F(P)SO Tandem Mooring Hawser System
This defines the requirements for hawser system design for use at Floating Production Storage Offloading (FPSO) and Floating Storage Offloading (FSO) units [F(P)SOs].
The theory is applicable to spread-moored F(P)SOs that normally discharge via a CALM buoy, but which may be outfitted to do tandem operations as a contingency in the case of unavailability of the CALM System.
There have been several cases where a single point failure in the tandem mooring system at a CALM or an F(P)SO has resulted in a break-out of the receiving tanker with pollution and potential for collision. In each of these cases, there was a component in the hawser system that was not redundant and when it failed, the tanker was no longer moored. In one case, the tanker grounded. In several others the Gall Thompson Marine Breakaway Coupling parted and resulted in a pollution event.
At a minimum, when designing and procuring equipment for the tandem mooring system, OCIMF recommendations should be followed. Two Hawser/Two Chain Stopper system is 100 percent redundant.
The system shall have two hawsers, two fairleads, and two chain stoppers on the F(P)SO (2 x 100 percent system).The system can be operated, with one hawser and/or chain stopper. Due to the 100 percent capability of each system, the ability to continue operations should one hawser system become inoperable can be high. At the Export Tanker, each hawser will individually connect to separate chain stoppers on the foredeck of the tanker.
The system will have each hawser connected to independent quick-release chain stopper attachments on the deck of the F(P)SO (2 quick-release chain stoppers and 2 fairleads for the chafe chains). The hawsers will be independent of each other in a configuration that will provide a system with 100 percent redundancy in all components.
Each quick-release chain stopper will have tension monitoring capability. The quick-release chain stoppers will be designed to be released individually by local activation and individually or together by remote activation.
Each hawser assembly will incorporate a weak link that will be located inboard of the F(P)SO side.The safe working load for all mooring components shall be derived from the OCIMF document, "Recommendations for Equipment Employed in the Mooring of Ships at Single Point Moorings." Safety factors for hawsers and chains shall be in accordance with the Manufacturer's specifications. Safe working loads for shackles shall be one-third of the ultimate breaking load. Safe working load should be based on the expected forces to be experienced by the tandem mooring system in the worst case metocean operating conditions for offloading operations.
It is recommended that computer based simulations be performed to verify limits for tandem mooring operations, determine optimum hawser length, tugboat requirements, and determine the maximum loads in the mooring hawsers and connections. This analysis should include the following phases of tandem operations:
The following criteria should be considered during the design and Manufacture of the tandem mooring system:
A floating hawser arrangement shall be provided based upon the maximum-size Export Tankers, the static and dynamic loads expected, and the results of the Dynamic Analysis. Hawser shall be supplied with a polyurethane coating with imbedded flotation.
Based on experience, a minimum 70-meter length hawser is to be provided, but a longer length hawser can be used to minimize hawser peak loads, to account for the relative motions between the F(P)SO and the Export Tanker, and to maintain a greater distance between the two vessels. Final hawser length determination will be based upon results of the Tandem Mooring Dynamic Analysis.Important considerations in hawser length determination are the length of cargo hose needed and ensuring that there is no unacceptable pressure drop and flow rate reduction in the Floating Hose System.
Hawsers can be left in the water between offloads or brought back aboard the F(P)SO on a reel or on the deck. Hawsers left in the water are subject to potential mechanical damage and potential increased fatigue due to continuous motion and fiber interaction and subsequent friction damage. Hawsers brought aboard the F(P)SO can be inspected more readily and protected from mechanical damage and fatigue from sea and swell cycling.
For tandem mooring operations with non-dedicated tankers, hawsers are generally deployed and retrieved with a workboat, line handling vessel, tug, or PSV. A special designed line-handling vessel often is the ideal platform to transfer hawsers to the offloading tanker due to their excellent maneuverability and the design features (such as jet drives to prevent propeller damage to the hawser) that make hawser operations safer and more efficient.
Chafe chains should be provided as defined in OCIMF Documents.Flotation for the Export Tanker chafe chain will be through floats (not a buoy).A flotation buoy will be attached to the F(P)SO end chafe chains (outboard of the F(P)SO side) to keep the chain assembly afloat in case of release.
Each hawser shall include an engineered weak link that is set to break at a predetermined tension level (typically 270 tons or 90 percent of the MBL of the Export Tanker's bow stopper with 300 tonne capacity). The weak link shall be fitted on the F(P)SO end in the chafe chain between the fairlead and the quick-release chain stopper. In all cases the weak link will be sized to ensure no other components in the tandem mooring system (including the Export Tanker bow stopper) will fail prior to the weak link failure.
