This study is concerned with mechanical damage experienced by steel pipes transporting gas and liquids for energy-related applications.

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1 MECHANICAL DAMAGE (1) Preamble This study is concerned with mechanical damage experienced by steel pipes transporting gas and liquids for energy-related applications. (2) Definition In the context of the study, mechanical damage is defined as follows: Mechanical Damage means Localized damage to the pipe resulting from contact with an object. (3) Glossary Localized means confined to some portion of the cross section or having limited extent along the pipe itself (typically less that five pipe diameters). Damage means any of a variety of changes to the pipe, or a related pipeline component that may degrade or reduce serviceability (ability to function as designed), including but not limited to a. Change of pipe shape (longitudinal and/or circumferential) b. Change in localized pipeline alignment c. Changes of pipe material properties d. Change to the localized distribution of stresses, strains, and/or loading e. The introduction of other anomalies or defects Each of the above is discussed in more detail in Section (4), Damage Features. Additional information, which clarifies the application of the definition of mechanical damage, is described in more detail below in Sections (5), Damage Categories, and (6) Activities, Events, and Objects Associated with Mechanical Damage. Contact results in a load or stress experienced by the pipe, or a related component, that has not normally been considered during the design of the pipeline. Such loads and stresses can be of short or long duration. Contact is usually unintentional, but may be deliberate (e.g., vandalism). Contact may occur during manufacture (excluding pipe-forming activities), transportation, construction or service. (Note that modifications to the pipe due to properlyconducted maintenance activities are excluded, but that damage due to incorrectlyconducted maintenance activities is included.) The scope of this study and the definition above exclude mechanical damage that does not result from contact with an object. Among such excluded forms of noncontact damage are; flattening, ovalization, bending, buckling and wrinkling due to ground settlement, landslip, frost heave, earthquake, land erosion, mud-flow, washout and heavy equipment operating over the buried pipe. 1

2 (4) Damage Features A. Change of Pipe Shape Cross sectional changes could be in the immediate vicinity of the contact loading or could extend along the pipe. Typical cross-sectional changes in shape are denting, ovalization, or flattening, of the normal cross section of the pipe, and wall thinning due to the associated radial and/or axial extension. B. Change in Localized Pipeline Alignment Introduction of a localized change in pipeline alignment due to enforced movement perpendicular to the pipeline route. Onshore pipe movement may result, for example, from hooking and dragging by an excavator bucket. Offshore pipe movement includes movement due to snagging or pulling by, for example, an anchor or fishing gear. C. Change in Material Properties These include changes in yield strength, ductility, hardness, toughness, magnetic properties, metallurgical structure etc. in the pipe steel due to the denting and rerounding. Steel property changes include the effects of cold working and other changes to the microstructure due to the extreme plastic deformation and rapid heating/cooling of the surface layer that may result from sliding contact between the indenter and the pipe during the damage event. D. Change in Localized Distributions of Stress and Strain, and/or Loading These include residual stress and strain fields in the damaged region, as well as additional secondary loadings introduced by the damage (e.g., circumferential flexural stresses that would act in concert with the hoop stresses due to pressure). They also include stress concentrations due to the shape of dents and localized anomalies or defects (see also below). Stresses and strains may be associated with elastic or plastic deformation; they may change during the removal of the indenter, during changes in pipe constraint, or due to shakedown associated with any subsequent load cycling. As was indicated above, stresses and strains associated with large-scale changes in axial forces and bending moments due to natural forces, for example, landslide, thaw settlement, frost heave, changes in overburden or lack of support due to wash out, are not considered within this definition of mechanical damage. E. The Introduction of Other Anomalies or Defects Anomalies and defects include localized shape changes, such as metal movement and removal, thinning, ploughing, gouging, scratching, deformation, or perforation of the pipe or a related component. In the pipe steel, localized shape changes typically result in a stress concentration. Anomalies and defects also include surface and subsurface cracking, and the development of voids or micro-voids. Cracking could result from the damage process itself (e.g., during extreme plastic deformation of the surface layer caused 2

