3.2 Mooring Equipment, Lines & Deck Fittings

Deck Mooring Fittings: Bitts, Bollards & Fairleads

Mooring deck fittings must withstand substantial cyclic loads imposed by winds, currents, passing vessels, and tidal fluctuations. These fittings transfer tensile loads from synthetic and wire mooring lines directly into reinforced internal deck girders and frames.

Bollards and Bitts

• Double Bitts (Deck Bollards): Heavy vertical cylindrical steel posts mounted in pairs on a common welded base plate. Found forward and aft, they are used to make mooring lines fast to the ship.
• Cruciform Bollards: Bitts incorporating horizontal cross-arms on each post. The cross-arms prevent mooring lines from riding up or slipping over the top of the bitt when lines lead upward at steep angles to high quays.
• Securing Procedure: Stopper the hauled line before transferring it from the warping drum to the bitts. Use the turns and lead specified for the fitting and line; single turns or figures of eight may be used. Keep hands clear as weight transfers.

        SECURING A LINE ON DOUBLE BITTS
          (Figure-Eight Configuration)

          [Bitt 1]           [Bitt 2]
            .--.               .--.
           /    \             /    \
          |   1  |           |   2  |
           \    /             \    /
            '--'               '--'
              \                 /
               \   FIGURE-8    /
                \   TURNS     /
                 \           /
     <=== From Fairlead

Fairleads and Chocks

Fairleads guide mooring lines from deck winches and bitts outboard to the pier bollards, altering line direction while minimizing chafe and frictional resistance:

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Mooring Winches & Capstans

Mooring winches provide mechanical pulling power to heave the vessel alongside, maintain holding tension at the berth, and spool mooring lines when unmoored.

Split-Drum Winch Architecture

Modern commercial vessels are equipped with split-drum winches, where the main spooling cylinder is divided into two distinct sections separated by a notched steel flange:

1. Storage Drum: Holds the slack, un-tensioned reserve length of the mooring line.
2. Tension Drum (Working Drum): Receives the working end of the line leading outboard through the fairlead.

          SPLIT-DRUM MOORING WINCH
   [Drive Motor]   [Storage Drum]   |   [Tension Drum]   [Brake]
         |       (Multiple Layers)  |  (ONLY ONE LAYER)     |
      +-----+    |================| | |==============|    +---+
      |     |----|================|-+-|   (Set turns)|----|   |
      +-----+    |================| | |==============|    +---+
                                    ^
                             Notched Divider

The Single-Layer Operational Rule

If a synthetic or wire line is subjected to heavy tensile load while spooled in multiple overlapping layers on a single drum, the top wrap acts as a wedge. It cuts down through the lower wraps—a catastrophic phenomenon termed line burial or cutting in. Line burial crushes internal synthetic fibers, distorts wire rope strands, and locks the line so tightly that the winch motor cannot pay it out, creating severe danger during unmooring or tidal drops.

Drive Types & Safety Controls

• Hydraulic Winches: Driven by low- or high-pressure hydraulic power packs; offer smooth variable speed control and high holding torque.
• Electric Winches: Driven by multi-speed or variable frequency drive (VFD) electric motors; require zero deck hydraulic piping but demand strict electrical enclosure sealing.
• Deadman Controls: Releasing a spring-return operating lever returns it to neutral and stops the drive. Follow the winch instructions for its drive brake, drum brake and clutch.
• Emergency Stop Buttons: Red emergency-stop buttons stop the powered operation through the installed safety system. The drum holding brake may be a separate manual control. Do not assume that an E-stop has secured the mooring line or made it safe to handle; use the specified brake and isolation procedure.

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Mooring Lines: Synthetic Fiber Materials

Synthetic lines have largely replaced natural fiber ropes on commercial vessels due to their superior strength-to-weight ratios, rot resistance, and durability. However, each synthetic polymer possesses distinct physical properties that directly dictate deck safety.

