Full Scale Measurements of Cyclic Bending Moments on Subsea WellHeads on the Norwegian Continental Shelf
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Résumé
Abstract Wellhead Fatigue is a growing well integrity concern when installing subsea wells, especially with exposure to harsh environments, like e.g. North Sea or East Canada, and extreme environmental loads, using deep water drilling rigs in shallow water. Consequently, subsea instrumentation has been employed in several projects to document the actual load levels experienced by the subsea wellheads during drilling. There are no standard methods for such instrumentation and measurement processing, but in these cases the bending loads on subsea wellheads during offshore drilling have been measured according to ref. 11/ and methods presented at OTC 2015, ref. /5/, and OMAE 2015, ref. /3/. The actual load levels presented here have never before been published, and are of an unprecedented magnitude. Previously, state-of-the-art riser analyses have been found to be conservative when comparing to full-scale measurements offshore. Recent drilling campaigns in shallow water on the NCS have shown that in some cases the analyses are no longer conservative as they match the actual loads measured. Cycle-range histograms have been established from the measured WH bending moment time series. In some cases the dynamic bending moments are excessively large, even in moderate storm conditions. The loads can be a significant fraction of the static capacity of a typical subsea wellhead. Not sufficient to threaten the structural strength of the wellhead, but representing a significant challenge for the fatigue capacity. Three important findings have been made from resent measurements campaigns in shallow water in the North Sea. The dynamic bending moments acting on the subsea wellhead may be exceedingly large in shallow water.The magnitude of the dynamic bending moments is not primarily governed by the vessel and vessel motions, but also by the drilling package, i.e. BOP and riser. The dominant effect is the direct wave loading on the riser and BOP system itself.There seems to be a shift in the relation between predicted loads and actual loads at some given water depth. For deeper waters, riser analyses are usually found to provide conservative results. However, the riser analyses may not necessarily be conservative when moving into shallow water. This is believed to be related to the first riser mode coinciding with the region with most wave energy, i.e. 8–10 seconds.Consequently the margin of safety in safety-by-design excerisises may vary with waterdepth The relation between load levels and selection of drilling package is described in more detail. Modern sixth generation drilling rigs with large Blow Out Preventers (BOP), will get a double penalty. The large height of the BOP increases the bending moments at the wellhead. In addition, the riser tension is increased by the large submerged weight of LMRP. Finally, if the BOP stack itself is set in dynamic motion, it can potentially give a dynamic amplification of the loads. This paper provides important new information to assessments of subsea well integrity in shallow water.
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