DOSSIER

 

Electrostatic Discharges on Board Ships Tanks


BY CAPTAIN DOMINIQUE PERROT, ASSOCIATION DES CAPITAINES ET OFFICIERS DE LA MARINE MARCHANDE (ACOMM), FRANCE


Does this kind of discharge present a real danger in ships tanks transporting dangerous goods? Indeed it does, this kind of discharge is present each time a cargo is loaded or discharged due to: tank washing, the speed of the fluids in the pipes, the decantation, and the wear of materials against others. The atmosphere of a tank could reach a very high potential of load. To keep it, then to dissipate it with time, or to bring on a disaster if the mixture of air and hydrocarbon is within the limit of explosiveness and an external element allows the discharge of energy contained in a tank atmosphere.


The causes of this risk are known:
- The flow of the fluids in the collectors or in the filters
- The settling of solid particles or another not miscible liquid, has fault a liquid.
- The passage of solid or liquid particles has fault a buzzard of reduced diameter.
- The projection of a fluid of wash on partitions and ele­ments of structure.
- The abrasion of certain synthetic substances, following frictions extended.

The electrostatic load accumulates over time, it needs a favourable atmosphere and a source of ignition, such as a spark. To determine the risk a metallic probe, acting as a type of lightning conductor, can be used as.
A number of accidents on tankers that take place are caused by this problem, although the exact number is unknown, a large number are likely to be caused by electrostatic discharge.
We now have a good knowledge of this phenomenon, so we are able to determine on which factor to act, to limit the creation of electrostatic charge and avoid electrostatic discharge or modify the gaseous characteristics of the atmosphere of tanks to avoid combustion.

Further to the loss of human lives, the risks of pollution and the financial losses which such accidents involve, OCIMF (Oil Companies International Marine Forum) created a department to regulate and so reduce the risks of explosions as a consequence to electrostatic discharges in flammable atmosphere. The ISGOT Guide (International Safety Guide for Oil Tankers and Terminals) is approved by the IMO (International Maritime Organization) and serves as a reference internationally. Although private or public bodies and shipping lines conduct research and publish their own safety instructions, the benchmark is the IMO Guidelines.


RISK OF EXPLOSION: Required Conditions
The development of an electrostatic charge does not alone present a risk of fire or explosion. There should also be a discharge or recombination of separated positive and negative charge which we name Electrostatic discharge. For this to be at the origin of a fire or an explosion, four conditions must be met:

  1. The necessary conditions for the development of a static load
  2. It has to exist in such a way as to accumulate the electrostatic charge
  3. There should be an electrostatic discharge of sufficient energy
  4. The discharge takes place in an environment with and explosive atmosphere.

DEVELOPMENT OF STATIC LOAD Flow in the conducts
The presence of solitary ions resulting from impurities and the corrosion inside the collectors are examples at the origin of the losses of electronic potential of the fluid. The flow of electrons in the liquid results from the oxidation

of the metal. Often, it is the conduct which gains electrons and takes charge negatively to the detriment of the hydrocarbon which loses electrons and takes charge positively. This action is marked all the more as the speed of the fluid is high.


Use of vapour or water of wash

The clouds of steam formed during the tank washing, using hot or cold water, can be charged electrostatically. The risks are all the more important if the tank is big. These charges in the vapour cloud can be strongly increased with the use of products in the wash. The steam creates a cloud of vapour highly charged very quickly, much more than a simple wash using water; we call this the cracking.


Load by the dome

When the fluid is loaded by the dome and when it falls from the height of the tank, it acquires a sufficient potential energy so that in the shock against the top of the ballast, molecules break up and create an ionized cloud. We find ourselves then in the configuration of a cloud of vapour emitted by characteristics appropriate to the fluid loaded. If it has a low conductivity, the risk of electrostatic discharge is real.


Introduction of air or inert gas at the bottom of the tank

The action of the surfacing bubbles when we introduce a gas in the bottom of the tank will shake the fluid which risks changing its molecular structure along the column of gas by ionizing. The gives a high risk of a difference in electrical potential.


PVC pipes

Pipes made of PVC being of use for the stripping of the cargo were implicated during the explosion of a barge. Plastics or pipes in polyethylene used for the ventilation of tanks generate a high static charge.
Crude oil washing
During crude oil washing, a jet of 12 bars collides with the bulkheads and the bottom of tank. The energy of the water pressure converted to kinetic energy by the reduction of diameter of nozzles is big enough to break the molecules in the crude oil residues and so to charge the wall and the cloud of hydrocarbon which results from it.


Oil/Bulk ore carriers

Holds on these ships spread out over the width of the ship and offer a large surface area, if ships are not equipped with "stowage automatic tanks", this displacement of cargo can produce clouds of charged dust which present risks of explosion if they are blown in the direction of zones with explosive atmosphere.

 

HOW TO LIMIT STATIC GENERATION

1. During the conception of ships and terminals:

Oil tankers and chemical tanker are built in agreement with the existing regulations (ICS, ISGOTT, SOLAS)

- Cargo pumps must be designed in steel, or steel stainless mouldings. The use of pumps at variable flow rates is recommended in view of the highest loading flow and progress must be carefully monitored during the duration of the load.

- Immersed portable pumps used in case of break-down by the main pump or in case of anti-pollution measures must be carefully bonded to the ship before use.

- Filters must be also bonded to the ship

2. Precautions in the load:

For static accumulators, liquids such as natural gasoline, fuel, white spirit, jet fuel oil, naphtha, heating oil, solvents, aromatic products ... The percentage of oxygen in the tank must never exceed 8%. A reduction of the flow rate should be adopted at the beginning of the loading operation until the drop-line is covered. The same precautions should be taken during discharge operations.

3. Blowing of lines:

To avoid any contamination of cargo during subsequent commercial operations, the lines must be blown into the tanks at the end of every load or into shore tanks in for discharge operations.

4. Washing:

The use of steam for the tank wash should never be used in a tank that has not been inerted.
A wash using hot water will always be preferred to steam being used due to the risk of static electricity gene-ration.

Reference should be made to the procedures for “Tank Cleaning guides" for these operations.

Insulating flanges should be used between the ship/shore connection and also on the Vapour Return Line/ Vapour Treatment Unit.


CONTROLLING VAPOURS

The control of the atmospheric composition in tanks was the most improved means to avoid the danger of explosion with the appearance of inert gas installations and nitrogen generators following the first accidents on tankers.

To avoid the risk of fire and toxicity for the crew, it is preferable that vapours are not left in the tank atmosphere but are returned to the shore tank by the use of a Vapour Return Line or a Vapour Treatment Unit. These 2 systems allow a high rate of transfer without overpressure or depression in the tank.


CONCLUSION

This phenomenon on board ships cannot be avoided. Thus it is necessary to make every effort to guard against this kind of accident by respecting "Ship/Shore safety list", procedures for washing, degassing and not giving in to commercial pressures to the detriment of the safety.


Human life is very precious and must be protected at all times.

 

 

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