A Precedent-Setting Pipeline
THE MAGAGINE DEVOTED TO NICKEL AND ITS APPLICATIONS
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STAINLESS STEEL PIPE positioned along the route of an 11-kilometre-long waterline in Mesa Verde National
Park, Colorado, U.S.A.
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CONFINED SET-UP for equipment to directionally drilling a hole, two metres below surface for
the water pipeline.
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DRILL RIG used to drill and ream the hole to its final diameter and to pull the strings of stainless
steel pipe into the hole.
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THE DRILL BIT is the only part of the drill string that rotates during directional drilling.
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STAINLESS STEEL PIPE, 168 millimetres (mm) in diameter and 7 mm in wall thickness (6-inch
Schedule 40) was supplied in 16-metre lengths.
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SOME OF THE HARDWARE needed to complete the 11-kilometre-long waterline.
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DEDICATED WELDING STATION where pipe was joined by welding into strings up to 500 metres in
length.
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ALL WELDING was manual GTAW.
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DISTURBANCE OF THE NATURAL environment was minimized by reducing the size and number
of trenches needed to join long strings of pipe.
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Nickel-containing stainless steel has been used for the first time to construct a buried water pipeline
in North America. By Steve Lamb
Nickel magazine, October, 2003 -- The United States' National Park Service was
recently faced with a challenge at Mesa Verde National Park -- how to transport snowmelt water safely from
the magnificent headwaters in the foothills of southwestern Colorado to a water treatment plant inside the
park boundaries without disturbing the natural beauty of the area?
Mesa Verde is the United States' first World Heritage site, and home to hundreds of 700-year-old stone villages of the Ancestral Puebloans, which are nestled in overhanging cliff ledges. As such it is considered a premier archaeological wonder, so digging a trench 11 kilometres long for a water pipeline through the national park was out of the question. A method of drilling a hole horizontally was therefore called into play.
Directional drilling involves operating a drill head in a horizontal manner. The drill rods do not turn; instead, they simply push a rotating drill head through the soil terrain. In this case, the initial pass made an 8-centimetre-diameter hole 1.8 metres below the surface. A larger, reaming head was then attached to the drill string and was pulled back through the drilled hole, expanding it to 17 centimetres in diameter. The water pipe, welded in lengths up to 500 metres, could then be attached to the drill rods and dragged backwards through the hole. The drilling mud, laden with bentonite clay, forms a hard coating around the pipe when it dries. Depending on the terrain and soil conditions, a 500-metre section of pipe can be installed using this technique within 96 hours to one week. Longer times are needed for drilling through "cobble," which has a large content of stone and rocks. This can result in collapsed or unstable holes.
Directionally drilling proved to be an effective and efficient means of installing a water line in Mesa Verde, as it caused minimal land disturbance and allowed the contractor, Triad Western Constructors, Inc. to achieve accurate depth and direction control.
In addition to this drilling technique, project engineers, RTW Engineers, specified a material that is new to this application in North America -- stainless steel S31600, containing 12% nickel. Stainless steel was supplied in 16-metre lengths compared with 6-metre lengths for the other materials, and welded to final length, resulting in fewer joints. Also, the fully welded line offered less opportunity for water leakage. High density polyethylene (HDPE) could not handle the high water pressures involved. With an elevation difference of 760 metres, pressures were in the order of 4.1 megapascals. And ductile cast iron was too bulky and heavy for use in combination with directional drilling. Other advantages of stainless steel in this project include the following:
- Stainless steel pipe could be welded on-site in lengths of up to 500 metres.
- It can be joined to itself, pumps and valves using a wide variety of joining techniques, including
mechanical, welded and flanged.
- Tapping sleeves, couplings and restrained clamps are also available in stainless steel (AWWA standards
C223 and C606).
- Its ductility and flexibility allow it to withstand soil movement.
- It does not embrittle in cold weather.
- Water quality is maintained (to American National Standards Institute / National Sanitation Foundation 61
drinking water standards), so no water treatment other than biocides (chlorination) is required.
- It is expected to have a long service life with minimal maintenance. It was not necessary to coat the external surfaces of the stainless, although sacrificial anodes were installed to ensure long life in variable soil types.
Stainless steel pipe, 168 millimetres (mm) in diameter and 7 mm in wall thickness (6-inch Schedule 40) was supplied by Marcigaglia U.S.A. with capped ends. These lengths were fully welded (four weld passes) into 150-to-500-metre lengths, ready for directionally drilled insertion. Two field weld stations were used for completing pipe spools; the first station made the root pass and cover pass, while the second station completed the weld with two additional passes (no shielding gas on the ID). The radiographic inspection was undertaken several lengths away from the field welds (only two welds were rejected in the 11 kilometres of piping). The buried sections of piping strings were joined together using S30403 stainless steel couplings. These were fillet welded to the outside diameter (OD) of the pipe. All welding was manual GTAW. Oversight of these activities was provided by PBS&J.
Socket weld fittings were welded both in the field and under shop conditions using argon gas coverage. Argon cover gases were used to protect the inside diameter (ID) and OD of the pipe during welding. ID root pass weld regions were checked using ID cameras.
Stainless steel box work and screens, supplied by Johnson Screens, were placed at the headwaters of the western fork of the Mancos River to redirect the melted ice field waters into the pipeline.
Together with directional drilling technology, stainless steel piping has proven to be an ideal solution
to the environmental challenges facing the National Park Services in Colorado.
Steve Lamb is a consultant to the Nickel Development Institute.
PHOTOS: by Tom Skudra for NiDI
 National Park Service RTW Engineering Post Buckley Schuh & Jerneig (PBS&J) |
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