The present paper describes the behaviour of chain segments that are subjected to pretension and a rotation angle at the segment end. The behaviour such segments has been investigated both experimentally and by finite element modelling. The purpose is to carry out fatigue life predictions. A full scale test with a studless chain segment with a diameter of 125 mm has been carried out to shed more light on the behaviour. The test corroborated the assumption that the chain segment behaves semi-rigidly under the given conditions due to locking of the inter-link hinge mechanism. The influence of various important parameters on the bending effect was studied. The chain segment has a significant flexural stiffness and an intra-link bending curvature for links that are in a flat position with respect to the inflicted end rotation. The associated Out of Plane Bending (OPB) stresses are significant for critical links close to the end hang-off. This effect must be taken into account when carrying out fatigue life predictions. There is still no guidance given in rules and regulations for this case. In the present work it is suggested that a hot spot method is applied for fatigue life predictions. Under a combined loading mode, defined by tension and OPB, the maximum principal stress range in the link bend area must be determined by refined finite element analysis. Subsequently, the fatigue life can be predicted by using an appropriate hot spot S-N curve. The paper points out how a good fatigue design of the hang-off area can reduce the effect of OPB. A Buoy Turret Loading (BTL) is used as a case study to demonstrate both the design proposal and the fatigue life prediction methodology.
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.
This paper addresses mooring design methods, reveals records of site tests, and also compares the conventional theoretical numerical modeling approach with a real full-scale disconnection of this challenging scenario. The FPSO BW Pioneer set a new world record for deepest mooring system on a production unit at 2,500m (8,200ft) of water depth in the US Gulf of Mexico. The unit also pioneered this type of facility (FPSO) in the US Gulf of Mexico. This area of the globe is prone to hurricanes. The decision was made to proceed with a concept that would allow the vessel to detach from its mooring and risers system and sail away to sheltered waters when necessary. The mooring as well as all risers and umbilicals sink to a depth that is safely below the waves in case of a major storm. This system installed is the first production module for two deepwater fields, Cascade and Chinook, both operated by Petrobras America Inc. with the FPSO BW Pioneer. Various connect - disconnect scenarios were identified during the design phase. Since the system is detachable, the disconnection and reconnection dynamics were analyzed in detail. Due to the complexity of the interaction between the STP™ turret buoy and the mating cone, a model was introduced to better evaluate the local effect of dragging and hydraulic suction combined with the vessel motion. This effect is paramount to the integrity of the riser and umbilical structure, as well as to prevent collision between the vessel and the buoy. The model is described and simulations emphasize vertical velocities and accelerations, as compared to traditional methods of computation. At the time, it was unknown whether any previous projects had ever compared the theoretical numerical model with a real full-scale disconnection. The decision was made to proceed with an instrumented site free-drop test, using customized equipment similar to what Petrobsas uses to monitor the trajectory of the Petrobras' torpedo piles during installation. This paper presents to the Industry these site test results and draws conclusions about how accurate or robust design premises are when performed by state of the art design tools used on this project. Introduction The Cascade and Chinook fields are located in the Walker Ridge area of the Gulf of Meixco and are being produced from the FPSO BW Pioneer that is located in WR/Block 249. The Cascade field was discovered in 2002 and Chinook field was discovered in 2003. The mooring with turret buoy was installed in late 2009 and the FPSO BW Pioneer was first connected to the mooring late 2010. Installation of the mooring and connection of the FPSO set a new world record for deepest mooring system on a production unit at 2,500m (8,200ft) of water depth.
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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