Modern chemical tankers are arguably the most technologically advanced vessels of all the major commercial ship types. The complexity of this vessels results from wide range of cargo that they are designed to carry and the diverse nature and characteristics of these cargoes. Many of the cargoes carried on the modern chemical tankers are considered hazardous materials that require careful handling.
The chemical tanker's mission is to transport liquid chemicals in bulk via the world's waterways while protecting life, property, the environment, and the quality of the cargoes being carried. Chemical tankers,as commercial vessels operating in a competitive global market, must carry out their mission while providing their shipowner/operators with measures of economic benefit.
The design of a chemical tanker may allow the vessel to carry hundreds of different liquid cargoes ranging from petroleum products,to inorganic acids,to fish oil,to specialty chemicals. The cargoes transported by chemical tankers can be categorized in several different ways. The cargoes may be divided into groups based on their chemical composition,such as inorganic and organic chemicals.
The chemical cargoes may be divide into following four groups based on their origin and a specific heavy group.
- 1) Petro-chemical products
- 2) Coal tar products
- 3) Carbohydrate derivatives
- 4) Animal and vegetable oil, and
- 5) Heavy chemicals
Petro-chemical products are those oil products and chemicals that are derived from the refining of crude oil and natural gas.
Coal tar products are derived from the carbonization of coal.The coal tar that results from this process is a source of hydrocarbons used for industrial purposes.
Carbohydrate derivatives include molasses and various forms of alcohols.
Types of Chemical Tankers
Chemical tankers can be categorized in a number of different ways. Unlike oil and product tankers there is no universally accepted size categorization of chemical tankers,however modern vessels typically fall into one of the following three categories:
Inland Chemical Tankers - 500 to 4000 tonne DWT. Typically in the form of self - propelled barges.Commonly used in the river sysyems of northwestern Europe to load cargo from larger tankers or coastal terminals and transport the material to inland industrial facilities.
Coastal Chemical Tankers - 3000 to 10000 tonne DWT. These small tankers,also referred to as short sea tankers,are used to transport chemicals coastwise and to transship cargoes into ports and terminals where larger tankers are unable to call because of any number of restrictions. These tankers may load or discharge cargo from a shore terminal or directly from a larger vessel. These vessels are commonly used in the intraEurope,intra - southeast Asia,and the north - central south American markets.
Deep Sea Tankers - 10000 to 50000 tonne DWT. These ocean going vessels typically have a large number of segregations and have either stainless steel and coated tanks.These vessels operate on the major trade routes between North and South America,Europe,the Middle East and Asia.
Alternatively,Chemical tankers can be grouped by the level of cargo containment designed into the vessel,known as Ship Type (ST).
The IBC code defines three specific ship types,with ST 1 providing the greatest level of containment for the transportation of the most hazardous cargoes.Conversely,ST 3 provides minimal containment for carriage of the least hazardous cargoes by the IBC code.Chemical tanker designs typically fall in one of three ship type arrangement:
- 1) ST 1/2
- 2) ST 2
- 3) ST 2/3
Commercially,the lining of the cargo tanks can be used to categorize chemical tankers. The tanks may be mild steel in construction and covered with a specifically formulated tank coating,or they may be constructed or lined with corrosion resistant stainless steel. Some vessels have a combination of both coated and stainless steel tanks. The cargo tank lining partially determines which cargoes a vessel can carry. Certain cargoes because of their inherent properties or aggressive nature, cannot be stowed in tanks lined with certain tank coatings. Similarly many characters demand stainless steel cargo tanks out of a concern for cargo quality when it would otherwise not be required.
- Hull form and principal particulars
The service speed of chemical tankers typically falls between 12 and 16 knots.This speed range, which is a function of the markets governing economics, allows for the use of full hull forms similar to product tankers. Concern for ballast condition speed is minimal as chemical tankers rarely sail in ballast condition for long distances. Block coefficients for chemical tankers range from 0.80 to 0.85 with some smaller ocean-going vessels having finer hull forms with block coefficients below 0.80. Chemical tankers typically employ bulbous bows and trapezoidal sterns. Model tests and CFD analysis are employed when developing new chemical tanker hull forms.
