Design Verification and Load Estimation of Subsea Wellhead in HPHT Applications

Abstract

Abstract Subsea equipment design verification and validation are to be performed by analysis and testing, respectively. Two of the most important aspects of this process for high-pressure, high-temperature (HPHT) applications are load estimation and fatigue assessment. This paper presents dynamic loading scenarios typically considered in the design of subsea wellhead in HPHT applications during service life as well as load estimation of these scenarios, particularly on riser loads. Fatigue damage to the subsea wellhead is also included to examine the significance and effect of the loads from each of the loading scenarios. Dynamic loading scenarios in drilling and production phases are evaluated in the paper. In the drilling phase, typical loading scenarios for the riser in connected conditions with wellhead are evaluated. A typical drilling system, generic sea state scatter diagram, soil condition and operational profile are selected to estimate the dynamic loads for each of the scenarios. In the production phase, typical loading scenarios are also evaluated. A typical subsea production system with the same sea and soil condition as drilling but with different operational profile is selected to estimate the dynamic loads for each of these scenarios. A typical wellhead system including wellhead housing, conductor, and casing system is selected for the riser load estimation and the wellhead fatigue assessment using fracture mechanics method is carried out in accordance with API 17TR8. Dynamic loads estimated from the scenarios in drilling and production phases are compared at the top of the wellhead in terms of maximum load and load histogram. The significance and effect of these loading scenarios are also evaluated from the fatigue standpoint at select hotspots on the wellhead system. From the study, it is observed that loading from drilling and production phases are equally important and both should be considered to verify the subsea wellhead's structural capability. In the paper, an analysis procedure to evaluate the riser load to the wellhead and fatigue assessment of the wellhead is proposed, which may be used for the design verification of subsea wellhead in HPHT applications.