Weathervaning Instabilities of a FPSO in Regular Waves and Consequence on Response Amplitude Operators

Abstract

A series of model tests in regular seas were performed on a 1:60 scaled model of a generic FPSO. The model was moored by means of four instrumented mooring lines attached to an internal turret. The wavelength to model length ratio was varied from 0.5 to 2.0, while the ratio of the wavelength and the wave amplitude was kept constant as per ITTC seakeeping recommendations. The goal of the tests was to evaluate the response of the hull to swell conditions that may happen in open sea environments and to assess the weathervaning characteristics of the internal turret arrangement. Three set of tests were performed. 1. Model free to rotate about the turret; test duration 15 minutes full scale and initial position set at head seas. In this case yaw instability was observed where the model showed a tendency to rotate towards starboard. 2. Model free to rotate about the turret; test duration 2 hours full scale and initial position was varied. The effect of initial condition on the instability and any slowly varying oscillations were observed. 3. Model slightly restrained by a constant line force applied at the stern. In this case, the model maintained its heading towards waves. The results of these runs were used to evaluate the Response Amplitude Operators for different headings of the model. During the tests the six accelerations and six displacements have been measured by means of an inertial and optical system respectively, together with the wave elevation in the basin. From the results, motion response amplitude operators were obtained at different heading conditions. The experimental results for the model displacements were compared with simulations from the well-known diffraction program WAMIT/WADAM. The paper will discuss possible reasons for the yaw instabilities, including the effect of moorings and test duration. The influence of the heading changes on the RAO of the hull with and without constraints will be discussed. Finally, RAO comparison with SESAM will demonstrate similarities and differences with RAOs at different sea headings.