Page 329 - Pipelines and Risers

302 Chapter 16

P(hit) = 3.105 .180/5542.0.468
= 4.6-107/lift
= 4.6-107nift.20 lifts/rig day500 rig days/
20 yeadtemplate
= 4.6.103/20 yeadtemplate
= 2.3.104/year/template . 17 templates
= 3.9.103/year
To calculate the probability of a dropped object hitting the flowlines outside the rockdumped
area, the procedure above is repeated considering the cone section between the end of the
rockdump and the end of the 99% cone area, giving:
A‘, = ‘II.. (173.2’ - 133’)ln = 38670 m’
A’f = 3 . (173.2-133) . lm = 120.6 m2 (length of pipe & umbilical within A, with a lm
corridor)
Probability of hit within A’=:
133 m167.2 m = 1.979 *P(-1.979<~<1.979) = 0.952
P(hit within A’J = 0.99-0.952 = 0.038
P’(hit) = 3~10~~ 120.6/38670.0.038 = 3.6.10-’/lift
= 3.6.10-9/lift 20 liftslrig day 500 rig days/20 years/template
+
= 3.6.105/20 years/template
= 1.8.106/year/template . 17 templates
= 3 .O. lO”/year

Energy absorbed by steel pipe
The energy required to produce a dent of 3.5% of OD is found from Equation 7 to be 5.2 M.
Only items of approx. 1 tonne will have an impact energy less than 5.2 kJ. It is assumed that
most dropped objects will be heavier than this, and consequently also assumed that all
dropped objects will damage the spool/flowline enough for repair to be required.

This assumption is conservative because the falling object area (the object will not necessarily
indent the pipe in a “knife edge” fashion) and the protection offered by the flowline coating is
neglected.

16.9.5 Consequence Analysis
As stated earlier this example analysis pays little attention to the consequence of pipeline
failure. The only consequence which is considered is the environmental damage that could be
suffered. The damage category which the environment is likely to suffer is ‘minor’.