Real Time Prediction of Second Order Wave Drift Forces for Wave Force Feed Forward in DP

The presented research is an extension of the development of an onboard wave and motion estimation system that aims to predict wave elevation and wave frequent vessel motions some 60–120 s ahead, using remote measurements of short crested waves. The main aim is to provide decision support during motion critical offshore operations. As an addition to this, an attempt is made to predict second order wave drift forces. This can be useful for condition monitoring of a Dynamic Positioning (DP) system [18] or for feed forward of wave drift forces into the control of DP systems. The paper describes the techniques used to predict second order wave drift forces real time from remote wave measurements. For validation, measurement data is used from model experiments during which wave elevation in irregular short crested seas was recorded by a large number of probes simultaneously. A method is described to obtain a 3D representation of a wave field in such a way that it can be used to predict both first order waves and motions and second order forces. The second order forces resulting from the wave field description as obtained from remote probe measurements can be compared to those that have been derived from the probes in the proximity of the prediction location, thus providing insight in the sensitivity of the 2^nd order wave force prediction error with respect to the first order wave prediction error. In a full scale field situation, remote wave sensing can be provided by X-band radar. Possibilities for application of the developed method with the WAMOS II X-band radar system is considered.