Calculation and Analysis of Operability Envelopes for Deepwater Drilling Riser System

Abstract

This article focuses on calculation and analysis of operability envelopes for deepwater drilling risers system. A solver is developed by modeling the drilling riser as a finite number of beam elements based on finite element method, and it is capable of performing static analysis to obtain connection operability envelopes, hang off operability envelopes and running/retrieval operability envelopes of the drilling riser system. A practical example of 1500m drilling riser system in south China sea is presented to illustrate the application of the proposed method. The main conclusions are:By modeling the drilling riser as a finite number of beam elements, the stiffness matrix, mass matrix, damping matrix, and load matrix in the static and dynamic governing equation are derived. A solver is developed by using direct stiffness method to model and calculate the operability envelopes of deepwater drilling riser system.For connection operation, the normal drilling envelope is determined by the LFJ and UFJ angle limit. When surface velocity is small it is affected by LFJ angle, and it is affected by UFJ angle when surface velocity is large, while the connected non-drilling and disconnection envelopes are determined by allowable stress of the conductor and the bending capacity of the wellhead.Hang-off and deployment/retrieval operability envelopes of the drilling riser are determined by the heave RAO and wave height in the analysis. Generally, the hang-off envelope is larger than the deployment/retrieval envelope because of the BOP being involved in the analysis model, which increase the weight of the suspended riser column, and therefore increase the stress level of the riser during the heave motion of the vessel. The related methods and conclusions have been put into drilling practice in south China sea in 2012, such as LW6-1-1, LH26-2-1, LH29-2-1 et al, those wells have been drilled successfully.