Assessing the Dynamic Stability of an Offshore Supply Vessel

ABSTRACT The dynamic stability was investigated of a typical offshore service vessel operating under stability critical operating conditions. Excessive roll motions and relative motions at the stern were studied for two loading conditions for ship speeds ranging from zero to the design speed. A linear frequency-domain seakeeping analysis was followed by nonlinear time-domain simulations of ship motions in waves. Based on results from these methods, critical scenarios were selected and simulated using finite-volume solvers of the Reynolds-averaged Navier-Stokes equations to understand the phenomena related to dynamically unstable ship motions as well as to confirm the results of the simpler analysis methods. Results revealed the possibility of excessive roll motions and water run-up on deck; counter measures such as a ship-specific operational guidance are discussed. INTRODUCTION A rational evaluation of the operational performance of an offshore service vessel (OSV) should include its seakeeping, specifically, the assessment of its dynamic stability [1]. Although the industry has addressed numerous efforts to reduce influences leading to crew fatigue, the operational safety is primarily governed by the ship’s motion performance in a seaway. Ship motions must be restricted to ensure not only an acceptable level of crew comfort, but also to avoid excessive roll motions that can lead to large lateral accelerations, green water on deck, or even loss of stability. The aim of this study was to investigate the dynamic stability of a typical OSV operating under stability critical conditions. Usually, excessive roll motions occur due to synchronous or parametric resonance. Synchronous resonance is excited by waves with periods close to the ship’s natural roll period. Parametric resonance is brought about when righting levers vary in resonance with natural roll motions. Two modes of parametric resonance are most relevant in practice: low-cycle (or principal) parametric resonance occurring when the wave encounter period is close to half the natural roll period and fundamental parametric resonance, for which the wave encounter period is close to the natural roll period. Although the first mentioning of parametric roll goes back to Froude [2], principal (or low-cycle) parametric rolling was first practically observed and studied for smaller ships with marginal stability in following seas, in relation to capsizing, in the thirties and forties, [3], [4] followed by [5], [6]. These and other early studies have revealed that principal parametric resonance occurs in following waves when the wave encounter period is close to the half of the roll natural period, and wave length is close to the ship length. As a result of these investigations, IMO approved operational guidance for ship masters, indicating dangerous situations in following and quartering seas [7]. Due to several accidents in head waves, notably with container ships [8], and later with other ship types [9], attention arose to principal parametric resonance in head waves. Model tests and full-scale observations have shown that principal parametric resonance can occur not only in long-crested longitudinally running (i.e., head and following) waves, but also at slightly oblique heading angles, with and without directional wave spreading [10]. Fundamental parametric resonance has attracted less attention so far because, for container ships, it occurs in loading conditions with low initial