A New Solution for Tandem Offloading of LNG

Abstract

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.

LNG Transfer in Harsh Environments - Introduction of a New Concept

“…It is a common design for flexible pipes in submerged or aerial applications and it guarantees long lifetime even in severe conditions (Ref. 3…”