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Subaqueous force main replacement using the horizontal directional drilling technique.

The directional drilling technique, used for the first time in Maryland, the triple benefits of completing a force main replacement expeditiously, economically, and in an environmentally sensitive manner.

A 22-in. diameter steel force main constructed in 1975 conveyed flow from the Cox Creek service area in northeast Anne Arundel County to the Cox Creek waste-water treatment plant. in 1983, a parallel 30-in. diameter force main was constructed to increase capacity. With completion of the new force main, use of the 22-in. main originally installed on wood pile bents at the Marley Creek crossing was discontinued due to numerous leaks from corrosion. In 1985, an unsuccessful attempt to lint the crossing resulted in the abandonment of the 22-in. pipeline across Marley Creek.

The Anne Arundel County department of utilities engaged Whitman, Requardt and Associates to prepare designs, contract documents, and provide engineering services during construction for a new 24-in. diameter force main to replace the existing creek crossing.

Construction of the parallel 30-in. Force main included interconnecting piping with the 22-in. force main and isolation valves in each pipeline. the distance between isolation valves on the 22-in. steel force main, approximately 1,117 ft, established the maximum limits of existing pipeline replacement.

Environmental Sensitivity

Marley Creek is a tidal estuary of the Chesapeake Bay. Consequently, limits of the proposed work placed the entire project within the state of Maryland's Chesapeake Bay Critical Area and Anne Arundel County's Sensitive Area. These area designations dictated from the beginning a need for environmentally sensitive designs.

At the creek crossing, the 22-and 30-in. diameter force mains straddle an existing 200-ft-wide Baltimore Gas and Electric Company right-of-way. Contained within the right-of-way are two 115 k VA transmissions lines with towers and a high pressure gas transmissions main protected by a impressed current cathodic protection system.

Site investigations revealed a number of additional conditions requiring attention during design. Soil resistivity reading of 2,000 ohm-cm close to the creek and pH values between 4.5 to 6.0 indicated corrosive soil conditions, Detection of stray current interference from the gas pipeline cathodic protection system, the presence of a 100-year flood plain, and discovery of tidal wetlands along both shores of the creek further complicated selection of the appropriate pipe material and installation procedure for the replacement force main.

The selected alignment for the replacement main closely parallels the existing pipeline, which limits interference with existing utilities and minimizes the area of disturbance. Topographic surveys and bottom sounding along the alignment showed a creek 475 ft wide with water and soft mud depths of up to 30 ft. As shown in Table 1, evaluations for the crossing included three installations methods and four pipeline materials. [TABULAR DATA OMITTED]

Hostile Environment

The hostile pipeline environment from corrosive soils and stray electrical currents combined with the existing steel line history at the crossing all dictated the use of high-density polyethylene (HDPE) as the pipe material.

A comparison of installation methods indicated that both the reuse of the existing pile bents and open cutting with oyster shell bedding were the least costly approaches. However, both methods required excavation of the creek bed and open cut construction through wetlands on both shores. The ability to avoid environmentally sensitive areas within the project site and potential restrictions on periods of construction warranted using the unique method of horizontal directional drilling for pipeline installation.

Horizontal directional drilling is a rapid "no-dig" technique of installing pipelines across watercourses and/or environmentally sensitive areas. The techniques involves initially drilling a small diameter pilot hole along a predetermined underwater water path extending from shoreline to shoreline. The pilot hole is then reamed to a larger diameter and the assembled pipeline pulled back through the oversized hole.

Project design criteria were determined largely by directional drilling requirements and HDPE characteristics. Drilling parameters included entry/exit angles; minimum drilling radius; uniformity and density of the material to be drilled; and the drilling rig pull force capabilities for pipe installation.

Pipe characteristics evaluated for both operating and construction conditions included: pressure rating, ring deflection (ovality), buckling resistance, expansion/contraction, crushing resistance, materials yield strength, and minimum bend radius.

With few large diameter pipelines installed using the directional drilling techniques, and even fewer using HDPE, the capability to withstand stresses exerted during installation in the even difficulties were encountered was deemed critical. To withstand the possible stresses imposed during installation without exceeding the material yield strength, a pipe wall thickness of 2.25 in. was selected. This is a standard dimension ratio (SDR) of 11 for 24-in. diameter pipe. The selected pipe SDR met all criteria for design and allowed exerting full pulling force of the drill rig during installations with the pipe still capable of relaxing to a pre-pull length.

Subaqueous Installation

Final design incorporated 1,075 ft of new HDPE pipe with slightly over 800 ft to be installed subaqueously using the directional drilling technique. Designs for the subaqueous portion provided for drilling to begin at elevation +20; proceed down under the creek at a maximum angle from horizontal of 15 degrees to elevations -49; turn upward and exit on the opposite shore a approximately the same angle and elevation as entry. The remaining pipeline installation anticipated using conventional construction methods.

