ssc-367 - Ship Structure Committee

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The water particle kinematicsare assumedto be unaffectedly
the presence of such members. When the cylinders are spaced
so that they are at least 3 or 4 diameters apart, the wave
inertia forces on one cylinder are relatively unaffected by
the presence of the other cylinders as the radiation effects
are small.
Since platform member diameters are typica” ly less than 3 feet
(2.Om) for braces and less than 6 feet (2.0 m) for legs, the two
basic premises are valid. Even if a 10 foot (3.Om) diameter leg is
utilized, for a wave period of 6 seconds the wave length-to-leg
diameter ratio is in excess of 18. Thus, diffraction effects are
smal1.
However, a45 foot (14m) diameter column of a tension leg platform
will have a wave length-to-columndiameter ratio of only about 4 for
a wave period of 6 seconds. The columns are likely to be only3 to
4 diameters apart. The column spacing is even less for a
semisubmersiblehavingthree columns on each pontoon. Thus, the two
premises are not applicable for structures made up of large
members. The water particlekinematicsat member centerlinesareno
longer valid for smallwave periods and the presence of such members
in the proximity of others affects the water particle kinematics.
Although the stickmodel ofa platformcan be modeled from one joint
node to another, the applied loads could be in error by 2% to 3%
because the loadings on member ends within the chord are computed
more than once due to member overlaps, Most software packages
include an option to define the member ends within the chord,
preventing multiple computation of the applied loads, buoyancy and
weight at each joint.
Accurate definitionofa ship’sdeck strength is importantto define
the box-girder-likeresponse of the entire hull. If a strip method
is not used, the plate elements of the model used in a diffraction
analysis (for loads) and the finite element analysis (for stresses)
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