The surface signature of internal waves

Oceans that are stratified by density into distinct layers support internal waves. An internal soliton gives rise to characteristic features on the surface, a signature of its presence, in the form of a ‘rip’ region, as reported in Osborne & Burch (Science, vol. 208, 1980, pp. 451–460), which results in a change in reflectance as seen in NASA photographs from the space shuttle. In the present paper, we give a new analysis of this signature of an internal soliton, and the ‘mill pond’ effect of an almost completely calm sea after its passage. Our analysis models the resonant interaction of nonlinear internal waves with the surface modes, where the surface signature is generated by a process analogous to radiative absorption. These theoretical results are illustrated with numerical simulations that take oceanic parameters into account.

Current Feed Forward Control in Dynamic Positioning

The presented research investigates to what extent current feed forward control improves the positioning performance of Dynamically Positioned (DP) Floating Production Storage and Offloading (FPSO) vessels in varying currents. Positioning performance refers to the accuracy of positioning (both heading and horizontal position) and the associated power usage. Tidal current reversals and so called ‘internal soliton’ currents are examples of varying currents, in which current feed forward control is expected to improve positioning performance. Time domain simulations are carried out using the DpSim software, as developed by the Maritime Research Institute Netherlands (MARIN). In this research project, the DpSim software is extended with a module containing current feed forward control. When current feed forward control is applied in DP in varying currents, the mean and standard deviation of the control point excursion are reduced. The heading performance and power usage do not change significantly while achieving this reduction.