Weld-After-Backfill, A Growing Industry Practice Authors: Bob Buchanan1 - Canusa-CPS
Dennis Dechant2 - Northwest Pipe Company Ralph Warner3 - National Welding Corp.
Keywords: Steel Water Pipeline, Welding, Backfill, Corrosion Coatings Abstract This study documents a series of eight trials of the weld-after backfill (W-A-B) sequence. This sequence of pipeline construction was first used in the late 1980s on a pipeline in Texas. Since then, more than 45 major pipeline projects in the United States have been constructed using W-A-B. These projects included a variety of pipe joint coating materials such as mortar diapers, concrete encasement, tape coatings and heat-shrinkable sleeves. This study focuses on heat-shrinkable sleeves for coating the pipe joint, prior to backfill. A growing practice in steel water pipeline construction entails fitting bell & spigot pipe segments together, installing field joint coatings and backfilling the pipe prior to welding. Welding is then completed from the inside of the pipe after the pipe has been backfilled. Weld-after-backfill has proven to be successful in minimizing the length of open trench, improving project safety and speeding up pipeline laying, resulting in design, economic and safety advantages over traditional steel water pipeline construction methods. These advantages benefit the owner, designer, contractors and suppliers. W-A-B is recommended for projects with lap welded joints, which is the predominant joint design for most water pipeline installations. The welding process, however, produces high temperatures on the pipe wall, which may damage some corrosion coatings. Research by Northwest Pipe, National Welding and Canusa-CPS was completed to evaluate various welding processes, measure temperatures and assess the affects of the W-A-B sequence on exterior field
1 Marketing Manager, Canusa-CPS, Toronto, Ontario 2 Chief Corporate Engineer, Northwest Pipe Company, Denver, Colorado 3 Operations Manager, National Welding Corporation, Salt Lake City, Utah
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joint coatings. Various heat-shrinkable sleeve-coating systems were tested in order to optimize current practices. This paper describes the W-A-B sequence, explains typical installation and welding processes, provides data from the research and makes recommendations as to best practices when this construction method is used. Introduction Trials were held at the Northwest Pipe Company's Denver yard in partnership with Canusa-CPS and National Welding in March and July 2004 respectively. The observations and data are summarized in the figures and tables below. The objectives of the trial applications were threefold: 1. Measure and understand the amount of heat produced during the welding process. 2. Evaluate various corrosion protection systems and how the heat affected the
materials. 3. Demonstrate the effective implementation of W-A-B with the products and
welding processes used. Two different welding techniques were used and eight trials were designed to evaluate various system designs consisting of an optional underlay and a primary corrosion protection sleeve. The techniques and trials were meant to evaluate the effects of different welding processes and the high temperatures generated on the field joint corrosion protection materials. The corrosion protection system configuration variables included adhesive and backing component types, thicknesses and shrink recovery ratios used for the underlay and outer sleeve. The trials were done on thin 0.250 wall (6mm) bell & spigot pipe with cement mortar lining. Since the pipe used were "drop ends" or remnants from Northwests production yard some gaps in the bell & spigot were greater than field specs may have allowed. Backfill conditions were also not completely representative of the field and these variables are believed to have created a worst-case situation, which affected results of the tests. Pipe and Backfill Set Up Eight pipe joints were assembled, fitted with various heat-shrinkable sleeves, backfilled and welded in the course of these trials. Pairs of short pipe sections with bell ends were attached by a short plain end by plain end pipe section. Four pairs of these pipe assemblies were used during these trials. Each assembly was then placed in a joint box designed to hold the pipe joints and backfill material, approximating trench conditions. The joint box allows the backfill to surround the pipe joints at a depth of least 12 inches all the way around the pipe. The backfill material was mortar sand from Northwest Pipes mortar coating and lining operations at the plant.
