Jack-up rigs explained

Added to the archive in 2026 · Reference note, not an original Bennett Offshore page

Picture a steel barge with three or four lattice towers standing up out of its deck. Towed to a site, it lowers those towers until they stand on the seabed, then keeps pushing until the barge climbs up them and hangs clear of the water. That is a jack-up. The industry also calls it a self-elevating unit, and it has long been the most numerous type of mobile offshore platform.

This page is a plain-language companion to two restored Bennett pages: the Jack-up Platforms service page and the jack-up primer that Bennett & Associates wrote with Keppel FELS in 2005. Read this first if the primer feels dense, then go back to it.

The parts

A jack-up has a buoyant hull, a set of legs and a jacking system that moves the hull up and down the legs. The hull is a watertight barge. It carries the drilling or work equipment, the quarters and the machinery, and it floats the whole unit while the legs are raised. Most jack-ups are triangular with three legs or square with four, though designers have built units with six or eight.

The legs come in two broad families. Open truss legs are built from three or four vertical chords tied together by bracing; the chords carry toothed racks. Tubular legs are large steel columns, usually moved by a pin-and-hole system. At the foot of each leg the designer fits a footing. On most modern units that footing is a spudcan, an inverted cone that spreads the load into the soil and gives the leg some resistance to sideways force. Some designs instead set all the legs onto a single bottom mat.

Modern jacking systems use rack and pinion gearing. Hydraulic or electric motors turn pinions mounted in the hull's jackhouses, and the pinions climb the racks fixed to the leg chords. The V-924 liftboat concept, for example, specified 64 hydraulic jacking units with planetary gearboxes, four hydraulic power units and a central console with leg-load and tilt indication.

From tow to working height

You can split a jack-up move into four stages. In transit the hull floats with the legs pulled up high above the deck. Most drilling jack-ups have no propulsion of their own and travel behind tugs or on a heavy-lift ship; liftboats and the newer self-elevating platforms carry their own thrusters.

On location the crew jacks the legs down until the footings touch bottom and start to penetrate. Then comes preloading. The crew pumps seawater into ballast tanks so that the legs carry more weight than they will see in service, and holds that load while the soil settles. Preloading tests the foundation before anyone relies on it. If a leg is going to sink further, the crew wants it to sink now, with the hull still close to the water, and not during a storm with the hull high up.

After preload the crew dumps the extra ballast and raises the hull to its operating height. The vertical distance between the underside of the hull and the wave crests is the air gap. With an adequate air gap, waves, tides and current push on the slender legs instead of slamming into the hull. Leaving the site reverses the sequence: lower the hull to float, pull the legs out of the soil, raise them, move off.

Why water depth sets the limit

A jack-up stands on the seabed, so its legs must be longer than the water is deep, plus the penetration into the soil, plus the air gap, plus enough leg above the jackhouse to hold the hull. Long legs are heavy and catch wind and waves. The Wikipedia summary puts the usual working range below about 120 metres of water, with premium and ultra-premium units able to work in roughly 150 to 190 metres. The tallest jack-up legs on record belong to the Noble Lloyd Noble, completed in 2016 with legs 214 metres tall. Beyond those depths operators turn to floating units, which the companion page on semi-submersible platforms covers.

A short history

Leon B. DeLong built one of the first self-elevating platforms. His DeLong Rig No. 1 of 1950, for Magnolia Petroleum, was a barge on six legs. In 1953 DeLong formed a joint venture with McDermott, and in 1954 they delivered the DeLong-McDermott No. 1 to Humble Oil. It stood on ten legs fitted with spud cans and is usually described as the first mobile offshore drilling platform; it later worked as Offshore No. 51.

The shape most people recognise came from R. G. LeTourneau, a builder of earthmoving machines. In 1954 Zapata Offshore, then run by George H. W. Bush, ordered a rig from him. LeTourneau built the Scorpion beside the Mississippi River near his Vicksburg plant: a barge on three electro-mechanically driven lattice legs. It launched in December 1955 and went to work in May 1956 off Port Aransas, Texas. A second LeTourneau unit, Vinegaroon, followed in 1957.

By the end of 2013 about 540 jack-up drilling rigs were working worldwide. Bennett's own entry into the type came in 1998, when Keppel FELS invited the firm to co-design what became the Keppel B-Class. A rig ordered by UMW Drilling in 2014 was the 75th B-Class unit ordered since 2000.

Three jobs, one principle

Drilling is the classic use. A jack-up drilling unit counts as a mobile offshore drilling unit, the same regulatory family as semisubmersibles and drillships. Service and construction is the second. A liftboat is a small, self-propelled jack-up with a large open deck and a crane, built to support work on fixed platforms; see Liftboats explained. Wind is the third. Turbine installation vessels jack up beside each foundation so the crane can lift nacelles and blades from a steady deck, described on the installation vessels page and on the restored wind farm vessels page.

What goes wrong

Most jack-up incidents start in the ground. Offshore soils vary across short distances, include materials such as carbonates and shallow gas, and are expensive to investigate. A spudcan that meets a strong layer over a weak one can break through suddenly and drop several metres. Engineers call that a punch-through, and it can tilt or overload the unit. Old footprints matter as well. The depressions left by earlier legs, called canholes, can push a new spudcan sideways.

The legs carry their own risk during jacking. When a spudcan rests on sloping ground or slides through uneven soil, the chords of one leg take unequal load and the racks shift slightly out of line with each other. The total offset is the rack phase difference. Excessive rack phase difference wears teeth and guides, overstresses brace welds and can damage the jacking drives, so the designer sets an allowable limit and the crew monitors it.

The structural side of these problems is what the restored structural engineering and naval architecture pages describe from the designer's chair. For the soil side, the companion page on legs, spudcans and preload goes further.

What the primer itself says

The sample of chapter 1 kept on the restored primer page defines a jack-up as a hull, legs and a lifting system that let the unit be towed to a site, lower its legs into the seabed and raise its hull into a stable work deck. It puts a typical modern drilling jack-up at water depths up to 500 feet, in wave heights up to 80 feet and winds above 100 knots. It also explains preload in one sentence: the unit is preloaded on arrival to simulate the largest leg loads it expects, so that the soil will hold once the hull is at full air gap. The rest of the chapter lists the questions the full primer set out to answer, from the components of a jack-up to the way an operator should choose one for a job. The glossary collects the terms with short definitions.

Next in the archive: Liftboats explained · Wind turbine installation vessels · Restored page: Technical papers