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. The BOP may be connected to the subsea well directly, which consists of a string of vertical pipe running into the seabed down to the reservoir. The part of the well above (2-5 m) the seabed is called the wellhead system. This is the interface to the equipment to be connected to the subsea well. Alternatively, the BOP may be connected to a valve tree, which in turn is connected to the subsea well. The valve tree is used to control the flow of oil, and is usually called a Christmas tree.
Challenge: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. The results from a fatigue test are plotted as stress (S) versus number of cycles to failure (N), which gives the S-N curve. However, fatigue testing is expensive and time consuming so it is usually possible only in certain cases.