Truncation Methods for Deep Water Mooring Systems for a catenary moored FPSO and a Semi Taut Moored Semi Submersible

With the trend towards ultra deep water field developments, the joint application of model tests and numerical simulations is becoming very important. The practical limits of model basins give rise to the concept of truncated mooring systems. The philosophy is that a numerical model of the truncated set-up is validated against model test results for the same truncated setup. The resulting calibration information is then applied in full-depth simulations in operational conditions. This approach requires a truncation strategy for mooring and riser truncation where the motion behaviour of the floater is as close as possible to the full depth system.
Recent experience in model tests has shown that the optimum truncated mooring system is not always obvious. Mooring line types, pre-tension values, pretension angles, floater size and floater type are aspects that play a major role. Experience learned that FPSOs, deep water CALM buoys and semis require a different approach to guarantee reliable results. Detailed analysis of the mooring and riser system static characteristics as well as the dynamic behaviour is consequently necessary. The present paper discusses an optimum strategy to come to such truncated mooring systems. Comparisons are made between the floater behaviour with full-depth and truncated mooring system and risers for FPSOs and Semi submersibles.