|Grain Cargo||Cargo Care||IMDG||Cargo Handling Equipment||Cargo Handling Safety|
|Oil Tanker||Cargo Measurement||Enclosed Spaces|
A tanker is a specialized ship intended for the carriage of bulk liquid cargo. An Oil tanker again is further divided into 2 basic types, namely Crude Oil Tanker and Product Oil Tanker.
For both of the above the cargo of oil is carried within the tanks similar to the holds of other ships, the difference being that the bulkheads are extra strengthened to take in the load, and the hatch or rather the tank openings are very small, the sole purpose of having them is for Man Entry and for small repair work in the dry docks.
The cargo of oil is loaded on to the ships tanks by pipelines, which are fixed on the ship (permanent structure), the shore pipelines are connected to the ships pipelines at the manifold on either side of the ship. Note that some special ships also have manifolds at the bow and at the stern.
The shore pipelines may be connected using flexible steel rimmed rubber hoses (small ports/ Ship to ship transfers/ SBM) – the flexible come in small lengths are connected to each other to make them long pieces.
The shore pipelines may also be connected with rigid loading arms – also called ‘chiksons’, which are remotely controlled and take in the roll of the ship to a certain extent but the fore and aft movement of the ship has to be kept to a minimum.
The combined pipeline system of the shore and the ship deliver the oil to the cargo oil tanks directly via the drop lines. These are as the name suggests pipelines, which drop to the bottom of the tanks vertically from the pipeline on deck – thus bypassing the pump room.
There are various cross- over valves, which are opened in order to load a group of tanks.
The shore system starts to pump/ delivers by gravity
To prevent this surge from affecting the pipelines the cargo valves have set times at which they close – this depends on the size of the valves – typically a 550mm valve would shut at about 24 seconds, whereas a 250mm valve would shut at 6-8 seconds.
After the ship completes her loading the stage is set for the unloading or discharging operation.
While loading the cargo had by passed the pump room, now however the cargo from the tanks is allowed to flow to the pump room through the bottom pipelines. Just within the pumproom and at the pumproom bulkhead are situated isolation valves known as ‘Bulkhead Master valves’, by opening the valves the oil is led to the pump suction valve and on opening that the oil flows to the centrifugal pumps. Turbines, which are situated in the Engine Room, commonly drive these pumps; the shaft penetrates the ER bulkhead and drives the pump situated at the bottom of the pumproom.
The pump accelerates the flow of the oil into the discharge pipeline and this oil is thus led on the deck pipelines and to the manifold from where it flow through the flexible pipeline or the hard loading arm to the shore pipeline system.
The Pump Room
This is a cofferdam kind of space – in fact it is accepted as a cofferdam, which begins on main deck and ends at the keel.
It may have more than 2 decks, however these decks are not normally solid decks but are partial decks made of expanded metal, so you are able to see right to the bottom.
There would be a companionway leading from the top to the next deck and so on right to the bottom.
At the lowermost deck are situated the Cargo Oil Pumps (COP’s). The numbers of pumps vary in number – for crude oil tankers it is normal to have 4 pumps, three being used at any one time.
For product oil tankers the number of pumps depend on the number of grade of oil that the ship is capable of carrying.
So if the ship can carry 4 grades of oil then she would be having 4 pumps.
Once the gravity flow to the COP’s is not possible the stripped pumps are started, these pumps are of the reciprocating type and have great capacity to create partial vacuum to suck out the remaining oil from the tanks. Again on a product oil tanker the number of stripped pumps would be equal to the number of grades of oil that it can carry.
Earlier on Crude oil carrier there would be stripper pumps of the reciprocating type however today largely eductors are used to remove the remaining oil from the tank. Generally 2 eductors are provided on each crude oil tanker. However 1 stripper pump is always provided to strip the cargo lines of any residual oil and to pump the same to the shore system.
The pumproom is a hazardous area as such the light fittings are gas tight and only tanker safety torches are used. The ventilation system is of the exhaust type and has intakes from all the levels with the intakes being fitted with closing devices so that if required only a certain level can be evacuated.
Hydrocarbon gases being heavier than air tend to settle at the bottom of the pumproom as such the main exhaust are always from the bottom level.
The pumproom lighting is devised in such a way that the lights do not come on unless the ventilation has been started and is kept on for 15 minutes.
