Drillships and semisubmersibles garner a lot of attention throughout the offshore drilling industry based on their ability to open up new frontiers in deepwater exploration and development as well as the high dayrates they command. Arguably though, the workhorse of the offshore drilling fleet is the jackup. Jackups have been earning wages on continental shelves around the world since the mid-1950s. Our goal here is to describe a few design characteristics of this type of drilling rig as well as to shed some light on its operational capabilities.
On a very basic level, a jackup’s major components include the hull, drilling package and legs with their associated jacking systems. In addition to housing living quarters, machinery and storage spaces, the hull provides buoyancy to the rig when it is in tow from one location to the next. The drilling package consists of the derrick and hoisting equipment, pipe racking area, drilling equipment and pipe handling tools. The legs of a jackup can be made of either thick-walled pipe or steel lattice depending on the design of the rig. Most modern rigs are designed with three legs but there are a few operational 4-legged jackups being used today.
As a jackup nears a predetermined drilling location, the legs are lowered to just above the seabed. Once on location, the legs make contact with the seabed and the hull is jacked a few feet out of the water. Jacking stops at this point and an operation known as preloading begins which involves taking on seawater to increase the weight of the rig beyond its operational weight. Preloading allows the legs to penetrate into the sediments of the seabed and compact the soil underneath to establish a firm base.
Once preloading is completed, the hull will be jacked up to a height appropriate for environmental conditions that can reasonably be expected while the rig is on location. The height, known as air gap, ensures that storm waves will not contact the bottom of the hull which could damage the legs or move the rig off location and compromise the integrity of the well.
Some older rigs were designed with a large notch in the stern section of the hull known as a slot. The drilling package was constructed on a substructure above the slot. If an operator needed wells drilled through an existing production platform, their rig choices were limited to those with slots wider than the platform. To remedy this issue the cantilever drilling package was invented.
No slot in the hull is needed for a cantilever rig which increases buoyancy and below deck storage. The substructure containing the drilling package is built on giant skid beams. While under tow, the drilling package is positioned near the center of the rig. Once on location and jacked up, it is skidded out over the stern of the rig. This allows cantilever rigs to work over platforms regardless of their width as long as it has enough leg length to elevate higher than the platform.
Perhaps one of the most identifiable images of the offshore drilling industry is that of a jackup drilling rig’s legs silhouetted against the sky. The first jackups were built in the 1950s and could only operate in maximum water depths of around 100 feet. As rig designs and technology improved, deeper waters became available to the jackup market. Today, designs such as the GustoMSC CJ70, can operate in up to 500 feet of water.
Within its water depth range, a jackup has a major advantage over floating rigs in that it provides a stable platform from which to carry out drilling and completion operations. With the base of the legs firmly implanted into the ocean floor, there is no need for expensive motion compensation equipment such as riser and guideline tensioners and drill string compensators. A by-product of this is that there are less things to break which results in increased operational efficiency and reduced maintenance costs. Another cost advantage the jackup has over its deepwater cousins is that they typically require less crew to operate the rig. Well control incidents become somewhat less complicated as well since most operations are carried out with surface BOP stacks.
Currently, over 500 jackups are operating throughout the world with 121 newbuilds under construction. By comparison, there are roughly 190 semisubs and just over 100 drillships, according to Rigzone’s RigLogix. This piece is intended to provide only a basic overview of jackup design and operational characteristics. For more information, please check the How It Works section at Rigzone.com.