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.
The paper describes the Submerged Turret Production System (STP) and a state of the art disconnectable FPSO. The STP technology is well proven in the North Sea, South China Sea and other offshore arenas worldwide for both disconnectable FPSO's and for permanent mooring configuration. Lately it is being used for turret mooring of one of the worlds largest FPSOs located in Mexican waters in Gulf of Mexico on the PEMEX KuMaZa field. The FPSO is scheduled for first oil April 2007 and a short update of this project is presented. The STP technology is built on the Submerged Turret Loading - STL technology used for Offshore Loading of Crude Oil, which is further developed for Discharge of Natural Gas and is the basis for the world's first Offshore LNG Receiving Terminal, Gulf Gateway - located in US Gulf of Mexico, Block WC 603. The paper presents this turret technology and how this technology can significantly reduce development risks in the Deep Water Fields in GoM and how it subsequently can have a positive impact on insurance premiums for the FPS/FPSO facility due to the ability to safeguard and sail away from Hurricanes. Introduction The ability to easily connect and disconnect a single point mooring and riser system in a safe way is attractive for a number of reasons. The most obvious reason in Gulf of Mexico is to be able to remove the vessel in case of a hurricane. The technology described, has been developed over a period of 15 years, starting in the North Sea with the STL loading system for shuttle tankers and the disconnectable STP mooring and riser system for FPSOs. Basically the technology is the same, utilizing a submerged buoy integrating riser buoy and turret in one compact module which all together is disconnected.
The use of FPSOs in the development of oil and gas fields in deep waters requires accessibility to safe and reliable off-take solutions. The Deep Water SAL (DW SAL) design moves the well proven Single Anchor Loading (SAL) concept to deep water applications. The SAL system has been used for the last ten years in the North Sea at eight different locations. The DW SAL system consists of a submerged buoy, the SAL Base, moored to the seabed by typically 6 mooring legs. The Oil Offloading Lines (OOLs) are connected at one end to the FPSO and at the other end to the centre of the SAL Base. The distance between the FPSO and the DW SAL is typically in the range of 1500m to 2500m. The SAL Base is equipped with a mooring swivel and a fluid swivel, allowing the moored export tanker to freely weather vane and serves as the connection point between the export tanker and the geo-stationary mooring system. The DW SAL can be applied to a wide range of water depths, as the system is virtually made water depth independent by placing the mooring swivel on the SAL Base. The target water depth of the SAL Base is about 45m, at which, the wave induced motions are reduced significantly compared to a surface buoy. The loads on the OOLs are consequently much lower and the fatigue life of the system components much higher. At the same time as the SAL Base will be at a safe distance below any service vessel or export tanker when idle and disconnected, it may still be reached by divers. However, the operational philosophy is based on emptying specially designed ballast tanks to blow the SAL Base to the sea surface so that maintenance and other work easily can be performed. The extreme tension in the mooring hawser is also much lower for a DW SAL system compared to a standard calm buoy. The main reason is the combination of slow drift motion of the export tanker combined with the first order motion of the calm buoy / SAL Base. Typically the slowly varying offset of the tanker creates the mean tension level; while the wave induced motion of the buoy creates the peak loads. The development of the DW SAL has been using cite specific data both from West of Africa as well as Santos Basin offshore Brazil. Introduction Various offloading concepts are available and in use today and the selection depends on location, weather conditions and infrastructure. APL has over the years developed systems like BLS (Bow Loading System), SDS (Stern Discharge System), STL (Submerged Turret Loading), SAL (Single Anchor Loading) and BTL (Buoy Turret Loading). Except for two BTLs, which essentially are specially designed calm buoys for deep water, these systems have been installed in moderate water depths, up to 350m, but designed to operate in the harsh North Sea environment. The SAL system, Figure 1, where a tanker is connected by a single mooring hawser in the bow of the vessel, has in particular shown to perform very well. More than 2000 shuttle loads have been lifted since the introduction of the first system on the Siri Field in 1999. Key elements are the mooring and fluid swivels, the mooring hawser and the flexible hose for fluid transfer. The anchor, which also serves as PLEM (Pipe Line End Manifold) is either suction anchor, pile anchor or gravity anchor. While disconnected, the mooring line and loading hose are lowered to the seabed until next hookup. Contrary to a Calm buoy, all equipment remains subsea on the seabed during storms and extreme weather. The maximum design sea state for the SAL systems in idle condition have been significant wave heights up to 13m.
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