strong element.

In an embodiment, the first and second wall supporting elements are substantially similar in shape and size. That means that, albeit the height of the wall supporting elements can change in accordance with changes in water depth at the designated location on the seabed, substantially similar manufacturing, transportation and installation processes are ensured, resulting in overall cost reductions. In an embodiment, the wall panel comprises one or more hollow compartments within the wall. Thereby the weight of the wall panel is reduced whereby the wall pane! floats more easily. This makes transportation and installation of the wall panel easier. The hollow compartments of the wall panel can subsequently (i.e. during or after positioning of the wall element at a designated position) be filled with a filler material, e.g. concrete, stone or sand. By the term 'hollow compartments' is intended a hollow section within the wall panel. These hollow compartments can have an opening through the top of the wall panel when in an upright position or they can be fully closed on all sides.

In an embodiment, the wall panel comprises a lower portion which is substantially vertical and an upper portion which is inclined from the vertical towards the side where the wall supporting elements are placed, said upper portion being configured to break the waves of the sea. Thereby, the peak load magnitude on the wall panel from waves is reduced, since waves impacting on the inclined portion of the wall panel will have a reduced peak load magnitude impact on the wall. The portion inclined from vertical is smaller than the substantially vertical portion, e.g. with a vertical height of about 5%, 10%, 15% or 20% of the height of the substantially vertical portion. The inclination angle of the inclined portion is less than 45 degrees from the vertical, e.g. about 5 degrees, 25 degrees or 35 degrees.

In an embodiment, the wall panel has such vertical dimension as to make it reach from the seabed to above the sea level; thereby, the water is separated into the reservoir side and the sea side.

In an embodiment, the wall panel has such vertical dimension as to make it reach from the seabed to near the sea level, but still below the sea level. Thereby wave loading on the wall panel during at least a part of the installation phase is reduced, as the wall panel is not exposed to the wave forces near the sea level where they are largest. Additional wall elements are later added to make the reservoir wall reach above the sea level. In an embodiment, these additional wall elements contain wave energy converters. The wave energy converters are e.g. floaters on lever arms that are driven by the wave motion and then drive a generator via a hydraulic system thus converting wave energy to electricity, or other wave energy converters.

In an embodiment, the height of the wall pane! is adapted to the water depth at the designated location on the seabed of the wall panel. The height of the wall panel above sea level is adapted to local wave conditions, but is typically a minimum of 2 meters above normal sea level. The overall height of the wall panel is at least 15 meters when it is in an upright position. Thereby the wall panel is suited for constructing an offshore reservoir with a possible height difference of more than 10 meters between the inside reservoir and the surrounding body of water, which is necessary for the reservoir to be of commercial interest.

In an embodiment, the wall panel has plane contact surfaces that lie in the same substantially vertical plane. Thereby manufacturing of the wall panel is simplified, as plane substantially vertical contact surfaces in the same substantially vertical plane are easier to manufacture than curved contact surfaces or surfaces with different inclinations. Thereby, the manufacturing costs of the wall panel are reduced.

In an embodiment, the first and second wall supporting element are installed with their first contact surfaces such that theoretical horizontal extensions of the contact surfaces meet between the first and second wall supporting elements. Thereby it is ensured that the contact surfaces of the wail supporting elements have a projected component in the direction parallel to the centreline between the wall supporting elements, such that a contiguous contact between the contact surfaces of the wall panel and the wail supporting elements can be established also if the wall panel is shifted in the direction of the centreline between the wall supporting elements. In an embodiment, the theoretical extensions meet substantially halfway between the wall supporting elements. Thereby a symmetric setup of the set of elements is ensured, which reduces manufacturing and installation costs.

In an embodiment, the wall supporting elements each has a significantly smaller vertical cross section area than the area spanned by the length and the height of the wall panel, e.g. about 20 %, 15 % or 10%. Thereby it is ensured that only a relatively low loading from sea waves and ocean currents on the wall supporting elements occurs while the wall supporting elements are installed standing alone on the seabed compared to an assembled set of elements. Thus a high survivability of standalone wall supporting elements is ensured. The high survivability of standalone wall supporting elements coupled with the fast installation of the wall panel on the wall supporting elements enables a shortening of the period from the installation of the first wall panel to the last wall panel, in which the reservoir structure is more vulnerable, as the wall supporting elements can be installed earlier and survive the offshore environment until a period suitable for installing the wail panels occurs. By the term 'contact surface' is intended 'a surface suitable for contact with another contact surface and for transfer of larger forces, e.g. hydrostatic forces on a reservoir wall'. The contact surface has a smooth surface without protrusions, but it can contain cavities and recesses for the purpose of sealing and/or interconnecting, e.g. by grouting. In an embodiment, the contact surfaces are smooth enough for a wall panel and a wall supporting element, with abutting contact surfaces forming a contiguous contact, to slide relative to each other when one of them is affected by a resulting force smaller than or equal to the weight of that element. The second contact surfaces located at the respective ends of the wall panel are not necessarily located on the end face of the wall panel: they may just as well be located on a side of the wall panel near the end of the wall panel, e.g. less than about 1 m, 3 m or 5 m from the end of the wall panel. By the term 'horizontally uniform shape' is intended that the contact surface is similar along a horizontal axis, e.g. flat or with constant curvature. The contact surface must have a component parallel to the centreline between the wall supporting elements, when the set of elements is in the designated position, in order for the contiguous contact to be formed also if the distance between the wall supporting elements deviates from the nominal distance. In an embodiment, the wall supporting elements comprise a pile for rooting the wall supporting element to the seabed. This configuration allows the pile to provide foundation for the wall supporting element which can then provide foundation for the wall panel and thus the offshore wall. The loads on the wall panel are transferred to the seabed via the pile and the offshore wall remains in position. Further, as the wall supporting elements are rooted to the seabed, the wall panel can utilize the wall supporting element as offshore fix points during positioning and as guides when it is lowered down to the designated position on the seabed.

In an embodiment, the pile is made from steel; thereby it has high strength in terms of tension, shear and compression along with a high ductility, which means it can have a relatively low pile wall thickness and can be driven into the seabed. Pile driving is a relative inexpensive pile installation method compared to e.g. drilling.

In an embodiment, the pile is driven into the seabed by a hammer or it is vibrated or drilled into the seabed or by any combination of the three which are techniques already known from pile installation.

In an embodiment, the pile diameter is at least 2 m, e.g. 4 m or 6 m; thereby ensuring a foundation capability sufficient for withstanding large hydrostatic forces. In an embodiment, the pile is made from reinforced concrete and is assembled from multiple short pile pieces. Thereby it is constructed from relatively inexpensive materials. Further, concrete is relatively insensitive to the corrosive effects of an offshore environment. In an embodiment, the pile is constructed at the reservoir site with deep- mixing techniques. Deep-mixing techniques are in situ soil treatment technologies whereby onsite material is mixed with cementitious and/or other materials. Such use of onsite materia! to construct the pile entails cost savings on materials and material transportation. Further a deep mixing pile is installed upon construction, meaning that costs are saved in that the pile is constructed and installed in one step.

In an embodiment, the pile is positioned in a channel running through the wall supporting element in a substantially vertical direction, when the wall supporting element is in an upright position; wherein the channel is configured to guide the pile through the channel and into the seabed; and wherein the channel is, at least at a lower portion, configured to accommodate a portion of the pile for fixating the pile and the channel in a rigid interconnection. This configuration allows the pile, serving as foundation for the wall supporting element, to be installed after the wall supporting element is positioned on its designated location on the seabed. Further, it ensures correct placement of the piles and wall supporting element relative to each other. Also, the channel provides a shield against the offshore environment which allows for subsequent interconnection e.g. by casting between the pile and the wall supporting element without complicated and time-consuming construction of formwork constructions exposed to the offshore wave and wind loads.

In an embodiment, the pile is an integral part of the wall supporting element, where the pile is integrated into the wall supporting element as part of the process of casting the wall supporting element. Thereby, a fast wall supporting element installation process is enabled, as the wall supporting element including foundation is fully installed on the seabed when the pile is rooted into the seabed. In an embodiment, the pile is positioned in a channel running through the wall supporting element in a substantially vertical direction, when the wall supporting element is in an upright position, before wall supporting element installation. The channel comprises guide pieces protruding from the inside surface of the channel towards the centre of the channel, and the guide pieces are configured to guide the pile through the channel. Thereby friction between pile and channel can be reduced e.g. by suitable choice of materials for the guide pieces, and the guiding pieces can hold the pile and the channel fixed relative to each other while a grouted connection is curing. A suitable choice of materials includes e.g. steel, concrete, polymer and/or a lubricant added.

In an embodiment, the wall supporting elements comprise a buttress structure, and the pile is a portion of this buttress structure either as an integral portion or part or positioned in a substantially vertical channel. Thereby the pile provides foundation for the buttress structure, while it becomes better at counteracting the overturning moment acting on the wall from the hydrostatic pressure, as the lever arm from the wall panel, where the hydrostatic pressure is acting, to the pile is increased. Thereby the stiffness of the pile can be reduced, as it is subjected to less bending. Reducing stiffness requirements of the pile reduces the pile costs.

By the term 'guiding the pile' is intended that the pile is prevented from significant movement in a substantially horizontal plane, but is allowed movement in a substantially vertical direction when the wall element is in upright position. Thereby it is ensured that the pile is aligned with the channel.

In an embodiment, a wall supporting element comprises a transition piece, fixed on top of the wall supporting element, where the transition piece is configured for the installation of a wind turbine. Thereby cost savings are achieved since the wall supporting element serves the dual purpose of being a wind turbine foundation and a support for the wall panel. The wall supporting element is fixated to the seabed to be able to support the wail panel. This fixation makes the wall supporting element well suited for also providing the foundation for a wind turbine. In an embodiment, the wall supporting element comprises a pile and the transition piece is secured to this pile. The transition piece can be secured to the pile by adapting means and methods known from the offshore wind turbine industry, e.g. a grouting process. Thereby installation techniques and equipment known from monopiie foundation for wind turbines can be implemented when installing the transition piece and the wind turbine. The use of well proven technology and existing equipment lower the installation costs.

