2.7.1 Unless deck structures are designed for
wave impact, a clearance acceptable to the Administration should be
maintained between passing wave crests and the deck structure. The
Administration should be provided with model test data, reports on
past operating experience with similar configurations or by calculations
showing that adequate provision is made to maintain this clearance.
2.7.2 For units designed to be supported by the
seabed the clearance in paragraph 2.6.2 should be maintained.
2.7.3 The structural arrangement of the upper
hull is to be considered with regard to the structural integrity of
the unit after the assumed failure of any primary girder. The Administration
may require a structural analysis showing satisfactory protection
against overall collapse of the unit after such an assumed failure
when exposed to environmental loading corresponding to a one-year
return period for the intended area of operation.
2.7.4 The scantlings of the upper structure should
not be less than those required for the loading shown in the deck
loading plan.
2.7.5 When an approved mode of operation or damage
condition in accordance with the provisions governing stability allows
the upper structure to become waterborne, special consideration should
be given to the resulting structural loading.
2.7.6 The scantlings of columns, lower hulls and
footings should be based on the evaluation of hydrostatic pressure
loading and combined loading including wave and current considerations.
2.7.7 Where a column, lower hull or footing is
a part of the overall structural frame of a unit, consideration should
also be given to stresses resulting from deflections due to the applicable
combined loading.
2.7.8 Particular consideration should be given
to structural arrangements and details in areas subject to high local
loading resulting from, for example, external damage, wave impact,
partially filled tanks or bottom bearing operations.
2.7.9 When a unit is designed for operations while
supported by the seabed, the footings should be designed to withstand
the shock of bottom contact due to wave action on the hull. Such units
should also be evaluated for the effects of possible scouring action
(loss of bottom support). The effect of skirt plates, where provided,
should be given special consideration.
2.7.10 The structure in way of components of the
position mooring system such as fairleads and winches should be designed
to withstand the stresses imposed when a mooring line is loaded to
its breaking strength.
2.7.11 Bracing members should be designed to make
the structure effective against applicable combined loading and, when
the unit is supported by the seabed, against the possibility of uneven
bottom bearing loading. Bracing members should also be investigated,
where applicable, for combined stresses including local bending stresses
due to buoyancy, wave forces and current forces.
2.7.12 The unit’s structure should be able
to withstand the loss of any slender bracing member without causing
overall collapse when exposed to environmental loading corresponding
to a one-year return period for the intended area of operation.
2.7.13 Where applicable, consideration should
be given to local stresses caused by wave impact.
2.7.14 Where bracings are watertight they should
be designed to prevent collapse from hydrostatic pressure. Underwater
bracing should be made watertight and have a leak detection system.
2.7.15 Consideration should be given to the need
for ring frames to maintain stiffness and shape in tubular bracing
members.