HDD Crossing Design Considerations and Requirements
HDD crossings are considered critical components of every pipeline project. A variety of design requirements and factors for a successful HDD crossing are studied and placed into perspective to ensure that the crossings are completed on time and without issues.
To properly construct the crossing and assure its success, a detailed examination of underground characteristics such as rock type, soil type, and their placement relative to the proposed drill crossing is required.
Some of the resources utilized to investigate the prospective crossing include aerial images, geological maps, topographical maps, past job histories (bridges, road cuts, and other crossings), hydrology, and geological analysis. Vertical exploration holes are typically drilled to obtain core samples, which are used to determine the depth of various subsurface conditions.
A cross-sectional profile and plan view for the anticipated crossing are prepared with a thorough geotechnical investigation report in hand. The next stage is to pick a drill path that is suitable with the surface layout, including entrance, exit, and pipe laydown locations.
The line pipe required for an hdd crossing has a thicker wall than the main line in most cases. The reason for this is that during the pulling/installation of the pipeline into the crossing, forces and stresses are applied to the pipe.
The diameter-to-thickness ratio should not exceed 50 while designing the pipeline. The drawbacks of having a pipeline that is too thin include a higher chance of pipe collapse and the cost of removing and replacing the existing pipe.
In addition to examining the diameter-to-thickness ratio, a stress study should be conducted to determine the forces acting on the pipeline and to ensure that the right grade of material is used.
The pipeline, including any parts that will be utilized for an hdd crossing, is treated with an anti-corrosion coating. To create a corrosion protection system for steel line pipe, anti-corrosion coating is employed in conjunction with cathodic protection.
The anti-corrosion coating has been engineered to have some flexibility, impact resistance, moisture permeation resistance, good adherence to the steel substrate, and compatibility with the cathodic protection system.
Anti-abrasion coatings have been developed to protect the anti-corrosion coating from mechanical harm. In addition to gouge and wear resistance, strong adhesion between the anti-corrosion and anti-abrasion coatings is critical.
The fact that the joint coating is applied in the field exposes it to additional problems that are not present in a plant application like degree of surface preparation/cleanliness, climate, and the level of quality control required to ensure correct application.
The joint area is often protected by liquid-applied, spray-applied, and heat-shrinkable sleeves. To safeguard the underlying primary sleeve in the latter instance, a sacrificial sleeve can be included in the design.
The process of drilling the crossing requires the following;
Develop a detailed execution plan.
The development of a precise execution plan is the first stage before drilling the crossing. Pilot hole, reaming, buoyancy control, and pullback methodology should all be included in the plan. It’s a good idea to make contingency plans in case of unforeseen events. The execution plan should also include preparations for emergency response, environmental response, and safety.
Make sure the equipment is appropriate for the job.
Check the drilling contractor’s records for equipment maintenance as well as a personnel experience, job histories, and training records.
Following the pullback step, evaluate coating integrity (pipe is installed in the crossing).
Pulling a couple of sacrificial joints through the bore prior to a pullback is one of the most typical procedures for determining the nature of the drill bore.
This way, any damage to the sacrificial joints may be assessed, and remedial measures, such as extra reaming and/or cleaning passes, can be taken. Pullback can begin once the drill bore has been found to be satisfactory.
The most crucial aspects for the crossing’s success are the geotechnical study and engineering design of the drilling and pipeline installation. Also, the anti-abrasion coating should be chosen carefully since it helps to safeguard the pipeline’s anti-corrosion coating. The lack thereof may result in a “failed” crossing. Thus, before the project begins, these factors and their potential impact on the crossing should be investigated.