Ocean Kinetics - The Engineering Experts

Four snapped chains caused collapse

FIVE days of disruption on the Yell ferry a month ago was caused by four steel chains simultaneously snapping on the terminal linkspan at Toft, council engineers reported on Thursday.

The linkspan collapsed on 28 April as the stand in ferry Fivla berthed at Toft in a low tide, trapping the ferry beneath its 78 tonne frame with 12 cars and 40 passengers on board.

The failure threw Shetland’s busiest ferry service into turmoil with ferries diverted to Vidlin, an extra hour’s journey away.

Reporting to Shetland Islands Council’s inter island ferries board on Thursday, transport manager Ken Duerden said that despite speculation to the contrary, the linkspan had been operating within its design limits.

Quayside experts had believed that the device had been stretched beyond its range by handling a small ferry when the tide was low, but Mr Duerden said this was not the case.

SIC ferries engineering superintendent Winston Brown explained that four steel “preventer” chains supporting the linkspan had snapped simultaneously. An investigation is ongoing to find out why this happened.

The incident caused Mr Brown’s department a huge headache because the islands do not have a crane large enough to lift such a huge piece of equipment.

“The linkspan weighed 78 tonnes and there wasn’t a crane available in Shetland with that lift capacity and two cranes couldn’t be used because there wasn’t enough room.

“We had to find a way to support the seaward side of the linkspan so we utilised the Dagalien ferry, putting the bow under the linkspan. We secured it with chains and used its buoyancy to lift the linkspan back onto its bearings,” Mr Brown explained.

He said that to hire in special lifting equipment would have added an extra 10 days to the job. “We didn’t think that was acceptable and we needed a more rapid solution than that,” he said.

The collapse caused chaos as the service struggled to inform passengers of emergency timetables and efforts were made to tie in public transport with the temporary service put in place.

A folk festival concert on Yell and a wedding on Whalsay added to the pressure as extra vessels were drafted in to cope.

Mr Duerden said the linkspans at Toft, Ulsta on Yell and Hamars Ness on Fetlar had since been modified to ensure there was no repeat of the incident.

He said the full cost of the repairs, the modifications and the extra service that had to be laid on had not been fully calculated, but it was “significant”.

The cause of the chains snapping is still subject to an investigation involving the council, its insurers, civil engineers and the linkspan’s designers.

North Isles councillor Laura Baisley praised the ferry department for tackling the problem in such an imaginative way.

“I think they did a remarkable job to deal with an unusual situation and used their engineering ingenuity to get the job done quickly,” she said.

Mr Duerden said that the council had learned several lessons from the incident about setting up an emergency timetable which could dovetail in with existing public transport, and how to effectively communicate with the travelling public.

He added that alterations were being made to the Holmsgarth ferry terminal in Lerwick where the NorthLink ferries dock to allow the Yell ferries Daggri and Dagalien to use the linkspan.

One of the major concerns during the five day incident was that farmed and caught fish would not reach the southbound ferry in time to get to market while still fresh.

“Last week we discovered that we can’t discharge large vehicles at the Lerwick linkspan so we are going to look at configurations so that we can in future.”