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Welding Comments and Data Two welding processes were used during the trials; Shielded Metal Arc Welding (SMAW), commonly referred to as stick welding, and Flux Core Arc Welding (FCAW), or wire welding. Each process was used to complete a full fillet weld on the inside of the pipe joints in these trials. These two welding processes are the most common processes used in production pipeline field welding. SMAW involves the use of a rod or stick electrode that has a flux bonded to the outside surface. The flux forms the shield for the weld puddle to prevent oxygen impurities and inclusions during the welding process. FCAW involves the use of a hollow wire electrode with flux in the core in addition to a shielding inert gas to protect the weld puddle and prevent impurities and inclusions during the welding process. The two processes differ primarily in the deposition rate of metal applied to the joint. SMAW progress is made in either an uphill or downhill direction on the pipe joint, depending on the electrode, while FCAW progress is usually made in an uphill direction on the pipe joint. Observations of several welding parameters were made during the weld sequence on each pipe joint. Data gathered includes the electrical parameters of amperage (I), voltage (E) and travel speed (V), the rate at which the weld is applied to the joint. These three parameters were used to calculate reasonable levels of heat input, using the following equation1):
Sample Heat Input Data and Calculations:
Amperage 210 Voltage 26 Travel Speed (in/min) 9 Heat Input (Joules) (210 x 26 x 60) / 9 = 36,400
Other data included the use of a "temp stick" grid on each welded joint. A temp stick is a crayon used in the steel industry to determine the temperature of a metal surface. The temp stick is formulated to melt at a pre-determined temperature. The grid was created, using temp sticks with melting points between 800F (427C) and 200F (93C), in 100F (56C) increments. Starting at the top with an 800F stick, a line was drawn perpendicular to the joint to be welded. Two inches below that a line was drawn with a 700F stick, and so forth down the joint, finishing with a 200F line. After welding, the melt back distances were measured and recorded. The temp stick grid gives a qualitative indication of heat transfer through the steel adjacent to the weld bead. Specifically, the grid indicates the distance from the weld root at which
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the metal temperature reached the temp stick melting point. This is a measure of heat transfer away from the weld. Some joints in the trials included two grids, one for each welding process as half of the joint was welded with SMAW and the other half with FCAW. In some instances the grid was impossible to measure due to the temp stick mark being blurred or obscured by welding smoke and soot or by smearing due to direct contact by the welder. Note that the welding produced a heat zone approximately 1.3" (35mm) wide with steel surface temperatures of at least 800F (427C) on the inside of the pipe. This was important to consider when analyzing the effects of the heat on the exterior coating. The data from Trial #2 is shown in Figure 1 below as a sample.
Trial #2 FCAW
-4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0
Bell Side (d)
Spigot Side (d)
Figure 1: Temp Stick Melt Back Distance
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Differences in Welding Processes It is assumed that both processes deposited approximately the same quantity of weld metal to complete the joint. The SMAW process required two passes to complete the weld on the pipe joints; a root pass and a fill pass. The heat input for the first pass using the SMAW process is generally low; in the 20,000 joules range. However the second pass is usually higher; in the 25,000 to 35,000 joules range. The FCAW process required one pass to complete the weld on a pipe joint. The heat input for the FCAW process was in the 27,000 to 40,000 joules range. The welding done in these trials was similar to that done under typical field conditions. Neither process used any special or non-standard methods. The electrical settings and welder techniques were the same as the welders use in the field. The joint assembly was also typical of field installed pipe joints. Coating Evaluation and Comments The various trials evaluated heat-shrinkable sleeves either as the only joint coating or combined with a protective underlay material. The variables were in the types of sleeves and underlay materials and in the design of these materials, i.e. backing and adhesive type and thickness. These variables are identified in Appendix B - Coating Performance. The welding temperatures had mixed effects between the trials but it was clear that both methods produced high temperatures below the field installed corrosion protection coating system. Figure 2 charts thermocouple readings taken below the joint coating as the welder passed the measurement point. Th