Vortex-induced vibration response characteristics of catenary riser conveying two-phase internal flow

The STRIDE JIP has performed test initiatives into several areas considered to be crucial to the design of steel catenary risers for harsh, deepwater environments. The following investigations are described:Water-tank testing of a ¼" diameter × 100ft long catenary riser model, looking at response to production vessel motions, in particular the response in the touch-down region for rigid and elastic seabed simulations.Vortex induced vibration tests on riser models 20ft long, 6" in diameter, in simulated currents up to 16.5 ft/s (5m/s - Reynolds 6.7x105). Helical straked and bare pipe models were towed at angles of up to 45° to the flow, simulating current flow over non-vertical riser sections of simple catenary and buoyant wave risers. The requirement and effectiveness of strakes on inclined sections was investigated.Vortex induced vibration tests on riser strings 650ft long, 10.75" diameter in simulated currents up to 5.8 knots (3m/s - Reynolds 6x105). Helical straked and bare pipe models were towed at angles of up to 75° to the vertical in a Norwegian Fjord between two tugs, continuing the investigation into non-vertical riser response to high currents. Introduction Phase II of the STRIDE JIP was initiated by 2H Offshore in March 1998, and is sponsored by 15 operators and 5 engineering contractors. During Phase I of the project, 2 areas were identified as being of prime importance to the design of deepwater steel catenary risers: the touch down point (TDP) and vortex induced vibration (VIV)1. TDP - A deepwater catenary riser TDP can actually involve a wide region of possible contact between different parts of the riser and different areas of the seabed, as dictated by the environmental loading on the floating production vessel and riser system. STRIDE Phase I identified this area of a catenary riser as critical with regards to the strength and fatigue life of the whole system. Confidence in analysis techniques was considered to be of paramount importance to such developments, and this lead to the proposed test program. VIV - Lack of empirical data on VIV of inclined and curved structures required a conservative design approach in STRIDE Phase I. In particular, it had to be assumed that VIV suppression strakes would be required over most of the riser length. The increase in riser drag due to such heavy straking produces a succession of technical and commercial knock-on effects. A high drag catenary riser needs extra ballast in the vertical section to accommodate the current loading, otherwise the riser can be pushed into shapes that overstress the TDP - Figure 1. Within the analysis, ballast was assumed to be most effectively applied as increased riser pipe wall thickness, and resulted in an increase of +40% on that required for pressure considerations alone. Increased wall thickness was shown to have a major impact on installation costs, particularly the amount of temporary buoyancy required for what was found to be the leading installation method at the time, controlled depth tow out. At the kick-off meeting for STRIDE Phase II, it was agreed by all participants that VIV and TDP be top of the list for further investigations. TDP Testing Program The main objective of these tests was to benchmark the ability of industry software to predict the response of a riser pipe catenary at its TDP, particularly as it is put into compressive loading or buckling as a result of extreme motions at the top end, for instance due to extreme storm motions on the floating production vessel.

The Steel Catenary Riser (SCR) concept offers advantages over other riser concepts and has been widely deployed worldwide. The first deepwater SCR was installed in the Gulf of Mexico in 1994. Since then, more than 100 SCRs have been installed for many types of deepwater floaters (Spars, TLPs, SEMIs, and FPSOs) in the deepwater fields of West of Africa, the Gulf of Mexico (GoM), and Offshore Brazil. As the second of two companion papers, this paper presents the state-of-the-art of key analysis techniques of deepwater SCRs while the first paper addresses the design methodology [R. Song, P. Stanton, Ref. 4]. First of all, the procedure for analysis of deepwater SCRs is discussed and presented in more detail than given in the first paper and is also illustrated in an analysis flowchart. Wave theory applicable to deepwater SCR analysis and time domain vs. frequency domain analysis approaches are described and discussed. More focus is given to the strength analysis including discussion and comparison of regular wave and random wave approaches. Attention is paid to the vortex induced vibration (VIV) analysis including discussion of modal response analysis and VIV parameter selections. For SCRs on semisubmersibles and FPSOs, vessel heave-induced VIV needs to be taken into account, and a corresponding time-domain approach is presented. Similarly, for Spars and deep draft semisubmersibles, vortex-induced motion (VIM) fatigue damage of SCRs is discussed in more detail. Particular attention is also given to the analysis of SCR compression in the touch-down zone (TDZ) and corresponding acceptance criteria are presented. The application of fracture mechanics to engineering criticality assessment (ECA) is explored. Two examples of deepwater SCRs corresponding to a semi and a Spar are given to illustrate the presented methodology.

