To transport liquefied natural gas from Yamal LNG, which is located in the Arctic and constitutes one of the world’s biggest LNG projects, we and our partners have designed a new type of ship: an ice-class LNG carrier. This innovative solution allows large shipments of LNG to be transported efficiently and at a steady pace throughout the year, without the assistance of icebreakers. The ship, which is 300 meters long and has a capacity of 172,600 cubic meters, can sail in temperatures as low as -52°C and through ice as thick as 2.1 meters. Fifteen ice-class LNG carriers were commissioned between December 2016 and December 2019. In this article, we delve into this technological microcosm.
LNG shipping: a strengthened hull
Under the Russian Maritime Register of Shipping, the ice-class LNG carried is graded Arc7, which means that it can sail in ice up to 1.7 meters thick, making it the biggest commercial vessel with this certification. To be able to sail in such conditions, the ship has a strengthened hull and the engine room is protected by a double hull. High-strength steel with a yield strength of 500 megapascals (MPa) has been used to reduce the ship’s weight — 125,000 tons when loaded — and ensure a full load draught of 12 meters.
A hydrodynamic bow
By opting for a scaled-down icebreaker bow, instead of a bulbous bow, the ship can sail more easily in open water and in light ice (up to 1.5 meters thick). In thicker ice, this double acting ship turns 180° and proceeds astern. The hull’s heavy icebreaker profile around the stern allows the ship to sail through 2.1-meter-thick ice. Numerous tank tests have confirmed the ship’s performance and maneuverability on the open sea and in different sea ice conditions.
LNG storage tanks: an innovative containment system
The ship’s tank containment system is equipped with NO96 GW membrane technology developed by Gaz Transport & Technigaz (GTT). Reinforced plywood boxes filled with glass wool provide insulation. Glass wool performs better over time than perlite, which is normally used, with regard to the vibrations associated with operating in ice. The ship is not equipped with a bilge keel, a first for an LNG carrier. This decision was approved after studying the sloshing of LNG in tanks subject to harsh sea conditions.
A total of six Wärtsilä 50 DF hybrid diesel-electric engines deliver 45 MW of thrust, the most ever for an LNG carrier. The ship can reach a speed of 19.5 knots in open water and 5 knots when proceeding astern in 1.5-meter-thick ice. The LNG shipment is used to power the ship. This solution lowers the vessel’s CO2 emissions by up to 30% on this Arctic route. The system that injects the LNG used as fuel is designed to provide high power at low temperatures, as well as to ensure that the natural boil-off rate is as low as possible. It comprises two low-duty compressors and a large-capacity LNG vaporizer system.
An effective Azipod® propulsion system
Propulsion is provided by three ABB Azipod® units. These azimuth thrusters make the ship easier to maneuver in ice. Each Azipod®, contained in a pod under the hull, comprises an electric 15 MW engine that is connected to a propeller; the entire unit can rotate 360° to change the ship’s direction. The Azipod® units crush the ice that is broken by the bow and stern and push it to either side of the ship. Their vorticity causes turbulence that greatly reduces friction between the carrier and the ice. This method is far more effective than the ramming-based approach used by conventional icebreakers.
An engine room adapted to the cold
The engine room is insulated by a 10-centimeter layer of fiberglass. Its ventilation system is not connected to the engine air supply system. The minimum ventilated air temperature is 5°C when the outside temperature is -52°C. To limit heat loss, almost one-third of this air is recycled. The engine cooling system includes a cold-water loop that is fed by a sea chest that acts as an ice/water separator. Some of the warm water produced by the cooling system is directed to the chest to prevent it from getting blocked by ice.
very high safety standards
The engine room is divided into two separate areas, partitioned by a firewall. Both areas house the systems that are vital for the ship’s propulsion. This design complies with class notation AVM-IPS, as certified by Bureau Veritas, and guarantees that the engines remain available in an emergency. In the event of a fire or flooding in one of the two rooms, the carrier can operate on half of its power with two Azipod® units. Tank tests have shown that the ship can continue to sail safely with the assistance of an icebreaker and reach an open-water harbor where it can undergo repairs. During the first months of operations, an LNG icebreaker in degraded mode was able to independently and safely reach a port with only two operational Azipods®.
A winterized deck
The equipment on the ship’s deck is designed and protected to operate at very low temperatures — down to -52°C. This winterization, mostly passive, is based on the use of special materials, extensive thermal insulation and electric motors, which are more reliable at very low temperatures. Thermal oil heaters keep the deck’s strategic areas free from ice and the lookout stations warm. The mooring points are covered to prevent snow and ice from building up and to protect the crew from the cold while maneuvering.
The LNG carrier has two pilothouses. The first conventionally faces the bow and comprises, on the port and starboard sides, two external winterized wings to monitor maneuvers. The second is located at the rear of the deck and is used when the ship proceeds astern. The pilothouses are connected by a walkway, which gives the double pilothouse a T-shaped layout. Each is equipped with all of the technology needed to sail in ice and in extreme remote areas: an ice radar, Xenon light, GPS and GLONASS positioning systems, GPS compass, Iridium satellite system and more.
CONCLUSIVE TESTS IN REAL ICE CONDITIONS
An extensive testing program was designed in advance to identify the most representative ice conditions and meet testing requirements, with assistance from a nuclear-powered icebreaker. Intended to check the performance of the first ice-class LNG carrier before delivery, the performance testing campaign in real ice conditions was a challenge for the shipyard, the designers and the ship owners. The vessel met and exceeded the guaranteed performance regarding speed in forward and astern operation through 1.5-meter-thick ice, turning circle in ice and penetrating ice ridges up to 15 meters high.
EARLY LNG PRODUCTION START-UP
Yamal LNG began production in December 2017 with the commissioning of Train 1. Trains 2 and 3 were started up in late 2018, nearly a year ahead of schedule. Only 10 of the 15 planned ice-class LNG carriers had been delivered at the time and the transshipment point at the Zeebrugge LNG terminal in Belgium was still being built. During the winter of 2018-2019, Total therefore transferred the LNG from these LNG carriers to conventional LNG carriers off the coast of Norway. This shortened the journeys of the Arc7 vessels and added flexibility to export the increased production from the port of Sabetta.
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