This paper gives a broad outline of model tests with single point mooring systems. This includes the purpose of the model tests, the information necessary to set up test programs, the scope of tests, the characteristics simulated, measurements carried out, possible sources of errors and the analysis of results. Furthermore, examples of test results are given which show phenomena characteristic of s.p.m. terminals. Finally, some of the phenomena observed during tests and a source of uncertainty in existing methods to calculate the behaviour of s.p.m. ship systems are discussed.
Single point mooring terminals are, as the name implies, facilities of small horizontal dimensions, to which large vessels are moored by means of a bow hawser or by any other means which allows the vessel to rotate 3600 around the mooring point. Generally, the single point mooring terminal can have two functions. Primarily, it affords a safe mooring to the vessels in question. Secondly, it can form a link in the chain for the transport of oil.
The single point mooring terminal can assume many forms as is shown in Fig. 1. The most common is the Catenary Anchor Leg Mooring system (CALM) consisting of a flat cylindrical shaped buoy which is anchored to the sea floor by means of up to 8 chains. This system employs the properties of the catenary to supply the elasticity required when holding large tankers in open seas. The Single Anchor Leg Mooring system (SALM) consisting of a cylindrical buoy attached to a heavy base on the sea floor by means of a single pre-tensioned anchor chain, obtains its elasticity from the angle between the single anchor leg and the vertical. Enlarged version of the single point mooring terminal are the Exposed Location Single Buoy mooring (ELSBM) and the SPAR. The second of these does not only fulfill the function of mooring point and link in the oil transport chain, but is also used as a storage unit.
The types of mooring system described here, are excepting the ELSBM and the SPAR primarily used in water depths of up to approximately 150 ft. Some of the systems can with modifications be used for even greater water depths, however.
Rigid spar buoys connected to the sea floor by means of a universal joint are at present being designed. These consist of a buoy at the surface rigidly connected to a frame work which extends down to the base. This type of mooring is intended for water depths of 300 ft. or more. Finally, there are the mooring towers which consist of a slender open frame work fixed to the sea floor. Unlike all other systems, this type of mooring terminal does not contribute to the total elasticity of the system.
Generally, the tankers are connected to the mooring terminal by means of large circumference synthetic ropes which are part of the operating equipment of the terminal itself. In one case where the vessel is permanently moored to serve as storage space, the connection consists of a rigid arm.