FPSO MOORING HOOK UP PROCEDURE
This article describes the procedure for mooring hook up of FPSO at operation site. It explains the methods applied for hook up.
FPSO General Configuration during Mooring Hook up
Normally FPSO are hooked-up by mooring chains at ballast draft, with level trim. Based on ballast draft loading condition, the final pretension of the mooring lines shall be set to calibrated 10 to 15% of mooring chain maximum breaking loads. As 100 % loaded condition draft will be more than ballast draft, With such pretension at ballast draft, lower pretension will be found for the “100% loaded” loading condition.
FPSO MOORING CHAIN PICK UP ARRANGEMENT
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FPSO MOORING CHAIN ARRANGEMENT
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Tugs General Requirements during mooring hook up
A minimum three tugs are recommended for the hook-up operations. An additional Installation vessel is used to install the mooring lines.
A bare minimum of two tugs are required to hold FPSO position on site, typically the two of the ocean tugs used for towing. However, a safe minimum of three tugs shall be used for hook-up. An additional Installation vessel is required to support during the hook-up operations.
The positioning tugs and assisting installation vessel are arranged in following way
- Lead tugs, tied to the bow of the FPSO
- Stern tugs, tied to the stern of the FPSO
- Support installation vessel, located on site to support the hook-up operation.
Mooring General Arrangement of the Hook-Up
Lead tugs are used to tow the FPSO to site, while the stern tugs are used to stabilize the FPSO fishtailing during sailing and installation.
Installation Vessels used on site to recover the chains and ensure the connection of the chains with the FPSO.
All the mooring chain,anchor and steel surface buyos shall be prelaid before FPSO or FSO approaches the field. Prevailing wind and current direction shall be determined and FPSO-FSO approach shall be fixed to minimize environmental loads on vessel so that vessel towing to the target coordinate will be smoother. For example if wind and current is coming from north, then approach shall also be towards north.
Buoys shall be prepositioned to give proper access to approaching vessel.
In the meantime,AHTS recover one end of Single mooring chain from buoy and will be waiting near design chain stopper location.Selection of first mooring chain to be hooked up shall be based on prevailing wind and current load. So that after connection vessel shall be able to weather vane around that point.
Once FPSO OR FSO reaches at designated coordinates,mooring chain shall be connected to chain stopper and chain shall be tensioned by vessel movement.
FSO or FPSO starts to weather vane against wind and current loads and rotates around single connected mooring chain. In this scenario excursion is possible from target coordinates of turret or chain stopper.Other mooring lines shall commence hooking up with respective chain stoppers on FPSO or FSO. Vessel shall be aligned in the direction of each mooring wire for facilitating mooring wire tensioning. So towing tugs shall be used every time when wire is pulled in through mooring winches and connected to chain stoppers. Finally winches shall be used for pretension of mooring wires and to achieve target coordinates.
Buoys shall be prepositioned to give proper access to approaching vessel.
In the meantime,AHTS recover one end of Single mooring chain from buoy and will be waiting near design chain stopper location.Selection of first mooring chain to be hooked up shall be based on prevailing wind and current load. So that after connection vessel shall be able to weather vane around that point.
Once FPSO OR FSO reaches at designated coordinates,mooring chain shall be connected to chain stopper and chain shall be tensioned by vessel movement.
FSO or FPSO starts to weather vane against wind and current loads and rotates around single connected mooring chain. In this scenario excursion is possible from target coordinates of turret or chain stopper.Other mooring lines shall commence hooking up with respective chain stoppers on FPSO or FSO. Vessel shall be aligned in the direction of each mooring wire for facilitating mooring wire tensioning. So towing tugs shall be used every time when wire is pulled in through mooring winches and connected to chain stoppers. Finally winches shall be used for pretension of mooring wires and to achieve target coordinates.
