The present invention relates to an offshore construction and a method of constructing an offshore construction, such as a so-called energy island, including a pumped hydro storage, at an offshore area, e.g. in the North Sea. The method comprises, at the offshore area, erecting a first outer wall including lowering a collection of wall construction elements onto or into the seabed such that the collection of construction elements collectively forms the first outer wall and encloses a containment.

INTRODUCTION

Pumped hydro storage is a well-known technology for storing energy, specifically electric energy. The principle is to use electricity to power a pump, pumping water from a lower reservoir to an upper reservoir, when there is a surplus of electricity available e.g. from wind turbines. When electricity is needed, water from the upper reservoir is led back to the lower reservoir while spinning a hydro turbine connected to a generator to thereby produce electricity.

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

It is foreseen that pumped hydro storages may cover an offshore area of about 0.2 to 4 square kilometres. The dynamic volume of pumped hydro storages may be in excess of 20 million cubic metres e.g. 50-70 million cubic metres. The dynamic volume of pumped hydro storages may generate about 1 to 6 Gigawatts of electrical power. Pilot plants for pumped hydro storage may be much smaller.

PRIOR ART

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

WO 2014/184312-A1 discloses an offshore pumped storage facility comprising a reservoir wall that comprises multiple prefabricated large diameter steel piles; wherein the prefabricated large diameter steel piles are positioned adjacently and rooted into the seabed by vibration or hammering. To connect the steel piles and seal the reservoir from the surrounding sea, arched steel sheets are installed between and fixated to two adjacent large diameter steel piles, such that a wall segment is established between the piles. The large diameter steel piles and the spaces behind the steel sheets are at least partially filled with seabed material. The offshore pumped storage facility is thus configured to dam up water and separate an inner reservoir from the surrounding sea. At least one pump-turbine system is configured for pumping water from the inner reservoir to the surrounding sea and for letting water from the surrounding sea drive turbines when running from the surrounding sea into the inner reservoir.

However, it is observed that there is a need for an island for one or both of a green energy harvesting plant, such as a wind turbine farm, and an energy conversion plant, such as a hydrogen factory running on electrical power.

SUMMARY

There is provided a method of constructing an offshore construction at an offshore area, comprising: at the offshore area, erecting a first outer wall (141 ) including lowering a collection of wall construction elements onto or into the seabed such that the collection of construction elements collectively forms the first outer wall (141 ) and encloses a containment (142) including a designated reservoir portion (101 ) and a designated island portion (102); positioning a first vessel (V1 ), including a dredger, to float on the water in the containment at the reservoir portion (101 ); lowering the water level in the containment (142), at least at the reservoir portion (101 ), by pumping sea water out of the containment until a predetermined range of depths (10m) in the reservoir portion (101 ) is reached.

The outer wall serves as a barrier between the surrounding sea and the containment at times when excavation of seabed material is in progress. In some examples it may require about 80% water to excavate about 20% seabed material. It is thus expected that a great amount of seabed material will be suspended in the water, however only inside the containment. Transport of seabed material from an excavation position, at the first vessel, to a deposit position, at the second vessel, takes place via a hose conveying the about 20% seabed material in a flow with about 80% water. Other ratios of seabed material to water may easily occur or be set as an operating parameter of the dredger. The example ratio of 20%/80% is only given as an example. An important advantage is that the containment enables an attractive trade-off between keeping a low resource consumption during deployment of the offshore construction and protecting the sea environment surrounding the containment while deploying the offshore construction. Further, the outer wall protects the area inside the containment from large waves in the surrounding sea. Thus, the first vessel and the second vessel in operation to perform the long-duration excavation process are protected from the open sea and can operate in smooth waters when the surrounding sea is rough. This significantly reduces the number of days when the excavation process otherwise had to be suspended.

In some examples the predetermined range of depths, in the reservoir portion, is in the range of 3 to 18 meters e.g. 5 to 12 meters. Thus, dredgers commonly available for deepening rivers and fairways to harbours can be used. In particular, the step of lowering the water level in the containment, at least at the reservoir portion, can be performed while causing only minimal suspension of seabed material in the water pumped out e.g. into the surrounding sea. An advantage is that the dredger can operate throughout the excavation process despite a limited depth capacity for effective suction of seabed material and despite the continued deepening of the reservoir portion. It is thereby enabled that the containment and in particular the reservoir can be deployed using conventional, inshore (coastal) equipment including dredgers. This greatly lowers resource consumption.

