Although it is being sporadically employed, trenchless technology should be used more widely as a solution to land acquisition issues and pilferage. With lesser above-ground destruction and shorter time consumption, there are several pluses in using trenchless for developing O&G pipeline networks, says Niranjan Swarup, who proposes that trenchless technology can be expanded to longer installation stretches.
In addition to crossing difficult terrains, trenchless technologies can also provide a major relief to proÂject owners on account of reduced land acquisition for construction of oil and gas (O&G) pipelines. As the technology utilises trenchless construction equipment, construction durations can be reduced with proper proÂject planning and execution. Current project execution approach is to use trenchless only for crossings, but shoÂuld this approach change and trenchless be applied at more locations with longer trenchless installation lenÂgths, those pluses could be harnessed for better projects. Next stages in the lifecycle of any pipeline project are the maintenance and replacement phases of old pipeÂlines. These works can also be executed by trenchless methods and such works can again help in limiting open cut works.
For better sensitisation about trenchless technology application in O&G pipelines, it is important to undÂerstand its various dimensions. These include scope of trenchless technology in laying and managing O&G pipeline, benefits of technology through its state-of-the-art properties that are helpful in simplification of the overall working, the relative advantages of this techÂnÂoÂlogy vis-Ã -vis the conventional practices of laying O&G pipelines in India, and avoidance of open cut trenches through its basic concept of 'no dig' that can help in laying the pipeline without digging the fertile land thus avoiding major land acquisition exercise.
The article attempts to provide such sensitisation to the reader.
The trenchless technology sector has been undergoing continual refinement and development. Improvements cover larger and smaller diameters, longer drives, greater accuracy, faster and curved driving, different soil conÂditions and the ability to work deeper into water tables. Records for new installations in terms of size, distance and routing (direction) are being broken all the time.
New installation applications: TrenÂchless techniÂques are generally categorised into two main groups, steÂered and non-steered. The difference between them is in their respective capabilities. The former techniques can develop tailored cavities with pre-defined grade and aliÂgnment, but the latter can just create a cavity without any control over grade and alignment. Non-steered techÂniques include moling where a pneumatically pulsating hammer pierces the ground to create a cavity or pipe raÂmming where a steel pipe is hammered into ground or auger boring where the cavity is created by the rotÂating actions of an auger train. Steered techniques incÂlude Horizontal Directional Drilling (HDD) or Guided Boring where the cavity is created by the cutting actions of a rotating drill head or, Microtunnelling & Pipe-jackÂing where the cavity is created by the cutting actiÂons of a cutter head. In all these cases the cutting actions are conÂtrolled remotely by operators from the ground surface.
HDD is used throughout the world, and the maxiÂmum application for this technique, particularly in India, is in the telecom sector. HDD is generally a two- or three-stage operation, initially to create a pilot bore, which is subsequently enlarged by reaming to the desiÂred size. The drill is usually surface launched and fluid lubricated. Machines range from compact, mobile rigs for small bores in confined spaces to very large units with large diameters and drives of more than 2 km. HDD is fully steerable, but until recently, the degree of accuracy has not been suitable for gravity pipeline installations. The technique is now being improved and many succÂeÂssful on-line and on-grade installations have been comÂpleted in India in sewerage works. The technique is widÂely used for water supply and cable ducts, as well as for gas and oil pipelines.
There are many variants of Pipe-Jacking, in which the product pipe is forced into the ground by jacks that are mounted horizontally in a launch shaft. The run is completed when the pipe string reaches an exit shaft. Both shafts are often used as service access points later. The ever-increasing length of runs and fewer access points is reducing project costs, making this technique increasingly popular.
The equipment used for Pipe-Jacking is sometimes termed Tunnel Boring Machine (TBM). TBMs can be categorised as:
- Auger TBM, in which the spoil is removed by an auger through the incoming pipe.
- Slurry TBM, in which the spoil and ground water are removed by pumping as a slurry.
Microtunnelling is a more advanced form of Pipe-Jacking, and uses a separate miniature TBM and is conÂtrolled from the surface. Specially designed pipes are jacked in behind the machine, which uses the leading pipe face to push forward as it cuts.
Initially used for large gravity sewers of 500 mm diameter and upwards in Japan where a high degree of accuracy was required, the method has been further developed for the installation of PVC ducting down to 150 mm diameter. Another recent development in this area has made it possible for curved driving when using Microtunnelling.
Impact Moling is primarily used for short drive croÂssings under roads for cable ducts and small diameter service pipes. It has the advantage of mobility and quick set-up time and is often used where there is no reqÂuirement for a high degree of accuracy.
Replacement applications: ReplaceÂment of defective or overloaded pipelines has been identified as an urgent need, particularly now that so much more is known about the condition of earlier installations. In congested areas, the existing defective pipeline route may be an “asset” which can be enlarged by a replacement pipeline. Here again, considerable progress has been made in terms of the degree of upsizing, dealing with the type of construction of the existing line, difficult ground conditions and the improved durability of the newly installed line.
Replacement systems are frequently grouped under Pipe-Bursting, although there are many variations and terms such as Pipe ing, Pipe Splitting and Pipe Eating. In pipe replacement, the defective pipeline is burst, generally by brittle fracture, using either a pnÂeumatic or hydraulic mole, and the fragments are forced into the surrounding ground or removed through the new pipeline that is pulled in behind the mole.
