The Art of Dredging

Dredging and shipping

Rupturing a gas pipeline 

This article is based on a topic in MARS"; (Mariners' Alerting and Reporting Scheme) in Seaways, August 2010, by The Nautical Institute

To dredgers, who seemingly always work in the vicinity of submarine pipelines and cables, this may prove interesting.

 


 

Containership "APL Sydney" arrived on Melbourne roads and anchors in the designated waiting area under pilotage.

 

When the pilot disembarked, the wind speed was gusting to 48 kts. A submarine gas pipeline lay 0.6 nm downwind.

Summary of significant events (see position plots in chart):

12:00: Sea pilot boarded

14:28: The ship's starboard anchor was let go on a heading of 108 with a 35-knot gale blowing from SW. (The heading was chosen to create a lee for the sea pilot to disembark safely, and the drift rate increased under this situation until the ship turned head in the wind at about 14:55.

14:36: Sea pilot off.

15:01: Master concluded that the anchor was dragging and requested permission from harbour control to move vessel. He was instructed to maintain position and await pilot.

15:27: Harbour control gave permission to move vessel. Master used ahead engine power to relieve stress on the anchor and commenced shortening cable.

15:48: Starboard anchor windlass breaks down, with two shackles still out.

15:49: Ship's starboard anchor presumed to have snagged the pipeline.

16:03: Sea pilot re-boarded the vessel.

16:20: On pilot's advice, attempt to drag the anchor clear by using engine.

16:21: Gas pipeline ruptured. The eruption of gas was seen about 50 meter upwind from the APL Sydney. Engine stopped, ventilation stopped, crew gets inside.

16:27: Vessel manoeuvred clear and anchor no longer fouling the ruptured pipeline.

16:34: Emergency shutdown valves of gas pipeline closed.

21:53: Starboard anchor cable cut at hawse pipe and anchor with about 2 shackles of chain abandoned.

 

Multibeam image of the ruptured pipeline (=blue line). Depressions in the seabed caused by outflowing gas under pressure. The South-North path of the dragging anchor is clearly recognisable.

 

Some findings from the investigation:

1. The rupture was the result of attempting to drag the anchor instead of slipping it immediately
2. The anchor had also been let go too close to the pipeline and "upwind" in poor weather conditions;

Important guidance to mariners on fouled submarine pipelines:

1. Australian Notice to Mariners 26 advises that in the event of any vessel fouling a pipeline, the anchor or gear should be slipped and abandoned without attempting to get it clear.

2.  The Mariner's Handbook notes that if a ship has fouled a gas pipeline with its anchor or gear, the ship 'could face an immediate hazard by loss of buoyancy due to gas aerated water or fire/explosion'.

 3. Many pipelines were laid before accurate GPS receivers became commonplace, so it would be prudent not to rely on the accuracy of their charted positions.

 

The full report (by the Australian Transport Safety Bureau) can be found here.

 

Another accident -with rupture of a gas pipeline- can be found here.

Tanker "Young Lady" was anchored off Teesport, UK, when the anchor started dragging. In an attempt to heave the anchor, the hydraulic windlass broke down. The anchor kept dragging for five miles, until the flukes ruptured a gas pipeline.

This accident is a copy / paste of the previous story.

 

What about dredgers ?

"any vessel fouling a pipeline, the anchor or gear should be slipped and abandoned without attempting to get it clear"

"Abandoning gear" is simply impossible for a dredger, be it a cutter, trailer or backhoe.

The dredging gear moves over the bottom with a certain force, and will most certainly damage a pipeline on first contact, anyway.

 

 

"Prins der Nederlanden",  draghead gantry

 

 

 

 

 

 
So, for dredgers, it's all about prevention ?

  1.  The position of pipelines in the vicinity of dredging area's has to be established. If positions are not proper known, the pipeline(s) must be found, with subbottom profiling if necessary. That may sound like farfetched and expensive, but it's a cheap deal compared to damaging a pipeline.
  2. A safety corridor can be established around the pipelines, and visualised on the navigation displayed onboard the dredger.
  3. One step further yet: systems exist that make the dredger hoist its dredging gear automatically (this is: without human intervention) when the dredger crosses into the safety zone around a pipeline. Dredgers have already worked safely to a distance of 50 meter from a live gas pipeline, with this safeguard.
  4. Positioning systems onboard the dredger have to be reviewed, focussing on redundancy, reliability and accuracy, prior to operating close to pipelines.
  5. If possible, operations should be planned "down weather". In case of blackout, failure to hoist the dredging gear, etc... the dredger drifts clear from the pipeline.

 

 

Marc Van de Velde

september 2010

 

 

 

 

 

 

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