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Effects of BOP Stack Modelling on Estimated Wellhead Fatigue Damage
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Marine Technology.
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Marine Technology.
2013 (English)MasteroppgaveStudent thesis
Abstract [en]

To date, there are no international codes or standards that deal with estimation of fatigue in subsea wellhead systems. Nevertheless have preliminary analytical methods for wellhead fatigue estimation been established. These analytical methods involve the use of global dynamic response analyses. Such analyses are commonly carried out in nite element software where the drilling system is modelled as beam or bar elements. Several uncertainties exist in regards to the mathematical modelling and simulation in global response analyses of drilling systems. In this thesis the uncertainties that are related to the goodness of the representation of the blowout preventer stack (BOP stack), are addressed. An overview of previous and ongoing work on analytical estimation of wellhead fatigue is given. Relevant theory on the subject is presented and described. The theory comprises of fatigue on structures, loads on a drilling system and static and dynamic response analysis. The main features of the preliminary analytical methods for estimating wellhead fatigue are summarized. The BOP stack is commonly assumed to have innitely high stiness when performing a global response analysis of a drilling system. The main objective in the thesis is to investigate if this is a good assumption. The investigation start with local modelling of an elastic beam element model of a BOP stack. Further, this elastic beam model is calibrated to have the same stiness properties as a detailed 3D element model of the BOP stack. The calibrated elastic beam BOP stack model is implemented in a global model of a drilling system. Global response analyses are carried out for two global models. One with an innitely sti BOP stack model and one with the calibrated elastic BOP stack model. Fatigue damage estimates are calculated with basis in the results from the global response analyses. The eects of BOP stack modelling are evaluated with regards to estimated fatigue damage in the wellhead. In addition to this main study, parameter studies and a sensitivity study are carried out to evaluate uncertainties and assumptions within a realistic frame. The results from the wellhead fatigue assessments conclude that the elastic BOP stack model imposes greater estimated fatigue damage in the wellhead compared to the innitely sti BOP stack model. The dierence, in terms of estimated fatigue damage, imposed by the two BOP stack models is, at maximum, 0.51% for the main study. The largest dierence observed in the parameter studies is 1.34%. Hence, it is concluded that the eects of improved BOP stack modelling in a global response analysis, with respect to wellhead fatigue estimation, is not signicant. There are though some uncertainties connected to the bending stiness of the wellhead connector and the LMRP connector.

Place, publisher, year, edition, pages
Institutt for marin teknikk , 2013. , 209 p.
URN: urn:nbn:no:ntnu:diva-21881Local ID: ntnudaim:9420OAI: diva2:644980
Available from: 2013-09-02 Created: 2013-09-02 Last updated: 2013-09-02Bibliographically approved

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