Overview The primary advantage of the jackup design is that it offers a steady and relatively motionfree platform in the drilling position and mobilizes mobilize s relatively quickly and a nd easily. Although Although they originally were designed to operate in very shallow water, some newer units, such as the ultra-harsh environment! "aersk "#$ $%&'-%(' "$, are huge ) Fig. 1* and can be operated in ((' ft in the +". ith &/.0-ft. leg length, a hull dimension of 12%3//3/2 ft, and a variable deck load )456* of %',''' long tons, it is mammoth and rivals some of the larger semis. This type of unit can be commercially competitive only in the 7orth #ea and in very special situations.
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Fig. 1—Maersk’s giant jackup (largest in the world) designed for deepwater use (55 ft in the !OM) and harsh "orth #ea environ$ent. %ourtes& Marine #tructure %onsultants.
'&pes of jackup unit There are two basic types of jackups8 '&pes of ackup nits The 9ndependent-6eg Type Type :ackup
The "at Type :ackup
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;sually three legs with lattice construction The legs are attached to a very large mat that rests on the ocean bottom
+ndependent,leg t&pe jackup unit =or the independent-leg units, preloading! is required to drive the legs into the ocean bottom before the hull is completely jacked out of the water. 5uring this procedure, the jackup "5; is at risk from weather and leg punch through!> i.e., one leg breaks through a hard crust, putting the other legs in a large bending movement. +enerally, (-ft swells and?or a combined sea of @ ft are the maimum seas in which these units can jack out of the water. 9f
the hull should roll, pitch, and heave to an etent that the legs come into contact with the ocean bottom, particularly if it is hard, the legs can be severely damaged. The preload sequence is usually done in stages, with the hull never rising more than ( ft out of the water to safeguard against having a leg punch through. 9f the ocean bottom is soft and consists of clay, it is not uncommon to take & or more sequences, with each sequence taking & to %1 hours. The unitBs pumps seawater into its preload tanks, adding weight to the hull and driving the legs. After the legs are driven and the hull goes into the water, the seawater is dumped overboard and the sequence is begun again. This process occurs until the legs no longer penetrate the ocean bottom. The concept is to load the legs to a level above that which the unit will encounter in the harshest predicted environment. The newer, enhanced premium units do a single preload in which the jacking system is strong enough to jack the unit with all the preload water onboard, the basic weight of the hull, and the full transit 456. This is a significant advantage in that a much smaller weather window! can be acceptable to move the unit. :ackups are most susceptible to major damage or loss when they are floating.
Mat,t&pe jackup unit The mat-type jackup also usually consists of three legs that are cylindrical and are from @ to %1 ft in diameter )Fig. -*. The mat is carried just under the hull during mobilization, usually with C (-ft gap. hen the unit comes onto location, it jacks the mat down to the ocean bottom, and because of its low bearing pressure, usually under ('' to '' psf, the unit jacks the hull out of the water without going through the preload sequence required for independent-leg units.
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Fig. -—'&pical ethlehe$ $at,t&pe jackup under construction. "ote si/e of $at and wh& work0oats and tugs $a& da$age it in shallow water.
;nfortunately, the "at-Type :ackup unit also has several disadvantages8 •
They are very susceptible to damage from any object on the ocean bottom.
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They tow very slowly because the mat and hull are large and create a lot of drag. Their mats are susceptible to being gouged by workboat propellers.
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Their upper hull has limited open deck storage space.
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Their legs sometimes form a wind-induced leg vibration known as vorte shedding at high winds, which can cause them to fail.
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4orte shedding is a form of severe vibration seen with smoke stacks without spoilers.
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"ost mat rigs have cylinders for legs and are structurally limited to shallower water depths, usually less than 1(' to 1&( ft.
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nly a few units have reached /'' ft, and these units have lattice-type legs.
=or these reasons, mat jackups have fallen into disfavor, although they are relatively inepensive and for some well types are more than adequate.
