This paper presents a simplified way of comparing the fatigue load on different subsea wells. The simplest comparison is done by accumulating the number of days BOP has stayed connected to the wellhead. The wellhead fatigue load is however heavily dependent on the vessels used, water depth and weather while connected to the well. An equation for deriving a benchmark load factor for each operation phase for a subsea well is proposed. This benchmark load factor takes into account the water depth, metocean season of the operation, BOP height and weight, and the stiffness of the marine riser lower flex joint. This benchmark load factor will represent a standard number of days with a BOP connected, correcting for some known effects. The goal has been to define a measure of ‘BOP days’ that accounts for the water depth, operational season, and BOP particulars. A base case (one MODU, 100 m water depth, and all year operation), equating to one standard BOP day, has been chosen as the reference for all cases discussed.
The validity of the benchmark load equation will be shown through a comparison with 31 different global riser analyses intended for wellhead fatigue. For each of the 31 data sets, time domain load analysis is done for all sea states in the wave scatter diagram. The different analyses covers different rigs, water depths and two operational phases (with or without subsea XT installed). To enable a large scale comparison of the bench mark factor, an approach where the fatigue load is summarized using the bending moment standard deviation on the wellhead datum is presented. This methodology is then compared to four full fatigue calculations using a typical subsea wellhead fatigue capacity. Then the simplified fatigue calculation is performed for all 31 global riser analyses. The calculated damage is then compared with the corresponding bench mark formula in each case.
Finally it is shown how this benchmark load formula has been implemented into the Statoil WellSpot database as a fatigue load criticality screening tool for the different Statoil subsea wells. It is further shown how this can be used as a tool during planning of future operations, and how to prioritize wells where a detailed fatigue analysis is recommended.