Current forces in tankers and bifurcation of equilibrium of turret systems: hydrodynamic model and experiments

Abstract

Listen

Translate article

In the present paper a heuristic hydrodynamic model is proposed to describe the forces and moment in the horizontal plane in a tanker caused by an ocean current. The obtained expressions depend only on the incidence angle of the current, on the main dimensions of the ship and on some well known hydrodynamic coefficients. The results from this model were confronted with experimental values obtained by Wichers ( A Simulation Model for a Single Point Moored Tanker, Publ. 797, MARIN, Wageningen, the Netherlands, 1988) and at IPT's test basin, the observed adherence being reasonably good for all current headings. A special device was used to experimentally study the stability of the equilibrium of a tanker free to rotate around a vertical axis and with the other's movements constrained, emulating a turret system with stiff mooring lines. In this situation the only bifurcation parameter is the turret position, and the proposed hydrodynamic model was able to predict, with reasonable accuracy, the critical value of this parameter. Furthermore, the adherence between the experimental and theoretical models in the post-critical region was very consistent, disclosing that the post-critical behavior is dominated by the parcel C YΨ¦Ψ¦ , where C Y is the lateral force coefficient in beam current and Ψ is the angle between the current and the ship's longitudinal axis. It turns out that, in the vicinity of the bifurcation point, the yaw angle increases linearly with the bifurcation parameter and not with its square root, as a standard approach would suggest, based on Taylor's series expansion (hydrodynamic derivatives).