Page 509 - Pipelines and Risers

476 Chapter 25

25.5 Fabrication Tolerance Example Using the Lifecycle Cost Model

25.5.1 General
The purpose of this calculation example is to demonstrate the validity of Life-Cycle Cost
modeling as a method by which to justify choices between design alternatives. This example
will look at the practicalities of assessing the failure probability, the cost of consequence, the
implementation of economic theory and the utilization of interval method. This calculation
example I inspired by NMand et a1 (1997), will follow the steps outlined in the introduction.

25.5.2 Background
Pipeline fabrication quality is one particular aspect of pipeline design that could give potential
cost savings over the life cycle. Good quality in the fabrication of pipeline can increase the
safety and thus reduce cost of unplanned maintenance and cost of consequences. However,
too stringent quality requirements can drive up fabrication costs and this increase of initial
cost may not compare favorably to lesser quality options. This design example will compare
the Life-Cycle Cost of two different fabrication qualities, in terms of the probability of failure
due to corrosion, and thus arrive at a judgement as to which fabricator is more economically
viable.
25.5.3 Step 1- Definition of Structure
The structure to be considered is a subsea pipeline.

25.5.4 Step 2- Quality Aspect Considered

The quality aspect that is to be considered in this example is the fabrication tolerance that is to
be used. This calculation example will consider two fabricators each of which produce a
different quality of pipe. The different qualities of pipeline will be implemented into the
problem through a random variable, modeling the uncertainty in wall thickness. The exact
nature of this variable is described fully in Step 5- Definition of Parameters and Variables.

25.5.5 Step 3- Failure Modes Considered
In order to simplify the scope of the example, the only design aspect which is to be considered
is the design criteria for corrosion allowance, from this only two failure modes are likely,
these are hoop stress and hydrostatic collapse.

25.5.6 Step 4- Limit State Equations
1. General
By considering corrosion depth as the load and wall thickness as the resistance, it is possible
to apply Load-Resistance Factored Design 0.W) methodology to pipeline corrosion
allowance design. This introduces a welcomed opportunity to take into account the range of
uncertainties inherent in corrosion rate calculations and residual strength of corroded pipelines
in the design.

2. Corrosion rate and defect length