Water depth influence on wave–structure-interaction

For seakeeping model tests and the experimental investigation of offshore operations the water depth is a parameter which is difficult to adjust to a particular problem and scale. It poses a limit for the feasibility of model tests by governing the model scale—especially in small testing facilities. Therefore, it is often desirable to violate or ignore the condition “water depth” and to use larger models in order to carry out tests at higher Reynolds numbers. In this paper its influence is examined in order to clarify how it affects wave kinematics, wave forces, and wave–structure interaction, and where the deep water assumption holds.After a short recapitulation of the water wave theory, a freak wave sequence is presented that has been reproduced at two different wave tanks at scales 1:75 and 1:81 and two water depths. The wave kinematics are approximated with asymmetric high order Stokes waves. The comparison of the computational wave kinematics reveals that moderate water depths are of low importance for extreme waves with identical surface elevations but different water depths.In order to examine the effects regarding wave–structure interaction with regard to seakeeping investigations model tests are conducted at various water depths using the huge semi-submersible crane vessel Thialf. Here, the effect on heave and pitch motion is investigated. The model tests are accompanied by numerical calculations utilizing the panel code WAMIT. A comparison of the hydrodynamic coefficients in deep water with those in decreasing water depths is shown including coupling effects taking into account viscous damping which is determined in experiments. It is shown that for the given scale of 1:75 a water depth of 1 m is sufficient to model deep water conditions.