AMET UNIVERSITY,CHENNAI.

B.Sc. Nautical Science Semester IV Cargo Handling and Stowage- IV/NS 1407 Unit-5 Oil Tanker operations and related pollution-prevention regulations

Objectives:- From this lesson the students should understand about: i) Precautions to be taken to prevent pollution of Marine environment during various ship operations; ii) Design/Construction/ Equipment requirements for various type of Tankers under MARPOL Convention; iii) Precautions to be taken during loading/discharging operations on Tankers including entry into enclosed spaces; iv) Proper use of Oxygen analyzer, Explosimeter, Tank scope , Multigas detector using tubes; v) Types of pumps used on Tankers and salient points of the International Safety guide for Oil Tankers and Terminals (ISGOTT); and vi) Cargo calculations for Cargo quantity taking into consideration various factors.

History of Oil Carriage and Pollution Prevention- OIL POLLUTION BACKGROUND-

World’s 1st Tanker was in late 19th century- Carried Kerosene for Lighting. Invention of motor cars fuelled demand for OIL. World War II, tanker size was 16,400 DWT. 1950 onwards Tanker fleet grew rapidly. 1959 – 1st tanker of 1,00,000 DWT. Mid 1960s – Tanker of 2, 00,000 DWT.V.L.C.C.

OILPOL 1954 Prevention of pollution by OIL by ‘UK’, 1958 IMO- Came into existence. 1950 – Cleaning of oil tanks, resulting mixture pumped into the sea. OILPOL 1954 – Prohibited oil mixture discharge into sea within certain distance from the land in special areas. 1967 ‘TORREY CANYON’ ran aground while entering English Channel & spilled 1, 20,000 tons crude oil into sea. Biggest ever pollution incident recorded. Above incident and chain of incidents that eventually led to the adoption of ‘MARPOL’ 1969 –OILPOL 1954 Convention amended ‘Load on Top’. Saving & reducing pollution. Water was pumped OBD & the oil pumped on the oil kept in the tank. Apart from oil tankers, chemical tankers increased. 1971 – IMO adopted amendments to Oil Pollution Act 1954, which limited the size of cargo tank in all tankers after 1972. 1973 – Oct-Nov. incorporated much of Oil Pol. 1954 and its amendments into Annexes I, while other Annexes covered Chemicals, Harmful substances in packaged form, sewage &Garbage. Introduced Shore Reception Facility at terminals and special areas more stringent regulations, Mediterranean Sea, Red Sea, Gulf Sea Area & Baltic Sea. Annex –I, Reg.13, Segregated Ballast tanks on new tankers over 70,000 DWT There was slow progress on ratifying Annex II. 1976 – 1977, tanker accidents near US coast. Trading of ‘Agro Merchant’ led to more stringent regulations on accidents & Operational Pollutions. ‘Agro Merchant’ ran aground in US waters in Dec. 1976 whereby 27,000 tons of oil spilled caused huge public concern. Oil slick threatened resorts & the fishing industry. 1978 Tanker safety & Pollution Prevention, crude oil tankers of 20,000DWT & all new product carriers of 30,000 DWT require Segregated Ballast Tanks. New tankers >20,000 DWT require to be fitted with Crude Oil Washing System. Just one month after the 1978 conference, Amoco Cadiz spilled 2, 23,000 tons of crude oil, off France. 130 cm thick oil slick spread across 130 beaches on the French coast. Sufficient nos. of States had ratified MARPOL by October 1982 & MARPOL 1973/1978 Convention, entered into force on 2nd October, 1983. March 1989 ‘Exxon Valdez’ 1,264,155 barrels of crude oil ran aground in the NE portion of Prince William Sound spilling about 1/5th of its cargo, off US.US introduce oil pollution act of 1990 (OPA 90), making mandatory in US waters, to have double hull. 1992 Jan. IMO decided on two designs, Double Hull & Mid-Height Deck. MARPOL Annexure I, Reg.13F – Double hull entered into force July 1993 for Tankers delivered on or after 6th July 1996.Existing tankers -30 years after the date of delivery. Tankers of 5,000DWT and above must be fitted with double bottom & wing tanks to full depth of ship side. Tankers of 600 DWT and above and <5,000DWT must be fitted with double bottom. Capacity of each tank not to exceed 700m3, unless they are fitted with double hull. Regulation 13G- Enhanced programme of inspection to be implemented for tankers >5 years of age. 12th Dec. 1999, 37,238 DWT Tanker ‘ERIKA’ broke into two off the coast of Britain & France, carrying 30,000 tons of heavy oil, spilled 14,000 tons of oil. More than 100 miles of Atlantic coast line was polluted. This accelerated the phase out of single hull

Revised MARPOL adopted in October 2004 and entered into force on Jan.1st 2007.

Segregated Ballast, Clean Ballast, Dirty Ballast, Slop Tank and handling slops, Load-on-top and ODMCS. SEGREGATED BALLAST: means the ballast water introduced into a tank which is completely separated from the cargo oil and oil fuel system and which is permanently allocated to the carriage of ballast or to the carriage of ballast or cargoes other than oil or noxious liquid substances as variously defined in the annexes of the present convention.

