Page 519 - Pipelines and Risers

486 Chapter 25

25.6.5 Step 4- Limit State Equations
Two limit state equations, one for each of the failure modes, these can be expressed as
follows.

Uplift Failure:
gi (X)= W-FL (25.20)

Lateral Failure:
g2(X)= Ru- FD (25.21)

where:
W= submerged weight of pipe
FL= hydrostatic uplift force
Ru= Resistance of Soil (friction)
FF Hydrostatic Drag force

25.6.6 Step 5- Definition of Variables and Parameters
As can be noticed from the equations 25.20 and 25.21, there are few variables to be
considered. However, a greater amount of complexity can be added to the model by
introducing probabilistic variables.

25.6.7 Step 6- Reliability Analysis
The reliability analysis could be performed using SYSREL (as in the previous example). It is
important to note the type of probability of failure that is determined in this procedure. For
this example the failure would be a time independent failure, since the forces causing failure
(currents and wave action) are random in nature.

25.6.8 Step 7- Cost of Consequence
Movement of the pipeline could result in buckling, this could result in similar consequence
scenarios as those presented in the previous example. Alternatively, the consequence may be
to stabilize the pipeline further. This is a very case-specific matter, which would require
further details.

In determining the cost consequence it is necessary to use the time value of money principles
to determine the NPV of cost of consequences.
25.6.9 Step 8- Expected Cost
By multiplying the cost of consequence and the risk found, it is possible to determine the
expected cost of failure.