In an embodiment, the wall supporting element comprises a pile and the transition piece comprises multiple legs to be secured to respective multiple piles, e.g. three piles, where one of these piles is the pile comprised in the wall supporting element. Thereby a solid wind turbine foundation is created, as this wind turbine foundation is better suited for resisting the overturning moment coming from the wind turbine, compared to a one pile foundation. Thereby different design constraints given by the wall supporting element design and the wind turbine design can be met with the same pile diameter, which is economically attractive.

By the term On top of the wall supporting element' is intended that the bottom of the wind turbine is on or above the top of the wall supporting element. This could be directly above the wall supporting element or anywhere above a horizontal plane at the top of the wall supporting element.

By the term 'wind turbine' is meant a wind turbine in the megawatt range, e.g. about 2 MW, 3.6 MW, 5 MW, 7 MW or 10 W in rated power. By 'transition piece' is intended a piece suitable for connection with the wall supporting element and a wind turbine. The transition piece can compensate if the wail supporting element is not as close to vertical as is required by the wind turbine. Examples of such transition pieces are found in the offshore wind turbine industry. The transition piece is typically made of steel.

In an embodiment, the set of elements comprises a buttress structure configured to counteract overturning moments acting on the wall panel, where the buttress structure is integrated in the set of elements or connected to the set of elements by mechanical fastening means, e.g. bolts. This configuration results in a reduction of the installation costs of a set of elements capable of resisting large overturning moments. The overturning moment acting on the wall face must be counteracted for the set of elements to stay upright in all weather conditions. The buttress structure utilizes the bearing capacity of the seabed in a distance from the bottom of the set of elements to counteract the overturning moment acting on the wall panel. The buttress structure converts at least a portion of the overturning moment acting on the wall panel to a substantially downwards force on the seabed or a pile or another buttress foundation. Thereby, the amount of the overturning moment to be counteracted by the foundation of the set of elements is reduced. This lowers the foundation costs for the set of elements and thus the installation costs. Further a buttress structure can result in material savings for the set of elements, as the buttress structure adds stiffness to the set of elements.

A buttress structure is a structure projecting from the set of elements to support and strengthen it. The buttress structure supports the set of elements by bracing it. The buttress structure may have a downwardly extending slope towards the seabed, at least for some length of the buttress structure. The buttress structure extends in a substantially transverse direction from the set of elements. In an embodiment, the buttress structure extends from a wall supporting element. Thereby the stiffness and ability to counteract overturning moments is added to the set of elements at an element already designed to support the wall.

In an embodiment, the buttress structure extends from a wall panel in a substantially transverse direction. Thereby material savings on the wall panel is enabled as stiffness is added to the wall panel.

In an embodiment, the buttress structure extends from the upper half of the set of elements, which is where the loading from sea waves mainly occurs. Thereby the stiffness of the upper half of the set of elements and its ability to withstand sea wave loading are increased.

In an embodiment, the buttress structure extends to a point substantially horizontal from the bottom of the set of elements and with a transversal distance to the set of elements of more than 15% of wall panel height. In an embodiment, the transversa! distance from the end of the buttress structure to the bottom of the set of elements is within the interval 25-65% of wall panel height. This is a good trade-off between ability to counteract overturning moments and material consumption of the buttress structure. In an embodiment, the buttress structure comprises a foot. Thereby the load from the buttress structure is distributed to a larger area of the seabed, reducing the pressure on the seabed and thus the needed bearing capacity per unit area of the seabed. This serves to ensure that the buttress structure will not sink into the seabed and thus lose its capability to counteract overturning moments acting on the set of elements. in an embodiment, the buttress structure extends from the set of elements on the side facing away from the highest water level, when the set of elements is damming up water. Thereby the forces are transferred to the seabed in compression which removes the need for anchoring the buttress structure and thus reduces the installation costs. Further, the buttress structure will be in compression which is advantageous if the buttress structure is made of concrete.

In an embodiment, the buttress structure is a solid wall.

In an embodiment, the buttress structure is a truss structure or a solid wall with a hole in it; thereby reducing material consumption compared to the solid wall buttress structure. In an embodiment, the set of elements comprises multiple buttress structures, thereby reducing the load on each buttress structure.

In an embodiment, the buttress structure is extending from the set of elements in a substantially perpendicular direction. Thereby it provides the best support for the set of elements against hydrostatic pressure acting on the side of the wall panel opposite that where the buttress structure is placed. Thereby the cost of the buttress support is reduced.

In an embodiment, multiple buttress structures of a set of elements are connected with a tensioned cable that is substantially horizontal. Thereby, firstly, stiffness is added to the buttress structures in the direction of the cable, and, secondly a slimmer structure of the buttresses is enabled. This reduces the material consumption and costs of the set of elements. In an embodiment, the lowest portion of the buttress structure is raised to a level corresponding to a seabed level, above the bottom of the set of elements, which is configured to sit in a trench in the seabed. Thereby, the volume of trenched material is reduced and so are the trench costs.

In an embodiment, the angle between the first contact surface of a wall supporting element and the centreline between the wall supporting elements is be!ow 45 degrees in a horizontal plane, and the contact surface is located on that side of the wall supporting element that is facing towards the side with the highest water level when the set of elements is damming up water. This configuration utilizes the nature of the load on the wail panel to ensure a good connection between the contact surfaces with good load transferring capabilities. The resulting hydrostatic forces acting on the wail panel from damming up water will primarily be transferred to the wall supporting elements as normal compression forces on the contact surfaces, thus pushing the contact surfaces together.

By the term 'centreline between the wall supporting elements' is intended a straight line from the centre of the first wall supporting element to the centre of the second wail supporting element. If the wall supporting element comprises a pile, the centre of a wall supporting element refers to the centre axis of the pile.

By the term 'below 45 degrees' is intended that the minimum angle between the contact surface and the centreline is below 45 degrees in a horizontal plane. This means that the angle between the contact surface and the centreline can be on both sides of the centreline. In case the contact surface of the wall supporting element is not plane, the direction of the contact surface refers to an average tangent of the contact surface.

In an embodiment, the first contact surfaces of the wall supporting elements are substantially parallel to the centreline between said wall supporting elements. Thereby the resulting hydrostatic forces on the wall panel are substantially perpendicular to the contact surfaces of the wall supporting element which will push the contact surfaces of the wall panel against the contact surfaces of the wall supporting element, thus ensuring a good connection.

In an embodiment, the first contact surfaces of the wall supporting elements form an angle to the centreline between the wall supporting elements, where this angle is more than 5 degrees, but below 45 degrees. Thereby the first contact surface of the wall supporting elements will help guide the wall panel into the right position, as normal forces from the first contact surfaces will steer a misaligned wall panel towards its right position between the first contact surfaces. Further, forces in a direction of ±90 degrees from the direction of the resulting hydrostatic forces on the wall pane! will have a larger component perpendicular to either of the first contact surfaces of the wall supporting elements, compared to first contact surfaces parallel to the centreline between the wall supporting elements. Thus, the connection between wall panel and the supporting elements is made stronger towards forces deviating from the direction of the resulting hydrostatic force, e.g. wave forces.

In an embodiment, the wall supporting elements each comprises a pile, and the first contact surfaces are oriented such that a perpendicular projection of the contact surface will intersect the pile surface and be fully absorbed on the pile surface area. Thereby normal compression forces on the first contact surface will be transferred to the pile mainly as compression forces within the wall supporting element. Compression forces reduce the risk of cracks building in the wall supporting element. Further, a wall supporting element made from concrete is strongest in compression. in an embodiment, the wall panel is arched around a substantially vertical axis, when the wall panel is placed in an upright position. The curvature is substantially homogeneous with a ratio of distance between wall panel end points to arc radius of between 1.67 and 0.25 or between 0.25 and 0.01. These configurations typically result in reduced materia! consumption, as the wall panel can be made thinner and longer. An arched wall face is stronger towards pressure forces on the outside of the arc compared to a straight wall, as the wall will experience less shearing loads within the wall.

When the ratio of distance between wall panel end points to arc radius is between 1.67 and 0.25, the arched wall face is locally transferring the forces acting on it to the wall supporting elements at the ends of the arched wall panel. The curvature of the wall enables the wall supporting elements at the end of arched wall panel to be distanced further apart and the arched wall panel to be thinner, compared to a straight wall panel. Thus a reduction in material consumption is achieved.

When the ratio of distance between wall panel end points to arc radius is between 0.25 and 0.01 , the wall panel is part of a larger arched reservoir wall with substantially the same radius as the wall panel. The arched reservoir wall will act as one arc, and the forces acting on the wall panel will be then transferred to other wall panels or other structures at the ends of the wall panel. Thus, the needed foundation capacity and the material consumption of the wall panel system are reduced. in an embodiment, the wall panel describes an arc with a radius substantially equal to the reservoir radius. Thereby the finished reservoir wall has substantially the shape of a circle when seen from above which enables equal loads on opposite sides of the reservoir to at least partially cancel out each other. Thus, the needed foundation capacity and the material consumption of the reservoir are reduced. In an embodiment, the arc axis is located on the side of the wall panel that will have the lowest water level when the wall panel is damming up water. Thereby the wall face arches out towards the higher water level, making the wall face stronger towards the hydrostatic forces from this higher water level towards the lower water level. In an embodiment, the lower water level when the wall element is damming up water is on the reservoir side of the wall element.

By the term 'homogenous curvature' is intended that the curvature is constant in the horizontal direction.

In an embodiment, the wall panel is arched around a substantially vertical axis, when the wall panel is placed in an upright position. The curvature is substantially homogeneous, and the arc radius at an upper portion of said wall panel is different from the arc radius at a lower portion. Thereby the different loading levels experienced at different heights of the wall panel are utilized to reduce material consumption, as wall curvature and thickness are adapted to the corresponding load of that height. Wave loading is most significant near the sea level, and the hydrostatic pressure increases with depth.

