Description
The pre-lay mooring system consists of lines with Anchors, anchor chain,JSS ground wire, excursion limiter chain, adjustment / dip zone chain and ground chain . The riser wire and top chain for the system will be installed as part of the buoy hook-up stage. Anchor handling tug supply (AHTS) vessel shall be utilized for the mooring operations as the primary installation vessel (PIV). The PIV will make trips offshore to pre-lay the complete mooring systems and if required shall be supported by a another AHTS for tensioning operations.
Each step of the pre-lay installation shall be ascertained
Following installation stages with the maximum installation design seastate shall be analyzed
Bollard Pull Requirement for tensioning
Bollard pull requirements for mooring line test load shall be achievable by AHTS.If primary installation vessel BP is not sufficient for this test,for additional BP another AHTS shall be used.While deciding combined BP, thruster losses and efficiency shall be considered.If two vessels are used for bollard pull, minimum distance between the vessels during the tandem pull shall be at least 500m, This distance is important to minimise the risk of shock loading and reduce any losses in the PIV BP efficiency due to the wash from the vessel in front.
Mooring Line Test Loading
In order to avoid the use of the vessels work wires and ensure that sufficient grounded length is maintained during the tensioning process, a method of test tensioning the system following the deployment of the following mooring line components has been developed:
Anchor Laydown
The anchors shall be deployed using AHTS vessels. The PIV will be connected to the anchor by the anchor chain and the ground wire, and the secondary AHTS will be connected via a wire pennant.The vessels shall be positioned stern to stern with approximately 50m separation.
Ground Wire Laydown
The lay down of the ground wire section is set-up to represent the end of the wire laydown where the upper end of the wire is locked off in the sharks jaw. This allows the maximum loads to be estimated during the lay down and also indicates the loads expected on the sharks jaw when making the connection between the wire and the excursion chain. A nominal layback distance of 50m shall be chosen for the lay of the ground wire. Reducing the layback will reduce the loading but may result in compression at the seabed. Increasing the layback distance will increase the tension in the lines due to the increased horizontal component of force.No additional rigging other than the vessels work wire and permanent mooring equipment is required for this stage of operation.
Laydown of the Excursion Chain (Adjustment chain on deck)
Lay down of the excursion chain shall be split into two sections. The first section is with the excursion chain (load chain and drape chain) deployed with dip chain/adjustment chain on deck. This model therefore indicated the maximum loading expected at the sharks jaws for any required cutting of the adjustment chain. A nominal layback distance of 50m shall be chosen for the lay of the excursion chain section. Reducing the layback will reduce the loading but may result in compression at the seabed. Increasing the layback distance will increase the tension in the lines due to the increased horizontal component of force. No additional rigging other than the vessels work wire and permanent mooring equipment is required for this stage of operation.
Laydown of the Excursion Chain ( Ground Chain Payout)
Lay of the excursion chain (load chain and drape chain) is set-up to represent the full excursion chain and dip chain suspended in the water column close to the touch down point. Ground chain has been paid out over the stern roller. A nominal layback distance of 50m has been chosen for the lay of the excursion chain section. Reducing the layback will reduce the loading but may result in compression at the seabed. Increasing the layback distance will increase the tension in the lines due to the increased horizontal component of force.No additional rigging other than the vessels work wire and permanent mooring equipment is required for this stage of operation.
Laydown of the Ground Chain
The model for the laydown of the ground chain is set-up to represent deployment of the upper end of the ground chain to the seabed. To calculate the maximum results for this operation the model has been set up with the upper end of the ground chain at the sharks jaw and the removal of adjustment chain from the system (Assuming anchor drag at its maximum during tensioning). In this case of ground chain, excursion chain, ground wire and anchor chain have been deployed outside of the stern roller. A nominal layback distance of 50m has been chosen for the lay of the ground chain section. Reducing the layback will reduce the loading but may result in compression at the seabed. Increasing the layback distance will increase the tension in the lines due to the increased horizontal component of force.The end of the ground chain is to be laid down with a ROV hook and recovery rigging for future recovery. The maximum loading will be used to define the SWL of the laydown rigging.
The pre-lay mooring system consists of lines with Anchors, anchor chain,JSS ground wire, excursion limiter chain, adjustment / dip zone chain and ground chain . The riser wire and top chain for the system will be installed as part of the buoy hook-up stage. Anchor handling tug supply (AHTS) vessel shall be utilized for the mooring operations as the primary installation vessel (PIV). The PIV will make trips offshore to pre-lay the complete mooring systems and if required shall be supported by a another AHTS for tensioning operations.
