Longwall mining operations could compromise the integrity of high pressure pipelines by way of surface subsidence and soil strains. Prior to implementing field programs for monitoring subsidence, a preliminary mitigation/stress analysis study should be designed to determine the possible effects of the longwall mining operations on the pipeline(s). If the stress analysis indicates possible high stresses beyond the allowable limits of a pipeline, then a mitigation plan should be developed and implemented. Regardless of the anticipated stress level in a pipeline, a strain monitoring program is usually recommended. The purpose of this paper is to discuss the design of a pipeline strain monitoring program, which includes the installation of strain gages at critical locations along two adjacent pipelines. The study area includes a 12 inch diameter steel pipeline (for natural gas transport) and a 12 inch HDPE pipeline for water transport. The study area is located in a mountainous region of West Virginia. Prior to the field program, a laboratory pilot study was performed with strain gages on a test section of HDPE pipe to determine the best mounting procedures. The field implementation program included the installation of strain gages on the gas and water pipelines. Multiplexers, data loggers, a solar array and a satellite modem for 24/7 data transfer were installed, and monitored throughout the study. During the field implementation program several meteorological and geologic events occurred which caused some design changes in the field program.
All buried pipes experience loading from the weight of soil overburden. When pipelines cross railroads, roads, parking lots or construction sites, the pipes also experience live surface loading from vehicles on the ground, including heavy construction equipment in some scenarios. The surface loading results in through-wall bending in pipes, which generates both hoop stress and longitudinal stress. Current standards limit the stresses in buried pipes to maximum values in terms of hoop stress, longitudinal stress and combined biaxial stress. An early approach to estimating stresses and deformations in a pipe subjected to surface loads dates back to Spangler’s work in the 1940s. Many models have been developed since then. API RP 1102 provides guidance for the design of pipeline crossings of railroads and highways following the model developed by Cornell University for the Gas Research Institute (GRI). The Cornell model was developed only based on experiments on bored pipes crossing a railroad or a highway at a near-right angle. The live surface loading distribution is also limited to the wheel-layout typical of railroad cars and highway vehicles. Most other existing models only focus on the hoop stress in the pipe. In this paper, a new approach to determine the stresses in buried pipes under surface loading is introduced. The approach is suitable for assessing pipes beneath any type of vehicle or equipment at any relative position and at any angle to the pipe. First, the pressure on the pipe from surface loading is determined through the Boussinesq theory. Second, both hoop stress and longitudinal stress in the pipe are estimated. The hoop stress is estimated through the modified Spangler stress formula proposed by Warman and his co-workers (2006 and 2009). The longitudinal stress, due to local bending and global bending, is estimated by the theory of beam-on-elastic-foundation. The modulus of foundation can be determined through the soil-spring model developed by ASCE. The hoop stress, longitudinal stress and the resulting combined biaxial stress can then be compared against their respective limits from a pertinent standard to assess the integrity of the pipe and determine the proper remediation approach, if necessary. The performance of the proposed approach is compared in this study with the experimental results in the literature and the predictions from API RP 1102.
Pipeline pressure-cycle fatigue analysis is typically performed by analyzing pressure data in the amplitude domain and then calculating incremental fatigue crack growth in accordance with the Paris Law. Alternatively, the stochastic pressure history is converted to an equivalent number of uniform-amplitude cycles using a cumulative damage rule. The fatigue life may then be estimated by integration of the Paris Law. This second approach is computationally less involved and therefore lends itself to a probabilistic analysis because of the large number of iterations necessary with techniques such as Monte Carlo analysis. However, studies have shown that for a broadband stochastic signal, applying linear cumulative damage can introduce large errors. The presence and magnitude of error cannot be easily determined by inspection of the pressure signal. This paper describes the analysis of the pipeline pressure signal in the frequency domain to determine the power spectral density. The result can be used to estimate correction factors to the estimated linear cumulative damage fraction. The corrections may then be applied with a simplified integration of the Paris Law in closed form to improve both accuracy and speed for probabilistic assessment. The computation time for a probabilistic assessment may potentially be reduced by a significant factor.
Pipeline operators commonly use means of temporary crossing such as timber-mat, airbridge, and slab to reduce surface loading induced stresses in a buried pipeline at locations where a heavy vehicle crosses the buried pipeline. When a temporary crossing has a continuous contact with soil, (e.g. timber mat, flexible slab) load distribution over the ground surface is not immediately known. Load distribution under a timber-mat or flexible slab is a function of the slab to soil stiffness ratio. The load distribution tends to become more uniform with increasing timber-mat or slab stiffness. In this work an analytical model using beam-on-elastic-foundation has been developed and Laplace transform has been utilized to find the solution and apply free-end boundary conditions. The analytical solution can be used for any arbitrary load distribution over a beam-on-elastic foundation. In this work the solution for a point load and a partially distributed uniform load were employed as these scenarios can accurately represent conventional vehicle foot-prints, while being computationally efficient. The analytical solutions are compared to finite element analysis to validate the model. This model can be used in conjunction with the Canadian Energy Pipeline Association (CEPA) surface loading calculator (or similar tools) to analyze pipeline encroachment problems when means of temporary crossing is installed. This model can help the operators determine dimensions and bending stiffness of timber-mat or flexible slab to assure a desirable load distribution will be achieved. The model can also be used for structural analysis of a timber-mat or flexible slab under vehicular load.
In this work, the Canadian Energy Pipeline Association (CEPA) equation for prediction of hoop stress in a buried pipeline was validated using the Milestone 1 experimental data. The Milestone 1 testing program included a 24-inch outside diameter (OD), 0.25-inch wall thickness (WT) pipe specimen in sand (24-inch Sand); a 12.75-inch OD, 0.5-inch WT pipe specimen in clay (12-inch Packed Clay); and a 24-inch OD, 0.25-inch WT pipe specimen in clay (24-inch Dumped Clay). Two different depths of cover (DOC) values of 2 and 3 feet were used in the testing and the test specimens were crossed by a variety of construction equipment, namely a dump truck, a bulldozer, a front loader, and a vibratory compactor. The testing was conducted at internal pipe pressures of zero, 550 and 750 psig.
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