The control of the water level may be performed dynamically as a substantially continuous process or at regular or irregular times. In some examples the water level is controlled based on a sonar on-board a vessel, e.g. on-board the first vessel. The water level is controlled such that the dredger operates effectively. To avoid excessive pumping, the first vessel swipes the reservoir portion in a pattern such that deepening takes place across the area of the reservoir, before further deepening.

During excavation, further pumping water of water from the reservoir portion to the surrounding sea can be performed by suction of water from a position distant to at least the second vessel. Thereby the sheer size of the containment itself, having a huge expanse, may serve to ensure that clean water, rather than dirty water is pumped out into the open surrounding sea. In some examples the distance between any one of the vessels and a pump station for pumping water from the containment to the open sea is at least 300 meters e.g. at least 500 or 800 meters. In some examples the suction side of the pump station includes a filter for removing suspended particles.

The containment is divided into the designated island portion and the designated reservoir portion by a wall, e.g. a steel wall or a shore with a desired slope. The slope may be less than 1 :5. During construction, the containment may be divided into the designated island portion and the designated reservoir portion by a barrage.

In some examples the outer wall has an oval shape, a substantially circular shape, a figure-of-eight (8-shape) or another shape e.g. a substantially rectangular shape. In some aspects the shape has a pointed portion serving as a breakwater in a direction towards a predominant wave and/or wind direction e.g. south-west.

In some examples, the wall construction elements comprises steel piles, e.g. circular steel piles, that are rammed into the seabed or floating caissons that are towed into position at the wall and sunk into place. A space established between the elements may be filled with seabed material to stabilise the wall.

In some embodiments the method comprises: installing water turbines (130) including at least one canal connecting the open sea surrounding the first outer wall and the reservoir portion, wherein the water turbines are configured to lower the water level in the reservoir portion while consuming electrical power and increasing the water level in the first portion while generating electrical power; and preparing the island portion (102) of the containment including installing infrastructure (131 ) for energy processing at the island portion.

An advantage is that the containment, including the island and the reservoir, can be converted into an energy conversion plant and in particular the energy conversion plant can be constructed with a low environmental impact on the surrounding sea. In some examples, the infrastructure for energy processing comprises one or more of:

AC/DC converters;

Electrical power transformers; and Hydrogen production.

In some embodiments the method comprises: establishing a first inner embankment (121 ) or wall (406) between the island portion and the reservoir portion; establishing a first sluice (505) in the first inner embankment (121 ) or wall (406) at a level above the water level in the reservoir portion and below a water level in the designated island portion at a time when the water level has been lowered; controlling the flow of water through the first sluice (505), into the reservoir portion, to maintain the predetermined water depth inside the reservoir portion until the predetermined water level at the reservoir portion is reached.

An advantage is a low-energy refill of the reservoir portion to maintain the water depth inside the designated reservoir portion within the predetermined range of depths during the course of excavating the seabed material. The island portion is thus deliberately filled to keep a higher water level inside the island area, at least in a region from the first inner embankment or wall, higher than in the reservoir portion. Another advantage is at least the reduction in dirty outlet water back into the surrounding sea by approximately 50%, wherein the dirty outlet water originates at least partly form the excavation process.

In some aspects, the method includes establishing a second sluice, 506, or pump station in the first outer wall at the island portion at a level above the sea level of the surrounding sea. In some examples, the second sluice or pump station is arranged at the island of the first inner embankment or wall. The island portion is thus deliberately filled to keep a higher water level inside the island area, at least in a region from the first inner embankment or wall where the second sluice or pump station is installed, higher than the surface of the surrounding sea. An advantage is that the flow of water into the open surrounding sea can be controlled by controlling the second sluice.

In some embodiments the method comprises establishing a water pump (204) to gradually lower the water level in the containment as excavation gradually reaches lower depths; wherein the water pump is controlled to maintain a predetermined range of water depths at the reservoir portion.

In some aspects, a GPS and a sonar is installed on-board the first vessel to measure the depth of water under the first vessel. In some aspects, the first vessel is controlled to sweep across the reservoir area and to perform the excavation to gradually lower depths. In some embodiments the method comprises installing a water cleaning or water purification plant for cleaning water before leading or pumping water out of the containment to the surrounding sea.

An advantage is that the impact on the water environment in the surrounding sea is kept at a very low level, wherein contamination by suspended seabed material is reduced.

In some aspects, sediment or retained material generated from the water cleaning or water purification plant is discharged at the island portion to contribute to the building of the island.

In some examples, the water cleaning or water purification plant includes filter sections e.g. based on a centrifugal turbine removing sand and other particles (also known as centrifugal sand filtration).