In an embodiment, the arc radius is gradually increasing from the bottom of the wail panel towards the top, with the arc axis on the reservoir side of the wall panel. Thereby the wail panel curvature is corresponding to the variations in hydrostatic pressure, as the arc radius gradually decreases with increasing hydrostatic pressure, forming a more curved and thus stronger wall with increasing hydrostatic pressure.

In an embodiment, the portions of the wall panel with different arc radiuses have constant arc radius themselves, and the transition between the portions is either gradual or a substantially horizontal step. In an embodiment, the first contact surfaces of the wall supporting elements and the second contact surfaces of the wall panel are substantially plane. Thereby the manufacturing of the contact surfaces is simplified and hence less costly. Further, the contiguous contact between contact surfaces of the wail supporting elements and the wall panel is more easily achieved.

In an embodiment, there is one substantially plane contact surface covering the entire width of a wall supporting element. Thereby, the deviation from the nominal distance between the wall supporting elements, at which the contiguous contact between the contact surfaces of the wall panel and the wall supporting element is still formed, is made as large as possible as a wall panel can connect to the entire width of the wall supporting element. In an embodiment, the wall supporting element comprises two substantially plane contact surfaces, one for each of the two neighbouring wall panels.

In an embodiment, the contact surfaces of the wall panel are substantially plane and are lying in substantially the same vertical plane; thereby the manufacturing process of the wall panel becomes easier, which, in turn, reduces manufacturing costs.

By the term 'plane' is intended that the contact surface is flat and without curvature or protrusions; however, it can contain cavities and recesses for the purpose of sealing and/or interconnecting.

In an embodiment, the first contact surfaces of the wall supporting elements or the second contact surfaces of the wall panel or both are arched around a substantially vertical axis, when the set of elements is in an upright position. This configuration ensures a contiguous contact between the contact surfaces also if a wall supporting element is misaligned in a direction transverse to the centreline between the wall supporting elements. A curved contact surface will maintain the contiguous contact - even with the rotation of the wall panel around a vertical axis caused by the misalignment of the wall supporting element. This reduces the accuracy required for the positioning of the wall supporting elements, and therefore also the requirements to the machinery performing this positioning.

In an embodiment, the contact surfaces that are connecting to the arched contact surfaces are plane. Thereby the ability to form a contiguous contact if the wall supporting elements are misaligned is maintained, and the manufacturing process of the element with plane contact surface is simplified.

In an embodiment, the wall panel is arched, and the contact surfaces of both the wall supporting elements and the wall panel are arched with a curvature substantially corresponding to that of the wall panel. Thereby a large contact area is established which ensures good load transferring capabilities. A contiguous contact over a large area reduces the stress within the contact surface material and thus the requirements and thereby the costs of the contact surface material.

In an embodiment, a wall supporting element comprises two curved contact surfaces forming a v-shape in a horizontal plane. In an embodiment, the wall panel comprises a foot that has a width of at least 10% of the height of the wail panel. Consequently, the weight of the wall panel is distributed onto a larger area of the seabed which reduces the need for additional foundation of the wall panel. Further, the reduced need for additional foundation of the wall panel reduces the requirements to the connection between the wall panel and the wall supporting elements, as the loads that need to be transferred by the connection are reduced. Further, a foot will increase the distance that water will have to travel through the seabed to move from outside the reservoir to inside the reservoir, thus impeding the flow of water through the seabed underneath the wall panel. In an embodiment, the width of the foot is within the interval of 15-45% of the wall panel height. Thereby the carrying capacity of the foot is ensured, while the material use for the foot is kept at a minimum. The exact width of the foot will depend on the bearing capacity of the seabed at the reservoir site and the wall panel weight.

In an embodiment, the foot is not extending in the full length of the wall panel. For instance, the foot may be divided into portions or multiple foots. Thereby material is saved by adding a foot only underneath one or more portions of the wall panel. Further, the foot may be cut off or may be reduced in size towards the ends of the wall panel so as to allow an easier connection between the wall panel and the wall supporting elements, since the foot is thereby not in the way close to the ends of the wall panel where the connection is to be established. In an embodiment, the wall pane! is made of concrete, and the foot is capable of supporting the wall panel by itself at the manufacturing site. Thereby the wall panel can stand by itself while the concrete is curing, thus reducing manufacturing costs. In an embodiment, the foot is placed asymmetrically on the wall panel, with the foot placed more to the reservoir side of the wall panel. Thereby the ability of the foot to resist any overturning moment acting on the wall panel from outside the reservoir is increased. !n an embodiment, the foot comprises bracing supports; thereby increasing stiffness of the foot, which possibly reduces material consumption and costs of the foot. In an embodiment, the wall panel comprises a skirt extending into the seabed below the bottom of said wall panel, where the skirt is configured to impede the flow of water through the seabed underneath the wall panel. Thereby the nature of the seabed, which can be penetrated by a skirt, is utilized to impede flow of water underneath the wall panel, which would reduce the height difference between the reservoir water level and the surrounding water level and thus the amount of energy stored within the reservoir. Further, a reduced flow of water underneath the wall panel will limit scouring around and underneath the wall panel. This is advantageous as scouring can lead to foundation failure.

In an embodiment, the skirt is connected to the wall pane! with a connection impermeable to water. Thereby flow of water between the skirt and the wall panel is prevented. In an embodiment, the skirt reaches into a layer with a hydraulic conductivity of 10^("6) m/s or a layer with a hydraulic conductivity of 10^("7) m/s or lower. Thereby the deeper layers of the seabed are utilized to keep the flow of water underneath the skirt to a minimum. In an embodiment, the skirt is fixed to the wall panel such that it extends below the bottom of the wall panel, before the wall panel is lowered into place on the seabed. Thereby the skirt follows the wall panel and is thereby automatically inserted into the seabed when the wall face element is lowered into place. Consequently, further installation work on the skirt is significantly reduced or avoided. Further an impermeable connection between the wall panel and the skirt can easily be established when the skirt is being installed on the wall panel before the wall pane! is lowered into position on the seabed.

In an embodiment, the skirt is inserted into the seabed after the wall panel has been installed on the seabed.

In an embodiment, the skirt is made from sheet piling.

In an embodiment, the skirt is made of steel or vinyl or a combination thereof.

In an embodiment, the wall panel comprises a foot, and the skirt is embedded within the foot of the wall panel; thereby the foot is used to guide the skirt and hold it place during the installation of the skirt. In an embodiment, the skirt is constructed at the reservoir erection site using deep-mixing techniques. Deep-mixing techniques are in situ soil treatment technologies whereby onsite material is mixed with cementitious and/or other materials. Thereby onsite material is used to construct the skirt, thus whereby costs for materials and material transportation are reduced. Further, a skirt constructed with deep-mixing techniques increases the soil bearing capacity and thus enables the skirt to add to the foundation of the wall panel, thus reducing material use and costs for other foundation for the wall panel. The skirt constructed from deep-mixing techniques can for instance be constructed as disclosed in connection with co-pending application with the title "Method of building an offshore power storage facility and an offshore reservoir", filed on the same day, by the applicant of this present application, e.g. in connection with figure 1 and the description thereof on page 3-5 and page 55-56, where the 'coherent foundation' is described and serves as a skirt. In an embodiment, the bottom face of the wall panel comprises a recess extending throughout the length of the wall panel, configured for encompassing the top portion of the skirt and for establishing an impermeable connection with the top portion of the skirt, e.g. by grouting the recess when the wall panel is positioned on the seabed. Thereby the skirt and the wall panel are interconnected in a fast and simple way, saving time and reducing costs.

By the term 'skirt' is intended a barrier with the purpose of hindering flow of water through the seabed. It is inserted into the seabed to a depth of e.g. about 1 m or 3 m or 5 m.

In an embodiment, the contact surfaces of either the wall supporting elements or the wall panel comprises a substantially vertical recess comprising a gasket or a bag installed within it; wherein said gasket protrudes from the recess; and wherein said bag is configured to be filled with a filler material during the installation of the set of elements. This configuration enables a sealed interconnection between the wall panel and the wall supporting elements to be established by the gasket or by the bag filled with a filler material. The sealed interconnection is formed by the gasket or the filled bag protruding from the recess and adapting to the abutting contact surface, thus creating an impermeable contiguous connection between the contact surfaces of the wall panel and the wall supporting element also if either element is misaligned from the vertical. Further, the placement within the recess protects the gasket or the bag during transportation and positioning of the wall elements.

In an embodiment, the filler material in the bag is grout or water or air or epoxy or polyurethane {e.g. known from expanding insulation foam from the building industry). In an embodiment, the recess and the gasket or bag has a length substantially equal to or higher than the height of the wail pane!. Thereby it is ensured that the connection between wall supporting element and wall panel is impermeable in the entire height of the wall panel. This prevents water from flowing into the reservoir between the wail panel and the wall supporting element.

In an embodiment, the bag is filled with a fluid filler material before the placement of the wall panel. Thereby the bag acts as a buffer between the wall supporting element and the wall panel, as it is known from marine fenders. This prevents collisions between wall panel and wall supporting elements from causing damage. In an embodiment, the wall supporting elements comprise an in situ cast portion that covers a substantial portion of the end face of the wall panel, where the in situ cast portion is constructed after the set of wall elements has been positioned at the designated location on the seabed. This configuration ensures a rigid and impermeable connection between the wall supporting element and the wall panel. The in situ cast portion adapts to the placement of the wall panel and wall supporting element relative to each other, which may vary due to the horizontally uniform shape of the contact surfaces and the way in which the set of elements is assembled. An impermeable connection is established that adjusts to the specific placement of each set of elements, and this connection enables loads to be transferred between the wall elements. This reduces the assembly costs as the connection does not need to be customized to fit the specific placement of each wall panel and wall supporting element. Further, the in situ cast portion is capable of transferring loads with a direction along the wall panel as compression stress within the in situ cast portion, as a substantial portion of the end face of the wail panel is covered by the in situ cast portion. This ensures a more durable connection as such loads are not transferred as shear forces between the contact surfaces of the wall supporting element and the wall panel.

