SELF-ELEVATING (JACK-UP) PLATFORMS

‘Jack-ups’ comprise about half of all the mobile rigs in the world, and are used for shallow-water drilling. They are self-contained platforms resembl- ing a flat bottomed barge hull with three, four or more vertical legs fitting through openings on the outer hull edges. These legs have ‘teeth’ notched into them and can be raised or lowered by a jacking mechanism on the deck that usually employs a hydraulic or electric rack and pinion arrangement.

In its drilling mode the barge hull is raised on its legs well out of the water and serves as the drilling, storage and living platform. It is thus firm and sta- ble and experiences none of the motions due to the sea that affect floaters.

After the well has been completed the ‘drilling package’, which includes the drill floor and the derrick, is ‘skidded’ clear of the wellhead, and the barge hull is jacked down the legs until it floats freely. Jacking continues, with the legs now being supported by the floating hull rather than the hull being supported by the legs. The legs are jacked up at least until their bottom ends are sufficiently clear of the sea bed to permit safe clearance during the rig-move to the next location.

In its ‘transit condition’ the legs of a jack-up rig can usually be seen tower- ing high above and around the drilling derrick, giving an impression of insta- bility. There have been several accidents during jack-up transits, but the vast majority of rig-moves are carried out in complete safety.

On arrival at the next location the legs are jacked down until they touch the sea-bed. The jacking continues after a test period called ‘pre-loading’, the legs penetrating the sea-bed for some distance while the barge hull begins to climb up the legs. When the hull is high enough to be clear of the highest waves expected at the location, the legs are locked and remain in this position until the well has been completed.

Most jack-ups have vertical legs, but several incorporate legs which slant outwards at the bottom to obtain a wider standing ‘spread’ and better stability. The legs are all independently jacked, and their position can be adjusted so that the barge stands horizontal on a sloping sea bed. The legs of some units are fitted with a large, flat steel frame at their lower ends, cal- led a ‘mat’. This affords better stability on some bottom soil types and reduces the danger of capsize due to scouring.

Cantilever jack-up rigs drill overside.

Jack-up rigs are normally stable in their drilling mode, although there have’been some instances of rigs collapsing and sinking when a leg has either sunk into the sea bed or its steel structure has failed. They are, however, designed to withstand strong wind and wave pressures and are usually only at risk from the sea during the critical hours of the jacking phase when the hull is either just leaving or just returning to the position, and during bad-weather transits when their low ‘freeboard’ allows water to come aboard the hull easily.

Early types were dogged by the excessive penetration by their legs of soft sea beds, but this has largely been eradicated in recent types by the fitting of large feet called ‘spud cans’ or ‘spud tanks’ at the bottom of the legs.

ever, as the legs of modern rigs get taller the problem of structural bending stresses in them is becoming more difficult to resolve. For this reason some jack-ups’ legs are shortened by dismantling during long transits when there is likely to be much rolling or pitching motion.

The deepest water normally operated in by jack-up rigs is about 400 feet, which means having legs almost 600 feet tall to enable the hull to be jacked clear. Most of the jack-up rigs worldwide, however, do not operate in water more than 350 deep. The vast majority of these are not fitted with propulsion and have to be towed or carried on special ships between loca- tions.

The following particulars are for a typical modern jack-up unit that can drill in water of 300 feet depth. This is a ‘cantilever’ type in which, for dril- ling, a strong platform that supports the entire drill floor and derrick (the drilling package) is moved outboard on tracks until it is projecting over the after end of the hull. An alternative to this design is for the drilling to be done through a drilling slot in the barge hull. The drilling equipment and its use

are described in more detail in Chapter 4 and Length overall

Width overall Depth of hull Deck area Leg length

Diameter of spud tank Depth of spud tank

Distance from centre of fwd leg to centreline of aft legs

Maximum distance of rotary centreline from stern edge

66

Projection of leg below hull in normal tow Transit displacement

Jacking speed

Cantilever beam spacing Maximum water depth Assumed penetration of legs

Air gap (between sea and underside of hull) Maximum wave height

Surface current velocity Bottom current velocity Maximum drilling depth

10,500 tonnes

(300 feet) 7

20m

7617m (25,000ft)

JACKING SYSTEM: electro-mechanical rack and pinion type, total jacking

capacity 9000 tonnes.

POWER SUPPLY: 3 x 12 cylinder supercharged diesel engines driving 3 x

alternators.

Height (147ft); base llm x llm (36ft x 36ft); load capac-

ity 1.39 million lbs. (620 tons)

DRAWWORKS: 2000 hp.

BOP STACKS: 1 x 5,000 psi stack (annular)

1 x 10,000 psi stack (single ram)

1 x 10,000 psi stack (double U preventer) 2,000 psi (annular)

1 x 2,000 psi (single ram) 1 x 10,000 psi choke manifold .

PUMPS: 2 x 1,600 hp

ACCOMMODATION: 38 x 2-men cabins

2 x l-man cabins 1 x 3-man hospital

Upper plan view and side profile of a cantilever jack-u

Top: The hull section of a jack-up rig. Bottom: A cantilever jack-up rig drilling through form jacket.

This rig is roughly triangular in hull shape, with the apex of the

forming the forward end and the cantilever beams projecting over the after end. The accommodation block stands on the hull just aft of the forward leg, and aft of that are the pipe racks. The drilling package, which is capable of ‘slant drilling’ (see Chapter 5) is further aft on the cantilever beams which extend the middle of the deck. It can be skidded completely off the rig and onto a platform jacket so that slant drilling can be carried out from the cor- ners of the jacket, but before transits it is skidded to the centre of the main deck.

Beneath the main deck, inside the hull, are large spaces for generating machinery, electrical equipment, mud tanks and mud pumps, and below this level, in an ‘inner bottom’, are numerous tanks for ‘pre-load water’, ‘drill water’, potable (drinking) water, fuel oil, and drilling fluid or ‘mud’. There are numerous designs of jack-up in service but most follow this general pat- tern.