After a decade of rising costs and technical challenges, project financial data indicates that offshore wind may finally be on a downward cost trajectory while the industry logged its best deployment year ever in 2015. Historically, rising offshore wind costs have been attributed to a myriad of hindrances, including increasing siting challenges (e.g., deeper water, greater distances from shore) and a wide range of installation and operational difficulties that have frustrated developers and offset gains made in technology, learning, and experience. The resilience of the European offshore wind industry to overcome these daunting cost challenges can be attributed to stable European policy commitments, the introduction of new offshore-class turbine and substructure technologies, and the creation of an offshore wind industry supply chain.
Floating spar platforms are widely used in the Gulf of Mexico for oil production. The spar is a bluff, vertical cylinder which is subject to Vortex Induced Motions (VIM) when current velocities exceed a few knots. All spars to date have been constructed with helical strakes to mitigate VIM in order to reduce the loads on the risers and moorings. Model tests have indicated that the effectiveness of these strakes is influenced greatly by details of their design, by appurtenances placed on the outside of the hull and by current direction. At this time there is limited full scale data to validate the model test results and little understanding of the mechanisms at work in strake performance. The authors have been investigating the use of CFD as a means for predicting full scale VIM performance and for facilitating the design of spars for reduced VIM. This paper reports on the results of a study to benchmark the CFD results for a truss spar with a set of model experiments carried out in a towing tank. The focus is on the effect of current direction, reduced velocity and strake pitch on the VIM response. The tests were carried out on a 1:40 scale model of an actual truss spar design, and all computations were carried out at model scale. Future study will consider the effect of external appurtenances on the hull and scale-up to full scale Reynolds’ numbers on the results.
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