Connections with the existing steel pipeline at their end of the new pipeline use reducer Dresser couplings and stainless steel stiffening rings inserted into the HDPE pipe. To prevent connection separation due to the high expansion/contraction rate of HDPE, approximately one in. per 10 [degrees] F change per 100 ft, each end of the new pipe is anchored with a concrete restraint buttress. The buttresses are designed to accommodated a change in pipeline temperature of up to 30 [degrees] F.

Upon completion of the contract documents, applications were submitted for all necessary permits and approvals. Ten separate permits and/or approvals were identified for the project, the same as required for conventional construction methods and materials. All necessary approvals were received within seven months of submitting the first application, thus clearing the way for construction.

Approximately five weeks following notice to proceed, twenty-eight 40-ft lengths of 24-in. diameter, SDR-11 HDPE pipe were delivered to the project site. Fusing of the pipe started the following week without any disturbance of the surrounding area. Approximately 30 minutes was required to complete each joint. The entire 1,120 lineal ft of pipe was fused into one continuous piece within three days. Charged with water shortly after fusing, a pre-pull hydrostatic test to 150 psi was performed without difficult. With pipeline integrity confirmed, the stage was set for directional drilling to begin.

Directional Drilling

The driller arrived two weeks after pipe fusing. Only one day was required to prepare a site on the shore opposite the pipeline. Pilot hole drilling for the subaqueous crossing began the following day. Drilling proceeded from the western to the eastern shore using an 8 3/4-in. drill bit with bent housing and 5-in. steel drill string. During pilot hole drilling, the drill string is not rotating while being advanced.

While the drill string is advancing, a bentonite slurry is pumped at 2,000 psi through the drill string to the drill bit. The high pressure slurry is used to drive the drill bit cutters, aid in the cutting, fill the hole to prevent collapse, and carry cuttings back out to the drill site. Slurry exiting the hole is captured in a pit and either recycled or pumped into a tanker for disposal off-site.

Guidance during drilling operations is provided by an electronic compass and inclinometer mounted in a the drill bit housing. Continuous reading of azimuth and vertical angle are transmitted to a control trailer where the drill position is monitored and recorded every 31 ft, one length of drill pipe. changes in director required to maintain pilot hole alignment are accomplished by rotating the entire drill string. Rotating the drill string causes the bent housing with drill bit attached to changes direction. Proceeding at a rate of over 2 fpm, a pilot hole 860 lineal ft long was completed along the design centerline within six hours.

Pipe Installation

Installation of a 42-in. diameter reamer on the launch side with the drill string still in the pilot hole followed the drilling. Reaming of the pilot hole proceeded from the west shore launch site to the east shore where the fused pipe awaited installation. Unlike pilot hole drilling, reaming the pilot hole to a larger diameter is accomplished by rotating the entire drill string and reamer. Bentonite slurry pumped through the drill string a 900 psi is primarily used to evacuate cuttings and fill the hole to prevent collapse. Guidance while reaming is provided by the previously drilled pilot hole. Accomplished over a three-day period, total reaming time was 27 hours.

While reaming proceeded towards the fused pipeline on the eastern shore,-pipe pulling assembly was attached to the end of the HDPE pipe. The pulling assembly included a 30-in. diameter tapered pull head, universal joint, swivel, universal joint, and a 32-in diameter pull back reamer. The pull head provides an attachment point to the pipeline; the universal joints allow flexibility in movement between drill string and pipe; and the forum for public review of the assessment documents. This was attended by some 700 scientists from 30 countries. Over the years NAPAP was criticized for some of its conclusions. These criticisms, however, should not be viewed as program failures. Rather, they are natural and desirable aspects of forming scientific and public understanding of the acid rain phenomenon. Basic research is the foundation of scientific and technological progress, and NAPAP demonstrated that basic research may provide answers without direct policy consequences. The most important of all lessons learned is that learned while going through the science and policy process itself. The process the helped bring the acid rain debate into the open and raise the awareness of the American public. In its best moments, the NAPAP process has provided a useful framework for assessing complex environmental issues. Among the worst impacts, special interests and disparate missions represented by various agencies are preventing definitive conclusions of research results. NAPAP has been instrumental in bringing about the convergence of scientific opinion on the effects of acid rain

"NAPAP, A Perspective." By Chantale Y. M. Wong, formerly environmental engineer/policy analyst, NAPAP; now policy analyst, Office of Management and Budget, Washington, D.C. Water Environment & technology, December 1991.

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Mr. Bathurst is a Project Engineer with Whitman, Requardt and Associates, Baltimore, Maryland, and Mr. Wile is Chief Engineer, Department of Utilities, Anne Arundel County, Annapolis, Maryland. The following paper was presented at the joint annual conference of the Water and Waste Operators Association of Maryland, Delaware, and District of Columbia and the Chesapeake Water Pollution Control Association.
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Author:Bathurst, Luther E.; Wile, Bruce D.
Publication:Public Works
Date:Mar 1, 1992
Words:1916
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