This paper discusses the challenges of manufacturing and installing large diameter Steel Catenary Risers (SCRs), based on the experiences of manufacturing and installing the 16-inch seamless and 24-inch UOE risers for the BP Mardi Gras project in 4,500 feet water depth. Some of the challenges included:
Pipe dimensional control (from stringent mill tolerances to an elaborate measuring, sorting and machining program)
Controlling the mismatch of the internal diameters between adjacent pipe sections at welds ('Hi/Lo') throughout the welding process
Obtaining reliable Non-Destructive Examination (NDE) records by Automatic Ultrasonic Testing (AUT) despite large variations in the ultrasonic attenuation on the forged material used for J-lay collars
Logistics coordination to assemble the risers comprising three different coating systems, strakes, J lay collars, flex-joint assemblies, and in one instance a riser monitoring system
BP and its partners are developing several deepwater prospects in the Gulf of Mexico. These include the Holstein, Mad Dog, Thunder Horse and Atlantis floating production facilities in water depths ranging from 4,500 feet to 7,300 feet. Oil and gas will be exported via the Mardi Gras Transportation System. The deepwater development consists of a combination of 16-inch to 28-inch oil and gas trunk and lateral pipelines which are tied-in to floating production platforms using steel catenary risers (SCRs).
Each SCR is effectively a suspended pipeline, obliged to move with the host facility under the influence of environmental forces, and subject to local hydrodynamic forces, including vortex shedding and associated vibration. Accordingly, the SCR design addresses the potential fatigue related failure modes by using a flexible connection to the host facility, and by control of weld root profile and permissible defect size throughout the riser length (with the highest weld quality focused in the most critical regions). These measures are combined with the selection of appropriate coatings to prevent abrasion in the touchdown region, and provision of VIV strakes. A further complication on some risers is the provision of a strain monitoring system to collect data for determining the effectiveness of the design measures. A typical SCR general arrangement illustrating these features is shown in Figure 1. The detailed arrangement drawings and specifications for any riser can be quite complex, and require logistics control both onshore and offshore, particularly to address the variables listed below:
Weld acceptance criteria (typically two to three criteria along the SCR length)
Weld procedure (onshore horizontal, offshore horizontal, offshore vertical in tower)
Pipe coating type (three types along the SCR length)
J-Lay collar joint coating (two types)
Field joint coating (two types)
VIV strakes (where required)
Strain monitoring system components (several types)
Materials sparing strategy
