- Volume:3 Issue:4, 2015
- تاریخ انتشار: 1394/02/30
- تعداد عناوین: 6
Pages 1-9This paper evaluates the added resistance of a torpedo shape AUV, moving inside a water pipeline, due to wall effects within the tube. Today, there are long length pipelines of water or petroleum that need to regular inspection. Early detection of failure will help to prevent breakdown and avoid the huge cost of an accident. As pipelines are enclosed environments and difficult to access, pipeline inspection robots are increasingly used for routine inspection and early failure detection. In the engineering cases, the pipes are full of liquid because there is no possibility for evacuating the pipe and interrupting the liquid transfer. It may be with or without flow of water. Therefore, the AUV must be able to afloat inside the pipe and perform non-contact inspection. In dry pipes, inspection device has contact with the walls and moves on them but in full pipes, the AUV moves such as a torpedo or submarine. The pipes have limited diameter and because of the wall effects on the fluid flow around the AUV, the resistance will be more than the free flow condition. This added resistance should be accounted accurately because it is necessary for determination of vehicle speed, power demand, range and duration of operation. This paper considers a torpedo shape AUV moving inside the pipes with the different diameter. The resistance of this modeling will be compared with the resistance of free steam modeling. This analysis is performed by the Flow Vision (V.2.3) software based on CFD method and solving the RANS equations.Keywords: AUV, torpedo, pipeline, hydrodynamic, added resistance, wall effect
A Practical Method for Investigation of Aerodynamic and Longitudinal Static Stability of Wing-in-Ground EffectPages 11-19The purpose of this paper is to present a fast, economical and practical method for mathematical modeling of aerodynamic characteristics of rectangular wing-in-ground effect (WIG). Reynolds averaged Navier-Stokes (RANS) equations were converted to Bernoulli equation by reasonable assumptions. Also Helmbold's equation was developed for calculation of the slope of wing lift coefficient in ground effect by defining equivalent aspect ratio (ARe). Comparison of present work results against the experimental results has shown good agreement. Finally, according to the calculated aerodynamic coefficients, height static stability of WIG was evaluated by Irodovs criterion in various ground clearance (h/c). A practical mathematical modeling with lower computational time and higher accuracy was presented for calculating aerodynamic characteristics of rectangular WIG. The relative error between the present work results and the experimental results was less than 8%. Also, the accuracy of the proposed method was checked by comparing with the numerical methods. The comparison showed fairly good accuracy. The evaluation of Irodovs criterion shows that height static stability (HS) increases with reduction of the height. Aerodynamic surfaces in ground effect were used for reducing wetted surface and increasing speed in high-speed marine and novel aeronautical vehicles. The proposed method is useful for investigation of aerodynamic performance and HS of WIG vehicles and racing boats with aerodynamic surfaces in ground effect. The proposed method has reduced the computational time significantly as compared to numerical simulation that allows conceptual design of WIG craft to do with economical.Keywords: Mathematical modeling, Aerodynamic characteristics, Static stability, Rectangular wing, Ground effect
Pages 21-35This paper reports results from an investigation into the suction caissons failure mechanisms under vertical pull-out loads. An insight to the failure mechanisms of suction caissons paves the path for developing analytical solutions to their pull-out capacity. The numerical models of suction caissons have first been calibrated by and verified against several experimental data from other researches. The verified numerical models have then been used to obtain the pull-out response of suction caissons under a variety of conditions. In principal, with suction caissons subjected to vertical pull-out load, four distinctive pull-out failure modes have been identified. Each failure mode has been found to be related to certain categories of soil/caisson/drainage condition. They varied from local failure modes, with weak soils under drained conditions and caissons of low penetration, to more global failure modes, for dense sands under undrained conditions. With global modes, the failure surface has been observed to well extend in the surrounding soil. It has been noticed that as the failure surfaces shift from local modes in the vicinity of the caisson towards global modes in the surrounding soil the pull-out capacity of suction caisson increases.Keywords: offshore structures, suction caisson, pull, out capacity, drained failure mode, undrained failure mode
Pages 37-50This paper presents a detailed structural reliability procedure in order to achieve an acceptable safety margin for template type offshore platforms located in the Persian Gulf. Probability of failure in this study is calculated by considering the cumulative effects of all levels of wave loading during the lifetime of the structure and uncertainties associated with soil, material properties, connection strength and environmental conditions in the reliability analysis. For this purpose, the conditional probability of failures is computed for different levels of wave loading and then converted to the rate of failure by applying the total probability theorem.
Annual explicit probability of failure is then computed by using probability distribution of wave heights in the Persian Gulf region. The calculated probability of failure is also compared with Reserve Strength Ratio of the platform considering different failure modes. The results show that RSR may not indicate a unique safety margin for assessing the existing platform in the Persian Gulf and carrying out a reliability analysis may help to overcome this deficiency.Keywords: Assessment, Probability of failure, Reliability analysis, Jacket type offshore platforms, Wave height distribution, RSR
Pages 51-64Hydrodynamic performance of a marine vessel mainly depends on the frictional and pressure resistance. Pressure drag reduction could be achieved by improving the shape of the vessels with implementation of modern hull forms. Hull forms optimization techniques could also be used for this purpose. Other techniques are needed to deal with the viscous portion of the total resistance, which is mainly frictional resistance. In this paper, an extensive literature review on the different methods applied to reduce the resistance of marine vessels was made, and the advantages and disadvantages of the implemented methods were identified. The related papers were categorized into three main categories and a summary of experimental and theoretical studies was provided.
On the basis of results obtained from the reviewed research studies, the combination of hull form optimization methods with other applicable drag reduction technologies such as antifouling coating is recommended to optimize the hydrodynamic forces.Keywords: Drag reduction, Hydrodynamic performance, Hull form optimization, Air lubrication, Fouling, coating
Pages 65-75The fatigue life of tubular joints commonly found in offshore industry is not only dependent on the value of Hot-spot stress (HSS), but is also significantly influenced by the through-the-thickness stress distribution characterized by the degree of bending (DoB). The determination of DoB values in a tubular joint is essential for improving the accuracy of fatigue life estimation using the stress-life (SN) method and particularly for predicting the fatigue crack growth based on the fracture mechanics (FM) approach. In the present paper, data extracted from finite element (FE) analyses of tubular KT-joints, verified against experimental data and parametric equations, was used to investigate the effects of geometrical parameters on DoB values at the crown 0˚, saddle, and crown 180˚ positions along the weld toe of central brace in tubular KT-joints subjected to axial loading. Parametric study was followed by a set of nonlinear regression analyses to derive DoB parametric formulas for the fatigue analysis of KT-joints under axial loads. The tubular KT-joint is a quite common joint type found in steel offshore structures. However, despite the crucial role of the DoB in evaluating the fatigue performance of tubular joints, this paper is the first attempt to study and formulate the DoB values in KT-joints.Keywords: Tubular KT, joint, Fatigue, Degree of bending (DoB), Axial loading, Parametric formula