STATIC ELECTRICITY RELATING TO OIL TANKERS
The Electrostatic Hazard
This describes the specific elements which contribute to the four stages of hazardous electrostatic discharge and vapour ignition during routine operations in cargo tanks.
The development of (static) electrical charges may not be in itself a potential fire or explosion hazard.
However there must be a discharge or a sudden recombination of separated positive and negative charges to bring about this hazard.
The Four Conditions Required for Explosive Ignition
- Effective means of static generation.
- Accumulation of charges and maintaining a suitable difference of electrical potential.
- A spark discharge and
- The discharge must occur in an ignitable mixture.
Two differing substances in contact with each other will often become charged
An electrical double layer is formed along the adjoining surfaces.
The separation of the two substances often causes them to remain disparately charged, an effect which is exaggerated by increased speed of separation and increased mechanical work (friction).
Piping of oil products
Charge generation and separation occur when liquids move in contact with other materials, as in operations involving:
Mechanisms which exacerbate static separation in cargo loading operations are the following:
- Turbulence and splashing
- Any mixing or filtering of the cargo
- Impurities in the cargo
- Disturbance of water "bottom"
- Pumping of entrained air or other gases
The cargo is also disturbed during unloading operations
Discharge of slops and contaminated ballast
Displacing of lines using air and water
Water mist and steam Mists formed during water washing or from the introduction of steam
The charge associated with water washing may be much higher if cleaning chemicals are used.
Loading overall (from the top of the tank)
Air release in bottom of tanks Air
Crude oil washing (COW)
Oil/bulk ore carriers (OBO)
Single cargo holds extending the full breadth of the ship are subject to severe sloshing effects if not pressed full, leading to the possible formation of electro statically charged mists.
Accumulation of charge and potential
Static accumulator and non-accumulator oils
The conductivity of a liquid determines whether or not it retains the generated static charge.
A non-accumulator oil, defined by an electrical conductivity of greater than 50 (pS/m) will relax quickly
Accumulator oils are defined as having a conductivity of less than 50pS/m
When accumulator oil is loaded, charges of similar sign repel from each other toward the liquid's outer surfaces, including that in contact with air.
The latter is called the "surface charge" and is usually of most concern.
ISGOTT states that, in general, black oils do not accumulate static charge and clean oils -distillates- do.
It classifies several oils as follows:
- Crude oils
- Residual fuel oils*
- Black diesel oils
- Natural gasoline’s
- White spirits
- Motor and aviation gasoline’s
- Jet fuels
- Heating oils
- Clean diesel oils
- Lubricating oils
From the above it may be seen that generally all the distillates from crude oil are purified to an extant that makes them accumulator oils. The residual oils and crude itself containing the impurities are stated a non- accumulator oils.
Loading rate precautions
Middle distillates require loading precautions such as below to prevent the build up and thereby the chance of an explosion.
- Equipment introduced into cargo tanks
· Polyvinyl chloride (PVC) hoses
· Fixed plastic pipe in cargo tanks
· Carbon dioxide
- Synthetic clothing
- Spark discharge.
- Incendive sparks
Known causes of incendive sparks are identified below:
- Insulated conductors
- Cargo measuring devices
- Falling water slugs
- Flammable vapour Oils
- Steam cleaning
- Switch loading.
- Temperature fluctuations
Corrective and Preventative Measures
Procedures for the prevention are stated in the following documents.
The documents are abbreviated as follows:
ISGOTT- International Safety Guide for Oil Tankers and Terminals
API 2003- API Recommended Practice 2003, 'Protection against Ignitions Arising out of Static, Lightning, and Stray Currents".
API 2015- API Publication 2015, "Safe Entry and Cleaning of Petroleum Tanks".
Texaco- Texaco Inc. Research, Environment and Safety Department, "Static Electricity Code".
Mitigation of static charge generation
The generation of static electricity cannot be prevented absolutely, but may be minimized or eliminated through the application of certain precautions.
The following are essential only when loading static accumulator oils (conductivity < 50 pS/m):
- Restrict initial loading rates
- Restrict initial unloading rates
- Keep water and other impurities out
- Avoid pumping entrained gases with cargo.
- Degassing (to <20% of LFL at tank bottom) or inerting a ship's tank eliminates loading rate restrictions due to static electricity.
- Reduced pumping speeds are used for discharge of slops
Clearing of cargo piping prevents cargo contamination (a suspected cause in explosion) and requires care.
