The Buoy will be moored on an spread mooring lines. The moorings will consist of lines with a top chain , spiral strand riser wire (optional) , ground chain, adjustment and excursion limiter chain , ground wire, ground chain and Anchors.
The anchor handling tug supply (AHTS) vessel will be utilized for the mooring operations as the Primary Installation Vessel (PIV). The PIV will make one trip offshore to complete mooring hook-up.
The weather will play a crucial role in the operation since it affects personnel on deck, crane loads, ROV and Diver operations. To ensure the safety of personnel and equipment during the execution of the works the following operation and environmental limitations shall be predetermined
The buoy is to be moored at the location to an pre laid mooring spread. Prior to hook-up the PIV will install riser wire and top chain to the existing ground components of the mooring system along with an additional pull through wire. The complete mooring line will be laid down with a ROV friendly recovery system prior to hook-up operation commencing.
The buoy is to be pre-rigged with a pull through messenger line fed through the chain stopper mechanisms and secured in the buoy hang off hooks. Both ends may be secured in the hang-off hooks during tow out but the lower end of the pull through must be released prior to operations.
Upon commencement of the buoy hook-up operations the PIV will recover each of the mooring lines from the seabed and connect to the buoy pull through wire hang-offs using ROV intervention. Once a minimum of 4 lines are complete, tow tugs may be disconnected however it is recommended that all eight lines be hung off prior to tug removal.
Following the hang-off of all lines the PIV will stroke up each of the lines with the vessel crane and in a minimum of two stages until the design connection point is locked in the chain stoppers. Air diver intervention will be required during the pull through to temporarily secure lines and for the final connections of the chain to the buoy.
The PIV will be mobilized with all the necessary equipment for completing the buoy hook-up operation.The equipment will be loaded onto the PIV. The order in which the equipment will be mobilized will depend on port logistics and dockside layout.
Order of Operations
Buoy Pre-Rigging
This section of the procedures indicates the pre-rigging requirements for the buoy prior to its arrival on field. On arrival to the field, two tugs are to be attached to opposite sides of buoy for positional purposes. Buoy to be pre-rigged with pull through wires fed through each chain stopper . Top end of pull through wire to be connected to the buoy hang-off hooks via Bow shackle.
Mooring Line Retrieval and First Hang-off to Mooring Buoy
This describes the tasks to retrieve the pre-laid moorings and transfer to the hang-offs on the buoy. This task is to use the ROV for subsea connections and Survey for monitoring position and clearances to the buoy.On arrival to the field, two tugs are to be attached to opposite sides of buoy for positional purposes.Tug 1 and Tug 2 to manoeuvre the buoy into the mooring location and orientate as close as possible to its final position.
First line to be connected will be the weather line . Connection of lines will then follow in a diametrically opposite manner.Ensure that the upper pull-through wire segment (pre-rigged on buoy) for the appropriate line number has its lower end free and hanging vertically down.IV to set-up on DP over 1st line to be connected and launch ROV
Following the connection of the first 4 lines indicated in item 3 the PIV the following options can be considered:
Removal of the tow tugs
This section of the procedures describes the tasks required to remove the tow wires from the buoy. As the connection point to the buoy will be at the splash zone following the hang-off of either four or eight lines, it is proposed to disconnect the line from the buoy at the connection point to the tow tug tow wires. The tow wires will therefore remain connected for later retrieval if
Retrieval of Pull Through Wires and Temporary Hang-off
This section of the procedures describes pull through of the mooring lines following the initial hang-off of the eight mooring lines.
The first line to be tensioned will be determined on site depending on the predominant environmental conditions. All following lines will be tensioned in a diagonally opposite pattern
The anchor handling tug supply (AHTS) vessel will be utilized for the mooring operations as the Primary Installation Vessel (PIV). The PIV will make one trip offshore to complete mooring hook-up.
- Two tug boats will stabilise the buoy at location and remain there until at least 4 primary lines are connected,
- The Primary Installation Vessel (PIV) shall be the stationed at location
- Diving support at location shall be provided.
- One weather window of few days required prior to commencement of the buoy hook up operations
- Mooring system excluding riser wire and top chain will be pre-laid down on seabed with ROV friendly recovery rigging prior to hook-up.
The weather will play a crucial role in the operation since it affects personnel on deck, crane loads, ROV and Diver operations. To ensure the safety of personnel and equipment during the execution of the works the following operation and environmental limitations shall be predetermined
- Maximum wind speed for ROV deployment
- Maximum wind speed for surface diver deployment
- Maximum surface current for ROV deployment
- Maximum surface current for surface diver
- Maximum combined wave height for ROV deployment
- Maximum combined wave height for surface diver
- Maximum operating depth for surface diver (based on thruster distance)
The buoy is to be moored at the location to an pre laid mooring spread. Prior to hook-up the PIV will install riser wire and top chain to the existing ground components of the mooring system along with an additional pull through wire. The complete mooring line will be laid down with a ROV friendly recovery system prior to hook-up operation commencing.
The buoy is to be pre-rigged with a pull through messenger line fed through the chain stopper mechanisms and secured in the buoy hang off hooks. Both ends may be secured in the hang-off hooks during tow out but the lower end of the pull through must be released prior to operations.
Upon commencement of the buoy hook-up operations the PIV will recover each of the mooring lines from the seabed and connect to the buoy pull through wire hang-offs using ROV intervention. Once a minimum of 4 lines are complete, tow tugs may be disconnected however it is recommended that all eight lines be hung off prior to tug removal.