The most common propulsion system for chemical tankers has been a slow-speed diesel engine driving a fixed pitch propeller. This arrangement often includes a shaft-driven generator to provide electrical power during the sea passage and three or more auxiliary diesel generators to provide power for the in-port electrical loads.
On a given port call a chemical tanker will often call at more than one berth.On a given voyage a chemical tanker will typically call at a number load and discharge ports. These two factors result in chemical tankers spending up to 40% of a total voyage's time in port. These factors make it crucial that the vessels have good maneuverability. A maneuverable vessel will be more efficient in port as docking and undocking can be completed more rapidly and require less assistance from tugs.Furthermore,good maneuverability provides an additional level of safety in the congested waters of the largest chemical handling ports. The maneuverability of chemical tankers is enhanced through the use of large bow thrusters and high efficiency rudders Becker Flap and Schilling Rudders.
The cargo systems of chemical tankers are what truly differentiate them from product and crude tankers. A vessel's cargo system, together with its cargo tank arrangement and safety systems determines the cargoes that the vessel can or cannot carry. A chemical tanker's cargo system includes tanks, pumping systems, piping, venting systems,cargo monitoring systems, environmental control systems and tank cleaning systems.
- Cargo Tanks
The IBC code provides for four types of cargo tanks: independent, integral, gravity and pressure tanks. The boundaries of an independent tank are not part of the hull structure and therefore do not contribute to the structural strength of the vessel. Independent tanks are designed to eliminate the transfer of stress from the vessels structure to the tank structure.On chemical tankers,independent tanks typically taking the form of deck tanks. The deck tanks used on the Stolt Innovation are examples of independent cargo tanks.On the other hand, the boundaries of an integral tank are formed by the hull structure and the sub dividing of the hull in the cargo area creates the individual integral tanks.Integral tanks are the most common type of tank used on chemical tankers.Gravity tanks are designed for a maximum pressure of 0.7 bar gauge at the top of the tank and may be of the integral or independent type. Pressure tanks are designed for pressure greater than 0.7 bar gauge and are not typically used in chemical tankers.The boundaries of both integral and independent cargo tanks can be stainless steel or coated mild steel. The cargo tank coatings that are used on chemical tankers are primarily of the epoxy or zinc silicate type. There are certain cargoes that are incompatible with stainless steel and cannot be stowed in stainless steel tanks but can be stored in coated tanks.Stainless steel tanks are also easier to clean after cargo discharge than coated tanks.
- Cargo pumping and piping systems
The cargo pumping and piping systems are the principal systems in determining a vessel's operational flexibility.Although the IBC code allows for a cargo pump room and shared piping systems, these features are no longer used in modern chemical tankers. The modern chemical tanker is based on the concept of the complete segregation of cargoes. Each cargo tank and its associated systems are independent from the vessel's other cargo tanks. At the heart of the complete segregation approach is the deepwell cargo pump. As its name suggests a deepwell pump is submerged in the fluid that it is pumping with its impeller placed in a well in the cargo tank tank top. Deepwell cargo pumps are of the centrifugal type and are driven by either a hydraulic motor located in the tank with the impeller or by an electric motor that drives a shaft that runs from the deck down to the impeller in the tank.
Hydraulic driven pumps of this type are more popular on chemical tankers than the electrically driven pumps because the electric motors can be eliminated from the cargo area,and because variable speed control makes them attractive. Since centrifugal pumps do not pump high viscosity cargoes well,ships that have centrifugal deepwell cargo pumps and that frequently carry high viscosity cargoes,such as molasses are typically outfitted with a deck mounted booster pump to assist in the discharge process. These booster pumps are of the positive displacement type,with screw pumps being used most often.
- Cargo venting systems
- Cargo monitoring and control systems
Proper cargo monitoring is a key component of cargo care,operational safety and environmental protection. The type of cargo monitoring system that is required on a particular chemical tanker is based on the cargoes that it will carry. The monitoring systems must be more advanced if the cargoes the vessel will carry are more hazardous. The IBC code provides requirements for tank gauging, high-level alarms, overflow control systems, temperature measurement and pressure measurement.
Chemical tankers that are designed to carry hazardous cargoes are required to be fitted with a closed gauging device such as float-type systems and tank radar. Closed devices can be used without the contents of the tank being released.