Civil Engineering Reference
In-Depth Information
The structural dead loads should be generated by the computer program
based on geometry and member properties.
Several load cases should be investigated in the analysis for the equivalent
static force. For each case, the program should calculate reactions, deformations
and member forces and should check all members for compliance with AISC
Code. A 33% stress increase is allowed for all load cases.
3.11 ON-BOTTOM STABILITY
Offshore jacket structures must be temporarily supported by the near-seafloor
soils before driving of the foundation piles. The foundation elements that
bear on the seafloor soils include the jacket leg extensions, the lowest level
of horizontal bracing and mud mats, as shown in
Figure 3.5
. All these founda-
tion elements must be designed to support the weight of the jacket plus any
additional loads imposed by environmental or construction conditions. To
achieve a safe yet economical design, the structural and geotechnical engineers
must coordinate their activities in sizing the foundation elements.
The first step in the design sequence requires that the geotechnical engineer
explore soil conditions at the proposed structure location. By drilling and sam-
pling a boring, geotechnical data are determined for the critical zone, generally
located 10 to 20 m below the sea floor. After the structural engineer provides
the sizes of the jacket leg extensions and lowest level of horizontal bracing,
the geotechnical engineer can then compute the soil resistance developed on
these two foundation elements. Depending on the soil resistance, the length of
the leg extensions may need to be shortened to ensure full penetration. If the
jacket leg extensions and lowest horizontal bracing do not provide adequate tem-
porary support, mud mats must be provided and should be designed based on
bearing capacity of the sea-floor soils, as determined by the geotechnical engi-
neer. Although mud mats come into consideration only later, almost all steel
jackets are built with mud mats as a means of temporary support. Therefore,
mud-mat components are most reliable for on-bottom stability prior to pile
installation.
The mud mat is a component designed for most steel jackets. It has become
a common practice among engineers to include the mud mat in their jacket
design even though the support provided by jacket leg extensions and the low-
est level of horizontal bracing is sufficient to stabilize the structure. The funda-
mental aspects that need to be considered in mud-mat design include the
vertical resistance of the soil provided by the bearing capacity of soil and the
resistance of the structure against sliding and overturning. Another aspect rarely
considered correctly is the settlement of the mud mat.
According to the API code, the bearing capacity analysis should take into
account the combined effect of vertical, horizontal and moment loading.
More heavily loaded mud mats may experience a lowering of soil stiffness,
which can allow load to be transferred to other mud mats.
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