Our customer elected to post-install suppression fairings for vortex-induced vibration (VIV) on the suspended gas export pipeline in Green Canyon Block 113 due to recent data acquired on the depth and magnitude of loop/eddy and cold core current profiles in the Gulf of Mexico. VIV suppression fairings minimize pipeline excitation during high current events, thereby preventing accelerated fatigue failure of pipeline components and minimizing current-related disruption on producing operations. VIV suppression hardware installed on the span was designed and fabricated by Shell Global Solutions (US) Inc. (Shell GSUS) and was in the form of short (6-foot) fairing sections. For hardware installation, our customer teamed with Shell GSUS to develop a method and associated tooling to allow the fairing assemblies to be installed. The method of installation included a deepwater construction vessel using a work-class ROV and heave compensation techniques. This method and equipment can be utilized without affecting flow line operations. This paper reviews the planning, procedures and results of a deepwater, horizontal pipeline span VIV suppression installation project. This paper will be presented by the author at Offshore Technology Conference 2004 in Houston, Texas. Case for Action Span Description. During an as-built pipeline survey of a deepwater offshore gas pipeline, a span of over 300 feet was discovered. The span is in the Green Canyon area of the Gulf of Mexico (Figure 1). The depth of the span at location is approximately -2300 fsw. The span length is over 300 feet long and at times over 10 feet above the sea floor. The Gulf of Mexico has recently verified currents that can potentially harm suspended pipelines in deepwater. Fig. 1 - Approximate Span Geographic Location(Available in full paper) To limit vortex-induced vibration (VIV) during deepwater current events, suppression hardware is required. Shell GSUS was tasked with installing the VIV suppression devices. Fairings were the specified device due to their low drag properties and low cost. VIV suppression fairings have in the past been installed on vertical pipeline steel catenary risers (SCR) and top tensioned risers (TTR) by Shell GSUS. This was our first attempt at a horizontal span retrofit. Installation Method Development VIV Suppression Device Decision. Previously Shell GSUS developed and patented VIV suppression fairings. Fairings were found to be superior to commonly used helical strakes due to their light-weight, low-cost and low-drag properties. VIV suppression fairings developed by Shell GSUS (Figure 2) weathervane to keep the vortex-interrupting fin on the downcurrent side of the tubular that they are attached to. Unlike helical strakes, VIV suppression fairings do not increase the diameter of a pipeline substantially. Most deepwater facilities utilize VIV suppression in some fashion. Shell Exploration and Production Company (SEPCo) chose VIV suppression fairings for their tension leg platform (TLP) production risers in the high-current Mississippi Canyon region of the Gulf of Mexico. The Ursa TLP production risers were installed with VIV suppression fairings attached. Mars TLP production risers were also installed with VIV suppression fairings as well as strakes in the splash zone, and additional VIV suppression devices have been installed since arriving on location, due to a deeper understanding of the current profile.

The method of pulling a steel catenary riser (SCR) through a steel tube (termed as a pull tube) is common practice for deepwater riser tie back applications. Vortex-induced vibration (VIV) of such a system is complex. VIV analysis programs, such as Shear7 [1], are suitable only for a single, chain-like structure. The application of such a software tool in VIV design of the SCR-pull tube system requires careful consideration of a number of structural and hydrodynamic factors. This paper presents a methodology for VIV analysis of the combined structural system of the SCR with the pull tube. Firstly, the entire SCR-pull tube system is modeled with the finite element program Flexcom [2]. The modes are then calculated for the entire structure with program Modes [3]. Afterwards, the structural nodes are rearranged for VIV analysis with Shear7. The pull tube is secured on the platform through a number of guides on the truss structure of the hull. The diameter of the pull tube is much larger than that of the SCR, and the pull tube is much stiffer in bending than the SCR is. If the entire structure is analyzed with Shear7, the mode for the pull tube (a mode involving a large motion of the pull tube section), which is very high in order, would be embedded in the analysis. It makes sense to single out the pull tube mode for study as if it is the first mode. A computer program, named as V-Span [4] for subsea span VIV analysis, is used to analyze both in-line and cross-flow VIV of the pull tube. A numerical example is presented to demonstrate this methodology. This is a deepwater SCR, which has a diameter of 9 inches. The water depth is 6,300 ft. The pull tube is 640 feet long and 20 inch in diameter. Both the loop-eddy and background currents are analyzed. The fatigue damage resulted from both in-line and cross-flow VIV is estimated.

