Overcoming challenges in horizontal directional drilling – a geotechnical engineer’s perspective

Overview

Horizontal directional drilling (HDD) is a trenchless method commonly used for pipeline and cable installation. It is ideally suited to crossings beneath natural and man-made obstacles and offers substantial cost, safety and environmental benefits. Despite the wide use of HDD, there remain concerns regarding project risk in certain situations. Key to HDD success is good management of the drilling fluid and the avoidance of excessive mud loss. 

Fundamental risks in HDD arise from pressure build-up in-bore leading to hydrofracturing of the ground, heave of the overburden and mud loss – either to formation, subsurface infrastructure or up to the surface. Poor cuttings transport is a main trigger for bore blockage, leading to potential hydrofracture and the manifestation of the hazards associated with these risks.

In this lecture, a case study involving HDD bores for cable landfall of an offshore windfarm is discussed. The aforementioned risks associated with mud loss are most severe in this type of HDD where drilling starts at a clifftop and descends to punchout in the seabed. In-bore pressures before punchout are elevated by the hydrostatic head of the drilling fluid from the clifftop entry pit.

Other geotechnical challenges encountered onsite will be discussed, along with the analysis of rare annular pressure monitoring data which provided valuable insights into the behaviour of cuttings in the bore.

This lecture concludes with some lessons learnt, recommendations to minimise the risks of hydrofracture and bore blockage, and advice to other young engineers interested in this field.