CLEAN BALLAST and DIRTY BALLAST: Clean ballast means the ballast in a tank which, since oil was last carried therein, has been so cleaned that effluent there from if it were discharged from a ship which is stationary into clean calm water on a clear day would not produce visible traces of oil on the surface of the water or on adjoining shore lines or cause a sludge or emulsion to be deposited beneath the surface of the water or upon adjoining shore lines If the ballast is discharged through an oil discharge monitoring & control system approved by the administration, evidence based on such a system to the effect that the oil content of the effluent did not exceed 15 ppm shall be determinative that the ballast was clean, notwithstanding the presence of visible traces. If the discharged ballast does not meet these criteria shall be deemed to be Dirty Ballast.

SLOP TANK (RETENTION OF OIL ON BOARD): -ADEQUATE MEANS FOR CLEANING CARGO TANKS + TRANSFERRING TANK WASHINGS INTO SLOP TANK -APPROVED BY ADMINISTRATION -CAPACITY OF SLOP TANKS MUST BE SUFFICIENT TO RETAIN SLOPS GENERATED DURING TANK CLEANING SLOP TANK CAPACITY: -3% OF THE TOTAL CARGO CARRYING CAPCITY OF THE VESSEL. -MAY IN SOME CASES BE REDUCED TO 2% OF CARGO CAPACITY AS DECIDED BYTHE ADMINISTRATION

LOAD ON TOP: In1969 –OILPOL 1954 Convention was amended suggesting to ‘Load on Top’ thus Saving & reducing pollution. Water was pumped OBD & the oil pumped on the oil kept in the slop tank. When a Crude oil tanker completes discharge , remaining quantity of oil (up to 200 tons) may be left adhering to bulkheads and bottom. Tank cleaning will be carried out in the normal way. On completion of Tank cleaning the slop tank will contain all the tank washings. This mixture will contain small particles of oil held suspended in the water and water droplets will be suspended in the oil. For this reason the slop tank contents must be allowed to settle for 2-3 days after which oil can be expected to be floating on water. Thereafter the entire slop tank content is pumped through an Oily Water separator, with clean water discharged overboard and predominantly oily water pumped back into the cargo tank. At next load port fresh cargo is loaded on top of this mixture which would have been quantified prior to commencement of loading of new cargo. During the loaded passage the old and new cargo oil homogenize and water sinks to the bottom of tank. On arrival at disport, water dips are taken and the water quantity calculated and pumped direct to shore side slop tank.

ODMCS (Over-side discharging and Monitoring System) + OWS (Oily Water Separator) PUMP ROOM: -TYPE APPROVED BY ADMINISTRATION -FITTED WITH ALARM + BILGE PUMP STOP- SYSTEM -CONTINUOS RECORD OF DISCHARGE IN Ltrs/NM -TOTAL QUANTITY DISCHARGED -OIL CONTENT -RATE OF DISCHARGE -TIME & DATE /TYPE APPROVED BY ADMINISTRATION -FITTED WITH ALARM + BILGE PUMP STOP- SYSTEM -CONTINUOS RECORD OF DISCHARGE IN Ltrs/NM -TOTAL QUANTITY DISCHARGED Inert Gas system: Tanker ships have an inherent danger of fire and explosion and it is desirable that the atmosphere above the oil cargo or in an empty tank is such that it will not support combustion, by keeping these spaces fill with IG (Inert Gases). Such a system serves two main functions: 1) Use of IG avoids fire or explosion risk, and 2) It inhibits corrosion inside cargo tanks.

Boiler Flu IG consists of the following mix(assuming a well adjusted boiler): Component Percentage of IG Nitrogen 83% Carbon dioxide (CO2) 13% Carbon Monoxide 0.3% Oxygen 3.5% Sulphur dioxide 0.005% Nitrogen oxides Traces Water vapour Traces Ash Traces Soot Traces

Flu gases leave the boiler at about 300° C, contaminated with carbon deposits and sulphuric acid gas. The gas then passes through a scrubber which washes out the impurities and reduces the temperature to within 1°C of the ambient sea temperature. The clean cooled gas is now moisture laden and passes through a demister where it is dried. It is then fan assisted on passage towards the cargo tanks passing through a deck water seal and then over the top of an oil seal to enter at the top of the tank. It is allowed to circulate and is purged through a pipe which extends from the deck to the bottom of the tank. (See fig. 5.17) There is a sampling cock near the deck water seal for monitoring the quality of the IG. Individual tank quality is tested by opening the purge pipe cover and inserting a sample probe. Excess pressure in the cargo tanks being vented through a pressure vacuum valve (P/V valve)set at 2 psi, which is then led to a mast riser fitted with a gauze screen. The excess is then vented to atmosphere as far above the deck as practicable. Gas Detection System and the Explosi-meter: Many accidents and loss of lives have occurred due to lack of knowledge of gas detection methods and correct procedure to follow. The Explosi-meter (there are several trade makes are available) is used to detect the presence of flammable gas and/or air mixture. The Explosi-meter is an instrument which is specifically designed for measuring the Lower Inflammable Limit (LFL). It will only function correctively if the filament has an explosive mixture in contact with it. It is contained in a hand-held size box with a battery power supply. As per MARPOL Regulations Tankers of 20000 Tons DWT and above engaged in carrying crude oil must be fitted with an IG System.