In an embodiment, the in situ cast portion covers a substantia! portion of the end face of a neighbouring wall panel as well. Thereby both wall panels are included in the same in situ casting operation, thus reducing the number of offshore in situ casting operations and thereby the installation costs.

In an embodiment, the in situ cast portion is made in an enclosed volume, whereby said enclosed volume is established with a formwork element extending between the wall panel and the wall supporting element or between the wall panel and a neighbouring wall panel. Thereby the in situ cast portion can be established fast and easily as the wall elements form some of the sides of the enclosed volume and only one formwork element has to be positioned. This is a cost-efficient way of establishing an enclosed volume for the in situ casting process. Said formwork element can be of e.g. steel plates.

In an embodiment, the in situ cast portion is made in an enclosed volume sealed off by a formwork element, where the formwork element is pulled towards the wall supporting element by one or more cables. Thereby the enclosed volume is made fast and easily, as the installation process of the formwork element is simplified. This is a cost efficient way of establishing an enclosed volume for the in situ casting process.

In an embodiment, the set of elements comprises multiple wall panels configured for a water impermeable interconnection with each other along a substantially horizontal division when stacked on top of each other. This configuration allows for easier handling of the wall panels as the full height wall is constructed by stacking wall panels that are lower than a single wall panel of full height. The reduced size of the wall panels enables easier handling during manufacturing, transportation and installation, which all contributes to a lower cost. Further, the water impermeable connection enables the wall element system to be used as a part of a reservoir for damming up water.

In an embodiment, multiple wall panels are configured for being stacked, and they are positioned at their designated positions by contact being established between the contact surfaces of the wall panels and the wall supporting elements, and by the wall panels being slid down along the contact surfaces of the wall supporting elements. Thereby, the positioning and alignment of the stacked wall panels are controlled by the wall supporting elements, which simplifies the installation process of multiple stacked wall panels.

In an embodiment, a gasket is installed between the horizontal divisions of the wall panels. Thereby, the weight of the wall panel is used to advantage to establish a water impermeable interconnection between the stacked wall panels.

In an embodiment, the connection between horizontal divisions of two wall panels seals off a confined enclosure, between the wall panels, which is isolated from the surrounding sea. Thereby, the weight of the wall panel is used to advantage to create a grouting enclosure that, when grouted, forms a rigid and water impermeable interconnection between the stacked wall panels.

In an embodiment, the stacked wail panels are interconnected using post tensioning methods. Thereby, the stacked wail panels are firmly interconnected by use of a well known and proven technology. This ensures that the assembled wall is strong and durable, while enabling easy manufacture and handling of the individual wall panels due to their reduced height compared to a full height wall panel. By 'post tensioning' is intended to apply tension to cables or rods running through channels in the structure after the structure has been constructed. Thereby, the structure is held together and reinforced by the cable or rods. The channels can subsequently be filled with a filler material, e.g. grout. Post tensioning is known from the construction industry.

In an embodiment, the wall panel is longer than 20 meters or 40 meters or 60 meters or 80 meters. Thereby, the total reservoir costs are reduced, as the longer the lengths of the wall panels, the fewer transportations and installations are necessary for a given total reservoir wall length. Transportation constraints on size and weight are less important at sea compared to land transportation. However, the longer the lengths of the wall panels, the more difficult they are to handle during manufacturing, transport and installation. Thus the length of a wall panel depends on manufacturing facilities and transport and installation equipment available.

In an embodiment, a wall panel is interconnected to more than two wall supporting elements, e.g. 3 or 4 or 5. Thereby, the wall panel can be made longer than if it connects to two wall supporting elements, as the wall panel is supported in more places, and thus can be longer without having a longer span between wall supporting elements interconnected with the wall panel. There is also provided an offshore reservoir comprising a set of elements.

In an embodiment, a first wall panel and a second wall panel are arranged to both abut on a first wall support element at their respective second contact faces. Thereby, the first wall supporting element supports two wall panels, thus making the process of erecting a reservoir fast and simple, as the first wall supporting element is utilized to connect two wall panels. The contact surfaces of the first wall supporting element is configured to form a contiguous contact with a contact surface of the first wa!l panel and a contact surface of the second wall panel. These contiguous contacts enable a sealed interconnection to be established and thus the assembled wall to form at least a part of a reservoir enclosure. The reduced positioning accuracy requirements of the wall supporting elements and their reduced size make the installation process of the wall supporting elements fast. Wall supporting elements installed at the seabed benefit from a high survivability in the offshore environment also if the wall panels are not yet installed. The complementary contact surfaces of the wall supporting elements and the wall panels ensure a fast assembly of the offshore wall at the offshore reservoir site. The fast and easy placement and connection of the set of elements reduce the offshore installation time, which, in combination with the high survivability, makes the erection of the offshore reservoir fast and safe.

As a cost saving measure, the set of element are manufactured onshore, where weather protected series production with conventional manufacturing techniques is possible and are then transported to an offshore erection site for installation.

In an embodiment, a first wall supporting element and a first and a second wall panel are interconnected by an in situ grouting process of one or more confined volumes between the wall supporting element and the wall panels. Thereby a rigid and impermeable interconnection is created which is able to adapt to the size of the confined volume.

In an embodiment, an intermediate piece is positioned between the ends of the first and the second wall panel for wet and/or dry interconnection with the wall panels. The intermediate piece may be positioned e.g. in front of the wall supporting element, on that side of the wall supporting element facing the surrounding sea. A backside (i.e. a side opposite that side facing the surrounding sea) of the intermediate element may abut that side of the wall supporting element facing the surrounding sea. Alternatively the intermediate piece is configured to leave a gap between the backside and the side of the wall supporting element facing the surrounding sea e.g. for being filled with a filler material. The intermediate piece may be constructed from a composition of materials similar to the wall panels. The intermediate piece may be constructed with a reinforcement structure similar to the wall panels. The intermediate piece may have a shape complementary to the shape of the ends of the wall panels.

In an embodiment, a wall supporting element and a wall panel are interconnected by means of a post tensioning process. Thereby a strong and durable connection is ensured by means of a well known and proven technology.

In an embodiment, the wall supporting elements for a reservoir are placed in a substantially circular pattern. Thereby the highest possible inner area for a given reservoir wall length is provided and thus the highest possible reservoir volume. A higher reservoir volume means that more energy can be stored within the reservoir.

In an embodiment, the overall shape of the reservoir, comprised by multiple sets of elements, is substantially circular when viewed from above. Consequently the reservoir provides the highest possible inner area for a given reservoir wall length, and thus the highest possible reservoir volume, but requires the wall panels to be arched with an arc radius substantially equal to the reservoir radius, and the wall supporting elements to be placed along the circumference of the circle. A higher reservoir volume means more energy can be stored within the reservoir. In an embodiment, the wall panels of a reservoir are straight and of substantially equal length, and the reservoir forms a substantially regular polygon. Consequently the reservoir provides the highest possible inner area for a given reservoir wall length and thus the highest possible reservoir volume, when considering a reservoir wall constructed from straight wall pieces. A higher reservoir volume means that more energy can be stored within the reservoir. In an embodiment, the substantially regular polygon formed by the reservoir has more than 12 sides, e.g. 14 sides or 20 sides. In an embodiment, the wall supporting elements of a set of elements are installed at the seabed, and the wall panel is then subsequently connected to the wall supporting elements. Thereby the wall supporting elements, which are easier handled than the wall panel due to their compacted size, are installed first providing local fix point for the installation of the wall panel for an easier installation of the long wall panel. Further, the compact size of the wall supporting elements gives them a high survivability in an offshore environment. Thereby the wall supporting elements can be installed at the offshore reservoir site and survive until the wall panel is installed. In an embodiment, the length of the wall panels comprised in the wall of the reservoir enclosure is substantially equal. Thereby the nature of the load which is a hydrostatic load is used to advantage to reduce the constructional complexity and cost of the set of elements. By the term 'sealed' is intended 'impermeable to water under a pressure corresponding to that at a water depth equal to the height of the offshore reservoir'.

In an embodiment, an offshore reservoir comprises a pump/turbine system, where, in a first mode, the pump/turbine system is configured to drain the reservoir by using electricity and, in a second mode, the pump/turbine system is configured to fill the reservoir with water from the sea while producing electricity. Consequently, the offshore reservoir is able to store energy as gravitational potential energy of water, where the height is determined by the difference between the surrounding water level and the water level inside the reservoir. The energy is stored by pumping water from inside the reservoir to the surrounding sea, thus emptying the reservoir. The energy is reproduced by letting water from the surrounding sea into the reservoir through a turbine driving a generator. The height difference between the sea and the reservoir determines how much energy can be extracted from a given amount of water. The offshore reservoir can e.g. be operated with a period when the reservoir is drained by the pump, where the pump consumes energy; and a period when the reservoir remains in an at least partially drained state; and a period when the reservoir is filled, where the turbine generates energy. These periods can be controlled e.g. by energy prices and/or energy demand.

In an embodiment, the pump/turbine system is configured to handle more than 50 m³ of water per second. in an embodiment, the pump/turbine system comprises a pumping unit and a turbine unit integrated in a pump turbine unit. Thereby the space and material needed for the pump/turbine system are reduced as fewer units are required. This reduces the costs of the pump/turbine system. In an embodiment, the pump/turbine system is installed at or below seabed level. Thereby the pressure provided by the water inside the reservoir is utilized to reduce problems with cavitation within the system which may lower the performance and cause damage to the system. Further, the possible height difference between the surrounding sea and the water inside the offshore reservoir, and thus the energy storage capacity of the offshore reservoir, is maximized. In an embodiment, the pump/turbine system is installed in a separate housing facility located inside the reservoir and communicates with the sea through a pipe system. Thereby the pump/turbine system is easily installed below seabed level, as the separate housing facility can be located and installed separately. Further a separate housing facility enables the pump/turbine system to be preinstalled on shore within the separate housing facility. In an embodiment, the pump/turbine system is installed in a separate housing facility located underneath a wall panel. Thereby the pump/turbine system has ready access to both sides of the reservoir wall.

