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Oil pollution overview

Updated 11 Oct 2019

Oil pollution

With the possible exception of the polluting potential of heavy fuel oil in the Arctic and a movement to ban its use as a fuel in the high latitudes, almost all recent environmental regulation has been concerned with exhaust emissions.

Most would consider MARPOL to be the source of most regulation against pollution and to some extent this is true but oil discharge from operational sources had been regulated for over 20 years before the first edition of MARPOL was drafted.

The International Convention for the Prevention of Pollution of the Sea by Oil (OILPOL) was formulated at London in 1954. It came into force in 1958 and was amended in 1962, 1969 and 1971. It was eventually superseded by the International Convention for the Prevention of Pollution from Ships (MARPOL) and its measures are now included there.

OILPOL did not put a complete ban on disposal at sea; it merely prohibited the dumping of oily wastes within a certain distance from land and in ‘special areas’ where the danger to the environment was especially acute. It also imposed a requirement for contracting parties to provide reception facilities but, more than half a century on, the lack of facilities is still a bone of contention for the industry. OILPOL was mainly concerned with operational discharges as was the 1973 version of MARPOL drawn up by the IMO.

This was to be amended by the Protocol of 1978 adopted in response to a spate of tanker accidents in 1976-1977. As the 1973 MARPOL Convention had not yet entered into force and thus could not be amended, the 1978 MARPOL Protocol absorbed the parent Convention. The combined instrument entered into force on 2 October 1983. Most of the measures in MARPOL are the province of the IMO’s Marine Environment Protection Committee (MEPC) which is also entrusted with the development of other environmental conventions.

Pollution of the seas by oil is covered by Annex I of MARPOL. Because of the recent pre-occupation at the IMO with ballast water, energy efficiency and emissions to air, any changes to the requirements of Annex I have generally been related to administrative matters rather than introducing any new requirements.

The driving factor behind MARPOL in the early days were numerous pollution incidents mostly involving oil spills but the rules also cover smaller scale operational pollution and in most jurisdictions these are treated as being more serious in terms of criminal negligence than accidental loss of cargo or bunkers as a result of grounding or collision.

As well as pollution by oil and related substances covered in ANNEX I of MARPOL, the same topics are also included in the US EPA’s VGP introduced in 2008.

The US also has its own oil pollution prevention and response laws in the form of the Oil Pollution Act 1990, better known as OPA 90. In March 1989, the Exxon Valdez spilled over 11M gallons of Alaskan crude into the water of Prince William Sound and many lessons were learned in its aftermath. Two of the most obvious were that the country (like most nation states) lacked adequate resources to respond to spills, and the scope of damages compensable under federal law to those impacted by a spill was fairly narrow.

The US response to the incident was the OPA 90 which amended the Clean Water Act and addressed the wide range of problems associated with preventing, responding to and paying for oil pollution incidents in navigable waters of the United States. It created a comprehensive prevention, response, liability, and compensation regime to deal with vessel- and facility-caused oil pollution to US navigable waters.

OPA greatly increased federal oversight of maritime oil transportation, while providing greater environmental safeguards by:

  • Setting new requirements for vessel construction and crew licensing and manning;
  • Mandating contingency planning;
  • Enhancing federal response capability;
  • Broadening enforcement authority;
  • Increasing penalties;
  • Creating new research and development programme;
  • Increasing potential liabilities; and
  • Significantly broadening financial responsibility requirements.

All ships arriving into US waters are now obliged to obtain insurance against any possible pollution they may cause. This is generally referred to as ‘OPA 90 insurance’.

Double hulls
The new requirements for vessel construction was the spur for all US tankers needing to be built with double hulls. This requirement was also incorporated into MARPOL which in 1992 was amended to make it mandatory for tankers of 5,000dwt and more ordered after 6 July 1993 to be fitted with double hulls, or an alternative design approved by IMO.

The requirement for double hulls that applied to new tankers was applied to existing ships under a programme that began in 1995. Under those rules all tankers would have to be converted (or taken out of service) when they reached a certain age depending on size but set to a limit of 30 years. This measure was adopted to be phased in over a number of years but after the Erika incident off the coast of France in December 1999, in April 2001, IMO adopted a revised phase-out schedule for single hull tankers, which entered into force on 1 September 2003.