In some embodiments the method comprises establishing a first inner embankment (121 ) between the island portion (102) and the reservoir portion (101 ); wherein the first inner embankment (121 ) is reinforced by means of chippings including one or more of crushed stone and crushed aggregate.

Advantages are reduced erosion caused by the turbines what the reservoir is in use as a pumped hydro storage. Also, e.g. for expansion of the island portion, it is possible to add more seabed material to the embankment and thereby increase the area and/or depth of the reservoir portion. In some examples, the chippings including one or more of crushed stone and crushed aggregate are delivered by a ship or barge and deposited by a grab crane or conveyor.

In some embodiments the method comprises: at the first inner embankment between the island portion of the reservoir portion, erecting a reinforcing steel sheet wall against the first portion of the designated area, which includes the first reservoir.

The reinforcing steel sheet wall enables further deepening and/or increase of expanse of the reservoir portion. Thereby, the efficacy of the reservoir can be further increased. In some embodiments the method comprises: establishing a first inner embankment between the island portion and the reservoir portion; wherein the first inner embankment has a slope with an inclination flatter than 1 :5; establishing a partial reinforcement of the first inner embankment at portions of the first inner embankment having a largest water flow velocity.

An advantage is that cost of the embankment can be kept at a low level during deployment and that the embankment is protected, longer-term against erosion.

As examples, reinforcement is made from one or more of: Steel sheets and chippings including one or more of crushed stone and crushed aggregate.

In some embodiments the method comprises erecting an inner wall (405), e.g. by ramming a collection of steel piles and arched sheet members into the seabed, dividing the containment (142) into a reservoir portion (101 ) and an island portion (102).

The steel piles may have a diameter in the range of 8 metres to 40 metres. The steel piles can be vibrated or hammered into the seabed.

In some examples the steel piles are rooted more than 25 meters into the seabed. In some examples the steel piles are rooted more than 30 meters into the seabed at portions of the wall enclosing the reservoir portion.

In some embodiments the method comprises: ramming a collection of steel piles into the seabed; wherein the steel piles have a top brim at least 5 meters above sea level at a final stage; wherein the steel piles have a diameter of at least 9 meters and are rammed into the seabed at a mutual distance, centre-to-centre, in the range of one diameter to two diameters; wherein the collection of steel piles forms a wall of the containment; wherein the steel piles have one or two pairs of longitudinal slots for accommodating an edge portion of an arched sheet member; and between two neighbouring steel piles in the collection of steel piles, lowering a pair of arched sheet member into the longitudinal slot of a steel pile and into the longitudinal slot of a neighbouring steel pile, and ramming the pair of arched sheet members into the seabed; wherein the pair of arched sheet members have substantially the same length as the steel piles.

An advantage is that the outer wall of the containment can be rammed into the seabed at a rapid pace. Once a steel pile is installed, it can be filled with seabed material to significantly improve its strength against waves in the surrounding, open sea and against overturning forces when the reservoir is drained.

In some aspects, the distance between two neighbouring steel piles is 2 to 4 meters, edge-to-edge. The arched sheet members may be custom made to fit between two specific steel piles. In some examples, the arched sheet members are manufactured with an oversize or under-size and then deformed, e.g. by hydraulic force, to fit the exact space between two neighbouring steel piles.

In some aspects a spacer comprising a buffer made from wood or rubber elements is arranged to keep a predefined distance to a neighbouring steel pile during installation of a pile. The spacer may have a collapsible or scissor portion e.g. operated by a crane and/or hydraulics to ease removal of the spacer.

In some aspects the outer wall is made from two or more rows of wall elements; wherein the space between the two or more rows of wall elements is filled with seabed material.

In some aspects, the outer wall includes a gate or sluice to enable passage of a vessel.

In some embodiments the method comprises: installing one or more floating units carrying solar photo-voltaic panels (132); wherein the one or more floating units are launched to stay anchored or moored at the designated reservoir portion (101 ); wherein the solar photo-voltaic panels (132) are coupled to the energy island via power cables and wherein the infrastructure (131 ) for energy processing at the island portion includes power inverters. An advantage thereof is the better green energy harvesting. In particular, the floating units are floating on the water in the reservoir protected by the outer wall. Thus, the floating units and the solar photo-voltaic panels are protected from the rough sea in the surrounding sea.

In some examples, the infrastructure for energy conversion comprises one or more Renewable Energy Source (RES). In some examples, the one or more Renewable Energy Source comprises one or more of:

Solar power plants;

Wind turbines;

Tidal Energy plants;

Wave Energy plants;

Oceanic Thermal Energy plants.

