Subsea Dispersant Application


Dispersants are products used in oil spill cleanup to enhance the natural degradation and accelerate the breakdown of oil in the environment through natural processes. Dispersants work by reducing the surface tension of oil, reducing the size of oil droplets to a diameter typically similar to that of a human hair. The smaller droplet size provides greater surface area for the same amount of oil, making it easier for microorganisms such as naturally-occurring marine bacteria to consume the oil droplets. Dispersants can be rapidly applied to surface accumulations of oil through vessel-based operations and aircraft, or they can be injected subsea directly into the source.

As described in API Technical Report 1152 (API 2013), subsea dispersant injection (SSDI) is an oil spill response strategy designed to be used during a loss of source control (well blowout) on the ocean floor. SSDI takes place at the subsea wellsite, with dispersants injected directly into the flow of the release.

The intention of SSDI is to chemically disperse the released oil, reduce oil droplet size to less than 100 micrometers (ìm or microns), and impede oil from rising to the sea surface (API 2013). This approach:

Potentially reduces volatile organic compounds (VOCs) at the sea surface, which are a threat to the health and safety of workers;

Prevents or reduces the potential for floating oil to impact sensitive environmental resources, including direct contact with wildlife on the surface ocean, reaching nearshore areas, and stranding on shorelines; and

Optimizes the potential for microbial biodegradation by decreasing oil droplet size (effectively increasing the droplet surface area.)

By reducing droplet size at the source, subsea dispersant injection (SSDI) can reduce the amount of oil rising to the sea surface, thereby protecting personnel working near the incident from exposure to the oil and its associated hazards. SSDI is an effective application method because it requires less dispersant to break up the oil (approximately 5 times less the amount of dispersant than aerial spraying), will help protect vulnerable surface resources, shorelines and wildlife instead of allowing the oil to spread out into large surface slicks.

The technique was first used in 2010 in the U.S. Gulf of Mexico in response to the Deepwater Horizon oil spill resulting from a loss of well control at the Macondo well in Mississippi Canyon Block 252 (MC252). Since 2010, the recommendations and best practices for SSDI planning, implementation, and approval have been well documented and practiced during drills and exercises in the U.S.

SSDI has matured as an acceptable response tactic for source control incidents in deep water. A history of this maturation and detailed guidance on SSDI is provided by both industry and U.S. government sources including the National Response Team (NRT), American Petroleum Institute (API), and the International Association of Oil and Gas Producers (IOGP) (e.g., API 2013, 2017; IPIECA–IOGP 2015; U.S. NRT 2013). API (2017) also provides an extensive bibliography of publications related to the topic.

To facilitate potential future access to SSDI tools and technologies in the event of a source control incident, HWCG is collaborating with multiple companies to develop specialized subsea dispersant monitoring programs to demonstrate the effectiveness of subsea injection in real-time, as well as enhancing our monitoring plan. While subsea application has numerous advantages, there are also areas under further investigation regarding the application process and dispersant effects on living organisms.


United States Coast Guard (USCG)

The USCG has the authorization to use dispersants when responding to oil spills. During the Macondo incident, the USCG (in conjunction with Regional Response Team (RRT) VI) approved the SSDI method and use. The effectiveness of SSDI was continuously monitored during application. During exercises and incidents, the USCG is represented on the RRT VI.

United States Environmental Protection Agency (USEPA)
The USEPA monitored the oil spill cleanup process following the Macondo incident and found that applying dispersants directly into the source helped prevent oil from reaching the surface by making the oil droplets smaller in size. The USEPA is represented on the RRT VI. 

The Gulf of Mexico Research Initiative (GoMRI)

The GoMRI is a 10-year independent research program that has been assessing multiple oil spills and discovered that subsea dispersants used in the cleanup process reduce oil droplet size. In 2015, their study discovered oil droplet size is a significant factor regarding the cleanup process for rig blowouts, like Macondo, because it can lessen the overall impact and amount of oil that reaches the surface and shoreline, which can cause billions of dollars in damage to shoreline and wildlife.  


HWCG works with Regulators and the API to gain approval for subsea dispersants. HWCG has facilitated access to a global dispersant stockpile for members. HWCG was instrumental in working with the RRTs and API to develop a standard guidance document for SSDI use by the industry (API 4719 – Industry Guidelines on Requesting Regulatory Concurrence for Subsea Dispersant Use). This API document should be the standard used by all Deepwater Operators to prepare and submit a comprehensive package for SSDI approval from RRTs. In addition, HWCG has prepared templates for SSDI operations and monitoring plans based on the API 4719 guidance and has facilitated contracts for monitoring equipment required by both API and NRT guidelines.