This paper presents key issues and lessons learned from the design, fabrication, weld qualification, fatigue testing and determination of defect acceptance criteria for the Matterhorn field export SCRs, the first reeled SCRs installed to a TLP in the Gulf of Mexico. The Matterhorn TLP, located in 2850ft water depth in Mississippi Canyon 243, exports oil and gas via 8-inch and 10-inch SCRs, operated by ChevronTexaco and Total. The design of these SCRs was one of the first to consider The Cold-Core Eddy Current (or submerged current) in the Gulf of Mexico and the design of the SCRs and other catenary risers was strongly influenced by its introduction. Reeled installation of the SCRs also affected the design, fabrication and fatigue testing process, particularly the pipe welding and matching process and the fatigue test and fracture mechanics analysis strategy. Both SCR designs were shown to be robust by early consideration of sensitivity to a wide range of design inputs. Designing to several such sensitivities was found to be a very valuable input to achieving robust SCR designs when managing change to base case assumptions in as-built SCRs and components. Being among the first to consider the Cold Core Eddycurrent in the Gulf of Mexico and the first reeled SCR to a GoM TLP, lessons learned have significant relevance to future SCR projects. Introduction The Matterhorn field consists of a SeaStar mini-TLP in 2,850ft of water in Block 243 of the Mississippi Canyon area of the Gulf of Mexico. The riser system consists of an array of top-tensioned production risers combined with steel catenary risers (SCRs) for gas and oil export with an additional flexible riser and steel tube umbilical riser to an injection well. The 10" gas export SCR is operated by Total and the 8" oil export SCR is operated by ChevronTexaco. Figure 1 illustrates the SeaStar TLP steel catenary riser and subsea layout. SCR Design and Construction Process The design, procurement and installation process for deepwater SCRs is a combination of interrelated tasks in several disciplines the interface between which is a critical prequesite to achieving a feasible, optimized and consistent SCR design. The maintenance of the interface between these disciplines and functions is an essential part of the SCR design process which requires effective interface management throughout the project. Figure 2 presents a flowchart of the SCR design and construction project process which illustrates the interfaces between the key SCR project stages:DesignProcurement of pipe and ancillary devicesWeld qualification and testingCoating and constructionInstallationOperation Also illustrated by this flowchart is the interrelationship between these processes and the important feedback loops that exist back to the design stage of the project. Although most inputs to design are typically defined by the project Design Basis, design is also influenced by key assumptions made at the design stage. Key elements of this feedback to design are provided by the pipe procurement (pipe weight), flexjoint procurement (flexjoint stiffness), pipe matcing (stress concentration factors), fatigue testing (design S-N curve).and installation (fatigue damage) stages of the project.