In December 2003, further revisions were made, accelerating further the phase-out schedule. These amendments entered into force on 5 April 2005. A new regulation on the prevention of oil pollution from oil tankers when carrying heavy grade oil (HGO) banned the carriage of HGO in single-hull tankers of 5,000dwt and above after the date of entry into force of the regulation (5 April 2005), and in single-hull oil tankers of 600dwt and above but less than 5,000dwt, not later than the anniversary of their delivery date in 2008.

Under the revised regulation, which entered into force on 1 January 2007, the final phasing-out date for Category 1 tankers (pre-MARPOL tankers) was 2005. The final phasing-out date for category 2 and 3 tankers (MARPOL tankers and smaller tankers) was brought forward to 2010, from 2015.

Although the double-hull requirement related only to tankers, it was noted that many vessels of other types would be carrying larger quantities of oil as bunkers than the cargo capacity of some of the smaller tankers. It was proposed that large vessels such as bulk carriers should be subject to similar rules and, although several bulk carriers have been constructed with double hulls, there is no regulatory requirement for this to be done.

There is however, an IMO requirement covering all ships with a bunker capacity above 600m3 for the bunker tanks to be in a protected location. For ships with a bunker capacity of between 600m3 and 5,000m3, the bunker tanks must be located at least 760mm inside the moulded line of the side shell plating. For ships with a bunker capacity over 5,000m3 the distance is increased to at least 1,000mm. The space between the bunker tank and the outer hull may be used for purposes such as fresh water or ballast water storage.

The major part of Annex I is actually concerned with construction and cargo operations of oil tankers over 150gt and the parts which affect other vessel types over 400gt is confined to a very few operational matters as well as the form and issuing of the International Oil Pollution Prevention Certificate (needed in most ports to obtain customs clearance) and the need for ships to have and maintain an oil record book.

The oil/water separator

The first demand as regards the operational waste oil from machinery is that the ship must be fitted with adequate holding tank capacity for any waste that cannot be dealt with by way of discharge or incineration.

Most ships generate large amounts of oily waste (waste contaminated with oil) and waste oils (such as spent lubes or sludge from fuel and lube treatment systems). As well as the oil in the bilge water, there will be grease, detergents and cleaning fluids along with contaminants that may have been removed from fuel and lube treatment systems, some of which may present more of a hazard to the marine environment than oil does.

Prior to the introduction of regulations, all of this waste would generally have been disposed of at sea. Today, all vessels above 400gt are required to filter the waste so as to reduce the oil content to a maximum of 15ppm (Canadian rules on the Great Lakes have a maximum of 5ppm) before discharging it at sea. Some classification societies also demand a higher standard of 5ppm to comply with their voluntary Clean Design notations. After filtration, any resultant waste must be retained on board for disposal ashore.

The filtering is done by a bilge- or oily-water separator – a piece of equipment that has gained an unenviable reputation in recent years. As well as the separator, all vessels subject to the regulation must also be fitted with an oil content monitor (OCM) and bilge alarm to detect if the treated bilge water being discharged meets the discharge requirements. Separators used on board ships are not generally unique pieces of equipment design specifically for marine use but will be versions of separators used in many industries ashore.

It is generally accepted that separators have not performed as well at sea as they do in applications ashore. There are many reasons for this, including the fact that the waste products are less easy to deal with, the conditions at sea with constant movement in many planes affecting operation and the fact that installed systems often lack the capacity to meet the demands placed on them. As a consequence, they require constant monitoring and frequent cleaning and overhaul, which has made them unpopular with many seafarers.

This, coupled with the operators’ desire to reduce the cost of disposing of treated waste ashore, has led to several instances where the separator has been by-passed and waste discharged illegally overboard. These are the so-called ‘magic pipe’ incidents that lead to regularly-reported prosecutions by port state control regimes and heavy fines and imprisonments especially in the US. It should be noted that the US authorities have no jurisdiction in case of illegal discharges outside of US waters and the fines and imprisonments are not for the acts themselves but for presenting the US authorities with falsified records, most notably the Oil Record Book, which is an offence under US law.

The limit of 15ppm oil allowed in discharges from separators was established in 1993 but did not extend to emulsified oils. Ten years later the requirements were amended to include emulsions with guidelines for equipment performance laid down in MEPC 107(49) introduced in January 2005.

The MARPOL regulations may lay down a maximum limit of oil allowed in discharges but they leave the means of achieving this open. As a consequence, several technologies are used across the diverse range of separators available and crew members may find themselves having to operate and service unfamiliar equipment. Those that use membranes, flocculation or absorption Filters in particular means valuable time must be spent searching out manuals and attempting to make sense of them.

