Workover operations in Shell’s Shearwater field in the North Sea are being aided by an unusual riser support system fitted to GlobalSanteFe’s Magellan drilling rig by Claxton Engineering. The system is designed to control the deflection of the workover riser caused by horizontal movement of the rig relative to the fixed jacket platform – and hence reduce loadings on the surface tree as well as on the riser itself. The ultimate goal is to maximise operational rig time.
The system is unusual for two reasons: first, it applies to the horizontal movement of the rig rather than the vertical, and second, it provides soft restraint using a combination of hydraulic dampers and compensating weights. The hydraulic elements of the system permit the riser to deflect along with the rig as a result of differential movement between the platform and the rig (during normal weather conditions), thereby ensuring maximum operational time. In addition, there is a “weak link” between the riser and the hydraulics in the form of fixed-weight anchors inside guide tubes, which are secured to the rig. As soon as the riser reaches its deflection limit, the weights lift to allow the rig to continue moving, while limiting any further deflection of the riser.
The system was installed on the Magellan during February 2007. So far, it has performed exactly as intended.