HWCG continues to be a leader in the subsea dispersant application conversation among the industry. We have presented on Subsea Dispersant Application to the API Spill Prevention Group (SPG) and other industry forums such as the Oil Spill Response Limited (OSRL) Forum. HWCG has presented three times to RRT VI in Dallas on the success of our program. HWCG has also presented and facilitated sessions at the Clean Gulf Conference in the past to help demonstrate the capabilities of our membership. Attendees for both presentations included operators and government agency representatives. Organizations like API and OSRL have utilized our knowledge and resources to assist in faster government acceptance of subsea dispersant approval. HWCG and its contractors have conducted three successful exercises with SSDI approval through the RRT VI process and with numerous other agencies assistance to develop a package that was safe, achievable, and reflected the capabilities of HWCG. HWCG continues to test dispersant issues to make sure process and procedures are in place for any future SSDI events a Member may have. 

CSA Ocean Sciences Inc. (CSA)

CSA is a marine environmental consulting firm that specializes in projects concerning potential environmental impacts of activities around the world. HWCG enlisted CSA’s assistance to develop the SSDI monitoring plan during HWCG’s 2017 Annual Drill. In collaboration with CSA, HWCG developed a subsea dispersant monitoring plan for each of its members, which was submitted by Stone Energy and approved by RRT VI.

National and Regional Response Teams

The NRT and RRTs are tasked with coordinating emergency preparedness and response. RRTs function in two ways – as a standing team and as an incident-specific team. RRT VI serves the states of Arkansas, Louisiana New Mexico, Oklahoma and Texas, and are co-chaired by the USEPA and USCG. As mentioned above, RRT VI recently approved the SSDI monitoring plan developed during HWCG’s 2017 Annual Drill, which was submitted by Stone Energy.



Selection of an appropriate response tactic requires a Net Environmental Benefit Assessment (NEBA), also known as a Spill Impact Mitigation Analysis (SIMA), during which the trade-offs associated with each response tactic are examined.

Although there are many response options, the use of dispersants (whether applied to the sea surface or directly to the subsea release site) is a commonly favored tactic owing to its demonstrable utility in mitigating the potential impact of spilled oil on both human health and safety and the environment.

While both surface (aerial) and subsurface dispersants work to reduce VOCs at the sea surface, break up oil, and speed up oil removal by natural processes, there are some advantages to each.

Advantages of Dispersants:

Work best on fresh oil that hasn't been significantly weathered

Work best on light oils and medium to heavy weight crude oils

Used to combat large oil spills

Can be applied over a broad range of weather conditions

Lower manpower and logistical requirements than other response options

Pre-approved areas in the U.S. are in place

Higher encounter rate compared to other surface options


Advantages of Subsea Dispersants:

Reduces the amount of oil coming to the surface, therefore making it safer to respond to as well as causing less damage to property onshore, nearshore and wildlife, fisheries, and bird habitats

Requires substantially substantially less dispersant by volume (approximately 5 times less) owing to the ratios of dispersant to oil (100 to 1 for subsea compared to 20 to 1 for surface)

Application can occur continuously (24-hour operations)

Reduces any VOC or LEL issues so work can begin above the wellhead

Can be applied in all but very severe weather conditions

High encounter rate




Use of dispersants has been shown to be beneficial by reducing the exposure of sensitive coastal environments to oil that comes ashore. These web-accessible resources from the National Institute of Environmental Health Sciences and the National Academy of Sciences provide information about dispersants and their use during the Deepwater Horizon incident:

Dispersants Frequently Asked Questions

Petroleum dynamics in the sea and influence of subsea dispersant injection during Deepwater Horizon



Since the Deepwater Horizon incident in 2010, the industry as well as government and non-governmental agencies have been working toward enhancing policies and guidance for subsea dispersant knowledge and use. Because of several research projects investigating the various aspects of the subsea dispersant injection process, more accurate guidelines for subsea dispersants could be in place in the future. These projects involve the effects of dispersed oil on living organisms, long term effects of oil at extreme depths and the rate of biodegradation.

In October of 2017, RRT VI approved the dispersant monitoring plan developed during the 2017 Annual Drill, which was later submitted by Stone Energy. Having a dispersant monitoring plan approved provides guidance on environmental monitoring and dispersant operations.

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For additional in-depth resources about subsea dispersants and how they're changing the industry, check out the following links:

Continue to our How Macondo Leads the New Responder Immunity Policy page about the latest court decision regarding responder immunity, how this new legislation would impact the recovery process and further action still needed for responder immunity.


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