There is also provided an offshore construction at an offshore area, comprising: a first outer wall (141 ) including a collection of wall construction elements installed on or into the seabed such that the collection of construction elements collectively forms the first outer wall (141 ) and encloses a containment (142) including a reservoir portion (101 ) and an island portion (102); wherein the reservoir portion is connected to water turbines (130) including at least one canal connecting the open sea surrounding the first outer wall and the reservoir portion, wherein the water turbines are configured to lower the water level in the reservoir portion while consuming electrical power and increasing the water level in the first portion while generating electrical power; wherein the island portion (102) of the containment comprises infrastructure (131 ) for energy processing at the island portion.

Aspects of the offshore construction are set out above, in the detailed description including the figures, and in the claims.

BRIEF DESCRIPTION OF THE FIGURES

A more detailed description follows below with reference to the drawing, in which: figs. 1A-1 C are top-views showing stages of deploying a first offshore construction; fig. 2 is a cross-sectional view showing stages of lowering a reservoir water level and excavating seabed material including depositing excavated seabed material; fig. 3 is a cross-sectional view of a combined energy island and a reservoir for storage of hydro-power energy; fig. 4A-C are top-views showing stages of deploying a second offshore construction; fig. 5 is a cross-sectional view showing stages of lowering a reservoir water level and excavating seabed material including depositing excavated seabed material to build up an island; fig. 6 is a cross-sectional view of a combined energy island and a reservoir for storage of hydro-power energy; fig. 7 is a top-view of a single-row wall section; fig. 8 is a top-view of a double-row wall section; fig. 9 shows a diagram for controlling water levels during excavation; fig. 10 is a top-view showing a third offshore construction; and fig. 11 is a top-view showing a fourth offshore construction.

DETAILED DESCRIPTION

Figs. 1A-1 C are top-views showing stages of deploying a first offshore construction. In fig. 1A a containment 142 is established by erecting a first outer wall 141. The containment 142 is protected from the surrounding sea 140. The first outer wall 141 is established by lowering a collection of wall construction elements onto or into the seabed. In some examples, the collection of wall construction elements includes substantially circular or oval steel piles interconnected by arched steel sheets. In other examples the collection of wall construction elements includes caissons. The caissons arrive as floating caissons and are sunk in position to resemble the outer wall. In some aspects the caissons are stacked. The caissons or steel piles are filled with seabed material excavated from inside the containment 142. The outer wall may include a gate or water lock for ships or other vessels to pass and get into or out from the containment 142. In some examples, one or more cranes are positioned to lift equipment, such as vessels and construction elements into the containment.

The containment includes a designated reservoir portion 101 and a designated island portion 102. At the designated reservoir portion 101 , seabed material is excavated using a first vessel V1 , including a dredger and a suction hose 123. Seabed material is conveyed to and deposited at the designated island portion 102. A second vessel V2 includes an outlet hose 124 for depositing the seabed material.

It can be seen on the right hand side, at a position 117, in the open sea surrounding the containment 142, that the water level 111 (indicated by a dashed line) has a first water level, e.g. 25 meters and, at the same position 117, that the seabed level is at a first seabed level 115 (indicated by unbroken line). Correspondingly, at position 116, inside the containment 142, the water level was at the first water level 111 and is then lowered some meters 112 to a second water level 113. Thus, the water depth at position 116 is thereby at a depth 114 e.g. 5-15 meters. The water level in the containment 142 is lowered from the first water level 111 to the second water level 113, e.g. by pumping sea water out of the containment until a predetermined range of depths (e.g. 5-18 meters, e.g. about 8-12 metres) in the reservoir portion 101 is reached. Thus, the depth is below the first vessel.

The first vessel V1 and the second vessel is then on an appropriate water depth enabling the effective use of the dredger installed on-board the first vessel V1 to excavate seabed material from the designated reservoir portion 101.

It is shown in fig. 1 B that the first vessel V1 and the second vessel V2 are coupled by a connecting hose 122 for conveying seabed material from the first vessel to the second vessel. The firs vessel V1 , has a suction hose portion 123 which is coupled to an outlet hose portion 124 via the connecting hose. The outlet hose portion is carried by the second vessel V2. Thereby, seabed material can be excavated from the reservoir portion and deposited or dumped at the island portion. After some time of excavating, the island portion 102 builds up and raises above the water level 113 and optionally after yet some time, above the water level 111 of the surrounding sea. As the island builds up, a shore 121 of the island gradually moves. The area or volume of the island increases and the area or volume of the reservoir decreases correspondingly. The vast majority of the excavated seabed material is thus (re-)used to build up the island and to stabilise the wall construction elements. Very low amounts of dirty water and/or seabed material may be discharged into the open sea while of excavated seabed.

