This master thesis is done in collaboration with Aker Solutions Subsea AS and proposes how one could securely cope with uncontrolled drift-off where the marine riser experience excessive moments and/or tension that will damage the well integrity and structure while the well is undergoing workover or intervention. This weak link design is meant to release the riser from the well structure before damage occurs, either when excessive moments, tension or both have the potential to damage the structure. The release mechanism should work as a 3rd barrier, and by itself, with no input from the operator, make the riser disconnect safely. The mechanism should also secure the well so that the surface vessel can safely drift-off from position.
The preliminary study project, which was finished in December 2010, produced several interesting concepts. One was chosen for further study and analysis in the master thesis by the candidate, Aker Solutions representatives and the supervisor from NTNU. In this thesis, the concept has been further developed, analyzed and dimensioned to cope with relevant example and extreme loading conditions provided by Aker Solutions Subsea AS.
The concept developed in this thesis is based on an idea of a standard ANSI flanged connection. By using pressurized CO2, the flange geometry produces a separation pressure which forces the flange bolts to break, thus separating the riser connection and disrupting the communication between the surface vessel and the subsea equipment. The system is initiated by a battery powered offset monitoring system which measures the riser offset at a position 55 meters above the EDP. When the separation has been initiated, the hydraulic pressure and control signal connection between the operator and the subsea well is terminated, thus closing the wellbore by utilization of the failsafe valves in the subsea tree and/or well control package.
Analyses of the final concept in Abaqus show that the dimensions and geometry copes well with the applied loading condition. The weak link bolts are designed to break if subjected to a limited tension force. This enables the present weak link to be applied as a replacement for existing links which are solely based on tension response.