| The North American Society of Trenchless Technology (NASTT,2001) defines Trenchless construction as " A family of methods, materials, and equipment capable of being used for the installation of new and replacement or rehabilitation of existing underground infrastructure with minimal disruption to surface traffic, business, and other activities". Compared with the traditional pipe-laying technology has great advantage, has been widely appreciated.Compared with the traditional pipe-laying technology, the pipeline crossing trenchless horizontal directional drilling technology has great advantages, such as the small environmental impact, the wide range of construction, the great adaptability and greater social and economic benefits. It can also be crossed with the shortest reasonable path and with high crossing precision. At the same time, it can be buried in depth and crossed in a long distance. However, there exist some urgent problems to solve, such as the slow velocity of drilling, the potential menace to the engineering due to the difficulties of borehole high-speed, the complex working process of drilling leakage because of the large amount of drilling leakage when the drill crosses the high porosity layer.Underbalanced drilling technology derives from the field of oil and gas drilling. Natural or artificial methods are used in the processing of drilling. This allows the drilling fluid density below the equivalent density of the pore pressure, at the same time allows the stratum fluid enter the well under control and circulate the fluid to the ground. This technology is still restricted to research status at the moment. If applied successfully, it can improve the drilling velocity, shorten the drilling period, extend the bit life and reduce the construction cost. It can solve the leakage problems effectively and decrease the frequency of complex situations. It can also avoid the drill sticking caused by differential pressure and affect the surrounding very slightly.This paper analyzes the hydraulics calculation model of underbalanced drilling, and describes the flow type of the two-phase conduit flow. It also studies the hydraulics of under-balanced drilling technology applied to the trenchless horizontal directional drilling pipe-laying technology. Moreover, this paper analyses the necessity and the significance of the under-balanced drilling technology applied to the directional drilling. It introduces the present state of the domestic and foreign research and predicts the development tendency of the trenchless technology and horizontal directional drilling technology respectively. Research objectives, research contents and technical routes are identified on such basis. It also analyses the advantages, difficulties and scope of application of the horizontal directional drilling. It states the construction technology of the horizontal directional drilling in detail. Advantages of the under-balanced drilling technology applied to the horizontal directional drilling are also explained.This paper studies the hydraulics calculation mode of the under-balanced drilling technology and describes the flow pattern vapor and liquid phases pipe flow. According to fluid mechanics. multiphase flow parameters and hydraulics mode optimized construction parameters are built up. Combined with the project of "Lan-Zheng-Chang" pipeline project used the horizontal directional drilling technology crossing Yangtze river, this paper describes the practical situation of this project. The types of drilling fluid, construction parameters and the solving methods are designed based on the reality. Different drilling fluid density and delivery capacity are used to calculate the distribution of the annulus velocity, the annulus pressure, the equivalent circulating density (ECD) and the density distributing rules of debris. At the same time, it develops sensitivity analysis on all kinds of parameters and forms a controlled method of borehole under-balanced main flow parameter. |
PDF Full Text Request