The application of single buoy mooring systems as offshore terminals for VLCC's is now generally accepted. Large diameter hoses - 2 or 3 strings parallel - are used to transfer the crude oil from the sea bottom pipeline manifold via the terminal to the tanker manifold, or vice versa.
When the submarine hoses are replaced by articulated piping (comprising a vertical pipe connected to the SBM and a horizontal pipe connected to the sea bed, both sections linked together and all connections being made by universal joints), the reliability of the terminal will be strongly improved if the components of this pipe system are properly designed.
The design criteria have been obtained from extensive model tests on single and double systems and through calculations, whilst the components of the flex pipe system are and have been tested on full-scale. Great care is given to fatigue strength to predict life-time of the system on site and maintenance procedures are studied and tested on full-scale to prevent any unexpected situations when installed.
The single buoy mooring system, apart from acting as a crude oil terminal, is also employed for the transfer of other fluids, in particular petro-chemicals, bunker oil, gas and even iron ore slurries. Until now the underwater link between pipeline manifold and the SBM has been provided by flexible hoses.
This hose string must be relatively long in order that the buoy may ride easily and without restriction on the sea surface at high tides and at maximum expected wave heights. At low tide, the hose string should not bend beyond the safe bending radius, nor chafe along the anchor chine or along the bottom. Finding the right length of hose string and configurations to meet all these criteria presents a certain amount of difficulties.
Two basic configurations in use are the Lazy-S and the Chinese Lantern, both based on a delicate balance between the rigidity and an equilibrium obtained by counter-balancing the hose weight with either buoyancy tank or floats. Factors which disturb this balance are currents, waves, buoy motions and changes in the specific gravity of the fluids pumped through the hose. Static model tests are usually carried out to define the optimal lay-out, and in some difficult situations the dynamic behavior of the underwater hose lay-out has been tested in the NSMB model tank basins in Wageningen.
The design of an underwater hose lay-out for Several large diameter hoses in parallel for rough areas with strong tidal currents is extremely difficult since the radius of permissible bend is obviously large for such hoses and the deviation that can be tolerated from the neutral position is therefore small.
In order to overcome the restrictive requirements for the application of an SBM using such hoses, the "Flexpipe" system has been developed. The system comprises a vertical pipe connected to the SBM, and a horizontal pipe connected to the sea bed pipeline manifold.