Spread Mooring System is suitable or shallow or deep water, but should be restricted to mild or moderate environments since it does not respond well to the changing wind, wave and currents However, spread mooring system is easily expandable to install additional risers during the life of the field.
1. General Requirements
The spread mooring system of the FPSO should include sufficient capacity to allow for up to two mooring lines to fail without impact on safety or operations.
The FPSO spread mooring system should meet the following requirements with respect to maximum horizontal offset:
- 5% of water depth with fully intact mooring system
- 6% of water depth with one mooring line damaged
- 7% of water depth with two mooring lines damage
- Minimum distance between mooring lines and any other line in water is three meters.
The horizontal offset should be calculated for all loading with a 100-years return period. The maximum offsets should not be exceeded during offloading with a Very Large Crude Carrier (VLCC) export tanker connected to the FPSO and with environmental loading corresponding to one (1) year return period. The load from the VLCC export tanker should be based on ballast condition and with a 1,000,000 bbl oil cargo or more based on new and modern tankers.
The mooring system should have the strength to sustain a powered collision during approach with an export VLCC with a 1,000,000 bbl oil cargo and a collision speed at 2 m/s. An added water mass factor at 1.1 should be used in the collision simulations for the export tanker and the deformation energy of the FPSO and export tanker should be ignored.
The FPSO spread mooring system design should have the chains attached at the keel level. There should be sufficient chain, storage and arrangements to relocate the FPSO 50 meter from design location in any direction without holding up the production operation.
Wires used in the design should be such that eventual future repairs/replacements in the mooring system, such as wire, jewellery or chain replacement should be possible to be carried out by offshore boats.
The mooring lines should consist of steel wire with chain segments at both ends.
Each mooring cluster on the FPSO hull should have its own set of mooring jacks or windlass capable of adjusting the length of each individual mooring line from that cluster. Local control consoles should be located nearby the mooring jacks or windlass for operation and control of the system.
The FPSO should be equipped with system for measuring mooring line tensions. The line tensions should be continuously displayed for each mooring line in the marine workstation in the Central Control Room (CCR). Load cells on each line should be redundant with two output signal lines connected to the Integrated Control and Safety System (ICSS). It should be possible to replace the load cells in the field without having to disconnect the mooring line.
3. Mooring Line Chain
The mooring line chains should be stud-less with a 10 mm corrosion allowance in addition to the calculated size based on Classification Society and applicable rules. The mooring line chain segments should be manufactured in one continuous length.
All mooring line connections to FPSO, anchor piles or between chain-wire segments should be designed for chain minimum breaking load (MBL) plus 20%. It should be possible to connect and disconnect the lower chain segment from the pile segment with ROV assistance.
Mooring system chains should be maximum grade R4S as per DNV OS-E302 Offshore Mooring Chain and should have a maximum surface hardness of 32 Rockwell C.
4. Connecting Links
Shackles for the mooring system of grade R5 or higher are not acceptable. Only chain connecting shackles of the D-type and links which have an external nut, a nut-locking device and a pin rotational restraint should be used in the mooring lines.
Connectors such as ‘Kenter’ links, ‘Baldt’ links and normal chain connecting shackles are not acceptable.
5. Mooring Piles
The FPSO mooring system should be restrained by suction piles. The load carrying capacity originating from the suction effect should not be used in the determination of the maximum load bearing capacity of the pile.
Pile chain connections should be able to support lateral loads resulting from failure of mooring lines.
6. Steel Wire
Spiral strand wire ropes should be heavy galvanized and sheathed; maximum tensile stress capacity should be 1770 MPa.
Sockets should have redundant means of electrical isolation between socket and wires and between the socket and the shackle. The surface sheathing of the wire rope should be extruded plastic jacket to prevent ingress of seawater and flushing out of blocking compound.
Sockets should be provided with bend stiffeners of polyurethane, to protect the wires during installation operations, and with anodes.
7. Fairleads and Chain Stoppers
Connection at keel level should provide freedom of motion in two planes on the mooring chains so as to prevent out-of-plane bending of chain links.
Fairleads, chain stoppers and load bearing connection systems should be tested to 100% MBL of each mooring line. Minimum design capacity of each fairlead and chain stopper should be 120% of chain MBL.
8. Moorings Corrosion Protection
The mooring systems structural and foundation elements should be provided with a cathodic protection system. The design of the cathodic protection system should allow for anodes to be replaced in situ. As chains should not be coated; additional exposed surface represented by the top segment of the chain of the mooring lines should be taken into account in the cathodic protection calculations.
9. Installation and Maintenance Equipment
The FPSO spread mooring system supplier should provide all necessary handling devices, which should include monorails, bitts, lugs, winches, and handling hoists, in order to assist the installation, maintenance and de-installation of the mooring lines on the FPSO.
Bitts and lugs should be provided at suitable places where necessary for the purpose of facilitating maintenance, diving and monitoring operations during pull-in/pull-out and mooring legs hook-up.
10. Spare Parts
One complete mooring leg consisting of chain, wire, chain, all necessary connectors and jewellery should be procured and maintained by the FPSO spread mooring system supplier at its shore base. Each component should be preserved and packed for onshore storage.