Sliplining in Northern California

An existing 20-in. steel pipeline that conveys an industrial fluid stream under a large body of water in Northern California showed signs of wear and aging. Due to the importance of the existing alignment to the client’s operations, and its environmentally sensitive location, a reliable rehabilitation method was pursued as a first and potentially only course of action to assure the integrity of the pipeline and its reliable operation.

A team was assembled to analyze the logistics and recommend a fully engineered solution. That team included Galindo Construction (Galindo) as the general contractor, J-C General Engineering, Inc. (J-C) as the trenchless subcontractor, and the client’s own engineering and operations group. The 7,000-lf project from the outside was daunting: the existing 20-in. steel pipeline was originally installed by a tow-in method across the body of water in the early 1960s. Because of the original installation method, the exact alignment of the pipeline was not known, aside from some early as-built information that showed sketched renditions of the final placement. The horizontal alignment was known to be fairly straight, but the vertical alignment followed the bottom elevation of the water body floor. This was particularly important for the deepest section of the crossing that was located roughly in the middle. The alignment variations at this location caused the pipe to curve downward, curve back to cross the deeper location, then curve back up and finally, to bend flat again.

This particular alignment variation meant that the pipeline couldn’t be drained, and further, would require the ability to navigate a curvilinear alignment in a proposed solution. The client’s engineering and operations team had already evaluated the required pipeline solution in terms of design flow and determined that they would not need the existing capacity of the 20-in. steel pipeline going forward. A smaller pipeline would work for the future required duty and thus sliplining was evaluated as a possible rehabilitation method.

A slipline solution focused the evaluation on possible replacement pipeline materials. The size and length of the existing pipe required a capable slipline pipe material along with a strong, low-profile joint. Additionally, since the primary concern with the original steel pipeline was corrosion, a non-corrosive material was desired. With these constraints in mind, the team’s analysis resulted in the selection of Fusible PVC pipe (FPVCP). The butt-fused FPVCP product afforded the required strength and minimal outer diameter of the pipe and joint and also met the non-corrosion requirement. Additionally, due to the strength of the FPVCP compared to other applicable slipline materials, the greatest flow area could be realized combined with the smallest outer diameter which: 1) reduces energy consumption throughout the operating life of the pipeline, and 2) greatly increases the likelihood of a successful slipline installation.

With a proposed pipe material and installation method set, the success of the project hinged upon making sure that the required sliplined pipe could be physically installed within its material limits. First, the existing pipeline needed to be characterized to assure that the minimum bend radius of the sliplined pipe would be met. Second, there also needed to be assurance that there were no obstructions or deformations in the existing line that would stop the installation. Third, the insertion action and technique would need to assure that the safe allowable pull force for the sliplined pipe would not be exceeded.

It was proposed early on in the process to use an American Augers 200,000-lb drill rig to perform the insertion and pullback operations for the slipline. J-C also used the HDD rig to perform a wireline alignment verification as a first pass alignment investigation on the existing pipeline. The wireline information only included inclination and distance as the drill rod was advanced, as there was no way to locate the steering tool in the middle of the body of water in which the pipeline crossed. The resultant wireline survey data, which was compared to the available as-built information, showed fairly close agreement. Most importantly, it provided areas of maximum bending in the existing alignment, which were then compared to the capabilities of FPVCP and found to be within the required bending limits of the FPVCP section required.

After this initial wireline verification of the alignment, J-C used the drill rig with a tooling setup and a bentonite based drilling slurry to ream, swab and flush the pipeline in order to extricate any tuberculation or build up in the line, as well as ensure that there were no major deformations or obstructions in the line. Materials removed from the pipeline included evidence of corrosion and a small amount of fine, sediment build up. As a final check on the applicability of the slipline technique, an 8-ft length of pipe (two 40-ft lengths of pipe with one fusion joint in the middle) was pulled through the entire length of the line and then checked for damage. With this operation successfully completed, the team turned their attention to the final insertion.

Since the existing pipeline could not be drained, insertion logistics needed to account for some sort of ballasting in the pipeline, if only for the critical, deeper ‘U-shaped’ section in the middle of the alignment. If the annular space between the host pipe and the new sliplined pipe is filled with fluid, either from the cleaning operation or present from the previous use of the pipeline, this would create an undesirable buoyancy force and resultant friction during the installation in an empty FPVCP sliplined pipe. The idea of ‘self-ballasting’ the FPVCP was discussed within the team as a way to eliminate this issue. Using an ‘open’ pull-head on the FPVCP would allow the drilling slurry in the host pipe to filter into the new pipe as it is installed. While this fills the new pipe with the slurry, which may not be desirable, it also creates as close to a neutrally buoyant installation as possible (See Figure 1). As a final means of cleanup, J-C and Galindo planned to pig the line, to flush out the slurry taken on during the installation, and backfill it with clean water, ready for hydrostatic pressure test and acceptance of the rehabilitated pipeline.

The end of the existing pipeline on the insertion side of the alignment was fitted with an oversized spool of steel pipe to extend the alignment and provide a consistent insertion angle of the sliplined FPVCP into the host pipe. Rollers and a cradle assembly were positioned to align the new pipe during the insertion, along with a roller outfitted flange assembly at the steel spool connection.

Based on the calculated buoyancy of the FPVCP section, pull force values were expected to top out between ~20 and ~25 k-lbs. as a worst case scenario to account for buoyancy loading, anticipated capstan loading on the curved sections and drag at grade. This was assuming that the drilling fluid used was basically water and that a standard friction factor common for HDD installations (0.3) would be present in the pipe. This value did not take into account any additional force needed to pull the pipe through all of the cumulative vertical alignment variations from a pipe originally laid on the bottom of the body of water floor. Figure 2 shows the range of drill rig pull forces expected based on the assumptions described, as well as those assumptions with an additional factor for the cumulative alignment variations along the length of the installation. The actual pull forces recorded at the drill rig at key locations were also gathered and shown in the figure. While these recorded and estimated forces include the actual force of pulling the drilling string, reamer and tooling along with the pipe, it represents a rough estimate or characterization of the force required as it is applied to the pipe string. As can be noted from the figure at the end of the pull, when the rig and string have the least impact on the pulling force registered, the expected pulling forces fell in between the two estimates of the force required. This indicates that the alignment variations did have a significant impact on the pulling forces required, however, they ended well within the capabilities of the pipe material (safe allowable pull force of 93,000 lbs.). The pipe was inspected within the first 15 ft behind the pull head for signs of any excessive scratching or gouging to assure that the pipeline did not pass through an area that would have damaged the pipe after the initial cleaning and proofing.

The success of this project was based largely on the honest and open collaboration of the project team. The end result of this teamwork was not only the successful rehabilitation of an inaccessible yet critical pipeline, but also delivering that solution in a very short window of time. The project was completed with minimal disturbance to the existing project area and the operations of the pipeline. Sliplining, one of the oldest and least environmentally intrusive forms of trenchless rehabilition, has added another successful chapter to its story.

Richard “Bo” Botteicher, P.E. is with Underground Solutions.