During the course of excavating the seabed material, the water level inside the containment, and in particular the designated reservoir portion 101 , e.g. at position 116, is controlled e.g. by pumping water to/from the reservoir portion to maintain the water depth inside the designated reservoir portion 101 within a predetermined range of depths e.g. 5 to 15 meters, e.g. about 8 to 12 meters.

It is shown in fig. 1 C that the vessels have been removed from the containment e.g. using a crane or water lock in the outer wall 141 .

The reservoir 101 is completed by installing one or more water turbines 130 including at least one canal connecting the open sea surrounding the first outer wall and the reservoir portion. The water turbines are configured to lower the water level in the reservoir portion while consuming electrical power and increasing the water level in the first portion while generating electrical power. Optionally, one or more floating units carrying solar photo-voltaic panels 132 are installed. The one or more floating units are launched to stay anchored or moored at the designated reservoir portion 101. The solar photo-voltaic panels 132 are coupled to the island and in particular the infrastructure 131 via power cables.

Further, the island 102 is completed by landscaping the seabed material forming an upper layer of island and preparing a desired surface. Further, the island 102 is completed by installing infrastructure 131 for energy processing. In some examples, the infrastructure includes one or more of the following:

1 . A hub for receiving power cables from: a. wind turbines e.g. installed in the surrounding open sea and/or installed at the island or on the outer wall 141 , and/or b. solar photo-voltaic panels; c. wave energy to electrical power units;

2. Transformers and/or inverters for electrical power conversion;

3. A power-to-X plant, e.g. for production of hydrogen

The outer wall may include a harbour portion (not shown) for loading ships with products from the power-to-X plant. The term power-to-X designates a class of plants for converting e.g. electrical power into other energy forms or products e.g. Hydrogen or Ammonia.

Fig. 2 is a cross-sectional view showing stages of lowering a reservoir water level and excavating seabed material including depositing excavated seabed material. It can be seen that during the course of excavating the reservoir, seabed material is gradually removed from the reservoir portion and conveyed to the island portion, such that the reservoir is gradually deepened and the island gradually builds up. The dashed lines 204 and 205 show the redistribution of seabed material taking place relative to the original seabed level 203.

A water pump 204 is installed to gradually lower the water level in the containment as excavation gradually reaches lower depths. The water level is lowered as illustrated by dashed lines L1 , L2 and L3 to maintain the water depth in the reservoir region, e.g. at least below a route passed by the first vessel V1 performing the excavation. The water depth is maintained within a predefined range e.g. about 10 meters, e.g. about 8 to 12 meters.

The water level at the surrounding sea is designated 140.

Fig. 3 is a cross-sectional view of a combined energy island and a reservoir for storage of hydro-power energy. It can be seen that the island has a shore with a slope, S, leading into the reservoir. The reservoir is operated as a pumped hydro storage having a dynamic depth, D1 . The dynamic depth may be in the range of 25 to 50 meters, e.g. 35 to 45 meters. In some examples, the turbine 130 is placed in a trench to increase the turbine at a lowest possible depth. The improves the dynamic depth of the reservoir without excessive excavation.

Fig. 4A-C are top-views showing stages of deploying a second offshore construction. Fig. 4A shown that the second offshore construction has an outer wall 405 roughly having the shape of a figure-of-eight (8) or a double-0 (OO). It is shown that a wall section is not yet installed between the reservoir portion 401 and the island portion 401 . The containment enclosed by the outer wall 405, at this stage has an open passage between the reservoir portion 401 and the island portion 401. The outer wall may be constructed as described in connection with fig. 1A and in connection with figs. 7 and 8 below.

Fig. 4B shows that, after some time of excavating, the island portion 404 builds up and raises above the water level in the island portion 402 and optionally after yet some time, above the water level of the surrounding sea. As the island builds up, a shore 406 of the island 404 gradually moves. The area of the island increases and the depth of the reservoir increases correspondingly, e.g. while discharging less than 0.001 % of excavated seabed material into the open, surrounding sea. At a point in time an inner wall 406 is installed. The inner wall 406 may be constructed in a similar manner as the outer wall 405. In some embodiments, the inner wall 406 is replaced fully or partially by a shore made from the excavated seabed material.

