trawl impact
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Transcript of trawl impact
By: Afifah Abdul Rashid
INTERFERENCE BETWEEN TRAWL
GEAR AND PIPELINES
References
DNV-RP-F111, Interference between Trawl Gear and Pipelines
PTS 31.40.10.17, Pipeline/Trawlgear Interaction
What is Trawling?
Method of fishing that involves pulling a fish net through the water behind one or more boats
Bottom Trawling
Towing the trawl along or close to the sea floor
Why considered bottom trawling?
Bottom trawling shall be considered for offshore pipelines, WHY?
Possible hazard and inconvenience to the fishermen in case of trawl gear hooking to the pipeline.
Possible hazard to the integrity of the pipeline due to loads from the trawl gears.
Typical Trawl Gears
Otter Trawls by use of trawl boards Beam Trawls by use of transverse beams Twin Trawls by use of clump weights
Figure i : Otter trawl gear crossing a pipeline
TYPICAL TRAWL GEARS
Figure ii : Beam trawl gear crossing a pipeline
Figure iii : Twin trawling with clump weight
Data Requirements
Trawling category (trawling with otter, beam trawl or twin trawling)
Trawling equipment in use (type, shape, size, mass, trawling speeds)
Expected frequency crossing over the pipeline
Expected/possible changes in the trawling equipment
WHAT ARE THE POSSIBLE HAZARDS?
1.Impact
2.Pull-over
3.Hooking
Impact
Initial phase when trawl gear hits a pipeline
The transfer of kinetic energy from the impacting trawl board to the pipe, its coating and the soil.
Time of transfer is so short (few hundredths of a sec)
Local deformations
Consequences
Cause dent in the pipe wall Extreme case - impede the passage pigs or
cause stress concentration leading to fatigue failure
Impact scenarios considered
Trawl direction normal to the pipeline
Trawl direction at an inclined angle to the pipeline
Condition of the pipelines (i.e buried/exposed and small/larger diameter)
Pull-Over
2nd phase where trawl gear pulled over the pipeline
Impact last from 1-10 secs More global response Subject to relatively large horizontal (lateral)
and vertical forces
Pull-Over Duration
Trawl velocity Pipeline induced movement at interaction Warp line stiffness Clump mass
Consequences
Displacement of pipeline Bending at point of contact Extreme case – local buckling
Hooking
Trawl gear wedged under the pipeline The trawler forced to stop and attempt to free
the gear by winch Lift up the pipeline Rarely occurring event
Free spans represent an increased risk of hooking
In a direction almost parallel to pipeline Other seabed obstructions in the area
Typical hooking scenario in a free span:
Consequences
Lateral deflection to the pipeline Large vertical load by trawler to free the trawl
gear Yielding and local buckling
SUMMARY
Impact – denting to pipeline Pull-over –deflection due to thermal
loading and trawl gear loading Hooking – deflection due to hooking load If the design is OK, protection is adequate If not, increase coating thickness or bury
the pipe