Wellhead Fatigue Analysis : Surface casing cement boundary condition for subsea wellhead fatigue analytical models

Material fatigue is a failure mode that has been known to researchers and engineers since the 19th century. Catastrophic accidents have happened due to fatigue failures of structures, machinery and transport vehicles. The capsizing of the semisubmersible rig Alexander L. Kielland in Norwegians waters in 1980 killed 123 people, and investigations pointed at the fatigue failure of a weld as one of the direct causes. This accident led to a number of improvements to the design of offshore structures. The noticeable safety principle ”No single accident should lead to escalating consequences” has since been adopted in a widespread manner. Since 1992 the Petroleum Safety Authority in Norway has enforced a risk based safety regime. Wells are designed to hold back reservoir pressures and avoid uncontrolled escape of hydrocarbons. In other words a well is a pressure containing vessel. Norwegian safety regulations require a dual barrier construction of wells. This safety principle ensures that one “barrier” is preventing an escalating situation should the other barrier fail. A wellhead is a heavy walled pressure vessel placed at the top of the well. The wellhead is part of the second well barrier envelope during drilling. The subsea wellheads are located at sea bottom and during subsea drilling the Blow Out Preventer (BOP) is placed on top of the subsea wellhead. The drilling riser is the connection between the BOP and the floating drilling unit. Waves and current forces acting on the drilling riser and drilling unit will cause dynamic movement. Flexible joints at top and bottom of the drilling riser protects the drilling riser from localised bending moments. The subsea wellhead is both a pressure vessel and a structurally load bearing component resisting external loads transmitted from a connected riser. These external loads can be static and cyclic combinations of bending and tension (compression). Cyclic loads will cause fatigue damage to the well. The well can take a certain amount of fatigue damage without failing. A fatigue failure of a WH system may have serious consequences. Should the WH structurally fail its pressure vessel function will be lost and for this reason WH fatigue is a potential threat to well integrity. The structural load bearing function will also be affected. Wellhead fatigue analysis can be used as a tool to estimate the accumulated fatigue damage. Analysis results then compares to a safe fatigue limit. This thesis addresses selected aspect