3 by sliding contact between the pipe and the indenter, or as the pipe re-rounds when the indenter is removed) or subsequently as a result of continued operation of the pipeline (e.g. stable crack growth, fatigue, SCC 1 ). Anomalies or defects may extend through the pipe wall. Defects may be comparatively small and circular, for example those caused by an excavator tooth or drilling equipment. Alternatively, defects may extend axially or circumferentially, up to or beyond the extent of the localized damage. (5) Damage Categories Damage has often been categorized according to the predominant features present, for example as follows: Plain (or smooth) dent: Kink: An inward indentation with a simple geometric shape, with no other associated anomalies or defects. Typically the associated indenter only moves radially and not over the surface of the pipe. A kink is a narrow transverse, inward indentation of the pipe diameter with a low radius of curvature. Kinks may also show bulging around their perimeter. Kinks are typically caused by construction equipment. (Note that kinks associated with field bending during construction are not considered in this study.) Dent with secondary features: A dent with associated gouging, scratching or cracking (occurring at the time of the damage event, or developing subsequently by fatigue or stress corrosion cracking), or a dent associated with corrosion, or a dent associated with a girth or seam weld; i.e., combined damage. Constrained dent: Unconstrained dent: Gouge: A dent that is prevented from re-rounding by the continued presence of the indenter (e.g. a rock dent). A dent where the indenter has been removed and rerounding can take place (due to internal pressure or due to removal of surrounding soil or indenter). Metal removal, with little or no associated denting, typically having sharp edges and resembling a deep scratch. Metal removal due to a gouge is often associated with metallurgical damage and residual stress, unlike metal removal due to corrosion. 1 The initiation and growth of cracks due solely to fatigue, stress corrosion cracking (SCC) or stable crack growth (i.e., in the absence of mechanical damage) are not considered in this study 3

4 Combined dent/gouge: A combination of a plain dent with a gouge (also described as a dent with secondary features; see above) Damage has also often been categorized according to its extent and timedependence, as follows: Leak: Rupture: Immediate failure: Delayed failure: penetrates through the pipe wall but does not extend significantly beyond the region of localized deformation (immediate or delayed failure; see below) penetrates through the pipe wall and extends substantially beyond the region of localized damage (immediate or delayed failure; see below) penetrates through the pipe wall at the time of contact or very soon thereafter is initially non-penetrating, but subsequently penetrates through the pipe wall (typically days to years after the damage event) However, as can be seen from the descriptions above, it is often difficult to use these categories in a precise way and they should be considered as descriptive rather than specific. (6) Activities, Events and Objects Associated with Mechanical Damage Mechanical damage may occur during the manufacture, transportation, on-site handling, laying and backfilling of pipelines. Usually such damage is found and remedied prior to completion of construction. However some damage, particularly that due to contact with rocks and buried objects during construction and installation or occurring during settlement of the pipe following installation, may not be detected until the pipeline is subsequently inspected. Such damage is included within the definition of mechanical damage used in this study. Information gathered by the U.S. Department of Transportation (DOT) and others indicates that, in general, the majority of mechanical damage occurs after the pipeline has been constructed, and results from activities in the pipeline right-of-way. Activities and events associated with occurrences of mechanical damage (as defined in the present study) include, but are not limited to: Earth-moving activities including excavation, drilling, horizontal drilling, tunneling, trenching etc. The purpose of the excavation may be for maintenance activities, fencing, drainage, agricultural activity, landscaping or infrastructure engineering (building, road-making, utility installation) Vessel impact, anchor drag, rock dumping, dropped objects, (offshore) 4

5 Exposure to missiles and projectiles: rocks, shrapnel, bullets, floating debris, vehicular impact, sabotage and vandalism, (exposed pipelines and above ground facilities) Heavy equipment crossing the pipeline (contact with exposed or inadequately covered pipeline.) Pipe inspection by ILI vehicles (internal damage, occurring quite rarely) It can be seen from the above that mechanical damage may occur slowly (e.g., contact with rocks and slabs due to ground settlement) or quickly (e.g., impact by excavation or drilling equipment); it may be accidentally or deliberately caused; and it may originate at the outside or, rarely, at the inside of the pipe. It can also be seen that mechanical damage may result from many sources, including the activities of pipeline operators or their sub-contractors, operating conditions, forces of nature or right-of-way loadings that cause contact between the pipe and moving rocks and other debris, other utility operators, landowners, farmers, infrastructure contractors or the general public. Damage due to earth-moving activities is often categorized by source, as: First party damage: Second party damage: Third party damage: caused by employees who work for the operator of the pipeline caused by service providers working under contract to the operator caused by employees of other underground owners and operators and their contractors, as well as unknown third parties such as farmers, general construction contractors, landscapers, other right-of-way property owners/users For clarification, it is pertinent to highlight intentional activities and situations that are not considered to result in mechanical damage so long as they are carried out correctly. These include authorized operations and maintenance activities, such as hot tapping and grinding to remove manufacturing defects or corrosion, etc. 5