Ann Rigmor Nerheim: All about the pressure

Since doing her PhD on natural gas hydrates she’d been working in Rogaland Research and Statoil in Stavanger, before she was nicely ensconced in Naturgass Møre AS in Ålesund in 2004, with only a passing eye turned to the nearby sea.

But her main working area centred on natural gas transport so around three years ago she was approached by Rolls-Royce’s marine arm: the company had realised it needed someone with her background if it was going to adapt successfully to the demands of the new LNG fuelled ships. She jumped at the chance: “I thought it would be an interesting challenge to take my understanding from land-based systems and apply it to marine,” she says.

What she hadn’t realised at the time was that the ‘challenge’ waiting for her had just erupted into a fully blown problem. Rolls-Royce was getting performance results back from their LNG-fuel a which were not entirely in line with what had been expected.

However if she’d had any qualms about leaving the well-known environment of land-based applications, they were blown away by the intensity of the work: rather than making her think twice about her decision she says it was “a fantastic experience” explaining that she immediately felt she could contribute something valuable: “It gave me the opportunity to see a project through from the initial problem to getting some answers.”

Sherlock Holmes style, she and her team worked their way through the ‘variable’ factors that could be causing trouble for the LNG fuel system: “First of all we had the data coming back in from various LNG fuelled ships with different designs, but at first we just couldn’t figure out why one system should be better or worse than another, so we began to look into it using CFD analysis.”

At the same time they started to investigate the only other variable, the LNG itself.

Some of the pieces started to fall into place “when it was realised that these problems started with colder bunkering temperatures”. She explains: “The early LNG deliveries came in by truck; these had the LNG onboard for a while so it was warmer than the later bunkers which came from terminals much closer to the production plant.”

But the problem “only occurred when ships were out beyond the coastline” and not in the calmer fjords or harbours, making it frustrating to pin down.

However the team started to suspect that movement itself was key: “In land-based systems the tanks just sit there with the gas on top, but when you take them out to sea sloshing can cause violent, fast movements on the surface of the fuel, and if it’s really very cold the liquid can start to behave like an active condenser,” she explains. “That has the potential of bringing down the pressure in the tanks till suddenly you don’t have enough gas running to the engines.”

Another part of the jigsaw fell into place when the CFD investigation into the onboard tanks started to throw up a few anomalies.

“There are often bulkheads inside a big C-type LNG tank that help it cope with the forces of sloshing impact in heavy seas,” she says “and these may have been exciting the surface a little.” Further, it seemed that the fledgling state of the industry had played a part and not all innovation was hitting its intended target: “A few manufacturers had combined the usual internal bulkheads with ‘anti-sloshing’ structures which were supposed to reduce any rough movement on the liquid’s surface,” she explains. “Unfortunately, given certain positions some of these introductions were in fact enhancing the mixing of gas and liquid – actually far worse than nothing at all.”

This pretty much completed the picture and underlined not just the need to get the information back to the tank manufacturers but also to those onboard. Although there are possible changes to the installation (a bigger pressure build-up unit or a cryogenic pump) she believes it’s not all down to the hardware: “The difficulties associated with cold LNG can be avoided by thinking about where and how to bunker. It means a different kind of planning to what the crew may be used to, but LNG operations demand a different kind of thinking.”

Plus, automation is a path to be trod carefully: “You could make a fully automated system, but if something happens nobody would know what the required action would be. Errors are almost always man-made although this can be anything from the programmer to the operator.”

“However you choose to design a system, whether you go down a fully automatic or semiautomatic path, you need to make sure the crew is confident and that they know how the system behaves, not just when it’s going well but when it starts to go wrong.”

“But that’s the same for any fuel system, if you want to add safety you have to make sure that the crew really understand it all.”