Blizzards rage over the Norwegian town of Hammerfest, located around 600 kilometres north of the Arctic Circle. It is hard to think of a more inhospitable setting for a high-tech industrial complex – let alone one of the largest natural gas liquefaction plants in the world. On Melkøya, a small barren island off the coast of Hammerfest, engineering experts from Linde successfully delivered a pioneering natural gas project. Here, natural gas sourced from below the Barents seabed is pre-purified, cooled to minus 160 degrees Celsius – which turns it into liquefied natural gas (LNG) – and loaded onto tankers. Most LNG plants are located in places with hot climates such as Algeria or the Arabian Peninsula, and none of them are exposed to six long months of snow and ice each year. Since no-one had ever designed a plant on this scale for this kind of climate before, the challenge facing the Linde engineers was immense. They began by simulating the local snow and storm conditions and even constructed a test rig of steel girders, pipes and valves on the island of Melkøya. The engineers used a webcam to see where the snow accumulated and whether it affected the rig.
From Norway to the four corners of the globe: Cryogenic LNG from Hammerfest travels around the world in the spherical tanks of the Arctic Princess.
Engineering a plant to withstand high winds and freezing temperatures
Engineers from Linde and Norwegian energy group Statoil first started discussing plans for a terminal on Melkøya back in the late 1990s. Everyone involved knew it would be a very unusual undertaking involving a number of challenges. Because of the harsh climate, Linde opted against building the main plant components locally. Instead, it distributed manufacturing across various sites in Europe, and had the components shipped to Hammerfest from the nearest ports. The coil-wound heat exchangers and the coldbox modules were manufactured in Germany. Belgium was the shipping point for the coldbox and the manufacturing centre for various other plant components and pipe elements. The slug catcher was manufactured in the Netherlands. A plant in Italy fulfilled the order for the gas turbines and compressors. In Spain, a special barge was built to ensure the safe transport of the process unit to Hammerfest. This modular approach proved to be the best option as it builds on the proven engineering expertise available at each of the production locations. It also saved on labour costs and resolved space issues during on-site assembly. The project was a planning and logistical masterpiece. To name but one example: the process unit could only be safely shipped by barge during the summer months because of choppy seas and high waves. Minutely detailed and far-sighted planning was required here, because otherwise the small window of opportunity would be missed and the delivery would be delayed by a whole year. The success of the project is a credit to the Linde engineers. The plant went on stream at the start of 2008 and has produced over four million tonnes of LNG each year ever since – enough to supply three million private households for a year.
Heat exchanger at the heart of it all
Every natural gas field is different – the pressure, composition and temperature of the gas deposits can vary significantly. The process chain, above all cooling and liquefaction, therefore has to be tailored exactly to these variables. On Melkøya, liquefaction takes place in a heat exchanger installed in a coldbox around 62 metres high and as large as a narrow office block. It functions along the same lines as a fridge. A compressed coolant condenses and expands as it is fed past natural gas in huge heat exchangers. This expansion process gradually extracts heat from the natural gas, with the temperature falling from around 40 degrees Celsius to minus 163 degrees, at which point it liquefies. In this state, the gas shrinks to 1/600th of its original volume – making it more suitable for transport by sea. The bundle of tubes that makes up the coil-wound heat exchanger is the heart of the coldbox. These boxes comprise numerous heat exchangers, separation columns and separators together with connecting pipes and instruments. Linde is the only manufacturer that produces both types of cryogenic heat exchanger typically deployed in industrial-scale LNG plants – coil-wound and plate-fin models.
The coldbox for the LNG plant in Hammerfest, Norway, was assembled at the Belgian port of Antwerp.
Harsh conditions for man and machine
Although Linde already had extensive experience with LNG plants when it took on the Hammerfest project, existing references were for smaller liquefaction capacities located in warmer climates. It was a huge step for Linde engineers, therefore, to ramp up to annual capacities of four million tonnes in Arctic conditions. The Melkøya project changed the global market for world-scale LNG plants. Ten years ago, two technologies dominated the international market for plants capable of producing millions of tonnes of LNG per year. Linde engineers raised the game by developing their own highly efficient liquefaction process specially for the Melkøya project, thus sending out a clear signal to the industry. Linde’s engineering experts have thus proven their expertise in delivering modular process plants and key components along with their ability to cover the entire spectrum from small terminals to world-scale plants operating under the harshest conditions.
At the heart of the coldbox – which was manufactured in Schalchen, Germany – lies a bundle of tubes that makes up the coil-wound heat exchanger.