Finite Element Analysis of Elliptical Chord: Tubular T-Joints

Finite Element Analysis of Elliptical Chord: Tubular T-Joints

K. S. Narayana (Department of Mechanical Engineering, Anil Neerukonda Institute of Technology and Sciences (ANITS), Visakhapatnam, Andhra Pradesh, India), R. T. Naik (Department of Mechanical Engineering, Indian Institute of Science, Bangalore, Karnataka, India), R. C. Mouli (Department of Mechanical Engineering, Anil Neerukonda Institute of Technology and Sciences (ANITS), Visakhapatnam, Andhra Pradesh, India), L. V. V. Gopala Rao (Department of Mechanical Engineering, Maharaj Vijayaram Gajapathi Raj College of Engineering (MVGRCE), Vizianagaram, Andhra Pradesh, India) and R. T. Babu Naik (National Geophysical Research Institute, Hyderabad, Andhra Pradesh, India)
DOI: 10.4018/ijmmme.2013100104
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The work presents the Finite element study of the effect of elliptical chords on the static and dynamic strength of tubular T-joints using ANSYS. Two different geometry configurations of the T-joints have been used, namely Type-1 and Type-2. An elastic analysis has been considered. The Static loading conditions used are: axial load, compressive load, In-plane bending (IPB) and Out-plane bending (OPB). The natural frequencies analysis (dynamic loading condition) has also been carried out. The geometry configurations of the T-joints have been used, vertical tubes are called brace and horizontal tubes are called chords. The joint consists of brace joined perpendicular to the circular chord. In this case the ends of the chord are held fixed. The material used is mild steel. Using ANSYS, finite element modeling and analysis of T-joint has been done under the aforementioned loading cases. It is one of the most powerful methods in use but in many cases it is an expensive analysis especially due to elastic–plastic and creep problems. Usually, three dimensional solid elements or shell elements or the combination of two types of elements are used for generating the tubular joints mesh. In tubular joints, usually the fluid induced vibrations cause the joint to fail under resonance. Therefore the natural frequencies analysis is also an important issue here. Generally the empirical results are required as guide or comparison tool for finite element investigation. It is an effective way to obtain confidence in the results derived. Shell elements have been used to model the assembled geometry. Finite element ANSYS results have been validated with the LUSAS FEA and experimental results, that is within the experimentation error limit of ten percentage.
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1. Introduction

1.1. Offshore Structures

The demand for the fuel resources from the oceans has increased the necessity of offshore drilling and production operation. For this purpose, continuous improvement in the design, development and construction of an offshore structure becomes imperative. A Finite element analysis has been done using ANSYS, to study the static strength of an offshore tubular T-joint.

Steel tubular framed structures are installed on the seabed for the exploration and the production of oil from the sea bottom called offshore platforms.These serve as bases, supporting the drilling and production facilities above the elevation of waves. Offshore structures used for oil and gas extraction have the common function of providing a safe, dry working environment for the equipment and personnel who operate the platform. Jacket supported drilling equipment as a guide for the piled foundations. The substructure referred to as the jacket is a three dimensional space frame made from large tubular steel members. The jacket which takes the loadings from the top side and the sea environment is piled to the sea bed. These piles must also be able to resist tension as hydrodynamic forces on the structure have a tendency to cause over turning. To construct a steel jacket it is necessary to join the large diameter tubular steel members in somewhere. These tubular joints or nodes are major sources of difficulty and high cost in the design of jacket. Tubular joints can be classified into four categories. They are the simple welded joints, complex welded joints, cast steel joints and composite joints.

Circular cross section tubes are preferable to other types of sections and are used extensively in offshore structures because their drag characteristics minimize wave forces on the structure, and their closed cross section allows for the needed buoyancy during installation in the ocean environmental. These circular tubes are also more convenient to use than other tube shapes because of their availability in different sizes. These joints are usually subjected to static and dynamic loading. The basic loading cases are axial tension or compression, bending and torsion or any combination of these cases. Experimental tests are generally the only way to ascertain the strength of any tubular joint .The Empirical equations have been developed for most types of circular joints but there is a still a paucity of experimental data for other shapes or cross sections. The department of energy UK report has been collected some of the related experimental tests to add to the existing empirical equations steel is the basic material used in these tubular joints. Different types of steels are being used, although mild steel is the more common material. Other types of materials such as lead –tin alloy are also used. The current design format for the ultimate strength of Tubular joints varies from one code to another.

There are two design formats that are widely accepted by the different design codes. The working stress format, which is based on the punching shear strength of the chord wall, and the limit state format, which is based on circular model .In the first method, joint failure is assumed to occur when the nominal shear stress in the chord wall exceed an allowable value. This assumption is usually valid for joints with small branch to chord diameter ratios. The circular model on the other hand is based on the plastic collapse of chord member. The chord is modeled by a circular ring with the same diameter and wall thickness as the chord and an effective length that is assumed to be three times the mean diameter of the chord .The brace load is modeled by two lines loads acting on effective length of the ring. The collapse load is computed from equilibrium of forces.

The design equations for T-tubular join subjected to axial tension, axial compression and in plane–bending moments are summarized for the four major design codes. The strength values computed from the equations are normalized for the ultimate strength after removing all factors of safety. However the accuracy of design equations depends on the quality and the range of applicability of the design equations depends on the assumptions that are used to derive their basic form. Two analytical models are discussed, one for joints subjected to axial load and another for joints with in-plane –bending moment. For joint under axial loads, the resulting equation contains a new term that accounts in a more natural way for strengthening effect observed in joints. For moment loaded joints the analytical model provides a linear equation.

Numerous experimental tests have also been carried out with different loading cases, but these were limited to testing circular tubular joints. On one shape of elliptical corn T-tubular joints and these were compared with circular chord tubular joints .The chord tube was either circular or of an elliptical cross sectional shape while the brace was a circular cross section tube. The strength of the welded joint can be tested with tensile, compressive, bending and shear loads.

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