01 Dec 2004
A typical ferry linkspan, built by Ravestein in the Netherlands for Port of Dunkerque Linkspans at Vlissingen in the Netherlands

A typical ferry linkspan, built by Ravestein in the Netherlands for Port of Dunkerque Linkspans at Vlissingen in the Netherlands

A new design code for the design of ro-ro ramps, linkspans and walkways - British Standard 6349 Part 8 - is due to be published early next year, enabling ro-ro infrastructure to be planned, designed and built to an acceptable common set of rules, as explains.

Linkspans and walkways are hybrid and generally complex items of port equipment, which have been subject to a number of serious incidents, in particular the collapse of the Ramsgate walkway in 1994, resulting in a number of deaths.

The life of such structures is related more closely to that of a ship, around 20 years, than the notional 120 year life of highway bridges covered by structural standards; yet the operating conditions are far more onerous, the support system having to accommodate the tidal range, wave effects and even ship impacts, which has led to industry calls for improvements in the area of structural safety, as well as control, operation and maintenance.

As Dr Richard Marks of Posford Haskoning, part of Royal Haskoning, explained in a paper he gave at a recent conference on roro shipping, there are a variety of problems which fit between ship and linkspans, caused by the fact that there are a variety of locations of ship openings (bow, stern and side and increased elevations), a variety of sizes and shapes of openings (width and height and opening mechanism arrangement), 'female' and 'male' ships and finger flaps or 'blunt end' ship ramps.

Then there are issues such as ship door and ramp clearances to shore structures, limiting gradients and transitions, standardisation of ship ramp shape, support shoes and finger flaps, standardisation of linkspan interface shape, and ship motion on berth. Where there is multi-deck vehicle access similar problems to single deck linkspans occur, but ship door and ship ramp operating mechanisms can also clash with the upper deck on shore.

Other problems with multi-deck vehicle access include standardisation/variation of the relationship between lower (garage) and upper deck; large adjustments that are sometime required to landing position of upper deck finger flaps, and the need for retraction of finger flaps or upper deck to give safe clearance at berthing and door opening. The increased effect of roll, the need for adjustment to match height/headroom between decks, ship motion and clearances between opening and finger flaps are also issues that need to be addressed, Marks told the conference.

A consultation draft of BS6349 Part 8 was produced early this year and the final standard may still be issued before year-end.

Companies that worked on the draft included Posford Haskoning Limited, Scott Wilson, BMT Fraenkel, Lloyd's Register of Shipping and Portsmouth City Council.

The need for the new standard is highlighted by the fact that until recently there have been no comprehensive, standalone rules specifically for linkspans, other than those produced by Lloyd's Register in 1998, the standard being LR's reaction to its own realization that there were no published standards either in Europe or worldwide for the design and construction of these vital links between ship and shore.

More recently, Transport Canada initiated a programme of work on ferry linkspans, and produced its own standard - CSA S826 - which recognised the fact that that ferry operators must adapt to new realities, including longer, heavier trucks and increasing numbers of vehicle movements.

MACHINERY, NOT STRUCTURE, USUALLY THE PROBLEM The new British Standard has been drafted by a panel set up by the Institute of Civil Engineers (ICE) and the British Standards Institute (BSI). As Steve Osborn, a consultant at Posford Haskoning who had input into the new Code explains, the draft code has been well received so far, although it has been criticised because it is essentially about the design of structures, and not the machinery associated with them.

"People are rightly concerned that machinery should be covered as it's the machinery that is usually the cause of problems and not the structures themselves. However, we believe that we have addressed many machinery issues, " says Osborn. "We think that this is the first draft structural standard that requires designers to start with a fundamental risk assessment - very much a 'machinery' type approach. We also require designers to contribute to the operations and maintenance manuals for the facility. Both these requirements are in the second and third clauses of General Requirements, which gives these issues a very high priority.

Asked what is the most controversial change in this code compared to previous design standards Osborn says: "Perhaps it is the problem that has been taxing people for some time now - what happens if one or more of the hydraulic cylinders or winches in the bridge lifting machinery should fail?"

"A ro-ro linkspan is one of the few structures where people and traffic are using the bridge while it is actually being supported by machinery. In the past it was considered acceptable to rely on a normal structural design approach without any special enhancements. But now there is a widespread view that there should be an alternative load path in case a cylinder or other part of the equipment fails in effect to 'catch' the structure."

"I warned the committee that Royal Haskoning have a direct interest in this issue because of our involvement in the RoSafe independent braking system with Bosch Rexroth (see below), but, nevertheless, the committee decided that it was right to include a requirement for a secondary load path in the code and put it forward for consultation. Of course, systems other than RoSafe will be developed. Already the solution of using double lifting cylinders is in use, though the additional moving parts can lead to other problems.

Industry must debate this issue and it is extremely important that they reach a consensus on these proposals, " Osborn says.

Asked why highway loadings were also included in the draft code when these are already covered elsewhere, Osborn explains that although another BS - BS 5400 Part 2 - covers highway loadings, it was felt that some modifications are needed. "We were concerned that with ro-ro spans you can find a large number of lorries on a ramp at one time, so we have devised the HRo highway loading system especially for ro-ro linkspans."

Changes have also been made to the requirements for linkspan geometry and Osborn and his colleagues are recommending a significant overhaul of the geometric standards for road surface transitions and other features in ISO 6912:1983 (BS MA 97:1984):

Specification for roll on/roll off ship to shore connection: interface between terminals and ships with straight stern/bow ramps.

"Although we have set out proposals in the draft standard, we will be sending out questionnaires to people in the port industry to gather feedback on these proposals. This is an additional initiative to the consultation process that the BSI will coordinate, " concludes Osborn.

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