Model Tests With a Single-Point Mooring System In ShortCrested Waves

Abstract

ABSTRACT Laboratory experiments with a deep-water singlepoint moored tanker model in irregular waves are reported. A rigid mooring arm is used. Wave induced forces and motions in both long-crested and short-crested sea are investigated. Short-crested waves are usually believed to decrease the wave loadings. In many cases this is not necessarily the case. The aim of the present paper is to point out possible significant differences between wave loadings of short-crested and long crested waves. INTRODUCTION The oil industry is applying floating production system in increasing numbers for the development of off shore fields. There are a number of floating production schemes currently in use or under design, which use a floating surface support vessel. The most important factor in successful design of any production riser concepts is thorough knowledge of the environmental conditions and the loads they apply to the structure. Both for determining design and operational ability, it is important to know whether or not short-crested sea should be taken into account. The results and conclusions from this paper are primarily useful when working with single point mooring problems. The magnitude of differences in responses in short-crested and long-crested waves indicate that the results will be of interest in analysis of non-rigid systems in general. In this paper we are going to describe tests which were conducted in both long-crested seas, with and with out wind at various heading angles, and in short-crested seas. In this paper, special emphasis is given to compare results from unidirectional head seas and multidirectional seas. These tests were a part of an extensive model test program carried out in order to investigate a deep water SALMRA (single anchor leg mooring rigid arm) moored storage tanker system. The tested SALMRA system is a double articulated system design for 375 m water depth and for North Sea conditions. Main features of the system are a gravity base, a structural riser, a large top buoy to provide the restoring capacity and a rigid arm of the truss type for permanent tanker mooring. Fig. 1 shows the system in general. EXPERIMENTAL SET-UP Ocean basin The model experiments were carried out at the Norwegian Hydrodynamic Laboratories (NHL) ocean basin. Fig. 2 shows a plan view of the Ocean Basin and the towing tank. Further, a cross-section of the Ocean Basin and some characteristic data are shown. In the basin, two different wave generators are installed. In one end a hydraulically driven double flap generator is located, which is able to generate unidirectional waves with a maximum wave height of 90 cm referring to regular waves. A beach is located in the other end of the basin. At one of the sides of the basin, a 62.5 m long wave generator is located. It consists of 144 individually controlled flaps which are able to generate unidirectional or multidirectional waves.