Recent years have seen the first uses of steel catenary risers with spread moored FPSOs for deep water field developments in West Africa. Acergy have been in charge of the design and installation of more than 20 Steel Catenary Risers (SCRs) on FPSOs in this area. The design, fabrication and installation of these risers have required many significant challenges to be overcome for the first time. Innovative solutions have been developed and implemented to overcome these challenges particularly in the areas of design, welding and installation. This paper presents some of these challenges and solutions with the applications of SCRs attaching to mono-hull floating production units. Strength and fatigue analysis, on bottom stability, interface with the FPSO, and fabrication issues are described in detail. Lessons learnt from previous projects as well as results of new developments /1/ to extend the suitability of Steel Catenary Risers to deeper developments and to turret-moored FPSOs are also presented. Introduction A steel catenary riser is a seemingly relatively simple system, when comparing to others, where the riser is in continuity with the flowline and is made up from welding a number of rigid steel pipe joints of standard length. The catenary riser is generally connected to a floating platform with a flexible joint, steel or Titanium stress joint to absorb the potentially large angular movement of the platform. The bottom end of the riser pipe rests on the seabed as a beam on elastic foundation. The main concerns for the design of steel catenary risers described in the following sections are:Interface management with the floater,Impact of as-built uncertainties on the static configuration of the riser,Dynamic behavior of the catenary riser,Welding requirements,Installation aids for the final transfer and pulling of the riser on the FPSO /2/. The last section presents some results of new analyses performed by Acergy to improve the dynamic behavior of SCR's with the aim of using them for turret moored FPSO's in West Africa. Interface with the floating vessel Design of the SCR is strongly linked with the characteristics of the floater. Main interfaces are:Location of the hang-off point alongside the hull,Flexible joints designed to sustain great temperaturesStiffness of the floater mooring system and maximum excursion,1st and 2nd order motions of the floater,maximum heel, yaw and pitch in extreme, damage and survival conditions,local structural detail design of hang-off supports and hull reinforcements,Integration of installation aids including transfer and pulling winches,Space and lifting equipment available for precommissioning activities. This list is not exhaustive and a huge number of pieces of information have to be exchanged all along the design phase of the SCR and of the floater /3/. Clear definition of all these needs and requests with associated schedule, open relationship between Contractors and support of the Company when necessary are the key drivers for success of the project. Some of the main lessons learnt are the following /4/:

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractExperiments and analyses have been made to determine the response of a downstream cylinder that is in the wake of a fixed upstream cylinder. This geometry is of great practical importance for risers in deepwater. It arises with jumpers in deepwater hybrid riser towers and in closely spaced risers. The theories of Huse [1], and Blevins [2] are applied to predict the steady response of the down stream cylinder. These predict that within the confines of the wake of the upstream cylinder, the drag on the downstream cylinder is reduced and there is a lift force that acts to draw the downstream cylinder into the wake of the upstream cylinder. There is apparently no simple, direct experimental confirmation of this predicted behavior.An experiment was made in the flow channel of the Scripps Institution of Oceanography to test these theories up to about 80 000 Reynolds number. The paper describes the methodology and results of these experiments to validate the parameters of the theoretical models.Preliminary measurements show that as the flow increases the downstream cylinder travels aft and inward towards the centerline of the wake.At small inline spacing the downstream cylinder moves upstream, implies negative lift, and impacts the fixed upstream cylinder.

As floating production systems move into deeper water applications, flexible risers are reaching hydrostatic collapse and axial tension design limits. It may be expensive to extend the shallow water flexible riser designs to deepwater applications due to its limited capacity in the deepwater applications. Therefore, a catenary or wave shaped steel risers offer an innovative alternative to flexible risers for these cases [1-2]. This paper presents the dynamic behavior comparison of two deepwater riser systems for a floating production system (FPS) with a Turret Mooring in the Gulf of Mexico (GOM) up to 4000 ft water depth. The deepwater riser configurations include a Lazy-Wave flexible riser and a Tension Leg Risers (TLR) with the combinations of Steel Catenary Riser (SCR) and a flexible riser. The work includes 6-inch and 8-inch risers for flowlines and water injection lines and 8-inch and 10-inch for gas export risers. The design methodology and design criteria for these riser configurations are presented. Other than the flexible riser design criteria, the sizing and wall thickness of SCRs are based upon the design pressure, collapse resistance, hydro-test and installation requirement. The configurations developed in these cases are very promising, with well acceptable stress levels. The estimated cost and proposed installation methods are also presented and compared. Furthermore, the concepts that minimize possible drawbacks related to high top hang-off angle, riser compression and soil-riser interaction are given. Introduction The objective of this study is to investigate the dynamic behavior comparison of two riser systems in a FPS with a Turret Mooring in the GOM up to 4000 ft water depth. The deepwater riser configurations include a Lazy-Wave flexible riser and a Tension Leg Risers (TLR) with combinations of Steel Catenary Riser (SCR) and a flexible riser. The work includes 6-inch and 8-inch risers for flowlines and water injection lines and 8-inch and 10-inch for gas export risers. The FPS selected is a typical mono-hull vessel employing Single Point Mooring (SPM) turret production buoy. The FPS can support oil and gas processing facilities and export the separated and treated oil and gas through pipelines to shore. This paper describes the riser system configurations, static and dynamic riser analysis results, installation procedures, schedules, and costs. Design Basis Environmental Conditions and Load Parameters The riser system is designed to operate in the Gulf of Mexico up to 4000 ft water depth. Four different environmental conditions as shown in Table 1 are considered in the riser analysis. Figure 1 shows how the environmental conditions are applied to the FPS. The current profiles assume piecewise linearity and varying with water depth, while the direction is constant with water depth. Table 2 lists drag and added mass parameters applied in the riser analysis. Vessel Data The riser configurations are studied for maximum vessel offset of 12% water depth associated with 100-year hurricane condition and 11.5% water depth associated with maximum loop current. The offsets apply to both near and far positions of the FPS relative to the riser touch down point. Wave-induced vessel motion are prescribed by a set of Response Amplitude Operators (RAO) at the center of the FPS. Riser System Configurations Riser systems evaluated are Lazy Wave Riser (Option 1) and Tension Leg Riser (Option 2). The risers are connected to the FPS, which is moored in 3,700 ft water depth. The overall ship length is 781 ft with 138 ft beam and 67 ft draft.

Measurements have been made of two dimensional motion of an elastically supported circular cylinder in the wake of a fixed upstream cylinder. The experiments were made in a water flow channel with 6.35cm(2.5in.) diameter cylinders with a maximum Reynolds number of 77,000. The elastically supported downstream cylinder moves downward and inward toward the centerline of the upstream cylinder’s wake with increasing flow velocity, indicating the presence of a transverse lift force and reduced drag in the wake. These forces can cause the cylinders to clash. The measured motions correlate with the theory.

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