A steel catenary riser (SCR) is often the preferred option for subsea tie-back to an existing deepwater platform due to its conceptual simplicity, ease of construction and installation, and simple interface with the flowline. Unlike new field development, which provides more room for the design of new SCR, the engineering design of a tie-back SCR has unique challenges and requires special solutions. This is mainly because initial prelimainry SCR design during design of the floating production system kacks project specific information for the tie-back. In addition, the early SCR design draws much less attention compared to the detail design. Based on deepwater tie-back SCR engineering experience, key tie-back SCR design challenges are summarized from the engineering execution perspective. These challenges include potential concept changes, platform stringent weight budget limits, existing riser porch strength limits, SCR top arrangement, thermal performance requirement, subsea layout, riser interference, SCR and flowline interface design, SCR fatigue design, and installation. They call for design optimizations and even innovative ideas. The engineering analysis and design to handle the challenges are discussed together with solutions. The lessons learned from recent tieback SCR engineering projects are outlined. These lessons should have significant relevance to future tie-back SCR projects. To demonstrate the challenges and solutions, an example is given based on a recent deepwater Spar tie-back SCR project with solution to challensge design that makes the design reliable and robust. Introduction In recent years, exploration and production activities have increased dramatically in deepwater nearly doubling the water depth compared to the activities a couple of years ago. The offshore industry is building systems today for production from even deeper water, progressively using new technology and significantly extending existing technologies. The targeted water depths for oil and gas developments in areas such as Gulf of Mexico (GoM), West of Africa (WoA), Brazil and North Sea are increasing every year. Regardless of the floater concept adopted for offshore field development, there is always a need of risers. Risers are some of the more complex aspects of deepwater developments. It became evident that riser system plays bigger and bigger role as part of the offshore infrastructures. There are different ways to group deepwater risers. The most widely used and field proven deepwater riser concepts include SCRs, hybrid risers with a single line or bundled multi-lines, flexible risers with different configurations, and top tension risers (TTR) as shown in Figure 1. Figure 1: Illustration of Deepwater Riser Concepts Among the riser concepts, SCRs have been enjoying a widespread acceptability for deep and ultra-deepwater oil and gas production in recent years. Up to today, more than 100 deepwater and ultra-deepwater SCRs have been installed worldwide mainly in the GoM, WoA, and offshore Brazil. The impetus for developing SCR technology is the continuing push into deep and ultra deep water, and the growing belief that SCRs offer the best technical and commercial solution.

Effects of vertical and lateral riser-soil interactions on vortex-induced vibration of a steel catenary riser

Deepwater development in Gulf of Mexico (GoM) started about two decades ago. It is the time to evaluate the global integrity of the offshore production unit and riser system and explore the possibility of life extension. This paper investigates life extension of deepwater risers used for a spar application in GoM. A spar in GoM has been put into service for the past 17 years since it was installed in 2000. It was expected to extend the riser design life of 20 years by 10 more years. This paper first introduces the configuration of Steel Catenary Risers (SCRs) and Top Tensioned Risers (TTRs) used for the Spar platform. It then discusses the philosophy for life extension of deepwater risers by assessing the integrity of the riser system. Latest technology and monitored data are used for the assessment. The TTRs and SCRs are modelled using latest analysis programs that were not readily available when the risers were designed. The interaction between the TTR and buoyancy can tensioning system is modelled as a pipe-in-pipe system that considers the function of centralizers with a gap size. The contact interaction between buoyancy can / stem and supporting guides of the hull platform is modelled as non-linear spring elements. The spring elements connect normally to the hull centerline and allow vertical sliding movement of the buoyancy can tension / stem system with friction. The base line inspections of the vessel, SCRs, and TTRs was carried out in the same inspection campaign with the spar hull structure. As-built information, production and operational data was also used for a continuous service assessment. Environmental condition data to the state of the art, including measured spar VIM data, was used in the assessment of VIM fatigue damage to the risers. The investigation was based on the up-to-date analytical tools including latest FEA program ABAQUS and VIV prediction Shear7. It further computes the riser global performance including dynamic strength and fatigue damage with the contributions from wave fatigue, VIV and VIM. This paper finally draws a conclusion for the life extension of the risers for the application in GoM.

