In 1975, the Dutch gas board Nederlandse Gasunie installed a 42" gas pipeline to transport gas between Norway and the Netherlands. The pipeline lands in Delfzijl and runs through the Eems estuary from Emden in Germany through the Paap sandbank to Spijk, in the north-east province of Groningen, in the Netherlands. The company recently decided it needed to lower the pipeline.
Gastransport Services (Gasunie) requested Design & Construct proposals from four contractors for lowering the gas main without having to disrupt service. VBMS (as part of Visser & Smit Hanab) was awarded the project, and its Technical Department performed the engineering, together with Hydraunamic (Boskalis). The project was executed by a consortium made up of Visser & Smith Hanab, its sister company VSF, Boskalis and Rederij Waterweg.
Various technical solutions were closely examined before starting on the project, including building a new crossing using HDD (horizontal directional drilling). Ultimately, Gasunie opted for carefully designed piled frames, which could be used to position and lower the existing, live, pipeline.
Removal of the stone layer
Boskalis removed the still extant stone layer with a large backhoe dredger. This was no simple task, since workers had to make sure the bucket of the backhoe did not touch the pipeline. By making a 3-D animation of the pipeline and the dam above and integrating this into the software of the backhoe, they created a safety zone around the pipe that the bucket could not enter. When the jack on the pile standard indicated that the pipeline was freely suspended, Boskalis would continue dredging underneath the pipeline.
Subsequently, in accordance with a method specifically developed and designed for this project by Visser & Smit Hanab’s Technical Department, the live gas pipeline was jacked down in fourteen stages, over a period of four days, to as low as 12 metres under the original level, at the deepest point. During each stage, the piled frames were manned by eight teams of two people each. Everything was co-ordinated from the central command post aboard the survey vessel. The pipeline was lowered in incremental stages, with the loads measured being reported back to the command post, where all of the data was assessed.
Boskalis secured the gas pipeline locally with the previously removed stone layer, and once it was determined that the pipeline was sufficiently safe and stable in the ground, the suspension brackets were removed. The piled frames were also removed, and this unique project was successfully completed.
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