Direct pipeline installations

Photo courtesy ASCE.

Pipelines of various types, sizes and purposes are routinely installed below the ground in communities around the world. The most common method of installation is open cut, also known as trenching, which results in obvious—sometimes significant—unwanted ground disturbance. Consequently, trenchless methods of pipeline installation have been developed to reduce surface impacts and public inconvenience, including pipe ramming, horizontal auger boring, microtunnelling, guided boring, shielded pipe jacking, horizontal directional drilling (HDD) and utility tunnelling.

Direct steerable pipe thrusting (DSPT), in particular, was developed relatively recently to fill a need for installation in ground conditions challenging for other trenchless methods. It combines some of the characteristics of HDD and microtunneling. Direct Pipe, developed in Germany by Herrenknecht, was the first commercially available system in this category. Since its initial use in 2007, more than 200 installations have been completed worldwide.

How it works

DSPT is a near-surface launched, thrusted microtunnel method of pipeline installation. It can be used to install steel pipe in a single pass along both horizontal and vertical curves.

The soil or rock in the heading of the pipe is removed through slurry microtunnelling methods. A clamping device on the exterior (perimeter) of the pipe is used in combination with a thrusting machine to hold and push a steel installation pipe—which is typically prefabricated on-site—along its heading. Excavated materials are removed by a closed slurry system, located within the installation pipe. The pipe is thus installed in compression by the thrusting machine.

DSPT incorporates the sequential process of a typical HDD installation, including site preparation, pre-welding long sections of thrust pipes, integrity testing and certification of joints within these long sections, application of joint coating and tunnelling. In most cases, even the necessity for intermediate or golden welds, non-destructive testing of the welds and final field-joint coating remains unchanged.

However, DSPT differentiates itself from traditional multi-step processes like HDD by combining the process of tunnelling and simultaneously installing the thrust pipe in one pass. The two key components for this process are the microtunnel boring machine (MTBM) and the pipe thruster or pusher.

More than 200 DSPT installations have been completed worldwide.

Comparable to a traditional jacking frame used in microtunnelling or to pipe jacking for pushing short pipe sections of 8 to 10 ft in length at a time, the pipe thruster or pusher provides the forward thrust in DSPT to the prefabricated pipe. However, instead of applying force to the end of the pipe as in conventional microtunnelling, the thruster or pusher clamps and grips the outside of the thrust pipe, pushing it forward into the ground.

The joining of prefabricated pipe strings requires connecting the internal umbilical lines of the MTBM, which run through their full length, and then joining the strings together with golden welds. Section welding is typically followed by non-destructive testing/examination (NDT/NDE) procedures and field-coating of the joint(s).

The tunnel is excavated by the MTBM, which is based on slurry-type microtunnelling machines and cutter heads or wheels. The MTBM is attached to the forward end of the thrust pipe and they are pushed into the ground together.

A rotating cutter wheel excavates the ground at the face of the tunnel. The excavated material is broken down by the cutting tools before entering the crushing chamber, where it mixes with drilling fluid and passes into the slurry circuit through openings in the chamber. The slurry is then pumped back through a dedicated discharge line inside the thrust pipe to the separation plant on the surface. At the separation plant, the cuttings are removed from the slurry and the clean drilling fluid is then returned to the face of the MTBM through a dedicated feed line, also located inside the thrust pipe. With the slurry circuit flow contained in the feed and return lines, DPST can operate at low annular pressures relative to HDD, which reduces the risk of hydraulic fracture and inadvertent drilling fluid surface release.

During the excavation process, the overcut or annular space created by the cutter wheel’s gauge cut is filled with a bentonite-based lubricant. This reduces friction between the bored hole and the thrust pipe. And if the lubricant is maintained slightly above ambient ground pressure, it supports the ground to maintain the integrity of the surrounding formation.

The bentonite lubrication system is separate from the feed and slurry return system. All of the control and support units are located above ground near the launch area, workspace permitting.

The method provides an important niche tool for difficult sites.

DSPT can install pipe as the final product or as a casing. Most of the activity and required workspace during construction is at and behind the entry location. After the drive is complete, the MTBM is disconnected and the umbilical removed from the jacking pipe. A relatively small workspace is typically required at exit, which is often unoccupied until just before the drive is complete and the MTBM punches out.

While DSPT will not completely displace HDD and conventional microtunnelling, it provides an important niche tool for trenchless engineers, contractors and owners to consider for challenging sites.

Jon L. Robison, PE, is a principal at GeoEngineers. This article is based on the American Society of Civil Engineers’ (ASCE’s) new Manual of Practice (MOP) 155, Direct Steerable Pipe Thrusting, which details how engineers and construction professionals can use the direct steerable pipe thrusting (DSPT) method to design and install pipelines. For more information, visit www.asce.org.

This article appeared in the May/June 2003 issue of Canadian Consulting Engineer.