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Stress Distribution at the Fillet of an Internal Flange

This paper deals with the determination of the stress distribution at the fillet of a ANSI B16.5 flanges attached internally to a hollow cylinder. A load parallel to the axis of the cylinder and of variable eccentricity acts on a bearing plate which rests on the flange. The strains are measured by means of electrical resistance wire strain gages. The ratios of the mean cylinder diameter to the cylinder wall thickness and of the mean cylinder diameter to the flange thickness are varied. The principal stresses at the fillet are given as functions of these parameters. The experimental results are compared with the stresses calculated on the basis of an approximate theoretical solution for both an axial and an eccentric load.

Abstract Joining of steel pipes and pipe flanges use today the conventional method of fusion welding, where the flange is girth-welded onto the pipe. However, fusion welding of flanges to pipes is associated with many disadvantages such as the final quality of the weld, degradation of the mechanical properties of the base pipe near the heat affected zone, defects and cracks appearing in the weld, misalignments, to mention a few. The current study proposes a novel pipe-flange connection to replace the fusion welding process of steel pipes with a method based on cold working. The method is based on that the steel pipe is inserted into the neck of the flange, in which two circumferential grooves are manufactured. An expansion tool having two teeth is entered from the open side of the connection and is expanded hydraulically such that the teeth deform the pipe and cold work it plastically into the grooves. This will provide a strong joint between the flange and pipe. In this study the performance of the connection is maximized by optimizing the design of the flange and the expansion tool.

The use of bolted flange connections in the offshore wind industry has steeply risen in the last few years. This trend is because of failings observed in other modes of joints such as grouted joints, coupled with enormous economic losses associated with such failures. As many aspects of bolted flange connections for the offshore wind industry are yet to be understood in full, the current study undertakes a comprehensive review of the lessons learned about bolted connections from a range of industries such as nuclear, aerospace, and onshore wind for application in offshore wind industry. Subsequently, the collected information could be used to effectively address and investigate ways to improve bolted flange connections in the offshore wind industry. As monopiles constitute an overwhelming majority of foundation types used in the current offshore wind market, this work focusses on large ANSI welding neck flanges in the primary load path of a wind turbine foundation, such as those typically found at the base of turbine towers, or at monopile to transition piece connections. Finally, a summary of issues associated with flanges as well as bolted connections is provided, and insights are recommended on the direction to be followed to address these concerns.

As per recent reports, the offshore wind sector could bring in £17.5 bn investment to the U.K. economy over the next few years after faster than expected cost-cutting slashed subsidies for the technology by half [1]. On top of that, the baseline scenario for the United Kingdom’s installations by the end of 2030 is to reach the capacity levels of 40 GW, four times the current state [2]. Additionally, the target of £100 per MWh set for the year 2020 regarding the levelised cost of energy (LCOE) of offshore wind was achieved in U.K. projects four years earlier in 2016 [3]. The above figures reinforce the need for new technological developments that will enable the utilisation of larger and more efficient offshore wind turbines (OWTs). In this direction, one of the most important concerns is the support structure of the turbine’s tower, which requires further study concerning not only the feasibility of future installations, but also current problems that need to be better understood and addressed.

OWT structures, which are quite large in thickness and diameter, operate in the hostile marine environment, where variable amplitude loads are constantly applied on different parts of the structure [4,5]. In the offshore industry, grouted connections were initially used to charge the transition piece (TP), with a certain overlap length, on the monopile (MP) foundations. Therefore, there is a tube-in-tube connection, wherein the space between the two tubes is filled with grout (Figure 1) [6]. Towards the end of last decade, numerous grouted connection joints between large diameter monopiles and connecting tubular steel transition pieces at the base of overlying support towers were found to be failing. For the majority of U.K. offshore MPs that experienced grout cracking and failures, the issue was recognised to be primarily owing to the widespread absence of shear keys (or weld beads) on straight MP and TP surfaces. Bending moments as a result of complex wind (which was the main difference in loading conditions compared with oil and gas platforms) and wave loading were important design considerations that were not accounted for during design of grouted connections for OWTs. Furthermore, axial connection capacity was found to be significantly lower than that assumed previously owing to the MP scale effect, lack of manufacturing and installation tolerances, and abrasive wear due to the sliding of contact surfaces when subjected to large moments. Typical failure modes included dis-bonding, cracking, wear, and compressive grout crushing failure.

The number of bolts depends on the ANSI plate flanges radius and thickness, type of tool used, size of the bolts, and predicted loads on the structure. These bolts serve the purpose of exerting a clamping force to keep the joint together [20]. The behaviour and life of the bolted joint depend on the magnitude and stability of that clamping force. The preload is created by the tightening process during the assembly of bolt and nut in the joint to provide enough clamping force on the joint. Therefore, the bolts need to be preloaded at the assembly stage in the flange connection. An intuitive analogy would be to think of the bolts and the joint members as elastic parts. In that way, they can be modelled as spring elements, where the bolts are stretched in their elastic region when tightened, in order to compress the joint. The joint has a much stiffer elastic constant compared with the bolts, depending on material and dimensions.

Further studies in the offshore wind industry can enable the optimal use of ANSI threaded flanges connections in design, manufacturing, installation, operation, maintenance, and decommissioning phases.

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