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