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Field observations show that a steel catenary riser (SCR) connected to a floating production platform will dig itself a trench of a few pipe diameters in depth through the touchdown zone (TDZ). This appears to happen during the early stages of the SCR life. The trench has a curved vertical profile that is shaped around the SCR in the touchdown zone (TDZ). The trench changes the geometry of SCR contact with the sea floor and potentially exposes the riser to stiffer soil at the bottom of the trench. The seabed trench may therefore be expected to have a significant effect on the SCR touchdown zone fatigue life. However, studies published to date have given contradictory indications as to whether the net effect is to increase or decrease the SCR fatigue damage in the TDZ. This paper presents a study of the main aspects that may impact fatigue damage for a given set of sea states: the trench vertical profile, seabed soil stiffness model (linear, non-linear) and the directionality of the predominant fatigue sea states relative to the orientation of the SCR. Three different approaches for analytical modeling of trench vertical profiles are considered, two based on previous literature and one new. For validation, the modeled trench profiles are compared with a measured trench profile for a 12-inch gas export SCR connected to a TLP in about 1000 m water depth. Fatigue damage is evaluated for realistic fatigue sea state bins for Gulf of Mexico and Western Australia, using either a conventional linear seabed model, or a hysteretic non-linear model, with soil parameters that are appropriate for the two different regional soil types. The overall conclusion, for both soil types, is that trenches have a beneficial effect, extending the fatigue life of the SCR in the touchdown zone. Introduction Steel catenary risers (SCRs) are often the most attractive riser solution for a deepwater field development, with fatigue life in the touchdown zone (TDZ) representing one of the main feasibility issues. As a result, the effect of soil stiffness variation and trenching on the long term fatigue life of SCRs has generated increasing interest, and many publications, in recent years. Field observations of SCR touch-down zones (Bridge and Howells, 2007) showed that a trench will almost always form in the seabed soil, since the top surface of the seabed soil is usually soft enough for self-trenching to occur. However, studies published to date have given contradictory indications as to whether the net effect of the touchdown zone trench is to increase or decrease the SCR fatigue life. Touchdown fatigue studies are routinely performed using a linear model for the seabed response, even though it is accepted that the actual seabed response is non-linear, with the operative stiffness varying with the displacement amplitude. Advancement in understanding of the soil-structure interaction has been achieved through a combination of analysis, centrifuge model testing, field experiments and trench observations for existing SCRs. Studies have included two joint industry projects, STRIDE and CARISIMA (Bridge et al., 2004). This body of work has led to development and implementation of numerical soil structure interaction models in software packages available for SCR analysis. A summary of some of the work relevant to this paper is presented here as background literature in respect of (a) seabed response models, and (b) analytical modeling of SCR trenches. Before that, key papers that address SCR trenches and their potential effect on touchdown fatigue are summarized.

A steel catenary riser (SCR) is a widely used concept for deepwater floating production facilities. Severe motions of a floating host facility such as a semisubmersible or FPSO may cause a significant compression load on SCRs at the touch down zone (TDZ). This paper investigates how to assess the compression that could be experienced by deepwater SCRs, including methodology, failure modes considered, acceptance criteria, computer modeling, and describe the steps necessary for assessing the compression forces. To demonstrate the proposed methodology and criteria, a recent example of the Independence Hub 20-inch Gas Export SCR in ultra deepwater (i.e. 8,000 ft) is given to illustrate the compression and buckling phenomenon. The behavior of the SCR compression and buckling at the TDZ is investigated by using a nonlinear finite element method to determine the mechanism and governing factors. Both beam and shell elements are used in the detailed analysis for comparison purposes. In addition a strain-based criterion is implemented to determine if the compression level is acceptable. Short term fatigue damage is also calculated by using the time domain rain-flow method. In general, the paper presents an analysis procedure outlining the steps necessary for evaluating the compression and buckling phenomenon of deepwater SCRs.

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.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper describes the challenges encountered during the design, fabrication and installation of the seven deepest Production Flowline Steel Catenary Risers (SCRs) which are connected via flexjoints to the Independence Hub Deep Draft Semi-Submersible located in Mississippi Canyon Block 920 (MC920) in 8,000 ft water depth. The SCRs are for high pressure and low temperature gas production service. The Vortex Induced Vibration (VIV) suppression system of these SCRs comprises of a combination of fairings and strakes to achieve the thermal requirements for the minimum target 18 o F arrival operating temperature of the produced gas.Among project firsts is the S-lay installation of the fairings on five of the seven Independence Hub "S-lay installed" flowline SCRs. The other two flowline SCRs were installed using J-lay method. All SCRs including the 20-inch export were laid on the seabed with several crossings. The pick up, cross haul and hang off installation of the SCRs was a significant challenge since the SCRs have to be cross hauled underneath the semi and supported by temporary installation aids while the installation vessel maneuvers to the other side of the semi. The design, fabrication and testing of the temporary installation aids were unique. This paper presents in some details the aforementioned challenges in addition to others that were encountered during the execution of the project. An example describing the 10-inch Spiderman West SCR fatigue re-evaluation based on the steps taken to provide robustness to the SCR design is also demonstrated.