Fracture mechanics can be simply described as the study of the resistance of a material to fracture. The concept was first studied by C.E. Inglis in 1913 and the study was extended in the 1920’s by Griffith A. A., an English aeronautical engineer. It was only in the 1950’s that Griffith’s work on fracture mechanics gained attention and since has proved to be of excellent help in understanding the failure of material due to the formation and propagation of cracks. The study of fracture mechanics can help optimize safety and cost factors for any pipeline project by choosing the right materials that will ensure minimal environmental impact and financial loss especially for gas pipelines that pass through wetlands and estuaries.
Fracture Factor for Trenchless Methods
Trenchless construction methods such as horizontal directional drilling (HDD), horizontal auger boring (HAB), pipe jacking and microtunneling subject the pipeline to tremendous amounts of strain and stress either during pulling or pushing of pipe. This can lead to fracture of the pipe if the pipes do not meet the standard requirement for the amount of stress it will be subjected to; causing considerable loss of time and money. Sub-surface pipes are also subject to dead or live surface loads. The elasticity of a pipe material plays an important role in determining whether a pipe will fail under such loads. Laboratory tests on materials can help determine which materials are best suited for specific soil and load conditions that the pipe will be subject to after commissioning.
Mechanics of Fracture
There are three factors that have significant influence on the type of fracture namely, temperature, rate of strain and the state of stress. For example; a material that is brittle at low temperature may become ductile at high temperature, while a ductile material may behave as a brittle material when the strain rate is slightly increased. The temperature at which this transition occurs is known as the ductile brittle transition temperature. Fracture can be broadly classified into ductile and brittle fracture.
Fracture in Ductile and Brittle Materials
In ductile materials plastic deformation occurs at the crack tip due to stress and energy is consumed as a result, leading to the blunting of the crack tip. This energy consumption means that less energy is available for the growth of the crack. Ductile materials are therefore more resistant to the propagation of cracks. In brittle materials the crack tip carries the maximum stress unlike ductile materials where the maximum stress lies a little ahead of the crack tip. This stress at the crack tip enables opening of the crack when the bonds are broken at the tip and thus helps propagate the crack.
Fracture Mechanics and Trenchless Installation
When it comes to transporting gas the most economical method is transportation through pipelines. Read Why the Oil and Gas Pipeline Industries are Eyeing HDD.) However; the distances involved in delivering gas from the source to the customer is considerable. Usually these pipes are made of steel and it is possible that these pipes contain some defects inherited either during the manufacturing process, by micro-voids present in the material, inclusion, during transportation to the site or by overloading. These points of defects can initiate cracks that may grow either nominally or to such an extent as to cause fracture of the pipe. A fractured gas pipeline can be hazardous to the environment and may herald heavy financial losses.
Causes of Failure Due to Fracture
Failure of gas pipelines can be just leaks or complete rupture. While most of these failures can be blamed on stress corrosion cracking or corrosion, other causes can be defects in welding, movement of soil, improper bedding, earthquakes, ground slips etc. Sometimes heavy machinery such as excavators operating in an area can also cause damage to underground pipelines. About 90 percent of pipeline service failures can be attributed to ruptures that initiate from sites where there is a concentration of stress forces or due to fatigue cracks. Fracture strength will also be weakened when there is a change in the pipe cross section such as the presence of a notch. (Read Methods for Preventing Corrosion in Infrastructure.)
Choosing the Right Pipe Material
Microtunneling, HDD, HAB and other trenchless methods are used for the purpose of laying pipelines whether for gas, water or sewer, especially when they are to pass through environmentally sensitive locations such as under rivers, palustrine and lacustrine wetlands, railway embankments, busy roads etc. These methods are excellent because they cause little to no disturbance to surface features or moving traffic as the case may be. However these pipelines will be subject to top loads from the soil and water over them or from moving loads such as traffic. While designing pipeline routes for installing pipelines all these factors including soil type have to be taken into consideration. Fracture mechanics can help designers to determine the right kind of pipe material for a particular soil type and expected loading.
Trenchless Rehabilitation for Fractured Pipes
Fractured pipes can also be repaired or replaced using trenchless rehabilitation techniques such as cured-in-place pipe, sliplining, pipe bursting, mechanical spot repair etc. Using non-destructive testing methods such as trenchless pipeline inspection with closed circuit television (CCTV) cameras and robotic inspection, fractured pipes can be detected and repaired. These trenchless methods are cost effective as it eliminates the need to dig; repair or replacement can be carried out from manholes.
Testing of Pipe Material for Trenchless Installation
The American Society of Testing and Materials (ASTM) has many guidelines laid down for the purpose of carrying out physical and mechanical tests including proper procedures for testing materials. This testing guide for materials enables designers and planners to select the best pipe material suitable for a particular project also taking into consideration what the pipe will transport. For example; ASTM E3039-16 provides a standard test method for determining the crack tip opening angle of steel pipes; ASTM C506-16b provides standard specification for reinforced concrete arch culvert, storm drain and sewer pipe; ASTM F1743-17 provides standard practice for rehabilitation of existing pipelines and conduits by pulled in place installation of cured-in place thermosetting resin pipe.
Safe Working Practices for Trenchless Installation
The Occupational Safety and Health Administration (OSHA) sets down many safety guidelines for undertaking pipeline installation using trenchless installation methods such as HDD. There is always a chance of hitting existing utility lines while sub surface drilling is in progress. Using the states utility location system and checking municipal records during the planning stages can help prevent related accidents. For ensuring workers safety and protecting environmentally sensitive areas, OSHA provides guidelines that can help a contractor to complete a project cost effectively and within the stipulated time period.