Crude Oil Washing (COW), Advantages and Disadvantages: A procedure that is conducted during the discharge and which has positive advantages over water-washing methods. As per MARPOL Regulations New Crude Oil Carriers of over 20000 DWT must now be fitted with and use a COW facility. The method employs a high pressure jet of crude oil from fixed tank-cleaning equipment. The jet is directed at the structure of the tank and ensures that no slops remain on board after discharge, every last drop of cargo going ashore. The advantages are that i)Tank cleaning at sea is avoided; ii) with less likely hood of accidental pollution; iii)less tank corrosion is experienced compared to water washing; iv) increased carrying capacity is available for the next cargo; v) full tank drainage is achieved; and vi) gas-freeing time is saved for Dry dock periods. Some disadvantages of the system include a) crew workload which is increased at disport; b) discharge time is increased; c) It has high installation cost; d) Maintenance costs are increased; and e) Crew need special operational training.

Aspects of COW; The operational principle of the COW system is to use dry crude from a full tank to wash tank being discharged. Crude containing water droplets from the bottom of a tank should not be used for washing purposes as this may introduce water droplets that have become electro statically charged and produce an unnecessary source of ignition in the tank atmosphere. To this end any tank designated for use as COW should be first de-bottomed into the slop tank or bled ashore with the discharge pump. One of the main cargo pumps is used to supply the COW line with pressurized crude for washing operations. The line, along the track, will carry branch lines to all of the fixed machines. Large and Very Large Crude carriers (VLCC) may have up to six (6) machines per tank.

COW- Preparation and activities: Items of Pre-arrival check list: Prior to arrival at the port of discharge- 1. Has the terminal been notified? 2. Is oxygen analyzing equipment tested an working satisfactorily? 3. Are tanks pressurized with good quality IG (maximum 8% oxygen)? 4. Is the tank-washing pipeline isolated from water heater and engine-room? 5. Are all the hydrant valves on the tank-washing line securely shut? 6. Have all tank cleaning lines been pressurized and leakages made good?

In port - 1. Is the quality of the IG in the tanks satisfactory (8% oxygen or less)? 2. Is the pressure on the IG satisfactory? 3. Have all discharge procedures been followed and ship to shore check list completed?

Before Washing - 1. Are valves open to machines on selected tanks for washing? 2. Are responsible persons positioned around the deck to watch for leaks? 3. Are tank ullage gauge floats lifted on respective tanks to be washed? 4. Is the IG system in operation? 5. Are all tanks closed to the outside atmosphere? 6. Have all tanks positive IG pressure?

During washing - 1. Are all lines oil tight? 2. Are tank- washing machines functioning correctly? 3. Is the IG in the tanks being retained at a satisfactory quality? 4. Is positive pressure available on the IG system?

After Washing- 1. Are all the valves between discharge line and tank washing line shut down? 2. Has the tank washing main pressure been equalized and the line drained? 3. Are all tank washing machine valves shut?

After departure- 1. Have any tanks due for inspection been purged t below the critical dilution level prior to introducing fresh air? 2. Has oil been drained from the tank-washing lines before opening hydrant to the deck?

Loading and discharging operations on a Tanker: Loading- Loading of tankers takes place at Jetties, from FSUs or from SBM. Where booms carrying oil-bearing pipes are to be connected, these will be insulated to prevent stray currents flowing, as from corrosion preventive systems employed both on ships and on jetties. The flow of current in itself should not be a problem, but it may give rise to a spark when making or breaking connections to the manifold. For this reason these sections are tested regularly for efficient insulation. Lines are often bonded to reduce static electricity effects which could also give rise to an unwanted source of ignition from the fast pumping of liquids.

These points are highlighted to illustrate that a high degree of awareness is required in all tanker operations whether loading, discharging or Gas freeing. Fire precautions are paramount because the risk of fire aboard the tanker is a real hazard and stringent fire precautions must be adopted throughout cargo operations of every kind.

Loading Procedural checklist: Company policy on loading procedures varies and Cargo officers should adhere to the Company procedures and take additional reference from te International Safety Guide for Oil Tankers and Terminals (ISGOTT). 1. Complete and sign the ship/shore checklist. 2. Establish an agreed communication network. 3. Agree the Loading Plan by both parties and confirm in writing. 4. Loading and Topping off rates agreed. 5. Emergency Stop procedures and signals agreed. 6. All effected tanks, lines, hoses inspected prior to commencing operations. 7. Overboard Valve sealed. 8. All tanks and lines fully inerted. 9. IG System shut down. 10. Pump room isolated and shut down. 11. Ships lines set for loading. 12. Offside Manifolds shut and blanked off. 13. All Fire fighting and Ship’s SOPEP Plan equipment in place. 14. Notice of Readiness served/Accepted. 15. First set of tanks and Manifold valves open. 16. Commence loading at a slow rate. 17. Check and monitor the first tanks to ensure cargo is being received. 18. Carry out Line Sample. 19. Check all round the vessel and over-side for any leaks. 20. Gradually increase the loading rate to full. 21. Check ullage at half hourly intervals and monitor flow rate to confirm with shore side figures. 22. Check that valves operate into next set of tanks prior to change over. 23. Reduce loading rate when topping off final tank. 24. Order ‘Stop’ in ample time to achieve the planned ullage/line draining. 25. When the cargo flow has completely stopped close all the valves. 26. After settling time, take ullages, temperatures and samples. 27. Ensure that all Log book entries are completed. 28. Cause an entry to be made into the Oil Record Book. N. B. The loading plan devised by Chief Officer and Shore-side Authority would take account of the ship’s stability and the possibility of stresses being incurred during all stages of loading operations.

