Pipeline integrity: price at the cost of safety?

the results from borehole monitoring pleased everyone’s expectations. A couple of years later, the National Energy Board (NEB) of Canada also recognised this great advantage of Direct Pipe on a difficult 48 in. highway crossing in Alberta.

In Serbia, the 48 in. gas pipeline interconnector to link Bulgaria and Hungary had to cross the Danube River over a length of 1409 m. As it seemed too risky using HDD in the highly permeable sand, silt, and gravel, the crossing was originally planned for pipe jacking. Due to the tight schedule, Direct Pipe was finally chosen as the preferred, efficient single-step installation method. Small-diameter Direct Pipe In Poland, the first small diameter Direct Pipe system using jet pump technology has accomplished two river crossings recently. An AVNS600 completed two 28 in. sections for the Gas Interconnector Poland-Lithuania. The first crossing of 340 m was completed by installing one single prefabricated pipe string, while the second crossing of 884 m was split into two pipe strings. The anticipated ground conditions on both crossings were sand, silt, clay, some peat, and some pebbles. However, the encountered ground conditions on the first crossing were significantly different and changed shortly after launch to heavy gravel and some boulders for almost the remainder of the drive.

Nevertheless, this prototype demonstrated what was considered impossible for decades: small diameter installations with AVN Slurry MTBM are feasible even for extended distances. Traditionally, small diameters were limited by centrifugal pumps being too large to fit inside the pipe. The new jet pump concept with its smaller size, creating the AVNS range, can be installed inside diameters as little as 20 in. Distances of up to 1500 m (depending on ground conditions and depth) are achievable – with a single discharge pump. For Direct Pipe® this opens up the available range for installing 24 - 60 in. pipeline diameters.

Direct Pipe in hard rock Along with the small-diameter AVNS series, Herrenknecht also developed a small-diameter AVN Rock series for traditional microtunnelling and incorporated it into the AVNS concept for Direct Pipe, which covers conditions of up to 150-200 MPa strength. This new AVN Rock series defines a separate line of MTBM specially designed with features such as stronger main bearings for improved penetration, performance, and higher RPM. Cutting wheels are equipped with TCI Tricone Bits or milled-tooth disc cutters alternatively. A high degree of wear-resistance allows for longer drive distances unheard of for such small diameters in the past. They have already demonstrated their ability to cope with conditions where serious gravel and/or boulders are the predominant subterranean formation.

Outlook The new AVNS concept featuring the jet pump and the AVN Rock series presents a milestone for microtunnelling and Direct Pipe, pushing the boundaries towards feasibility for longer drives in small diameters. Future developments in surveying technology by Herrenknecht’s partner VMT are expected to make these systems even more reliable, simpler, and more versatile. While Direct Pipe has already established its place for land crossings and shore approaches by offering a number of arguments and benefits over other trenchless technologies, its strong benefits towards sustainability and environmental impact just now are becoming more and more important. Fuel consumption, CO2 footprint, and other aspects are expected to be key decision drivers in the future.

Figure 6. Borehole pressure comparison Direct Pipe versus HDD.

Figure 7. Site installation for 28 in. Direct Pipe river crossing in Poland.

Figure 8. Small diameter Direct Pipe MTBM (AVNS600).