An inadvertent release or inadvertent return (IR) of horizontal directional drilling (HDD) fluid is a condition that develops despite: i) appropriate subsurface investigation, ii) engineering design and analyses of the drill path, iii) evaluation of subsurface pressures, iv) use of appropriate drilling fluids, v) following the drill path that was designed, and vi) monitoring and adjusting drilling fluid pressures throughout the drilling process.
An IR can lead to an interruption of the HDD installation with project schedule delays, expensive clean-up costs, environmental damage and fines, deterioration of community relations, and increased regulatory scrutiny. Avoidance of IR is thus in the best interest of pipeline owners, contractors and all other stakeholders.
A comprehensive approach to mitigating IR requires an emphasis on upfront geologic evaluation and engineering assessment during the project planning phase to objectively determine the feasibility of using HDD for pipeline crossing installations. Typical ground conditions that can lead to IR include the presence of previously unidentified gravel and cobbles, highly fractured rock, and karst. Engagement of engineering geology or geotechnical engineering professionals, with demonstrated HDD experience, to scope and execute the geologic evaluation, and to provide specialist support through all phases of an HDD installation, is an important part of any IR mitigation effort.
IR may also develop from large elevation differences between entry and exit, on long crossings with limited cover, and in weak soils that will not contain drilling fluid pressures or allow a stable opening. Assessment of IR risk must consider both the likelihood of occurrence (the potential hazard) and the consequences. A small volume IR to a highway ditch near an HDD exit point, which can be easily contained and cleaned up, presents significantly less risk that an IR to the bed of a high value fish habitat stream or to an aquifer that supplies drinking water.
Some of the IR mitigation options include: i) adjust the drill path to increase the depth of cover or to encounter a more suitable geologic formation; ii) lower the entry and/or exit point elevations to reduce drilling fluid circulation pressures; iii) utilize surface (conductor) casing(s); iv) use the intersect drill method on long crossings; v) pre-grout permeable ground or fractured rock; vi) install relief wells to provide a preferential pathway for drilling fluids to migrate to the surface; vii) plan for use of special drilling fluids; and viii) select an alternative installation method. In some instances, HDD may not be the best pipeline installation method. HDD is only one of a number of available trenchless pipeline installation methods and alternatives, including conventional trenching, should be considered and compared critically during the project planning phase.
Every HDD pipeline crossing installation should include an IR mitigation plan developed for, and implemented during, all stages of a project (from feasibility evaluation, to geological investigation, through engineering design analysis, and finally during drilling and pipeline installation). The IR mitigation plan must include emergency response and cleanup procedures to be implemented in the event of a release, but an emergency response plan alone does not constitute a comprehensive approach to mitigating inadvertent release of HDD drilling fluid.
Tony Rice is a Senior Principal Geotechnical engineer with more than 35 years of experience as a consulting professional to the energy, transportation and mining industries. Over the past 25 years, he has worked extensively with clients in the oil and gas transmission pipeline business. He assists clients in pipeline route selection, geohazard assessment, geological site characterization, HDD feasibility assessment and engineering design, forensic evaluation of failed HDD installations, and implementation of alternative remedial crossing designs. He has worked on HDD pipeline installation projects in North America, Asia, and South America.
Rodolfo Sancio is a Senior Principal Geotechnical Engineer with 19 years of experience in most areas of geotechnical practice. Rodolfo's experience has mainly focused on onshore and offshore geotechnical engineering issues related to the development of large infrastructure projects for the upstream and midstream oil and gas and petrochemical industry. Learn more about Rodolfo Sancio: https://www.geosyntec.com/people/rodolfo-sancio