This paper deals with some of the issues that were encountered during the design and construction of the steel catenary risers (SCRs) for the Prince Project in Ewing Bank Block 1003 in water depth of approximately 1500 ft in the Gulf of Mexico. The SCRs are two 12 inch oil and gas export SCRs to transport the oil and the gas from the Moses TLP to a connection into pipeline systems on the shelf in shallower water. The Prince SCRs represent a leap in the SCR applications by having many firsts in the industry. They have been designed and installed for less than 1500 ft water depth that represents the shallowest water for SCRs so far, breaking Morpeth SCR's water depth record of 1670 ft. The Prince SCRs had the largest departure angle of 24 degrees and FlexJoint angle variation of +/-20 degrees to accommodate the TLP motion in such relatively shallow water depth. The VIV strakes are the first to be pre-installed on the SCR pipes during an S-lay operation. The main issues that every designer and owner should be aware of during the design, procurement, fabrication and installation are SCR geometry and configuration, material selection, welding and other contracting aspects, particularly the interface requirements and installation methods. The Prince project has provided a great opportunity for uncovering many of the details that require close attention to avoid delays and mishaps during the typical accelerated schedule of nowadays projects and to ensure proper design in accordance with applicable design codes and industry standards. This paper will focus on the lessons learned during the execution of the Prince SCRs activities and help the industry with better understanding for the issues that should be and could be considered as common sense. Introduction Steel Catenary Risers represent one of the most significant challenges ever encountered by project teams designing and constructing deepwater marine pipeline systems. Contracting philosophy and interfacing of disciplines go hand-in-hand to achieve timely completion of pipelines incorporating SCRs. All pipeline facilities share common ‘building blocks’: design and engineering, material specification and procurement, and installation. These building blocks have to be integrated when constructing SCRs. The Prince SCRs project was a model for team integration and cooperation among all parties involved. The Prince SCRs also introduced many firsts in the industry that without the teamwork and close relationship between the management, the design engineers and the suppliers would not have been achieved. These first include the following:The Prince SCRs are the first to be installed in water depth less than 1500 ft.The Prince SCRs are the first to have large departure angle of 24 degrees connected to the TLP hull by flexible joints (FlexJoints) with angle variation of +/- 20 degrees, which is another first.The Prince SCRs are the first to have VIV strakes preinstalled during the S-lay operation of the SCR pipes. The VIV strakes have to go over the S-lay vessel stinger and suffer some deformation before bouncing back after leaving the stinger. This paper presents the highlights of the Prince SCRs project and discusses in more details the relationship factor that played a significant roll in the success of the project. Design and engineering, material specification and procurement, and installation issues are also presented. Design/Engineering Issues Engineering design represents an early project contracting philosophy. Catenary risers are attached to and suspended from a floating hull.

Three-dimensional nonlinear vortex-induced vibrations of risers under internal gas-liquid and external shear flows

Time-domain analysis of vortex-induced vibration of a flexible mining riser transporting flow with various velocities and densities

Flexible risers and steel catenary risers often provide unique riser solutions for today’s deepwater field development. Accurate analysis of these slender structures, in which there are high-speed HP/HT internal flows, is critical to ensure personnel and asset safety. In this study, a special global coordinate-based FEM rod model was adopted to identify and quantify the effects of internal flow and hydrostatic pressure on both flexible and deepwater steel catenary risers, with emphasis on the latter. By incorporating internal flow induced forces into the model, it was found that the internal flow contributes a new term to the effective tension expression. For flexible risers in shallow water, internal flow and hydrostatic pressure made virtually no change to effective tension by merely altering the riser wall tension. In deep water the internal pressure wielded a dominant role in governing the riser effective tension and furthering the static configuration, while the effect of inflow velocity was negligible. With respect to the riser seabed interaction, both the seabed support and friction effect were considered, with the former modeled by a nonlinear quadratic spring, allowing for a consistent derivation of the tangent stiffness matrix. The presented application examples show that the nonlinear quadratic spring is, when using the catenary solution as an initial static profile, an efficient way to model the quasi-Winkler-type elastic seabed foundation in this finite element scheme.

