In order to unlock the promising offshore market potential in deeper waters, where the wind blows more strongly and steadily, wind-farms are moving beyond the immediate continental shelf while increasing in size. In the range of intermediate water depths (30-60m), where the industry’s favorite monopile becomes gradually inefficient, alternative existing foundations are gaining ground. One of the most promising foundation types employed at the moment by the offshore wind industry is the suction caisson.Regarding their use as OWT foundations, suction caissons can serve either in monopod or multipod installations. Contrary to the lack of commercial applications as monopod foundations, caissons have already been deployed commercially as multipod foundations supporting jacket structures in intermediate water depths, with installations taking place at the Borkum Riffgrund 1, Borkum Riffgrund 2, and Aberdeen Bay offshore windfarms.Operating as the foundation system of multipod OWT support structures, suction caissons are subjected to a V-H-M (Vertical – Horizontal – Moment) loading combination, fundamentally different compared to that of their monopod counterparts.&nbsp;Resistance under the high-amplitude overturning moments that originate from environmental (wind and wave) loading is provided by the frame structure configuration of the jacket. As such, the windward jacket legs are subjected to compressive axial loading, while the opposite legs (leeward legs) are loaded in tension.The present paper focuses on the bearing mechanics of suction caisson foundations under axial loading, since the latter is considered to govern foundation design in the case of Suction Bucket Jackets (SBJs).&nbsp;The tensile load carrying capacity of a suction caisson in clay is investigated numerically through a series of coupled pore fluid diffusion – effective stress analyses with a hypoplastic constitutive model for clays. The latter allows for the examination of loading rate effects on the amount of negative excess pore pressures (passive suction) generated within the conﬁned soil plug and their contribution to uplift caisson resistance. Prior to the investigation of rate effects, the efficiency of the numerical model to simulate the pull-out capacity of suction caissons in clay is validated against published results. In the ensuing, foundation capacity for a range of loading rates is assessed; initially at the caisson level, and subsequently at the OWT system level, in order to shed light upon the magnitude of uplift capacity that should be accounted for in the design of multipod caisson foundations.

Suction Caissons as Multipod OWT Foundations in Clay: Investigation of Loading Rate Effects

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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