Mooring accidents can be horrific and insurance companies are increasingly concerned to reduce their incidence, as are those operating ships and ports. The act of mooring brings together theship, which is designed to move, and the shore – which is not. New books from The Nautical Institute can help ports to understand the issues ships face when berthing. The Londonbased membership organisation has produced two volumes – Mooring and Anchoring Ships – which set out to explain the principles involved in such operations, to help spread best practice in an area where there are no international standards of operation and few local ones.
Fenders to protect quays
Ports, fenders and their mountings are primarily designed to resist a ship’s onshore motion, and tend to be less able to withstand ship movements along a quay. Fenders must protect the quay and the ship when impacts happen, without being ripped off the quay or damaged by friction. Rubber fenders are particularly vulnerable to friction, and should be protected from shear forces by sacrificial covers made from a material such as wood, polyethylene, Teflon or thin aluminium sheet. Alternatively, some fenders, particularly those fitted to the knuckles of a wharf, are built to act as rollers so that a ship can slide along the wharf without ripping them out. Some roller fenders are even made to lock so that they resist fore and aft movement when a ship is in position alongside. The restraining force exerted by fenders is also transmitted to wharves, which is not usually a problem for ships up to about 20,000 tonnes of displacement berthing on quaysides built on a riverbank, the shoreline or the edge of an excavated dock. Any impact from ships at excessive speed may locally chip part of the wharf facing, but ships are likely to suffer greater damage than the wharf.
Large tankers and bulk carriers often tie up alongside jetties at the end of long, relatively light and flexible finger piers, which are vulnerable to damage from even the gentle impacts with ships in excess of 100,000 tonnes of displacement. Short-range and high precision laser-ranging systems should be installed on berths taking such large ships, in order to monitor the speed at which a ship is coming alongside.
Pre-tensioning mooring lines
Although fenders on a berth can be damaged by the friction of a ship moving against them, friction is very good at preventing the fore and aft movement of a ship when it is heaved tight up against the fenders. This is known as ‘pre-tensioning’ the mooring lines. If the mooring lines are pre-tensioned against very hard fenders, then any small movement of the ship from the berth is likely to break the contact between the ship’s side and the fenders, so there will be no friction to oppose the ship’s fore and aft motion. In order to allow effective pre-tensioning, fenders require a degree of softness, which depends on the size of the ships normally handled on the berth. Some complex fenders on berths for very large ships are actually designed so that changes of compression in the middle region of their operating range require no significant change in the compressive force at all. If a ship’s lines are pre-tensioned to squash the fender into this ‘flat’ region of compressibility, then relatively small movements of the ship on and off the berth will not change the frictional force opposing any fore and aft motion of the ship.
The long breast lines, which are often used to hold tankers alongside oil jetties, usually remain relatively close to horizontal and perpendicular to the berth, as the height of the ship relative to the jetty changes with the tide and/or its loaded state. Consequently, the increase in the breast line tensions, beyond the force of the wind, is the about same as the compressive force on the fenders. The extra load on pre-tensioned head and stern lines, which invariably have relatively poorer leads for holding a ship alongside, will be much greater than the force of the ship against the fenders.