General Hook-Up Sequence
The general hook-up procedure is as follow:
On-site, the positioning tugs are used to guide the FPSO to final location in order to start connection of the mooring lines. Assisting installation vessel is used to pick up the lines and help transfer the chains to the FPSO. First inner line of spread mooring system is recovered and the line hook-up procedure is followed. This mooring line is hooked up with 50% of final pretension. Same procedure would be repeated for other three lines at the four corners and on inner most locations. At the end of this stage, these four lines are connected and their tension shall be adjusted to get the final tension.
To compensate new mooring line tension,At each new line connected, Lead Tug and Stern Tug shall work to keep the FPSO around +/-15m offset from its final location .
The line hook-up procedure would be repeated for next set of middle lines .These Mooring lines also shall be re-adjusted to get the final pretensions. In this way, The line hook-up procedure would be repeated for next set of lines. So basic is that starting from inner most lines tugs shall work on next set of lines. At the end of this stage, all mooring lines are connected. Once all the mooring lines are connected and tensioned, adjustment of the mooring line tension shall be performed to get the correct positioning of the FPSO (midship location, riser porch location and FPSO heading). The following sequence is to be used to hook-up each mooring lines to the FPSO.
Retrieve mooring line end using the subsea or pennant buoy (at the line end location). Chain end is recovered and locked onto the installation vessel. Buoys and pennant wires are stored on AHT deck Moves towards FPSO stern corner, relocating the line up to fairlead of mooring line. Installation vessel shall (with the end of the chain) move as close as possible towards FPSO, so that it will have minimum distance from the appropriate fairlead. Meanwhile on the FPSO, mooring wire and winch sockets are prepared and connected to fairlead. The mooring wire shall be placed through the chain stopper (open) and stored into the chain chute. At the end of the mooring wire, A messenger line is attached to the socket. Once the installation vessel close to the FPSO, the synthetic rope (messenger line) free end is sent to the installation vessel (while the other end remains attached to the socket). Once the messenger line is connected to a winch on the installation vessel, the mooring wire is paid-out from the mooring winch through the chain stopper and fairleads; and the synthetic rope in paid-in from the AHT to guide the open spelter socket from the FPSO to the installation vessel. Once the socket at end of the mooring wire is on-board the AHT deck, the open spelter socket is connected to the last chain link. Once the chain has passed 15 links inside the chain stopper, chain stopper shall be closed and messenger line shall be retrieved which is ready to be used onto the next line. Then the chain stopper shall be opened and the mooring winch is used to set the chain at 50% of final pretension.
On-site, the positioning tugs are used to guide the FPSO to final location in order to start connection of the mooring lines. Assisting installation vessel is used to pick up the lines and help transfer the chains to the FPSO. First inner line of spread mooring system is recovered and the line hook-up procedure is followed. This mooring line is hooked up with 50% of final pretension. Same procedure would be repeated for other three lines at the four corners and on inner most locations. At the end of this stage, these four lines are connected and their tension shall be adjusted to get the final tension.
To compensate new mooring line tension,At each new line connected, Lead Tug and Stern Tug shall work to keep the FPSO around +/-15m offset from its final location .
The line hook-up procedure would be repeated for next set of middle lines .These Mooring lines also shall be re-adjusted to get the final pretensions. In this way, The line hook-up procedure would be repeated for next set of lines. So basic is that starting from inner most lines tugs shall work on next set of lines. At the end of this stage, all mooring lines are connected. Once all the mooring lines are connected and tensioned, adjustment of the mooring line tension shall be performed to get the correct positioning of the FPSO (midship location, riser porch location and FPSO heading). The following sequence is to be used to hook-up each mooring lines to the FPSO.
