For more than 30 years LNG ship to ship loading has been addressed by several Gas operators. One of the main technical challenges that have been identified has been how to extend and combine the proven technologies of static small bore diameter cryogenic piping and offshore ship to ship transfer of oil to large diameter offshore LNG transfer. In the last years the market has pushed for flexible piping suitable for LNG offshore loading systems, and the industry has responded with different technical solutions based on very different design criteria. In this paper the basic requirements for a LNG offshore loading system are presented. As a minimum the requirement of the European code prEN 1474 - II is quoted. Design Classification of LNG Flexible Pipes Today there are two completely different flexible pipes designs existing on the market, both are proven in their applicationsComposite hosesFlexible metal pipes based on corrugated stainless steel pipes (sometimes called " bellows?? although not necessarily based on the bellows technology, which is in principle limited in length) The composite hose is a proven technology for a wide range of applications, amongst others the offshore loading of all kind of ambient temperature liquids. In the LNG business they are available as emergency unloading hoses. The flexible metal pipe has been used in smaller diameters for more than 30 years for all kind of cryogenic applications, transfer lines for Liquid Nitrogen, Helium and even Hydrogen and Oxygen. So for both design options, the LNG ship to ship loading is a new application of a well known technology
Many oil companies presently work on plans for production of Liquefied Natural Gas on a Floating LNG Production and Offloading (FLPO) vessel. All these schemes require suitable systems for offloading of the LNG to a transport tanker. Safety and regularity are important issues in design of offshore LNG production and loading systems. In addition to the offloading operation the tanker approach, connect, disconnect and departure phases include critical operations which are weather dependant and may impose restrictions on operability and regularity. The OCL system is a stern to bow, crane and flexible pipe solution that is developed, based on extensive experience from tandem offloading operations of crude in the North Sea in harsh weather conditions. The system is well suited for LNG transfer where high regularity is required and a large number of cargoes are shipped every year. The LNG tankers are dedicated transport vessels with modifications in the bow to suit purpose built hawser and loading facilities. When offloading the LNG carrier in a harbor a conventional LNG manifold offloading system in the center of the LNG tanker is used. In offshore loading, the two vessels are moored together in a "crowfoot" hawser configuration reducing the relative movements between the crane tip on the FLPO and the bow of the tanker. The LNG carrier is operating on constant stern thrust to ensure the required stability of the offloading system in both calm and harsh weather conditions. Components and sub-systems are selected on basis of functional requirements. Critical components have been qualified in a step-by-step process. The qualification includes development of mathematical tools that have been verified through material testing, model testing and full scale testing. Verification includes design, manufacture and testing of the complete pull-in and connection system as well as full flow testing. Figure 1 - The OCL LNG transfer system(available in full paper) Vessel model testing is used both as verification and calibration of the mathematical models and it ensures accurate calculation of the two vessel's motions in the loading area and thus good prediction of the loads on the transfer system. The flexible pipe is the most critical part of the transfer system. The OCL system is based on a longitudinally welded, thin-wall stainless steel tube corrugated to provide the required bending flexibility. The finished flexible transfer pipe is a lightweight, double-wall construction with vacuum in-between the pipes as thermal insulation. The qualification process includes full-scale flow testing with both water and LNG. All tests and calculations show that the described solution will allow safe and economic transfer of LNG from a floating production vessel to a tanker. The OCL LNG transfer system will allow development of stranded gas reserves in areas with no infrastructure, or where a pipeline to a shore facility is not viable or economical.
Offshore production of LNG will soon be a reality as several oil companies are working on projects aimed at building large floating LNG production units in the near future. Tandem loading has been used to transfer crude oil between two floating vessels for several decades worldwide and the time has now come to transfer of LNG in a similar manner. In areas with harsh weather, the vessel motions will require the loading to be performed in a Tandem configuration and flexible pipes / hoses are required as transfer arrangement between the two vessels to accommodate the relative vessel movements. Framo Engineering has together with Aker Pusnes, Kongsberg Oil&Gas Technologies (KOGT), Nexans and MIB developed a tandem loading system for LNG named the " Offshore Cryogenic Transfer System (OCT)??. This paper describes the successful development, manufacture, testing and approval of the double walled corrugated, stainless steel vacuum insulated flexible pipe designed for offshore transfer of Liquefied Natural Gas in the OCT system. The Goals and the Challenges Offshore transfer of LNG has been discussed for decades. The obvious solution is to " copy?? the methods and procedures that have been developed and refined by the crude oil loading industry in more than 20.000 offloading operations in areas such as the North Sea. One main challenge with a offshore offloading system for LNG is the need for a flexible hose that can maintain its functions and integrity at a temperature of −164 degrees C. Bonded reinforced rubber hoses are used to transfer crude oil and are also used for LPG transfer, however, the exposure temperature is limited to approximately - 50 degrees C. None of the conventional hose materials will offer the required mechanical properties at -164 degrees C. However, some metals maintain or even improve their properties at cryogenic temperature and 316 Stainless Steel is one such material. Nexans in Hanover has manufactured and supplied corrugated stainless steel vacuum insulated pipes to the cryogenic industry for many decades and have developed extensive know-how on the manufacturing process. A Joint Industry Project was carried out in 2000–2004 to develop, manufacture and test a 10.5?? ID flexible pipe. The pipe design was based on Nexans manufacturing technology and KOGT riser technology. The project was successful, but in the years to come it became clear that a larger pipe with higher flow capacity would be required. A second JIP was carried out in 2007–20010 with the purpose to design, build and test a 16?? ID flexible pipe, based on the experiences gained in the first JIP.
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