Modeling of wave propagation in drill strings using acoustic transfer matrix method
In order to understand critical vibrations of a drill bit such as stick-slip and bit-bounce and their wave propagation characteristics through a drillstring system, it is critical to model the torsional, longitudinal, and flexural waves. The objective is to model these waves propagating through the drillstring in a computationally efficient way. Here, a modeling method based on an acoustic transfer matrix between two sets of wave variables at the ends of a cylindrical pipe is proposed. For a drillstring system with multiple pipe sections, the total acoustic transfer matrix is calculated by multiplying all individual matrices of which each is obtained for an individual pipe section. Since drillstring systems are typically extremely long, conventional numerical analysis methods such as FEM require a large number of meshes, which makes it difficult to analyze these drillstring systems. On the contrary, the analytical acoustic transfer matrix method requires significantly low computational costs. For the validation, experimental and numerical data are obtained from a laboratory measurement and by using a commercial FEM package, ANSYS, respectively. They are compared to the modeling results obtained by using the proposed method. It is shown that the modeling results are well matched with the experimental and numerical results.
The impact of vibration mechanism’ installation place on the process of retrieving stuck drill pipe
The authors express their sincere gratitude to Dr. Ja. Kuntsyak (Joint Stock Company “Scientific Design Bureau for Testing of Drilling Tools”, Kyiv, Ukraine) and Prof. V. Vekeryk (Ivano-Frankivsk National Technical University of Oil and Gas, Ukraine) for consultations on technical issues of borehole drilling. 
Scientific advice was obtained from The National Academy of Sciences of Ukraine, Academic Society of Michal Baludyansky (Bratislava, Slovak Republic), National Lviv Polytechnic University (Lviv, Ukraine). 
We greatly appreciate Prof. B. Kopej (Ivano-Fran-kivsk National Technical University of Oil and Gas, Ukraine) for his valuable improvements to this manuscript.
EFFECT OF SAMPLE PREPARATION ON WATER TREE RESISTANCE OF XLPE/SEBS BLENDS
The effects of an aromatic polymer, such as poly[styrene-b-(ethylene-co-butylene)b-styrene] (SEBS), and its preparation processes on water tree resistance, morphology, crystallinity, dielectric constant and dielectric loss tangent of crosslinked polyethylene (XLPE)/SEBS and XLPE/SEBS/ethylene vinyl acetate (EVA) blends were investigated. The XLPE/SEBS samples were characterized by means of differential scanning calorimetry, scanning electron microscopy, dielectric spectral analysis and the accelerated water treeing experiment. The results clearly showed that although SEBS was partially compatible with LDPE and it was dispersed evenly in XLPE matrix, the blends demonstrated excellent dielectric performances, such as dielectric constant and dissipation factor. The crystallinity of XLPE/SEBS blends was decreased with increase in SEBS content and it was further decreased with the addition of EVA. SEBS could effectively improve the water-tree resistance of XLPE and both SEBS and EVA were found to improve the water tree resistance of XLPE synergistically as expected. The differences in sample preparation processes could change the phase morphology and dielectric performance of the blends, and could also influence the dispersion of SEBS in XLPE matrix as well as the crystallinity of the blends. Different preparation processes led to different water tree resistance behaviours of the samples. The improvement of SEBS dispersion in XLPE matrix helped to improve the water tree resistance of XLPE.