Discharging: Flexible hoses are connected to ship’s manifold. As at the loading port, and ship to shore check list would be completed. Good communications between the ship and the shore authority is essential. All overboard discharges should be checked and if all valves are correct, discharge would be commenced at an initial slow rate. This slow rate is commenced to ensure that if a sudden rise in back pressure is experienced in the line, discharge can be stopped quickly. Such an experience would probably indicate that the receiving lines ashore are not clear, Back pressure should be continually monitored during the discharge operations and the ship, using ship’s pumps, should be ready to stop pumping at short notice from a signal from the terminal. The waterline around the ship should also be kept under regular surveillance in the event of leakage occurring. As with loading operations, the deck scuppers should all be sealed and SOPEP recommendations followed. All Fire-fighting equipment should be kept readily available throughout the operation.

Procedure for entry in enclosed spaces on Tanker: An enclosed space is one that has been closed or unventilated for some time, any space that may, because of cargo carried, containing harmful gases; any space which may be contaminated by cargo or gases leaking through a bulkhead or pipeline; any store room containing harmful materials; or any space which may be deficient of oxygen, like Chain locker, pump rooms, void spaces, CO2 rooms, cofferdams etc. A ‘Permit to work’ should also be obtained and a ‘Risk Assessment’ completed prior to any person entering an enclosed space. Any person intending to enter such an enclosed space must seek correct authorization from the ship’s Master or Chief Officer-in-charge. Entry would be permitted in accordance with the conditions stipulated by a ‘Permit to work’ for entry into enclosed spaces. The Senior officer would also complete a ‘Risk Assessment’ prior to entry taking place and all safety procedures must be monitored by an appropriate safety checklist. A suggested line of action for permitted entry into enclosed spaces is suggested as under: 1. Obtain correct authorization from the Chief Officer. 2. Ensure that the space to be entered has been well ventilated and tested for oxygen content and/or toxic gases. 3. Check that ventilation arrangements are continued while persons are engaged inside the tank space. 4. Ensure that rescue system and resuscitation equipment are available and ready for immediate use at the entrance to the space. 5. Persons entering have adequate communication equipment established and tested for contact to a stand-by man outside the enclosed space. 6. A responsible person is designated to stand by outside the space to be in constant attendance while persons are engaged inside the space (to raise the alarm in the event that difficulties are experienced by those persons entering the space). 7. Ensure that the space to be entered is adequately illuminated prior to entry and that any portable lights are intrinsically safe and of an appropriate type. 8.Regular arrangements for the testing of the atmosphere inside the space should be in place. 9. A copy of the ‘Permit to work’ must be displayed at the entrance of the space to be entered. 10. Prior to the entry, all operational personnel must have been briefed on a with drawl procedures from the space, in the event that such action is deemed necessary. N.B. When the atmosphere inside an enclosed space is known to be unsafe, entry should not be made into space.

Where the atmosphere in the compartment is suspect, the following additional safety precautions should be adopted with the use of “Breathing Apparatus” (B/A): 11. Endure that the bearer of the B/A is fully trained in the use of the B/A. 12 Thorough checks on the B/A equipment must be made and the ‘Mask seal’ on the face of the bearer must be proper fit. 13. The stand-by man should monitor the times of entry and exit of all personnel to allow adequate time for leaving the enclosed space. 14. Rescue harness and lifeline must be worn. 15.If the low pressure whistle alarm is activated the bearer must leave the space immediately. 16. In the event of communication or ventilation system breakdown, persons should leave the space immediately. 17. Operational personnel should never take the mask of the B/A off when inside the space. 18. The function of the stand-by man is only to raise the alarm if necessary. He should not attempt to affect a single-handed rescue with possible consequences of escalating the incident. 19. Emergency signals and communications should be clarified and understood by all affected parties. 20. A risk assessment must be completed by the officer-in-charge to take account of the items covered by the Safety check list, the age and experience of the personnel involved, the prevailing weather conditions, the reliability of equipment in use, the possibility of related overlap of additional working practices ongoing, the technical expertise required to complete the task and the time factor of how long the task is expected to take.

In all cases of enclosed space entry, the use of protective clothing, suitable footwear and the need for eye protection must be considered as an essential element of any risk assessment.

The Use of Oxygen Analyzer: In order for an atmosphere to support human life it must have the oxygen content of 21%. The oxygen analyzer is an instrument that measures the oxygen content of an atmosphere to establish whether entry is possible, but it also employed for inerted spaces which must be retained under 5% oxygen to affect a safe atmosphere within the tank.