The minimum design strength of the hawser shall be greater than that of the mooring chain and connectors in the system.
The hawser shall never be the weakest component in the system.
Depending on the design of the tandem hawser system chosen, a single or double fairlead shall be located in the F(P)SO deck bulwarks, sized to allow the largest component of the tandem mooring hawser (usually the triplate, weak link, or joining shackle) to pass through.
Quick-release chain stopper connection(s) shall be installed on the deck inboard of and in line with the fairlead. The quick-release chain stopper(s) shall allow the chain to be released locally while under load in an emergency situation. This release shall be activated either from a local station near the hawser connection, remotely from the CCR, or manually at the quick-release chain stopper location.
A strong point will be installed in the vicinity of the F(P)SO quick-release chain stopper(s) to provide the capability to test and calibrate the load monitoring system, test the quick-release chain stoppers under tension, and test system alarms. A hydraulic jack, or other suitable equipment and devices to connect between the quick-release chain stopper(s) and test point, will be provided to carry out these tests. The strong point will have the same SWL as the F(P)SO quick-release chain stopper(s).
A load monitoring system (tension measurement) shall be provided in the F(P)SO quick-release chain stopper. The load monitoring system shall measure instantaneous loads. Load monitoring information can be stand-alone/portable computer-based and/or linked directly into the F(P)SO DCS. The following functions shall be provided and the information recorded for the hawser string:
Studies and actual experience with spread-moored tandem operations using a 200 meter hawser show there do not appear to be benefits of using a 2 hawser system for this operation. The extra hawser length has proven to help mitigate dynamic peak loads in the hawser that are sometimes experienced in tandem operations with a shorter hawser. Longer hawsers for turret moored operations are generally not preferred because they can result in fish-tailing and increased risk of jack-knifing and can result in dynamic loading of the hawser system.
In spread-moored tandem operations 3 tugboats are used to keep the Export Tanker in position and minimize fish-tailing and excessive surge and sway events.
This defines the requirements for hawser system design for use at Floating Production Storage Offloading (FPSO) and Floating Storage Offloading (FSO) units [F(P)SOs].
The theory is applicable to spread-moored F(P)SOs that normally discharge via a CALM buoy, but which may be outfitted to do tandem operations as a contingency in the case of unavailability of the CALM System.
There have been several cases where a single point failure in the tandem mooring system at a CALM or an F(P)SO has resulted in a break-out of the receiving tanker with pollution and potential for collision. In each of these cases, there was a component in the hawser system that was not redundant and when it failed, the tanker was no longer moored. In one case, the tanker grounded. In several others the Gall Thompson Marine Breakaway Coupling parted and resulted in a pollution event.
At a minimum, when designing and procuring equipment for the tandem mooring system, OCIMF recommendations should be followed. Two Hawser/Two Chain Stopper system is 100 percent redundant.
The system shall have two hawsers, two fairleads, and two chain stoppers on the F(P)SO (2 x 100 percent system).The system can be operated, with one hawser and/or chain stopper. Due to the 100 percent capability of each system, the ability to continue operations should one hawser system become inoperable can be high. At the Export Tanker, each hawser will individually connect to separate chain stoppers on the foredeck of the tanker.
The system will have each hawser connected to independent quick-release chain stopper attachments on the deck of the F(P)SO (2 quick-release chain stoppers and 2 fairleads for the chafe chains). The hawsers will be independent of each other in a configuration that will provide a system with 100 percent redundancy in all components.
Each quick-release chain stopper will have tension monitoring capability. The quick-release chain stoppers will be designed to be released individually by local activation and individually or together by remote activation.
Each hawser assembly will incorporate a weak link that will be located inboard of the F(P)SO side.The safe working load for all mooring components shall be derived from the OCIMF document, "Recommendations for Equipment Employed in the Mooring of Ships at Single Point Moorings." Safety factors for hawsers and chains shall be in accordance with the Manufacturer's specifications. Safe working loads for shackles shall be one-third of the ultimate breaking load. Safe working load should be based on the expected forces to be experienced by the tandem mooring system in the worst case metocean operating conditions for offloading operations.
It is recommended that computer based simulations be performed to verify limits for tandem mooring operations, determine optimum hawser length, tugboat requirements, and determine the maximum loads in the mooring hawsers and connections. This analysis should include the following phases of tandem operations:
- Approach
- Connection
- Station-keeping
- Disconnection
- Emergency
The following criteria should be considered during the design and Manufacture of the tandem mooring system:
- Size of tankers operating at the facility
- Metocean conditions at which operations are designed to take place
- Storm conditions that may be encountered
- Maneuvering area, obstructions, grounding areas
- Size of the F(P)SO
- Thrusters/heading control on F(P)SO, if applicable
- Tugboat assist expectations
- Limitations of the cargo transfer system
- Position requirements for the cargo transfer system
- Workboat assistance
- Expected static and dynamic loading
- Pre-tension assumptions for station-keeping normal operations
A floating hawser arrangement shall be provided based upon the maximum-size Export Tankers, the static and dynamic loads expected, and the results of the Dynamic Analysis. Hawser shall be supplied with a polyurethane coating with imbedded flotation.
