Jack-Up Rig

A type of MODU

A jack-up rig is a type of mobile offshore drilling unit (MODU) designed for shallow-water operations. It consists of a buoyant hull fitted with three or four retractable legs that can be lowered onto the seabed. Once positioned, the hull is raised above the water surface, providing a stable platform for drilling. The cantilever can be extended over a platform to align the drillfloor rotary table above the well access.

A jack-up rig is normally associated with shallow water drilling and generally operate in water depths up to 400ft. For depths larger than this a semi-submersible or drillship is required.

A jack-up has some advantages inherent in its design

Mobility - Although the rig is not designed to move under its own power, tow boats are used to move the rig from location to location.

Stability - When on location, the rig floor and working area is raised above sea level providing a stable platform that is not affected by sea state limiting the potential for weather interrupting operations.

Cost - Jack-up rigs cost less to build and operated compared to semi-submersibles and drillships.

Well Construction - Since a jack-up is elevated above sea level, all the well construction equipment (wellhead / BOP) is simplified and cheaper compared to subsea items.

Disadvantages of a Jack-Up

Leg Penetration - Since the legs interact with the seabed, a study is required to ensure the seabed is stable to support the rig weight. Rapid penetration and punch through issues can cause the rig to capsize during placement.

Leg Stuck - If the rig is in place for a long time, the legs can become stuck and difficult to retrieve

Leg Integrity - RPDs (Rack Phase Difference) can be generated while getting into position which can damage or break the legs.