Exclusive:Ferguson Marine boss on 'the unknown' in Glen Sannox ferry sea trials 19 days from deadline

However, in an exclusive interview with The Scotsman, interim chief executive John Petticrew said "I can't be any more certain than I am certain" the vessel will be finished by its new delivery deadline of October 14 - just 19 days away.

Glen Sannox is then due to undergo six-and-a-half weeks of “delivery to operations” trials with CalMac ahead of its expected start of passenger service on the operator’s busiest route, to Brodick in Arran, in December.

But it will sail from Troon in South Ayrshire because the upgrading of Ardrossan harbour in North Ayrshire to accommodate the ferry has still to be agreed.

The vessel should have been completed at the Port Glasgow yard six years ago and will cost around four times its original price, or some £200 million, mainly due to construction problems. It will be CalMac’s second largest ferry, after Loch Seaforth on the Ullapool-Stornoway route, with space for more than 850 passengers and 127 cars.

Mr Petticrew said last weekend’s three-day trials of the ferry’s novel liquefied natural gas (LNG) fuel, which it will use along with traditional “marine gas oil” [a dense form of diesel], had been “very successful”.

LNG is primarily methane, used in central heating boilers and gas cookers.

But the CalMac vessel will now be tested to its limits, in its last major hurdle before being delivered, in five days of “acceptance” sea trials by its Scottish Government-owned owners, Caledonian Maritime Assets Limited (Cmal), starting on Monday.

Cmal chief executive Kevin Hobbs has described this as the “acid test”, which will include a 12-hour endurance trial at full power.

Mr Petticrew said: “It really is a stage of the unknown. Hopefully they’ll go as well as the LNG trials went. We will then come back, get sign off by the MCA [regulator UK Maritime and Coastguard Agency], sign off by [certification firm] Lloyds, and the customer [Cmal] is happy with the vessel.

“The vessel is going to be monitored very, very strictly when it’s out, because, by definition, it’s an acceptance trial.”

Andrew Milligan, the yard’s head of engineering, said of the trials: “I don’t expect any problems to come from LNG, but it is the first in class and things do happen. If a problem arises, we just have to shut it down, and that’s what we have been doing.”

The trials will also include the ship’s galley being used for the first time to ensure it works - and to provide catering for those taking part in the tests.

After the trials are completed on Friday, October 4, the yard will work through snagging lists, known as “punch lists”, such as minor finishing work in areas like cabins, walkways and engine rooms. Paintwork will also be touched up and the ferry deep cleaned.

Mr Petticrew said: “There might be something extra [to do], but we do know what we need to finish. They [Cmal] have waited a long time on this vessel. We don’t want to hand something over that’s not nice and polished.”

The latest two-week delay to the ferry’s completion followed the LNG tests being put back because it took longer than expected to cool pipes using liquefied nitrogen to transfer LNG on board from a tanker at -165C.

Mr Milligan said there had also been problems recruiting the necessary specialists to install the complex pipework, which is supercooled to convert the gas into a liquid that reduces its volume by 600 times.

The tests, which were a UK first, took place at the Inchgreen dock in nearby Greenock where the ferry is being completed because of lack of space at the yard, where sister vessel Glen Rosa is being built.

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Mr Milligan revealed that when LNG is loaded aboard the vessel, its auxiliary engines will have to keep running, although not necessarily round the clock, to prevent the fuel from over-pressurising. Otherwise, some of it would have to be vented through a pipe at the top of a mast.

Mr Petticrew said of the fuelling: “We were super, super cautious. We were aware it was brand new. Even just looking at it with the frozen pipes, it was actually quite scary, but it is a fascinating process.”

Loading the LNG, known as “bunkering” in a reference to ships’ past use of coal, involves creating a 20m (66ft) exclusion zone around the fuel tankers - which come from a terminal 450 miles away in Kent - and a round-the-clock security guard.

CalMac has said the distance transported was “not ideal”, but necessary to promote LNG use in Scotland. The body said it would take about two-and-half hours to refuel the ferry with LNG compared to about 45 minutes with MGO.

The dual fuel system was chosen when the ferry was ordered nine years ago as new, green technology at the time, which has lower emissions than MGO.

CalMac said by comparison, LNG eliminated sulphur oxides, cut nitrous oxide by 85 per cent, particulates by 90 per cent and carbon dioxide by 25 per cent. 

LNG is also being fitted to Glen Rosa, which is due to be completed in September next year to also run on the main Arran route.

But four CalMac ferries being built in Turkey will run on MGO only, although they could be retrofitted in future for another fuel.

CalMac said it intended to “maximise the use of LNG” by Glen Sannox, and the fuel’s lower environmental impact rather than cost was the key factor.

Its spokesperson said: “Price will not influence fuel use. The main driver for LNG use is the reduced carbon dioxide emissions from the consumption of LNG compared to MGO.”

The ferry will be able to switch between MGO and LNG seamlessly within seconds.