Numerous attempts have been made to develop unconventional submarine liquefied natural gas (LNG) transfer systems, as opposed to the conven-tional, costly insulated pipes laid on an artificial trestle at LNG jetties connected to loading arms. Total was linked in the past with the Chagal project, based on a proposed cryogenic single-point mooring, with subsea flowlines, flexible hoses, PLM and a cryogenic swivel. More recently, Coflexip Stena Offshore promoted a joint industry project for the development of a cryogenic flexible to replace conventional articulated loading arms.
Underwater view of LNG offload via submarine pipeline.
The main problems traditionally associated with submarine cryogenic pipe are:
- Construction material should be resistant to low temperatures
- Pipe shrinkage must be accommodated
- The insulation system needs to be effective thermally, watertight, and strong
- A fail-safe system is needed to keep the line operational in the event of damage.
Several solutions have been submitted, the latest being developed by ITP InTerPipe in Louveciennes, near Paris, for a partnership involving TotalFinaElf and Gaz de France. This differs from previous versions in that it offers the following features:
- Double-wall pipe technology already in use on subsea high-pressure/high-temperature projects in the North Sea (Total Dunbar, Shell ETAP) and West Africa (Elf Tchibeli). This consists of two co-axial pipes - one inner pipe inserted within an outer pipe, both pipes being linked at their ends. The sealed annular space between them allows application of a high performance insulation material, which significantly reduces the heat transfer between the inner pipe, containing the LNG, and the outer pipe, which is in contact with the seawater
- The inner pipe (16-40-in. diameter) is made from a nickel alloy (36% Ni), named INVAR, which is already used widely for LNG carrier membranes. Due to its low expansion co-efficient, the thermal stresses and strains arising from the temperature difference are kept low (approximately 10-times less than for stainless steel material). The INVAR also provides a high level of mechanical strength and good welding properties
- Izoflex, the insulation material (40 mm thickness), has been developed and patented by ITP and is in use on various offshore fields insulating hot effluent pipelines, including pipes where a significant temperature difference exists between the transported multiphase product and the outside environment. Izoflex's insulation properties permits design of a high thermal performance, double-wall pipeline, and also greatly reduces the line's overall diameter. The material's thickness is four times less than what would be required to achieve equivalent thermal performance using conventional polyisocyanate resin insulation. That in turn brings cost savings.
Mechanical tests in a cryogenic environment have confirmed the material's suitability for LNG transport, and being mechanically resistant, Izoflex supports the weight of the inner pipe when filled with LNG much more effectively than would be possible using conventional materials, ITP claims. The good mechanical behavior at any temperature, combined with limited shrinkage effect at low temperatures and a good anular filling, ensure concentricity of the inner and outer pipes. Due to the fabrication procedure and insulation material installation method, the thermal-bridging effect along the overall pipeline is kept to a minimum, and thermal performance overall is therefore optimized - The strong steel outer pipe is designed to give added protection to the insulated cryogenic piping system. The Izoflex panels are positioned safely under two layers of pipe, with no risk of moisture penetration damaging the insulation. Hence, maintenance/repair costs should be minimized
- The outer pipe is made of conventional carbon steel, with no need for a specific cryogenic alloy, as the outer pipe is at ambient temperature due to the high thermal performance of the insulation material. The outer pipe is protected by conventional anti-corrosion coatings and cathodic protection devices. Concrete mattresses can provide on-bottom stability. The pipe could also be buried.
The pipe system has been designed to withstand loads induced by the installation process (string weight and associated tension, hydrostatic pressures, temperature variations), and loads in operation (thermal, internal/external pressures, differential pressures, seabed stability, fatigue) during the pipe's lifespan. These criteria have been reviewed in accordance with the main codes and standards for refrigeration and subsea pipelines.
The inner pipe and the insulation material limit the impact of the temperature variations and the outer pipe ensures the system's integrity. The continuous annulus throughout the pipe allows placement of additional control devices. Pressure and temperature control gauges and secondary annular protection provide increased system safety.
Fabrication process
A process has been devised for fabrication of the cryogenic double-wall line, the sequence being driven by the installation procedure, based on the towing method. There are several fabrication steps, the first being to pre-assemble insulated units, 6 or 12-m long. These are welded together to produce 250-m strings or longer. The strings are then welded onshore before being pulled to the appropriate location. A double-wall insulated riser provides the connection between the subsea pipe and the offloading facility at one end. The other end will be connected to the storage tanks' manifold. Conventional tugboats (using bottom tow) could be used to pull up to 5,000-m long pipe sections to their position on the seabed.
According to ITP, risk of water ingress, or failure of external pipe, can be reduced to an acceptable level, assuming proper levels of external protection, such as trenching and concrete.
Full-scale thermal trials were conducted during April 2001 at Gaz de France's LNG facility in Nantes, to study the thermal behavior of the ITP system in LNG conditions. The tests involved a full-scale, double-wall section (8-m long, 32-in. outside diameter), insulated with two layers of Izoflex and tied to a 500 cu m LNG tank for LNG circulation. The inner pipe was filled with LNG - measurements were taken of gas flowing out through vaporization of the methane and temperature was measured with an optical fiber and conventional temperature probes.
According to Gaz de France, the results were a little better than anticipated, and proved that no modifications would be necessary to the design. The pipe section achieved the requested level of insulation properties with only 40 mm of insulation. On the outer pipe, the non-freezing condition was achieved with a 10° C ambient temperature.
TotalFinaElf has estimated that substituting a conventional trestle, as used on existing LNG transfer facilities, with an ITP cryogenic subsea pipeline, could bring savings of $20-40 million. The maximum length would probably be limited to 8 km, as the diameter of the INVAR material cannot be extended over much longer distances. Typically, this would involve two lines of 32- or 36-in. diameter, 5-km long, and with throughput of 5,000 cu m/hr.
