Blockage and relative velocity Morison forces on a dynamically-responding jacket in large waves and current

Abstract

This paper documents large laboratory-scale measurements of hydrodynamic force time histories on a realistic 1:80 scale space-frame jacket structure, which is allowed to respond dynamically when exposed to combined waves and in-line current. This is a follow-on paper to Santo, Taylor, Day, Nixon and Choo (2018a) which used the same jacket structure but very stiffly supported. The aim is to investigate the validity of the Morison equation with a relative velocity formulation when applied to a complete space-frame structure, and to examine the fluid flow (and the associated hydrodynamic force) reduction relative to ambient flow due to the presence of the jacket structure as an obstacle array as well as the dynamic structural motion, interpreted as wave–current-structure blockage. Springs with different stiffness are used to allow the jacket to respond freely in the incident wavefield, with the emphasis on high frequency modes of structural vibration relative to the dominant wave frequency. Transient focussed wave groups, and embedded wave groups in a smaller regular wave background are generated in a towing tank. 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 and modelled as a uniformly distributed Morison stress field derived from the relative velocity form. A time-domain ordinary differential equation solver is coupled internally with the CFD solver to account for feedback from the structural motion into the Morison distributed stress field. An approximate expanded form of the Morison relative-velocity is also tested and is recommended for practical industrial applications. Reasonably good agreement is achieved in terms of incident surface elevation, dynamic model displacement as well as total hydrodynamic force time histories, all using a single set of Morison drag (Cd) and inertia (Cm) coefficients, although the numerical results tend to slightly overpredict the total forces. The good agreement between measurements and numerical predictions and the generality of the results shows that the Morison relative-velocity formulation is appropriate for a wide range of space-frame structures. In these tests, this gives rise to additional damping of the dynamic system which is equivalent to 8% of critical damping. This is significantly larger than both the structural and hydrodynamic damping combined (which is about 1%) as quantified through free vibration (push test) in otherwise stationary water.