On the Determination of Impact Forces, Mooring Forces and Motions of Supertankers at Marine Terminal

Abstract

ABSTRACT This paper provides a method to predict:dynamic response of supertankers moored at sea-berth excited by water waves; andrelated mooring forces and impact forces induced to berthing structures. Significant results included:effects of dolphin-fender stiffness on hydrodynamic mass,effects of elastic characteristics and initial tensions of mooring lines on impact forces, mooring forces, and ship motions, andeffects of wave characteristics on these forces and motions The information will assist planners, designers, and operators of marine terminals for supertankers. INTRODUCTION The analytical treatment of problems in the berthing and mooring of ships was reviewed at the NATO Advanced Study Institutes held respectively in July 1965 (1), and in May 1973 (2). It was found that effort was greatly needed in solving the problem of coupled motions in six degrees of freedom of a ship moored in waves under linear or non-linear restraints from mooring lines and dolphin-fenders. Furthermore, the sway motion of a moored ship is considered more dangerous than surging but it has not received adequate attention until recently. Some progress has been made on the lateral motions of ships in oblique waves at zero or finite forward speed respectively as reviewed by Tasai (1972)(3). It was concluded that the slender body or strip theory method can be used to predict the dynamic response of moored ships with reasonable accuracy. However, experimental verifications are badly needed not only for the hydro dynamic coefficients but also for the exciting forces and moments in order to improve the analytical treatment of lateral motions of a ship. The motions of ships or platforms spread-moored with conventional line-and-anchor lines in open seas has been studied by several investigators (4–10). These studies all involved the theoretical determination of the responses of the ship to sea waves by application of the linear theory of ship motions. Relatively, very little information is presently available on the dynamic response of a ship moored at a sea-berth. Much attention was given in the past on the surge action of moored ships under the stimulus of long-period standing waves or seiches (11,12). Although the effect of seiches on swaying, yawing, and rolling motions are also important but it has not been investigated rigorously. In the case of mooring of supertankers at a sea-berth it involves multiple combinations of bow and stern lines, fore and aft spring and breast lines. Because the mooring lines are usually short and hang in air as non-co planar catenaries of pretensioned as non-catenaries, the dynamic analysis required must involve coupled motions. The effects of surge, sway, and yawing motions on the mooring forces are more important than those of heave, pitch, and roll motions because the hydrostatic restoring forces for the lateral motions are much greater than mooring restoring forces. Wilson noted that the vessel hull simulated by a rectangular block of suitable dimensions, is reasonably accurate particularly for supertankers (13,14).