Page 328 - Pipelines and Risers

Risk Anabsis applied to Subsea Pipeline Engineering 301

Two flowlines and one umbilical are assumed for each template. The probability calculated
considers a hit on any of these three items, for simplicity it is modeled as a total hit area of 3
x(one generalized spool length)x(a lm corridor around each item).

The assessment is based on objects being dropped through the moon pool of the drill rig.
Although objects may be dropped from the cranes, drops through the moon pool are assumed
to be the worst case, as these will normally happen closest to the spools.
A drill rig will be present on the field for the whole lifetime of the field (20yrs). A total of 17
templates has been assumed. This means that the time spent on one template will be 20yrd17
= 425 days. 75 days is added to this to account for increased drilling activities in the pre- and
early production phase, after the lines are installed, giving a total of 500 days of drilling
operations. There will be an average of 20 liftdday during these 500 days, giving a total of
10000 liftsl20 years.
Table 16.9 Basic Data and Assumptions.

Item Unit Value
Water dwth m I 300
II Cone angle I 0

Rig activity: rig daydtemplate/20 years 500
Number of liftdrig day 20
Design life Years 20
Pipeline Outside Diameter mm 259.8
Pipeline Wall thickness mm 15.6

16.9.4 Results

Probabilities
Cone radii are found using simple geometric principles.
Cone radius, end spools: (302 + 3O2)In = 42
Cone radius, end rockdump: (1302 + 3O2)In = 133
X= 300 m . tan30 = 173.2 m
From Equation 16.6 and Table of the standard normal distribution:
o = W2.575 = 67.2 m (In a normal distribution; P(-2.575ut<2.575) = 0.99)
The cone area of the cone section encompassing the spools is:
A, = n . (42)2 = 5542 m2
The spool area within this cone area is:
Af = 60m - 3 . 1 m = 180 m2 (length of pipe & umbilical within A, with a lm
corridor)
Probability of hit within A,:
42 d67.2 m = 0.625 *P(-0.625<x<0.625) = 0.468