AT the top of the pumproom a harness and lifting arrangement is provided to lift out a person from the lowermost deck, for this reason a clear passage is left vertically from the top to the bottom of the pumproom.
Fire man’s outfit are also placed at the top of the pumproom, the pumproom may have different types of fixed fire fighting appliances such as total flooding by CO2 or by foam applicators fitted in the bilges (below the floor plates under the lowermost deck).
Bilge alarms are fitted which give alarms when the bilges are filled – a high level and a low level alarm is fitted which gives indications in the Engine room as well as in the Cargo Control room.
Picture shows the main deck layout of a Product tanker (capable of carrying 4 grades of oil):
The same tanker – with the tank layout.
And part of the pump room layout of the same tanker.
The above shows the location of the drop valves; drop lines, line master, bulkhead master and the bottom lines.
Cargo Oil Pumps (COP)
A centrifugal pump, in the pumproom bottom platform. The dark green pipeline is the discharge line. The pump consists of an impeller which rotates within the casing. Due to this rotation which is generally about 1000 – 1700 rpm the oil is speeded up and this increase in velocity causes the oil to flow out at a great pressure. These pumps are capable of delivering a very high rate of discharge (up to 4000 m3/hr). With this type of pump the level of oil has to be above the pump – as such the pump is situated at the bottom of the pump room.
Another detail of the same centrifugal pump.
The earlier centrifugal pump situated in the pumproom is driven by a shaft which is connected to the steam turbine – situated in the ER. The shaft passes from the ER to the pumproom through the pumproom bulkhead via a gas and oil tight gasket.
The turbines are driven by superheated steam from the boiler in the ER.
Positive displacement pumps such as the reciprocating pump work on the principle of a hand pump – the movement of the piston creates a vacuum which sucks out the fluid. However the size of the pump is dependent on the size of the piston and the length of the strokes so for discharging at a high rate is practically impossible. In general these pumps are used to discharge small quantities of oil such as the strippings – the balance that the centrifugal pump cannot discharge due to the oil going below the level of the pump. The pump is used today on crude tankers to strip out the pipelines after discharging and then collecting these line content (small) and then pumping them to shore.
Eductors work on the principles of Bernoulli’s Principle.
A driving fluid is pumped down the main line, with very high velocity, through a constriction, and past a relatively smaller opening, thus creating a vacuum.
When eductors are used for clean ballast, the driving fluid is seawater.
When used for stripping crude oil, the driving fluid is the cargo itself- delivered by means of a bypass from one of the main cargo pumps.
When used for stripping tank washings, the driving fluid is from the secondary slop tank and then re-circulated back to the primary slop tank. In the latter case the driving fluid is either crude oil or seawater, depending on the tank cleaning method.
Eductors are simple and rugged, have no moving parts, and do not become air locked like other type of pumps. They are widely used on tankers of all types and sizes.
Tank layout of a crude oil tanker:
The Pipeline system:
Pipeline systems on tankers differ in their degree of sophistication, depending on employment of the tanker.
ULCC’s and VLCC’s have relatively simple pipeline systems usually the direct line system.
Some product (parcel) tankers may have very sophisticated piping systems. This could be the ring main system or in case of a chemical product tanker it could mean an individual pipeline and an individual pump for every tank on board.
Basically there are three systems of pipelines found on tankers, and the fourth system being the free flow system found on large crude carriers
Ring Main System
Direct line system
Single line to Single tank system (Chemical/Product ship)
Free Flow system
Ring Main System:
It is generally of a square or circular layout.
It is used mostly on product tankers, as segregation of cargo is required.
Though the system is expensive, as more piping, and extra number valves are used.
However if the vessel is carrying many grades of cargo, the advantages compensate for the extra cost of the original outlay.
Direct Line System:
This system is mainly found on crude oil carriers where up to 3 grades of cargo can be carried as most of the direct pipeline systems is fitted with three direct lines.
This system is cheaper to construct. The disadvantages over the ring main system, is that line washing is more difficult, the system has fewer valves which make pipeline leaks difficult to control, as the system lacks versatility there is problem with line and valve segregation.
This system provides the vessel to carry as many grades as there are tanks.
disadvantage is the cost factor having a multitude of pumps on board.
Free flow Tanker:
This system is usually found on large crude carriers, where the cargo piping is not used for the discharge of cargo.
