Hard-live bottom areas affected by the 2001 construction of Gulfstream Natural Gas System LLC’s 36-in., 419-mile offshore pipeline between Mobile Bay, Ala. and Tampa Bay, Fla. have largely recovered, in the face of permitting expectations that this process would take roughly 100 years.
The term “hard-live bottom” refers to seafloor that is both hard (as opposed to such other possibilities as sandy, silty, etc.) and life-sustaining. Community structure does not appear to be significantly different between reference and affected areas. Visible disturbance also appears to be minimal as compared to predicted effects.
Background
Hard-live bottom makes up the most significant and sensitive marine benthic resource in the federal waters portion of the Gulfstream pipeline (Fig. 1). Gulfstream conducted numerous pre-construction field investigations of the biological distribution and physical characteristics of hard-live bottom near the pipeline route and reduced potential damage through not only its siting efforts but also use of technologies such as the submarine plow and the deployment of buoys on construction barge anchor cables.
US Army Corps of Engineers and Minerals Management Service permits and Federal Energy Regulatory Commission certificate requirements obligated Gulfstream to measure and mitigate effects of the construction of the marine pipeline and to monitor the recovery of affected areas.
Before construction, Gulfstream prepared a federal mitigation plan that established the monitoring and mitigation protocols to be implemented to satisfy permit conditions. This plan required Gulfstream to conduct monitoring activities to determine the extent and severity of project construction effects on hard-live bottom areas within the trench corridor and associated spoil mounds and anchor strike-cable sweep locations.
Part 1 of this article, presented here, details the methodology of Gulfstream’s monitoring activities. Subsequent parts will discuss the results of its monitoring program and examine the use of pipeline habitat replacement structures by fish and epifauna.
Gulfstream established and monitored random 25-m transects perpendicular to the trench corridor and spoil mounds, within identified anchor strike and cable sweep locations in hard-live bottom areas, and also in unaffected hard-live bottom reference habitats.
Design stratification
The federal mitigation plan considered depth to be a key variable in hard-live bottom recovery due to the specifications of pipeline siting and biological community characteristics. In waters less than 200-ft deep, Gulfstream lowered the pipeline into an excavated trench with the expectation that the trench spoil would naturally backfill over time. During construction, hard substrate in some areas prevented excavation of a trench with a postlay plow. Pipeline anchors stabilized these portions of the pipeline. Plans did not require the pipeline to be trenched beyond the 200-ft contour.
The hard-live bottom habitat at all water depths along the route generally consists of low-relief algal sponge communities of algae, sponges, bryozoans, ascideans, and ahermatypic hard corals, with octocorals dominant in shallower depths.
To assess potential depth-related effects on hard-live bottom recovery, Gulfstream stratified the monitoring design into three depth zones; Depth Zone 1 (40-70 ft), Depth Zone 2 (70-100 ft), and Depth Zone 3 (100-200 ft). Gulfstream did not visit depths beyond the 200-ft contour during the 2005 assessment due to above-average hurricane activity.
Monitoring equipment
The scope of the 2005 field survey required use of monitoring vessels, depth profilers, differential global positioning systems (DGPS), navigational software, remotely operated vehicles (ROV) with video cameras, and underwater digital still camera systems.
The retractable rod allowed divers to accurately maintain a fixed distance from the seafloor during photograph collection and subsequently maintain a fixed scale in each transect photograph for analysis.
The camera flash and two diver lights mounted on the camera housing provided lighting for the transect photographs.
Transect establishment
Gulfstream established a total of 136 transects within the stratified depth zones in compliance with the federal mitigation plan (Table 1). All transects contained hard-live bottom habitat. Survey divers collected photograph transect data in Depth Zone 1. Either survey divers or ROV collected transect data in Depth Zones 2 and 3.
The analysis used randomly selected 25-m transect lengths within both affected and unaffected hard-live bottom areas to facilitate comparison. Gulfstream established the random transects perpendicular to the pipeline to assess trenched pipeline corridor, reference habitat, and anchor strike areas.
Transect coordinates provided to the field survey crews and entered into the vessel’s navigation system before deployment allowed the survey vessel to navigate to the appropriate locations, where either survey divers or ROV were deployed.
Survey divers navigated to each Depth Zone 1 transect coordinate end point and established a transect end point marker. After establishing the transect end points, survey divers measured each transect with a weighted tape measure to verify the transect length.
In Depth Zones 2 and 3, survey divers used one end point coordinate and a defined azimuth for each transect. After navigating to the end point, survey divers extended a weighted tape measurer 25 m along the defined azimuth direction and established the second transect end point. The digital still camera then collected photographs along the transect length.
Collecting sequential, overlapping photographs that contained a clear view of the tape measure within each frame ensured complete transect coverage and photograph position references. An ROV first navigated to one transect end point coordinate utilizing the Hypack Integrated Navigation System to ensure accurate positioning. Once at the end point, the ROV navigated down the length of each transect, collecting video 2 ft above the seafloor. The ROV’s two-point laser system ensured consistent height above the substrate.
Reference habitat transects represent delineated hard-live bottom areas close to the pipeline corridor but not affected by project construction activities. These transects serve as a reference for statistical comparison to affected hard-live bottom areas. Comparisons between the two allowed assessment of the effect of pipeline construction.
Delineating of hard-live bottom areas relatively close to the pipeline route took place before pipeline construction, during the permitting phase of the project. The survey team entered hard-live bottom polygons, the pipeline corridor route, and depth-zone demarcations into a GIS. Unaffected hard-live bottom polygons outside the pipeline corridor served as potential reference habitat transect locations. GIS then randomly selected 10 transect coordinates per depth zone.
