Background
Today there are approximately 1,222 tankers in the world that transport crude oil over long distances. Of these, 99.3 percent use large diesel engines for propulsion. The remaining 0.7 percent use diesel electric propulsion and are engaged in the specialized trade from Alaska to the west coast of the United States. Next to the propulsion system, the most important engineering system is the cargo discharge system. The cargo discharge system consists of a set of pumps, pump drivers and pipes carefully designed to allow the tanker to discharge its crude cargo in an efficient and safe manner. These pumping systems are usually steam driven as they are very compatible with the large diesel engines.

General Pumping System Description
The cargo pumps used for crude oil ships are normally installed in a pump room located at the aft end of the cargo tanks. The pumps are connected to the cargo tanks through a network of piping that runs through the cargo tanks at a distance of several feet above the bottom of the tanks. To prevent damage in the event of grounding and to avoid oil from leaking from the tanks, the piping is prohibited by international regulations from being run below the tanks.

Crude oils are a mixture of many hydrocarbon liquids, and many crude oils have highly volatile components that easily become vapor as the pressure of the liquid is lowered. When a tanker begins pumping cargo, the pressure of the liquid at the pump suction is relatively high because the liquid level is well above the pump. Pressure is lost due to pipe friction as the oil flows through the suction piping to the pump. This pressure loss is proportional to the square of the flow rate. As liquid is discharged ashore, the level falls, thereby reducing the pressure at the pump suction. Eventually, the friction losses equal the pressure due to the height of the liquid and the pressure at the pump suction is equal to atmospheric pressure. Continued pumping lowers the pressure even more, and the reduced pressure causes the more volatile products in the crude to flash into vapor. This can fill the pump with vapor and cause it to lose suction in the tanks. In order to delay the loss of suction to the last possible moment and remove all of the oil from the tanks, the pumps must have the ability to be slowed down. This reduces the friction losses in the pipeline and improves the pumps ability to work without becoming vapor bound.

Therefore, crude oil pumping systems must have the ability to reduce speed. Ideally, they should be able to run at any speed from idle to full speed.

Regulations to prevent explosions in the pump room prohibit electric motors and similar equipment from being located in a pump room. The drive equipment must be located in the ship’s engine room and drive the pumps through shafting that passes from the engine room to the pump room through gas tight seals.

Other Systems Required While Pumping Cargo
According to international regulations, while a tanker is discharging its cargo, the space inside the tank must be filled with inert gas in order to provide a safe non-explosive atmosphere. The inert gas can be any non corrosive gas mixture with less than 5 percent oxygen. The most common sources of this gas are the cleaned and cooled stack gases from the ship’s boilers or the cleaned and cooled combustion products from an inert gas generator.

An inert gas generator is a packaged set of equipment consisting of a burner, a scrubber, blowers and a control system that burns marine diesel oil, and then passes the combustion products through a scrubber where they are cleaned and cooled by seawater. The result is mostly nitrogen with some CO2 and less than 5 percent oxygen.

Pumping System Choices
There are three principle types of cargo pump drives that can be used on tankers: hydraulic, electric, and steam---all of which are located in the engine room.

Hydraulic
Hydraulic drive systems consist of hydraulic pumps that supply hydraulic oil at high pressure (3,500 pounds per square inch gauge [psig]) to a supply main on deck. Branch lines from this main supply hydraulic motors that drive the cargo pumps. Hydraulic oil is returned to a return main on deck then back to the supply pump. The hydraulic supply pumps can be driven by diesel engines or by electric motors. If the system is directly driven by a diesel engine, the efficiency of the drive system is about 27 percent. If the system is driven by electric motors that supplied power from diesel generators, the similar efficiency is about 25 percent.

The stack gasses from the diesels driving the hydraulic pumps are not suitable to be used as inert gas because the oxygen content is too high and not easily controlled. Therefore, a separate inert gas generator must be in operation during pumping to provide the inert gas. By adding the fuel required for the inert gas generator to the fuel required by the diesel engines, the overall thermal efficiency for the hydraulic drive system is about 16 percent for direct diesel drive or about 15 percent if for diesel electric.

For hydraulic-driven pumps, the ship must have a large bank of diesel generators or diesel engines devoted to driving the cargo pumps. A typical VLCC would require about 12 megawatt (MW) and 16,000 HP of power in addition to the normal ship’s power requirements to drive these systems.

Electric
Relatively new technology is now available to allow ships to use variable speed AC drives for the cargo pumps. With this system, power is generated by ship’s generators and supplied to the speed control device through large transformers. The drive devices vary the frequency of the electrical power to allow the pumps to operate at variable speeds. The overall drive efficiency from the diesel engine driving the generator to the pump input for this system is about 38 percent.

