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occur during Preload Operations. To alleviate the potentially
catastrophic results of such an occurrence, the hull is kept as close
to the waterline as possible, without incurring wave impact.
Should a soil failure or leg shift occur, the leg that experiences the
failure loses load-carrying capability and rapidly moves
downward, bringing the hull into the water. Some of the load
previously carried by the leg experiencing the failure is transferred
to the other legs potentially overloading them. The leg
experiencing the failure will continue to penetrate until either the
soil is able to support the leg, or the hull enters the water to a point
where the hull buoyancy will provide enough support to stop the
penetration. As the hull becomes out-of-level, the legs will
experience increased transverse load and bending moment
transferred to the hull mostly by the guide. With the increased
guide loads, some braces will experience large compressive loads.
During normal preload operations it is important to keep the
weight of the hull, deck load, and preload as close to the geometric
center of the legs as possible, as this will assure equal loading on
all legs. Sometimes, however, single-leg preloading is desired to
increase the maximum footing reaction of any one leg. This is
achieved by selective filling/emptying of preload tanks based on
their relative position to the leg being preloaded. Preload is water
taken from the sea and pumped into tanks within the hull. After the
preload is pumped on board, it is held for a period of time.
The Preload Operation is not completed until no settling of
the legs into the soil occurs during the holding period while
achieving the target footing reaction. The amount of preload
required depends on the required environmental reaction and the
type of Jack Up Unit. Mat Units normally require little preload.
JACKING TO FULL AIR GAP OPERATIONS
Once Preload Operations are completed, the Unit may be
jacked up to its operational air gap. During these operations it is
important to monitor the level of the hull, elevating system load
and characteristics, and for trussed-leg Units, Rack Phase
Differential (RPD). All of these must be maintained within
design limits. Once the Unit reaches its operational air gap, the
jacking system is stopped, the brakes set, and leg locking systems
engaged (if installed). The Unit is now ready to begin operations.
ELEVATED OPERATING CONDITION
When the Unit is performing operations, no particular
differences exist between the various types of Units. Likewise,

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