Jackup Terminologies and Types

In early 1955 ( before I was born in '59 and I started to work only in 1980, see my blog article on pressure vessel design ), the first 3-legged jack-up appeared on the offshore scene. The rig was the R.G. LeTourneau jack-up, the Scorpion, for Zapata Offshore Company. The Scorpion, an independent leg jack-up, used a rack and pinion elevating system on a truss framed leg. The rig worked very successfully for several years but was lost during a move in the Gulf of Mexico. The Scorpion was closely followed by The Offshore Company Rig No. 54. For Rig No. 54, however, a hydraulic jacking system on a trussed leg was used. These jack-ups were followed by Gus II, a mat supported unit using a hydraulic jacking system, which was built by Bethlehem Steel Corporation.

Those early breed of jack-ups were primarily designed to operate in the U.S. Gulf of Mexico area in water depths up to 200 feet. Wave heights in the range of 20 to 30 feet with winds up to 75 mph were considered as design criteria for these units. In most cases, in the event of a pending hurricane, the rigs were withdrawn to sheltered areas. Jack-ups can be either self-propelled, propulsion assisted, or nonpropelled. The majority of jack-up rigs are non-propelled. The self-propelled unit, although very flexible, requires a specially trained crew of operators as well as a better trained rig drilling team.

Jack-ups have been built with as many as 14 legs and as few as 3 legs. As the water depth increases and the environmental criteria become more severe, we find that to use more than 4 legs is not only expensive but impractical. The prime forces on a jack-up are generated from the waves and currents, hence, the less exposure to the waves and currents the fewer the forces being developed on the unit. From this standpoint the optimum jack-up is the monopod or single leg unit.

Problems other than wave forces, however, must be overcome with the monopod type unit. But in areas such as the North Sea with very rough' seas there is a need for the monopod jack-up.

When evaluating which type of jack-up to use, it is usually some of the criterias to consider :

1. Water depth and environmental criteria.

2. Type and density of sea bed.

3. Drilling depth requirement, environmental conditions.

4. Necessity to move or stop during hurricane or storm season.

5. Capability to operate with minimum support.

6. How often it is necessary to move.

7. Time lost preparing to move.

8. Operational and towing limitations of the unit.

The independent leg unit depends on a platform (spud can) at the base of each leg for support. These spud cans are either circular, square, or polygonal, and are usually small. Nowadays, spudcan bottom comes with tips for better holding on ground. The larger spud can being used to date is about 56 feet wide. Spud cans are subjected to bearing pressures of around 5,000 to 6,000 pounds per square foot, although in the North Sea this can be as much as 10,000 psf. Allowable bearing pressures must be known before a jack-up can be put on location.

Jackup Slides Ckw


Le Thourneau rigs have been the majorities in the Gulf of Mexico and most of them operating in the region are coming to thirty years or more in operating life. Some have gone through many upgrades, eg, increasing the cantilever outreach and hook load increase.

Le thourneau jackup