Instead, gate valves are provided on the bulkheads of the tanks which when opened; allow the oil to flow freely in the aft most tank and into the COP.
The advantages of this system are primarily the cost factor, it allows for fast drainage and efficient means of pumping the cargo tanks. Disadvantages are of single crude being shipped.
This layout is not very common in the tanker trade.
This system is quite normal on chemical ships.
There are some Product Tankers that have this system fitted on the ships.
This is a single line servicing an individual tank through an independent pump that could be either a submersible pump or a deep well pump.
Enclosed Space Entry
An enclosed space is one with restricted access that is not subject to continuous ventilation and in which the atmosphere may be hazardous due to the presence of hydrocarbon gas, toxic gases, inert gas or oxygen deficiency. This definition includes cargo tanks, ballast tanks, fuel tanks, water tanks, lubricating oil tanks, slop and waste oil tanks, sewage tanks, cofferdams, duct keels, void spaces and trunkings, pipelines or fittings connected to any of these. It also includes inert gas scrubbers and water seals and any other item of machinery or equipment that is not routinely ventilated and entered, such as boilers and main engine crankcases.
Many of the fatalities in enclosed spaces on oil tankers have resulted from entering the space without proper supervision or adherence to agreed procedures. In almost every case the fatality would have been avoided if the simple guidance in this chapter had been followed. The rapid rescue of personnel who have collapsed in an enclosed space presents particular risk. It is a human reaction to go to the aid of a colleague in difficulties, but far too many additional and unnecessary deaths have occurred from impulsive and ill-prepared rescue attempts.
Respiratory hazards from a number of sources could be present in an enclosed space. These could include one or more of the following:
Respiratory contaminants associated with organic vapours including those from aromatic hydrocarbons, benzene, toluene, etc.; gases such as hydrogen sulphide; residues from inert gas and particulates such as those from asbestos, welding operations and paint mists.
Oxygen deficiency caused by, for example, oxidation (rusting) of bare steel surfaces, the presence of inert gas or microbial activity.
During the carriage and after the discharge of hydrocarbons, the presence of hydrocarbon vapour should always be suspected in enclosed spaces for the following reasons:
Cargo may have leaked into compartments, including pumprooms, cofferdams, permanent ballast tanks and tanks adjacent to those that have carried cargo.
Cargo residues may remain on the internal surfaces of tanks, even after cleaning and ventilation.
Sludge and scale in a tank which has been declared gas free may give off further hydrocarbon vapour if disturbed or subjected to a rise in temperature.
Residues may remain in cargo or ballast pipelines and pumps.
The presence of gas should also be suspected in empty tanks or compartments if non-volatile cargoes have been loaded into non-gas free tanks or if there is a common ventilation system which could allow the free passage of vapours from one tank to another.
Lack of oxygen should always be suspected in all enclosed spaces, particularly if they have contained water, have been subjected to damp or humid conditions, have contained inert gas or are adjacent to, or connected with, other inerted tanks.
Other Atmospheric Hazards
These include toxic contaminants such as benzene or hydrogen sulphide, which could remain in the space as residues from previous cargoes.
ATMOSPHERE TESTS PRIOR TO ENTRY
Any decision to enter an enclosed space should only be taken after the atmosphere within the space has been comprehensively tested from outside the space with test equipment that has recently been calibrated and checked for correct operation.
It is essential that all atmosphere testing equipment used is:
Suitable for the test required;
Of an approved type;
Frequently checked against standard samples.
A record should be kept of all maintenance work and calibration tests carried out and of the period of their validity. Testing should only be carried out by personnel who have been trained in the use of the equipment and who are competent to interpret the results correctly.
Care should be taken to obtain a representative cross-section of the compartment by sampling at several depths and through as many deck openings as practicable. When tests are being carried out from deck level, ventilation should be stopped and a minimum period of about 10 minutes should be allowed to elapse before readings are taken.
Even when tests have shown a tank or compartment to be safe for entry, pockets of gas should always be suspected. Hence, when descending to the lower part of a tank or compartment, further atmosphere tests should be made. Regeneration of hydrocarbon gas should always be considered possible, even after loose scale has been removed. The use of personal detectors capable of continuously monitoring the oxygen content of the atmosphere, the presence of hydrocarbon vapour and, if appropriate, toxic vapour is strongly recommended. These instruments will detect any deterioration in the quality of the atmosphere and can provide an audible alarm to warn of the change in conditions.