As with the hydraulic systems, the stack gases from the diesels are not suitable for use as inert gas. Again a separate inert gas generator must be employed to meet the requirements for inert gas during the pumping operation. Adding the fuel required for the inert gas generator to the fuel required for the diesel engines, the overall efficiency up to the pump input becomes about 20 percent.

The electric drive system requires the installation of large diesel generator sets with engine power of approximately 11 MW (15,000 HP).

Steam
The steam pumping system consists of a pair of boilers producing saturated steam feeding turbines that drive the pumps and exhaust the steam to a seawater condenser from which it is returned to the boiler. The pump speed can be controlled by throttling the steam to the turbine. The overall drive efficiency of these systems is low since they operate only on low-pressure saturated steam. Typical efficiency for the system up to the pump input is about 14 percent.

The stack gases from the boilers are an excellent source of inert gas typically having oxygen contents well below the 5 percent limit allowed by the regulations. The combustion process can be controlled to maintain this level throughout the cargo discharge process. With the steam system, no separate inert gas generator is required and no additional fuel must be burned to make the gas.

There is no need to install any additional diesel generators with the steam system, as the small electrical requirements for the circulating pumps and the feed pumps are well within the existing generator capacities.

Retrofitting
The choice of system depends on the propulsion system. Less than 10 percent of the pumping systems are either electric or hydraulic. There is little or no opportunity for retrofitting a different cargo pump drive system since the equipment and its arrangement in the engine room is an integral part of the entire ship’s machinery design. In addition, the electric drive and hydraulic drive systems require more space at the upper levels of a machinery space than a steam plant and may not be capable of fitting into an existing engine room.

Comparison of Pumping Systems
The simplest system for operation and maintenance is the steam system. Capital cost is also very low compared to the electric or hydraulic systems for the component parts of each system. The only weak point of the steam system is its fuel consumption which is slightly higher than the other systems. This is only a minor problem, since the difference in fuel when the inert gas generator fuel consumption is included in the competing systems is minimal.

Hydraulic pumps are quite common in smaller tankers that are typically carrying many different petroleum products simultaneously and the products can not be intermixed. In this application, an individual hydraulic pump can be located in each tank and there is no possibility of contamination of products. Ships of this type are usually around 40,000 DWT or less although some ships as large as 70,000 DWT are engaged in the products trade. A few large tankers have been completed with hydraulic pumps, but they are typically for special trading areas or shuttle service.

Electric-driven pumps have the highest efficiency, but are probably the most expensive system to buy and install in a new ship. They require a very large installation of electric power generators that are required only for the pumping operation. For the rest of the ship’s operating time, the electric load is about 8 percent of the power required during pumping. When the fuel required for the inert gas generator is added to the engine fuel, the advantage over the steam system is not significant compared to the capital and operating costs of the electric system. There are several new ships in the Alaska/California trade with electric-driven pumps, and there are some large shuttle tankers with electric pumps. These ships are all designed for relatively short voyages and spend a proportionately high percentage of their time in port pumping cargo so that the fuel savings of the electric drive may be significant enough to overcome the capital and maintenance costs.

Other Considerations
Most large tankers are designed to load crude oil in the Arabian Gulf and then transport the cargo to ports in Europe, Japan or the East or West coasts of the United States. These are typically voyages of 30 days or more. Including the return trip for the next cargo, there are about 60 days, including a day or two for the loading operation between each cargo discharge operation. This means that the drive system for the cargo pumps only operates for about 1 day in 62 days or roughly 6 times per year.

There is little incentive to change from the steam turbine systems that have proven so reliable over the years. The hydraulic systems do not save fuel over the steam system when the inert gas generator is included. In fact, the hydraulic systems requir a large installation of diesel engines, either driving generators or driving hydraulic pumps, that must be purchased and then maintained over the life of the ship. The electrical systems have generally been applied only to shuttle tankers or to some Alaska trading vessels that are closer to shuttle service than to intercontinental service. Many of the ships that have had electric pumps installed have been built with electric propulsion systems. If the large electric plant is already required for propulsion, then adding the pumps to the existing plant is a simple and less costly change. However, these ships cannot compete with direct slow speed drive diesel on long sea routes due to the inefficiency of the electric propulsion drive.

The ship operator’s primary objective when arriving in port with a cargo is to discharge that cargo safely and return to sea as rapidly as possible. The simplicity, reliability, and economy of the steam turbine system have made it the choice for all VLCCs and the vast majority of smaller crude tankers for many years. There is no system currently available, or in the foreseeable future, that offers simpler, less costly or more reliable operation than steam driven cargo pumps.

Conclusion
Discussions with tanker owners, ship brokers and review of the literature all indicate there is no trend, either now nor in the future, for tanker pumping systems on large crude oil ships to become electric driven. If, however, main propulsion were to change to electric drive, the electric cargo pump drives would be utilized. The efficiency and reliability of the present system overrides any move to change to electric drives.