Page 183 - Pipelines and Risers

156 Chapter I I

113.2 Largest Trawl Gear in Present Use
Table 11.2 indicates presently applicable data for the largest trawl boards in use in the North
Sea in 1995 as follows:

Consumption Industrial
Polyvalent V-board V-board

mass 0%) 3500 2300 1525
length x breadth (m) 4.8 x 2.8 3.8 x 2.25 3.1 x 2.4
trawl velocity (mls) 2.8 2.8 1.8
L

As for future developments or changes in equipment, these must be accounted for by
investigating possible changes within the lifetime of the pipeline. Trends are going towards
improved design in order to optimize trawl board shape and in this way reduce the power
needed to drag the trawl, hence minimizing fuel consumption and improving the economy.
Although there may be fewer, but larger trawlers in the future, this indicates that there will be
a negligible increase in the mass of trawl board and the velocity of trawling.

11.3 Acceptance Criteria

The acceptance criteria corresponding to accidental loads and environmental loads from NPD
(1990), is that no leak should occur. The acceptance criteria 'no leak' is interpreted below.

113.1 Acceptance Criteria for Impact Response Analyses
When the trawl loads are considered as accidental loads, the present study proposes a dent
depth acceptance criterion as below.

In past practices, Dent Depth was limited to 2% of OD (Outer Diameter) according to ASME
B31.8 (1992). This was a conservative assumption. A rational criterion on dent acceptability
can be argued based on residual strength assessment. Up to 5% of OD can be allowed based
on the following considerations:

- serviceability limit state: the limit for allowing pigging operation is 5% OD

- burst strength: the pipe corrosion coating is not likely to be penetrated by the impact. It is
then assumed that no cracks (gouges) will be given to pipe steel wall due to impact.
Therefore, burst strength of the pipeline will not be reduced significantly because the dent
depth is 5% OD with no cracks in the dented area.