In an embodiment, the pump/turbine system is installed within a wall supporting element or a wall panel. Thereby, the pump/turbine system can be preinstalled in the wall supporting element onshore and thus reduces the number of offshore installation operations.

In an embodiment, the opening from the pipe system to the sea is located close to the seabed, e.g. less than 3 m away. Thereby pipe losses in the pipe system are reduced as the pump/turbine system is also located close to the seabed, and the length of the pipe system can be minimized.

In an embodiment, a method of installing a set of elements comprises: installing at least one additional wall panel on top of a first wall panel, where the additional wall panels are connecting to the same wall supporting elements as the first wall panel. This configuration allows for easier handling of the wall panels as the full height wall is constructed by stacking of wall panels that are lower than one single wall panel of full height. The reduced size of the wall panels enables easier handling during manufacture, transportation and installation, all of which contribute to a lower cost. In an embodiment, the top wall panel comprises wave energy converters. Thereby the reservoir wall for damming up water is utilized as foundation for wave energy converters, thus reducing the total cost of reservoir and wave energy converters. The wave energy converters can be e.g. floaters on lever arms or oscillating water column wave converters.

In an embodiment, a method of installing a set of elements comprises: manufacturing a set of elements at a manufacturing facility onshore, where the manufacturing facility has launching means for launching the set of elements into sea. A first wall supporting element is ferried to an erection site for a reservoir and is installed at a designated location on the seabed. A second wall supporting element is ferried to the erection site for the reservoir and is installed at a designated location on the seabed, as a neighbouring element to the first wall supporting element, with about the nominal distance to the first wall supporting element. A wall panel is ferried to the erection site of the reservoir and is placed at a designated location on the seabed, with the contact surfaces of the wall panel abutting on contact surfaces of the first and the second wall supporting element.

Thereby manufacturing and installation costs are lowered. An onshore manufacturing environment is more controlled than an offshore manufacturing environment, which results in a standardized manufacturing process that lowers manufacturing costs and ensures a homogenous quality of the wall elements. Further, the installation process of installing wall supporting elements at the seabed and then subsequently installing the wall panel reduces the installation costs, as the wall supporting elements are easily handled, and the wall panel is installed while using the wall supporting elements as guides. In an embodiment, the wall supporting elements or the wall panel or both are kept afloat by use of external floaters, Consequently, the elements are not required to be self-buoyant to be transported afloat, thus reducing material consumption by enabling a less constrained design of the wall element system.

In an embodiment, one or more external floaters are attached to a wall supporting element or a wall panel with cables and keep the element floating in an upright position. Thereby the need for heavy duty offshore handling vessels is reduced as the element can be lowered to the designated location on the seabed at the reservoir site in an upright position by slackening of the cables.

In an embodiment, a wall supporting element or a wall panel or both are towed on a barge; thereby using a known and well proven method for transportation at sea.

In an embodiment, a wall supporting element or a wall panel or both are self- buoyant, thereby enabling them to be ferried with the use of tow boats only.

In an embodiment, the wall supporting elements comprises a pile as an integral part of the wall supporting element. Thereby the wail supporting element is installed on the seabed when said pile is rooted into the seabed. In an embodiment, a wall supporting element comprises a pile positioned in a channel and is installed on the seabed by horizontally positioning the wall supporting element above its designated location on the seabed. Then the pile is lowered down through the channel to the seabed, where the channel guides the pile during the lowering process. Then the wall supporting element is lowered down to the seabed, where the pile guides the wall supporting element during the lowering process. Then the pile is installed into its final position within the seabed. The pile and the wall supporting element are then interconnected. Thereby the pile and the channel are utilized to position the wall supporting element on the seabed in a cost-efficient way. In an embodiment, a wall supporting element comprises a pile positioned in a channel and is installed on the seabed by positioning of the wall supporting element in its designated position on the seabed; then the pile is rooted into the seabed, where the pile is guided by the channel; and then the pile and the wall supporting element are interconnected.

By the term 'launching into sea' is intended 'transferring the set of elements from land to sea, where the set of elements is either floating by internal or external means or positioned on a boat or a barge'. By the term 'ferrying' is intended 'transported at sea'. It can be e.g. onboard a boat, a barge towed by a boat or the set of elements itself is towed by a boat, where the set of elements is either self-buoyant or kept afloat by external floaters attached to it, or the set of elements can be ferried by external floaters attached to it, where the external floaters have propulsion means.

By the term 'external floaters' is intended objects with high buoyancy designed for making wall supporting elements and/or wall panels float, , e.g. a buoy or a rigid shell structure. The wall elements can either float one by one, together as a set of elements or as multiple wall elements.

In an embodiment, the set of elements is installed at seabed by: installing a third wall supporting element at a designated location on the seabed as neighbouring element to a second wall supporting element, with about the nominal distance to the second wall supporting element. A wall panel is placed at a designated location on the seabed, with the contact surfaces of the wall panel abutting on contact surfaces of the second and third wall supporting element. This configuration allows for an offshore reservoir to be constructed from multiple sets of elements. The sets of elements are interconnected by sharing wall supporting elements and are installed sequentially, allowing for the individual elements to be manufactured onshore in a more controlled environment than the offshore reservoir site. On shore manufacturing lowers manufacturing costs.

In an embodiment, a method of installing a set of elements comprises: dredging a trench in the seabed at the designated location of a set of elements before installation of the set of elements. The trench is at least the length of the wall panel and wide enough for the set of elements to be installed at the bottom of the trench. The set of elements is then installed in the trench. Thereby any irregularities of the seabed are removed and a plane surface for the set of elements to rest on is ensured. Thus, any load concentrations within the set of elements due to an uneven seabed are avoided and the risk of unforeseen settling of the set of elements is reduced.

In an embodiment, the trench is substantially level. Thereby, it becomes easier to install the wall supporting elements in a substantially vertical position, which makes connection with neighbouring wall panels easier and enables installation of wind turbines that need to be substantially vertical on the wall supporting elements.

In an embodiment, the trench is constructed in substantially level sections, where each section has a length corresponding substantially to one or more wall panels. Thereby minimum dredging is required to provide a substantially level seabed for all wall panels, as each dredged section needs to compensate for water depth differences only along the length of a single wall panel. Further it is ensured that the steps between substantially level sections of the trench with different heights are located at the wall supporting elements which means that each wall panel is easier to manufacture as it will have a substantially level bottom and a constant height corresponding to the water depth at its substantially level section of the trench. The wall supporting elements are configured to be installed in the steps of the trench. This configuration reduces the costs of both the dredging of the trench and the manufacture of the wall panels.

In an embodiment, a gravel bed is laid in the trench. Thereby a level and stable surface for the set of elements is ensured. In an embodiment, a wall panel or a wall supporting element comprises a buttress structure, and the trench has such width that the buttress structure rests at the seabed outside the trench. Thereby the width of the trench is reduced which reduces the amount of dredging needed to construct the trench. in an embodiment, a method of installing a set of elements comprises: establishing a first cable connection between a first wall supporting element installed at the seabed and a floating wall panel and establishing a second cable connection between a second wail supporting element installed at the seabed and said floating wall panel. The first and second cable connection are used to pull the floating wall panel into position, such that there is contact between the contact surfaces of said wall panel and the contact surfaces of the wall supporting elements. This configuration allows the floating wall panel to be positioned in an easy way without the use of heavy duty offshore vessels. Thereby the cost of installing the wall panel is reduced. The wail panel can be made to float by the internal buoyancy of the wall panel or by one or more external floaters attached to the wall panel.

In an embodiment, the floating wall panel is lowered down to the designated position on the seabed while the tension is kept on the cable connections. Thereby contact between the contact surfaces of the wall panel and the wall supporting element is ensured during the lowering process, where said contact surfaces are sliding relative to each other. This prevents uncontrolled movement of the wall panel during the lowering process to its designated position at the seabed.

In an embodiment, the cable connections have a downward angle from the wall panel to the wall supporting elements. Thereby the wall panel is pulled both towards the seabed and towards the wall supporting elements and thus towards the designated position of the wall panel at the seabed.

In an embodiment, an in situ casting process is performed when the wall pane! is at its designated position at the seabed, where said cable connection is encapsulated within the in situ cast portion. Thereby the cable connection will act as reinforcement in the in situ cast portion.

In an embodiment, a vessel is connected with a cable to the wall panel as well, but located on the side of the wall panel which is opposite the wall supporting elements. Thereby the vessel is capable of counteracting the pull from the cables and thus of controlling the movement of the wall panel to a steady and controlled positioning. The vessel could be e.g. a tow boat.

By 'cable connection' is intended a connection capable of transferring mainly tension forces and, to a lesser degree, compression or shear forces, such as a cable. The mentioned cable connection could also be a connection with a rope, a wire, a chain or a wire rope.

In an embodiment, a method of installing a set of elements comprises: lowering a wall panel from a floating transport mode to a floating positioning mode and positioning the wall panel such that there is contact between the contact surfaces of the wall panel and the contact surfaces of a first and a second wall supporting element. Then the wall face is lowered to its designated location on the seabed. Thereby the wind loading on the wall panel, during the final positioning to its designated position on the seabed, is reduced as the wall panel floats in a lower position and is thus easier to position. Further, the vertical distance from the wall panel to the designated location on the seabed is reduced, such that the wall panel can reach the designated location on the seabed faster once it is positioned correctly with contiguous contact between the contact surfaces of the wall supporting elements and the contact surfaces of the wall panel. In an embodiment, the lowering process is performed by means of external floaters attached to the wall panel with cables, by slackening of the cables. Thereby the need for heavy duty offshore vessels to lower the wail panel onto the seabed is reduced. This reduces installation costs. In an embodiment, the wall panel is lowered by reducing the buoyancy that keeps it afloat, e.g. by filling ballast rooms with water. Thereby the need for heavy duty offshore vessels to lower the wall panel onto the seabed is reduced. This reduces installation costs. By 'floating transport mode' is intended that the wall panel is floating with a low draft, e.g. about 6 m or 8 m or 10 m; such that the wall panel can be ferried out from the manufacturing facility to the reservoir erection site.