While personnel remain in a tank or compartment, ventilation should be continuous and frequent atmosphere tests should be undertaken. In particular, tests should always be made before each daily commencement of work or after any interruption or break in the work.
Sufficient samples should be drawn to ensure that the resulting readings are representative of the condition of the entire space.
To be considered safe for entry, whether for inspection, cold work or hot work, a reading of not more than 1% LFL must be obtained on suitable monitoring equipment.
Checks for benzene vapour should be made prior to entering any compartment in which a cargo that may have contained benzene has recently been carried. Entry should not be permitted without appropriate personal protective equipment if statutory or recommended Permissible Exposure Limits (PEL’s) are likely to be exceeded. Tests for benzene vapours can only be undertaken using appropriate detector equipment, such as that utilizing detector tubes. (Benzene causes cancer, and has a delayed action which may be up to 20years)
Detector equipment should be provided on board all vessels likely to carry cargoes in which benzene may be present.
Although a tank which has contained sour crude or sour products will contain hydrogen sulphide, general practice and experience indicates that, if the tank is thoroughly washed, the hydrogen sulphide should be eliminated. However, the atmosphere should be checked for hydrogen sulphide content prior to entry and entry should be prohibited in the event of any hydrogen sulphide being detected. Hydrogen sulphide may also be encountered in pumprooms and appropriate precautions should therefore be taken.
Before initial entry is allowed into any enclosed space, which is not in daily use, the atmosphere should be tested with an oxygen analyzer to check that the normal oxygen level in air of 21% by volume is present. This is of particular importance when considering entry into any space, tank or compartment that has previously been inerted.
Generally nearly all substances have been assigned Permissible Exposure Limits (PEL) and /or Threshold Limit Values (TLVs). The term Threshold Limit Value (TLV) is often expressed as a time weighted Average (TWA). The use of the term Permissible Exposure Limit refers to the maximum exposure to a toxic substance that is allowed by an appropriate regulatory body.
The PEL is usually expressed as a Time Weighted Average, normally averaged over an eight-hour period.
Short Term Exposure Limit (STEL), is normally expressed as a maximum airborne concentration averaged over a 15-minute period.
The values are expressed as parts per million (PPM) by volume of gas in air. Toxicity can be greatly influenced by the presence of some minor components such as aromatic hydrocarbons (e.g. benzene) and hydrogen sulphide. A TLV of 300PPM, corresponding to about 2%LEL, is established for gasoline vapours.
A responsible officer prior to personnel entering an enclosed space should issue an entry permit. An example of an Enclosed Space Entry Permit is provided in ISGOTT.
Suitable notices should be prominently displayed to inform personnel of the precautions to be taken when entering tanks or other enclosed spaces and of any restrictions placed upon the work permitted therein.
The entry permit should be rendered invalid if ventilation of the space stops or if any of the conditions noted in the checklist change.
No one should enter any cargo tank, cofferdam, double bottom or other enclosed space unless an entry permit has been issued by a responsible officer who has ascertained immediately before entry that the atmosphere within the space is in all respects safe for entry. Before issuing an entry permit, the responsible officer should ensure that:
The appropriate atmosphere checks have been carried out, namely oxygen content is 21% by volume, hydrocarbon vapour concentration is not more than 1% LFL and no toxic or other contaminants are present.
Effective ventilation will be maintained continuously while the enclosed space is occupied.
Lifelines and harnesses are ready for immediate use at the entrance to the space.
Approved positive pressure breathing apparatus and resuscitation equipment are ready for use at the entrance to the space.
Where possible, a separate means of access is available for use as an alternative means of escape in an emergency.
A responsible member of the crew is in constant attendance outside the enclosed space in the immediate vicinity of the entrance and in direct contact with a responsible officer. The lines of communications for dealing with emergencies should be clearly established and understood by all concerned.
In the event of an emergency, under no circumstances should the attending crew member enter the tank before help has arrived and the situation has been evaluated to ensure the safety of those entering the tank to undertake rescue operations.
Regular atmosphere checks should be carried out all the time personnel are within the space and a full range of tests should be undertaken prior to re-entry into the tank after any break.
The use of personal detectors and carriage of emergency escape breathing apparatus are recommended.
Reference should be made to ISGOTT for additional guidance on entry into pumprooms.