MeyGen Phase 1A Project is the first stage of the 398MW MeyGen tidal turbine farm, being the world’s first utility-scale tidal turbine array. Phase 1A comprises four 1.5MW turbines using turbines from two different suppliers, installed south of the island of Stroma in the Pentland Firth (Scotland).
The site was chosen for its significant tidal stream: as the tide flows from the Atlantic Ocean into the North Sea and back, the flow is concentrated to produce powerful tidal races. Because of the irregular seabed, the flow is highly turbulent, containing strong eddies and vortices at both small and large scales. The chosen concept is a gravity-based structure comprising a fabricated steel tripod with two 200t ballast blocks per leg.
The 158-tonne structure is 25mx20m on plan with the cylindrical pylon supporting the turbine 15m above seabed. Robert Bird Group has supported the project from earliest concept through to detailed design of the turbine support structures.
Key technical challenges
The MeyGen Project is the first successful full-scale commercial installation of tidal turbine of this kind. The novel structural form resists long-term dynamic forces in some of the most extreme environmental conditions including highly turbulent current of average 4.4m/s, 1m/s storm surge and 50-year return period wave of 15m height.
RBG worked with Imperial College and the Danish Hydraulic Institute (DHI) in the early project stages to better understand the turbine hydrodynamics and oceanic interaction. The team helped develop the draft IEC standard for marine energy devices, and continues to support development of the marine energy industry generally through industry associations and conference presentations, sharing lessons learnt.
Based on research and first principle studies, in-depth dynamic analysis procedures were developed, including multi-axial fatigue analysis to efficiently design the structure, and behavioural assessment of combined substructure-turbine system under dynamic loading. Bespoke software tools were developed and validated by the team to support this work.
The typical solution for a fixed marine structure is a tubular jacket, but in such shallow water (relative to turbine diameter) this would have interfered with the turbine operation and attracted significant drag. RBG developed an alternative low-profile structural form to enable development of the shallow water MeyGen site. This low profile form also minimises frontal-area exposed to environmental drag, thereby reducing the size of ballast required, and enables the turbine to be efficiently placed in a high tidal energy region.
Articulated feet (employing spherical bearings) were developed. Without this innovation, twisting of the legs due to seabed irregularity would have governed the leg design for fatigue purposes requiring a significantly more costly structure.
Being the world’s first tidal turbine array, many design aspects lie outside existing codes. In particular, the behaviour of structures in marine environments with large waves on strong currents is not fully understood, and is poorly documented. A scaled tank test (1:36 model) was performed at the HR Wallingford fast-flow facility to calibrate the basis of hydrodynamic loading assumptions.
Renewable Tidal Energy
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