%o$$on factors that i$pact 0oth t&pes of jackups Air gap, or the distance from mean water level to the bottom of the hull while the unit is jacked up in the operating condition, is a critical issue. The bottom of the hull must have a large enough air gap that the largest wave crest will not hit the hull and turn over the rig. Air gaps usually are /( to (' ft, with the larger air gaps in shallower water, because wave heights build as water depth decreases. 9f a unit should work over a platform with a very high deck, air gaps of up to %'' ft are not uncommon> however, this obviously reduces the water depth rating. :ackup water depth ratings generally use a minimal leg penetration of %( to 1( ft, which may not be the case in actual operation. 9ndependent- and mat-leg jackups also come in two types of drill floors, slot and cantilevered. As previously discussed, slot units were initially built in the %2('s through the late %2&'s> however, with bigger platforms, the ability to cantilever the drill floor over the platform had an advantage over the slot units, which could only swallow! minimal-size platforms. As the cantilever moves out to position itself over a well, it generally loses combined drillfloor load rating. The combined loading consists of the hook, setback, rotary, and drive-pipe tension if that tension is hung off the drill floor substructure. +enerally, a minimum cantilever length )C %0 to 1' ft* is required for moving blowout preventers )<Ds* and other items net to the hull. =ull rating is usually accomplished at center positions, but decreases as the cantilever moves further out and the drillfloor moves either side of center )usually E%( ft*. The rating on the etreme cantilever and etreme off-center can decrease by as much as @'F, leaving the unit capable of only light workovers.
'echnological advances of jackup unit ;nlike typical earlier %-million-lbm cantilever load units, the new premium jackups have ratings of at least 1 million lbm. ith the advent of etended reach wells )GHs*, deeper gas wells, and high-pressure?high-temperature requirements, the higher load ratings are required, so many older jackups have been upgraded and enhanced, although not to the etent of some
of the newer premium units built in the late %22's and early 1'''s. The Atwood
Accommodations for a crew of %1'
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1-million-lbm cantilever load rating
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&,(''-psi-working-pressure mud system
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&'-ft cantilever
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0''-ft water depth rating
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&,(''-kip 456 )which is typical of the dozen or so units like the Atwood
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Fig. —2twood eacon enhanced pre$iu$ jackup t&pical of the large and ver& capa0le vintage units of the -s. "ote that the rig is setting a s$all fi3ed platfor$ 0& jacking up to a 141,ft air gap at which ti$e the platfor$ will 0e righted to vertical and then the rig will jack down and set the platfor$ on the sea 0otto$. %ourtes& 2twood Oceanics.
There are more jackup-type "5;s than any other type of "5;. 'a0le 1 shows general information about the various types of major units. "arathon 6e Tourneau )now 6e Tourneau* has designed and built more of these units than any other designer and builder. As shown, the size and capabilities of these units vary widely, with the general trend being for them to get bigger and more epensive with higher drilling and marine capability.
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'a0le 1,ackup *esign and 'heir !eneral %haracteristics
2dvantages of jackup units ;nlike platform rigs, submersibles, and ships, jackups and semis are upgradeable from a technical and commercial standpoint. Howan $o. and 7oble 5rilling, both large offshore drilling contractors with large jackup fleets, have done etensive upgrades and enhancements to units built in the %2&'s and %2@'s. ;pgrading usually consists of8 •
$onverting slot to cantilever units
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6eg strengthening and lengthening with more preload tanks
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9ncreasing environmental capability
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;pdating the drilling package with higher hook loads and installation of top drives
riginally, "5;s were considered to have a life of %1 to %( years, but, through rigorous hull and equipment maintenance and technological updating, some /'-year-old units are considered modern! and well fit for select purposes. hy use a jackupI =or water depths of 1( to /'' ft, there are many units to choose from. #ome can be used in greater than 0''-ft water depth. The jackup is the rig type of choice in certain water depths because of its8 •
#table work platform,
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Helatively inepensive mobilization costs
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Availability
