Mooring Lines and Towing Gear

Mooring Lines and Towing Gear covers the rope sections of a mooring leg and the gear that connects a rig to its tugs. It includes six-strand and spiral strand mooring wire, polyester and other fibre ropes, spelter sockets and terminations, pennant wires and buoys, and towing bridles with triangle plates, recovery gear and emergency towing equipment.

What the lines and towing gear do

On a chain and wire leg, wire forms the upper part between the winch drum and the chain. That keeps the leg lighter and lets the rig moor in deeper water. Mobile units use six-strand wire with an independent wire rope core, because it spools on drums and survives fairleads. Spiral strand wire, often sheathed, suits pre-laid and long-term moorings. Polyester rope, in taut or semi-taut legs, cuts weight in deep water and gives the leg compliance through stretch. Pennant wires and buoys let an anchor handling vessel set and recover anchors. During a rig move, a bridle with a triangle plate connects the rig's towing brackets to the tug's towline.

Configurations

Line	Use	Notes
Six-strand wire	Mobile chain and wire legs	Spools on drums, needs lubrication
Spiral strand wire	Pre-laid and long-term legs	Higher strength and stiffness, often sheathed
Polyester rope	Deepwater taut or semi-taut legs	Must stay clear of the seabed
Pennant wire	Anchor running and recovery	Short lengths, heavy wear
Towing bridle	Rig moves	Chain or wire legs to a triangle plate

Key specifications

Parameter	Why it matters	Typical classes or units
Diameter and construction	Fit to drum, sheaves and sockets	mm (in), strand count, core type
MBS	Strength in the mooring analysis	kN (kips)
Terminations	Weakest point of many lines	Open or closed spelter sockets, spliced eyes
Corrosion protection	Service life	Galvanised, sheathed, lubricated
Bridle safe working load	Must suit the tug's bollard pull	tonnes-force (kips)

Standards

API RP 2SK and ISO 19901-7 set line tensions and safety factors.
API RP 2I covers in-service inspection and discard of wire and fittings.
DNV-OS-E304 covers offshore mooring steel wire ropes.
DNV-OS-E303, API RP 2SM and ISO 18692 cover fibre ropes for station keeping, including handling and inspection.
DNV-ST-N001 covers marine operations, including towing, and is the basis for many marine warranty surveyor approvals.

Failure modes and inspection

Broken wires, crushing, birdcaging and kinks. Inspect the full length visually and gauge the diameter.
Corrosion and loss of lubrication inside the rope. Open a few strands at a sample point.
Cracked sockets and wire breaks at the socket neck. Use magnetic particle testing on the socket and check the resin or zinc cone.
Electromagnetic rope testing finds internal broken wires and loss of metallic area that visual checks miss.
Fibre rope cuts, jacket damage, abrasion and soil ingress. Check the full length and ask for the rope's storage and handling record.
Towing brackets crack at their welds, and bridle chain and plates wear at the pins. Use NDT on the welds and measure pin holes.

Documentation

Manufacturer's certificate with MBS and construction.
Socketing records, with resin or zinc type and the socket's certificate.
Inspection reports, including electromagnetic testing where done.
Service history, with location, duration and peak loads.
For fibre rope, the test insert results and storage history.
For towing gear, proof test certificates and the last inspection of each component.

Integration

Wire diameter must match the winch drum grooving and the fairlead sheaves. The chain-to-wire connector must pass over the wildcat and through the fairlead. Fibre rope needs connectors and handling that respect its minimum bend radius. Towing gear must suit the rig's brackets and the tug, and match the tow plan approved for the move.

Service life and the used, surplus or refurbished choice

Mooring wire and pennants have a finite life. Used wire is mostly worth buying as spare or short-term rope, with a recent electromagnetic test. Unused surplus wire and fibre rope with mill certificates is the stronger choice for a main mooring. Towing bridles and plates are often reused after inspection and proof testing. The chain sections these lines join are in Anchors and Chains, and the machines that run them are in Anchor Winches and Windlasses.

FAQ

Frequently asked questions

How is used mooring wire rope inspected?

Used mooring wire is inspected visually over its full length and tested electromagnetically for internal broken wires and loss of metallic area. Inspectors gauge diameter, open sample strands for internal corrosion and check each socket by magnetic particle testing. API RP 2I and the maker's discard criteria set the limits.

Can used polyester mooring rope be reused?

Sometimes, if inspection and a documented handling history show no damage. Cuts, abrasion, jacket damage or seabed contact rule it out, because soil particles inside the rope wear the fibres. Test inserts broken after service give the best evidence of remaining strength.

What towing gear does a semi-submersible need for a rig move?

A rig tow needs towing brackets on the rig, a bridle of chain or wire, a triangle plate and the tug's towline, plus emergency towing gear and recovery pennants. The tow plan and the marine warranty surveyor set the exact gear and its safe working loads.

Why do mobile rigs use six-strand wire instead of spiral strand?

Six-strand wire tolerates spooling on winch drums and running over fairlead sheaves far better than spiral strand. Spiral strand is stronger and stiffer at the same diameter, but it suffers when bent repeatedly. It suits pre-laid moorings where the line stays in place.

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