The Oxygen sensor will be either an electromagnetic heated filament or an electrochemical resistor cell. The instrument was designed to measure the oxygen content only and will not detect the presence of any other gases. As sown in the figure above the resistor filaments R3 and R4 are of equal rating. The resistor filament R3 is surrounded by a magnetic field. The atmosphere sample drawn past the filament will depend on the permitted current flow through the coil and meter, depending upon the amount of oxygen in the sample. Oxygen analyzers are portable instruments which draw a sample of the atmosphere for testing through a sampling hose by means of a rubber aspirator bulb. The principle of operation is a self-generating electrolytic cell in which the electric current is directly proportional to the percentage oxygen in a salt solution connecting to the electrodes. The electrodes are connected to a micro-ammeter, so that the current read by the meter can be calibrated to indicate directly the %age oxygen of the sample.

Explosimeter , Tankscope and Multi-gas detector using tubes: An explosimeter is a device used to detect the amount of combustible gases present in a sample of the given atmosphere. This gives the reading in terms of percentage of the LFL (lower flammable limit).

“Resistance proportional to heat" is the working principle. The equipment consists of a Wheatstone bridge in which one of the resistances is variable. The circuit is shown below:

It consists of four resistances in which one varies according to the amount of the gas present. A hand pump is used to draw the gas or the atmosphere containing the gas inside the device. A filter and flash back arrestor is used to filter the gas and also acts as a flame arrestor.

The device is switched on. As the hand pump is operated to suck a sample of gas from the cargo tank, simultaneously the filament gets heated. Any combustibles in the sample will land on the filament in the sample chamber. The combustibles will burn as the filament is already hot causing an increase in resistance which disturbs the Wheatstone bridge. The reading can be read from the indicator. The instrument gives the reading in percentage of the Lower Flammable Limit or Lower Explosive Limit which is 1%.

This type of gas meter can only be used if the gas content is very low (i.e.) this instrument should not be used if the atmosphere contains: http://www.brighthubengineering.com/marine-engines-machinery/84331-gas-detection-meters-for-ships/#

· H/C + inert gas – then the gas will not burn as there is no oxygen

· H/C + oxy-acetylene – then the burning will be too violent

· H/C + oxy-hydrogen – Same as above

· Lead petroleum vapors – Lead oxide deposits on the filament cause a reduction in sensitivity

All meters require calibration. This meter requires the following before using:

· Zero check

· Span check

· Battery check

Proper working of the equipment can be achieved by regular maintenance. Clean the filters regularly and it is advisable to have it serviced by the manufacturer once every six months.

Tankscope or Non-combustible Gas Indicator A Tank scope is a device used for measurement of hydrocarbon gas content in a sample of given atmosphere. This instrument is meant for measuring the hydrocarbon vapor in inerted atmospheres. This instrument is not as sensitive as the explosimeter. The reading is only in percentage of the volume of the hydrocarbon vapor and hence used only during the gassing up operations and during inerting. This is purely meant for measuring the volume of the hydrocarbon vapors present inside any enclosed space, and hence it is not meant for measuring during a man-entry. It works on the same principle as that of an explosimeter except that the gas does not burn inside the sample chamber; there is an alteration in the temperature of the heated filament which enhances the change in resistance. It is always advisable to flush the sample tube with fresh air after every use. The following checks are done to ensure the proper working of the instrument:

· Zero check

· Span check

· Voltages check (battery check)

Multi-Gas Analyzers

Multi-gas analyzers are used to detect only targeted gases and vapors. It is very specific to that type of gas only, so care has to be taken to ensure that correct tubes are used for the particular type of gas. The multi-gas analyzer consists of a portable bellows pump and detector tubes. The detector tube is like a vial filled with reagent that will react with the specific chemical. Both the ends of the tube are closed. In order to use it we have to break the two ends of the tube and insert it into the pump according to the directions mentioned on the tube. Now start pumping 3-4 times (or as specified by the manufacturer) to suck in the particular gas from the atmosphere. If the atmosphere contains that particular gas or vapor, then the color of the tube changes. The length of the color change can be read from the tube and compared to obtain the level of that particular gas or vapor. Some of the gases include carbon monoxide, chlorine, hydrogen sulphide, organic arsenic compounds, arsine, and phosphoric acid esters. An extension hose is provided to measure the concentration of vapor present at a different height. In a situation like this, we have to insert the hose with the pump and the tube connected to the other end of the hose.

Cargo Pumps (Centrifugal, Reciprocating, Eductors):

The function of any pump is to transfer liquid from one point to another and this involves the use of piping. Such a transfer in a tanker can be divided into two parts:-

1. The movement of liquid from the tank to the pump. This is a function of the pump and its installation design. These factors are beyond the control of the ship provided the design ratings of the pump are maintained.

2. The onward movement of the liquid from the pump to its destination. This is an area where the efficient operation of the pumps is essential if optimum results are to be obtained.

The major factors influencing pumping performance are discussed below. The flow of liquid to and from the pump must be matched exactly and this requires the flow on the suction side to be equal or greater than the discharge rate of the pump. Where the flow to the pump suction falls below the pumping rate cavitations will occur with the possibility of loss of suction and pump damage. Centrifugal pumps do not suck liquids. The only factors which cause liquid to flow to the pump are:-

Fig: Submerged pump

• Pressure acting on the surface of the liquid.