The following precautions are useful:
- Asphalts and heavy fuels only may be cleared with air or inert gas.
- Heavy fuels and crude oils may be cleared with water, which must thereafter be debottomed.
- When necessary to use inert gas to displace clean products, a minimum amount must be used, particularly for aviation fuels.
- Clean product lines should never be blown with air.
- To clear water from a product line, pump twice the line fill volume of product at 3 ft/sec (fast enough to prevent water persisting at low points and at the correct speed to minimize static generation).
Protection against mist and steam
Steam must not be injected into the tank
When required, steam cleaning is only allowed in a gas free atmosphere.
Industry standards allow the use of steam driven blowers; this practice is now in question because of the AMERICAN EAGLE accident (for the full report please see appendix 1).
For COW, use of "dry crude oil"
For overall loading Non-volatile petroleum
Texaco allows loading from the top only for crude oil and cutback asphalts.
For all other products, temporary hoses must extend to the bottom of the tank with free ends secured against movement.
Air injection precaution, no air to be introduced into an inerted atmosphere
Precaution for combination carriers, due to large beam and that the hatches may extend from ship side to ship side, sloshing effect gives rise to undue static electricity generation.
Prevention of charge accumulation
The following safety precautions have been developed to prevent the accumulation of static charge.
API 2003 recommends that these additives be introduced at the beginning of the "distribution train", and notes that their positive effect may be reduced by repeated shipments or passage through clay filters.
Safety precautions for the handling of static accumulating oils have historically been waived for those treated with antistatic additives.
These precautions have, however, recently been extended to residual oils and oils treated with anti-static additive to raise conductivity above 50pS/m (May 1991 amendment to ISGOTT).
The document is silent as to treated oils at or above 100pS/m.
Relaxation of static accumulators
The charge which accumulates in a poorly conducting liquid will slowly dissipate after loading is completed and the cargo is still. Relaxation time of 30 minutes is recommended after loading of static accumulating oils before introduction of cargo sensors into the tank.
The USCG has recommended that all pipes, hoses, and fittings be conductive, electrically continuous, and bonded to ship's structure. The following precautions have been adopted for stripping and cleaning hoses:
- Hoses must be non-corrosive, nonmagnetic, non-sparking, and conductive
- Aluminium wands with brass tips may be used
- The USCG has recommended against hoses made of certain conductive materials such as aluminium -and magnesium since they can spark in contact with rusty hull steel
- Electrical continuity (bonding) of this equipment is Critical.
The recommended use of conductive materials for fixed piping also addresses this problem.
The USCG has specified that plastic pipes (such as post-chlorinated polyvinyl chloride (CPVC) in tanks containing combustible and flammable fluids have a maximum resistance to ground of 1 mega ohm (106 ohms).
200kilohms is specified in tanks which are adjacent to pump rooms and which contain static accumulating cargoes (conductivity > 100pS/m).
The International Maritime Organization (IMO) is considering a similar standard: the resistance of plastic piping would not exceed 100kilohms/meter, and nowhere should exceed 106 ohms.
Prevention of static accumulation is critical during all tank washing operations because of the vigorous agitation of liquids involved. Detailed precautions for all tank atmosphere conditions are given in ISGOTT.
The most important are:
Mixing of immiscible liquids is inevitable during water wash and is a source of static electricity.
The following precautions apply, particularly in undefined or too lean atmospheres:
- The tank should be kept drained during washing and washing stopped in case of water buildup. ISGOTT
- Recirculated wash water should not be used for tank cleaning. ISGOTT
- Chemical additives in wash water must not be used in an undefined atmosphere. ISGOTT
- The last cargo carried must be determined by examination of the Material Safety Data Sheet (MSDS).
- Prior to washing, tank bottoms, cargo piping, and cargo pumps must be stripped to the greatest extent possible.
- Ground or bond the vessel to the facility prior to opening cargo tanks.
- Filters - Loading rates should be adjusted to ensure that 30 seconds elapse between the time the cargo leaves the filter and the time it enters the cargo tank.
- This restriction applies primarily to micropore or clay filters. Coarse filters (less than 50 mesh per inch, if kept clean, do not generate significant charge. ISGOTT
- Mopping After stripping, crew often remove residue from tank bottoms by mopping. The mop head must be 100% cotton (non-accumulating and non-conductive), attached to a stainless steel handle.
- Both the mop bucket and the handle are bonded to the hull.
