Evaluation of excessive wellhead motions: Reliability assessment and wellhead integrity index (Part II)

Abstract

As discussed in part I of this work, the wellhead is an important component in the context of well integrity and operational safety, as it serves as a link between well and riser in offshore oil wells. The wellhead must sustain loads from the casings (soil reaction), which are hung into it through the casing hangers, as well as sustain loads from the riser (waves, current), which is connected at the top of the BOP (right above the wellhead) through the lower flex joint. In Part I of this work, we proposed a deterministic methodology to evaluate excessive wellhead motions, considering a mechanical system that couples the vessel, the riser, the wellhead itself, the conductor and surface casings, and their interaction with the soil. Now, in Part II, we move onto a stochastic analysis, which allows us to calculate the wellhead reliability. The simulation of this system provides the motions experienced by the wellhead; then, criteria such as maximum allowable lateral deflection and maximum allowable rotation are applied to verify if they will cause wellhead failure. Several interesting results have been observed: modern 5-phase wells with cemented conductor confer higher integrity to the wellhead compared to old 4-phase wells with jetted conductors; current speed/profile is the variable that affects the most the wellhead reliability; and heavier inner fluids reduce the reliability, posing a problem for later drilling phases. Finally, an integrity index for the wellhead throughout the well life cycle is suggested, aggregating the literature related to wellhead fatigue failure. The wellhead integrity index can then be combined with other indexes from other barrier elements to evaluate the overall integrity of barrier envelopes.