Reducing discharges from chemical tankers

Kjell Larsson,
Professor Emeritus in Maritime Science,
Kalmar Maritime Academy, Linnaeus University,
Sweden

Several million tons of chemicals, in addition to crude oil and mineral oil products, are transported by tankers each year in the Baltic Sea. The transportation of chemicals by specialised chemical tankers or combined chemical/oil product tankers is growing worldwide, both in respect to the number of substances and the total volume transported. A diversity of chemicals are carried by chemical tankers in the Baltic Sea, including acids, bases, alcohols, ammonia, vegetable oils, biofuels, fuel additives and a large number of different hydrocarbons including benzene, styrene, xylenes, acetone, phenols etc.

The majority of the chemicals carried by chemical tankers will have detrimental effects on the marine environment if discharged or spilled to the sea. The safety standards on chemical tankers are usually very high, but accidents leading to large discharges do sometimes occur. However, a more common pathway by which noxious chemicals from the ships’ cargo tanks enter the marine environment is through the recurrent tank washings.

Because chemical tankers after unloading of a tank usually will load a different non-compatible substance in the same tank, meticulous tank washing operations are needed after unloading for safety and commercial reasons. How washing operations should be performed, and if, when and where the contaminated wash water could be released back to sea is regulated by the MARPOL Convention Annex II, named “Regulations for the Control of Pollution by Noxious Liquid Substances in Bulk”, and by the IBC code, as well as by the approved ship specific manual. Note that whereas discharges of noxious chemicals from chemical tankers to some extent actually are allowed in the Baltic Sea outside 12 nm from land, discharges of oil cargo residues from oil and oil product tankers is completely prohibited in the whole Baltic Sea and North Sea according to regulations in MARPOL Annex I.

Noxious chemicals carried by chemical tankers are divided into four categories, that is, categories X, Y, Z and OS, other substances. Category X-chemicals are deemed to present a major hazard, Y-chemicals a hazard, and Z-chemicals a minor hazard to the marine environment and human health. Chemicals in OS category are considered to present no harm.

A tank from which a category X-chemical has been unloaded shall be prewashed before the ship leaves the port of unloading and the resulting wash water and residues shall be discharged to a reception facility. The same rules applies when a tank has contained Y-chemicals which are highly viscous, solidifying or are persistent floaters. There are, however, a large number of noxious or harmful chemicals in the Y and Z categories which are transported by chemical tankers and where the unloaded tanks are not prewashed in ports but washed legally at sea. Such tank washings at sea occur regularly and according to the Swedish Coast Guard the number is increasing. Tank washing and release of residues of noxious chemicals from chemical tankers are today even legal in marine protected areas and marine Natura 2000-sites in the economic zone.

In addition to the legal discharges of tank residues, illegal discharges do also occur. Recently, several slicks of large quantities of tall oil and biodiesel, classified as Y-chemicals, have been detected along the Swedish coast and in offshore marine protected areas. For example, in March 2021, a slick of fatty acid methyl ester, FAME, which covered 25 square kilometres of sea surface was detected within a protected Natura 2000-site east of Öland, that is, in an area which hosts large numbers of threatened waterbirds and Baltic harbour porpoises.

As a measure to reduce climate change, larger volumes of biofuels will in future be produced and transported by chemical tankers in the Baltic region. It is important to realize that biofuels and vegetable oils, when released to the sea, have similar effects on the marine life as chemically similar fossil fuels. Vegetable oils and biofuels can form noxious degradation products, damage the plumage of waterbirds, create anoxic environments or absorb other toxins.

To reduce the legal and illegal discharges of noxious chemicals from chemical tankers, and the associated negative effects on the marine environment, the chemical industries in the Baltic region must take full responsibility for the transport of their chemical raw material and products. It is now also the time for authorities to strengthen the regulations regarding discharges of noxious chemicals from chemical tankers. A first step should be to make it mandatory to perform a prewash procedure at the port of unloading when the tanks have contained any of the Y-categorised chemicals. A total ban to discharge residues of noxious chemicals from chemical tankers in marine protected areas should be uncontroversial and could be adopted immediately by the countries around the Baltic Sea.

Email: kjell.larsson@lnu.se

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