The fatigue problem associated with the Touchdown Point (TDP) of a SteelCatenary Riser (SCR) may be less serious than is commonly understood, particularly for "soft" seafloor sediments typical of the Gulf of Mexico. Forsuch soft sediments, a SCR will dig for itself a trench having a curvedvertical profile that minimizes the shear forces in the SCR at the TDP andconsequently reduces the fatigue damage in the vicinity of the TDP. In thispaper this trench is modeled as a rigid curved surface upon which the SCRlands. Quasi-static analysis predicts that the fatigue life of a SCR with the TDPtouching down on a rigid curved trench is much greater than that of an SCRlanding on a rigid flat seafloor. The maximum fatigue damage either occurswithin the trench past the TDP, where riser motions are small, or occurs up theriser away from the TDP, where shear forces are small. Dynamic analysesutilizing finite element computer models are less clear. One early FE analysisincorporating a rigid curved trench showed a fatigue life improvement factor ofabout 2.3, whereas other analyses were inconclusive. Therefore, it is too earlyto draw general conclusions about potential fatigue life improvements untilfurther research is done. Introduction Since 1994 the population of Steel Catenary Risers has grown from zero tomore than 60 worldwide, connecting marine pipelines and flowlines with TLPs, SPARs, Semisubmersible FPSs, Ship-Shaped FPSOs, and other fixed and floatingplatforms. Many more such risers are planned in sizes ranging from 4" to 24", and water depths ranging from about 1000 feet to more than 6000 feet. The primary design criteria for Steel Catenary Risers are the usual internaland external pressure requirements, and a requirement that the predictedminimum fatigue life of the riser must exceed the field life by a specifieddesign factor. (The usual factor of 10 is thought to realistically cover themany unknowns in material strength and oceanographic loadings.) Fatigue damageis caused by cyclic bending of the riser, induced by the combination ofplatform motions and direct wave and current loadings on the riser, includingvortex-induced vibrations. Fatigue analysis of a SCR involves counting thebending cycles that will be applied to the riser over the full range of theoceanographic loading conditions that will occur during its lifetime. Fatigue damage is concentrated in the girth welds at both the upper end ofthe SCR where the riser is connected to the platform, and at the TouchdownPoint where the SCR first touches down onto the seafloor. This paper focuses onthe conditions at the TDP.
Progress has been made in the design of buckle arrestors, or more precisely collapse arrestors, for deepwater pipelines. Empirical relationships have been developed for the design of both integral ring and grouted sleeve arrestors, forming the basis of a simple and straightforward design procedure. The good agreement between the latest design formulas and the crossover pressure data obtained from large scale tests by Shell E&P Technology Company and by Professor Kyriakides at U.T. Austin over the past few years, should result in more efficient and reliable buckle arrestors for deepwater pipelines. Introduction An offshore pipeline which has been damaged locally may fail progressively over long distances by a propagating collapse failure driven by the hydrostatic pressure of the seawater. The pressure required to propel a propagating collapse is much smaller than the pressure required to initiate collapse of an undamaged pipe. For deepwater pipelines it is often uneconomical to design the pipeline with sufficient strength to prevent a propagating collapse failure. Such pipelines are designed to prevent buckling and collapse failures due to normal combined bending and external pressure loads, but are left vulnerable to propagating collapse failures initiated under extraordinary circumstances. In such cases, it is feasible to install buckle arrestors, such as thick-wall rings, at intervals along the pipeline. A series of such arrestors, each sufficiently strong to stop a propagating collapse failure, will limit the extent of damaged pipe in event of a mishap. In general, the distance between buckle arrestors is selected to enable repair of the flattened section of pipeline between two adjacent arrestors, at "reasonable" cost. For pipelines installed by J-Lay, the buckle arrestors also serve as pipe support collars. In this case the distance between arrestors is simply the length of each J-Lay joint. Three types of buckle arrestors are in common use, namely Grouted Sleeve arrestors, Integral Ring arrestors, and Thick Wall Pipe Joints. Grouted Sleeve arrestors are steel sleeves that are slid over the ends of selected pipe joints andare grouted in place, as shown in Figure 1, before being installed offshore. Grouted Sleeve arrestors are preferred, where feasible, because of their low cost. However, this type of arrestor has limited usefulness in deep water because, as external pressure increases, a collapsed pipe will transform from its normal flat "dogbone" cross section into a C-shaped cross section which then passes through the arrestor. Hence, for sufficiently deep water, even an infinitely rigid Grouted Sleeve arrestor is ineffective. Integral Ring arrestors are thick-wall rings that are welded into selected pipe joints, as illustrated in Figure 2, before being installed offshore. Integral Ring arrestors are used for pipelines in which the strength of sleeve type arrestors is not adequate, and for J-Lay applications that require a support collar on each pipe joint. These arrestors are very efficient in terms of strength for a given amount of steel, but are more expensive than sleeve arrestors because of the additional welding required. Thick Wall Pipe Joint arrestors are special pipe sections, each designed to prevent collapse propagation, that are welded into a pipeline at intervals. A Thick Wall Pipe Joint is essentially a very long integral ring arrestor, but is much less efficient in the amount of steel used.
The Auger export pipelines are connected to the TLP by steel catenary risers (SCRs). This is believed to be theftrst time steel pipe has been used for catenary risers. SCRs offer advantages over tensioned risers, since SCRs need no heave compensation and no subsea connections, and over risers made of "flexible pipe", since SCRs are much less expensive. However, significant design effort was required to prove that the SCRs could safely withstand environmental loads and the effects of TLP motions. The design effort consisted of extensive dynamic analyses as well as full scale fatigue testing of both the riser joint welds and the flexible joint that connects the riser to the TLP pontoon. Devices which suppress vortex induced vibrations were also tested. SCR installation is accomplished by lowering the riser on the abandonment and recovery cable from the J-Lay installation vessel and transferring the riser on a chain that is run through a chain jack hung from the TLP upper deck structure. A special Installation and Maintenance System was built for this purpose. INTRODUCTION Oil and gas export pipelines are connected to Shell's Auger Tension Leg Platform (TLP) by steel catenary risers (SCRs). Each SCR is essentially an extension of the pipeline [1], suspended in a near-catenary shape from a TLP pontoon to the seafloor. See Figure 1. The SCRs are composed of steel pipe sections welded end-to-end, terminating at a flexible joint which is supported by a receptacle mounted on the pontoon. Piping is routed from the deck down a TLP column and along the pontoon, where there is a flange connection to the top of thei1exible joint. This is believed to be the first application of steel pipe for catenary risers. SCRs offer advantages over risers made of "flexible pipe" since SCRs are much less expensive. SCRs also offer advantages over top tensioned risers since SCRs need no heave compensation, no subsea connections, and no flexible jumpers to transition to fixed piping at the production deck. For some applications a disadvantage of catenary risers compared to top tensioned risers is the length of active footprint on the seafloor, but this is not the case for Auger. Each SCR has an outside diameter of 12.75 inches and a wall thickness of 0.688 inch. The pipe material is API 5LX-52. The entire riser has a triple coat epoxy/polyethylene coating for corrosion protection and high abrasion resistance in the touchdown area. The upper 500 feet has a 0.5-inch thick neoprene coating for additional protection and marine growth prevention, plus triple-start helical strakes for suppression of vortex induced ibration (VIV). The flexible joint provides a rotation capacity for the upper end of the riser of ±14 degrees from the installed orientation of the riser, which is 11 degrees from vertical. See Figure 2. The maximum operating pressure is 2160 psi and the maximum operating temperature is 100 degrees F.
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