It is shown in fig. 4C that the vessels, V1 and V2, have been removed from the containment e.g. using a crane or water lock in the outer wall 141. It is also shown that a power cable 407 connects to a mainland and/or to a wind turbine farm.

The energy island 404 and the reservoir 401 is completed e.g. as described in connection with fig. 1 C.

Fig. 5 is a cross-sectional view showing stages of lowering a reservoir water level and excavating seabed material including depositing excavated seabed material. Since an inner wall 406 or barrage is installed between the island portion and the reservoir portion, a different method for controlling the water depth inside the reservoir portion during excavation is needed. An inner wall 406 between the island portion and the reservoir portion is constructed. Also, a first sluice 505 or controlled valve is installed in the wall 406 at a level above the water level in the reservoir portion and below a water level in the designated island portion at a time when the water level has been lowered. Thereby, when the valve or sluice is open, water can freely run from the island portion into the reservoir portion. This is advantageous because excavation by the dredger requires about 80% water to excavate about 20% seabed material. Thus, the water depth inside the reservoir would be drained too fast. However, the sluice 505 or controllable valve makes it possible to have a return flow of water. The surrounding sea is thereby not polluted by dirty water with suspended particles from the excavation.

During the course of excavating the reservoir, the island builds up and will eventually force out water from the island portion of the containment. To provide a controlled outlet of that surplus water, a second sluice 506 or controlled valve is established in the outer wall 405. In some aspects, a water cleaning or water purification plant is installed for cleaning the surplus water before leading or pumping water out of the containment to the surrounding sea. An advantage is that the impact on the water environment in the surrounding sea is kept at a very low level, wherein contamination by suspended seabed material is reduced. In some aspects, sediment or retained material generated from the water cleaning or water purification plant is discharged at the island portion to contribute to the building of the island. In some examples, the water cleaning or water purification plant includes filter sections e.g. based on a centrifugal turbine removing sand and other particles (also known as centrifugal sand filtration).

The water pump 204 is run initially to drain water out of the reservoir to establish a predefined depth e.g. 10 meters below the first vessel V1 to ensure appropriate operating conditions for the dredger on-board the first vessel. This water will be sufficiently clear and clean to be discharged into the surrounding sea.

Fig. 6 is a cross-sectional view of a combined energy island and a reservoir for storage of hydro-power energy. It can be seen that the island has an embankment or wall 406 separating the island from the reservoir. The reservoir is operated as a pumped hydro storage having a dynamic depth, D2. The dynamic depth may be in the range of 25 to 40 meters or deeper.

Fig. 7 is a top-view of a single-row wall section. The wall section 700 may be a section of the outer wall 141 or 405 or the innerwall 406. The wall section includes substantially circular or oval steel piles 701 interconnected by arched steel sheets 702. The steel piles are installed with their centre positions on a curve 703 representing the centre of the wall. The wall section may be installed by ramming a collection of steel piles 702 into the seabed e.g. by hammering or vibration.

Preferably, the steel piles have a top brim at least 5 meters above sea level at a final stage i.e. when they have reached a final depth e.g. including a cap or top portion mounted subsequently to the piles having reached a final depth. The steel piles have a diameter of at least 9 meters and are rammed into the seabed at a mutual distance, centre-to-centre, in the range of one diameter to two diameters, 01 . In some aspects, the steel piles have one or two pairs of longitudinal slots for accommodating an edge portion of an arched sheet member.

The arched steel sheets 702 are installed between two neighbouring steel piles e.g. by lowering an arched sheet member 702 into a longitudinal slot of a steel pile and into the longitudinal slot of a neighbouring steel pile and ramming the arched sheet member into the seabed. The arched sheet members may have substantially the same length as the steel piles.

As illustrated by the hatched areas, the steel piles and the space formed by the arched steel sheets are filled e.g. with excavated seabed material to stabilise the wall. Further details are described in WO 2014/184312-A1 .

Fig. 8 is a top-view of a double-row wall section. This double-row wall section is configured as described above in connection with fig. 7, however along two substantially parallel curves 801 and 802. This configuration, including two parallel rows of steel piles is advantageous for steel piles with smaller diameters, 02, e.g. below about 10 meters. Also, this configuration enables easier handling of the steel piles during production and during transport to the offshore location. Additionally, the space 803 between the rows of steel piles is filled with seabed material to stabilise the wall. Further details are described in WO 2014/184312- A1 .