Following the hang-off of all lines the PIV will stroke up each of the lines with the vessel crane and in a minimum of two stages until the design connection point is locked in the chain stoppers. Air diver intervention will be required during the pull through to temporarily secure lines and for the final connections of the chain to the buoy.
The PIV will be mobilized with all the necessary equipment for completing the buoy hook-up operation.The equipment will be loaded onto the PIV. The order in which the equipment will be mobilized will depend on port logistics and dockside layout.
Order of Operations
- PIV to pick-up mooring system ground chain and install riser wire, top chain, and pull through wire segment before laying down with ROV friendly recovery rigging
- Mooring buoy pre-rigged with Pull through wires through the chain stoppers shall be provided, and the chain stoppers held open with fibre rope. Buoy to have two positioning vessels attached to buoy with sufficient capacity to hold the buoy in position for the installation environments.
- PIV to retrieve mooring system from seabed
- PIV to connect mooring system to buoy pull through hang-off rigging by ROV
- Once all lines hung-off, PIV to pull through first pull through wire on all lines. Diver intervention will be required for temporary hang-off.
- PIV to pull through second pull through wire on all lines and lock chain end into chain stopper.
- PIV to determine any required modifications to the top chain length until buoy is within design tolerances.
Buoy Pre-Rigging
This section of the procedures indicates the pre-rigging requirements for the buoy prior to its arrival on field. On arrival to the field, two tugs are to be attached to opposite sides of buoy for positional purposes. Buoy to be pre-rigged with pull through wires fed through each chain stopper . Top end of pull through wire to be connected to the buoy hang-off hooks via Bow shackle.
Mooring Line Retrieval and First Hang-off to Mooring Buoy
This describes the tasks to retrieve the pre-laid moorings and transfer to the hang-offs on the buoy. This task is to use the ROV for subsea connections and Survey for monitoring position and clearances to the buoy.On arrival to the field, two tugs are to be attached to opposite sides of buoy for positional purposes.Tug 1 and Tug 2 to manoeuvre the buoy into the mooring location and orientate as close as possible to its final position.
First line to be connected will be the weather line . Connection of lines will then follow in a diametrically opposite manner.Ensure that the upper pull-through wire segment (pre-rigged on buoy) for the appropriate line number has its lower end free and hanging vertically down.IV to set-up on DP over 1st line to be connected and launch ROV
- PIV will deploy ROV hook assembly
- ROV to guide hook down to seabed for connection to Recovery grommet
- ROV to connect ROV hook on work wire into Dyneema sling on laid down pull through wire
- PIV to recover work wire
- PIV to manoeuvre stern first towards the buoy and hold position with the stern approximately 15 m from buoy connection point.
- PIV to deploy ROV to a working depth
- ROV to swim to bottom end of pull-through wire from buoy and connect ROV hook into the bottom end of the 5m pull through wire. PIV deck to provide sufficient slack in tugger as directed by ROV.
- PIV to retrieve tugger wire to deck and lock end of 15m pull through wire in sharks jaw.
- PIV to re-position during recovery so that stern roller has approximately 5m clearance from buoy connection point.
- Available fendering to be used as required under direction of vessel and deck crew
- PIV to connect ends of the 15m upper and lower pull through wires on deck via the 5m pull through section from the buoy end.
- Connection to be made with shackle with masterlink included in connection between 5m section and 15m lower pull through wire.
- PIV to connect a wire sling to end of vessel work wire
- Overboard pull through wire connection and pay out on work wire until the load is transferred to the buoy hang-off.
- ROV to monitor mooring line as work wire is paid out to determine when load transfer occurs.
- Once load is on the buoy hang-off, release the end of the sling from the ROV hook and recover work wire.
- ROV to monitor free end of sling to ensure that there are no snags as it is pulled through masterlink
Following the connection of the first 4 lines indicated in item 3 the PIV the following options can be considered:
- If weather conditions are appropriate the tow tugs can be removed from the system
- If necessary tow tugs can remain connected until all lines are connected.
Removal of the tow tugs
This section of the procedures describes the tasks required to remove the tow wires from the buoy. As the connection point to the buoy will be at the splash zone following the hang-off of either four or eight lines, it is proposed to disconnect the line from the buoy at the connection point to the tow tug tow wires. The tow wires will therefore remain connected for later retrieval if
- Tow tugs to reduce holding power and retrieve work wire until end of 30m wire section can be secured in shark jaw.
- Feed a sacrificial sling through end fitting on the 30m wire section and connect both ends back to work wire
- Pay out work wire until weight of 30 m sling is hung off the buoy
- PIV ROV to monitor until support tug work wire is slack
- PIV ROV to cut sacrificial sling and release tow tug
Retrieval of Pull Through Wires and Temporary Hang-off
This section of the procedures describes pull through of the mooring lines following the initial hang-off of the eight mooring lines.
The first line to be tensioned will be determined on site depending on the predominant environmental conditions. All following lines will be tensioned in a diagonally opposite pattern
- Position PIV so that the buoy is on one side of the vessel inline with the vessel crane.
- PIV to connect wire sling to crane hook and ROV hook to other end
- PIV to manoeuvre crane so that ROV hook is accessible by divers
- Connect wire liftsling to ROV hook
- PIV to pull through upper pull through wire until top end of the lower pull through wire is approximately 1.5m from the buoy hang-off point. Crane to stop and hold position
- PIV crane to lower system until weight is taken on the buoy hang off
- Diver to disconnect the top of lifting sling at the shackle and place end of lifting sling in buoy hang-off hook for later retrieval.
- PIV crane to recover upper pull through wire and shackle to deck
