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Mooring capabilities
Quick disconnection of the platform is required in certain regions and is one of the main features of this design, and incorporates a safe reconnection option. Feasibility design has been performed for a mooring system and riser buoy located in the center of the lower part of the hull, with all risers and mooring lines connected to the buoy. The design calls for 16 chain/polyester tautleg mooring lines connected to the submerged buoy and 21 flexible riser connections. The total number of lines depends on the environmental conditions at the field, accounting for the 100-year intact condition, the 100-year condition with one-line failure, and the 10-year condition with two-line failure. Suction anchors are assumed for the sea bed interface.
The main phases for the platform installation include:
- Initial installation of the buoy involves towing it to the field with parts of all mooring lines pre-rigged. One by one, the mooring lines would be installed. At completion the buoy will be left at installation draft of -127 m (-417 ft)
- The floater with topsides installed is positioned at safe draft of -117 m (384 ft) to give a 10 m (33 ft) clearance between the buoy and platform
- A combined deballasting and anchor/wire would pull and guide the buoy into position
- At completion, the combined buoy and platform would be deballasted to operating draft of -112 m (- 367 ft); this will give the final tensioning of the mooring system
- Riser pull-in would be done one-by-one from the topsides through the buoy and the pre-installed casing in the center shaft of the platform. Ballast levels in the buoy will be adjusted as required using ROV-assisted deballasting to give the specified vertical load into the platform.
For quick release, the risers would be disconnected at topside level and dropped through the conductors to the top of the buoy. The weight of the risers are be balanced by the buoy buoyancy system.
Typically, the buoy would have a ballast system with closed and open compartments that can be operated by an ROV to ensure the appropriate vertical load is transferred to the platform.
Main phases for the platform disconnection and reconnection are disconnect and drop risers from the topsides and down onto the buoy, disengage the buoy by ballast/air release and by disconnecting the locking bolts, buoy will rest at a safe draft below iceberg keel and reconnect as per the initial installation.
Buoy lay-out
The cone shaped buoy is designed to take all the required risers and mooring line loads. During release the buoy will be lowered as one complete unit with risers and mooring lines connected. During reconnection the buoy must be deballasted prior to being pulled upwards and guided into position where it is then locked, similar to initial installation. Access to the water filled void above the buoy would be by ROV through the tricells. The center shaft will be dry from the top to the lower dome. Typical buoy dimensions could have an overall diameter of 34 m (112 ft) and a total height of 15-20 m (49-66 ft).
The towing spread will position the floater typically within ± 10°. The guiding system will be designed to take this offset. Guide and locking bolt arrangement will hold the buoy into its final position. Vertical loads and torsion forces will typically be transferred through the load bearing ring at top of the buoy. During the release operation, the buoy will be deballasted and lowered to safe distance allowing for proper under keel clearance for any iceberg.
The top of the buoy is designed to allow for all 21 flexible risers to be lowered to rest on top of the buoy. Overall lay-out is designed for working space between risers.
Disconnection schedule
Having all mooring lines and risers connected to one submerged buoy allows for an effective disconnect and reconnect operation. With riser hang-off at topsides elevation during normal operation, a safe and quick procedure can be developed for the release. When the buoy is released, it will go to a predefined draft using the buoy ballast system. The floater will be free from the mooring and riser system for tow.
In disconnected mode, the buoy is positioned in floating equilibrium at a predefined draft to give sufficient under keel clearance from any approaching icebergs or other threats.
During reconnection, the process would be reversed. The possibility of ballasting and deballasting the platform allows for posttensioning and adjustment of the mooring lines as required.
The time required to disconnect the riser and mooring buoy from the floating platform has been assessed for two cases:
- An extreme disconnection where the risers, flowlines, and subsea wells are prepared for quick restart to production mode
- A survival disconnection whereby the platform is disconnected as quickly as possible during an emergency situation and the impacts of a potential future reconnect is not considered.
Typical disconnection schedule for a controlled disconnect scenario, prepared for quick restart:
- Preparations: 3-9 hours, plus mobilization time for towing vessels
- Disconnect operation: 1-2 hours
- Total time until start platform tow to safe haven: 4-11 hours
Controlled disconnect preparation activities include:
- Mobilization of vessels and required manning
- Demobilization of platform manning
- Preparations for process plant shut down
- Production stop, subsea wells; close subsea SSIVs (subsea isolation valves)
- Prepare risers and production lines (depressurize and fill with inert gas as applicable, close riser EVs)
- Disconnect and drop risers from topsides down onto buoy
- Connect towing vessels to floater
- Releasing buoy from platform by ballasting/air release
- Disconnect locking bolts
- Drop buoy to safe draft (below iceberg keel)
- Tow platform to safe haven.
The minimum preparation time for emergency/survival disconnect is estimated to about three hours and includes production stop at subsea wells, closing of the riser SSIVs, depressurizing the risers/closing the EVs at platform topside, shut down the processing plant, prepare the buoy for disconnect and connection of the towing vessels.
The time required to reconnect risers and production restart has not been assessed at this stage, as this will be closely related to the specific field and platform operating philosophy and design.
Acknowledgment
Based on a paper presented at the Deep Offshore Technology International Conference & Exhibition held in Aberdeen, Scotland, Oct. 14-16, 2014.
