Tethered BOP Stacks: Performance and Monitoring

Subsea wells present challenges for fatigue utilisation throughout the life of these high-value assets. New wells may be constructed at seabed locations where soil properties may be unfavourable or uncertain. Existing wells may require workover and eventual abandonment. This can lead to relatively small conductors supporting higher BOP masses than was envisaged when these wells were constructed. To address such stability issues during drilling and work-over operations, a BOP stack may be tethered to seabed anchors to provide lateral retention. The increased foundation stiffness raises the BOP characteristic frequency which suppresses motion of the BOP and significantly reduces fatigue of the conductor and wellhead. Real-time monitoring of the tether tensions and BOP characteristic frequency provide on-going verification of the efficacy of this fatigue suppressing technology as well as tracking structural integrity. For example, the ability of the monitoring system to detect loss of tether tension and the success of ROV intervention to reapply tension is demonstrated.