Oil/water separation technology

Early separators were mostly of the gravity separation type that employ plate or filter coalescing technology to separate oil and water. The bilge water is usually heated gently to improve separation with the oil gradually settling out above the water content.

The oil is then pumped to the holding tank and the water discharged to sea after passing through the oil content monitor (OCM; see below). Without further refinements, gravity separators can have difficulty in meeting the 15ppm standard especially when the bilge water contains emulsified oils which do not separate easily.

Centrifugal separators also work using the different densities of oil and water but with the centrifuge greatly multiplying the gravity effect as the centrifuge accelerates. This type of separator is more efficient and can generally deal with emulsified oils. Many crew members are familiar with this type of equipment which is also used for preparing fuels and lubes before use by removing sludge and homogenising the fuel or lube. They are more compact than gravity-type separators but have the disadvantage of requiring power to operate the centrifuge and, because of their moving parts, often have a higher maintenance requirement.

One way for separator performance to be improved is to add a polishing device into the circuit. Several makers’ current systems include a polishing stage but for older vessels, adding a polishing unit between separator and monitor will improve the performance sufficient to prevent alarms sounding constantly.

Other technologies are also used for cleaning bilge water including absorption and adsorption, flocculation, biological and membrane separation. Absorption and adsorption are very similar physicochemical processes and for the purpose of this guide can be considered together.

In both cases, the bilge water is forced through the sorption media in a reactor or contactor vessel and the oil is removed. When the sorption material has reached its full capacity, it is removed and replaced with fresh material. Some sorption materials can be regenerated onboard, but others will need to be delivered to shore. Popular absorption materials include bentonite and zeolite used as substrates or in cartridges.

Typically, 100m3 of bilge water will require 10kg of media. Flocculation and coagulation make use of an emulsion-breaking chemical to treat emulsions after any free oil has been separated. The chemical breaks down the emulsion and the released oil comes together to form flocks which can then be skimmed off leaving the remaining water to go through further filtration stages. This method tends to produce large amounts of sludge and requires an outlay on the chemical reagent.

Biological treatment employs microbacteria in a bioreactor to literally consume the organic chemicals in the oil, converting it to carbon dioxide and water. It is a slow but effective treatment for oil and emulsions as well as also removing some of the other solvents often found in bilge water. Capital outlay can be high but operating costs are low. Care must be taken to avoid overload on the microrganisms and maintaining operating temperature within the safe range to avoid destroying them.

Membrane technology, ultrafine filtration and reverse osmosis are all physical means of preventing oil and other large molecules from remaining with the water that can pass through the filter barrier. They are efficient but require attention to prevent blocking of the filter or membrane.

Avoiding problems with separators begins long before the device is switched on and involves a proper plan for managing waste streams and doing as much as possible to prevent emulsions forming, especially if they are chemical emulsions resulting from the use of cleaning chemicals and detergents.

So called primary emulsions in which larger drops of oil are dispersed in water generally separate through gravity within 24 hours. Secondary emulsions caused by turbulent conditions where oil droplets are very fine become stable and will not separate easily. Solid material should also be prevented from contaminating the bilge as much as possible. Not only does it promote emulsification, it also creates false alarm situations and shuts down the separator requiring crew intervention to restart the separation process. Filters and removal of solids before treatment will allow the separator to operate more effectively and for longer.

Oil content monitor

Under MARPOL regulations, ensuring that oil/water separators function correctly and no discharge exceeds the permitted 15ppm limit, an oil content monitor must be used. The monitor must be fed with a continual flow from the same outlet of the separator as the overboard discharge pipes.

In the event that the monitor detects a level above 15ppm, the separator is either shut down automatically or a valve is operated which sends the outlet water back to the bilges. In normal operation, a monitor may be fooled by suspended solids such as rust and scale which are quite innocuous, but they may not detect the presence of some clear liquid chemicals that could be toxic to marine life when discharged into the sea.

It is also possible to fool the monitor by various deliberate means including closing the valve from the separator so that the sight glass of the monitor is continuously monitoring a clean sample. This may not be possible if the monitor includes a flow-measuring component.

The monitor is a crucial component of separators and is often not an in-house product of the separator maker. There are a small number of specialist monitoring device manufacturers which produce systems for measuring many of the other different discharges permitted from ships such as ballast and scrubber washwater. Many of these monitoring systems have versions that are designed to prevent any deliberate cheating.

