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and in non-design events. The legs of a Jack Up Unit may extend
over 500 ft above the surface of the water when the Unit is being
towed with the legs fully retracted. Depending on size and length,
the legs usually have the most detrimental impact on the afloat
stability of the Unit. The heavy weight at a high center of gravity
and the large wind area of the legs combine to dramatically affect
the Unit’s afloat stability. For Units of the same hull configuration
and draft, the Unit with the larger legs will have less afloat
stability. When in the Elevated Mode, the legs of a Jack Up Unit
are subjected to wind, wave, and current loadings. In addition to
the specifics of the environment, the magnitude and proportion of
these loads is a function of the water depth, air gap (distance from
the water line to the hull baseline) and the distance the footings
penetrate into the seabed.
Generally, the larger the legs and footings, the more load
wind, wave, and current will exert on them. Legs of different
design and size exhibit different levels of lateral stiffness (amount
of load needed to produce a unit deflection). Jack Up stiffness
decreases with increases in water depth (or more precisely, with
the distance from the support footing to the hull/leg connection).
Furthermore, for deeper water depths, flexural stiffness (chord area
and spacing) overshadows the effects of shear stiffness (brace).
Leg stiffness is directly related to Jack Up stiffness in the elevated
mode, thereby affecting the amount of hull sway and the natural
period of the Unit (which may result in a magnification of the
oscillatory wave loads).
EQUIPMENT
The equipment required to satisfy the mission of the Jack
Up Unit affects both the hull size and lightship weight of the Unit.
There are three main groups of equipment on a Jack Up Unit, the
Marine Equipment, Mission Equipment, and Elevating Equipment.
“Marine Equipment” refers to the equipment and systems aboard a
Jack Up Unit that are not related to the Mission Equipment.
Marine Equipment could be found on any sea-going vessel,
regardless of its form or function. Marine Equipment may include
items such as main diesel engines, fuel oil piping, electrical power
distribution switchboards, lifeboats, radar, communication
equipment, galley equipment, etc. Marine Equipment, while not
directly involved with the Mission of the Jack up Unit, is
necessary for the support of the personnel and equipment
necessary to carry out the Mission. All Marine Equipment is

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