Analysis of Peak Mooring Force Caused by Slow Vessel Drift Oscillation in Random Seas

Abstract

INTRODUCTION Traditional naval architecture technology has not devoted extensive efforts to understanding of the requirements for mooring a vessel in the open ocean under storm conditions. Perhaps it can be said that conventional maritime practice would consider mooring under such circumstances an act of foolishness and therefore not deserving of serious technological effort. The demand on the offshore petroleum industry for mooring under trying conditions has, however, created the need for a clearer understanding of the physical phenomena involved. The offshore industry has experienced major difficulties in mooring under storm conditions and has suffered extensive financial loss. Over the years, attempts have been made to solve offshore mooring problems, utilizing a variety of vessels and mooring techniques. Results of experience and practice offer conflicting indications of the relative merits of various mooring systems. Various engineering and scientific studies have contributed toward an understanding of many factors influencing forces; however, it appears that previous studies have, for the most part, ignored a governing phenomenon. Specifically, there has been little attention devoted to the effects of slow vessel drift oscillation in random or irregular seas. It is this phenomenon which is the prime subject of the present paper. Fig. 1 illustrates results obtained from model tests of a moored vessel in irregular waves. Shown in the figure, as a function of time, a-re the variations of wave height and period, the surge or drift position of the vessel and the tension in the primary mooring line. It will be noted that the surge motion of the vessel involves both a direct wave induced surge and a gradual slow drift taking place over a period of 1 minute or more in prototype time. The drift behavior shown in Fig. 1 is the phenomenon of critical importance. This type of drift motion is found in the motion records of moored ships in an actual ocean storm environment. Moreover, the basic behavior of slow oscillations is not unique to moored vessels. For instance, such behavior has been observed in tests involving vessels towed through irregular waves with a constant towing force. In such case, it has been observed that the vessel velocity exhibits slow oscillations with periods in the range of 1 to 2 minutes. When an ocean wave is propagated toward a moored vessel, part of the wave is reflected, the remainder being transmitted on beyond the vessel. The conservation of wave momentum results in a net force applied to the vessel for each wave. For regular waves the consequence is a steady drift force resulting in a static shift of the position of the moored vessel. For irregular waves, on the other hand, a varying sequence of drift forces arises in correspondence to changes in wave height and period. Investigations leading to this paper show that the ensuing long period drift of the vessel can for many cases be the completely dominating influence in determining maximum mooring line tension.