Application of Probability to the Design of Single-Point Moorings

Abstract

HARING, R.E., MEMBER AIME, ESSO PRODUCTION RESEARCH CO., HOUSTON, TEX. ADAMS, R.B., ESSO STANDARD OIL (AUSTRALIA) LTD., SYDNEY, AUSTRALIA BEAZLEY, R.A., ESSO RESEARCH and ENGINEERING CO., LINDEN, N.J. KIPP, K.L., ESSO PRODUCTION RESEARCH CO., HOUSTON, TEX. Abstract A procedure has been developed to analyze mooring forces and to aid the selection of design parameters for single-point mooring systems. The parameters for single-point mooring systems. The results of a study indicate that a conventional SPM buoy could be used successfully to permanently moor a 52,000-dwt tanker in 260 ft of water under moderate sea conditions. The procedure utilizes scaled-model data and environmental observations to derive force-probability functions, which define The probability of exceeding any given force in the mooring system. Introduction There has been a steady increase in the application of monomoorings in loading terminals for exporting crude oil from shore storage and in discharging terminals for handling products and importing crude to refineries. Almost all the monomoorings have been single-point mooring buoys. Their general characteristics are illustrated in Fig. 1. The buoy itself is circular and of all-welded construction. It is subdivided into watertight compartments with provisions to handle the anchor chains, rotating manifold, and bow mooring hawser attachment. About 60 systems of this type have been installed or are under construction in the free world. Their main application has been in water depths less than 150 ft and in relatively calm, sheltered areas. Increased use of super tankers has stimulated further interest in loading buoys for deeper water. In at least two instances single-point mooring buoys have been used for floating, crude-oil storage vessels. As offshore oil fields are discovered in areas far from existing crude-oil markets or gathering systems, the use of single-point mooring systems for floating storage vessels will increase. The most important considerations in the design of single-point mooring systems for this application will be the continuous exposure to environmental forces, the more severe conditions typical of the open ocean, and water depths greater than are found at existing installations. A design procedure should allow the engineer to identify the probability of exceeding specified forces induced in the mooring system by the ocean environment. Mooring systems are usually designed well within the nominal strengths of their components to allow for fatigue, corrosion and short-duration high stresses. Therefore, even if the induced forces exceed the design forces, failure will not necessarily result. Rather, this condition alerts the marine operator to the need for corrective action or suspension of operations to prevent continued exposure of the system to excessive forces. SPEJ P. 219