Towing-tank experiment and analysis of nonlinear roll damping for a drillship with different appendages

Abstract

Offshore platforms with ship-like hull structures have relatively low roll damping moment. In this paper, variations of roll reduction appendages based on increasing damping strategy are proposed. The appendages are an ordinary bilge keel, a newly adopted variation involving extended platforms with ship-like hull structures, such as drillships or reversed flaps, and a stinger type. To validate the roll damping performance of newly designed appendages, a series of free-decay data for the roll motion in calm water and in regular waves are obtained from towing tank tests. The free-decay data for the roll motion is analyzed using a linear-plus-quadratic damping method as well as an alternative method based on the Hilbert transform in order to overcome the possible problem of nonlinearity for non-conventional roll stabilizers. The Hilbert transform technique has the advantage over polynomial damping models for not assuming the form of the roll characteristics. A comparison of the analysis methods for the different appendages is carried out for linear, nonlinear, and equivalent linear damping methods. The results are crosschecked in regular beam waves for different amplitudes near their resonant frequencies. From the experimental analysis, the linear and nonlinear roll characteristics for various appendages are discussed.