Riser Inertia Management and ControlControl Flow Inc. / WesTech HMD presents an innovative system which recognizes and resolves problems inherent with conventional Riser Tensioning Systems.
SYSTEM PURPOSE AND ARRANGEMENT
A Riser Inertia Management and Control (
RIMAC®) system allows the drilling vessel to disconnect rapidly from the subsea well head in a safe manner. The RIMAC® controls the riser tensioning system in a manner such that the stored energy within the tensioners is "managed" to prevent undesirable upward acceleration of the riser immediately following it's subsea disconnection. The riser tensioning force is again revised at the point where the risers upward motion has ceased, to provide a controlled riser elevating tension. This elevating tension is maintained until the riser is seated within it's capture ring.The proposed
RIMAC® can be utilized to manage any size and number of riser tensionsers that are used to tension semi-buoyant marine risers in deep water. The described system is proposed for the WesTech Model VCT tensioner line but is applicable for any ram type of unit with an air/liquid accumulator. All of the ram tensioners that apply tensioning to the marine riser are grouped together and viewed as a system from a controls standpoint.In a typical system, eight or more tensioners could be utilized, in which 4 of the tensioners would be considered "master" units with the remaining being slave units. Each master tensioner would be paired with at least one slave unit on the opposite side of the vessel. This provides a redundancy provision as a deficient tensioner must be shut down with an operable unit, i.e. a pair arrangement to ensure a balanced force application to riser tensioning ring. Each master tensioner would be fully equipped with control sensors with the slave units having limited sensors sufficient for safety reasons.
RIMAC® CONCEPT
The operational concept for the proposed RIMAC® is based on varying the total tensioning force being applied to the marine riser during the disconnect cycle. Therefore the controls must identify what part (or phase) of the disconnect cycle is occurring. With phase identified, the riser tensioner performance requirements are pre-determined based on the marine riser data. Therefore the preset tensioner performance is switched from phase to phase as they occur during the disconnect cycle. This permits the RIMAC® performance to be easily revised to suit the marine riser being used.
(Note) "4 phases" of the disconnect cycles are those identified from the OTC papers that were given on a system installed and tested onboard the SONAT drillship, Discoverer 534.
I. Timing of the actual riser disconnect event as related to the drillship or semi heave cycle.
2. Tensioner energy limitation delivered to the riser during its dynamic release of riser and wire rope pre-stretch energy to prevent slack wire conditions.
3. Tensioner limited force timing and application when the riser and wire rope stored energy has been dissipated, allowing the riser to be elevated to its most upward position.
4. Tensioner force timing and application when the marine riser is being landed against it's hardpoint capture device.
The proposed
RIMAC® control concept addresses these 4 event cycles in that the four cycles are identified at occurrence and the tensioner performance is modified as required for each cycle. The previous RIMAC® system that Western Gear built addresses the first disconnect cycle phase but does not address the remaining phases - the tensioners simply are stopped".The proposed
RIMAC® control concept identifies master tensioner performance by monitoring ram cylinder pressure and wire rope speed, direction and position. Slave tensioner performance is monitored by wire rope speed and direction only.The wire rope tension requirements of the tensioners is determined as a factor the marine risers total mass and buoyancy, which are easily pre-determined. The position of the tensioner rams and marine riser is determined by monitoring tensioner wire rope position with automatic center stroke correction. The tension being developed by the tensioners is determined by monitoring the pressure being applied to the ram cylinders. The APV pressure is separately monitored and displayed by the individual tensioner controls but it is not utilized by the
RIMAC® controls. The ram cylinder pressure and wire rope position is also utilized by the RIMAC® controls to develop "ton/mile" wire rope service data.Proprietary Information of Control Flow Inc.