Dissertation for Master's degree
in the field of civil engineering – Marine structures
September
92
Abstract:
Pipe-in-pipe systems have been widely used in oil and gas transmission line applications where the use of line thermal insulation is very important. This system includes inner and outer thin wall tubes with rings between them that These rings are completely filled by a selected insulator. In this research, using the linear buckling method, the elastic buckling behavior of pipe-in-pipe systems with cores of different thicknesses and Young's coefficients under the external hydrostatic pressure of water has been investigated using Ensys software, and the results show that the buckling pressure strongly depends on the Young's coefficients of the core and pipe filling materials, the thickness of the core, and the thickness of the inner and outer pipes. Models with high modulus of elasticity of the core, their critical buckling pressure is similar and almost stable and the same regardless of the thickness of the core, but in models with a weak modulus of elasticity of the core, two situations occur: 1- A model where the thickness of the inner tube is less than the outer tube, as the thickness of the core decreases, the critical pressure of buckling also decreases. find Also, the results show that two different buckling phenomena occur, one is the overall buckling of the combined pipe and the other is the buckling related to The pipe is external.
Key words: critical pressure, modulus of elasticity, linear buckling, pipe-in-pipe systems, cylindrical shells, ANSYS software
Chapter One
Introduction
-1 Introduction related to pipe-in-pipe systems
Pipe-in-pipe systems [1] They are widely used in pipeline applications where the use of line thermal insulation is very important. Usually, the space between two pipes in these systems can be empty or contain non-structural insulating materials. In deep water, the outer pipe must be designed to be resistant to damage caused by the external pressures of the environment, while the inner pipe, in the first stage, is designed to be resistant to the pressures of liquid hydrocarbons present in them. In addition, analyzes in which many other factors are considered are also available in this field. Therefore, pipe-in-pipe systems in deep water should be designed to withstand external pipe failure. As with single pipelines, during the installation and operation of structures, conditions outside of the design can occur in the structures, which will lead to internal failures in the structure[1].
In marine pipe systems, factors such as high bending during pipe installation, additional stress caused by the roughness of the seabed, the impact of external factors such as ship anchors, fishing tools, and the reduction of pipe wall thickness due to processes such as corrosion, abrasion, and erosion can be among the main causes. buckling [2,3].
Buckling and failure due to external pressures are very important consequences that should be considered in the design of pipelines such as pipe-in-pipe systems installed in the sea[2] be considered The second problem that exists in this field, and usually its importance is not less than the one mentioned above, is related to the propagation of buckling [3], which can ultimately cause problems for the survival of the line. The propagation of buckling can start from a weakened section in the pipe, for example, from a depression, and this weakening can be caused by phenomena such as the impact of foreign objects. When buckling starts, it can be seen to propagate at high speeds, which can eventually lead to a very fast failure of the entire pipeline. PP release pressure is the minimum pressure at which we will see buckling release. This pressure is also known as the characteristic pressure in the pipeline, this pressure is usually between 15-20% of the failure pressure of the PCO, and as a result, in many projects, pipeline design based on the release pressure is impractical. To deal with this problem, the design is based on the failure pressure, and instead, tools called buckling are used at regular intervals along the line.At the time of initiation of buckling, these buckling arresters limit the damage to a length of pipe [4].
Experiments conducted by Kriakides [5][1] showed that the propagation of buckling in pipe systems is also visible in a pipe under hydrostatic pressure. This diffusion is very similar to that which occurs in single pipe systems. Due to the propagation of buckling in two-pipe systems, damage to the inner pipe can also be seen. As a result of such a situation, we will have the dynamic propagation of buckling and this propagation will flatten both sides of the pipe. Experiments on the pipes showed that in most cases, the collapse of the outer pipe will lead to the simultaneous collapse of the inner pipe, and subsequently, the simultaneous collapse propagation will cause the collapse of both pipes. In this case, the release pressure is indicated by Pp2 (index 2 related to the two-pipe system). The second mode of collapse propagation discovered is that the carrier tube collapses and the inner tube remains intact. If we consider the diameters and properties of the outer and inner tubes to be constant, for each D/t of the outer tube, there will be a certain thickness of the inner tube wall. The pressure at which this occurs corresponds to the pressure that results in similar buckling propagation in a system where the inner tube system is replaced by a solid rod. He called this corresponding pressure PPS. If the wall of the inner tube [6] in tube-in-tube systems is larger than the diameter of this solid rod, and when a limited length of the outer tube [7] is associated with failure and failure, the second quasi-static instability will occur at pressures higher than the PPS pressure. This time, the inner tube will also fail. He observed that when this double buckling propagation event occurs, the inner tube is not completely destroyed because it has a high wall thickness and hardness. Kriakides et al.[5] in the second part of their study showed that for tubes with common application geometries, the internal failure of the outer tube will usually lead to failure in both tubes. If the external pressure is high enough, the failure will propagate dynamically and can randomly flatten large sections of the pipe. The minimum pressure at which the failure of the pipe-in-pipe systems in this study started was the pressure in the system designed with the PPS pressure value, this is the pressure at which the pipeline should be protected by bucklers.
Researches conducted in recent years in connection with axial and symmetric buckling in circular cylindrical shells have led researchers to draw attention to the effects of stress wave propagation. The characteristics of inertia (the state of the incoming force) in a shell along with the characteristics of its materials, including the elastic coefficient, density and the like, determine the special patterns of the axial propagation of stress [8]. As a result, this problem will lead to the development of dynamic buckling, both in the form of plastic buckling and in the form of progressive buckling [28].
The pipelines in the pipe in which oil and gas are transported in deep ocean wells are designed in such a way that they can withstand high hydrostatic pressures. However, pipelines in pipes can sometimes form internal kinks due to random erosions, and if the hydrostatic pressure is high enough, this kink will spread along the pipeline and lead to the flattening of the pipeline and the creation of irregular geometric shapes known as dogbone [9]. Sustained buckling propagation pressure is the lowest pressure at which the propagation of buckling can be seen with the initiation of buckling along the pipeline [29]. Propagational buckling will stop only when the external hydrostatic pressures become less than the propagation pressure. The phenomenon of buckling propagation was observed for the first time in the 1970s, and the first experimental study in this field was conducted by Maslow [10] and colleagues [6].
Generally, three different types of performance are expected from marine pipelines, the first and most important of which is resistance to damage caused by hydrostatic pressure in deep waters [11] up to 2000 meters.