A first for Gasunie: first ever HDD-crossing removed!Groningen
It’s widely known that Gasunie has laid hundreds of km of pipeline in the last few years. What fewer people know is that Gasunie also regularly removes gas pipelines. Gasunie aims to remove as many pipelines as possible if they become obsolete. If an obsolete pipeline cannot be removed, pipe caps will be installed and the pipeline will be filled with nitrogen. In the next few years approximately 90 km of obsolete pipelines will be removed in the southern Netherlands, including five HDD-crossings.
HDD stands for “horizontal directional drilling”, a well-known and widely-used technique in which pipelines can be drilled under obstacles such as roads, rivers and existing underground infrastructure. A recent example of a Gasunie HDD-crossing is the pipeline crossing the “Ketelmeer”.
A first for Gasunie
Residents and schoolchildren in the Scharwoudestraat in Tilburg were witnesses last week to the first ever removal of a HDD-crossing which had been in use by Gasunie for several years. In this case it was a HDD-crossing which had been drilled in 1999, with a length of approximately 190m under the river Donge and its surrounding nature reserve. The removal was carried out by the joint venture Grondig (Denys and Dura Vermeer) and van Leeuwen Boringen.
In order to avoid any potential subsidence caused by removal of the pipe, the drill hole was filled with “dämmer” while the pipe was being removed. Dämmer is a cement-based liquid which gradually becomes harder through time and which ensures that the hole left by the pipeline remains permanently filled.
Technique and traction used
In an ideal world, engineers would be able to calculate the exact resistance keeping a HDD-crossing in contact with the ground surrounding it, but unfortunately this is impossible. Because of this, a plan was made in which the maximum traction to be applied to the pipeline would be gradually increased, by using different techniques. If the resistance proved to be extremely high, the last available technique would be what is termed a “washover” tool. The washover tool applies a new layer of “lubrication” to the HDD, which substantially reduces the resistance so that the HDD can almost definitely be removed.
During the work it became clear that the HDD would move with a relatively low traction of approximately 30 tons, after initial support for the first six metres from a “pipe rammer” (see photo). The pipe rammer is a sort of pneumatic hammer, which pounds the other end of the pipeline in the direction of the drill by using high-pressure air. This loosens the pipe in the ground and makes it easier for it to be pulled out. After the initial use of the pipe rammer, the HDD-crossing could be carried out purely with the “pipe pusher” (see photo).
The experiences gained from the removal of this HDD-crossing in Tilburg will be used to fine-tune the procedure for removal of other obsolete HDD-crossings in future.