Conventional offshore pipeline installation methods have become more difficult because of the growing demand for bigger diameter pipelines with longer installation runs, deeper water depths and greater bottom-hole pressures. Therefore, there is a need to develop unconventional methods in order to allow its use in deeper waters.
The HDD method was developed by the geotechnical engineers for installation of the pipeline in place of conventional methods by using an open-trench excavation method.
A number of research studies have shown that it is more environmentally-friendly compared to conventional open-trench methods. This is due to the fact that HDD method can be designed in a way which limits environmental impact during pipeline installation process by using specialised drilling fluids and minimising damage to surface.
Horizontal directional drilling is also gradually being used for installation of offshore pipelines. Researchers have identified that the environmental risks associated with HDD pipeline installation are low compared to other open-trench methods. It has been also noted that soil conditions can affect the outcome of this type of installation technique and site specific evaluation needs to be carried out before design of HDD method is finalised.
What are the Advantages of the HDD Method?
The horizontal directional drilling method allows for installation of pipelines in environmentally sensitive areas where the soil conditions are unstable. This method requires that the soil conditions are stable enough to support the pipeline wall against pressure by means of bearing on the bottom of the trench.
It also makes it possible to install pipelines under bodies of water or marshland, where traditional installation methods would be impossible because long conventional open-trench crossings require that the trench needs to be empty before excavation. With this conventional method, crossings cannot be dug through water or marshland because the trench will immediately fill up with water and construction of crossing will turn into a major engineering project.
HDD allows for installation of pipelines in environmentally sensitive areas where the soil conditions could not support heavy machinery involved in conventional methods.
Moreover, this method is the most promising technology for installing pipelines in deep, soft soils especially when there are environmental concerns. However, it may become more challenging with increasing depth and longer crossing because of the complicated geotechnical engineering problems.
HDD design has to be carried out by taking into account various factors affecting pipe stability during installation. This includes soil properties, fluid pressure in the trench, potential caving or burying of the pipe
A typical HDD installation is done by excavating a trench in the seabed, laying the pipe inside the trench and filling up the hole with backfill material. The design criteria for horizontal directional drilling method include proper site evaluation, covering soil conditions, depth of water, current flow, selection of pipe and backfill materials, and pipe installation.
What About Risks?
The main issues that need to be considered in HDD design include: soil properties, pipe sticking during installation, caving or burying of the trench walls, leakage from broken pipes and fluid pressure inside the trench. The geotechnical engineering challenges include accurate evaluation of soil strata stability such as sedimentary sequence, water content, geotechnical investigation to determine soil strata properties which are contained in the final design.
During drilling of the hole into the seabed, piping can get stuck if they are not rigid enough or large enough. Also when the pipe is installed in partially fluid-saturated ground it may collapse due to liquefaction, which means that the material loses its strength and behaves like a fluid. This can be prevented by adopting appropriate excavation and grouting procedures which ensure that the soil is sound enough to support pipe walls without problem.
Broken or damaged pipelines during HDD installation result not only in damage of pipes but also can contaminate ground water with oil and other chemicals, which can harm the environment. This risk can be minimised by adopting appropriate construction procedures which ensure that the pipe is properly supported during drilling and grouting phases.
As fluids inside HDD pipes are under pressure it may cause leakage into surrounding soils. If there are any defects in the welds or other structural parts of tubing this may result in breakage. This can be prevented by adhering to the construction specification and obeying the pressure limitations.
The main purpose of HDD is to reduce environmental impact during pipeline installation process. However, in some cases HDD may damage the environment due to unknown soil conditions which are not discovered until excavation starts. If there are any hazardous materials hidden under the ground there is high risk of contamination.
If the installation process goes wrong or if drilling fluid enters into ground water then this will contaminate the surrounding ecosystem due to oil spillage. If the pipe fails during operation then leakage may occur which can be hazardous for aquatic life and other forms of marine life. It has been recommended that HDD methods be used when conventional open-trench methods are not feasible.