By 'floating positioning mode' is intended that the wall panel floats with a high draft, such that the distance from the seabed to the wall panel is minimal, e.g. [ess than 1 m.

In an embodiment, a method of building a reservoir installed at seabed by: installing neighbouring sets of elements sharing a wall supporting element; where the sets of elements, at their respective designated locations at the erection site, collectively forms an enclosure. This configuration provides a cost-efficient erection of an offshore reservoir, as sets of elements manufactured onshore can be interconnected to form an offshore reservoir. Onshore manufacturing lowers the manufacturing costs of the sets of elements.

Brief description of the figures

fig. 1 shows a set of elements;

fig. 2 shows the orientation of contact surfaces of different sets of elements; fig. 3 shows an assembled set of elements comprising piles, foot and a skirt; fig. 4 shows different connections between a wall supporting element and wali panels;

fig. 5 shows different reservoir walls assembled from sets of elements with arched wall panels;

fig. 6 shows a set of elements comprising multiple wall panels;

fig. 7 shows a reservoir wall being assembled and installed in a trench in the seabed;

fig. 8 shows a reservoir for storing power; and

fig. 9 shows a wall panel being ferried from the on shore manufacturing facility to the offshore reservoir site.

Detailed description

Figure 1 shows an exploded view of a set of elements 101. The set of elements 101 comprises a first and a second wall supporting element 102; 103 each with a first contact surface 104, and an arched wall panel 105 having at each of its ends a second contact surface 106. The wall supporting elements 102; 103 are installed with a nominal distance between them, and the first contact surfaces 104 are facing towards the highest water level when the offshore wall is damming up water, i.e. the wall supporting elements 102; 103 are on the reservoir side of the wall panel 105. The arched wall panel 05 has a length matching the nominal distance between the wall supporting elements 102; 103, such that the wall pane! 105 can connect to both wall supporting elements 102; 103. The arched wall panel 105 is arched around a substantially vertical axis when the wall panel 105 is placed in an upright position. The curvature is substantially homogeneous and the arc radius at an upper portion 107 of the wall panel is different from the arc radius at a lower portion 108. The arc radius is gradually increasing from the bottom of the wall element towards the top, with the arc axis on the reservoir side. The reservoir side of the wall panel 105 is the side where the wall supporting elements 102; 103 are placed. This is the right side of the figure. The wall supporting elements 102; 103 comprise a pile 109; 110 for rooting the wall supporting elements 102; 103 to the seabed. The seabed is not shown. One wall supporting element 102 comprises a pile 109 that is an integral part of the wall supporting element 102 and is integrated in the wall supporting element 102 as part of the process of casting the wall supporting element 102. One wall supporting element 103 comprises a pile 1 0 positioned in a channel 111 extending through the wall supporting element 103 in a substantially vertical direction. The contact surfaces 106 of the wall panel and the contact surfaces 104 of the wall supporting elements are substantially plane. The wall supporting elements 102; 03 both have one substantially plane contact surface 104 covering the entire width of the wall supporting element 102; 103. The contact surfaces 106 of the wall panel are substantially plane and are lying in substantially the same vertical plane.

Optionally, for the purpose of installation, a first and a second cable connection 112 are established between the wall panel 105 and the first and second wall supporting elements 102; 103 installed at the seabed. These cable connections 12 can be used to pull the wall panel 105 into position such that there is contact between the contact surfaces 106 of the wall panel 105 and the contact surfaces 104 of the wall supporting elements 102; 103. The cables 112 are shown without a winch or other means necessary for pulling the wall panel 105 towards the wall supporting elements 102; 103. The arrows show how the wall panel 105 and the wall supporting elements 102; 103 are placed relative to each other when the wail panel reaches its designated position on the seabed.

Figure 2a-c shows a top view of three different sets of elements 101 with different orientations of the contact surfaces 204; 205; 206 of the wall supporting elements 102; 103; 201 and the contact surfaces 106; 207; 208 of the wall panels 202; 203. The wall panels 202; 203 have at each of their respective ends a second contact surface 106; 207; 208. The angle 210 between the contact surfaces 204; 205; 206 of the wail supporting elements 102; 103; 201 and the centreline 209 between the wall supporting elements 102; 103; 201 is below 45 degrees in a horizontal plane. The contact surfaces of either the wall panel 106; 207; 208 or the wall supporting elements 204; 205; 206 have a horizontally uniform shape such that a contiguous contact is formed also if the distance between the wall supporting elements 204; 205; 206 deviates from the nominal distance. The contact surfaces of the wall panel 106; 207; 208 and the wall supporting elements 204; 205; 206 are complementary in the sense that they form a contiguous contact over a substantial portion of the height of the wall panel 106: 207; 208, when adjacently positioned.

Figure 2a shows an embodiment with a straight wall panel 202 connecting to two wall supporting elements 103. The contact surfaces 204 of the wall supporting elements are substantially parallel to the centreline 209 between the wail supporting elements 103. Theoretical extensions 2 1 of the contact surfaces 204 of the wall supporting element 103 are coincident. The wall supporting elements 103 comprise a pile 110 positioned in a substantially vertical channel 111. The contact surfaces 106 of the wall panel 202 and the contact surfaces 204 of the wall supporting elements 103 are substantially plane. The contact surfaces 106 of the wall panel 202 are substantially plane and are lying in substantially the same vertical plane. Each wall supporting element 103 comprises two substantially plane contact surfaces 204, one for the wall panel 202 and one for a neighbouring wall panel. The neighbouring wall panels are not shown.

Figure 2b shows an embodiment with an arched wall panel 203 connecting to two wall supporting elements 201. The contact surfaces 205 of the wall supporting elements 201 and the contact surfaces 207 of the wall panel 203 are arched around a substantially vertical axis, when the set of elements 101 is in an upright position. The wall panel 203 is arched and the contact surfaces 205 of the wall supporting elements 201 and the contact surfaces 207 of the wall pane! 203 are arched with a curvature corresponding to that of the wail panel 203. The angle 210 between the contact surfaces 205 of the wail supporting elements and the centreline 209 between the wall supporting elements 201 is below 45 degrees and theoretical extensions 211 of the contacts surfaces 205 of the wall supporting elements 201 meet substantially halfway between the wall supporting elements 201. The wall supporting elements 201 comprise two curved contact surfaces 205 forming a v-shape in a horizontal plane.

Figure 2c shows an embodiment with an arched wall panel 203 connecting to two wall supporting elements 102. The wall supporting elements 102 comprise a pile 109 integrated in the wall supporting element 102 as part of the process of casting the wall supporting element 02. The contact surfaces 208 of the wall panel 203 are on the end surfaces of the wall panel 203. The angle 210 between the contact surfaces 206 of the wall supporting elements 102 and the centreline 209 between the wall supporting elements 102 is below 45 degrees and theoretical extensions 211 of the contacts surfaces 206 of the wall supporting elements 102 meet substantially halfway between the wall supporting elements 102. The contact surfaces 206 of the wall supporting elements 102 are oriented such that a perpendicular projection of the contact surface will intersect the pile 109 surface and be fully absorbed on the pile surface area. Figure 3 shows an assembled set of elements 101. The wall panel 203 is arched around a substantially vertical axis, when the wall panel 203 is placed in an upright position, and the curvature is substantially homogeneous. The wall panel 203 comprises a straight foot 301 that has a width that is at least 10 % of the height of the wall panel 203. The foot 301 is not extending in the full length of the wall panel 203. The wall panel 203 is positioned adjacently to two wall supporting elements 103; 304, and the contact surfaces of the wall panel 203 and the contact surfaces 205 of the wall supporting elements 103; 304 form a contiguous contact 303. The contact surfaces 205 of the wall supporting elements 103; 304 are arched around a substantially vertical axis with a curvature corresponding to that of the wall panel 203. The wall supporting elements 103; 304 each comprises a pile 110 positioned in a channel 111 for rooting of the wall supporting element 103; 304 to the seabed. One wall supporting element 304 comprises a transition piece 305, fixed on top of the wail supporting element 304. The transition piece 305 is configured for installation of a wind turbine. The shown transition piece 305 is installed on top of a pile 110 rooting the wall supporting element 304 to the seabed. Using a pile as foundation for a wind turbine is known from the offshore wind industry. The wall pane! 203 comprises a skirt 302 extending into seabed below the bottom of the wail panel 203. The skirt 302 is configured to impede flow of water through the seabed underneath the wall panel 203. The seabed is not shown. The skirt 302 shown is a sheet piling skirt and is shown only along a part of the wall panel 203. Typically, a skirt 302 extends in the full length of a wall panel 203, but it can also extend only a portion of the length of a wall panel 203. In an embodiment, the skirt of an offshore reservoir is installed continuously along the perimeter of the entire offshore reservoir.

