CBM system ensures reliability of Shetland ferries

Edited by the Engineeringtalk editorial team Oct 14, 2009

A Maintelligence standalone condition-based maintenance (CBM) system implemented by AV Technology (AVT) is helping to ensure the reliability of Shetland Islands Council's fleet of 12 ferries.

The contract involves the onboard setup and support of the CBM system under the guardianship and tutelage of John Sykes, AVT's senior marine engineer.

Shetland Islands Council runs a cost-effective network of inter-island ferries that makes it quick and easy to travel to and from the isles, with or without a vehicle, providing services between mainland Shetland and the islands.

Services run from 16 terminals serving eight islands and there are 190 sailings per day.

In addition to smaller passenger ferries, roll-on/roll-off vehicle services operate to and from the larger islands of Yell, Unst, Fetlar, Whalsay and Bressay.

Four out of the 12 vessels are Lloyds registered and are, therefore, assessed annually for a range of parameters.

Collectively, the fleet makes more than 70,000 crossings each year, carrying around 750,000 passengers and more than 350,000 vehicles.

Given the tight schedules, the frequency of journeys and, in some cases, the complexity of routes, ship reliability is important for both operational and safety reasons.

However, when assessing maintenance and repair requirements in such environments, it is important to strike the right balance between over-caution and ensuring that premature failure does not occur.

Bringing ships in for unnecessary repairs has double cost implications: the cost of the repair itself, including docking fees, and the cost of providing a replacement vessel for the lifeline services.

To maintain a high level of reliability for its fleet, Shetland Island Council wanted a comprehensive standalone CBM system that, following initial setup and suitable training, could be effectively managed centrally by local marine engineers.

Maintelligence allows engineers to collect, analyse and evaluate data centrally within a short time period.

As the system is managed locally, if anomalies arise, results can be re-verified under working conditions, without the need for external support.

Over a two-week period, AVT's Sykes set up an asset database covering more than 1,500 points across the fleet.

Although the majority of data collected is vibration based, data related to temperature, pressure and rotational speed is also included.

Equipment within the asset register includes propulsion systems, motors, engines, pumps, gearboxes and heating systems.

Collecting and interpreting CBM data on such vessels is said to be a complex task.

The operational conditions vary considerably, with factors such as weather, tides, loading and ballast playing a significant part.

Unlike onshore CBM systems, the vibration patterns at sea vary according to these operational conditions.

The setting of normal peak tolerance levels from measurement points can be meaningless unless other factors are brought into play.

It is, therefore, important for operators to be able to interpret and react to real, meaningful data collected under suitable operating conditions.

The setup programme presented a number of operational and logistical challenges in order to complete it within the allotted time.

The shortest ferry crossing takes only seven minutes, giving a representative measuring window of less than five minutes.

Setting up the asset register for this took eight consecutive crossings.

At the other end of the time scale, the longest crossings last more than two hours.

However, these ferries do not always turn straight around and may even go on to other islands, meaning that Sykes faced long waits.

To overcome this, the strategic placements of vehicles on the mainland, together with a knowledge of the timetables and extended working days, enabled the schedule to be met.

Winston Brown, Shetland Islands Council's engineer superintendent, said: 'During the short time our system has been in place, we have been able to predict abnormal component performance in several cases on board our vessels.

'As a result, we were able to plan and execute corrective action without any disruption to our schedules,' he added.