THE EFFECT OF MINING SUBSIDENCE ON THE RELIABILITY OF API 5L X52 GAS PIPELINE AND CIRCUMFERENTIAL SEMI ELLIPTICAL CRACK IN GIRTH WELD USING FINITE ELEMENT METHOD

• Main Author: Rasyid Hanifa, Luthfi
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/77360
• Institution: Institut Teknologi Bandung

Coal is still the largest contributor to the energy mix in the world. The longwall mining is one of the most popular underground coal mining methods in Indonesia. Besides high productivity, this can induce subsidence in large areas. This is supported by the characteristics of overburden rocks in underground mines in Indonesia which are very weak and tend to be prone to deterioration due to ground water. As is well known, most natural resources such as natural gas, oil and water are transmitted through buried pipelines. Steel pipe is currently the main choice in its application because it has a high value of strength to weight ratio so it can reduce material costs. Subsidence can cause bending and deformation of the buried pipeline that lies through the underground coal mining area. As a result, the potential for failure, economic loss, and disaster can be the greatest possible risks. In Kalimantan, Indonesia, in an underground coal mining plan a simulation has been carried out on the possibility of land subsidence in accordance with the underground mining plan. The subsidence occurred right in the area where there is a mining panel. This research will be the first to analyze the effect of data-based coal mining subsidence on the integrity of the API 5L X52 gas pipeline buried along the mining area using analytical methods that are available in the literature and the finite element method using ANSYS software. The stress calculation results obtained are compared with the standard to obtain safety predictions for mining operations and pipe life. From the predictions made using the UK NCB method, there are 10 curved segments with varying subsidence values. An analysis was carried out regarding the effect of mining subsidence on the additional axial stress distribution. The interaction between the pipe and the soil is ignored considering that the subsidence value is too small when compared to the pipe length. In addition, the factor of safety of the allowable stress is analyzed using the von Mises equivalent stress parameter. The presence of girth welds is also considered considering that the area is considered as the weakest link due to the longitudinal direction of tensile stress caused by the movement of the ground due to seismic activity. Simulations were also carried out with the possibility of the presence of semi-elliptical crack with variation in the location and dimension. Variations in the location of cracks are carried out in the weld middle and weld toe areas as well as the types of internal and external cracks. Dimensional variations were carried out with an aspect ratio (a/c) of 0.2; 0.5; 1 and relative crack depth (a/t) of 0.2; 0.5; 0.8. Both the simulation and calculation results show the highest von Mises stress at point x=1834 m in segment 9. Even so, this value is still lower when compared to the safety factor at Class 3 locations according to pipe specifications. Assuming a constant corrosion rate and no further subsidence, the pipe can be used safely for the next 30 years. The existence of a girth weld has the potential to increase the von Mises equivalent stress in the presence of stress concentrations due to geometric discontinuity and inhomogeneous mechanical properties. The presence of crack increases the risk of pipe failure. The stress intensity factor (SIF) value for crack located in the weld toe section is higher than in the weld middle. Crack located on the internal parts also tend to be more dangerous due to the influence of the Hoop tension. In general, semi elliptical crack with a lower aspect ratio have higher SIF values. In addition, the deeper the crack as indicated by the increase in the relative crack depth also increases the SIF value. With the given assumptions , the results of the FEM simulation show good agreements with the results of analytical calculations based on the literature for both stress evaluation and stress intensity factor. Therefore, the method used in this study is feasible to be used practically to evaluate the mechanical behavior of buried pipeline when the mining subsidence phenomenon occurs.