Retrieve mooring line end using the subsea or pennant buoy (at the line end location). Chain end is recovered and locked onto the installation vessel. Buoys and pennant wires are stored on AHT deck Moves towards FPSO stern corner, relocating the line up to fairlead of mooring line. Installation vessel shall (with the end of the chain) move as close as possible towards FPSO, so that it will have minimum distance from the appropriate fairlead. Meanwhile on the FPSO, mooring wire and winch sockets are prepared and connected to fairlead. The mooring wire shall be placed through the chain stopper (open) and stored into the chain chute. At the end of the mooring wire, A messenger line is attached to the socket. Once the installation vessel close to the FPSO, the synthetic rope (messenger line) free end is sent to the installation vessel (while the other end remains attached to the socket). Once the messenger line is connected to a winch on the installation vessel, the mooring wire is paid-out from the mooring winch through the chain stopper and fairleads; and the synthetic rope in paid-in from the AHT to guide the open spelter socket from the FPSO to the installation vessel. Once the socket at end of the mooring wire is on-board the AHT deck, the open spelter socket is connected to the last chain link. Once the chain has passed 15 links inside the chain stopper, chain stopper shall be closed and messenger line shall be retrieved which is ready to be used onto the next line. Then the chain stopper shall be opened and the mooring winch is used to set the chain at 50% of final pretension.
FPSO MOORING CHAIN PROFILE
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FPSO MOORING CHAIN PICK UP
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Tugs Requirements
Lead and Stern Tugs
The tugs used for the hook-up operations are usually the same the one used for the towing operation. Towing is usually designing but in any case, following requirements are needed for the Lead and Stern Tugs.Bollard pulls of tugs are calculated based on the force required to keep the FPSO in position with only two lines connected in specified wave heights, wind and current load in beam and quartering directions.
The tugs used for the hook-up operations are usually the same the one used for the towing operation. Towing is usually designing but in any case, following requirements are needed for the Lead and Stern Tugs.Bollard pulls of tugs are calculated based on the force required to keep the FPSO in position with only two lines connected in specified wave heights, wind and current load in beam and quartering directions.
Installation Vessel
An Installation vessel(INSTALLATION VESSEL) is required to support the hook-up operation.
The installation vessel shall be fitted with Anchor Handling systems. The installation vessel shall be able to fit a studless link hydraulic stopper. Installation vessel shall be fitted with auxiliary winch to heave the messenger line with drum capacity of about 50m.
The installation vessel shall be fitted with Anchor Handling systems. The installation vessel shall be able to fit a studless link hydraulic stopper. Installation vessel shall be fitted with auxiliary winch to heave the messenger line with drum capacity of about 50m.
MEASURES AND TOLERANCES
Positioning System
Recommended systems for the operation are:
DGPS1 with an accuracy of this system lower than +/- 1m. Gyrocompass with an accuracy lower than +/- 0.2 degree. A DGPS1 surface positioning system shall be used on the FPSO to position the vessel in the target box. A survey Gyrocompass shall be sued to install the FPSO with the proper heading. An additional DGPS1/2 system shall be placed at the riser porch tie-in location to monitor the good location of the riser connection. Positioning systems shall be set with Projection UTM Zone 48, local datum.
Recommended systems for the operation are:
DGPS1 with an accuracy of this system lower than +/- 1m. Gyrocompass with an accuracy lower than +/- 0.2 degree. A DGPS1 surface positioning system shall be used on the FPSO to position the vessel in the target box. A survey Gyrocompass shall be sued to install the FPSO with the proper heading. An additional DGPS1/2 system shall be placed at the riser porch tie-in location to monitor the good location of the riser connection. Positioning systems shall be set with Projection UTM Zone 48, local datum.
Positioning Tolerances
The positioning of the FPSO using above equipment shall be as follow:
FPSO horizontal position tolerance is a box with +/-5m sides from reference point FPSO heading tolerance is +/-2 deg from reference heading, with a maximum accuracy of +/- 0.5 deg
The positioning of the FPSO using above equipment shall be as follow:
FPSO horizontal position tolerance is a box with +/-5m sides from reference point FPSO heading tolerance is +/-2 deg from reference heading, with a maximum accuracy of +/- 0.5 deg
Mooring Lines Pretensions
The chain tension will be confirmed by a chain angle measurement gauge that will be fitted to each mooring chain outboard of the chain chutes.ROV survey between the distance between the fairleads and the mooring lines touch-down point on the seabed can be used as confirmation results of the proper tensioning.
