Performance of a novel concept of a monopile adapted with pre-tensioned tethers for intermediate waters

Abstract

Monopiles are the Industry's first choice today to support an Offshore Wind Turbine (OWT) due to the complexities associated with fabrication, transportation, and installation of various foundation solutions. These are, however, considered suitable for water depths up to around 30 to 35 m, beyond which these become massive and complex to handle. Increasing monopiles’ application range without making them too large by adopting some suitable means may significantly impact the rapid growth of OWTs at intermediate water depths (∼50 m), where existing solutions like jackets or concrete gravity foundations involve large capital expenditure. In this paper, a novel approach of utilizing pre-tensioned tethers along with a 6.0 m diameter monopile has been explored for 50 m water depth. Under extreme load case, the stress ratio has been noted to be reduced from 1.17 to 0.44 when the pre-tensioned tethers are used. Similarly, it is found that by adjusting parameters associated with the proposed concept, the fatigue damage can be brought down from a very high value to 0.63 or less. Thus the results show that the proposed approach can open new avenue to bring competitiveness to the cost of offshore wind energy at the intermediate water depths.