Effects of BOP Stack Modelling on Estimated Wellhead Fatigue Damage
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.
IdentifiersURN: urn:nbn:no:ntnu:diva-21881Local ID: ntnudaim:9420OAI: oai:DiVA.org:ntnu-21881DiVA: diva2:644980
Leira, Bernt Johan, ProfessorYttervik, Rune