• The height of the liquid level in the tank in relation to the pump suction. Since no centrifugal pump can generate a total vacuum at its suction inlet, only a proportion of the atmospheric pressure can be usefully employed. Therefore, before a pump can operate satisfactorily, a certain pressure must exist at the pump suction and this is known as the required Nett Positive Suction Head.

Centrifugal PumpsThe centrifugal pump has for many years been the most suitable pump where a high pumping capacity is the most important factor. The size and cost of such a pump does not increase in proportion with the throughput, as it is not a positive displacement pump. It requires either the provision of ancillary self-priming equipment for the removal of air in the system or a separate stripping system.In a centrifugal pump the motive force is provided by a rotating impeller which takes its suction at its centre and centrifuges the pumped liquid outwards to the casing discharge. The head generated is dependent on the diameter, blade angle and speed of rotation of the impeller. Flow rate is affected by the pressure in the discharge system and can fall to zero. Reverse flow through the pump can occur if a non-return valve is not fitted and operational on the discharge side of the pump. The correct and efficient use of centrifugal pumps requires the observance of certain basic operating principles. Guidance on these principles is given here however, as manufacturers may incorporate special design features to meet operational requirements, the information given here must be read in conjunction with the manufacturers operating instructions and on board procedures organized. The basic characteristics of a centrifugal pump are:-

· Throughput varies with speed.

· Head varies as speed squared.

· Power required varies as speed cubed.

These relationships are subject to appreciable variation caused by the system in which the pump operates. CENTRIFUGAL  pump is basically a kinetic pump which increases the flow when through the pump—it centrifuges the liquid into the discharge line.

If the flow into the pump suction is less than the delivery the pump will cavitate causing gassing and loss of suction. They are resistable to solids in cargo. The big disadvantage is its inability to evacuate air/ gas from its casing. The pump casing must be filled with liquid before starting—the pump must be stopped to do this. When the impeller starts to turn the liquid is driven to the periphery of the housing by centrifugal forces. This results in a positive pressure on the outside of the impeller and a negative pressure in the centre.

Centrifugal pumps can be operated with dischg valve closed as long as the pump does not grow too hot. The last part of dischg is more effective with pump running at low RPM and against a throttled delivery valve. For hot liquids and those which flash into vapour at pressures below atmospheric pressure, the liquid must flow into the pump suction under positive pressure. Never run a centrifugal pump dry. The pump bearing temp must not exceed 90C. No liquid will be delivered if the pump rpm is too less when the dischg head is too high or suction lift is too high . The viscosity of liquid cannot be too high. There cannot be any leaks on the suction side pipelines or air leak in the shaft seal or vaporization in the suction line. Never try to reduce pump capacity by throttling the valve on the suction side.

The casings of centrifugal pumps without a self priming system should be vented of gas and primed full of liquid before starting the pump.

The pump suction valve should be full OPEN and a flow of liquid should be available to the pump suction when the pump is started.

Positive Displacement Pump (Reciprocating Pump)Unlike the centrifugal pump, the positive displacement pumps used in dedicated stripping systems are capable of a low suction pressure and the ability to pick-up suction without external priming. This type of pump includes steam reciprocating pumps and ‘screw’ type pumps. Both types are now mainly used for stripping tanks or as specialized cargo pumps.The suction and discharge valves of a positive displacement pump must always be open before starting the pump and must remain open until the pump is stopped. These pumps must not be operated in excess of their design speed and particular care must be taken to avoid these pumps over-speeding when they lose suction. Pressure relief devices must be checked at regular intervals to ensure their correct operation. Use of eductorsEductors may be used for ballast stripping purposes. To strip efficiently, an eductor used for tank cleaning operations should have a capacity of about twice the rate of liquid being introduced to the tanks.

• Eductors are always to be operated at or near their design driving pressure as, in general, lower driving pressures will considerably reduce eductor efficiency. Higher back pressures in the system than the eductor was designed for can also reduce suction capacity. • The eductor drive liquid must always be flowing before the suction valve is opened to prevent back flow of the driving liquid to the tank suction. • When shutting down an eductor the suction valve is to remain open until the eductor is stopped to prevent the eductor drawing a vacuum on the suction line. • If, during use, the eductor driving pressure falls below the required operating pressure, the eductor suction valve is to be closed to prevent backflow of the driving liquid. The tank suction must not be used to prevent backflow as the suction pipe work is not designed for such high operating pressures.

