Wellhead Fatigue, Effect of Uncertainty in Directional Variation and Environmental Conditions for Operations of Short Duration

Fatigue of subsea wellhead systems due to wave-induced loads from riser and rig motions has been subjected to increased attention in recent years. Major accidents due to fatigue failure has not been experienced up to now, but with increasing size of drilling rigs and BOPs in combination with longer drilling campaigns on wells, proper documentation of adequate fatigue capacity for a planned operation is becoming increasingly important. Achieving this has turned out to be a challenging issue for some wells, where application of existing fatigue analyses methodology provides rather short fatigue lives. It is expected that such calculated fatigue life results are somewhat conservative due to conservatism in the assumptions commonly applied as both input parameters and methodology are associated with some uncertainty. However, it is difficult to quantify the degree of conservatism in such analytical results. One area of conservatism relates to wellhead fatigue analyses typically being performed under the assumption that the environmental actions and response is unidirectional (in one plane only) while assuming the most unfavorable direction. This is clearly not the case in real life, and one would like to take some benefit of distributing the loading more accurately around the circumference of the pipe section. For offshore structures subjected to continuous loading over many years, one may use directional scatter diagrams for wave height and wave period based on average over years of data. However, for wellhead fatigue, the duration of a drilling or well intervention operation may be relatively short, perhaps weeks or a few months, and it is more likely that there one prevailing environmental direction may dominate such short term exposures. The environmental conditions will vary from one year to another, i.e. the environmental conditions in one particular year may be less or more severe than the average represented by the scatter diagram. Both these variation effects are studied in the present paper by doing fatigue analysis for different durations; 3 days, 10 days, 1 month, 3 months, 6 months and one year. The same period is taken from each of the 56 years of environmental data, and individual results for that period within each year are calculated. Statistics quantifying the directional effects is determined. The directional effect is here defined as the ratio of the fatigue life where the directional information is taken into consideration and the fatigue life using an omnidirectional scatter diagram considering one direction only. Directional effect with reference to both head sea and beam sea are considered. Similarly, statistics of the fatigue life variation between years are obtained. Statistics are estimated separately for the six different durations. Finally, the statistics derived are applied in a structural reliability analysis (SRA). The SRA results are then used to propose reasonable factors that can be applied in conventional wellhead fatigue analysis to account for directional effects and for variation in environmental conditions from one year to another, depending on duration of the planned operation.