The skirt can be installed on either the sea side or the reservoir side of a wall panel, or anywhere on the foot of a wall panel. Figure 4a-c shows a top view of different connections between a wall supporting element 201 and wall panels 202; 203; 401. Only respective end portions of the wall panels 202; 203; 401 are shown. Figure 4a shows an embodiment with two neighbouring arched wall panels 203 connecting to a wall supporting element 201 which comprises arched contact surfaces 205 with a curvature corresponding substantially to the curvature of the arched wall panels 203. Substantially vertical recesses 402 are shown in a contact surface 205 of the wall supporting element 201 and in a contact surface 207 of a wall panel 203. The purpose of the recesses 402 is to contain and protect a gasket or a flexible bag to ensure a sealed connection between wall panel 203 and wall supporting element 201. The recesses 402 are shown without a gasket or a flexible bag installed within them. The gasket will protrude from the recess 402, and the flexible bag will be configured to be filled with a filler material. The recesses 402 are proportionally too large compared to the rest of the figure. This is done to make the figure clearer. Typically, the recesses 402 are located on both contact surfaces 205 of the wall supporting element 201 or on the contact surfaces 207 of both the wall panels 203, but they can also be located as shown. Two formwork elements 403 are sealing off confined enclosures 404 to be used for an in situ casting process for creating in situ cast portions of the wall supporting element 201 that covers the end face of the wall panels 203. The in situ cast portions fixate the wall panels 203 to the wall supporting element 201 with a rigid and impermeable connection. Each formwork element 403 is positioned after the wall supporting element 201 and the wall panel 203 it connects to have been positioned at their designated locations on the seabed. Each in situ cast portion is made after the formwork element 403 sealing off the confined enclosure 404 is in place. Figure 4b shows an embodiment with two neighbouring wall panels with straight walls 202 connecting to a wall supporting element 201. The wall supporting element 201 comprises one plane contact surface 104 covering the entire width of the wall supporting element 201. Substantially vertical recesses 402 are shown in the contact surface 104 of the wall supporting element 201 and in a contact surface 106 of a wall panel 202. The purpose of the recesses 402 is to contain and protect a gasket or a flexible bag to ensure a sealed connection between wall panel 202 and wall supporting element 201. The recesses 402 are shown without a gasket or a flexible bag installed within them. The gasket will protrude from the recess 402, and the flexible bag will be configured to be filled with a filler material. The recesses 402 are proportionally too large compared to the rest of the figure. This is done to make the figure clearer. Typically, the recesses 402 are located on the contact surface 104 of the wall supporting element 201 or on the contact surfaces 106 of both the wall panels 202, but they can also be located as shown. A formwork element 403 is sealing off a confined enclosure 404 to be used for an in situ casting process for creating an in situ cast portion of the wall supporting element 201 that covers a substantial portion of the end face of a neighbouring wall panel as well. Thereby the end face of both the neighbouring wall panels 202 is included in one in situ cast portion. The in situ cast portion fixates the wall panels 202 to the wall supporting element 201 with a rigid and impermeable connection. The formwork element 403 is positioned after the wall panels 202 and the wall supporting element 201 is positioned at their designated locations on the seabed. The in situ cast portion is made after the formwork element 403 is in place, as the formwork element 403 is sealing off the confined enclosure 404.

Figure 4c shows an embodiment with two neighbouring wall panels with straight walls 401 connecting to the same wall supporting element 201. The contact surfaces 405 of the wall supporting element 201 are arched around a substantially vertical axis. The contact surfaces 106 of the wail panels 401 are substantially plane. The wall panels 401 are rotated around a substantially vertical axis due to a misalignment of the wall supporting elements (not shown) that the wall panels 401 connect to. The contiguous contact 303 between the contact surfaces 106 of the wall panels 401 and the contact surfaces 405 of the wall supporting element 201 is maintained. Formwork elements can still be installed and in situ casting processes can still be performed, but this is not shown.

Figure 5a-d shows a top view of different reservoir walls 501 assembled from sets of elements 101 with arched wall panels 203. The wall panels 203 are arched around a substantially vertical axis when the wall panel 203 is placed in an upright position, and the curvature is substantially homogenous. Multiple set of elements 101 share wall supporting elements 201 with the neighbouring sets of elements 101 , such that a wall supporting element 201 is comprised in two neighbouring sets of elements 101. Figure 5a shows an embodiment where the wall pane! 203 has a ratio of distance between end points 502 to arc radius 503 of between 1.67 and 0.25. The wall supporting elements 201 are placed on a straight line. The wall supporting elements 201 need not be placed on a straight line. They can be placed e.g. along an arc with a lower curvature than that of the wall panels 203. The curvature of the wall panels 203 is higher than the curvature of the line on which the wall supporting elements 201 are placed. Therefore there will be load concentrations at the wall supporting elements 201 when considering a hydrostatic load acting on the outside of the arched wall panels 203. Due to the high curvature of the wall panels 203, they can cover a long distance between the wall supporting elements 201.

Figure 5b shows an embodiment where the wall panel 203 has a ratio of distance between end points 502 to arc radius 503 of between 1.67 and 0.25. The wall supporting elements 201 are placed on a straight line. The wall supporting elements 201 need not be placed on a straight line. They can be placed e.g. along an arc with a Sower curvature than that of the wall panels 203. The curvature of the wall panels 203 is higher than the curvature of the line on which the wall supporting elements 201 are placed. Therefore there will be load concentrations at the wall supporting elements 201 when considering a hydrostatic load acting on the outside of the arched wall panels 203. Due to the high curvature of the wall panels 203, they can cover a long distance between the wail supporting elements 201.

Figure 5c shows an embodiment where the wall panel 203 has a ratio of distance between end points 502 to arc radius 503 of between 0.25 and 0.01. The wall supporting elements 201 are placed along an arc with substantially the same radius 503 as the wall panel 203. The wall panels 203 are part of a larger arched reservoir wall with substantially the same radius as the wall panel 203. This means there will be little or no load concentrations at the wall supporting elements 201 when considering a hydrostatic load on the outside of the arched wall panels 203. The loads are transferred through the neighbouring wall panels 203. Due to the low curvature of the wall panels 201 , they can cover a medium distance between the wall supporting elements 201. Figure 5d shows an embodiment where the wall panel 203 has a ratio of distance between end points 502 to arc radius 503 of between 0.25 and 0.01. The wall supporting elements 201 are placed along an arc with substantially the same radius 503 as the wall panel 203. The wall panels 203 are part of a larger arched reservoir wall with substantially the same radius as the wall panel 203. This means there will be little or no load concentrations at the wall supporting elements 201 when considering a hydrostatic load on the outside of the arched wall panels 203. The loads are transferred through the neighbouring wall panels 203. Due to the low curvature of the wall panels 201 , they can cover a medium distance between the wall supporting elements 201. Figure 6 shows a set of elements 101 comprising multiple wall panels 601 ; 602. The wall panels 601 ; 602 are configured for a water impermeable interconnection with each other along a substantially horizontal division when stacked on top of each other. The first wall pane! 601 is arched and comprises an ached foot 605. The first wall element 601 is positioned adjacently to the wall supporting elements 603; 604. The contact surfaces of the first wail panel 601 and the contact surfaces 104 of the wall supporting elements 603; 604 form a contiguous contact 303. In this embodiment the contiguous contact 303 is formed as both contact surfaces are substantially plane and substantially vertical. But the contiguous contact can also be formed by contact surfaces with other shapes as long as they are complementary, e.g. a substantially vertical and substantially plane contact surface and a substantially vertical and arched contact surface. The second wall panel 602 is arched and comprises plane contact surfaces 106 that will form a contiguous contact with the plane contact surfaces 104 of the wall supporting elements 603; 604 when adjacently positioned. When the second wall panel 602 is positioned, it will form a water impermeable connection between the top 606 of the first wall panel 601 and the bottom 607 of the second wall element 602. The second wall panel 602 will connect to the same wall supporting elements 603; 604 as the first wall panel 601. The wall supporting elements 603; 604 both comprise a buttress structure 608; 609 configured to counteract overturning moments acting on the wall panels 601 ; 602. The wail supporting elements 603; 604 comprise a pile 109; 110 for rooting the wall supporting element 603; 604 to the seabed. The seabed is not shown. One wall supporting element 603 comprises a pile 109 that is an integral part of the wall supporting element 603 and is integrated in the wail supporting element 603 as a part of the process of casting process the wall supporting element 603. The buttress structure 608 has a downwards slope towards the seabed. One wall supporting element 604 comprises a pile 110 positioned in a channel 111 extending through the wall supporting element 604 in a substantially vertical direction. The pile 110 is comprised in the buttress structure 609 where it is positioned in a substantially vertical channel 111. The buttress structure 609 has a downwards slope towards the seabed, at least for some length of the buttress structure.

In an embodiment the wall supporting elements are of a similar design to simplify the manufacturing processes and thus the manufacturing costs.

The wall panels 601 ; 602 can also be configured for a water impermeable interconnection with each other along a substantially horizontal division when stacked on top of each other if the wall supporting elements 603; 604 comprise a smaller buttress or no buttress at all.

Figure 7 shows a reservoir wall 501 being assembled and installed in a trench 701 in the seabed. The reservoir wall 501 comprises three wall supporting elements 103 with piles 702; 703; 704 positioned within channels 111 and two wall panels 202; 705 both comprising a foot 605; 706. One foot

605 is arched and not extending in the full length of the wall panel 705. One foot 706 is straight and extends in the full length of the wall panel 202. The piles 702; 703; 704 are shown in different installation levels. One pile 702 is installed such that the top of the pile is substantially in level with the top of the wall supporting element 103. One pile 703 is installed such that the top of the pile is about the same height as the top of the wall supporting element 103. This can be advantageous for instance if a transition piece is to be installed on top of the pile. One pile 704 shown reaches high above the wall supporting element 103, as it is not fully inserted into the seabed yet, but shown in a position it would be in during the installation phase. The pile installation equipment is not shown. The pile installation equipment could for instance be a hydraulic hammer. The wall supporting elements 103 are neighbouring wall supporting elements. One wall panel is straight 202 and is located adjacently to two wall supporting elements 103 forming a contiguous contact 303. The other wall panel 705 is arched and is yet to be positioned at the designated position adjacently to the wall supporting elements 103 with the contact surfaces 106 of the wall panel 705 at the contact surfaces 104 of the wall supporting elements 103, such that a contiguous contact is formed. The different types of wall panels 202; 705 are shown for illustration purposes. Typically either straight wall panels 202 or arched wall panels 705 would be used for the reservoir wall 501. But a reservoir wall 501 can also comprise different types of wall panels 202; 705. The seabed shown is a cutout section of a seabed that is not horizontal. A trench 701 has been dredged in the seabed at the designated location of the set of elements 01 before installation of the set of elements 101. The trench 701 is wide enough for the set of elements 101 to be installed on the bottom of the trench 701. The trench dredged in the seabed 701 is constructed in substantially level sections, where each section has a length substantially corresponding to one or more wall panels 202; 705. The sections in the trench 701 have a vertical distance between them. Such vertical steps are located at the designated location of a wall supporting element 103, such that the wall panels 202; 705 can be placed on a plane and level surface in the trench 701. This means that the heights of the wail supporting elements 103 and wall panels 202; 705 are configured for the depth at the designated location in the trench 701. The trench 701 can also be dredged such that there is oniy one level of the trench. The trench 701 is shown with a grave! bed 707 laid in a portion of the trench 701. The gravel bed 707 is shown in a portion of the trench 701 for illustration purposes. Typically a gravel bed would be added in the entire trench or not at all. One wall panel 705 is shown in a floating positioning mode. The arrows illustrate how the wall panel 705 in the floating positioning mode is positioned such that there is contact between the contact surfaces 106 of the wail panel 705 and the contact surfaces 104 of the wall supporting elements 03 and is then lowered down to its designated position on the seabed. The means to keep the wall panel 705 afloat and the means to position and lower the wall panel 705 are not shown. In an embodiment, the wall panels used to assemble a reservoir wall have the same shape.

