In 1974 SBM, Inc. designed and built a SALM for the Pulai Field in the South China Sea, where the water depth is 245 feet. The terminal was designed to serve a permanent storage tanker of 190,000 DWT, which will remain moored in the 100 year storm with a significant wave height of 23.5 feet. At that time only two SALMs were installed (Okinawa and Brega). The third one was to be installed at Tembungo, also in the South China Sea. All three were of the base-(riser)-chain-buoy type. (Fig. 1) Extensive model tests were conducted for Pulai at the Netherlands Ship Model Basin in Holland. The test showed that a riser-sparbuoy SALM, in which the chain between riser and buoy is eliminated, has more desireable motion characteristics. In addition lower mooring forces were measured. (Fig. 2).
The knowledge obtained from designing and building the Pulai BALM was used to start the study, model tests and design of a riser-sparbuoy SALM for 533 feet water depth, to be installed at the Thistle Field in the North Sea. The terminal will serve 80,000 DWT tankers in wave heights of 15' significant. Survival conditions are 100 foot waves and 100 knot winds. In this paper the theoretical background, as well as construction of this SALM will be discussed. (Fig. 3).
For the design of an offshore terminal the mooring force is one of the main design criteria. However, in the severe conditions of the North Sea, the forces acting on the structure during survival are as important. The Thistle study and model tests showed that motions of the SALM, induced by the wave action are of extreme importance. It was shown by computer simulations and confirmed by model tests that correctly chosen dimensions and weight distributions on riser and buoy reduce the bending moments in the riser. Slightly different buoy dimensions and weight distributions will increase the bending moments occuring in an otherwise identical riser. Moreover, the mooring force level can be reduced significantly by applying the following theory.
The basic reason for the motion and force phenomena are the wave period and the energy level of the waves. The wave period can vary between 5 and 20 seconds. Energy levels of waves below 5 and above 16 seconds are so low that they may be overlooked. In the wave spectrum for which a SALM is designed, two frequency levels are of importance. From vibration theories it is known that a single column like the riser can vibrate in its own natural frequency, known as the beam frequency. On the other hand, the combination riser-buoy has its pendulum frequencies. Any of the frequencies can be of the same order of magnitude of the frequency of the high energy level waves, if the dimensions and weight distributions are not correctly chosen. Resulting resonance would put severe strains on the system as a whole.
