Page 327 - Pipelines and Risers

300 Chapter 16

Probability of object landing within a cone area containing the flowline or spool.
Probability of object hitting the spool or flowline (inside the cone area).

This is expressed in Equation 16.7.
A (16.6)
P(hit) = P(drop).P(A,).-
A,
where:
P(hit)=Probability of a dropped object hitting flowline, spool andor umbilical
P(drop)=Probability of an object being dropped
P(&)=Probability of a dropped object hitting the cone area &
AFArea of flowline, spool andor umbilical within Ac, assumed = length x 1 m.

Energy absorbed by steel pipe
The energy required for a knife edge indentor to produce a dent in a pipeline may be
calculated as follows:

E, = 25.SMYS. t2 (16.7)
where:
SMYS= Specified Minimum Yield Strength
t= wall thickness
A= dent depth, assumed max. 3.5% of OD based on serviceability
OD= outside diameter

The effect of coatings and surface area of the falling object is conservatively neglected in
Equation 16.8.

Basic Data and Assumptions for risk analysis
This example will consider the hit probabilities for a generalized L-spool. Table 16.9 presents
the basic data for these calculations.
A lOOm section of rockdump is assumed to follow directly after each spool. The hit
probabilities are calculated for two areas:

Probability of hitting the spool between the template and the start of the rockdump
Probability of hitting the pipeline outside the rockdump, but inside the 99% cone area

The probability of the line being hit outside the 99% cone is considered negligible.