Incidental and operational discharges

Beyond major catastrophes involving cargo or bunker leakages, illegal discharges as a result of deliberate acts undoubtedly cause most of the pollution from shipping but there are other operational sources of oil waste which can occur.

Lubricant leaks from propeller shaft and rudder bearings are also a common cause of pollution and have attracted attention in recent years. Until around 50 years ago, many ships were fitted with propeller shaft bearings made from lignum vitae – an extremely dense timber with a high degree of natural lubricity – but these were abandoned in favour of metal bearings and mineral oil lubricants. Now, as environmental regulations tighten, water-lubricated propeller shaft bearings are becoming a popular alternative to oil-lubricated bearings for commercial vessels.

This was already happening before the US EPA revised the VGP in 2013 but that action is likely to accelerate take-up of water-lubricated bearings and new seal types and also a greater use of new approved Environmentally Acceptable Lubricants (EALs).

Conventional shaft seals inevitably leak over time due to wear and damage, such as may be caused by a rope fouling a propeller, but water-lubricated bearings avoid oils and grease lubricants altogether.

In such a bearing, seawater is pumped into the bearing and it simply discharges to the sea. It lubricates and dissipates heat from shaft friction and most manufacturers of water-lubricated seals say their products provide equal performance to conventional seals. The need for more environmentally-friendly seals is not limited to conventional propulsion systems with a tail shaft and propeller but is also a necessary consideration for makers of podded propulsion systems and thrusters. Each new generation of these brings improvements in seals and designs that limit the possibilities of lubricant leakage.

Under the VGP introduced in the US in 2013 and extended beyond its December 2018 expiry date, the list of permitted substances and the quantity each ship above 300gt will be allowed to discharge was reduced – quite dramatically in some cases. One of the changes under the VGP affects lubricants in any equipment or system that has an oil-to sea interface. In essence, that affects all propulsion systems and also deck machinery where run-off over the ship’s side could occur.

Under the earlier 2008 VGP, operators were free to use any lubricant they wished but since December 2013 the rules require environmentally acceptable lubricants (EALs) unless doing so would be ‘technically unfeasible’. EALs are defined as biodegradable, which rules out all mineral- based lubricants and even some synthetic alternatives.

The exact definition of an EAL is contained in an EPA document, EPA 800-R-11-002 November 2011, which can be accessed via the organisation’s website.

Operators have to apply for a VGP before a vessel enters US waters and to do so they need to identify all oil-to-sea interfaces and lubricants involved. Although ship operators can expect product makers to advertise compliant products, there are some labelling schemes that may give added assurance. EPA does not consider on-deck equipment that comes into contact with rain, splashed with waves, wave-generated spray or subject to icing to be a form of immersion, and therefore not an oil-to-sea interface.

Vessel operators are not required to use EALs in on-deck machinery that is not subject to immersion. However, discharges from deck machinery are subject to other discharge requirements, such as those for Deck Washdown and Runoff (Section 2.2.1 of the VGP), which recommends the use of EALs.

Among the most obvious systems that are considered oil-to-sea interfaces are the stern tube, rudder bearings, CP propellers, thrusters, and fin stabilisers. In addition, winches, cranes, hatch covers and even crane wires and the like must be considered.

The ship will be required to document all lubricants and any reason why the use of an EAL would be technically unfeasible. Most major oil companies and some specialist suppliers have formulated compliant products that are readily available, although with premium price tags.

However, these products are not necessarily compatible with some makes of seals, especially conventional rubber seals. This is a known problem and most combinations of lubricants and seals have been tested for compatibility over normal drydocking cycles of two to three years.

In selecting an EAL, operators must therefore seek advice from the seal manufacturer and great care must be exercised if the vessel makes use of enhanced or extended dry docking strategies. Inspections with regard to EALs would involve visual sheen tests and inspections of deck runoff.

Use of EALs and water-lubricated bearings are of course not restricted to ships that may trade to the US as a leaking stern tube can be the source of action against a ship anywhere in the world.

If switching to an EAL from a conventional stern tube lubricant, care must be taken to ensure that all of the old lubricant is removed and the whole system thoroughly cleansed. This is because there have been several instances of a mixture of conventional lubricants and EALs and even some EALs alone causing sludge formation and lacquering of bearings leading to bearing failure and leaks or even shaft fractures.

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