The chain tension will be confirmed by a chain angle measurement gauge that will be fitted to each mooring chain outboard of the chain chutes.ROV survey between the distance between the fairleads and the mooring lines touch-down point on the seabed can be used as confirmation results of the proper tensioning.
Tensions Tolerance
The chain angle accuracy shall be measure with accuracy of 0.5 deg. If ROV assistance is used, the distance from fairleads to TDP shall be measured with an accuracy of 5m.Tension calculation using above measurements shall be +/- 5MT from recommended pretension.
The chain angle accuracy shall be measure with accuracy of 0.5 deg. If ROV assistance is used, the distance from fairleads to TDP shall be measured with an accuracy of 5m.Tension calculation using above measurements shall be +/- 5MT from recommended pretension.
Mooring Chain Twist
The installation shall minimize the twist of the mooring lines. The twist tolerances shall be as follow:
There shall not be more than 1 twist every 100m. Not more than 3 twists all along the mooring lines shall be occurring along the whole line. A ROV survey of the mooring lines shall be conducted to ensure that allowable twist and no kinked chain link occurs along the mooring lines. Marking of the chain can be used to help control of the twist during ROV survey.
The installation shall minimize the twist of the mooring lines. The twist tolerances shall be as follow:
There shall not be more than 1 twist every 100m. Not more than 3 twists all along the mooring lines shall be occurring along the whole line. A ROV survey of the mooring lines shall be conducted to ensure that allowable twist and no kinked chain link occurs along the mooring lines. Marking of the chain can be used to help control of the twist during ROV survey.
Emergency Response Plan
The FPSO approach will be timed for the slack water prior to the start of the weaker ebbing tidal stream. As an overall statement, weather condition shall not exceed for mooring lines hook-up operation:
Significant Wave Height less than specified Wind speed less than specified Current velocity less than specified Otherwise, Mooring Line Hook-up is aborted and the line is retrieved and laid on seabed by the Support Tug. Mooring lines hooked-up remains attached to the FPSO. The Lead and Stern Tug sets back into a towing configuration in order to face the up-coming weather.
The FPSO approach will be timed for the slack water prior to the start of the weaker ebbing tidal stream. As an overall statement, weather condition shall not exceed for mooring lines hook-up operation:
Significant Wave Height less than specified Wind speed less than specified Current velocity less than specified Otherwise, Mooring Line Hook-up is aborted and the line is retrieved and laid on seabed by the Support Tug. Mooring lines hooked-up remains attached to the FPSO. The Lead and Stern Tug sets back into a towing configuration in order to face the up-coming weather.
FPSO Preparation
Prior to FPSO entering the field, a dedicated rigging team will board the vessel to make final preparations for the hook-up works.
The Tow Master will be in overall charge in Positioning of the FPSO, he will liaise closely with the Clients Offshore Installation Manager to instruct the different tugboats for the approach and to hold the FPSO in position during the hookup.
Mooring winches shall be used for the tensioning of the mooring chain legs. Chain angle measurement gauge shall be prepared.For the hookup operation, synthetic ropes and appropriate equipment will be prepared on deck at the fore and aft bundles of the FPSO. All safety equipment shall be as per SOLAS specification, National Regulation and International applicable standards mooring hookup.
Prior to FPSO entering the field, a dedicated rigging team will board the vessel to make final preparations for the hook-up works.
The Tow Master will be in overall charge in Positioning of the FPSO, he will liaise closely with the Clients Offshore Installation Manager to instruct the different tugboats for the approach and to hold the FPSO in position during the hookup.
Mooring winches shall be used for the tensioning of the mooring chain legs. Chain angle measurement gauge shall be prepared.For the hookup operation, synthetic ropes and appropriate equipment will be prepared on deck at the fore and aft bundles of the FPSO. All safety equipment shall be as per SOLAS specification, National Regulation and International applicable standards mooring hookup.
