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Fact Sheet - Discharge to Ocean

Past Practices

In early offshore oil and gas development, drilling wastes were generally discharged from the platforms directly to the ocean. Until several decades ago, the oceans were perceived to be limitless dumping grounds. During the 1970s and 1980s, however, evidence mounted that some types of drilling waste discharges could have undesirable effects on local ecology, particularly in shallow water. When water-based muds (WBMs) were used, only limited environmental harm was likely to occur, but when operators employed oil-based muds (OBMs) on deeper sections of wells, the resulting cuttings piles created impaired zones beneath and adjacent to the platforms. At some North Sea locations, large piles of oil-based cuttings remain on the sea floor near the platforms. Piles of oil-based cuttings can affect the local ecosystem in three ways: by smothering organisms, by direct toxic effect of the drilling waste, and by anoxic conditions caused by microbial degradation of the organic components in the waste. Current regulatory controls minimize the impacts of permitted discharges of cuttings.

Stricter Regulation of Drilling Waste Discharges

In the late 1970s, the U.S. Environmental Protection Agency (EPA) began placing stronger restrictions on ocean discharges of drilling muds and cuttings through National Pollutant Discharge Elimination System (NPDES) permits. Among the early restrictions were prohibitions on the discharge of OBMs and cuttings. In 1993, the EPA adopted enhanced national discharge standards for the offshore oil and gas industry. These established additional requirements for discharging WBMs and cuttings from wells drilled at least 3 miles from shore but prohibited discharges within 3 miles of shore.

During the mid-1990s, mud companies developed and promoted synthetic-based muds (SBMs) that offered strong drilling performance like OBMs but were much closer to WBMs in environmental impact. Unfortunately, EPA's 1993 offshore discharge regulations did not consider SBMs, so there was considerable uncertainty about whether offshore operators could use the SBMs and discharge the resulting cuttings. The EPA, DOE, the former Minerals Management Service, and numerous companies and industry associations worked together following an innovative expedited rulemaking process to finalize new effluent limitations guidelines (ELGs) for SBMs in 2001. Those rules allow for discharge of SBM cuttings, subject to various restrictions but prohibit the discharge of SBMs themselves. A summary of the 1993 and 2001 discharge requirements are shown in the box below.

Summary of U.S. Offshore Requirements for Drilling Wastes

Baseline Requirements

  • No discharge of free oil (using a static sheen test) or diesel oil
  • Acute toxicity must have a 96-hour LC50 > 30,000 ppm (using EPA's mysid shrimp toxicity text)
  • Metals concentrations in the barite added to mud must not exceed:
    • 1 mg/kg for mercury
    • 3 mg/kg for cadmium
  • No discharge of drilling wastes allowed within 3 miles of shore (except for Alaskan facilities in the offshore subcategory)

Additional Requirements for Synthetic-Based Muds (SBMs)

  • SBMs themselves may not be discharged
  • Cuttings coated with up to 6.9% SBMs may be discharged
    • Ester SBMs can have up to 9.4% SBM on cuttings
  • Polynuclear aromatic hydrocarbon (PAH):
    • Ratio of PAH mass to mass of base fluid may not exceed 1 x 10-5
  • Biodegradation rate of chosen fluid shall be no slower than that for internal olefin
    • Base fluids are tested using the marine anaerobic closed bottle test
  • Base fluid sediment toxicity shall be no more toxic than that for internal olefin base fluid
    • Base fluid stocks are tested by a 10-day acute solid-phase test using amphipods (Leptocheirus plumulosus)
    • Discharged cuttings are tested by a 4-day acute solid-phase test using amphipods (Leptocheirus plumulosus)
  • No discharge of formation oil
    • Whole muds are tested onshore by GC/MS analysis
    • Discharged cuttings are tested for crude oil contamination by fluorescence method
  • Conduct seabed survey or participate in industry-wide seabed survey

Unlike the EPA's decision to allow discharge of SBM cuttings in the Gulf of Mexico, governments in the North Sea area decided to phase out discharges of SBMs or cuttings. Other oil-producing countries around the world have followed various strategies for regulating discharge of SBMs and cuttings. Based on the scientific evidence, the discharge of the very low quantities of SBMs that adhere to cuttings meeting all of the required regulatory criteria appear to have minimal and brief impact on the ocean environment.

Treatment Processes Prior to Discharge

Shale Shaker Photoclick to view larger image
Shale Shaker

After coming to the platform, drilling wastes are placed on a series of vibrating screens called shale shakers. Each successive shale shaker uses finer mesh screen, so the collected particles are smaller in size. The liquid mud passes through the screens and is sent back to mud pits on the platform to be reused. If the recycled mud contains fine particles that would interfere with drilling performance, the muds are treated using mud cleaners or centrifuges to remove very fine particles. At the end of a drilling job or at the end of a particular interval that uses a specialized mud, the bulk mud will either be returned to shore for recycling or discharged to the sea.

Dried Cuttings Photoclick to view larger image
Dried Cuttings

The solid cuttings coated with a film of mud remain on top of the shale shakers and are collected at the opposite end of the shakers. If the cuttings are able to meet the discharge standards at this point, they are generally discharged. If they are unable to meet the discharge standards (particularly relevant when SBMs are being used), the cuttings must be treated further by vertical or horizontal cuttings dryers, squeeze presses, or centrifuges. The cuttings dryers recover additional mud and produce dry, powdery cuttings.

Horizontal Cuttings Dryer Photoclick to view larger image
Horizontal Cuttings Dryer
Centrifuge Photoclick to view larger image
Vertical Cuttings Dryer Photoclick to view larger image
Vertical Cuttings Dryer

Economic Considerations

Generally, the cost of treating and discharging drilling wastes is lower than the cost of hauling them back to shore or applying other management options. The cost of transporting wastes is eliminated when wastes are discharged. Where offshore discharge is an option approved by the regulatory agency, most operators choose that option. There may be situations in which a company elects not to discharge for public relations reasons or because of concerns over long-term liability.

Minton and McGlaughlin (2003) provide drilling waste costs as a fraction of the cost spent on drilling fluid. North Sea costs have the highest reported fraction (>0.5) among 13 regions, while the U.S. Gulf of Mexico fraction is reported as being about 0.12. This reflects the fact that many more offshore operations are discharging drilling wastes in the Gulf of Mexico than in the North Sea. North Sea operators must select other, more expensive waste management options.


Minton, R., and J. McGlaughlin, 2003, "Rising Drilling-Waste Costs Promote Novel Technologies, Approaches," Oil & Gas Journal, August 11.

OGP, 2003, "Environmental Aspects of the Use and Disposal of Non Aqueous Drilling Fluids Associated with Offshore Oil & Gas Operations," report no. 342, prepared by the Non-Aqueous Drilling Fluids Task Force for the International Association of Oil & Gas Producers, May (Available at:

Sumrow, M., 2002, "Synthetic-Based Muds Reduce Pollution Discharge, Improve Drilling," Oil & Gas Journal, December 23.

Veil, J.A., 2001, "Offshore Waste Management – Discharge, Inject, or Haul to Shore?," 8th International Petroleum Environmental Conference, Houston, TX, November 6-9.