Abstract
This paper evaluates the application of lightweight drilling riser systems, comprised of aluminum alloy material that is significantly lighter than steel riser systems. The paper describes the system's design and specifications, and presents operational and maintenance procedures particular to lightweight riser use. The paper evaluates the comparative costs and benefits of upgrading a rig versus using lightweight alloy risers to increase water depth capability, and reviews the performance of these systems after more than a decade of operation.
The first lightweight alloy drilling riser system was deployed in Brazil in 20031. The initial aluminum alloy riser replaced a steel riser on a drill ship to reduce riser weight by 30% and increase the rig's water depth capacity by 2,000 ft. By the end of 2005, lightweight riser systems were deployed on another drill ship and two semisubmersible rigs, also working in Brazilian waters. By switching to lightweight risers, these units could operate in deeper waters without undergoing expensive upgrades. The lightweight riser systems on three rigs of the original rigs are still in use and continue to be in excellent condition.
As water depths increase, risers for drill ships and semisubmersible rigs become some of the most critical components of the drilling operation. Risers must be strong enough to maintain integrity during deployment and operation, be light enough to be handled and supported by the floating drilling unit, and be constructed of materials that resist corrosion while exposed to seawater. Lightweight alloy risers provide a means of increasing the water depth rating of floating rigs by as much as 50% without requiring expensive upgrades. Because of their lightweight construction, alloy risers are easier to handle, require fewer buoyancy modules, and can enable earlier generation floating rigs to increase their water depth capabilities, when compared to using conventional steel risers. The first generation of lightweight deepwater (DW) alloy risers has been used successfully for more than a decade while drilling in water depths up to 7,200 ft.
