Required research for long distance horizontal directional drilling
The soil investigation for the design of horizontal directional drilling (HDD) normally consists of cone penetration tests (CPT) and vertical borings at regular intervals along the proposed pipeline track. In  case of off shore  conditions, this standard approach would lead to a very costly offshore site survey. For a drilling in the North of the Netherlands below the sea at different approach was carried out. The drilling line with a proposed drilling length of 4.6 kilometres was assessed at the start of the project. The assessment pointed out that the potential drilling risks related to the geological conditions were essential. First, the drilling line was surveyed with seismic profiling and standard soil investigation was done only at the entry and exit points. To determine the characteristics along the possible drilling line, a method was elaborated using HDD test drilling with a combination of site investigation and feasibility testing. The applied test drillings needed minimal adaptation compared to a regular HDD during the pilot phase. The soil investigation method was based on sampling of the return flow during drilling. On the rigs automatic monitoring was done of drilling parameters such as pushing force and torque. The achieved data set was used to deduct geotechnical characteristics of the soil. The measurements of the all the parameters including fluid losses, and indirectly measured drilling risks like borehole instability were evaluated to assess the feasibility of the project.  Recently the pipeline was constructed successfully, an achievement with  record status for HDD.

dr Henk M.G. Kruse, Deltares
Senior geotechnical consult and is working in the field of underground infrastructure. He is a specialist in geo engineering of pipelines especially in the disciplines of trench less installation techniques and the effects of installation activities on the surroundings.