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It will all come out in the wash

Ports and harbours Developments

Waves produced by the new generation of fast ferries can affect swimmers and small vessels hundreds of miles away.

Engineers studying the effects of unusual waves from new high speed ferries have developed a computerised tool for ship's captains giving them guidance on how to handle their vessels and avoid creating dangerous waves.

Studies at Queen's University Belfast (QUB) have been carried out on the new generation of fast ferries introduced in the mid1990s. They can travel at 40 knots, almost twice the speed of conventional ferries and produce unusual, small but fast, high-energy waves which could pose a danger to other vessels and people using beaches because they appear without warning.

'Fast ships produce very long period waves of low amplitude close to the ship. They are not particularly big, perhaps only 0.4m high, but they store a lot of energy and travel hundreds of kilometres. We can easily detect waves in Belfast which originate in Scotland, ' says QUB professor of coastal engineering Trevor Whittaker.

'The problem is these waves are unexpected: as they are coming into shore you won't see them, but as they arrive in shallower water they start to shoal, building up very quickly and breaking. By the time you see them, it is too late. It is especially dangerous on calm days when they are least expected.'

Approaching shallow inshore waters or beaches, the momentum of the waves in the reduced depth causes them suddenly to surge in height, affecting unsuspecting craft and beach users.

The urgent need for study came after an inquest linked the phenomenon to the death of a man who fell from a boat off the Essex coast in 1999 when it was hit in calm conditions by fast ferry wash waves from the Stena HSS Discovery. The waves suddenly surged to 4m in height after travelling over a sandbank near the small craft.

Extensive modelling to replicate the effect was undertaken.

Whittaker, a modelling enthusiast, created radio-operated scale replicas of five vessels in a massive 50m by 17m tank with probes to measure wave patterns.

'We were able to map the wash pattern against vessel speed and depth of water and could then simulate conditions in Belfast Lough. But we have extensive field measurements of fast ship wave wash data for Belfast Lough, so were able to calibrate the mathematical model we had developed to the field model, ' he says.

The unusual waves occur when the ship speed becomes 'super critical' - when its Froude number (proportional to the velocity divided by the square root of the water depth) exceeds 1, giving a 'delta' wake formation behind the ship. Subcritical flow with slower speed means the waves are similar to conventional craft and pose no hazard, but slowing down defeats the purpose of high speed vessels.

Whittaker says the Froude number value of 1 is analogous to Mach 1 for supersonic aircraft.

'Because of the sonic boom, Concorde could not travel above Mach 1 near land but only out on the open sea. In the same way, fast ferries can only reach their cruising speeds on the open sea, ' he says.

The studies have enabled development of computer software to give masters information on wave effects generated by different speeds and depths and to help route selection to minimise undesirable wave action.

Whittaker believes that ship's captains will soon be required to have 'vessel-specific' training using similar software before being allowed to command particular types of high speed ship, in the same way that airline pilots are trained for operating particular plane models.

High speed craft operators will also consider relocating terminals away from inshore berths, to allow travel with less inshore speed restrictions, he adds. More work is needed on the effects of acceleration and deceleration and changing course, ' adds Whittaker.

The fast ferry study group at QUB has been working on the ship wash impact management (SWIM) programme since 2000 with consultants Kirk McClure Morton, Posford Haskoning and Marinetech, along with Southampton and Portsmouth universities.

Moving story

Port operations can now be modelled from the moment the vessel enters the shipping channel until the cargo leaves for its final destination.

A package which models ship and shore operations at roll-on roll-off ferry terminals has been developed by UK consultant Posford Haskoning.

Ro-Ro Terminal Simulation (RTS) is the third in the Posport suite of packages, following Container Terminal Simulation (CTS) and the Marine Traffic Risk Assessment Model (MARTRAM).

The system allows desktop modelling of all port activities, both on and offshore, and is suitable for use both in planning new facilities and optimising existing port infrastructure.

'A terminal is dealt with as a whole, to ensure that the complex interaction between all the activities there are modelled precisely, ' says Posford Haskoning ports division director Henry Rowe.

Development stemmed from the need to develop quick but accurate models for clients.

Basic models can be produced in under an hour using graphical output, a key feature of the system.

'A lot of work has gone into creating a userfriendly interface to eliminate the need for specialist computer skills, ' adds Rowe.

The model is detailed down individual freight movements, and allows port-specific characteristics such as arrival and dwell times to be modelled, as well as the efficiency of plant and loading operations.

Schedules for transport and shipping serving a port can be included, or the model can suggest optimum alternatives.

CTS allows similarly detailed models of container terminals to be developed. MARTRAM is effectively a traffic modelling system for shipping, enabling risk analysis of collisions and grounding for given port and vessel conditions. In analysing the risk, optimum shipping schedules can be developed.

Combining the three packages enables a complete interactive model to be developed, covering port traffic, shipping and container movement.

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