A significant portion of the supply chain systems of world trade based on maritime transportation depend on commercial shipping, which carries cargo in liquid form.
About Chemical Tankers
When we think of liquid cargo, the foremost name that comes to mind is an oil tanker, a vessel carrying petroleum and natural gas-related products. Chemical tankers are another type of commercial vessel transporting bulk cargo in liquid form.
Chemical tankers, as the name suggests, are commercial cargo ships designated to carry chemical substances in bulk liquid form from one point to another. Chemicals are widely procured, distributed, supplied, processed, and consumed for various purposes, ranging from industrial to agricultural to domestic. The philosophy of design encompassing chemical tankers is close to that of oil tankers and has the following chief considerations that act as cornerstones:
- The containment of liquid cargo in bulk quantities, like in oil tankers
- The cargo contents are usually very flammable, as these dangerous chemicals not only have a critically low flash point but are also highly reactive, leading to a great degree of risk associated with fire outbreaks.
- The highly reactive nature of the chemical contents that are can pose a great deal of risk when it comes to interference with the hull structure
- The purview of water and air pollution arising from accidental leaks or release of these substances into the environment, even in small quantities.
Brief About Design and Construction
Chemical tankers are, for all practical purposes, smaller than oil tankers, ranging from as low as 5000 tonnes deadweight tonnage to at most 50000-60000 DWT for the biggest sizes.
The small size of these vessels can be due to several factors, such as port restrictions for loading and discharging these noxious substances, channel voyage restrictions for passage of these dangerous chemicals, the specialized nature of the cargo, and, of course, the lesser demand for chemical transport compared to other similar conventional types like oil and gas or freight cargo in bulk.
However, owing to the dangerous classification of liquid cargo in bulk that it carries from one point to another, the rules, guidelines, and codes dictating the design and construction of chemical tankers are more conservative than those of oil tankers.
Generally, the hull form of chemical carriers is like that of other conventional bluff from commercial ships such as oil tankers or bulkers. They do not have high-speed requirements. They are driven mainly by heavy-duty diesel or diesel-electric propulsion systems that provide sufficient power to travel through all sea conditions and, simultaneously, provide a constant supply to facilitate the internal systems constantly running onboard.
The main difference or typicality of chemical carriers arises from their internal structures, the cargo spaces, and the various systems running for the containment of such dangerous cargo in bulk that we shall discuss briefly.
Codes and Rules
One of the most essential bases for chemical tankers carrying chemical substances is Annex-II of MARPOL itself (Regulations for the Control of Pollution by Noxious Liquid Substances in Bulk), which governs the transport of noxious substances in any quantities across the sea.
Annex II of MARPOL technically defines chemical tankers as “vessels deemed to carry noxious liquid cargo in bulk quantities”, as defined in Chapter 17 of the International Bulk Chemical Code or IBC code, which is at the heart of the design and construction of chemical carriers.
IBC is the set of rules, guidelines, codes, and advisories encompassing the transport of dangerous chemicals in bulk by sea. Thus, by default, ALL CHEMICAL TANKERS come under the purview of IBC. They are considered special requirements or extensions stemming from MARPOL Annex-II itself.
Characteristics of Chemical Tankers
For all practical purposes, different chapters and regulations of the IBC code dictate the design indices for chemical tankers. Without delving deeply into each of them, let us have a glance at some important characteristics of these vessels:
Hull structure: All modern chemical tankers have a double-hull configuration in line with the regulatory requirements of SOLAS and MAPROL (special requirements like IBC and BCH catering to that). Cargo, like oil tankers, is stored in the central tanks only, and the wing double-bottom tanks are used for other utilities like freshwater, ballast, etc.
Chemical tankers are, for all practical purposes, both longitudinally and transversely stiffened to accommodate not only all kinds of external and internal loads but also dynamic time-varying loads that may arise due to the erratic motions of the dense liquid chemical cargo.
Another important trait of these vessels is that there are not many stiffeners in the way of the tanks to ease cleaning and minimize the accumulation of chemical residues. Transverse stiffening in the way of decks is provided instead to compensate.
Subdivision and compartmentalization: As these vessels carry high-risk cargo, they have higher subdivision per regulations to increase vessel survivability and minimize environmental damage due to spillage.
Moreover, the higher degree of subdivision in the chemical tankers also aids in minimizing problems like sloshing of the liquid cargo, which, in turn, can pose hazards like ignition due to heavy motions and frictional effects of the cargo contents with the holds and can also induce high transient pressure loads on the primary structural members.
Even the lowest deadweight chemical tankers are at least 2-compartment type, whereas, for an oil tanker, it is only when the length exceeds 150 m.
Stability guidelines: Stability is important in these vessels as low stability characteristics can lead to severe problems like overheating and fire outbreaks arising from the excessive motions of the cargo, spillage, etc. The codes allow a maximum heel angle of 25-30 degrees, even under worst-case scenarios.
Cargo, tankage, and general arrangements: This is one of the most crucial aspects of a chemical tanker. Chapters 2 and 3 of the IBC code mainly deal with the stowage and disposition of cargo, the internal arrangements, and how the holds or spaces are arranged in such vessels.
Maintenance and stowage of cargo contents: IBC stresses the four most critical aspects central to such dangerous cargo types – a) inerting, b) padding and insulating, c) drying and temperature control, and d) ventilation.
Inerting mainly involves introducing a constant supply of inert gases to the cargo and associated piping so that the risk of explosions due to reduced oxygen levels is minimal. Inert gases are distributed by pump sets that are kept at a constant rating.
Next, the insulation part is very crucial. Multi-layer robust insulation material is used for cargo tanks in chemical carriers and is approved by recognized bodies. An eclectic range of materials are used, some common ones being polyurethane foam, mineral wool, perlite, and so on.
Temperature control in chemical tankers is mainly done with the help of heating or cooling coils, as per the cargo’s requirement. Heating or cooling systems in these tankers are subject to stringent guidelines and are regularly maintained. Lastly, ventilation is an indispensable aspect when it comes to transporting hazardous materials. Various types of ventilation for chemical cargo are mentioned in Chapter 8 of the IBC Code.
Materials: The materials for constructing chemical tankers are not very much highlighted in the IBC Code and are primarily in line with classification rules. Mild steel and stainless steel are two materials used for construction. While tankers designed to transport one type of chemical contents are easier to construct, those designated to carry multiple types of chemicals are complicated.
The choice of materials used in the holds is dependent on the type of chemical contents. Austenitic grades of stainless steel, for example, are more resistant to corrosion and are thus used for highly reactive chemicals. Other breeds, like molybdenum-free ones, are more compatible with lesser corrosive chemicals. In chemical tankers, there is essentially a trade-off between the strength and the reactive nature of the chemicals.
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Source: MarineInsight
