Peter Broughton, Richard Davies and Otto Carlisle, of Marine Engineering Energy Solutions; and Jean-Charles DiSchino, Garron Lees, of ODE Group, assess an articulated wind column concept developed by the engineering firms, that seeks to provide support for large wind turbine structures
In the UK to date, the development of offshore wind turbines has been mostly restricted to relatively shallow water locations, many of which are a considerable distance from shore, resulting in high power transmission costs.
However, the development of ‘floating’ or ‘compliant’ substructures, used widely in the oil and gas industry, could allow the development of deeper water offshore wind opportunities, which are closer to shore.
ODE, part of the DORIS Group and Marine Engineering Energy Solutions have developed an articulated wind column (AWC) concept. The AWC provides a structural support for the largest wind turbines currently available (8MW) for 45-100 m water depths.
The design can be constructed using steel or, preferably, reinforced concrete which allows for greater cost reductions and lends itself to easier mass production local to the offshore windfarm development location.
Articulating the benefits
The compliant column is able to achieve a near vertical position through the natural buoyancy of the column.
The structure consists of two main elements, the compliant vertical column and the base, which sits on the seabed and is made stable using heavy iron ore ballast.
The vertical column and the base are constructed separately, then connected by an articulated joint, which allows rotation about both horizontal axes, prior to marine transport and installation. Because of the articulation between the column and base, the design allows for an uneven seabed.
The articulated joint and support consists of the cast articulated joint, upper supports to the underside of the column, and lower supports to the top of the transfer beam on the base.
Articulated column technology is well proven and has been deployed for more than 40 years in the North Sea, where 13 such structures (one concrete and 12 steel) have operated successfully.
In the early 1980s, DORIS Group created the world’s only concrete offshore loading column, the Maureen articulated column, which operated successfully in 93 m water depth in the North Sea from 1982, until it was decommissioned at the end of the field’s life in 2000.
The Maureen column had to handle the extreme hawser load – about 210 tonnes – when an offloading tanker was attached. This is a comparable figure to the extreme design load of a large offshore wind turbine. The AWC draws on the Maureen design.
Dimensions, volumes and weights for an AWC for 90m water depth, hosting an 8MW turbine
The AWC concept has been model tested using simulated conditions based on a site on the UK’s west coast, with 4-25 m/sec operating wind speeds and extreme wind speeds up to 54.5 m/sec. Wave height was 1.55 m, with a 10.2 m, 50-year wave height, with peak wave period of 5.75 sec for operating waves and 12 sec for the 50-year waves. Currents were 1.42 m/sec in operating conditions, with a high of 1.5 m/sec. The turbine rotor was 164 m diameter, with a 482-tonne nacelle. The mast was 86.7 m tall and weighed 494 tonnes.
A model (scale 1:42.5) underwent three weeks of testing. Using allowable tilt criteria, based on the indicative operating conditions, a 17m diameter column (2,106 m3 of concrete) was designed. Using this design, it was concluded that a tilt of 6ᵒ or more would not occur for more than 0.002% of the time and a tilt of 4ᵒ or more would not occur for more than 16% of the time.
A sensitivity study was performed for the same turbine size and weight, but for 90-130 m water depth, with 13 m diameter column (2,386 m3 of concrete). The column inclination for the operating conditions increased slightly to 7ᵒ – below the 8ᵒ considered to be acceptable. This indicates a relative insensitivity to the volume, weight and cost of the column due to increased water depths – ie 2,106 m3 of concrete needed for <90 m water depth and 2,386 m3 for 130 m water depth.
The compliant vertical column units would be constructed as two half units, joined together and post-tensioned prior to mating with the base in water. Two half column sections, each about 45 m long, will be constructed in the vertical using variable slipform techniques. Completion rates of a half column section per 20 days is expected, allowing for end slab construction.
Various options exist for placing the completed column units into the water, including the possible use of a synchro lift system or similar. It is planned to construct the base units on floating pontoon/barges using a gantry slipforming technology. On completion of each base unit, the pontoon/barge would submerge to allow the base unit to float off to be towed to a column and base mating area.
Mating and marine installation
Once connected the completed base and column unit will be towed either to wet storage or directly to site. Upending is achieved by water ballasting the base, at which point there would be 21 m of clearance above the seabed. Initial positioning on the seabed is done using partial seawater ballasting the column. Final placement is with heavy iron ore ballast into the 10 base cells together with the deballasting of the column. It is estimated that the solid ore ballasting would take 6-8 hours. Final placement of the upper steel shaft, turbine and blades would be done using a heavy-lift crane vessel.
The next step for the AWC is large-scale prototype testing. A number of sites in the UK are being explored.
This type of structure provides a unique and economic solution for offshore turbines in deeper water close to shore. The estimated levelised cost of energy in adopting the AWC in deeper water close to shore is marginally less than £90/MWh, with a rate of return of about 17%.