Page 145 - Pipeline Pigging Technology

Pipeline Pigging Technology

prior to shipping offshore to the laybarge. This reduced the amount of
welding on the laybarge, and therefore increased the laying rate. After the
welding was completed onshore to form this double joint, a layer of bitumen
was applied for corrosion protection, followed by reinforced concrete infill
- see Fig.7. At the start of pipelaying, where the concrete thickness was
4.875in, it was found that the concrete infill was cracking and spalling due to
lack of reinforcement. The double joints were therefore returned to shore,
and the concrete infill cut off and replaced with stronger reinforcement. All
11 features that were reported by the British Gas vehicle proved to be within
these double-jointed areas. Therefore, we could confidently link all features
to a common construction process, and conclude that the features were
caused by the cutting off of the field joint prior to replacement.
It is comforting to conclude that the 11 features reported by British Gas
could independently be traced back through the pipeline history to a
common fabrication process.
In parallel to investigating the cause of the features, a fitness-for-purpose
assessment was performed. This assessment included:

a determination of the significance of the features with respect to
current pipeline operating conditions; and
a consideration of the fatigue life of the features. The actual tensile and
toughness properties of each pipe joint was used in the calculations.

As all 11 features were located in the line pipe itself and not associated with
girth welds, plastic collapse analysis was used in determining their signifi-
cance.
All the 11 features proved to be insignificant with respect to current
operating conditions, and analysis has indicated that all the features would
have survived the stresses imposed during pipelaying, hydrotest and maxi-
mum operating conditions. Fatigue-life calculations have shown that the
features have a lifespan of over 60 years (the longest time calculated).

CONCLUSIONS

Total Oil Marine believes that the use of intelligent inspection vehicles is
a necessary item within the overall inspection programme of a major pipeline
system. The quality of the equipment now available is able to give the pipeline
engineer reliable information with respect to:

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