Fig. 9 shows a diagram for controlling water levels during excavation. The water levels are controlled by a controller 901 , e.g. in the form of one or more programmed computers or a system of computers and sensors. The controller is coupled to receive position data of a vessel e.g. the first vessel and/or the second vessel e.g. via a GPS receiver and to receive depth data from a sonar on-board the first vessel or another vessel. The controller receives or is programmed with a route plan 904 representing an excavation route that the first vessel follows during the course of excavating the reservoir.

The controller provides control signals by wired or wireless connections to a station 907 that comprises a first pump 905a and a second pump 905b. The first and second pump sits on each side of water cleaning station 906. The water cleaning station may comprise mesh-type filter elements and include outlets for discharging sediment material. In some aspects, the station 907 includes a centrifugal turbine removing sand and other particles (also known as centrifugal sand filtration). The pump 905b may be dispensed with since the centrifugal turbine may provide sufficient output flow without a separate pump on the output side. In some aspects the water cleaning station 907 may be submerged to the seabed of the reservoir portion and provide sufficient output flow to without pumps 905a and/or 905b. The pumps receives power supply from a power source 908.

The pump 204 and/or the sluice/pump 506 may be configured like the station 907 as described above to include water cleaning and discharge of water from the reservoir to the surrounding sea 140. An advantage is that the water environment in the surrounding sea is protected from discharge of dirty water.

Station 910 provides water cleaning, e.g. as described in connection with station 907. The controller 901 is also configured to control sluices 906 and 505 (described above). The controller 901 is thereby enabled to control the water depth inside the reservoir 401 or inside the containment 142.

Fig. 10 is a top-view showing a third offshore construction. The third offshore construction includes an outer wall 1001 , e.g. constructed as described above. The third offshore construction is advantageous in places wherein the surrounding sea is expected to be generally less rough, e.g. protected by natural breakwaters or the like and where the surrounding water environment is expected to be less sensitive to discharge of dirty water from the containment. The reservoir is located towards a predominant wind direction, e.g. south-west, SW. The island portion may be constructed to be located leeward of the island relative to the predominant wind direction.

Fig. 11 is a top-view showing a fourth offshore construction. The fourth offshore construction includes an outer wall 1001 , e.g. constructed as described above. In accordance with the fourth offshore construction, the island is protected by the outer wall 1101 and surrounds the reservoir. In some examples, the reservoir is concentric with the outer wall 1101. In some other examples, the reservoir 101 is offset from a centre position of the outer wall 1101.

In some embodiments one or more of the connecting hose 122, the suction hose 123 and the outlet hose 124 are supported by one or more buoyancy elements. The buoyancy elements may be configured to prevent the one or more hoses from sinking e.g. when empty or when conveying seabed material.

ITEMS

There is also provided a second method of constructing an offshore construction at an offshore area, denoted item 1 , as follows:

1. A method of constructing an offshore construction for pumped hydro- power storage at an offshore area, comprising: at the offshore area, erecting a first outer wall (141 ) including lowering a collection of wall construction elements onto or into the seabed such that the collection of construction elements collectively forms the first outer wall (141 ) and encloses a containment (142) including a designated reservoir portion (101 ) and a designated island portion (102); positioning a first vessel (V1 ), including a dredger, to float on the water in the containment at the reservoir portion (101 ); using a dredger installed on-board the first vessel (V1 ) and having a suction hose portion (123) for removing seabed material, excavating seabed material from the designated reservoir portion (101 ); wherein the suction hose portion (123) is coupled to an outlet hose portion (124); wherein the outlet hose portion is carried by a second vessel (V2) at the island portion (102); wherein suction of seabed material includes suction of sea water.

As a comment to the second method above, the outer wall serves as a barrier between the surrounding sea and the containment at times when excavation of seabed material is in progress. In some examples it may require about 80% water to excavate about 20% seabed material. It is thus expected that a great amount of seabed material will be suspended in the water, however only inside the containment. Transport of seabed material from an excavation position, at the first vessel, to a deposit position, at the second vessel, takes place via a hose conveying the about 20% seabed material in a flow with about 80% water. Other ratios of seabed material to water may easily occur or be set as an operating parameter of the dredger. The example ratio of 20%/80% is only given as an example. An important advantage is that the containment enables an attractive trade-off between keeping a low resource consumption during deployment of the offshore construction and protecting the sea environment surrounding the containment while deploying the offshore construction. Further, the outer wall protects the area inside the containment from large waves in the surrounding sea. Thus, the first vessel and the second vessel in operation to perform the long- duration excavation process are protected from the open sea and can operate in smooth waters when the surrounding sea is rough. This significantly reduces the number of days when the excavation process otherwise had to be suspended.

