The highs and lows of building at sea
Tidal Lagoon Power’s Swansea Bay project would be the first man-made, energy-generating lagoon in the world. But creating power from the tide is not a new concept: mills driven by ebb tides, known as tide mills, have existed for more than 1,000 years in the UK. In more recent times a barrage at La Rance, northern France, has been generating electricity since 1966 and in South Korea the Sihwa Lake Tidal Power Station came into operation in 2011.
The key concepts for a tidal lagoon are well understood, and the turbine technology is tried and tested. The 9.5km U-shaped breakwater will run from a point next to the mouth of the River Tawe, by the port of Swansea, and then rejoin land next to Swansea University’s new Bay Campus.
A concrete structure at a midpoint in the breakwater – to be built by Laing O’Rourke – will house 16 low-head bulb turbines that will generate electricity as the tide flows in and out of the lagoon. In a similar manner to run-of-river hydropower schemes, flow is created by gravity through the difference in head – or water height – inside and outside the lagoon walls.
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Atkins is the client’s engineer. Costain, which is also an investor, will provide project management support to Tidal Lagoon Power.
The Swansea proposal differs from the often-mooted Severn and existing barrages elsewhere (box, overleaf): unlike these schemes, it does not straddle the estuary and fully obstruct flow. Construction of the £1bn project has been divided into three main contracts: the turbines, the concrete turbine house and sluice gate structure – together known as “the civils package” – and the marine wall.
There are also separate contracts for ancillary works, such as the breakwater surface, roads, slipways, utilities and landscaping, building the visitor centre and the turbine assembly plant. The largest contract by value is the £300m marine works package to construct the 9.5 km wall, for which China Harbour Engineering Company has been named as preferred bidder.
Cross section of a turbine unit
The wall will rise from 5 metres at the shore, where construction will begin, to 20 metres out in the bay. At high tide 3.5 metres of the wall will be visible and at low tide up to 12 metres. It will be constructed over three summer seasons (March to October). The wall will act as a breakwater, protecting an area of water against waves and the project itself from being washed away, but it will also act like a dam, retaining a water-level difference between the open sea and the enclosed bay area.
“We need to create a big swimming pool at sea,” says Ton Van der Plas, senior marine engineer at Tidal Lagoon Power. “It is a dam structure, built offshore, which is designed to withstand alternating head differences four times a day. At the same time, it needs to be stable during storms and wave attack. So it is more than a typical breakwater or a typical dam, and combines the function of those two structures.”
The construction sequence for the breakwater wall is as follows. First, 2 metre-high barrier walls of “quarry run” – randomly shaped stone – are laid parallel under water with the space between them filled with sand.
Further progressively narrower layers of rock and sand are placed on top of the construction until a barrier with a triangle-shaped section is formed (see diagram). Larger “rock armour” is then positioned on top of the structure to protect it from the sea.
The fast-flowing water in the estuary means the construction process throughout has to be carefully managed, explains Van der Plas: “A large tidal range [the difference between the high and low tide] comes with tidal flows, which will tend to erode the newly placed material. It is key that any sand placed is protected as quickly as possible by rock or other means to prevent erosion and loss of progress.”
The wall is structured to withstand alternating differences in the height of the tide four times a day
The marine contract also include sourcing and transporting the 5 million tonnes of rock to Swansea Bay. To promote the project’s green credentials, Tidal Lagoon Power intends to use as few trucks as possible, and instead transport the rock by sea.
One plan being explored is to move the rock from a quarry on the Lizard Peninsula in Cornwall, by sea. Tidal lagoon Power’s plans to re-open a disused quarry in Cornwall have come up against local opposition, although the final decision on sourcing the quarry run will lie with China Harbour Engineering Company. The 7 million tonnes of sand required will be dredged from Swansea Bay. Around 80% of the material for the sea wall will be placed using marine equipment rather than manpower.
Although the structure is not technically demanding – Van der Plas describes it as “everyday” – the large tidal range that makes the site ideal for generating power creates complications, as it makes for an extremely variable construction environment.
The bund wall will be constructed with a combination of marine plant, such as barge- and pontoon-mounted equipment, and land-based equipment, with the tide determining what equipment can be used when. This will change throughout each day, explains Van der Plas.
“With planning and the right equipment, it is actually possible to take advantage of the tides and increase production.”
Ton Van der Plas, Tidal Lagoon Power
“With low tide, the land-based plant has a maximum work area, but six hours later most of the site will be flooded. With high tide, there will be sufficient water depth for most of the site for marine plant access, but six hours later one-third of the site will be dry or have only limited water depth, prohibiting most vessels from working there.” However, by working on several fronts at the same time, vessels and land-based equipment can move between locations depending on the tide to continue working.
In fact Van der Plas points out that the tides in the estuary can be advantageous for construction: “With planning and the right equipment, it is actually possible to take advantage of the tides and increase production. For instance, inspection of completed work can for a part be done in dry conditions as opposed to underwater surveys or even through diver work.”
Connected to the sea wall, an oval 2km coffer dam will create a dry environment to allow the turbine structure to be constructed, as the final element of the marine contract. Laing O’Rourke was named in May as preferred bidder for the £200m contract to deliver 410 metre concrete turbine house and sluice structure block.
At the south-west of the lagoon the reinforced concrete structure comprises 16 turbine housings and eight sluice gate housings with a 137 metre-wide dividing structure between them. Each housing unit for the 16 bi-directional turbines – supplied by General Electric and Andritz Hydro at a cost £300m – will be 15 metres wide. Set to be assembled in a plant to be built in Swansea, these 7 metre-diameter bi-directional turbines generate electricity from tidal water both entering and leaving the lagoon.
The sluice gate units will each be 16 metres wide. The steel vertical lift sluice gates will act as an additional mechanism to control the water entering and leaving the lagoon.
The structure will also contain operations and maintenance rooms, some below the road level and some above in the “offshore building”.