Modelling for safer solutions
Mooring ships can be dangerous. Every year thousands of terminal and ship staff are injured in mooring line accidents, and in some cases, the accidents can be fatal. David Foxwell reports on the risks and how to mitigate them.
Workers must be trained, equipped, and protected by operating procedures, and to keep costs under control, more and more ports are looking at automated mooring technology.
The hazards associated with mooring operations are well known and have been highlighted by a number of high profile cases recently.
Ferry and terminal operators face unsustainable costs due to the high number of injury accidents to workers handling mooring lines, to which must be added the personal and social costs incurred as a result of workplace accidents including a number of direct costs such as higher worker accident insurance premiums, employee training, the use of employee safety equipment, management and administration time - and indirect costs, such as harm to employee relationships.
Then there are operational problems associated with mooring. Long waves, swell and vessel wash can reduce cargo handling rates, particularly in the container terminal sector where mooring problems with larger, higher capacity vessels can adversely affect terminal productivity.
As Stephen Cork, a specialist in these kinds of problems at HR Wallingford in the UK explains, a container terminal affected by longwave swells suffers from poor cargo handling rates, but the cost of constructing additional breakwaters to reduce swell is often unacceptably high.
Ship motions at LNG berths is becoming a serious issue too, he explains, primarily because of the rapidly growing number of offshore LNG terminals, at which - because they are 'offshore' rather than in sheltered terminals - ship motions whilst berthed are much greater.
Ferry berths are another key area for study, Cork notes, because in the ferry market keeping to schedule and a fast turn-around are essential.
To help ports tackle issues such as these, companies such as HR Wallingford can conduct mooring analyses of wave swell and currents, and can provide assessments of mooring forces, and as Cork's colleague, Jane Smallman, who is now managing director of HR Wallingford, told a recent PIANC conference, there are a number of issues to be considered when addressing the vessel motions and mooring forces.
These include wave, wind and flow effects on ship; wave, wind and flow effects on tugs; wave, wind and flow effects on pilot boarding; and wave, wind and flow effects in channel, manoeuvring and berthing areas; each of which can affect the design of moorings, limit operations, and have a direct effect on safety.
As Dr Smallman explains, ship mooring assessments can be carried out prior to the construction or modification of a port in the form of desk studies, basic static/quasi-static modelling, dynamic modelling, and physical model studies which take into account factors such as the mooring and fendering system used, the mass properties of ships using the facility, laden and ballast conditions, natural periods of motion/resonance, and the range of ships that are likely to use the facility. There are design codes and design recommendations that should be consulted, she explains, produced by organizations such as the Oil Companies International Marine Forum (OCIMF) and the Society of International Gas Tanker and Terminal Operators (SIGTTO), and much experience can be gleaned from reviewing existing port operations.
Dynamic ship mooring modelling techniques can also be used to look at all types of wave effects, including first-order, second-order, and nonlinear effects, as well as shallow water effects, multi-directional waves, types of quayside and reflections of ship generated waves, non-linear effects of mooring lines and fenders. Techniques such as time-domain modelling allow for statistical analysis, thus reducing the conservatism inherent in more basic forms of modelling, and ship mooring studies provide information on important issues such as downtime, safety limits, and the moored ship motion and mooring forces.
SAFETY AND EFFICIENCY Manufacturers continue to develop new, safer and more efficient mooring technology. In 2005, TTS Marine's Material Handling Division delivered the first two examples of a new type of SemiAutomatic Bollard (SAB) to the Norwegian ferry ports of Horten and Moss on the Oslo Fjord, for the ferry service operated by Basto Fosen. The main aim of the development of the SAB was to reduce the number of shore personnel needed during mooring operations.
The SAB operates as a vertical telescopic bollard that can be tilted upright, against the ship's hull. It consists of a hydraulically operated telescopic arm with tilting cylinder and tiltable bollard;
the system's hydraulics; and a control panel. The bollard is operated by radio control from the winch deck, or, when departing, from the ship's bridge. Once the vessel is in position, the bow ramp is lowered and the operator activates the SAB using push buttons on the radio remote control. The bollard is then automatically tilted towards the ship, and a wire loop is hooked onto the bollard. The bollard frame is then tilted automatically back to the vertical position. The mooring cable is tightened from the winch deck, which completes the mooring. The system automatically sustains the tension in the cable.
To release the mooring, the operator releases the brake on the winch deck, and activates a button on the remote control to tilt the upper bollard which releases the wire loop thus making the vessel ready for departure.
TTS Marine says that, apart from a reduction in manning, and thus in costs, the SAB also makes mooring safer and is particularly wellsuited for use in applications such as ferry ports where berthing operations are frequent.
More recently, the Port of Salalah and Mooring Systems Limited's (MSL) licence holder Cavotec Group Holdings (CGH) concluded a deal for four new MoorMaster 600 (MM600) automated mooring units for delivery and installation in June this year. The order follows the successful trial of two MoorMaster 400 (MM400) units at Salalah in August and September last year, the trials being the first use of MSL's technology in a container terminal.
The new MoorMaster 600 (MM600) is an evolution of the MM400 design, and was developed following a technical review of the results from the trials in Oman. During last year's trials, ship movements typically induced by long waves that are present in Salalah during the Khareef season, were substantially reduced. Vessels subjected to long waves and moored conventionally with ropes often 'surge' alongside the piers, motion which adversely affects container transfer rates.
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