The below items set out aspects of the second method. 2. A method according to item 1 , comprising: beginning excavation at the designated reservoir portion at a time before the first outer wall is fully enclosing the designated island area and reservoir area.

As a comment to item 2, the step of item 2 enables construction of the offshore construction during a shorter time period. In some aspects, the excavation begins at a time before erection of the first outer wall is complete and/or after erection of the first outer wall has reached about 20% of its circumferential length. In some aspects erection of the first outer wall progresses in a direction against or with a predominating ocean current at the offshore area.

3. A method according to item 1 or 2, comprising: forgoing controlling the water level in the containment at least until excavation using the dredger is more than 75% complete in terms of seabed material by volume; and forgoing establishing a first inner wall or dam (406) between the island portion and the reservoir portion at least until excavation using the dredger is more than 75% complete in terms of seabed material by volume.

As a comment to item 3, the steps of item 3 enable maintaining a substantially unchanged surface water level in the containment, during the course of excavation. This is particular useful, e.g. to save power consumption, in situations where the water depth at the offshore area is shallow or where a dredge with a relatively deep suction (excavation) depth is available.

4. A method according to any of the preceding items, comprising:

Installing at least one water turbine (130) including at least one canal connecting the open sea surrounding the first outer wall and the reservoir portion, wherein the water turbines are configured to lower the water level in the reservoir portion while consuming electrical power and increasing the water level in the first portion while generating electrical power. 5. A method according to any of the preceding items, comprising: lowering the water level in the containment (142), at least at the reservoir portion (101 ), by pumping sea water out of the containment until a predetermined range of depths (10m) in the reservoir portion (101 ) is reached; controlling the water level, at least at the designated reservoir portion

(101 ), including letting water in or pumping water to/from the reservoir portion to maintain the water depth inside the designated reservoir portion (101 ) within the predetermined range of depths during the course of excavating the seabed material.

6. A method according to any of the preceding items, comprising: establishing a first inner embankment (121 ) or wall (406) between the island portion and the reservoir portion; establishing a first sluice (505) in the first inner embankment (121 ) or wall (406) at a level above the water level in the reservoir portion and below a water level in the designated island portion at a time when the water level has been lowered; controlling the flow of water through the first sluice (505), into the reservoir portion, to maintain the predetermined water depth inside the reservoir portion until the predetermined water level at the reservoir portion is reached.

7. A method according to any of the preceding items, comprising: establishing a water pump (204) to gradually lower the water level in the containment as excavation gradually reaches lower depths; wherein the water pump is controlled to maintain a predetermined range of water depths at the reservoir portion.

8. A method according to any of the preceding items, comprising: installing a water cleaning or water purification plant for cleaning water before leading or pumping water out of the containment to the surrounding sea.

9. A method according to any of the preceding items, comprising: establishing a first inner embankment (121 ) between the island portion

(102) and the reservoir portion (101 ); wherein the first inner embankment (121 ) is reinforced by means of chippings including one or more of crushed stone and crushed aggregate.

10. A method according to any of the preceding items, comprising: at the first inner embankment between the island portion of the reservoir portion, erecting a reinforcing steel sheet wall against the first portion of the designated area, which includes the first reservoir.

11. A method according to any of the preceding items, comprising: establishing a first inner embankment between the island portion and the reservoir portion; wherein the first inner embankment has a slope with an inclination flatter than 1 :5; establishing a partial reinforcement of the first inner embankment at portions of the first inner embankment having a largest water flow velocity.

12. A method according to any of the preceding items, comprising: erecting an inner wall (405), e.g. by ramming a collection of steel piles and arched sheet members into the seabed, dividing the containment (142) into a reservoir portion (101 ) and an island portion (102).

13. A method according to any of the preceding items, comprising: ramming a collection of steel piles into the seabed; wherein the steel piles have a top brim at least 5 meters above sea level at a final stage; wherein the steel piles have a diameter of at least 9 meters and are rammed into the seabed at a mutual distance, centre-to-centre, in the range of one diameter to two diameters; wherein the collection of steel piles forms a wall of the containment; wherein the steel piles have one or two pairs of longitudinal slots for accommodating an edge portion of an arched sheet member; between two neighbouring steel piles in the collection of steel piles, lowering a pair of arched sheet member into the longitudinal slot of a steel pile and into the longitudinal slot of a neighbouring steel pile, and ramming the pair of arched sheet members into the seabed; wherein the pair of arched sheet members have substantially the same length as the steel piles.