The use of carbon dioxide as a fire extinguisher or for inerting must be avoided unless the formation of solid particles is prevented.
Prevention of spark discharge
Bonding and grounding
The most important measure to prevent electrostatic hazard is to bond all metal objects together, eliminating risk of discharge between objects, and to assure that all components in the cargo handling system are at the same, electrical potential.
Grounding (or earthing) to earth is effectively accomplished by bonding to the hull, which is naturally earthed through the water. Equipment should be designed to facilitate bonding and, in particular, to avoid the insulation of any conducting metal.
Bonding of cargo transfer piping
Hoses used in terminal transfer operations must be continuously bonded, and grounded to the hull.
It is important to note that cargo transfer piping must be insulated from the land-side terminal since electrical potential may differ from that of the vessel due to stray current or cathodic protection of the pier. Insulating flanges, joints, or sleeves are sometimes used to divide the cargo hoses into electrically isolated halves - onboard and shore side. Each half is bonded and grounded to its respective base potential.
Texaco does not allow ship-to-shore bonding except where required by statute. In such a case, insulating flanges are still required in cargo lines, and numerous other precautions are specified regarding the bonding wire.
Texaco adds the following precautions:
- De-energizing the pier's cathodic protection system is not reason to waive precautions.
- Non-conductive hose can become conductive with use and is not an acceptable substitute for an insulating flange.
- Flange location is separately specified for all flexible, all metal, and combination connections.
- Insulating device is periodically tested for resistance of at least 1000 ohms.
- Cargo hoses must be tested for conductivity when new, and periodically thereafter.
- Insulating flanges must be used when connected to submarine pipelines which have cathodic protection.
Bonding of portable tank washing machines Bonding wires should be incorporated within all water hoses and bonding established between water hoses, the tank washing machine and the cleaning water supply line.
Hoses must be indelibly marked to show identification, and a record of continuity testing kept. All hose connections must be made up and tested for electrical continuity · before the machine is introduced into the tank and not broken until after the machine has been removed. ISGOTT
When suspended in a tank, portable washing machines must be supported by a natural fiber rope and not by means of the water supply hose. ISGOTT
Bonding of overhead stripping and cleaning systems
Portable or "overhead" systems are often used for cleaning and stripping tanks in the absence of fixed piping.
The following is the most thorough treatment of the safe procedure for the overhead stripping operation:
- Each length of hose is tested for continuity and visually inspected for damage.
- Hose is bonded to pump.
- Pump has permanently attached bonding wire, which is attached to hull by a C-clamp. One jaw of the clamp has a sharp conical point to assure penetration of painted or rusted surfaces.
- Final electrical continuity check on assembled stripping system is required, from end of conductive wands (see discussion below) to hull, and from pump to hull.
- Personnel do not wear insulating gloves while bonding equipment, but do while stripping.
- Falling liquid in tank must be avoided.
- Hoses (of approved construction) must be extended to the bottom of the tank while in use to prevent possible discharge in the middle.
- Bond is maintained until operation is finished and wand is completely withdrawn from tank.
- After stripping, pump is run to clear hoses.
- Checklist maintained throughout operation.
Dipping and ullaging
When loading static accumulator oils, metallic dipping, ullaging, or sampling equipment must not be introduced or remain in the tank during loading, and for 30 minutes after completion of loading, to allow for relaxation of accumulated static charge.
Bonded equipment which is grounded to hull structure may be used after the 30 minute stand down. Ropes used must be made of natural, not synthetic fiber. ISGOTT
The foregoing precautions also apply during water washing of tanks in uncontrolled atmospheres and for five hours thereafter, which period may be reduced to one hour if the tank is continuously vented after washing. ISGOTT
Operations carried out in sounding pipes are permissible at any time. ISGOTT
Permanently fitted float level gauges do not present a hazard if they are properly grounded and the guide wires are intact. ISGOTT
Every effort must also be made to ensure removing all loose objects from a tank and to prevent loose metal objects from falling into a tank. ISGOTT
Free fall of liquid:
It is essential to avoid the free fall of water or slops in the cargo tank or a tank used for receiving slops. ISGOTT
Portable fans or blowers should only be used if they are hydraulically, pneumatically, or steam driven. Their construction materials should be such that no hazard of incendiary sparking arises if the impeller touches the casing. ISGOTT
It should be noted that the U.S. Navy, as well as some operators, is considering discontinuation of use of steam driven blowers (such a machine was the suspected cause of the AMERICAN EAGLE casualty). Portable fans should be bonded to the deck. Air suction and discharge hoses should be bonded for electrical continuity to the hull. ISGOTT
Inert gas precaution
If the inert gas plant breaks down during discharge, operations should be suspended.
