Wave forces can form a serious threat to offshore platforms and ships. The damage produced by these forces of nature jeopardizes their operability as well as the well-being of their crews. Similar remarks apply to coastal defense systems. To develop the knowledge needed to safely design these constructions, in close cooperation with MARIN and the offshore industry the numerical simulation method ComFLOW is being developed. So far, its development was focussed on predicting wave loads (green water, slamming) on fixed structures, and for those applications the method is already being used successfully by the offshore industry. Often, the investigated object (ship, floating platform) is dynamically moving under the influence of these wave forces, and its hydrodynamic loading depends upon the position of the object with respect to the oncoming waves. Predicting the position (and deformation) of the body is an integral part of the (scientific and engineering) problem. The paper will give an overview of the algorithmic developments necessary to describe the above-mentioned physical phenomena. In particular attention will be paid to fluid-solid body and fluid-structure interaction and non-reflecting outflow boundary conditions. Several illustrations including validation, will demonstrate the prediction capabilities of the simulation method.

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Simulating the hydrodynamics of deformable, floating structures using a partitioned strategy poses a major challenge when the ratio of the added mass to the structural mass is considerate. Existing computational procedures for fluid-structure interaction become less efficient or even unstable. In these situations, it is advisable to modify the coupling to allow the fluid to respond better to the solid motions. A simultaneous solution of the equations governing fluid and solid-body would be a stable choice but is often not feasible. Usually the numerical problems are taken care of with subiterations between fluid and structure, but their convergence can be slow. In this paper we present a more powerful, quasi-simultaneous approach, which tries to mimic a fully simultaneous coupling in an affordable way. It makes use of a simple approximation of the body dynamics, based on the (6 DOF) solid-body modes and the main elastic modes of the structure. The method will be demonstrated in offshore practice, with a falling life boat, a floating CALM buoy, an elastic membrane and a rubber gate.@en

The physical level of interaction between fluid and structure can be either one-way or two-way depending on the direction of information exchange at the interface of fluid and solid. The former can be solved by a partitioned approach and weak coupling. In problems involving two-way fluid-structure interaction, using a partitioned approach and strong coupling, sometimes stability restriction is encountered. This is an artificial added mass effect, which is independent of the numerical time step. Unfortunately an accurate and efficient method to deal with all the different levels of interaction is scarce. Conventionally, relaxation is applied to remedy this problem. The computational cost is directly related to number of sub-iterations between fluid and structural solver at each time step. In this study, the source of this instability is investigated. A discrete representation of a basic added mass operator is given and instability conditions are assessed. A new method is proposed to relax this restriction, the idea essentially is to remove the instability source from the structure and move it to the fluid and solve it monolithically with the fluid. We call this an interaction law. An estimate of the structural response is derived from structural mode shapes. As a test case, a 2D dam break problem interacting with an elastic vertical flexible beam is selected. The interaction of fluid with the beam undergoes several stages. The breaking waves on the beam can increase the added mass drastically, therefore the added mass ratio increases as well. In such a cases, the asset of interaction law is better elaborated, while the stability condition requires very high relaxation without interaction law, but the relaxation can be lowered by only using first five beam mode shapes. As a consequence, the number of sub-iterations reduces by one order. The numerical observations confirm the reduction in computational time due to utilization of the interaction law.@en