Drilling and production risers experiencing vortex-induced vibrations (VIV) may adversely impact operations in two ways: by increasing deflection due to amplified drag and causing structural fatigue damage due to VIV induced dynamic stresses. Complex hydrodynamic mechanisms controlling correlation length and force dependence on motion and frequency of response are major obstacles in predicting the VIV response, except for a limited number of cases for which extensive experimental calibrations were carried out. A pragmatic VIV analysis methodology has been developed based on correlation length measurements, and riser specific experimental tests to establish basic CD and CL databases for realistic riser configurations with auxiliary lines and buoyancy modules. Correlation length properties are established using flow visualization methods, in particular Digital Particle Image Velocimetry (DPIV), which allows whole-field quantitative information on the velocity and vorticity fields to be obtained. Identification of the role of two basic VIV mechanisms, the formation of braid vortices and the impact of vortex splitting, has provided a robust correlation length model. A VIV analysis program has evolved from these experimental tests in conjunction with extensive research in structural and hydrodynamic riser responses. Introduction The design of drilling risers for mobile offshore drilling units (MODUs) is governed by operational considerations, specifically, the need to keep the riser deflection angle within 1° to 2° during normal drilling operations. This necessitates stringent measures:keep the rig within a tight watch radius of around 0.5% to 2.0% water depth,maintaining high riser top tension, andminimizing riser net in-water weight by the use of buoyancy modules. The occurrence of VIV may adversely impact drilling operability because VIV causes an apparent increase in hydrodynamic drag, resulting in increased riser deflection angles. In addition, fatigue damages appear to be a second concern, however less so since it is feasible to frequently inspect risers. In the event of strong current, the need to keep riser deflection angles at a minimum may dictate the use of faired riser sections. This is, however, a very costly option and must be avoided if at all possible. For example, increased tensioner capability may prove to be just as effective without encountering the same handling difficulties. As for production rigid risers and steel catenary risers for permanently installed platforms, they are much more tolerant of riser deflections and require minimal riser tensions. As a result of low top tension these risers become much more compliant and VIV responsive than rigid drilling risers. Fatigue considerations may dictate that these risers be protected with VIV suppression devices, such as strakes, in ections exposed to strong current. This paper highlights a pragmatic VIV analysis tool (VIVA) based on riser-specific experimental tests and mathematical models. This is followed by a review of VIV analyses for representative rigid drilling and production risers for 3000-foot water depth. VIV response sensitivity is investigated for 1.5-knot slab, linear, and reverse current profiles that are indicative of the 1-year extremes West of Shetland and Brazil. A Pragmatic VIV Analysis Methodology VIVA is a computational tool developed to predict the vortex-induced vibrations and fatigue damage of marine risers in shear flows based on strip theory and an extensive database of hydrodynamic data on sectional force coefficients and correlation length (Reference 1). The program is generically written to allow the input of additional hydrodynam

API RP 2RD was published in 1998, API RP 1111, Third Edition was published in 1999, and DNV-OS-F201 was published in 2001. Both 2RD and F201 are dynamic riser codes while RP 1111 is a pipeline code that includes dynamic pipeline risers. This paper describes a new code for the design of dynamic risers for floating production installations. This new code is scheduled to become ISO 13628-12 and also to be adopted by API as a replacement for API RP 2RD. A JIP formed in 2008 expedited the development of the new code, which is currently scheduled for ISO ballot in 2010 as a draft international standard (DIS). The new code has both mandatory (normative) and recommended (informative) practices. This paper will show the new Table of Contents and highlight the major differences between the new code and API RP 2RD. A significant difference is the way combined loads are checked. The new code allows designers to use any one of four methods:Method 1 based on the current API RP 2RDMethod 2 based on ISO 13628-7 (for completion and workover risers)Method 3 from DNV-OS-F201Method 4 from API 1111 Annexes illustrate the use of the new code for top-tensioned and steel catenary risers. Background A task group was formed to write a new recommended practice for floating production system risers in the fourth quarter of 1992. API RP 2RD Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms (TLPs) was finally published in June of 1998, almost six years later. In 1992 there were only three TLPs installed and no Spars. The only free-hanging compliant risers were made of flexible pipe. The first steel catenary riser was being designed for the Auger TLP, but was not installed until 1994. Five more TLPs with top-tensioned risers (TTRs) and a few more SCRs were installed before 2RD was published. The first Spar was installed in 1997. It used TTRs for production and catenary flexible pipe risers for export. Thus at the time 2RD was being written, industry experience was based mainly on top-tensioned risers for floating drilling vessels and TLPs. In January 2001, Det Norske Veritas (DNV) published Offshore Standard DNV-OS-F201, Dynamic Risers. There are major differences between the 1998 API RP 2RD and DNV-OS-F201. Consequently, in 2003 a small API task group was formed to write a " bridging document?? between the two codes. At the summer 2003 API Standardization Conference, this plan was scrapped in favor of writing a new code based on 2RD and F201. A new task group for this task met for the first time in early 2004. In keeping with API goals at the time, the new code was to be written as an ISO document and adopted back by API. At the first task group meetings, another API document was introduced into the mix, API RP 1111, Third Edition, July 1999, Design, Construction, Operation, and Maintenance of Offshore Hydrocarbon Pipelines (Limit State Design). As its names implies, RP 1111 is a pipeline recommended practice. However, it also gives guidance on how to design SCRs, which in essence are a continuation of pipelines and flowlines.

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.