Tanker operation is by its very nature a specialist subject and while the principles of pumping and storing cargoes are quite simple, tanker crew are required to have extensive training and knowledge of the pumping and valve arrangements so as to ensure safe management of cargo atmosphere, tank pressure and cargo operations.
The full detailing of the arrangement and design of tanker operation systems are outside the scope of this article but the following is an overview of some elements.
Inert gas production and its uses onboard has been covered elsewhere. One of the purposes of inert gas on a tanker is to ensure that an explosive or flammable atmosphere in and outside the tanks is avoided.
Liquid cargoes are prone to changes in volume due to expansion or contraction caused by ambient temperatures. Expansion will cause an increase in pressure inside tanks so it is essential that some form of venting arrangement is in place to relieve the pressure which might otherwise cause structural damage.
Many tanker cargoes give off volatile organic compounds (VOCs) so the relief of pressure by venting is not something that can simply be a matter of a pressure relief valve without some method of controlling the risk of explosion or fire.
The design of cargo tank venting and inert gas systems is governed by SOLAS regulation II-2/11.6 and 5. Most crude oil tankers have a common cargo tank venting and inert gas main pipeline which is also used for vapour emission control. Branches to each cargo tank are provided with isolation valves and blanking arrangements. The isolation valves and blanks are typically only used in connection with tank entry.
SOLAS chapter II-2 requires that the isolation valves are to be provided with locking arrangements to prevent inadvertent closing/opening of tanks. The cargo tank venting/inert gas main is connected to a mast riser. The mast riser has a minimum height of 6m with an IMO approved flame arrestor at its outlet. An isolation valve is provided between the cargo tank venting/inert gas main and the mast riser. Some designs have a small capacity pressure/vacuum valve fitted in a bypass across the isolation valve. This latter enables thermal breathing from cargo tanks when the isolation valve is closed.
A liquid-filled Pressure Vacuum (PV) breaker is typically connected to the cargo tank venting/inert gas main. The PV breaker has a capacity to accommodate the gas flow from cargo tanks during loading (125% of the loading rate and discharge rate). The cargo tank venting/inert gas main is typically used during loading and discharging operations. During loading the mast riser valve is open (unless vapour emission control is performed) and VOC is expelled to air.
During discharge the same valve is closed and inert gas used to replace the tank atmosphere. The cargo tank venting/inert gas main is also used during voyage but the mast riser valve will be operated only in the event of increasing pressure.
In addition to the common cargo tank venting/inert gas main, each cargo tank is required to have a PV relief device for thermal breathing in the event the cargo tank is isolated from the common cargo tank venting/inert gas main. Although classification societies accept that these devices have the capacity to accommodate gas volumes resulting from variations in cargo temperature only (ie thermal breathing), latest industry practices have led to the installation of devices with the capacity to accommodate the full gas flow from loading of cargo tanks.
Tank vent system outlets are located at a safe distance from all areas where personnel who are not involved in cargo work may be present, to ensure that toxic vapours are diluted to a safe level of concentration before they can reach such an area. The safe distances specified depend on the severity of the toxic hazard.
MSC 95 adopted amendments to SOLAS regulations II-2/4.5 and II-2/11.6, clarifying the provisions related to the secondary means of venting cargo tanks in order to ensure adequate safety against over- and under-pressure in the event of a cargo tank isolation valve being damaged or inadvertently closed, and SOLAS regulation II-2/20 relating to performance of ventilation systems.
Tanker operation – Tank Gauging
Tanks are not designed to be filled until overflowing because of the need to allow for thermal expansion of the cargo so it is necessary to be able to determine the volume of cargo in a tank both on loading and at other times during the voyage. It is possible to know the total volume of cargo pumped aboard a tanker using flow meters but this will not take account of leaks in the tanks or pipelines which could mean that cargo is not delivered to the intended tank. A method of measuring the level in each individual tank is therefore essential.
In times past this would have been done using an ullage tape and although these are still useful in some instances or for measuring volume of liquid in non-cargo tanks, most ships today use more sophisticated means. As mentioned, liquid cargoes can be hazardous and as well as the risk of explosion or fire with volatile cargoes, there are other risks to health caused by toxic fumes of some chemical cargoes.
The IBC Code details different methods for gauging the level of a liquid in a tank – open, restricted or closed – depending on the health hazard of the product. Many chemical cargoes may not be gauged by manual dipping because to do so requires an opening to the atmosphere during operation. The use of completely closed gauging systems is necessary, so that no vapour is emitted.
