Q OFFSHORE STRUCTURES
complete units per year from any one site location. In order to achieve this high production rate at any one site, significant modern procedures and processes have been adopted for the construction of both the columns and bases. The compliant vertical column units are
planned to be constructed as two half units, which will be subsequently joined together and post-tensioned prior to placing in the water for mating with the base. Two half column sections, each about 45m
in length, will be constructed in the vertical by the use of variable slipform techniques, as suggested by Bygging-Uddemann of Sweden. Rates of vertical slipforming will be about 3.5m/day, with the completion of a half column section being in about 20 days, allowing for end slab construction. It is planned to construct the base units on
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floating pontoon/barges using a Gantry Slipforming Technology, as suggested by Bygging-Uddemann of Sweden or Gleitbau of Austria. An example of this technique is shown in the photo right. Recent very extensive experience by Bygging-Uddemann has indicated that slipform construction of each base unit can be completed in 1 week. However, during the construction of the caissons for the Øresund Bridge Crossing from Malmo to Copenhagen construction rates for caisson structures, having dimensions of 49m x 22m x 9m and weighing 7,125 tonnes, were as short as 30 hours. Completion of each base unit would allow
the pontoon/barge unit to be sufficiently submerged to allow the base unit to be floated off in order to be towed to the column and base mating area in front of the main quay.
Marine installation Upending of the intended offshore site is achieved with water ballasting of the base. At the end of this phase the units would have
Methods of construction of base units using the gantry slip-form technique
approximately 21m of clearance above the seabed. Initial positioning of the units on the seabed is achieved with partial ballasting of the column with seawater. Final placement of the upper steel shaft,
turbine and blades is achieved with the use of a heavy lift crane vessel.
Full-scale testing Whilst 13 articulated column structures have operated successfully in the North Sea for many years (for different purposes), and whilst detailed hydrodynamic model testing has been carried out for the application of a wind turbine, it is considered necessary to execute full-scale trials of an articulated column supporting a large offshore wind turbine. Hence the immediate way forward is to execute large scale prototype testing. To date three possible sites have been identified and are being explored: offshore
Dounreay in 100m of water in conjunction with the Highlands and Islands Enterprise Board, which would involve a full scale prototype test; offshore Orkneys (Stromness) in 90m of water in conjunction with The European Marine Energy Centre (EMEC); and in the Celtic Deep offshore Pembrokeshire.
Summary The basic technology being used in the column and base of the AWC has been adopted from the oil and gas industries, which had up to 13 articulated columns operating successfully in the North Sea. The AWC is based on a proven, robust
technical solution suitable for the harshest environmental conditions, and it has a simple installation and removal process, based on a design successfully used by the offshore energy sector in the North Sea. The solution allows for installation on an
uneven seabed without the need for any seabed preparation. The use of deep water locations relatively
close to shore (approximately 25km) allows shorter cable connections to shore, and eliminates the need for the adoption of transformer units offshore, which otherwise would provide a significant component to the overall development costs. In order to achieve construction
efficiencies, and to optimise schedule and costs modern methods of mass/multiple fabrication of both the column sections and bases have been adopted from other engineering and construction sectors. This type of structure provides a unique
and economic solution for offshore turbines located in deeper water close to shore (approximately 25km). The estimated Levelised Cost of Energy (LCOE) in adopting the AWC in deeper water close to shore is marginally less than £56/MWhr, based on the adoption of 9.5MW turbines.
PETER BROUGHTON BSC, PHD, FRENG, CENG, FICE, FISTRUCTE, FRINA, FIMAREST
A three-dimensional view of the Cast Steel Articulated Joint which would use CSN 400 grade steel and weigh around 120 tonnes
With over 40 years’ experience in the project management of major offshore structures, Peter is now a director at Marine Engineering Energy Solutions Ltd, which he helped set up in 2013 to develop renewable energy projects within the marine environment. He was elected a Fellow of the Royal Academy of Engineering in 1996, and has been a visiting professor at the Civil Engineering Department of Imperial College, London, and a Royal Academy of Engineering visiting professor at Oxford University. The Articulated Wind Column (AWC) concept has been developed to support large offshore wind turbines in deep water locations close to shore.
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