When installing and operating transmission pipelines in mountain regions, one has to account for specific risks in order to mitigate potential lost time, increased costs, and accidents with human and economic costs.

Climate is an important consideration during both pipeline construction and operation in mountain areas. Some seasons can be harsh, with heavy snowfall or rain, and extreme temperatures and winds, restricting the access to the right-of-way (RoW) during construction. In addition, weather patterns are usually rapidly changing – quick temperature changes and flash rains – which can delay the construction of the pipelines. During pipeline operation, pipelines in permafrost regions face stability issues.

Geography is another significant risk factor with two sub-categories – topography and geology. Mountain areas can have a challenging topography such as steep slopes, river and lake crossings. Geology can also raise issues during both construction and operation, with companies faced with hard rock, wet or frozen ground conditions, earthquake and fault zones, erosion and landslides, karst and sinkholes.

Mountains often include environmentally sensitive and protected areas, such as national parks. Pipeline projects have to be designed, installed and operated with a minimum footprint on flora and fauna, usually translated in reduced RoW and temporary workspace. Minimising any risk of accidental harmful discharge or contamination is also important.

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Supplying some of the required materials such as sand for padding, and safely disposing of surplus materials can be difficult during the construction phase of a mountain pipeline.

Finally, building and operating pipelines in mountain regions can be dangerous for the people involved. Clear safety standards and operating procedures have to be in place to avoid accidents.

In order to address the above-mentioned risk factors, the pipeline industry is dedicating a lot of effort to prepare and standardise the construction and operation of pipelines in mountain regions.

The International Pipeline and Offshore Contractors Association (IPLOCA) presents ten different pipeline construction environments in its recently released recommended construction practices for onshore pipelines, Onshore Pipelines: The Road to Success. Three of these environments directly describe mountain areas – the side slope, ridge and rock RoW scenarios. Another two refer to arctic conditions and the environmentally sensitive area often encountered in mountain terrain. In all the scenarios, one of the most efficient ways of mitigating the pipeline construction and operating risks is to protect the steel pipe against mechanical damage from impact and penetration.

The need for supplementary mechanical protection

Mechanical damage to the pipe can occur during all phases of the pipeline construction and operation, for example during transportation, handling (loading in and out), storage, lowering-in, backfilling, and during pipeline’s service life. Impacts and penetration damage can be caused by many factors:

* Other pipes or pipe handling equipment; * Lowering-in; and, * Rocks in the trench bottom or impact from the backfill material.

Steel pipe is impact resistant by itself and some of the external coatings applied on steel increase this basic mechanical protection. However, in order to ensure an incident-free service life for the pipeline, the steel pipe and the anti-corrosion coating have to be intact during construction and operation. This cannot be guaranteed by the basic mechanical protection of the steel and anti-corrosion coatings. Therefore, the industry has developed supplementary mechanical protection systems that are aimed at reducing or eliminating the risk of mechanical damage.

As the industry uses a wide range of supplementary mechanical protection systems, this article will focus on the systems that protect the entire diameter and length of the pipe, and which are the most efficient in protecting the pipe and its coating against impact and penetration. Today, most pipeline projects use the following supplementary mechanical protection system: concrete coatings, sand padding and select backfill (mechanical padding), as well as non-woven geotextiles and rockshield materials.

Mechanical protection systems

Concrete coatings for mechanical protection have been developed during the last 25–30 years in North America, Australia and Europe, and are usually applied in specialised coating facilities.

Steel wire mesh reinforced concrete coatings that are usually 20–25 mm thick and fibre-reinforced concrete coatings (8–10 mm thick) are the two types of mechanical protection concrete coatings. Wire-mesh concrete coatings are applied using a side-wrap process, while the fibre-reinforced concrete coatings are applied using a spraying process, without any damage to the pipe and the pipe coating during application.

The concrete coatings offer excellent damage resistance – minimum impact resistance of 150 J for the fibre-reinforced concrete coatings and 450 J for the wire-reinforced concrete coatings. The mechanical protection concrete coatings are fully bendable according to the industry standards; do not need additional equipment or manpower for installation; and, do not have any usage limitations in terms of terrain configuration, ground conditions or climate. Concrete coatings are currently the only supplementary mechanical protection systems that protect steel pipe through all the construction and service life phases – from transportation, handling and storage to lowering-in, backfilling and long-term service life.

Sand bedding and padding is still the most frequently used supplementary mechanical protection system. Sand is usually supplied to the RoW, where it is used in the trench to protect the pipe against impact and penetration from rocky outcrops in the trench bottom or impacts from rocks in the excavated trench material. The sand layer usually has a thickness of 20–30 cm around the pipe and has a minimum impact resistance of 300–450 J. Sand padding needs additional equipment such as sand trucks, padding machines, temporary work and storage space at the RoW, as well as additional manpower and materials. Additional costs are usually incurred for the transportation and disposal of the original trench material that becomes surplus material after the use of sand. This system protects the pipe during the pipeline backfilling and operation phases, and industry experience shows that potential sand washouts can reduce long-term protection.

Sand padding also has limitations in terms of terrain configurations, such as steep slopes, climate, and wet or frozen sand. More recently, select backfilling (or mechanical padding) has been used as a sand padding technique. This requires special equipment that can screen the material excavated from the trench and install the finer grades around the pipe, while using the coarser material for closing up the trench. This system has terrain configuration, soil type (clay, silt) and climate limitations that are similar to sand padding, but has the advantage of re-using the trench material and thus avoiding most surplus material disposal costs.

Non-woven geotextile materials are polypropylene fibre-based rolls, and rockshield materials are polyethylene or PVC-based solid sheets or open-cell rolls that are installed around the steel pipe in the field, usually before the lowering-in phase. These materials are available in different styles and thicknesses, with the usual thickness per layer at 4–14 mm for non-woven geotextiles and 6–11 mm for rockshield materials.

These materials also have a wide range of technical performance. As an example, their minimum impact resistance is in the 25–35 J range. These materials protect the pipe during lowering-in, backfilling and the pipeline service life. The installation of these materials is very slow, taking approximately 15 minutes for a team of three people to protect just one pipe joint, and the quality of the protection is highly dependent on the skills of the field installation team.

The impact resistance of these materials is limited. When rocks more than 10 cm in size are present in the backfill material, other protection systems such as sand padding have to be added, which further increases the total protection cost. Industry sources, such as a recent Interstate Natural Gas Association of America (INGAA) report entitled Emerging Bedding, Padding, and Related Pipeline Construction Practices, also mention concerns regarding the negative impacts of some of these materials – the solid sheet type – on the active anti-corrosion protection of the pipeline.

Conclusion

Building and operating pipelines in mountain areas is a challenging endeavor that can be facilitated by properly protecting the steel pipe and its anti-corrosion coating with supplementary mechanical protection systems.

The optimum supplementary mechanical protection system will be selected based on a well defined series of technical performance, design and constructability, environmental impact and economical criteria. The pipeline industry is using its field experience to improve the existing systems and to develop new ones. The above-mentioned INGAA study has identified two main areas as the focus for future innovations in mechanical protection systems – coatings that are resistant to damage, and new crushing/padding equipment.