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Oilfield Technology
May
2016
Leg strength is governed by two factors: the first is the elevated survival
and operational condition. The analysismethods required for these
conditions are prescribed by classification societies and are thus
comparable for all jack-up designers. The second factor is the installation
conditions (leg-to-seabed first impact of the spudcan, pre-load,
punch-through and leg pulling) – class typically provides very little
guidance on how to approach the required strength analyses for this
criteria.
Themain difference between these two conditions for amodern
design is the engagement/disengagement of the fixation or rack-chock
systems. In the elevated conditions the fixation systems are engaged,
resulting in all leg bendingmoments and vertical forces being transferred
through the leg chords (the vertical elements of the leg) into the fixation
systems and the surrounding steel of the jacking structures and this results
in almost no forces being transferred by the leg brace system. During
installation, the legs are not fixed to the hull as they are still beingmoved
by the jacking systems; this results in a very different force distribution into
the jacking structures, where the vertical forces are transferred through the
jacking systems and the leg bendingmoments aremainly transferred by
the leg upper and lower guides. The guide structure reaction forces in such
a case result in large shear forces being transferred through the legs, which
are taken by the brace system. As classification rules have neither design
criteria nor a prescribed analysismethod it is up to the designer to choose
the best solution for the brace design.
The design philosophy is fundamentally different for designswithout
fixation systems. In such designs, the upper and lower guides resist the
leg-bendingmoments, whichmeans that the full survival leg bending
moments are transferred as shear by the leg brace system. This results in a
very heavy leg design because it represents amuch less effective use of the
structural components and thereforemust be compensated for through
the use of additional steel.
The leg brace design on the CJ43 is based on the X-brace set-up used in
all current CJ-series designs, in comparison to other designs, which use the
outdated K-brace system (when typically no fixation or rack-chock system
is installed) or the newer inverted-K bracing system. Themain advantage
of the X-brace over the inverted-K brace is that every bracemember is
fully utilised for shear force transfer, where the horizontal member of the
inverted-K systemonly acts as a spanbreaker of the chords but does not
transfer any forces.
Shear force transfer capability determines for a large part the
allowable installation conditions, an important parameter for getting the
rig on location and the legs of the CJ-series are therefore designed such
that the leg X-brace system is capable of transferring a large part of the leg
survival forces. By employing this design philosophy, the robustness of
the leg is enhanced and downtime caused by leg damage isminimised,
an itemnot to be overlooked as this results in significant cost to repair and
also causes lost revenue for the oil company and/or contractor as such
repairs usually take at least two to threemonths.
Anothermajor design parameter typically required for working on
fieldswhere the older generation of units have beenworking before is
themaximumspudcan bearing pressure of 39 t/m
2
. The CJ43 canmatch
this value by reducing themaximumpreload capacity, whilemaintaining
improved environmental capability.
Drillingenvelope
The X-Y-cantilever skidding systemprovides the large drilling envelope
of 70 x 36 ft (21 x 11m) and additional deck space as it raises the entire
cantilever and drill floor structure above themain deck by approximately
12 ft (4m). This allows for improvedmaterial handling as forklifts can drive
underneath and thus reduces crane lifts fromPS to SB side and vice versa.
It also allows additional container storage space and provides additional
means to handlemud return, cuttings and SWARFwaste.
The 36 ft transverse reachwith full combined cantilever load
independently of the transverse position, enablesmore flexibility for the
operator to performdevelopment drillingwith long horizontal sections
without the typical hook load capacity limitations encountered by
traditional drilling rigs at the outer transverse position of the cantilever
reach.
VSDcontrolled jackingsystem
Additional safety and reliability is introduced by the VSD-controlled jacking
system. This is a proven jacking system, which allows the rigmaster to get
his rig on site in a safe, smooth and fully controlled process. The system is
fully redundant and each pinion is torque controlledwith stepless-speed
ability. The controlled ramp up and down, with brakes still engaged, results
in a significant reduction of peak loads on the electric systemand reduces
thewear and tear of the leg rack, pinions, gears and brakes. The system
improves the safety of each rigmove as it will monitor RPD values on the
fly andwill informthe jackingmaster if critical limits, set in advancewith
reference to rig capability, are being exceeded.
The fixation (or rack-chock) systems are also VSD
controlled. This system is characterised by smooth and efficient
engagement/disengagement. The total time required for engagement of
all 18 systems, one on each side of the nine leg chords, is approximately
1 hour.
Disengagement of the fixation systems, an issue typical of other
designs, is ensured through the ability to transfer the vertical force fromthe
fixation systems into the jacking systems at zero speed, thus relieving all
forces acting on the fixation system. This, together with the natural ability
of the fixation systemdesign of sliding in/out under the same angle as
the teeth of the leg rack, results in a very low force required to extract the
fixation blocks and thus no uncertainty for the operator during this critical
stage of the operation. Failure to disengage the fixation systemswould
lead to lost time and revenue for the contractor, as it would not be possible
tomove the rig to the next location.
Innerhull anddeckspace
The large inner hull space allows for belowdeck placement of the P-tanks,
where typical jack-up designs in this range have to store this equipment on
themain deck. This arrangement ensures the shortestmaterial handling
routing between the sack store, mixing equipment andmud pits. Themud
pits are arranged so that there are no protruding girders or stiffeners on the
inside and therefore have the best possiblemud quality since there are no
dead zoneswhere solids can settle down, which also eases the cleaning
process of the pits. This in turn reduces corrosion occurringwithin themud
pits.
This rig design also features an additional ‘tween deck level inside the
hull, which allows additional space formachinery and other equipment.
The deck covers approximately half the inner hull area at ‘tween deck level,
providingmuchmore space overall and improved spatial separation of the
different types of equipment.
The cantilever on the CJ43 design is sized such that all mud treatment
equipment is located inside the enclosed part, consisting of two decks,
which createsmore availablemain deck space for the ever increasing
amount of third-party equipment. Being able to place all this equipment
within enclosed spaces also reduces the overall maintenance costs, as it is
no longer exposed to the outside environment.
Reducedweight
Onemight assume that having all the above advantageswould come
at the cost of additional weight. However, by employing newdesign
techniques, modern calculationmethods and using high strength steel in
the highest loaded areas, the CJ43 has a lower weight than comparable
jack-up designs.
