Improving Offshore Platforms Wave Measurements

Abstract

Offshore platforms are equipped with wave instrumentation at deck extremities to measure incoming wave elevations. When those instruments are close to large structural members they record the diffracted wave as well as the incident wave. This paper studies the effect of the diffracted wave on the measured wave height. First and second order diffracted wave elevations are computed for a model Tension Leg Platform (TLP) that was tested in the Offshore Basin of the Maritime Research Institute Netherlands (MARIN) offshore basin as part of the CresT Joint Industry Project (JIP). Their respective contributions to the wave spectrum are compared at locations near the structure. These calculations are useful for identifying the best locations for wave probes. The diffraction solution is used in forward calculations to compute the wave height and wave crest at locations under the deck from the undisturbed wave. These calculations can be used to set the air gap under the deck. Conversely, this paper introduces an inverse method to retrieve the undisturbed wave height and crest from the measured data by inverting the diffracted wave coefficients. The calculations are verified using measurements of undisturbed and diffracted waves under the TLP model. This work was sponsored by the Climatology and Simulation of Eddies (CASE) JIP.