MOORING OF SHIPS.pdf

Marine Outfittings Prof. Dr. Yousri Welaya

MOORING OF SHIPS

Mooring systems are used to secure a ship to a pier, wharf, mooring buoy, or

another ship. Mooring systems include the lines, fenders, ship/pier fittings and

related machinery that would be used to bring a ship into a moored configuration.

Purpose of Mooring

Loading/Unloading Loading and unloading items such as stores, cargo, personnel, ammunition, etc.

Maintenance/Repairs Scheduled maintenance and simple repairs can be performed at sea. Certain actions require shore services, special parts, and outside assistance that can only be provided while moored.

Ship Storage Ships in an inactive or reserve status are stored at moorings. This reduces costs to simple upkeep while ensuring the ship can be made available again if needed.

Mooring Arrangements

When a ship approaches a berth, lines are passed to establish the positive motion control required to position the ship alongside the fixed structure without damage. The first lines ashore are the bow and stern lines which are used to provide positive longitudinal alignment with a desired berth location and check residual forward or astern motion, due to current, wind, or forces applied by the tug or ships screws. After alignment has been established, additional lines are passed to heave the ship into the berth and provide the security necessary to restrain it against involuntary adverse forces.

Mooring lines which tend fore and aft are referred to as Spring Lines and resist surge forces, while lines tending at right angles are referred to as Breast Lines and resist sway and yaw forces. These lines may also be referred to as bow, stern, waist, or quarter lines depending on their location on the ship.

2


It is common practice to use a minimum of four lines, two at the bow and two at the stern. On large ships where large mooring loads are involved more lines can be used. Ideally, for maximum line efficiency, all lines should be horizontal from the ship to the pier fittings; lines acting in the same direction should be of equal length and size to share the load equally. However, in practice lines are usually attached on the weather deck of the ship well above the pier, reducing the capability of the line to resist horizontal forces.

Design Considerations

Based on available studies, 90% of the winds experienced at commercial ports are below 35 knots, discounting gusts of less than 5 min duration. As far as current forces are concerned, maximum current velocities of 2 to 3 knots are assumed to act simultaneously with the maximum wind force, parallel or perpendicular to the ship to determine line loads for design. Wave forces are not a significant design factor in typical mooring calculations and are generally not included.

Shipboard Mooring Equipment

The majority of mooring systems aboard ships are relatively simple consisting of, the mooring lines, a few accessories and deck fittings, and two speed capstans. Ships do not normally carry the fenders to which they may moor against.

1. Mooring Lines

The size and strength of mooring lines is matched to the ship and generally increases as the size of the ship increases. Synthetic lines have replaced ropes made from natural fibers because of their superior strength, durability, and reduced weight. While Mooring lines are used to secure a ship to a wharf, pier, dock or another ship. natural fiber ropes require special drying and storage to prevent dry rot and mildew, synthetic lines rarely mildew and never rot. Nylon is the strongest and most elastic providing high energy absorption capacity. Polyester is second in strength to nylon and polypropylene is the lightest of the three and the least expensive. Wire ropes are commonly used with wire rope winches which stow the wire on the winch drum. Wire and traditional synthetic fiber rope must not be mixed as primary load carriers in a heavy weather mooring system. In a mixed wire and polyester/nylon fiber system, the wire lines will take virtually the entire load because of the wide difference in elasticity between the wire and fiber ropes. They may fail first, leading to a cascading failure in which lines fail in succession as each comes under load. Wire ropes are very strong for their diameter, but have an extremely high modulus of elasticity and very low elongation until failure (1 2%). Wire lines can provide better positioning of the ship because of their low stretch characteristics. Properly rigged and tensioned wire rope mooring lines will keep the ship from moving significantly and therefore will prevent the ship from building significant kinetic energy. Nylon and polyester mooring lines are the best choice where high storm

3


surge is expected, and especially where the ship can be breasted out into the middle of a slip. The elasticity of these materials provides excellent load sharing and shock absorbing as the ship moves within its mooring.

2. Mooring Fittings

Mooring Bitts, Cleats and Rings

The bitts consist of two vertical hollow steel cylinders, called barrels (welded or casted) rigidly attached to a base. The bitt barrel is fitted with a top plate and in certain designs, a rope guard to keep lines from riding up the barrel. Their function is to secure the shipboard end of the mooring line. Mooring bitts are located a minimum of 1.8 to 2.4 m from chocks to allow space for stopper (which allows transferring the tensioned line from the gypsy or capstan to the bitt). The bitt location should not require a sharp bend around the chock.

4


Cleats perform a similar function but are used primarily for mooring barges or other small craft along a ship. Cleats are mounted on the top of the bulwarks or on deck near the side of the ship.

Mooring rings are installed in the side shell between the water line and the weather deck at a convenient height for barge or small craft crews to handle their mooring lines.

Fixed and Roller Chocks

Chocks are installed at the sides of the ship to lead the mooring lines from their point on shore to the hauling end aboard ship. Fixed chocks are used for fiber rope hawsers which do not require adjustment under load. When chocks are located in way of bulwarks, the chock consists of a heavy ring welded into the bulwark. In way of open rails, chocks are mounted on the deck and would either be of the open or closed type.

Roller chocks and fairleads are used to lead mooring lines around obstructions and provide alignment with the gypsy heads. Chocks intended for constant tension winches with wire lines are fitted with four pipe rollers (2 vertical and 2 horizontal). The Panama fairlead is an almost elliptical opening formed in a casting which is fitted into a suitably stiffened aperture in the bulwark. The following figure shows the (b) Multi-angle fairlead; (c) Pedestal fairlead; (d) Two-roller fairlead; (e) Panama fairlead.