Routing the pipeline to avoid large areas of hard-live bottom minimized potential effects on these areas within the trenched pipeline corridor. Further precautions included use of construction methods determined to significantly reduce hard-live bottom damage during pipeline lowering.
Plans called for the pipeline to be placed in an excavated trench 3 ft below the natural seafloor elevation in water less than 200-ft deep. This approach would provide pipeline stability under severe storm conditions based on storm models. Predictions foresaw trenching impacts 75-ft wide, based on a 25-ft wide trench corridor, with associated spoil mounds spreading 25 ft to either side of the trench. The anticipation that the trench would backfill through natural sediment transport made backfilling in federal waters unnecessary. Plans did not require trenching the pipeline corridor in water more than 200-ft deep.
Gulfstream divided the assessment of hard-live bottom affected by pipeline trenching between the trench corridor and the associated trench spoil mounds. Spoil containing rock would serve as hard substrate for epifaunal-epifloral colonization. Independently collecting and analyzing transects within each category determined the amount of damage and recovery attributed to each feature.
Preconstruction studies indicated that some hard-live bottom would be affected during construction as a result of mooring the lay barge and plow barge. Vessel anchors can affect hard-live bottom when anchors are placed, set, and lifted from the seafloor. Anchor cables can also affect benthic habitats when the barges are winched forward with anchors in place.
A portion of the heavy anchor cable rests and sweeps along the seafloor in an arc as the barge is pulled ahead. Gulfstream minimized potential damage from anchor handling by attempting to place the anchors in areas where hard-live bottom did not occur and by using midline buoys on anchor cables to minimize the size of cable sweeps.
The federal mitigation plan required postconstruction monitoring at up to 25 anchor strike and cable sweep locations documented to have occurred in hard-live bottom areas.
Data recorded by the anchor-handling vessel’s onboard computer identified potential anchor strike and cable sweep locations based on construction-collected coordinates. Identifying each anchor drop and lift location with a unique number produced a pair of coordinates for each anchor drop-lift event and associated cable sweep.
Overlaying these coordinate pairs on the affected hard-live bottom polygons in GIS generated a list of potential anchor strike or cable sweep locations. Postconstruction side-scan sonar records, examined for evidence of seafloor disturbance from anchors or anchor cables, also identified anchor strike and cable sweep locations. A random-selection process then determined locations to be visited for monitoring.
Divers or the ROV deployed at each random transect location searched for evidence of seafloor disturbance attributable to anchors and anchor cables. Evidence of damage would prompt measurement of the affected area’s areal extent and establishment of transects within the affected area.
If no disturbance was identified at or near the given location, the divers or ROV searched the periphery of the area in order to survey all potentially affected areas. Failure to find evidence of damage after all of these steps had been taken would prompt the assumption that the area had recovered naturally over time.
Acknowledgments
Erik Danielson of ENSR provided GIS support for the monitoring and reporting processes during the Gulfstream project. We also acknowledge Tracy Gayle Charters of Holmes Beach, Fla., for its vessel and diving support during the survey period and Walt Jaap, formerly of the Florida Fish and Wildlife Research Institute, for his support with field operations and review of this work.
Bibliography
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Based on paper presented at International Pipeline Conference (ASME), Calgary, Sept. 25-29, 2006.
The authors
Jon Schmidt ([email protected]) is a vice-president at ENSR in Tallahassee, Fla. Schmidt runs ENSR’s LNG and pipeline services and has worked for ENSR for over 10 years. He has also served as program manager and project manager at ecology and environment. He holds a PhD (1987) from Florida State University in marine ecology. He also holds an MS from University of Bridgeport and a BS degree from University of Massachusetts in Dartmouth.
Steve Ellsworth ([email protected]) is a senior project manager at ENSR in Anchorage where he is responsible for impact assessment services including NEPA environmental assessments and environmental impact statements. Ellsworth specializes in assessing impacts from oil and gas projects in both marine and terrestrial environments, and has done so for the past 14 years domestically and internationally. He holds an MS (1983) from Louisiana State University in wildlife management, and a BS in biology from the State University of New York, Plattsburg.
R. Allen Brooks ([email protected]) is a project specialist at ENSR in St. Petersburg, Fla. Brooks is a member of the natural resources department and works on projects involving coastal and oceanographic systems. Prior to joining ENSR, he was employed by the USGS offshore and deep-coral program. He holds a PhD (2002) and MS (1997) from the University of South Florida in biology. He also holds a BS degree from Ohio University.
Darren Bishop ([email protected]) is a project specialist working out of the ENSR St. Petersburg, Fla., office. Bishop has experience in permitting and regulatory compliance for natural gas projects focused on impact analysis for wetlands, soil, and mitigation. Prior to joining ENSR, he worked for The Nature Conservancy and federal (NOAA), regional, and state governments. Darren holds an MS (2006) from the University of Florida, and BS and BA degrees from the University of South Florida and Boston University, respectively.
Michael Aubele ([email protected]) is an environmental scientist at Gulfstream Natural Gas System in Houston. Aubele has been with Gulfstream/Williams for 6 years and provides environmental permitting and compliance support primarily to expansion and integrity projects in the Southeast and Northwest US. He holds an MS (2001) from Texas A&M University in zoology and a BS (1996) from Humboldt State University.
Herbert (Ed) Watkins ([email protected]) is president of A&E Project Management & Consulting LLC and a managing partner of W&W Marine LLC. Watkins has worked on the offshore portion of the Gulfstream Natural Gas System Project since its inception in 2001. His team has also conducted offshore surveys in the northern gulf to assess hurricane damage, completed an archaeological and hazard survey for a proposed pipeline in Tampa Bay and a coral removal and relocation project for the USCG at Key West. He holds an MS in civil engineering and a BS in mechanical engineering from Oklahoma State University.