Characteristics of Common Mooring Polymers

1. Polypropylene: The most widely used general-purpose mooring line on small and medium cargo ships. It floats, making recovery and handling easier, but slack floating lines can still foul ship thrusters or tug propellers and must be kept clear of them. However, ratings must never allow polypropylene to surge or slip on a rotating winch drum; frictional heat above 165°C melts the outer fibers instantaneously, fusing the line into a dangerous glassy crust.
2. Polyamide (Nylon): Possesses outstanding shock-absorption capacity, making it valuable for towing hawsers and offshore moorings. Its elastic stretch stores substantial potential energy. If the line parts under load, that energy produces rapid, potentially fatal recoil.
3. Polyester: Outstanding resistance to surface friction, sunlight (UV), and cyclical flexing. Often blended with polypropylene (polyolefin dual-fiber ropes) to combine polyester's abrasion resistance with polypropylene's reduced weight.
4. High Modulus Polyethylene (HMPE): Manufactured from gel-spun ultra-high molecular weight polyethylene fibers. HMPE ropes have replaced heavy steel wire ropes on modern ultra-large container ships and crude oil tankers. They match the tensile breaking strength of an identical-diameter steel wire at roughly 15% of the weight, and stretch much less than nylon. They can still recoil dangerously; an elastic tail can add substantial stored energy to the system.

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Steel Wire Mooring Ropes & Inspection Standards

Many bulk carriers, tankers, and vehicle carriers utilize galvanized steel wire ropes for permanent mooring lines due to their dimensional stability, non-stretch characteristics, and resistance to chafe against rough pier faces.

Construction

Standard mooring wires utilize flexible constructions, most commonly 6x24 (6 strands of 24 wires each around a central fiber core) or 6x36 with an Independent Wire Rope Core (IWRC). The steel wire rope core prevents cross-sectional crushing under multi-layer spooling loads.

Lubrication & Maintenance

Mooring wires must be lubricated regularly with approved, environmentally non-toxic wire rope dressing. The dressing penetrates between individual wire strands, minimizing internal friction and preventing corrosive oxidation from trapped salt water.

Inspection and Discard Criteria

Inspect mooring wires, including the eyes and splices, using the manufacturer's criteria recorded in the ship's Line Management Plan. Remove an unsuitable wire before failure; report damage immediately and do not use a suspect line pending assessment.

• Broken Wires: Check their number, concentration and location against the criteria for that rope construction and inspection length. Protruding broken ends also cause serious hand injuries.
• Kinks, Crushing and Bird-Caging: Permanent distortion or strand separation can seriously reduce strength and calls for withdrawal and assessment.
• Diameter Change: Measure and compare with the original diameter and the maker's discard limits; local reduction can indicate internal wear or core failure.
• Corrosion: Examine for pitting, internal corrosion and loss of section, not merely surface colour.
• Records: Record inspections, deterioration and replacement. Do not substitute a generic crane-rope percentage for the applicable mooring-wire criteria.

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Synthetic Tails (Mooring Pennants)

Steel wire ropes and HMPE lines possess negligible elasticity (less than 2% elongation). If a moored vessel is subjected to wave surges, tidal swells, or dynamic wake from passing ships, the unyielding wire rope transmits brutal shock loads directly to the deck bitts, risk snapping lines or shearing deck foundations.

Synthetic tails add elasticity to wire or HMPE mooring arrangements and reduce peak loads. Their use, length and connections must suit the ship and berth requirements. IMO guidance calls for considering tails on high-stiffness lines; their extra stored energy must also be included in the snap-back assessment.

       WIRE-TO-TAIL CONNECTION
  [Wire eye] -- [Approved joining shackle] -- [Tail eye]

       HMPE-TO-TAIL CONNECTION
  [HMPE eye] -- [Maker-approved connection] -- [Tail eye]

Key Tail Specifications

• Length: 11 metres is a common tail length; 22-metre tails provide additional stretch at exposed berths. Use the specified arrangement.
• Tail Design Break Force (TDBF): MEG4 recommends 125% to 130% of the ship design MBL, the reference used to size the mooring system. Check the tail certificate and approved Line Management Plan.

Mandal & Tonsberg Joining Shackles

Purpose-designed joining links include Tonsberg, Mandal and other approved mooring connectors:

• Tonsberg: The synthetic tail eye bears on the straight pin; the wire eye bears around the body.
• Mandal: The wire eye bears on the curved roller; the tail eye bears around the body.

For an appropriate HMPE line, the manufacturer may approve a cow hitch joining the two synthetic eyes without metal hardware. Protect eyes with suitable chafe sheaths and check the completed connection before loading it.

Inspect tails before use for abrasion, cuts, glazing, hardening, damaged eyes and pulled strands. Keep their identity, certificates and service history traceable. Apply the maker’s inspection and retirement criteria in the Line Management Plan, including any specified service-life limit; withdraw damaged or unsuitable tails before failure.