Water Jet Eductor

Contents and application of the Internationa Safety Guide for Oil tankers and Terminals ISGOTT Purpose and Scope This guide makes recommendations for tanker and terminal personnel on the safe carriage and handling of crude oil and petroleum products on tankers and at terminals. It was prepared originally by combining the contents of the ‘Tanker Safety Guide (Petroleum)’ published by the International Chamber of Shipping (ICS) and the ‘International Oil Tanker and Terminal Safety Guide’ published on behalf of the Oil Companies International Marine Forum (OCIMF). In producing this fourth edition the content has been reviewed by these organisations, together with the International Association of Ports and Harbors (IAPH), to ensure that it reflects current practices and legislation. This latest edition takes account of certain changes in recommended operating procedures. Comments made by the International Maritime Organization (IMO) on the terminology used in the guide have also been noted and changes made in order to avoid possible misinterpretation. The purpose of the guide is to provide operational advice to assist personnel directly involved in tanker and terminal operations. It is emphasised that the ship’s operator should always be in a position to provide positive support, information and guidance to the master who is in charge of the day-to-day running of the ship, and that the terminal management should ensure that its concern for safe operating practices is known to the terminal personnel. It should be borne in mind that in all cases the advice in the guide is subject to any local or national terminal regulations that may be applicable, and those concerned should ensure that they are aware of any such requirements. It is recommended that a copy of the guide be kept — and used — on board every tanker and in every terminal to provide advice on operational procedures and the shared responsibility for port operations. The contents of the guide are arranged in two parts. Part I covers operational procedures and is designed to provide guidance on safe operating practices. The approach adopted has been to arrange the material so that each chapter is concerned with a particular type of operation. However, some chapters deal with the general precautions to be observed in conjunction with the specific guidance for the particular operation concerned. Each chapter has a small introductory paragraph describing its scope and drawing attention to related advice contained in other chapters. Part II contains more detailed technical information and provides the reasons for many of the precautions described in Part I.Certain subjects are dealt with in greater detail in other publications issued by ICS and OCIMF or by IMO. Where this is the case an appropriate reference is made, and a list of these and other related publications is given in the bibliography. It is not the purpose of the guide to make recommendations on design or construction. Information on these matters may be obtained from national authorities and from authorised bodies such as classification societies. Similarly, the guide does not attempt to deal with certain other safety related matters — e.g. navigation, helicopter operations, and shipyard safety — although some aspects are inevitably touched upon. It should also be noted that the guide does not relate to cargoes other than crude oil and petroleum products which may be carried in tankers and combination carriers. It therefore does not cover the carriage of chemicals or liquefied gases, which are the subject of other industry guides.

ISGOTT (International Safety Guide for Oil Tankers and Terminals) 5th Edition

This is the definitive Guide to the safe carriage and handling of crude oil and petroleum products on tankers and at terminals.

ISGOTT was first published in 1978 and combined the contents of the ‘Tanker Safety Guide (Petroleum)’, published by the International Chamber of Shipping (ICS), and the ‘International Oil Tanker and Terminal Safety Guide’, by the Oil Companies International Marine Forum (OCIMF). In producing this Fifth Edition, the content has again been reviewed by these ICS and OCIMF, together with the International Association of Ports and Harbors (IAPH), to ensure that it continues to reflect current best practice and legislation.

This edition takes account of recent changes in recommended operating procedures, particularly those prompted by the introduction of the International Safety Management (ISM) Code, which became mandatory for tankers on July 1^st^, 1998. Also, account has been taken of latest thinking on a number of issues including the generation of static electricity and stray currents; the use of mobile telephones and pagers, which are now ever present; the use of new materials for mooring lines and emergency towing-off pennants; the toxicity and the toxic effects of benzene and hydrogen sulfide; and, importantly, the introduction of the principles underlying the International Safety Management (ISM) Code and the International Ship and Port Facility Security (ISPS) Code. The Ship/Shore Safety Check-List has been completely revised to better reflect the individual and joint responsibilities of the tanker and the terminal.

The Guide is now divided into four sections:

· General Information

· Tanker Information

· Terminal Information

· Management of the Tanker and Terminal Interface

The Guide provides operational advice to directly assist personnel involved in tanker and terminal operations, including guidance on, and examples of, certain aspects of tanker and terminal operations and how they may be managed. It is NOT a definitive description of how tanker and terminal operations are conducted.

Care has been taken to ensure that, where the guidance given in previous editions was still relevant and accurate, it has not been changed or deleted in moving to the new format. ISGOTT continues to provide the best technical guidance on tanker and terminal operations. All operators are urged to ensure that the recommendations in this Guide are not only read and fully understood, but also followed.

It is a general industry recommendation that a copy of ISGOTT is kept and used onboard every tanker and in every terminal so that there is a consistent approach to operational procedures and shared responsibilities for operations at the ship/shore interface.

Hazards of Petroleum

In order to appreciate the reasons for the practices adopted to ensure safety in tanker and terminal operations, all personnel should be familiar with the flammable properties of petroleum, the effects of the density of petroleum gases and their toxic properties. This Chapter contains a brief summary, and more detailed information is given in Chapters 15 and 16.

FLAMMABILITY

When petroleum is ignited, it is the gas progressively given off by the liquid which burns as a

visible flame. The quantity of gas available to be given off by a petroleum liquid depends on its

volatility which is frequently expressed for purposes of comparison in terms of Reid vapour

pressure. A more informative measure of volatility is the true vapour pressure but unfortunately

this is not easily measured. It is referred to in this guide only in connection with venting

problems associated with very volatile cargoes, such as some crude oils and natural gasolines.

