Page 400 - Pipelines and Risers

Use of High Strength Steel 367

19.2 Potential Benefits and Disadvantages of High Strength Steel
It is clear that the obvious advantage for using higher strength steels is cost-saving. However,
new approaches to design, manufacture and construction and the use of high grade materials
will expose potential pipeline projects to increased levels of technical and commercial risks.
This section identifies the benefits and disadvantages associated with the use of high strength
steels.
19.2.1 Potential Benefits of High Strength Steels

Potential Cost Reduction
Increasing the grade of linepipe used for construction of a pipeline provides the opportunity to
reduce overall material costs. The cost reduction is based on the premise that increasing
material yield strength reduces the wall thickness required for internal (or external in the case
of deep waters) pressure containment and hence the overall quantity of steel required. The
implications of using high grade material are considered in relation to linepipe manufacturing
and pipeline construction.

Price (1993) considered both direct and indirect consequences of using a high strength steel,
and estimated a 7.5% overall project saving for a 42-inch offshore line laid with X80 instead
of X65. Although the X80 pipe cost 10% more per tone, it was 6% less per meter. Further
savings were identified for transportation, welding consumables, welding equipment rental
and overall lay time.

On the recently completed Britannia gas pipeline, cost studies during detailed engineering
showed that by increasing the linepipe material grade from X65 to X70, an approximate cost
reduction of US$ 3.5 million could be achieved. The project CAPEX is approximately US$
225 million.

Although not directly related to the use of high strength material, other potential cost savings
identified in the same study include:

Tighter than normal (API 5L) definition of dimensions. Consideration should be given to
reducing linepipe tolerances on ovality and wall thickness from API 5L requirements. If
reliability-based limit state design is to be used wall thickness tolerances will have to be
specified tighter, according to limit state requirement. The actual tolerances required will
be determined by evaluating potential cost reductions anticipated during pipeline
construction and mechanical design. The cost of reducing tolerances should be compared
to the expected increase in pipeline construction rates and wall thickness reductions for
mechanical design.

Use of fracture mechanics acceptance criteria for determination of maximum allowable
defect sizes in pipeline girth welds. Traditionally, the acceptance criteria for weld defects
is based on workmanship standards. More recently, alternative criteria such as ECA have