Civil Engineering Reference
In-Depth Information
Local eddies formed around the legs due to currents will lead to scour and
will reduce the lateral capacity.
Steel mats are usually built onto the bottom of the legs, so that when the legs
are jacked down they take their temporary support from the sea floor. A short
stub pipe sleeve may penetrate below the mat to provide shear resistance against
sliding. The main leg is jacked down through the sleeve. Since jack-up perfor-
mance is so highly dependent on sea-floor soils, it is essential that a thorough
geotechnical evaluation, including at least one boring, be made at each site. Of
particular concern are layered soils, in which a leg may gain temporary support
but then suddenly break through.
A general rule of thumb is to plot the previous leg positions (if known) and
to space the new leg locations 4
5 diameters away. In clay soils, where jack-ups
have previously worked around the site, holes will have been left that now may
be partially empty or filled with loose sediments. If a leg is seated adjacent to
such a hole, it may kick over into it, losing both vertical and lateral support and
bending the leg. Of course, another advantage of mat-supported jack-up legs is
that the mats can span local anomalies. Furthermore, jack-up rigs are usually
equipped with two sets of six or eight legs.
One disadvantage of the jack-up occurs during the transfer of loads from
barges or supply boats. In this case, the jack-up concept again becomes weather
sensitive, because the barges must not be allowed to contact the legs or they
may damage them.
In general, jack-ups provide a fixed platform, free from motion in response
to the seas.
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5.11 TRANSPORTATION LOADS
All structures should be checked for the inertia loads applied during sea trans-
portation. Consideration should be given to the support points used for sea fas-
tening. The following should be considered:
Structure self-weight
●
Equipment and bulk self-weight
●
Transportation inertia loads
●
Roll, 20
º
; Period, 10 s
●
Pitch, 10
º
; Period, 10 s
●
Heave:
±
0.2 g
●
Center of rotation is 60% above barge keel at longitudinal midship of the
transport barge
●
The transportation inertia loads should be combined as roll
±
heave and
●
pitch
±
heave
Wind loads for a return period of 10 years (1 minute mean) should be
included
●
The support points should reflect the support points adopted during load-out
●
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