Mooring & Anchoring

What a mobile mooring spread looks like

A moored semi-submersible usually runs eight or more legs in a symmetric pattern, two or more from each corner column. Each leg starts at a winch or windlass, passes through a fairlead at the column base and runs to an anchor on the seabed. A leg can be all chain, all wire, or chain at the anchor end with wire above. In deep water, a polyester section in the middle of a pre-laid leg cuts weight and lets the leg work in taut or semi-taut form. Anchor handling vessels run and recover each anchor with pennants, buoys or a permanent chaser.

The three sub-Categories and how they fit together

Every component in a leg sits on one strength chain. The weakest component's minimum breaking strength (MBS) sets what the leg can hold. A wildcat suits one chain diameter and type, and a fairlead pocket or sheave suits one chain or wire size. Start from your mooring analysis and buy each piece to match.

Standards across the group

• API RP 2SK covers design and analysis of station-keeping systems, including MODU moorings, with safety factors on line tension for intact and damaged cases.
• ISO 19901-7 covers station-keeping systems for floating structures and aligns closely with API RP 2SK.
• API RP 2I covers in-service inspection of mooring hardware for floating structures, including chain, wire and connector discard guidance.
• IACS UR W22 and DNV-OS-E302 cover offshore mooring chain in grades R3, R3S, R4, R4S and R5. API Spec 2F is the API specification for mooring chain.
• DNV-OS-E301 covers position mooring and the POSMOOR notation, including windlasses, winches, stoppers and fairleads.
• DNV-OS-E303 and API RP 2SM cover fibre ropes, and DNV-OS-E304 covers steel wire ropes.

For a used component, these documents ask for the same things. You need the original certificate with grade, size, proof load and MBS. You also need traceable marking that ties the item to that certificate, and measured condition against discard limits.

Integration questions

• Chain size and type must match the wildcat, fairlead and stopper. Stud link and studless chain need different wildcats.
• Wire diameter and construction must match the drum, sheaves and sockets.
• Winch rated pull and brake holding capacity must suit the leg's MBS and the design tensions in the mooring analysis.
• Anchor shackles and connectors must match the chain size, and pass through fairleads and anchor handling vessel equipment where the plan needs it.

Where used mooring equipment fails first

• Chain at the fairlead and the seabed touchdown zone, where interlink wear and corrosion are highest.
• Connectors and sockets, which often carry the lowest MBS in the leg and crack in fatigue.
• Winch brakes and wildcats, which wear with every anchor run and recovery.
• Fibre rope that has touched the seabed or run over a sharp edge.

Ask for the inspection report on each of these before you look at anything else.

Sourcing used mooring equipment

Unused surplus chain, wire and anchors from cancelled projects often carry full mill, proof load and break load certificates. Used chain and wire are worth considering when a recent inspection shows measured diameters within the limits of your class and the condition criteria in API RP 2I. Winches and windlasses are long-life machines, and an overhaul of brakes, gears and seals can return them to near new condition. Naventr can arrange an independent inspection on request and pass the Seller's certificates to you.