Directional Drilling with Percussion
Gas Main Installation in Germany

Any type of new underground utility installation in Europe is highly scrutinized. In many cases ancient streets, buildings and landscapes face potential damage from conventional construction methods. Needless to say, trenchless technology flourishes in Europe.

The recent expansion of natural gas service by German gas provider Municipal Works Lippstadt required the installation of two new gas mains in the cities of Benninghausen and Heringhausen, Germany. The new lines would serve both commercial and residential customers in the area. Because the installations needed to cross a local river, only trenchless installation was considered. Directional drilling was chosen for what was expected to be a routine drilling project. By the time the project was complete, however, it took added percussive power and rock drilling equipment to successfully cross the river.

Job Site Layout
The utility provider planned to pull in two, 6-inch HDPE pipes together with the new gas main in a three-pipe bundle. One would be used as a spare potable water line, the other as an accommodation pipe to be leased at some time in the future. To complete the installations, Municipal Works Lippstadt turned to trenchless equipment manufacturer Tracto-Technik, Lennestadt, Germany (known as TT Technologies in North America).

Because of numerous sub-utilities at the Herringhausen site, as well as a sheet piling on the riverside, determining the proper bore path was critical. Both river crossings would be approximately 395 feet in length. Several types of steel pipe were evaluated based on the minimal bending radius. A maximum cover of 23 feet was determined and a special HDPE coated 10.5-inch diameter steel gas pipe was chosen for the project.

Because local soil conditions were already well known from a crossing successfully completed in the past, the Municipal Works crew did not expected any soil problems with the project. The sand/clay soils were expected to remain consistent throughout each bore. Soil investigation at deeper levels was deemed unnecessary.

Drilling at the Herringhausen site began in August with a Grundodrill 20S from Tracto-Technik.

Percussive Drilling
Standard directional drills use hydraulic thrust and rotational torque to complete their pilot bores. With the addition of water/bentonite slurry most small- to mid-range drills operate adequately in moderate soil conditions. But as soon as densely compacted or rocky soils are encountered, these drills reach their thrust force, rotational torque and flushing pressure capacities and cannot complete the bore. The same is true during extremely long bores.

The Grundodrill is a combination hydraulic/percussive steerable drilling system. According to TT Technologies' President Chris Brahler, directional drills with percussion are relatively new to North America and promise to have a large impact on directional drilling industry here, like they have in Europe.

Brahler said, "Often contractors look to larger, more expensive and less practical directional drill rigs to get more hydraulic thrust force when they encounter difficult situations. By adding percussion to the drilling process, the hydraulic/percussive drill is able to steer the drill head in soil conditions where standard, comparably sized drill rigs cannot."

The percussive force is created by impacting the drill stem at the drill rig. This provides the dynamic energy needed to steer in difficult and rocky soils. The percussive impacts on the highly flexible drill stems can be adjusted at a rate up to 1,000 impacts per minute.

The Bore
After a good start, progress slowed greatly at 100 feet into the bore at a depth of 16 feet. An obstacle, possibly a large boulder, seemed to be the reason. But this could not be confirmed. The percussive hammer was operating at full capacity and was able to keep the bore progressing. After a while it became evident that the crew was dealing with more than just rocky soil. They were unable to get past the obstacle and eventually there was hardly any detectable thrust.

A solid rock formation was now suspected, which could not be penetrated without rock drilling equipment. The percussive action of the drill had advanced the bore to a favorable point to start rock drilling operations. The Grundodrill was quickly re-equipped with rock drilling equipment and the pilot bore resumed without further problems. The large rock formation the crew encountered continued over the next 300 feet.

A substantial amount of drilling fluid was used for the rock drilling operation. For this project approximately 5.5 tons of boring fluid was used. The drilling fluid was properly disposed of with some used as fertilizer on the surrounding farm fields.

The Pullback
In order to accommodate the collective diameter of the three pipes, special steel chisel backreamers were used to expand the pilot bore. The first backreamer was 8 inches in diameter. After successfully backreaming to 8 inches, the second, 12-inch backreamer was used. The third backreamer was 20 inches in diameter. The large diameter backreamer was specifically made for this particular project. All three pipes were pulled in together in one bundle, while attached to the 20-inch backreamer.

Thanks to good prep work, pullback was completed in less than three and half hours, including a 45 minute delay for welding on the second pipe string. Welding the pipe during operations was necessary because there was not enough space to totally lay out the pipe length completely at the beginning of the project.

After the pullback was complete, the new gas line was successfully pressure tested and readied for service. Everyone was very satisfied with the results of the project, especially considering the unexpected encounter with the large rock formation.

In Benninghausen, approximately one and a half miles away from the Herringhausen job site, the second crossing was complete. No problems were encountered during the pilot bore on the second crossing. Crews at the Benninghausen site used a smaller Grundodrill 10S. No rock drilling equipment was required.

Pipeline & Gas Journal, May 2001