Civil Engineering Reference
In-Depth Information
Furthermore, high temperatures tend to induce residual stresses in the structure.
Because of the difficulty associated with thermal distortion, it is normal to
“
cor-
rect
C.
For joints, elastic deflections are also a source of difficulty in maintaining
tolerances. Foundation settlement or displacements under the skid beams and
temporary erection skids must be carefully calculated and monitored.
In the construction plan, each assembly should be completed before the lift-
ing process is started, which requires definition of the location, orientation and
face-up or face-down placement of each assembly to be matched with the lifting
procedure.
Central coordinates for each assembly are usually shown on the layout
drawings. The central coordinates are then used as local benchmarks for erec-
tion of the assembly.
Note that the QC check of dimensions should be performed before and after
welding to verify the measurements by check-through measurement. During
sub-assembly, fabrication should aim for the positive allowable tolerance to
compensate for the shrinkage that will happen due to welding. Perhaps the
most fundamental rule in fitting is avoidance of applying excess force on the
member during fitting prior to welding or to force stresses on unwelded mem-
bers through the welding sequence, since such conditions cannot be foreseen by
the designer and will account for more stress than was considered in the design
calculation.
The sequence of events that applies to all types of assembly is:
”
all measurements to a standard temperature, 20
º
Preparation of assembly support and staging
●
Rough setting of assembly main structure and position tacking. Dimensional
control of assembly main structure.
●
Infilling of secondary structure and position tacking. Dimensional control of
assembly and secondary structure.
●
Pre-weld inspection. Weld-out of structure subject to continuous inspection
and according to approved sequence.
●
Installation of appurtenances, such as anodes, supports, walkways, risers, J-
tubes and caissons, of grouting and ballasting, and of scaffolding, lifting
aids, erection guides and temporary attachments.
●
Overall NDT, dimensional control
●
Blasting and painting or touch up. Removal of temporary assembly supports
and staging.
●
Preparation for transport, lift and erection.
●
In this phase, assembled, sub-assembled and fabricated structures, together
with loose items, are incorporated into the final structure according to the
sequence of erection shown in
Figure 5.16
.
Jacket frames are typically laid out flat and then lifted upward by more than
one crawler crane. Coordinating such a rigging and lifting operation requires
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