Coupled Eulerian Lagrangian Approach to Model Offshore Platform Movements in Strong Tidal Flows

Abstract

Offshore platforms are subjected to wind, waves and tidal flows. Tidal flow will generate a steady current, which induces a lift force and a drag force on the platform legs. In addition, water particle velocities induced by waves give rise to an oscillatory flow. As a result, the structure will experience a lift, drag and inertia force when subjected to wave-induced flow patterns. On top of that, a turbulent Von Karman vortex street can appear in the wake of the platform legs for certain combinations of dimensions and flow velocities. Vortex shedding can lead to vortex induced vibrations, which may jeopardize the integrity of the entire offshore platform. Environmental loads can cause significant deformations of offshore structures, which can in turn influence the fluid flow. Multiphysics modelling is required to capture the mechanisms governing fluid-structure interaction. In this paper, a Coupled Eulerian Lagrangian (CEL) approach is pursued to simulate offshore platform movements in strong tidal flows. In a CEL analysis, the fluid flow is modelled in an Eulerian framework: the water is described by an equation of state, and can flow freely through a fixed mesh. The offshore platform is modelled as a compliant structure in a traditional Lagrangian formulation, where the nodes move with the underlying material. Interaction between the fluid domain and the offshore structure is enforced using general contact conditions. The strongly coupled problem is then tackled with an explicit solver. Here, the CEL approach is demonstrated to simulate the movement of an offshore jack-up barge. The response of the vessel is calculated for different flow conditions. The multiphysics model allows evaluating the added value of structural redundancy, e.g. in the number of platform legs required for a safe design. In addition, it provides a valuable tool to predict the tidal windows allowed for given operational conditions.