This paper describes stress analysis methods and results used to evaluate and improve the design of a new blowout preventer. A 30 inch, 3000 psi blowout preventer had not been previously manufactured and the design procedures involved extrapolation of experience with smaller designs. To evaluate the new design and eliminate the need for testing prototypes, finite element stress analysis was performed on the original design and subsequent design modifications. The stress analysis ensures that stresses and deformations in the BOP under expected test and service conditions are within generally accepted values and conform to standards of the ASME Boiler and Pressure Vessel Code.
A coarse finite element model is used to determine nominal stresses in the BOP and identify high stress regions. Refined models are used to determine stresses in critical areas. Results of the analysis were used to reduce the overall weight by removing steel from low stress areas and adding steel to regions that required additional strength to satisfy fatigue life design criteria. The size of the BOP and the pressures it must withstand result in deformations larger than those typical of smaller designs. Accurate calculations of these deformations were important to determine if a particular seal design is sufficient. The large span also results in greater variations in flange bolt loads and contact stress around the ring gasket than what occurs in other designs.
Most existing blowout preventers have been designed with hand calculations and prototype testing, before finite element analysis techniques became widely available or accepted. The subject of this paper is unique because of the size and pressure rating of the BOP and because computer models were used to thoroughly evaluate the design prior to any physical testing.