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.
"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 ?
Marc Van de Velde