Wave–current-structure blockage: Validation of one-way hydro-structural coupling with large-scale model tests of a stiffness-similar jack-up in elevated condition

Abstract

This paper documents extensive comparison between one-way hydro-structural coupling model (OF-ABAQUS) and key findings from the physical model tests of a jack-up performed in TCOMS Ocean Basin. A generic jack-up model in elevated condition is considered at 1:30 scale. A series of model tests involving waves and current is performed on one of the jack-up legs for calibration of hydrodynamic coefficients, as well as on the whole jack-up for characterisation of its dynamic response. With the novel use of polycarbonate rods for the jack-up legs, stiffness similarity can be maintained at model-scale which allows direct strain measurements on the legs and meaningful connection force measurements at the spudcan modules and leg-to-hull connection modules. Using the OF-ABAQUS coupling model, the jack-up legs are first numerically represented as porous blocks and simulated in OpenFOAM to derive the hydrodynamic forces and additional fluid damping following Santo et al. (2018a), where static simulations are performed. The forces and damping are subsequently mapped into a Finite Element (FE) model of the jack-up in ABAQUS as a one-way transfer to derive its dynamic response following Li et al. (2022), where dynamic simulations are performed. The novelty of this paper is on the demonstration of the coupling model in capturing dynamic structural modes and responses in multiple degree-of-freedoms (DOFs) which is more realistic, unlike the earlier work by Santo et al. (2018a) who demonstrated the coupling model for an idealised single DOF system.Comparisons with model test results yield reasonably good agreement for all the cases considered, covering not just bending moment at one of the spudcan modules but also strain and leg-to-hull connection force of one of the legs. From the dynamic response spectra, the total system damping at the surge resonance frequency is adequately represented using the coupling model, which accounts for the additional fluid damping contributed by the effect of wave–current-structure blockage, or more precisely by the relative velocity. Most importantly, the use of single and consistent set of hydrodynamic coefficients for the derivation of hydrodynamic forces and additional fluid damping is demonstrated throughout all the cases. In contrast, the use of standard Morison equation will require tuning of the hydrodynamic coefficients and arbitrary increase in the hydrodynamic damping to match the measured peak response of each wave condition, resulting in huge uncertainties when evaluating the structural reliability in larger and unseen sea-states. This paper demonstrates that, once the hydrodynamic inputs are adequately represented, the corresponding dynamic structural responses can be derived accurately for any sea-states. The proposed approach can be regarded as a step improvement in accurate prediction of hydrodynamic forces and dynamic responses of space-frame structures, which has its important applications covering new-builds, re-assessment of old platforms, and structural health monitoring or digital twinning of offshore assets.