The Generic Mud Concept for NPDES Permitting of Offshore Drilling Discharges
Authors: Ayers R.C. , Sauer T.C. , Anderson P.W. 
Published: December 2007

Summary
U. S. Environmental Protection Agency (EPA) Region 11 granted the first offshore drilling National Pollution Discharge Elimination System (NPDES) permits to the Mid-Atlantic operators in 1978. The generic or standard mud concept was developed by the Offshore Operators Committee with EPA Region 11 to provide the agency with an understanding of and control over mud components and discharges without requiring the operators to perform redundant bioassay and chemical tests every time mud was discharged. The generic mud concept subsequently has been incorporated into permits issued by EPA Regions I, III, IV, VI, IX, and X. Eight basic mud types were defined that encompassed virtually all water-based muds used on the outer continental shelf (OCS). A bioassay test procedure, adapted from the EPA/U.S. Army Corps of Engineers' procedure for dredged material, was developed, and a test program was conducted on field mud samples representing each of the eight basic types. Operators were allowed to discharge muds of the eight types without conducting additional bioassays. Results of the test program indicate that all eight generic muds demonstrated low toxicity. The eight generic muds contain primarily major components and do not include specially chemicals sometimes required to remedy special drilling problems (e.g., lost-returns material, lubricants, and defoamers). This situation is being handled by using an "approved additive list" that names additives that do not affect mud toxicity significantly. The API is developing a standard bioassay procedure based on the Region II protocol for testing specialty chemicals in standard muds, and service companies already are using parts of this procedure to gain approval for use of their products.

Introduction
In 1978, EPA Region II granted the first offshore drilling NPDES permits to operators drilling on leases in the Baltimore Canyon. As a permit condition, the operators were required to perform a jointly funded drilling-mud bioassay program. Before initiating the test program, the Offshore Operator Committee Task Force on Environmental Science, with EPA Region II, developed an acceptable drilling-mud bioassay procedure and a spectrum of eight general mud types that included essentially all water-based compositions. Previously, the agency had not recognized differences in water-based mud systems and had classified all muds in only two categories, oil- or water-based. The generic mud concept and the EPA Region II bioassay procedure evolved from this effort. The approach has proved to be practical, and the generic mud concept, supported by the Region II bioassay results, subsequently has been incorporated into permits issued by EPA Regions I, III, IV, VI, IX, and X.

Generic Mud Concept
The generic muds were identified by reviewing the permit requests and selecting the minimum number of mud permit requests and selecting the minimum number of mud systems that would cover all those named by the prospective permittees. Eight different mud systems were prospective permittees. Eight different mud systems were identified that encompass virtually all water-based muds used on the OCS (Table 1). Instead of naming a set concentration for each component in each mud system, concentration ranges were specified to allow the operators sufficient flexibility to drill safely. In the eight generic mud systems, only major components are specified. Specially additives (e.g., lost circulation materials and lubricity agents) needed for special drilling situations are not named. If an unanticipated need for a specially additive arises, the operator is required to submit data on the additive's chemical composition, usage rates, and toxicity to the EPA before its use. On the basis of this information, the EPA regional administrator approves or disapproves discharge of mud containing the additive on a case-by-case basis. If there is a continuing need for the additive, the operator then can submit bioassay data on mud containing the additive. Discharge would be allowed if the additive does not greatly increase mud toxicity. Once an additive becomes "approved" in this way, future discharge of muds containing the additive is allowed without conducting additional bioassays. Note that any of the generic muds may contain one or more specially additives; however, the presence of an approved specially additive in a generic mud does not change the generic mud type.