Alternative method for salvaging containers with water soluble HNS

A special problem arises with containers of water-soluble hazardous materials. The moment the container is lifted out of the water, the water and hazardous cargo inside leaks out through the corners and holes of the container due to hydrostatic forces. This is when most of the hazardous substance escapes into the marine environment during the salvage operation.

Proposed method 1

The damaged container could be lowered into an oversized, open top container large enough even for a deformed containers and the top secured with a piece of covering plastic.

The salvage method consists of the following steps:

1. Lower oversized containers to seabed next to container to be recovered.
2. Disconnect lifting gear from oversized container and connect to container to be recovered using divers or ROV.
3. Lift container into the oversized container.
4. Disconnect lifting gear.
5. Attach the plastic top cover to securing points on front, back and sides of the oversized container, using divers or ROV's.
6. Reconnect lifting gear to the hoisting points on the oversized container.
7. Lift oversized container to just above sea level.
8. Remove plastic top cover.
9. Insert suction hose into oversized container.
10. Start pumping seawater / hazardous liquid mixture into storage tank on vessel or tank container(s)

Thus, all of the hazardous cargo is contained inside the oversized container, which permits no subsequent leakage, though it need not

Be lifted completely out of the water, thus minimising the lift-capacity of the onboard crane. Thus, while the oversized container is partially supported by buoyancy, the contents can be pumped into to a tank container or storage tank on board the salvage vessel.

Proposed method 2

BDS Harlingen has designed another method to reduce environmental hazards and improve safety in the salvage of leaking containers of hazardous substances.

The method consists of fitting a watertight plastic oversized bag around even a deformed container.

Picture 82 Oversized plastic bag Copyright photo ACSS

The container plus cover are hoisted out of the water, and the contents are pumped out using a pump connection on the bottom of the plastic bag.

Picture 83 Pump connection on container cover Copyright photo ASCC

A great difference from the earlier approaches is that the cover is put on when the container is just below the waterline, most of the leakage occurring only when the container would otherwise be lifted out of the water.

The pumping is started when the container is still partly below the waterline, and the pressure is equal on both sides of the cover. This lowers the total weight and facilitates the lifting.

The salvage method consists of the following steps:

1. Attach lifting gear to container on seabed
2. Lift container till just under the waterline
3. Apply plastic oversized bag from the bottom upwards, and secure onto the four hoisting points on each corner of the container
4. Attach suction hose to the pump connection on the bag
5. Start pumping
6. When almost empty, lift up the container so the remaining liquid drains to the bottom pump connection

This method has some advantages and disadvantages compared to the previous method:

Advantages

• No additional underwater operation required, other than connecting the hoisting gear to the container
• No heavy lifting gear required, as there is no heavy metal construction and most of the water is pumped out before the container is lifted above sea level.
• Easy application of low-cost plastic

Disadvantages

• From the seabed to water surface, there is no protection against leakage to the environment, though this is limited by the inside pressure being the same as the outside pressure..
• The plastic cover is vulnerable to damage, and must be handled with care

Proposed method 3

A third method quite similar to the former, uses,

instead of a plastic bag, an oversized metal container with adjustable buoyancy to keep part of it the temporally under water.

The salvage method consists of the following steps:

1. Put oversized container overboard partly immersed as regulated by air flotation. (See Figure 85-1)
2. Attach lifting gear to container on seabed (see Figure 85-1)
3. Lift container to be recovered till just under the water surface line (see Figure 85- 2)

Picture 84 Container to be recovered is kept near the water surface; preferably just underneath the surface so no hydrostatic forces exist to press the water inside the container into the marine environment Copyright photo IMARES

1. Pull container into the partially floating oversized container (see Figure 85- 3)
2. When in place put sufficient air in the submerged part of the oversized container to adjust the horizontal bouyancy (see Figure 85- 4)
3. When the oversized container completely floats it covers the container to be recovered. The lifting of the container to be recovered can now slowly start. (see Figure 85- 5)
4. Attach suction hose to the pump connection on the oversized container (see Figure 85- 5)
5. Start pumping and continue the lifting process slowly till almost no dripping can be seen from the recovered container (see Figure 85- 6)
6. Put the recovered container t on a drip tray on board the salvage vessel
7. When all liquid has been removed from the oversized container this can also be brought on board.

Picture 85 Container is put in a drip tray on board salvage vessel. Copyright photo IMARES

Figure 85 Third method to salvage a container leaking hazardous substance