Real-Time-Forecast of Wave-Structure Interaction in Natural Sea States: Influence of the Wave Encounter Angle
Authors: Kosleck Sascha 
Published: September 2014

In an offshore environment the ability to predict the development of a natural wave field can be of significant interest for a wide range of operations, such as for example on/off loading of cargo, installation or heavy lift processes, helicopter landing or even dynamic positioning. Being able to deterministically predict natural sea state just in time also allows for the prediction of wave structure interaction. Hence, not only motions in 6 degrees of freedom, for an arbitrary number of vessels or structures, but also velocities and accelerations at certain points of interest can be derived before they occur. Especially in difficult, slightly unclear weather conditions at the edge of operational limitations additional information on a potential exceedance of operational limits, such as for example wave heights, motions or accelerations, help to differentiate between safe and critical situations.

Objective of the work conducted within the last years by Clauss, and Kosleck et al. is the development of a linear, deterministic approach for the just-in-time prediction of an ocean wave field of arbitrary specification. The survey presented within this paper focuses on the forecast of natural sea states and wave induced vessel/structure motions based on information gathered from a series of surface elevation snapshots of the surrounding free water surface. In order to investigate the approach a typical North Sea sea states with an underlying JONSWAP spectrum is generated and investigated at model scale. Simultaneous measurements of the surface elevation at over 450 different positions along the direction of wave propagation enable the artificial generation of surface elevation snapshots, used as input for the prediction method. Furthermore, these measurements also provide the basis for a comparison of calculated predictions and measurements at a huge variety of positions along the tank. The development of algorithms based on frequency domain analyses and the characterization of the sea state using a linear approach, enable the prediction of natural sea states in time and space — inside a well-defined range of validity. Knowing the general motion behaviour (transfer functions) of a structure/vessel together with the wave train to be encountered in the near future, its motion behaviour can subsequently be derived deterministically, taking into account that for the sailing vessel the transfer functions change depending on the actual cruising speed. The motion forecast procedure is compared and validated using measurements of the motion behaviour of an LNG carrier, again at model scale. So far, it has been shown (please see [1]) that the developed linear methods for the prediction of the motion behaviour of a stationary or cruising vessel/structure deliver excellent results for the vessel heading at different speeds but constant heading, namely 180° and 0° (defining head seas and following seas).

This paper presents, for the first time, investigations with a vessel at zero speed but changing heading, hence a time varying change of the angle of wave attack.