Effect of Hurricanes on the Cumulative Fatigue Life Consumption of a GOM TLP

Accurate assessment of cumulative tendon fatigue is of importance for evaluation of the overall design life of TLPs. This cumulative fatigue damage can be derived from direct tendon load measurements. However, in some cases the quality of the tendon load measurements decreases or direct measurements are not available and assessment of cumulative tendon fatigue is not possible. An alternative approach to derive cumulative tendon fatigue is presented in this paper, based on the measured accelerations of the topsides. The quality of this approach is verified using data measured on board the Marco Polo TLP. To investigate the dynamic behavior of a Tension Leg Platform (TLP) in real life conditions and therewith provide insight in the physical mechanisms, as well as data to verify and validate the current design methods, a full-scale monitoring campaign on the Marco Polo Tension Leg Platform (TLP) has been conducted as part of a Joint Industry Project (JIP). Within this JIP measurements of the loads in four tendons have been collected continuously since June 2004. In addition to the tendon loads the wave frequency TLP motions and wave elevations were also recorded. The alternative approach to derive the cumulative tendon fatigue is verified using Marco Polo measurements recorded during 2005 and 2006. From the Marco Polo TLP motions recorded during the two year period the axial loads were calculated in all tendons. Statistical and spectral comparisons were made between the calculated and measured tendon loads. Based on those results a frequency dependent correction function was computed and applied on the computed tendon loads. Next the fatigue consumption based on the corrected calculated tendon loads was computed and verified with the fatigue consumption based on the actual measured tendon loads. This comparison provides the accuracy of the fatigue calculation when the computations are based on the measured TLP motions.