Figure 8 shows an embodiment of a reservoir 801 for storing power. The reservoir 801 comprises a pump/turbine system 802 in a separate housing facility. The offshore reservoir 801 comprises multiple sets of elements 101 sharing wall supporting elements 201 with the neighbouring sets of elements 101 , such that a wall supporting element 201 is comprised in two neighbouring sets of elements 10 . The sets of elements 101 form a wall of a reservoir enclosure 801. The shown reservoir 801 comprises six wall supporting elements 201 and six arched wall panels 203. The wall supporting elements 201 are placed in a circular pattern. The wall supporting elements 201 comprise two contact surfaces 206 with an angle between them, configured for the construction of a reservoir with a general shape of a regular polygon. The wall panels 203 comprise an arched foot 605. The pump/turbine system 802 is installed in a separate housing facility located inside the reservoir 801 and communicates with the sea through a pipe system 803. Figure 9 shows a wall panel 901 being ferried from the onshore manufacturing facility 902 to the offshore reservoir site by a boat 903. A set of elements 101 has been manufactured at a manufacturing facility 902 onshore and is ready to be launched into the sea and ferried to the erection site for the reservoir. Two wall supporting elements 102 have already been ferried to the erection site for the reservoir and installed at their designated locations on the seabed as neighbouring wall supporting elements with about the nominal distance between them. A wall panel 901 is being ferried to the erection site of the reservoir. The wall panel 901 being ferried is straight and kept afloat with the use of external floaters 904. The wall face 901 is later assembled with the wall supporting elements 102 to form an offshore wall for damming up water. A skirt 905 is installed at the erection site for the reservoir prior to the installation of the wall panel 901. The skirt 905 shown is constructed at the reservoir erection site with deep-mixing techniques. The piles 906 shown for rooting the wall supporting elements 102 to the seabed are constructed at the reservoir site with deep-mixing techniques. Constructing both skirt 905 and piles 906 with deep-mixing techniques gives constructional benefits that save costs. Further it ensures that the reservoir has an impermeable barrier in the seabed along the entire perimeter of the reservoir.

Claims

1. A set of elements (101 ) configured to be assembled at an offshore location as a portion of a reservoir (80 ) for damming up water, comprising:

- a first and a second wall supporting element (102; 103) each with a first contact surface ( 04; 405);

- a wall panel (105; 202) having at each of its respective ends a second contact surface (106; 207); wherein the wall supporting elements and the wail panel are configured for installation of the wall supporting elements at a nominal distance; wherein the first and second contact surfaces are complementary in the sense that they form a contiguous contact over a substantial portion of the height of the wall panel when positioned adjacently; and wherein the contact surfaces of either the wall panel or the wall supporting elements have a horizontally uniform shape such that said contiguous contact between the contact surfaces of the wall supporting element and the wall panel is formed also if the distance between the wall supporting elements deviates from the nominal distance.

2. A set of elements according to claim 1 , wherein the wall supporting elements comprise a pile (109; 110) for rooting of the wall supporting element to the seabed.

3. A set of elements according to any of claims 1-2, wherein a wall supporting element comprises a transition piece (305), fixed on top of the wall supporting element; and wherein the transition piece is configured for installation of a wind turbine.

4. A set of elements according to any of claims 1-3, comprising a buttress structure (608; 609) configured to counteract overturning moments acting on the wall panel, wherein the buttress structure is integrated in the set of elements or connected to the set of elements by mechanical fastening means.

5. A set of elements according to any of claims 1-4, wherein the angle between the first contact surface of a wall supporting element and the centreline (209) between the wall supporting elements is below 45 degrees in a horizontal plane; and wherein the contact surface is located on that side of the wall supporting element that is facing towards the side with the highest water level when the set of elements is damming up water.

6. A set of elements according to any of claims 1-5, wherein the wail panel is arched around a substantially vertical axis, when the wall panel is placed in an upright position; and wherein the curvature is substantially homogeneous with a ratio of distance between wall panel end points to arc radius of between 1.67 and 0.25 or between 0.25 and 0.01. 7. A set of elements according to any of claims 1-6, wherein the wall panel is arched around a substantially vertical axis, when the wall panel is placed in an upright position; and wherein the curvature is substantially homogeneous and the arc radius at an upper portion of said wall panel is different from the arc radius at a lower portion.

8. A set of elements according to any of claims 1 -7, wherein the first contact surfaces of the wall supporting elements and the second contact surfaces of the wall panel are substantially plane. 9. A set of elements according to any of claims 1-7, wherein the first contact surfaces of the wall supporting elements or the second contact surfaces of the wall panel or both are arched around a substantially vertical axis, when the set of elements is in an upright position.

10. A set of elements according to any of claims 1-9, wherein the wall panel comprises a foot (301 ; 605) that has a width that is at least 10% of the height of the wall panel. 1 . A set of elements according to any of claims 1 -10, wherein the wall panel comprises a skirt (302) extending into the seabed below the bottom of said wail panel; where the skirt is configured to impede the flow of water through the seabed underneath the wail panel. 12. A set of elements according to any of claims 1-11 , wherein the contact surfaces of either the wall supporting elements or the wall panel comprises a substantially vertical recess (402) comprising a gasket or a bag installed within it; wherein said gasket protrudes from the recess; and wherein said bag is configured to be filled with a filler material during the installation of the set of elements.

13. A set of elements according to any of claims 1-12, wherein the wail supporting elements comprise an in situ cast portion that covers a substantial portion of the end face of the wall panel; wherein the in situ cast portion is constructed after the set of wall elements is positioned at the designated location on the seabed.

14. A set of elements according to any of claims 1-13, comprising multiple wall panels configured for a water impermeable interconnection with each other along a substantially horizontal division when stacked on top of each other.

15. A set of elements according to any of claims 1-14, wherein the wall panel is longer than 20 meters or 40 meters or 60 meters or 80 meters.

16. An offshore reservoir comprising a set of elements according to any of claims 1-15.

17. An offshore reservoir according to claim 16, wherein a first wall panel and a second wall panel are arranged to both abut on a first wall supporting element at their respective second contact faces.

18. An offshore reservoir according to claim 17, wherein the contact surface of the first wall supporting element is configured to form a contiguous contact with the contact surfaces of the first and the second wall panel.

19. An offshore reservoir according to any of claims 16-18, comprising a pump/turbine system, wherein, in a first mode, the pump/turbine system is configured to drain the reservoir by using electricity and, in a second mode, the pump/turbine system is configured to fill the reservoir with water from the sea while producing electricity.

20. A method of installing a set of elements according to any of claims 1-15, comprising: installing at least one additional wall panel on top of a first wall panel, wherein the additional wall panel are connecting to the same wall supporting elements as the first wall panel.

21. A method of installing a set of elements according to claim 1-15, comprising:

- manufacturing a set of elements at a manufacturing facility on shore, wherein the manufacturing facility has launching means for launching the set of elements into sea;

- ferrying a first wall supporting element to an erection site for a reservoir;

- installing the first wall supporting element at a designated location on the seabed; - ferrying a second wall supporting element to the erection site for the reservoir;

- installing the second wall supporting element at a designated location on the seabed, as a neighbouring element to the first wall supporting element, with about the nominal distance to the first wall supporting element;

- ferrying a wall panel to the erection site of the reservoir;

- placing the wall panel at a designated location on the seabed, with the contact surfaces of the wall panel abutting on contact surfaces of the first and the second wall supporting element.

22. A method of installing set of elements according to claim 21 , further comprising:

- installing a third wall supporting element at a designated location on the seabed as neighbouring element to the second wall supporting element, with about the nominal distance to the second wall supporting element; and

- placing a wall panel at a designated location on the seabed, with the contact surfaces of the wall panel abutting contact surfaces of the second and third wall supporting element. 23. A method of installing a set of elements according to claim 21 or 22, comprising:

- dredging a trench (701 ) in the seabed at the designated location of a set of elements before installation of the set of elements, where said trench is at least the length of the wall panel and wide enough for the set of elements to be installed on the bottom of the trench; and

- installing the set of elements in the trench.

24. A method of installing a set of elements according to claim 21 , comprising:

- establishing a first cable connection (112) between a first wall supporting element installed at the seabed and a floating wall panel; - establishing a second cable connection (112) between a second wall supporting element installed at the seabed and said floating wall panel; and

- using the first and second cable connection to pull the floating wall panel into position such that there is contact between the contact surfaces of said wall panel and the contact surfaces of said wall supporting elements.

25. A method of installing a set of elements according to claim 21, comprising:

- lowering a wall panel from a floating transport mode to a floating positioning mode;

- positioning the wall panel such that there is contact between the contact surfaces of the wall panel and the contact surfaces of a first and a second wall supporting element; and

- lowering the wall face to its designated location on the seabed.

26. A method of building a reservoir according to claim 22, which reservoir is installed at seabed, by:

- repeating the steps of claim 22, wherein the sets of elements, at their respective designated locations at the erection site, collectively form an enclosure.