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Drilling Riser

The marine (or drilling) riser is the communications link between the drilling

vessel and the subsea wellhead through which downhole equipment is guided and

mud is returned to the surface. It also serves as a running string for the BOP stack.

Improvements in materials and construction have greatly reduced the probability of

riser failure, but proper care and use is still required to reduce riser deterioration.

Without vessel motion, the marine riser would be just a line of pipe like

those used in land operations. Because of these motions, however, the marine riser

becomes a more complicated system, requiring additional special equipment. In

more complex and hostile offshore environments, drilling risers must be designed

to withstand the maximum combinations of riser tension, internal pressure, and

bending moments without overstress or fatigue. The physical demands imposed by

drilling vessel excursions; wind, current, and wave forces; and thermal expansion

require extremely reliable components.

A typical riser system is composed of a lower marine riser package, riser

joints with high-strength connectors, a telescopic joint with terminal fittings and a

tensioning ring, and a diverter. Of course, a tensioning system must be

incorporated and, in some cases, buoyancy modules. Some risers also include a

fillup valve that prevents riser collapse in the event of drilling-fluid pressure loss.

Sometimes, in deep-water drilling, an upper ball (or flex) joint is used with the

telescopic joint. However, the necessity of the additional ball joint is now being

questioned since wells have been drilled in over 2,000 ft of water without an upper

ball joint. The choke-and-kill lines may be attached integrally to the riser.

The lower marine riser package is a preassembled unit that connects the

marine riser to the BOP stack. Usually it is composed of a lower marine riser guide

frame enclosing a hydraulic connector, an annular preventer, a flex joint, and

choke-and-kill flex lines. If a double female hydraulic control system is being

used, the guide frame also houses the intermediate female receptacles, which in

turn carry the control pods.

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