As drilling rigs continue to move into deeper waters, the frequency of riser hangoff operations during storm conditions will increase. In the Gulf of Mexico, tropical weather systems may develop rapidly, leaving little preparation time to secure a well and pull the riser completely, an operation that can take days in deep water. Since a large portion of the riser may be left suspended from the rig, hard hangoff in a storm can induce very substantial load variations in the riser. This can lead to riser failure by main tube compression or extreme tension, especially in riser systems with low wet to dry weight ratios. The soft hangoff method, hanging the riser on tensioners that act as springs to absorb heave motions, is an attractive alternative that can greatly reduce the tension fluctuations experienced during a storm hangoff and increase the allowed maximum environment in which a riser can survive a disconnected situation.
This paper discusses the possibilities of using the soft hangoff option and compares the performance characteristics of the drilling riser in the soft hangoff and hard hangoff configurations, particularly for ultra deepwater applications down to 10,000 ft. The stresses in the riser and the loads on the drill floor and wire-line tensioners with the two configurations are compared and discussed. Limiting conditions in which to use the soft hangoff system, particularly with respect to stroke limitations in the tensioners and telescopic joint, are identified and reviewed.
In certain circumstances, such as with the approach of a storm, drilling vessels (semisubmersibles or drillships) will need to disconnect the riser and hangoff until it is practical and safe to reconnect. It is preferable to hang the riser off the vessel, rather than retrieve and re-run later, in order to reduce drilling downtime after the storm passes and decrease the operational risks associated with handling riser in heavy seas. In general the practice has been to collapse and lock the telescopic joint, hangoff the riser from the diverter housing, and release the tensioners. This is defined as a hard hangoff. An alternative is to use a soft hangoff.
In a soft hangoff system the tensioners and telescopic joint remain active, just as they are during connected mode operations, with the tensioners supporting the weight of the riser from the telescopic joint outer barrel down to the LMRP, and possibly the BOP. The benefit of this practice is that the telescopic joint and tensioners absorb the vertical motions of the vessel, thereby significantly reducing the load variations on the riser system. In a hard hangoff the vessel motions are transferred directly to the riser and may induce severe loads on it. This potentially leads to extreme top tensions or possible compression in the buoyant riser sections.
A detailed review of soft hangoff has been conducted and is presented in the following sections. Key assumptions and sensitivity cases are provided along with detailed results indicating operability ranges for two vessel types, small and large drillships, with two different wire line tensioners.