Marine riser failure analysis for offshore conventional drilling and managed pressure drilling operations

Abstract

The objective of this paper is to present an analysis of marine riser integrity in conventional and Controlled Annular Mud Level (CALM) offshore drilling operations. A mathematical model was developed to predict the three-dimensional (axial, tangential, radial) and the Von Mises Equivalent (VME) stresses along risers for a typical API X-65 55.88-cm OD, and 2500-m long marine riser. A sensitivity analysis was carried out to investigate the effect of riser thickness, bending, surface backpressure, liquid level, buoyancy modules, and tension from surface tensioners on riser mechanical behavior. To evaluate the riser integrity, the calculated VME stress was compared against the maximum allowable stress recommended by API RP 16Q. The key findings of this study are as follows. First, when maximum VME stress at riser surface is less than the maximum allowable stress, increasing riser thickness does not improve riser integrity. Second, increasing dog-leg severity from 0 to 4 deg./100 ft increases the riser surface VME stress 57% and the riser bottom VME stress 1%. The former VME stress is insignificant because high dog-leg severity occurs near riser bottom where VME stress is small. Third, when tension of 20% of the riser buoyant weight is applied at the surface, the riser integrity is improved more than when using buoyancy modules, which have a density of 4.5 ppg. Fourth, lower liquid level in the riser during CALM drilling operations results in an increase of VME stress, with the highest increase at the top of the riser liquid level. However, under severe conditions, VME stress is often less than maximum allowable stress, indicating that riser collapse is not a concern during controlled annular mud level drilling operations.This study has introduced a simple and accurate method to evaluate the riser's integrity during conventional offshore and Controlled Annular Mud Level drilling operations. The results of this study improve not only the design of risers, but also ensure safe and economic offshore drilling operations.