Each step of the pre-lay installation shall be ascertained
- Maximum dynamic line loading expected during the installation of each mooring line component
- Line lengths, vessel positioning and bollard pull requirements for the tensioning operation
- Dynamically analyse the anchor tensioning operation using vessels for the maximum installation design seastate
- Indicate the required line components and payouts during anchor tensioning operations to ensure that sufficient mooring line remains on the seabed to maintain a horizontal pull at the anchor.
- Calculate the bollard pull requirements to achieve the minimum test tension at the anchors.
Following installation stages with the maximum installation design seastate shall be analyzed
- Lowering of the 25te Stevpris Mk6 drag anchor to the seabed
- Lay of the ground wire
- Lay of the excursion chain
Bollard Pull Requirement for tensioning
Bollard pull requirements for mooring line test load shall be achievable by AHTS.If primary installation vessel BP is not sufficient for this test,for additional BP another AHTS shall be used.While deciding combined BP, thruster losses and efficiency shall be considered.If two vessels are used for bollard pull, minimum distance between the vessels during the tandem pull shall be at least 500m, This distance is important to minimise the risk of shock loading and reduce any losses in the PIV BP efficiency due to the wash from the vessel in front.
Mooring Line Test Loading
In order to avoid the use of the vessels work wires and ensure that sufficient grounded length is maintained during the tensioning process, a method of test tensioning the system following the deployment of the following mooring line components has been developed:
- Anchor
- Anchor Chain
- Ground Wire – Jacketed Spiral-Strand Wire (JSS)
- Excursion Limiter Chain
- Adjustment Chain
- Ground Chain
- Loadcell
- Work Chain
Anchor Laydown
The anchors shall be deployed using AHTS vessels. The PIV will be connected to the anchor by the anchor chain and the ground wire, and the secondary AHTS will be connected via a wire pennant.The vessels shall be positioned stern to stern with approximately 50m separation.
Ground Wire Laydown
The lay down of the ground wire section is set-up to represent the end of the wire laydown where the upper end of the wire is locked off in the sharks jaw. This allows the maximum loads to be estimated during the lay down and also indicates the loads expected on the sharks jaw when making the connection between the wire and the excursion chain. A nominal layback distance of 50m shall be chosen for the lay of the ground wire. Reducing the layback will reduce the loading but may result in compression at the seabed. Increasing the layback distance will increase the tension in the lines due to the increased horizontal component of force.No additional rigging other than the vessels work wire and permanent mooring equipment is required for this stage of operation.
Laydown of the Excursion Chain (Adjustment chain on deck)
Lay down of the excursion chain shall be split into two sections. The first section is with the excursion chain (load chain and drape chain) deployed with dip chain/adjustment chain on deck. This model therefore indicated the maximum loading expected at the sharks jaws for any required cutting of the adjustment chain. A nominal layback distance of 50m shall be chosen for the lay of the excursion chain section. Reducing the layback will reduce the loading but may result in compression at the seabed. Increasing the layback distance will increase the tension in the lines due to the increased horizontal component of force. No additional rigging other than the vessels work wire and permanent mooring equipment is required for this stage of operation.
Laydown of the Excursion Chain ( Ground Chain Payout)
Lay of the excursion chain (load chain and drape chain) is set-up to represent the full excursion chain and dip chain suspended in the water column close to the touch down point. Ground chain has been paid out over the stern roller. A nominal layback distance of 50m has been chosen for the lay of the excursion chain section. Reducing the layback will reduce the loading but may result in compression at the seabed. Increasing the layback distance will increase the tension in the lines due to the increased horizontal component of force.No additional rigging other than the vessels work wire and permanent mooring equipment is required for this stage of operation.
Laydown of the Ground Chain
The model for the laydown of the ground chain is set-up to represent deployment of the upper end of the ground chain to the seabed. To calculate the maximum results for this operation the model has been set up with the upper end of the ground chain at the sharks jaw and the removal of adjustment chain from the system (Assuming anchor drag at its maximum during tensioning). In this case of ground chain, excursion chain, ground wire and anchor chain have been deployed outside of the stern roller. A nominal layback distance of 50m has been chosen for the lay of the ground chain section. Reducing the layback will reduce the loading but may result in compression at the seabed. Increasing the layback distance will increase the tension in the lines due to the increased horizontal component of force.The end of the ground chain is to be laid down with a ROV hook and recovery rigging for future recovery. The maximum loading will be used to define the SWL of the laydown rigging.
