To regulate drilling activities proposed by oil and gas companies in the oil-rich Gulf of Mexico (GOM), governing agencies require the submittal and approval of certain plan documents before drilling activities can be initiated. For drilling activities in the Western GOM, these plans typically consist of one of the following:
- EP, or Exploration Plan – required to conduct exploratory drilling on any offshore lease
- DOCD, or Development Operations Coordination Document – required for development and production drilling activities on any offshore lease in the Western Gulf of Mexico
For the western GOM, the EP and DOCD plans and associated documentation are submitted, reviewed, and approved (or disapproved) by the Bureau of Ocean Energy Management Regulation and Enforcement (BOEMRE), a federal agency regulated under the Department of the Interior. (BOEMRE was previously known as the Minerals Management Service, or MMS). BOEMRE is responsible for overseeing the safe and environmentally responsible development of energy on the Outer Continental Shelf (OCS). Regulations governing the requirements for EP and DOCD plans are codified under Title 30 of the Code of Federal Regulations at 30 CFR Part 550.
An EP or DOCD plan must include air emissions information for the proposed activities. In general, this information includes projected emissions, frequency and duration of emissions, bases for calculations, equipment information, emissions reductions measures (if applicable), and dispersion modeling (if applicable).
Impacts to Air Quality — Importance of Air Dispersion Modeling
Before submitting an EP or DOCD plan to BOEMRE for approval, the applicant must demonstrate that onshore air quality impacts from proposed offshore activities, as compared to the National Ambient Air Quality Standards (NAAQS), will not adversely affect human health or the environment. Emissions from offshore sources must be quantified using EPA-approved methodologies. These sources include fuel burning sources, such as drilling rigs, generators, flares, and supply vessels as well as sources of fugitive emissions. If these emissions exceed pollutant-specific thresholds, then air dispersion modeling may be required.
There are several dispersion models available for modeling sources over water, including CALPUFF and OCD (Offshore and Coastal Dispersion Model). BOEMRE typically recommends OCD for the Western GOM because it is usually less time consuming than CALPUFF.
The OCD model was developed by EPA in conjunction with BOEMRE’s predecessor agency, the MMS, in the late 1980s, and the model was formally approved for use in January 1988. It is an hourly, steady-state model used to predict the onshore concentration of air pollutants emitted from offshore sources.
As detailed in Table 1, compared to CALPUFF, the OCD model’s relatively simpler data processing makes it an efficient model for use in predicting pollutant impacts from offshore sources.
Criteria for Using OCD Modeling
According to BOEMRE, OCD modeling is appropriate when:
- the emission source is located in the OCS; and
- emissions from the offshore source are greater than certain thresholds based on the following formula used by BOEMRE to calculates emission thresholds:
where E = emission threshold, tons/year
D= distance to shore, miles
k and n = pollutant-specific constants
As evidenced by this formula, emission thresholds are based on the distance from the source to the shoreline. The farther the distance to shoreline, the greater the emission threshold will be for a particular pollutant. EP and DOCD plans typically include the pollutants and thresholds shown in Table 2.
Typical offshore activities produce emissions from temporary (well testing, construction and installation) as well as permanent (production) operations. Traditional New Source Review permit modeling requires sources to be modeled in a manner representative of the source’s operations that are classified as permanent emissions. However, BOEMRE requires facilities to provide dispersion modeling results that also consider the additional emissions generated during the source’s construction phase (classified as temporary emissions). The addition of emissions from the construction phase results in exceedances of modeling thresholds.
Model Input Data
Model input data comprises source-specific data as well as meteorological data. Source-specific data is typically provided by the facility. Meteorological data may be obtained from various sources identified in Table 3. The following are the principal components for source specific data:
- Location of offshore activities - usually available in either Latitude-Longitude coordinates or Universal Transverse Mercator (UTM) coordinates
- Emission rate information for all sources associated with activities at the given location. This includes heat input ratings of emissions sources and estimated hours of operation. For the Western GOM this information is typically provided in a BOEMRE-approved AQR format.
- Stack parameters for each source - stack diameter and height, gas exit velocity and temperature, and the stack angle. Some sources, such as support vessels, may be modeled as one single source using total combined emissions and conservative default stack parameters.
As detailed in Table 3, the OCD model requires both over-land and over-water meteorological data to determine the potential onshore impacts of the offshore operations. These data include: overland surface characteristics such as surface roughness, and over-water data such as water temperature, over-water air temperature, over-water dew point, over-water wind speed, and over-water wind direction. These data are usually obtained from the offshore buoy closest to the source at three different mixing heights: 300 m, 600 m, and 900 m .
After obtaining the required input data, a model is set up by choosing on-shore locations (receptors) at which the OCD model will predict the pollutant concentrations of the modeled emission sources. Receptors are setup on the shoreline and at nearby Class I areas per guidance provided by BOEMRE on receptor placement and spacing.
The air quality modeling results are generally submitted in the form of a modeling report to BOEMRE along with the EP or DOCD plans per 30 CFR 250 Subpart B. The report typically includes a comparison of the maximum modeled concentrations with the relevant modeling significance level thresholds for the pollutant-specific averaging periods. If any of the maximum modeled concentrations exceed the corresponding significance levels, additional discussions with BOEMRE may be required.
Offshore platforms and rigs typically comprise combustion units that produce high NOx emissions when compared to the threshold. Hence, facilities are typically required to model NOx emissions. For NOx, the averaging periods under consideration are the annual and the 1-hour standards for NO2. All modeled NOx is assumed to be NO2. Table 4 provides an example of model results.
Typical offshore activities require modeling for emissions resulting from construction as well as permanent production operations. Although the OCD model does not include algorithms for parameters such as regional haze and acid deposition, it has streamlined data processing and relatively fast model run times. OCD is currently the preferred dispersion model for modeling in the western GOM for EP and DOCD plans submitted to BOEMRE, although CALPUFF may be preferred for some situations.