Fabrication and Installation - Offshore Structures: Design, Construction and Maintenance - page 307

dimensions within the allowable tolerance. Tolerances should be checked at
each stage in accordance with the fabrication procedures and the final survey
should meet the defined tolerances. Fitters should be doing their own checking
as the job progresses and their tape measures and straight edges do not need
calibration. The final survey should be on complete sub-assemblies and on
the completed structure after PWHT if applicable.
Based on ISO, personnel responsible for the final survey either should be
qualified surveyors or should have had at least 5 years

experience of similar
work. Instruments used should be in accurate adjustment and should have cur-
rent, valid calibration certificates.
According to ISO, tolerances are guided by the following:

'

Local tolerances should be controlled for structural components and sub-
assemblies so that the accumulation of such tolerances does not affect the
specified global tolerances.

●

The specified tolerances should apply at all stages of fabrication and
assembly.

●

Allowance should be made for weld-gap tolerances and weld shrinkage in
all component, sub-assembly and global tolerance calculations.

●

Where tolerances have to be derived from a formula to be expressed in terms
of a component dimension, such as wall thickness, the results should be
taken to the nearest mm (0.04 inch).

●

Tolerances should be based on theoretical setting-out points and centerlines
of the structure referenced to permanent approved datum points, such as the
coordinated survey stations, and should be corrected to a temperature of
+20

●

º C (68 º F).

1
4
inch) of the appropriate position shown on the approved workshop drawings.
Where no such drawings exist, fabrication should be carried out from a level
plane to within

Fabrication and yard assembly supports should be set to within ± 5mm( ±

●

1

± 5mm( ±

4 inch).

The dimensional tolerance of launch-way centerlines should be within ± 20mm ●

1
4
inch) of its reference elevation. The variation in elevation between any
two points on a launch-way should not exceed 3 mm ( 8 inch) within any 3
m (10 ft).

3

( ±

4 inch) of the theoretical position and should also be within

± 6mm( ±

Leg-spacing Tolerance
The global (horizontal) tolerances for leg spacing at plan bracing levels are as
shown on Figure 5.6 and are detailed below.

The horizontal center-to-center distance between adjacent legs at the top of a
structure where a deck or other structure is to be placed (stab-in nodes)
should be within 10 mm ( 8 inch) tolerance from the construction drawings.

●

The horizontal center-to-center distance between legs at other locations
should be within 20 mm ( 4 inch) of the design values.

●