Petroleum gases can be ignited and will burn only when mixed with air in certain proportions. If

there is too little or too much petroleum gas the mixture cannot burn. The limiting proportions,

expressed as percentage by volume of petroleum gas in air, are known as the lower and upper

flammable limits. They vary amongst the different possible components of petroleum gases. For

the gas mixtures from the petroleum liquids encountered in normal tanker practice the overall

range is from a minimum lower flammable limit of about 1% gas by volume in air to a maximum

upper flammable limit of about 10% gas by volume in air.

As petroleum liquid is heated the concentration of gas in air above it increases. The temperature of the liquid at which this concentration, using a specific measuring technique, reaches the lower flammable limit is known as the flashpoint of the liquid.

FLAMMABILITY CLASSIFICATION There are many classification systems for defining the flammability characteristics of petroleum liquids, most of which are based on flashpoint and Reid vapour pressure data. For the purpose of this guide, which deals only with the particular conditions in petroleum tanker cargo handling, the division of such liquids into the two broad categories of non-volatile and volatile, as defined below, is in general sufficient to ensure that proper precautions can be specified. · Non-Volatile Flashpoint of 60ºC or above as determined by the closed cup method of testing.

· Volatile Flashpoint below 60ºC as determined by the closed cup method of testing. If there is any doubt as to the characteristics of a cargo, or if a non-volatile cargo is being handled at a temperature above its flashpoint minus 10ºC, it should be treated as volatile petroleum. Owing to their particular characteristics, residual fuel oils should always be treated as volatile (see Chapter 24).

GAS DENSITY The gases from normal petroleum liquids are heavier than air and inert gas, thus the possibility of layering of gases is very important in cargo handling operations. The density of the undiluted gas from a high vapour pressure distillate, such as motor gasoline, is likely to be about twice that of air and about 1.5 times that from a typical crude oil. These density differences diminish as the gases are diluted with air. Flammable mixtures usually contain at least 90% by volume of air and consequently have densities almost indistinguishable from that of air. Detailed information on the density of petroleum gases is given in Chapter 15.

TOXICITY Comparatively small quantities of petroleum gas when inhaled can cause symptoms of diminished responsibility and dizziness similar to drunkenness, with headache and irritation of the eyes. The inhalation of a sufficient quantity can be fatal. These symptoms can occur at concentrations well below the lower flammable limit. However, petroleum gases vary in their physiological effects and human tolerance to these effects also varies widely. It should not be assumed that because conditions can be tolerated the gas concentration is within safe limits. The smell of petroleum gas mixtures is very variable, and in some cases the gases may dull the sense of smell. The impairment of smell is especially likely and particularly serious if the mixture contains hydrogen sulphide. The absence of smell should never be taken to indicate the absence of gas. More detailed information on the toxic properties of petroleum, and of substances associated with the carriage of petroleum, is given in Chapter 16.

Cargo Calculation for Quantity and Ullage of Oil cargo based on volume and height of space, density of cargo and temperature change.

Measurement of liquid cargoes: The volume of oil in a tank is ascertained by measuring the distance from a fixed point on the deck to the surface of oil. The distance is known as Ullage and is usually measured by means of a plastic tape. A set of tables are supplied to every ship, which indicate for each cargo compartment, the volume of liquid corresponding to a range of ullage measurements. The ullage opening is usually set as near as possible to the center of the tank so that for a fixed volume of oil, the ullage is not appreciably affected by conditions of trim and list. If a favourable siting is not possible then the effects of list and trim should be allowed for. The important measure of oil is weight and this must be calculated from the volume of oil in each tank. Weight in tons is quickly found by multiplying the volume of oil in Cubic meters by the Relative density (RD) of the oil. This density is a fraction and may be taken out of Petroleum Tables when te Rd of the oil is known. Weight of oil= Volume x density.

Oil expands when heated and it’s RD decreases with a rise in temperature. In order that the weight may be calculated accurately, it is important that when ullages are taken the RD of oil should also be known. This may be measured directly by means of a hydrometer.

The RD of a particular oil may be calculated if the temperature of the oil is taken. The change of RD due to a change of 1ºC in temperature is known as the RD coefficient. This lies between 0.0003 and 0.0005 for most grades of oil and may be used to calculate the RD of an oil at any measured temperature if te RD at some standard temperature is known.

Change in RD = Difference in Temperature x expansion coefficient.

Use of the Whessoe Tank Gauge: The function of the gauge is to register the ullage of the tank at any given time, in particular when the liquid level in the tank is changing during the loading and discharging periods. The gauge is designed to record the readings not only at the top deck level of the tank but also remotely at a central cargo control room. A transmitter is fitted on the head of the gauge for just this purpose.

The unit is totally enclosed and various models manufactured are suitable for use aboard not only oil tankers, but Chemical and Gas carriers as well. (Fig. 5.13)

Inside the gauge housing is a calibrated ullage tape, perforated to pass over a sprocket wheel and guided to a spring-loaded tape-drum. The tape extends into the tank and is secured to a float of critical weight, As the liquid rises or falls, the tape is drawn into, or extracted out from, the drum at the gauge head. The tape-drum, being spring loaded, provides a constant tension on the tap, regardless of the amount of tape paid out. A counter window for display is fitted into the gauge head, which allows the ullage to be read on site at the top of the tank.

_______________________