INHERENT SAFETY

The potential for catastrophic failure is minimised by having a ‘close to neutral’ submerged weight of the SDV and structure.


TOW RIGGING FAILURE
If there is rigging failure between the tow vessel and the tow chain clump weight, the chain clump weight will sink slowly to the seabed and act as an anchor for the SDV.  If the water depth is greater than the length of rigging between the SDV and the tow chain clump weight, the SDV will be pulled under and will float at a known height above the seabed.  Otherwise the SDV will float on the surface but will still be anchored by the weight of the chain clump weight.


If there is rigging failure between the SDV and the tow chain clump weight the SDV will float to the surface.

 

VIDEO - Orcaflex Simulation of Tow Rigging Failure - Real Time - Edited Version (220s)

The animation shows failure of rigging at the connection between the tow chain clump weight and the tow wire. The initial screen shows both the towing vessel and the SDV to give an impression of the tow depth of the SDV. The second screen shows the actual breakage, the response of the SDV and its subsequent soft descent to the seabed. The final screen shows the SDV response as the tow chain clump weight lands on the seabed.

 

VIDEO - Orcaflex Simulation of Tow Rigging Failure - 10 x time - Full Version (60s)

The animation shows the full sequence of the rigging failure described above but at 10x the frame speed.


LOSS OF BUOYANCY
During tow the SDV and structure will be marginally buoyant and will therefore be able to withstand some loss of buoyancy.


It is likely that any loss of buoyancy will be gradual.  The trim of the SDV will be monitored and will give early warning of any loss of buoyancy.  Consequently the SDV may be brought to a safe location for rectification.  This could be simply by removing ballast chain.

 

ADVERSE WEATHER
Although adverse weather is unlikely to affect the SDV and structure, there may be occasions where the weather is unsuitable for the tow vessel.  In such cases the tow chain clump weight will be lowered to the seabed leaving the SDV safely anchored.