An open system is one in which an ullage tape would be used and this can also be used for taking cargo samples. A restricted system is similar to the open system but one in which the opening into the tank is small allowing only limited amounts of vapour to escape. An example is a sounding pipe that reaches right into the liquid.
Virtually all toxic cargoes require either restricted gauging or closed gauging. Closed systems come in several types and include radar, ultrasound and mechanical means such as float gauges. A float gauge is similar to the ullage tape but inside a closed system and with a float rather than a bob at the end of the tape.
As the tank fills the float rises and the tape retracts into an indicating device that allows for the level to be read. It is important that the float is secured during the voyage unless measuring is taking place as it could easily become damaged.
The latest technologies to be employed for tank gauging are radar, ultrasonic or microwave gauges. All of these are closed systems and work by transmitting and receiving pulses which are reflected back from the surface of the liquid. They have no moving parts within the tank and can be serviced from outside. They may need occasional calibration and replacement of failed components.
Pressure gauges are found in some ships and again these are closed systems. They make use of the difference between atmospheric pressure and the pressure in the liquid near the tank bottom.
Sometimes an additional sensor is mounted near mid-depth in a tank to improve accuracy and reliability. In some cases a system can adjust for cargo density by use of sensors at known levels, and in other cases a correction factor associated with specific gravity is necessary.
A thermometer is usually included in the sensors, but a disadvantage of pressure gauges is that the cargo temperature may vary widely in a tank and, because correct density is dependent on temperature, an error in readings may develop. The delicate nature of sensors and their susceptibility to cargo leakage makes regular servicing essential.
Tanker operation – Tank Cleaning
Tanks used for carrying oils and chemicals need to be cleaned between cargoes to prevent contamination – sometimes very dangerous in the case of chemical tankers – of future cargoes. Even with crude oil cargoes there are differences between grades and contamination can occur if tanks are not cleaned.
In the past tank cleaning was done using water which would then be disposed of overboard. Some of the tanks were also used for carrying both seawater ballast and cargo and the ballast would become contaminated with cargo residues but still disposed of overboard. Clearly this is very polluting so ships were initially required to separate residues and make use of a slop tank for storing until they could be disposed of to shore and to cease using tanks for dual purposes leading to a requirement for segregated ballast tanks.
In 1978, the practice of crude oil washing was made mandatory for all tankers above 20,000dwt and with various amendments over time this is still the situation. Crude oil washing (COW) is actually beneficial to all because it increases the amount of cargo delivered and eliminates discharges into the sea.
Crude oil washing involves using the cargo itself to remove residues from tank walls and ceilings by way of diverting some of the cargo to tank cleaning guns which spray the tank surfaces at high pressure dislodging residue. COW is a hazardous operation as the atmosphere in the tanks during discharge can become an explosion hazard and the movement of the gun nozzles could produce static electricity which might spark an explosion. For this reason it is necessary to control the atmosphere in the tanks using inert gas and ensuring oxygen levels remain below the lower explosion limit.
The oil used for washing may need to be heated in order for it to be suitable for washing. This is done using a COW system that is usually housed above deck. All ships are obliged to carry a COW manual describing the system, its operation, any hazards and communications between ship and shore that are needed when COW is taking place. The IMO publication Crude Oil Washing Systems describes the requirements of the COW regulations. It is a 95-page document so too long to reproduce or summarise here. Suffice it to say that COW operations are part of the specialist training needed by tanker crew and most will be familiar with the document and the ship’s own COW manual.
Tank Cleaning Guns
Manual tank cleaning in chemical and oil tankers is done using hoses and attachments such as high pressure lances and water with or without cleaning chemicals. Obviously care must be taken to ensure a safe environment and appropriate protective clothing must be worn. Manual cleaning can be slow and laborious but may be necessary to ensure the highest standards of cleanliness that is needed with some chemical cargoes.
A faster method is to make use of tank cleaning guns either in place of or as a preliminary step to manual cleaning. Tank cleaning guns can be fixed or portable. Fixed systems comprise of a pumping system with a chemical dosage injection. The cleaning fluid is pumped through the gun’s nozzles which may be single or multiple depending on the model.
The nozzles rotate to a set pattern which gradually covers all areas of the tank except those areas which may be blocked by pipes of the cargo pumping system or heating elements. The speed of the nozzle rotation and the subsequent spray pattern ensures the cleaning is done much quicker than by manual cleaning. The speed and rotation of the nozzles in a tank cleaning gun are usually controlled by a magnetic clutch system. Portable tank cleaning systems also make use of guns with a nozzle pattern system and can either be placed on the floor of a tank or suspended from a suitable opening in the ceiling.