Page 526 - Pipelines and Risers

Design Examples 493

The cost of the pipeline was estimated at approximately 7 billion NOK ($1 billion US), where
approximately 66% is material costs, 29% installation costs, 2% management and design and
the remaining 3% insurance and miscellaneous items.

26.3.2 Design Approach
The project approach was to focus the design effort where the largest cost benefits to the
project could be made. The areas that were focused on were:

Wall thickness design
With tight control of the material characteristics for linepipe manufacture and welding during
installation it was possible to have a hoop stress usage factor of 0.80. This results in a 10%
saving in wall thickness, representing a 5% saving of the overall development cost.

Reliability design: the transported gas presented some potential corrosion risk to the pipeline
over the 50 year design life. Based on conventional approaches a nominal corrosion
allowance of 3mm was defined which would accommodate a limited number of ‘upsets’. This
approach was reviewed based on the increased ‘reliability’ of adding corrosion allowance
compared with having no corrosion allowance and controlling the number of ‘upsets’. The
increased reliability of the system with and without the corrosion allowance was relatively
small - and on this basis it was recommended to not have the corrosion allowance and use
some capital to better control the potential upsets. The saving would represent 5% of the
overall development costs. However, the project elected not to proceed with this approach due
to the uncertainties of gas composition of the 5 future tie-ins dong the AT route.

Pressure control regulation: the normal design is based on incidental pressures going up to
110% of design pressure during operation. If the system can ensure that incidental prcssurc
does not exceed 105% then the wall thickness can be reduced accordingly. This represented a
3% saving in wall thickness corresponding to another 1.5% saving in the overall development
cost. However, the project elected not to proceed with this approach due to the uncertainties
of controlling the incidental pressure from the 5 future tie-ins along the AT route.

Intervention design
Due to the extremely rough terrain a conventional design approach represented an
intervention cost of 15% of the overall development cost, this presented itself for large
potential optimization.

Trawl board protection
Based on stress design a trawl board impact would result in unacceptable stresses, this implied
that the pipeline would not be permitted to freespan. The design demonstrated, that if strain
based design could be applied, then the associated bending moments would be low. Removal
of trawl pullover as a limiting design criteria reduces the intervention by 60%, representing a
2.5% saving in the overall development cost.