Deepwater Production Riser
Authors: Panicker N.N. , Yancey I.R. 
Published: December 2007

The design of a deepwater production riser is described. The compliant riser design consists of a lower rigid section terminated about 200 ft [60 ml below the sea surface with a tensioning buoy assembly and a flexible upper section made of a bundle of flexible pipes. It is designed to be compliant to waves, currents, and pipes. It is designed to be compliant to waves, currents, and vessel motions and applicable to both mild and severe environments. Design, analysis, testing, and installation are discussed.

Introduction

The deepwater production system consists of clustered well production manifolds on the seafloor, a floating production production manifolds on the seafloor, a floating production facility at the sea surface, and a production riser to link the two. The system (Fig. 1) contains five major components.The subsea atmospheric system (SAS) consists of a multiwell template through which directional wells are drilled, split trees in the wet consisting of master valves and wellhead connector assemblies, which also provide vertical access, and a dry one atmosphere habitat that houses the well control valves, chokes, manifolding, and instrumentation required for remotely controlled producing operations. producing operations.The flowline bundle connects SAS units to a central riser manifold. The special installation method was described in Ref. 3.The central riser manifold or subsea atmospheric riser manifold (SARM) is designed to manifold production from several SAS units and interface with the production riser.The production riser is the subject of this paper.The floating production facility (FPF) accommodates producing facilities and the equipment for injection of producing facilities and the equipment for injection of gas and water, and also provides segregated storage and direct offloading capability.

In this system, the production riser serves to transfer the produced fluids from the seafloor to the surface and injection produced fluids from the seafloor to the surface and injection fluids from the surface to the seafloor. It is a bundle of pipes that also includes conduits for communication, control, power, and habitat life support. The flexibility needed for production requirements and for withstanding waves and currents for continuous service through weather extremes makes deepwater production riser development a technological challenge.

Description of the Compliant Riser

The compliant riser shown (Fig. 2) consists of a rigid lower section of steel pipes and subsurface buoy and a compliant upper section of a flexible pipe bundle. The lower section stays relatively undisturbed and requires minimal maintenance, although each line is individually retrievable. The upper section, being compliant, is continuously in motion and is considered the most vulnerable section. Also, each pipe can be replaced individually from the FPF. The riser is designed to provide these services: production, gas injection, water injection, gas lift, well test, TFL service, air supply, vent (two lines), and nitrogen supply. It also accommodates power bundles and electrical and fiber optic cables. The number and size of lines depends on the production rate and other requirements of field development. The compliant riser can be designed to meet all practical requirements because there are no inherent limits on production rate or water depth imposed by the configuration. Production rates are accommodated by line size and multiplicity of lines. The compliant upper section design is unaffected by water depth. The rigid lower section is dependent on water depth and environment. Studies to 6,000-ft [1800-m] water depth have established its feasibility for all environments within normal ocean current conditions.

Rigid Lower Section

The rigid lower section comprises a circular array of steel pipes, about 6 ft [ 1 .8 m] in diameter with a structural core. A subsurface buoy assembly terminates the lower section about 200 ft [60 m] below mean sea level and keeps the riser under tension. The structural core is made of 48-ft [ 15-m] pipe joints of 12-in. [30-cm] OD. Its bottom connection to the base of the riser manifold and top connection to the subsurface buoy are through tapered joints of varying section modulii. The tapered joints at the top and bottom avoid excessive bending stresses in the structural core. The structural core is not a fluid conduit but supports guide funnels at every 16-ft [5-m] interval through which the flowlines are vertically guided. In deep water, additional buoyancy is provided to the core by syntactic foam modules between the guide funnels, shaped to accommodate the flowlines. All lines can be retrieved individually from the floating platform. To prevent individual lines from becoming disconnected at the joints prevent individual lines from becoming disconnected at the joints and to enable disconnection from the bottom attachment, antirotation tool joints will be provided. The subsurface buoy assembly (Fig. 3) has a main buoy at the top and an auxiliary buoy directly below. The smaller auxiliary buoy is used to support the structural core during installation. The fabricated steel main buoy is compartmentalized and air-pressurized with redundant compartments for safety in case of damage or failure.

JPT

P. 1392