Design of Systems Single-Point for the Open Mooring Ocean

Abstract

ABSTRACT A procedure has been developed to analyze mooring forces and to aid the selection of design parameters for single-point mooring systems. The results of a specific study indicate that a conventional SPM buoy could be used successfully to permanently moor a 52,000dwt tanker in 260 ft depth of unsheltered water. The procedure utilizes scaled-model data and environmental observations to derive force-probability functions. These functions define the probability of exceeding any given force in the mooring system. INTRODUCTION Recent years have seen 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. l, 2 Almost all of these 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 40 installations of this type have been made in the free world to date. Their main-application has been in water depths less than150 ft and in relatively calm, sheltered areas. In at least two instances single-point mooring buoys have been used for floating, crude-oil storage vessels. 3,4 As offshore oil fields are discovered in remote 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, more severe environmental conditions typical of the open ocean, and greater water depths than are found at existing installations. A design procedure should allow us 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 unexpected 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. By identifying the probability of exceeding design forces, the effects of deterioration and operating shutdowns can be properly considered in the design. As a result of a study made for a particular open ocean area in 260 ft of water, we developed the procedure described in this paper. It incorporates scaled model-data and observations of sea conditions in the area of interest. MODEL DATA The mooring forces for a tanker at a single-point mooring were obtained from a model test program. This test program was conducted at the Netherlands Ship Model Basin and consisted of over 90 tests with model tankers moored in different environments. Tests were conducted with waves and currents. The waves were irregular and described by the Roll-Fisher spectra. Significant wave heights up to 20 ft were tested.