5


6


3. Mooring Machinery

Mooring machinery facilitates the handling and securing of mooring lines. It includes the capstan head and related machinery, as well as constant tension mooring winches.

The capstan is a warping head with a vertical axis used for handling mooring and other lines. Capstans are generally designed to rotate in both directions. All capstans consists of a capstan head, drive machinery, and operator control. A vertically mounted capstan is referred to as a capstan while a horizontally mounted capstan is typically referred to as a warping head. The capstan/warping head is sized to accommodate the largest mooring line which must be handled. The operating surface of the capstan/warping head is machined smooth and should be kept free of paint, nicks, and burrs to avoid damaging the mooring lines.

The windlass is a machine used to hoist and lower anchors. The gypsy head is a cylinder like fitting on the end of a winch or windlass shaft. The drive machinery for the capstan is frequently located below deck, often times in the overhead. On some capstans part of the machinery is located within the hollow of the capstan head and is hidden from view. On anchor windlass systems equipped with capstans, the drive machinery for the capstan is integrated with the machinery that drives the windlass.

Steam, D.C. and electrohydraulic drives are designed to give speed variation from creeping to a maximum of about 43 m/min. A.C. electric drives usually have two speeds; full and quarter.

Constant Tension

Ships that change draft rapidly when loading or unloading, such as tankers and ore carriers and ships which must moor alongside a pier where there is a large rise and fall of tides, frequently are fitted with constant tension mooring

7


winches and wire rope lines. These winches may haul in or payout lines as the tension varies from the limit for which the controls are set.

Passing and Securing Mooring Lines

The method for passing and securing mooring lines depends on the mooring plan, the type of pier fittings and the parts of line to be secured. The sequence of operations is to pass the line and secure the initial lines on pier fittings. Next remove the slack from these initial lines and secure a single part on the ship fitting, in turn securing the ship. Then deploy the remaining mooring lines or parts of line in accordance with mooring plan and adjust the initially deployed lines as required (slack or tension). The procedure for securing mooring lines to shipboard fittings may also include taking lines to power for pre-tensioning.

Fendering Systems

Fenders and fendering systems are designed to absorb or dissipate the energy of impact that occurs during berthing and mooring of ships. Fenders provide a load dampening interface, protecting ship hulls and berthing facilities (piers, wharfs, etc.) from excessive damage, which could otherwise occur in the absence of proper fendering between ships and piers.

Marine fenders are classified as either solid or resilient. The solid type, which have minimal capacity to absorb impact energy, may be timber facing on a flexible pier structure or mounted on the face of a rigid pier or wharf structure that is not subjected to large docking forces. Resilient fenders have the ability to absorb sizable amounts of impact energy without causing damage to the ship, the pier, or the fender.

Fenders are also classified as either fixed or portable (floating). Fixed fender systems could be comprised of floating fenders attached to a quay, or could be non-floating fenders attached to a pier, wharf or quay. Fixed fender systems are engineered for a specific site, to provide the desired protection from impact loads. Floating fenders are normally easily portable, so they can

8


be deployed in a wide range of situations to meet ever changing mooring requirements.

Deck Layout

A practical mooring equipment layout keeps the number of winches to a minimum and they are driven by a common power source.

At least one chock is installed at each mooring station to provide a fairlead for mooring lines between the ship and shore. Chocks should be arranged to accommodate Panama Canal Requirements. One bitt should be provided for each mooring line.

Shore Mooring Fittings Mooring fittings provided on piers are cast metal structures bolted into the concrete structure. Common pier mooring fittings are a combination of cleats,

9


double bitts and bollards of various shapes, sizes and strengths. Fenders are used for protection between a ship and a pier. Fenders are needed to absorb energy, to cushion against impact loads, and to provide standoff between ships and piers. Cleats on piers are similar in function and appearance to shipboard cleats, but they differ in size and strength. Pier cleats are bolted in place, whereas shipboard cleats are welded to the ship structure. Double bitts have the same basic barrel construction as shipboard bitts. They differ in that their caps are more one sided. A bollard is a strong, cylindrically shaped, upright fitting found on a pier or wharf. The eye of a ships mooring line is placed around it during mooring.

10


11


Automated Mooring Systems A new mooring system has been developed to replace the traditional system using ropes. The new system uses vacuum pads to provide the mooring attachment. Each pad has a measurable working load, providing a powerful physical attachment between ship and shore. The MoorMaster is the name given to the range of generic shore based mooring systems. It does not require specific installations on the ship and can directly attach to the hull of most commercial and military vessels. The quay face model has the advantage of compact storage when not in use. This enables the system to rest behind the maximum fender impact line during berthing. For instance, The MoorMaster 400 is ideally suited to ships from 70 up to 250 metres in length. The number of units required to safely moor a ship will be dependent on the hull windage area and environmental weather patterns. In most instances between 2 and 6 units will be required for a single berth. The unit has a design load capacity of 40,000 kg and an outreach of 900mm.

12


The new system is assumed to have the following advantages:

Risk of injury to shore and ship personnel by mooring ropes eliminated. Continuous load monitoring and sophisticated alarm functions, relayed in

real time to operations personnel. Multiple redundancy of vacuum pads and inherent fail safe features ensure

secure mooring even during power cuts or loss of control signals. Robust mechanical design ensuring reliable operations. Fast attachment ( typically 12 sec) and instant release. No more delays while waiting for mooring crews to become available. Only one operator required, based ashore or onboard, to activate and

remotely monitor the mooring system. Eliminating costs for enlarging pier structures when larger ship arrivals are

scheduled.

Documents

MOORING OF SHIPS.pdf