Pipe-bursting is usually used in soft ground conditions and is often not suitable for gravel or rock. It has been widely used in the gas industry to replace older cast iron mains that lend themselves to brittle fracture. More recently, Pipe-bursting has been used on defective and overloaded sewers, where the ability to increase the size of the new pipe is an advantage.
Rehabilitation applications: Perhaps the largest share of the trenchless market is represented by the reqÂuirement to rehabilitate defective pipelines with some residual structural and physical life, which can be used as a structure for the new line. Examples of rehaÂbilitation techniques include Cured-in-Place Lining (CIPP), Close-Fit Lining, Slip-Lining, and Spray Lining, all with their own patented variations, as well as various other localised repair techniques. Variations relate to the matÂerial used, wall thickness provided to offset structural or physical defects, the rate of rehÂabilitation, and the miniÂmum time of shut down for the existing service.
The rehabilitation of small-diameter underground pipes is a new area where the cost competitiveness of trenchless technologies is well recognised. Many utility pipelines, sewage in particular, become defective due to the corrosiveness of modern effluents. They also suffer from overloading and loss of capacity. One of the advÂantages of rehabilitation is that the new lining materials have a much lower surface friction coefficient, thus it is often possible to increase the capacity of the refurbished pipe without increasing its diameter.
In CIPP, a fabric impregnated with polyester or epoxy resin is inserted into the defective pipe and inflÂated to fit against the pipe wall. It is then cured by hot water, steam or ultraviolet light. The system has many variants and can be designed to provide different wall thicknesses to meet specific needs. An advantage is that the lining adjusts to variations in the size of the pipe. It is widely used for the rehabilitation of gravity sewers, including laterals, and usually results in no loss of capacity.
Spirally wound liners are a form of close fit in which a PVC strip is fed through a small access into the defÂective pipe. The PVC strip is then helically wound into place against the pipe wall using a winding machine operated from within the pipe. This technique is parÂticularly useful for emergency repairs and for adding strÂength to pipelines that have been weakened.
Slip-lining involves putting a pipe within a pipe and grouting the resulting annulus between the new lining and the old pipe. This causes a reduction in capacity and the process has now been modified using polyethylene to reduce the thickness of the liner and to minimise the size of the annulus. Spray linings using cement or resin are widely used on water pipelines.
Spray lining materials have to be used carefully and approved by regulatory authorities due to the potential for release of solvents and residues. Spray linings are suitÂable for dealing with leaks but not where there are structural defects.
Can trenchless technology save costs?
New installation trenchless techniques are used for developing subsurface networks. These are one of the most visible set of trenchless activities on account of equÂipment presence over ground. These techniques are appÂlied through entry and exit pits or shafts and require land areas equivalent to the equipment and material haÂndling needs. Beyond these spaces, no further land is generally needed and therefore no further land acquiÂsition is required.
The required land can be of two types – one to house permanent structures like manholes or valve chambers and others needed for construction and installation actiÂvities for a limited time. The second type of land cateÂgory requires temporary land possession and the only encumbrance is the cost of crops/shifting of struÂctures, if present. As highlighted earlier, a major appÂlication of these techniques in O&G pipeline proÂjects is mostly for installing pipelines on natural and man-made crossings. All other tracts where open cut working is possible, trenchless is generally not adopted. The priÂmary cause for this practice is the notion that trenchless is more expensive as compared to conventional construÂction techniques.
The basic point being made out here is that cost evaluation is only done for the direct costs only. Direct costs of land acquisition are covered in the analysis but costs of delays on account of getting the land possession, legal issues and indirect costs for land acquisition are seldom accounted. In addition, the possible savings due to reduction of project durations are also not evaluated. However when the final tally is drawn all these are spent on the project. A project owner must, therefore conduct a comparative economic evaluation of different scenario (usage of different lengths through trenchless application) and identify the most optimum solution.
In general, in comparison to the use of open cut metÂhods, trenchless projects are characterised by miniÂmal surface disruption over a much shorter period of time. For a well-managed trenchless project, the public may not be aware that major construction work is actÂually going on underneath. There are unlikely to be any resÂtrictions on the general public during working hours and less social disruption. Thus, trenchless projects are less hazardous and are generally viewed as being more envÂironmentally sound. Interestingly trenchless techÂnoÂlogy was initiated in the nation by O&G sector pipelines when a GAIL pipeline had to cross the Chambal river and HDD technique was applied for the first time. Over 17 years, several developments have taken place and many different trenchless techniques are being applied in various sectors like telecom, which represents one of the major success stories for HDD, urban sanitation where many new installation techniques and rehaÂbiÂlitation techniques are currently being applied succÂessÂfully, and likewise other utility sectors.
Trenchless applications for oil and gas sector, howÂever, have remained at the levels of installing pipelines at crossing locations like rivers, canals, railway tracks, or difficult tracts, with the remaining tracts being developed through conventional construction techÂniques. This approach necessitates acquisition of large tracts of land on the pipeline route making the project challenging on that front. In addition to new installation activities, rehÂabilitation and replacement of buried pipelines can also be done through trenchless technology so that large scale excavations for exposing/extracting the old pipeÂlines could be avoided. In addition to avoiding open cut works, trenchless also helps in faster project completion faster. With an intelligent combination of open cut and trenchless, project cost optimisation can be achieved by selecting different lengths of trenchless sections, diffÂerent trenchless and conventional techÂniÂques and evalÂuating the direct and indirect costs on such combinations. The optimum result thus evaluated can be selected.
The author is Executive Director, Indian Society for Trenchless Technology, New Delhi.