A drilling riser, also sometimes known as an oil riser, is a large-diameter pipe or series of concentric pipes which connects the drilling string in a seafloor oil well to a surface drilling platform. Drilling risers are divided into two categories, based on whether they are suspended from a fixed or highly stable floating platform or a free-floating drilling vessel.
Tie-back drilling risers are intended for use with fixed platforms and high stability floating platforms, whether or not they are moored. Tension-leg platforms create this stability with four corner tethers, the spar design with so deep a hull draft that as much as 90% of its structure is underwater. The stability is necessary because tie-back drilling risers consist of a single rigid pipe or series of concentric pipes with little give. This design is used with a surface blowout preventer.
Marine drilling risers are designed for use with floating rigs without deep stability or any kind of mooring. The only part of the rig which makes contact with the sea floor is the riser itself. This design is used with sea floor blowout preventers. Thus, in addition to the basic low pressure extensor pipe, a marine drilling riser must also include auxiliary lines for the continued operation and control of the blowout preventer, such as power lines and high pressure choke and kill lines.
When drilling in anything other than the most shallow of waters, the marine drilling riser must also be tensioned against the forces of wave and wind. Otherwise, each time the drilling ship rises and falls on the waves, the riser would be stretched or buckled, forces which would also be transmitted back to the casing on one side and the rig on the other.
To protect the entire rig structure, marine riser tensioners must be mounted between the rig and the riser. The springs themselves are enormous hydraulic cylinders, sheaved at either end, which absorb and release vertical tension as the waves go up and down. The specific design and degree of tension required is a sophisticated equation which takes into account the expected range of water forces in the area, the various masses of the equipment, the drilling fluids, and the floating platform, the drilling riser buoyancy, and an extra margin to allow for equipment failure.
Regulations governing the operation, selection, design, and maintenance of marine riser systems are governed by the International Organisation for Standardisation, document ISO 13624-1:2009.
