EXPANSION JOINTS FOR LNG-/LPG CARRIERS

LPG-/LNG carriers are vessels designed especially for the transportation of Liquefied Petroleum Gas and Liquefied Natural Gas.
There are primarily three different ways of transporting these liquefied gases. They are categorised as follows:

• Fully pressurised
• Fully refrigerated
• Semi-pressurised and Semi-refrigerated

Expansion joints for LNG-/LPG carriers are customised solutions designed to fit the application.
They are available in all designs, all sizes and all materials.

All these ways of transporting the gas, set various demanding requirements for the tanks and the tank system design. Normally the gas onboard the Carriers are stored in cylindrical or spherical steel tanks.

The vessels usually are designed with 4-6 tanks along the centreline of the vessel. A combination of ballast tanks, cofferdams and voids are surrounding the tanks. Inside the tanks pumps are installed. All the cargo pumps are discharged into a common pipe which runs along the deck of the vessel. It branches off to either side of the vessel to the cargo manifolds used for loading or discharging. On the Carrier each gas tank is mounted at an anchoring, which allows the gas tank to move.

In order to absorb these movements an expansion joint is installed at each anchoring. The tank is supported around its circumference by the equatorial ring which is supported by a large circular skirt which takes the weight of the tank down to the hull structure. This skirt allows the tank to expand and contract during cool down and warm-up operations. During cool down or warm-up the tank is able to expand or contract by 2 feet. Because of this expansion and contraction all piping enters into the tank via the top, and is connected to the ships’ lines via flexible bellows, which will result in movements in the tank settlements.

Due to the demanding design parameters and the severe consequences, following a possible failure, the expansion joints used for such applications must also meet very high demands:

• The design phase is rather complex as the design must meet both requirements in the classification Society Rules (e.g. Bureau Veritas) and the International Rules for Gas Carriers (IGC Code)
• Every design has to pass a burst test of 5 times the design pressure, and the fatigue life of the expansion joint must be verified
• The mechanical properties of the materials used for the expansion joint must be verified through comprehensive testing, especially the strength and ductility of the materials must be verified
• Every expansion joint has to pass a pressure test of 2 times the design pressure
• Special requirements regarding welding procedures and consumables are also applicable

All of these tests/documents must be witnessed and approved by a third party surveyor.