Horizontal directional drilling (HDD) is a fantastic method for pipeline installation: it’s trenchless and non-invasive, with minimal impacts on the surrounding environment and community. One common question is, how accurate can a typical HDD pipeline installation be? In this article, we take a look at the levels of accuracy in HDD and what factors can impact upon HDD accuracy.
What determines the accuracy of HDD?
The accuracy of HDD is determined by several factors. Selecting the right tools for the job is the most critical factor in ensuring accuracy from the beginning. Knowing the composition of the ground, as well as the location of any existing buried assets, enables you to choose the right steering method and the right drill for the conditions. Working with reputable professionals prior to selecting your tools is the best way to ensure accurate drilling.
A recent job completed by AHD trenchless. The target marker post was knocked to the side by the drill head, demonstrating pin-point accuracy. The job was 260m long and 18m deep, through very hard rock.
Magnetic vs gyro steering
Aside from the basic walk-over system where a technician literally walks above the tool tracking its position, there are two primary methods of remote steering (often referred to as wireline steering): magnetic and gyro. Magnetic steering is a common choice that uses an inline tool to communicate with a remote surface tracking software system to verify the precise location of the tool while underground, improving both accuracy and worker safety.
Magnetic steering uses either direct current (DC) or alternating current (AC) tracking. The DC method is the more accurate, as it uses a wire looped in a rectangular shape which starts and ends at the power source to create a magnetic field on each side of the loop. The AC method is similar but uses only one side of the wire instead of both.
Gyro steering, on the other hand, transmits its data through an optical fibre cable direct to the operator. The Gyro relays navigation information from the drill head to the operator in real-time as it continually tracks movements in pitch, yaw, and roll and measures this against the actual distance traveled. It also requires less surface equipment to operate than magnetic systems.
While both steering methods work in most cases, it is important to do your research to determine the best method for your project.
Magnetic steering is not suitable for projects where induced magnetic fields may be present at any stage of the drilling process, such as railways, as it may impact the accuracy of steering. On the other hand, gyro steering is unaffected by magnetic interference. However, the lack of external sensors with gyro steering can make it difficult to physically confirm the actual location of the drill, so an additional GPS check is used during course corrections.
Proven levels of accuracy
One of the reasons HDD is such a great method of installation is because of how accurate it is. Once a course is plotted by a guidance engineer, the pilot hole is drilled tracking the drill in real time. HDD accuracy has proven accuracy of between 0.01 to 0.04 degrees of the target. This level of precision and accuracy makes HDD a great option for all types of trenchless pipeline installation.
In addition, in our experience, we are finding that on many projects, the use of HDD is also improving the time frame in which project owners are able to obtain permits to commence work. When drilling under heritage zones, sensitive habitats, and in close proximity to buried critical assets like energy pipelines, HDD eliminates risk and speeds up the approvals process.