If air enters the tank, no dipping, ullaging, or sampling equipment should be introduced into the tank for at least 30 minutes, after which securely earthed equipment may be used; this restriction should be applied for five hours. ISGOTT
Carbon dioxide should not be injected into tanks which may contain flammable gas mixtures. ISGOTT
Control of vapour composition
Control of tank atmospheres has historically been used to control fire hazards, particularly since inert gas systems (IGS) was mandated following the VLCC explosions of 1969. A number of different approaches are used to prevent flammable gas mixtures.
Definition of tank atmospheres
Crew members should always check the Material Safety Data Sheet(s) (MSDS) prior to any operation, for cargo previously in the tank as well as that to be handled at that time.
MSDS includes the important physical properties of the material and all pertinent safety warnings.
This knowledge is critical to tank atmosphere control.
The flammable constituents of a tank atmosphere are defined by ISGOTT as follows:
- Inerted An atmosphere made incapable of burning by the introduction of inert gas and the reduction of oxygen content below 8% by volume.
- Too lean an atmosphere made incapable of burning by the deliberate reduction of the hydrocarbon content to below the lower flammable limit.
- Undefined an atmosphere which may be above, below, or within the flammable range.
- Over rich an atmosphere made incapable of burning by deliberately maintaining the hydrocarbon content of the tank over the upper flammable limit (usually 15%).
Water washing of tanks may be carried out in any of these atmospheres provided specific precautions for each tank condition are complied with.
These are specifically identified by ISGOTT.
Some companies exceed particulars of ISGOTT safety measures.
In washing of inerted tanks, for example, Sun specifies the following:
- Oxygen levels below 5%, instead of 8%
- Measurements required in each section of a tank divided by swash bulkheads
- Continuous monitoring of pressure and oxygen content during washing.
Gas freeing is one of the most hazardous operations on a tanker, since the tank atmosphere is likely to pass through the flammable range as fresh air replaces tank gases.
All electrostatic precautions should be observed at this time.
All cargo piping lines should be discharged and flushed with water, and the tank stripped afterward. Valves should be closed and secured. ISGOTT
Portable fans should be bonded to the deck. Final gas measurements should be done 10 minutes after completion of ventilation at several levels in the tank, and, in large tanks, at widely separate locations.
Periodic checks of the atmosphere should be made, particularly when cleaning disturbs residual product in the tank.
Steam cleaning of tanks
After carriage of certain products, some tanks require cleaning by steam.
This should only be done in tanks which have been inerted or water washed and gas freed. The concentration of flammable gas should not exceed 10% of the LFL prior to steaming.
Steaming should be avoided when there is any risk of a flammable atmosphere in the tank. ISGOTT
Switch loading is defined by Texaco as loading a low vapour pressure (high flash point) product, such as AVJET A, into a compartment in which the previous load was a high vapour pressure (low flash point) product, such as gasoline".
Merely changing product is called "cross loading".
Care must be shown to avoid contamination of static accumulators, such as middle distillates, with low flash point products. Thorough flushing of cargo lines, stripping, and gas freeing are obvious precautions, which may not suffice to prevent disturbing liquids and gases absorbed by rust and sludge in the tank.
Certain products such as lube oils are not allowed to precede high static fuels such as "Av-jet JP 4" as the last cargo. Texaco requires management approval for certain types of switch loading.
Ships staff should check the MSDS for the previous cargo as well as that to be loaded and proceed with extra caution (with .regard to loading rates, hand dipping, etc.) if a static accumulating oil is being loaded where a highly volatile cargo was previously carried, or vice versa.
Securing of covers
Stripping or cleaning of cargo tanks should be undertaken one at a time. All others (including WB tanks and void spaces) must be closed and dogged in order to keep their atmospheres above the UFL and to prevent migration of hydrocarbon gases across the deck.
No antistatic precautions are necessary while the tank is maintained in an inert condition or if the non-volatile static accumulator oils are being handled in a gas free tank at a temperature of less than their flashpoint minus 10ºC. ISGOTT
ISGOTT presents tables indicating necessary precautions and exceptions for all loading situations.