Survivability study of deepwater drilling riser in freestanding mode

Abstract

The conventional practice of retrieving the whole length of deepwater drilling riser during imminent typhoon is tedious, time-consuming, and has huge risk of untimely evacuation of personnel and facilities for safety. A proposed operational solution is one which has the drilling riser disconnectable near surface while the longer section is left tensioned by buoyancy can (BC) in freestanding mode. This prompts the need to investigate the survivability of drilling riser when disconnected in freestanding mode and reuse for continued drilling operation after typhoon. In this paper, a mathematical model is presented to analyze the mechanical behavior of drilling riser freestanding mode (DRFM). Finite-difference method with derived end-point formula to reduce imaginary nodes was used to obtain the solution. BC upthrust ratio required to support the DRFM weight and maintain its response while keeping stress and lower-flex joint angle within allowable limits has been figured out to determine survivability threshold for DRFM in extreme sea current condition. BC effect shows potential region for DRFM failure. Accuracy of the threshold condition was verified with numerical simulation in Abaqus. Threshold conditions for various BC positions were established. Influence of current loadings and riser thicknesses on the threshold condition have also been investigated.