A rotating tension ring using fluid bearings provides a means to rotate a drilling vessel relative to the marine riser system or a riser system relative to the vessel. A fluid bearing is used in a test fixture to determine the "break-away" coefficient of friction. This information is used to determine the expected resistance to rotation of a rotatable marine riser tension ring using fluid bearings. The fluid bearing is capable of balancing the total existing load from the riser tensioners. For this particular application, the requirement is to balance a load of 1,200,000 lbs. and allow rotation. The fluid bearings are incorporated into the rotatable tension ring without having to sacrifice important design parameters such as size, weight, and total remote operation. Rotating tension rings have been designed for outside diameters as small as 70 inches, with an inside diameter of 47-½ inches, and to handle as many as 16 riser tensioners.
Marine riser tension rings are used as a means to transmit the load produced by the riser tensioners to the marine riser system. Wire rope connects the riser tensioners to the tension ring. The tension ring is connected to the outer barrel of the slip joint by hydraulically actuated dogs.
The need for rotating tension rings was initiated with the existence of dynamic positioned drilling vessels. A dynamic positioned vessel does not have the limitation of being confined to a particular orientation or heading, as is the case with conventionally moored drilling vessels. A moored vessel can only change its heading a limited number of degrees without resetting its anchors.
Each drilling vessel will have different limitations that determine the outside physical dimensions of the tension ring. Some vessels may require as small an outside diameter as possible to prevent interference problems. Height limitations may have to be considered when the tension ring must be stored in a specific location. Some vessels will store their tension ring directly under the rotary opening with all tension lines attached. Others may have to remove half of the tensioner cables and swing the tensioner aside for storage. Requirements may be to remove all of the tensioner lines and store the tension ring in another area. Therefore, the tensioner cables must be easily attached and removed, and the tension ring must be capable of being handled and moved efficiently.
To provide uninterrupted running of the slip joint, all functions are designed to be operated remotely. This is accomplished by having all of the hydraulics located on the tension ring to eliminate the need of making and breaking hydraulic connections during the running and attachment process to the outer barrel of the slip joint. The dogs which make the connection to the riser system are hydraulically actuated at a remote location.
Rotation of the tension ring is required for several reasons.