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.
The Auger TLP wel 1 system permits use of proven deep water dri 11 ing methods and platform-1 ike completion techniques. Key features enabling these operational advantages are a lateral mooring system, a spread wel 1 pattern, an open well bay with special wel 1 handl ing equipment and production risers having surface well heads and trees. Significant advances in the understanding of Gulf Of Mexico eddy currents, riser interference and vortex-induced vibrations were made in support of the Auger TLP project, OTC 7617
Summary Estimating fatigue damage under wind-driven sea loading is of primary importance in the design of steel catenary risers (SCRs) serving floating hosts. For design, the wind-driven sea is modeled as a stationary random process. The resulting dynamic stress in the SCR is also a stationary random process. Spectral methods provide closed-form fatigue damage estimates in terms of statistics for stationary random stress processes. Cycle-counting methods, such as rainflow cycle counting, provide an alternative damage estimation approach that is generally applicable and requires simulation of stress time series. The cycle-counting approach requires more computation than spectral methods. Damage estimates using the cycle-counting method may be lower than spectral damage estimates; however, a substantial amount of simulation may be required to quantify the difference. This paper considers fatigue damage in SCRs attached to both tension-leg platform and semisubmersible hosts. Spectral and cycle-counting estimates are generated and compared. Accuracy of the estimates is discussed, and guidelines for damage estimation are presented. It is demonstrated that the differences between spectral and cycle-counting estimates of lifetime fatigue damage arise primarily from assumptions made regarding the spectral shape of the stress processes of interest.
Estimating fatigue damage under wind-driven sea loading is of primary importance in the design of steel catenary risers (SCRs) serving floating hosts. For design, the wind-driven sea is modeled as a stationary random process. The resulting dynamic stress in the SCR is also a stationary random process. Spectral methods provide closed-form fatigue damage estimates in terms of statistics for stationary random stress processes. Rainflow cycle counting provides an alternative damage estimation approach that is generally applicable and requires simulation of stress time series. The rainflow approach requires more computation than spectral methods. Damage estimates using the rainflow method may be lower than spectral damage estimates; however, a substantial amount of simulation may be required to quantify the difference. This paper considers fatigue damage in SCRs attached to both tension-leg platform and semi-submersible hosts. Spectral and cycle-counting estimates are generated and compared. Accuracy of the estimates is discussed, and guidelines for damage estimation are presented. It is demonstrated that the differences between spectral and cycle-counting estimates of lifetime fatigue damage arise primarily from assumptions made regarding the spectral shape of the stress processes of interest. INTRODUCTION SCRs accommodate floating host motions through self-flexure. Since host motions persist over the lifetime of the SCR, the task of estimating fatigue damage under wind-driven sea loading is of primary importance in the design of SCRs serving floating hosts. This paper investigates three basic approaches to this task:Spectral damage estimation using results of frequency-domain analysis (Rayleigh and bimodal)Rainflow cycle counting of stress time histories obtained from time-domain analysisRainflow cycle counting of stress time histories created from frequency-domain analysis results Cycle-counting techniques require the simulation of stress time histories. The accuracy of the resulting fatigue damage estimates depends on the amount (and accuracy) of simulation. Practical computational efficiency considerations have led to the development of spectral methods that match cycle-counting damage estimates for certain forms of stress spectra. Spectral fatigue damage estimation procedures represent powerful and efficient tools for estimating fatigue damage caused by stationary random stress processes because they relate fatigue damage to the statistics of the stress process using a closed-form relationship and do not require creation of stress time histories. The spectral approach will overestimate damage (relative to cycle-counting estimates) when the spectral shape of the process is more broad-banded than the form assumed in developing the spectral procedure.
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