Current blockage and extreme forces on a jacket model in focussed wave groups with current

This paper documents large laboratory-scale measurements of hydrodynamic force time histories on a realistic 1:80 scale space-frame jacket structure exposed to combined waves and in-line current. The aim is to investigate the fluid flow (and the associated hydrodynamic force) reduction relative to ambient fluid flow due to the presence of the jacket structure as an obstacle array, interpreted as wave-current blockage. Transient focussed wave groups, and embedded wave groups in a smaller regular wave background are generated in a towing tank, and the jacket is towed under different speeds opposite to the wave direction to simulate wave loading with different in-line uniform currents. The measurements are compared with numerical predictions using Computational Fluid Dynamics (CFD), with the actual jacket represented in a three-dimensional numerical wave tank as a porous tower modelled as a uniformly distributed Morison stress field. Good agreement is achieved, both in terms of incident surface elevation as well as total force time histories, all using a single set of Morison drag (Cd) and inertia (Cm) coefficients. Substantial force reduction is observed under transient large crest relative to prediction from the present industry design guideline with the same Morison coefficients. We demonstrate the generality of our findings: without influence of Keulegan-Carpenter (KC) number effect, a single invariant set of Cd and Cm is all that is required to numerically explain and reproduce the measured total force time histories on a realistic jacket model for a large range of wave heights and non-zero current speeds.