eSubsea do wellhead fatigue analysis on subsea wellheads to evaluate the wellhead integrity, which is a growing concern in the subsea oil and gas industry. The cyclic shear loads are generated from waves, rig drift off and current induced vortex vibrations .
These loads on the riser acting on top of the BOP during drilling generate large bending moments through the BOP stack and into the subsea wellhead and conductor which can cause fatigue in critical parts of the well system. The most critical parts are most often the welds in both the high pressure hosing (18 3/4 Inch) and the low pressure housing (30/36 Inch).
There are mitigating solutions developed to minimise the bending moments into the well system, wellhead load relief system. eSubsea can model, analysis and estimate the efficiency of such wellhead load relief WLR systems which are used to avoid or minimise the wellhead fatigue on the wellhead system.
eSubsea has extensive experience with wellhead fatigue design and analysis. We use finite element analysis to model and analyse the critical components of the well system from the riser drilling loads.
DNV Method and NORSOK Loads
The wellhead fatigue analyses are done according to DNV wellhead fatigue method using finite element analysis. The wellhead integrity, wellbay area and the subsea template or satellite structure is modelled and analysed to establish the system stiffness matrix. Drilling loads from NORSOK U-001 with BOP size and water depth or project specific riser analyses are applied to the FEA model, the load paths established and hot spot stresses identified.
The fatigue damage or fatigue life calculation is done using annual load histograms, Miner-Palmgren cumulative damage theory and S/N curves from DNVGL-RP-C203 Fatigue Design of Offshore Steel Structures .