Based on experience, a minimum 70-meter length hawser is to be provided, but a longer length hawser can be used to minimize hawser peak loads, to account for the relative motions between the F(P)SO and the Export Tanker, and to maintain a greater distance between the two vessels. Final hawser length determination will be based upon results of the Tandem Mooring Dynamic Analysis.Important considerations in hawser length determination are the length of cargo hose needed and ensuring that there is no unacceptable pressure drop and flow rate reduction in the Floating Hose System.
Hawsers can be left in the water between offloads or brought back aboard the F(P)SO on a reel or on the deck. Hawsers left in the water are subject to potential mechanical damage and potential increased fatigue due to continuous motion and fiber interaction and subsequent friction damage. Hawsers brought aboard the F(P)SO can be inspected more readily and protected from mechanical damage and fatigue from sea and swell cycling.
For tandem mooring operations with non-dedicated tankers, hawsers are generally deployed and retrieved with a workboat, line handling vessel, tug, or PSV. A special designed line-handling vessel often is the ideal platform to transfer hawsers to the offloading tanker due to their excellent maneuverability and the design features (such as jet drives to prevent propeller damage to the hawser) that make hawser operations safer and more efficient.
Chafe chains should be provided as defined in OCIMF Documents.Flotation for the Export Tanker chafe chain will be through floats (not a buoy).A flotation buoy will be attached to the F(P)SO end chafe chains (outboard of the F(P)SO side) to keep the chain assembly afloat in case of release.
Each hawser shall include an engineered weak link that is set to break at a predetermined tension level (typically 270 tons or 90 percent of the MBL of the Export Tanker's bow stopper with 300 tonne capacity). The weak link shall be fitted on the F(P)SO end in the chafe chain between the fairlead and the quick-release chain stopper. In all cases the weak link will be sized to ensure no other components in the tandem mooring system (including the Export Tanker bow stopper) will fail prior to the weak link failure.
The minimum design strength of the hawser shall be greater than that of the mooring chain and connectors in the system.
The hawser shall never be the weakest component in the system.
Depending on the design of the tandem hawser system chosen, a single or double fairlead shall be located in the F(P)SO deck bulwarks, sized to allow the largest component of the tandem mooring hawser (usually the triplate, weak link, or joining shackle) to pass through.
Quick-release chain stopper connection(s) shall be installed on the deck inboard of and in line with the fairlead. The quick-release chain stopper(s) shall allow the chain to be released locally while under load in an emergency situation. This release shall be activated either from a local station near the hawser connection, remotely from the CCR, or manually at the quick-release chain stopper location.
A strong point will be installed in the vicinity of the F(P)SO quick-release chain stopper(s) to provide the capability to test and calibrate the load monitoring system, test the quick-release chain stoppers under tension, and test system alarms. A hydraulic jack, or other suitable equipment and devices to connect between the quick-release chain stopper(s) and test point, will be provided to carry out these tests. The strong point will have the same SWL as the F(P)SO quick-release chain stopper(s).
A load monitoring system (tension measurement) shall be provided in the F(P)SO quick-release chain stopper. The load monitoring system shall measure instantaneous loads. Load monitoring information can be stand-alone/portable computer-based and/or linked directly into the F(P)SO DCS. The following functions shall be provided and the information recorded for the hawser string:
- Sample loads at regular, user-settable intervals.
- Calculated average loads and recorded and tabulated significant loads.
- A recording chart included in CCR of tension readouts for each hawser for entire time a tanker is moored.
- Allowed user pre-set limits on either instantaneous, average, or significant loads, and provide an audible alarm on tripping one or more pre-set limits. At least one low tension and several high tension settings must be available.
Studies and actual experience with spread-moored tandem operations using a 200 meter hawser show there do not appear to be benefits of using a 2 hawser system for this operation. The extra hawser length has proven to help mitigate dynamic peak loads in the hawser that are sometimes experienced in tandem operations with a shorter hawser. Longer hawsers for turret moored operations are generally not preferred because they can result in fish-tailing and increased risk of jack-knifing and can result in dynamic loading of the hawser system.
In spread-moored tandem operations 3 tugboats are used to keep the Export Tanker in position and minimize fish-tailing and excessive surge and sway events.
