Oil Tankers

Oil tankers are ships built to carry large quantities of liquid cargo within their cargo spaces.¹ They are most frequently used to transport crude oil or petroleum products including gasoline, diesel fuel or fuel oil.² Tankers vary in size, from small vessels of about 60 metres long, carrying 1,500 to 2,000 deadweight tons (dwt), up to leviathans over 400 metres long that can carry 550,000 dwt.³

Why are some oil tankers built with two hulls?

Huge tanks form part of an oil tanker’s structure.⁴ These tanks hold the vast quantities of oil that they transport.⁵ If tankers have a single hull, there is just one plate of steel separating their oil from the ocean.⁶ On the other hand, a double hull has two plates of steel with space in between them.⁷ It, therefore, creates a buffer zone between the ocean and the cargo of oil.⁸ Two hulls can reduce the chance of oil spills following damage to a ship’s hull by more than 60 per cent compared to a single hull.⁹

With the improved effectiveness that a double hull offers in preventing oil spills from ships, the United States (US) introduced the Oil Pollution Act in 1990.¹⁰ The Act required all oil tankers entering US ports to have double hulls.¹¹ Similar regulation was then passed by the International Maritime Organisation (IMO) in 1993.¹² It mandated double hulls for all newly built tankers of 5,000 dwt or above.¹³ Tankers with partial double hulls were permitted to continue operating until they were 25 years old or until 2017.¹⁴ This gradual phase out was to avoid major disruption to international shipping, due to the difficulty of converting single hull tankers to double hulls.¹⁵

Which ships are required to have double hulls?

Although double hulls reduce oil spills, they are only required for tankers and tank barges in the IMO’s regulations.¹⁶ However, container ships, freighters, cruise ships and other vessels may also carry a large quantity of fuel oil.¹⁷ They are, therefore, also at risk of causing serious oil spills.¹⁸ The single-hulled container ship Cosco Busan caused a significant oil spill in the San Francisco bay in 2007.¹⁹ A gash in the vessel’s hull released 53,000 gallons of fuel oil into the water.²⁰ Thousands of local birds, fish and shoreline animals were killed as a result.²¹

How many oil tankers are there in the world?

In April 2020, there were 810 very large crude carriers (VLCCs) worldwide.²² The average VLCC can transport over two million barrels of crude oil.²³ The global oil tanker fleet has a capacity of about 601 million dwt.²⁴ It accounts for just under 30 per cent of global seaborne trade.²⁵

Risk reduction policies for oil tankers, such as ensuring they have two hulls, has had positive repercussions. The average annual number of large oil spills worldwide between 1970 and 1979 was 24.5.²⁶ This decreased to 1.8 from 2010 to 2019, though they have yet to be eliminated entirely.²⁷

How are oil tankers being built to prevent oil spills?

The double hull is the most significant precaution enforced in the design of oil tankers to prevent oil spills.²⁸ All single-hulled tankers were phased out of US waters by 2015.²⁹ Amendments to the IMO’s regulations were adopted in 2001 to accelerate the phase out of single-hulled tankers by 2015.³⁰ This legislation has been ratified by 150 countries, representing over 99 per cent of all merchant tonnage shipped worldwide.³¹ It, therefore, ensures that all oil tankers over 5,000 dwt will be built with a double hull.³² Nevertheless, other large ships can cause serious oil spills too.³³ It would reduce the risk to marine wildlife worldwide if double hulls were required for other large vessels too. 

For a similar article, please see here.

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