Subsea drilling is conducted through a string of steel pipe called a rigid riser that runs from the oil rig to the blowout preventer (BOP). The primary purpose of the BOP is to cut off the flow of oil in the event of an emergency during drilling.
As the oil rig moves with waves and currents it pulls the rigid riser along, transferring loads to the equipment on the seabed. The cyclic loading from the wave motion can lead fatigue damage. The most critical areas from a fatigue standpoint are usually welds between pipes and stress concentration points, such as notches or connections between the different components.
The traditional approach to estimate fatigue life for underwater applications is based on the nominal stress at a given location. The calculated stress is related to test results through S-N curves, which give a required amount of stress cycles to cause fatigue failure.
Sometimes fatigue test results are extrapolated to other geometries but this often leads in inaccuracies. The VCCT and weight function method gives more conservative results than the S-N curve, this can be explained by the fact that the part geometry closely matches the specimen used for S-N testing and the crack growth methods assume the presence of a crack.
4Subsea AS has used crack growth calculations to overcome these challenges. To calculate the crack driving force, a weighting function was used. This method uses the stress determined by finite element analysis and it has been proven for a range of applications. However, it is considered more physically accurate to be able to simulate the growing crack through a finite element model. MSC Software’s Marc FEA solution, supports this approach through use of the Virtual Crack Closure Technique (VCCT). Therefore, 4Subsea compared results from both methods in order to validate the crack growth results. In turn, these results were compared with the results found from S-N method.
The result in some cases is that the life of subsea equipment can be extended considerably. In a few cases, oil companies have gained financial benefits by increasing the extractable amount of oil from a well without any additional capital investment.
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