Stress design of petrochemical pipeline

In the current offshore oil engineering, pipeline design has become an important link, and the quality of pipeline design determines the success or failure of the offshore petrochemical engineering. Therefore, under this background, we should fully realize the importance of pipeline design in offshore oil engineering, and fully realize the impact of stress conditions on pipeline design. This paper starts from reality, and combines the theoretical research results in recent years, The relevant details of stress design in petrochemical pipeline are analyzed in depth.

1. Preface

In the current rapid development of industrialization, more and more scholars pay attention to the stress design in petrochemical pipelines, especially in offshore petroleum engineering, the stress design is particularly important. With the continuous progress of industrialization and the level of science and technology in China, the quality of petroleum pipeline stress analysis is also improving, and scholars at home and abroad have more research on petrochemical pipeline stress. At the same time, in the future development, stress design is bound to become the key to petrochemical pipeline transportation, strengthen the study of petrochemical pipeline stress, to promote the sustainable development of China’s petrochemical industry has certain practical significance.

2. Analysis of the concepts related to petrochemical pipeline stress

2.1 Overview of stress

In the current offshore petrochemical engineering, the pipeline will be affected by the weight of the fluid inside the pipeline, the chemical properties and the environment faced by the outside of the pipeline. Generally speaking, in the normal transportation process, various fluids inside the pipeline will produce external loading force, pressure inside the pipeline, thermal expansion force inside the pipeline and other types of loads, and this phenomenon is objective, so in the actual offshore petrochemical pipeline In operation, the pressure carried by the pipeline is usually greater than the maximum limit of the pipeline, and this situation, if it exists for a long time, will lead to different degrees of deformation of the pipeline. Therefore, in the design of petrochemical pipelines, designers need to correctly distinguish the impact of various types of stress on the stability and safety of pipeline operation, fully analyze the various stresses, find ways to resist stress, and develop certain design measures. At the same time, the designers should also adopt different stress resistance designs for different pipeline types based on the perspective of pipeline classification.

2.2 Primary Stress

Primary stress mainly emphasizes “once”, that is to say, in the design of offshore oil pipelines, the pipeline is mainly subject to the pressure of external load conditions, generally by the pressure, gravity and other physical factors, making the stability of petrochemical pipelines is greatly affected. Primary stress is the stress required to balance external forces, and the more external loads, the more stress is required to balance these external forces. From a professional point of view, primary stresses are non-self-limiting in nature. If the shaped area within the pipeline reaches its limit state, the pipeline will still experience unrestricted flow or even be completely destroyed even when the external forces are no longer increasing. In addition, the primary stress can effectively balance the applied load force faced by the petrochemical pipeline, and the balance force is positively related to the external load force. If the increase is large and exceeds the limits of the pipeline material, plastic deformation of the pipeline material will occur, and damage will occur.

2.3 Secondary Stress

The biggest difference between secondary stress and primary stress is that the secondary stress is very self-limiting. If in the process of actual operation, the local yielding phenomenon and small deformation of the pipeline occurs, the size of the stress will also change significantly, often leading to a continuous decline in stress. When the value of secondary stress reaches a certain level, it will directly cause permanent fatigue damage to petrochemical pipelines. In general, secondary stresses are mainly caused by physical phenomena such as endpoint displacement or thermal expansion and contraction.

2.4 Peak Stress

Theoretically speaking, peak stress is usually a phenomenon of accumulation of a large number of stresses in a pipeline or pipeline accessories during actual operation, and when a certain value is reached, the stress will reach its highest value, and we usually call such stresses peak stress. From the external performance, peak stress is very hidden and not easy to detect, and the deformation caused by the existence of peak stress is not very obvious, so it is not easy to be found in the normal operation of petrochemical pipelines. From the current practice, when the total amount of peak stress is high, it will directly lead to surface cracking of petrochemical pipelines due to fatigue phenomenon, which undoubtedly seriously affects the life and use of the pipeline. In addition, from the point of view of its location characteristics, peak stress is generally generated in the pipeline accessories near the small radius corners or the presence of petrochemical pipeline welding does not achieve complete weld through the location.

3. Petrochemical pipeline stress design analysis

In the process of actual operation, it is obviously unrealistic and unnecessary to analyze all the pipelines in petrochemical engineering, and it is very important to select the appropriate object of analysis. From the physical characteristics, the stress size of petrochemical pipelines depends on the diameter of the pipeline, the operating temperature and the mechanical equipment connected to the petrochemical pipeline. At the same time, the nature of the fluid transported in the petrochemical pipeline and the stability of the pipeline junction are also one of the main factors determining the effect of stress resistance in petrochemical pipelines. Therefore, the designers of petrochemical pipelines need to start from the actual situation of the using unit, and carry out reasonable design for different chemical properties of fluids and materials of pipelines, and need to ensure the flexibility and scientific nature of the design work, so that the petrochemical pipelines can maintain a stable operation in the long run.

3.1 Basic content

Generally speaking, the stress analysis of petrochemical pipeline mainly consists of two parts: “dynamic analysis” and “static analysis”. Under the same objective conditions, the performance of unified petrochemical pipeline is exactly the same when dealing with static and dynamic loads. The former means that the load is applied to the petrochemical pipeline in a slower form. Since the static load on the pipeline is interfered by the time factor, the petrochemical pipeline can have sufficient time to adjust itself to ensure the evenness of pressure distribution and the stability and normal operation of the petrochemical pipeline in the face of static load. In the face of dynamic loads, the forces given to the petrochemical pipeline by the dynamic loads can be eliminated under special circumstances. However, due to the relatively fast change frequency of dynamic loads, the pipe system cannot complete the average distribution of load forces in a short period of time, and thus it is difficult to completely effect the forces brought about by dynamic loads. At the same time, under the influence of dynamic load, there will be load imbalance inside the petrochemical pipeline, which is directly manifested as the phenomenon of pipeline vibration.

3.2 Exploration of flexible design of pipeline

At present, the flexible design of petrochemical pipeline has become the main development trend of pipeline design. Flexible design refers to the petrochemical pipeline can maintain a certain degree of flexibility, reduce the deformation of the pipeline, vibration, displacement and other phenomena brought about by a large number of negative impact. Generally speaking, if the pipeline is not designed to be flexible, the petrochemical pipeline will be fatigued in the process of actual operation because the internal stress of the pipeline exceeds the standard of the pipeline, thus reducing the safety of the pipeline operation, accelerating the aging of the petrochemical pipeline, and even the problem of pipeline leakage. In serious cases, due to the pipeline thrust or torque is too large, but also make and petrochemical pipeline connected to the mechanical equipment affected by stress, so that there are different degrees of deformation. This will not only lead to a reduction in the stability of the operation of the equipment, but also on the safety of the connected equipment caused by a very large impact.
In the actual design process, we need to take into account not only the physical properties of the pipeline such as thermal expansion and contraction, but also the additional displacement of the end points of the pipeline. First, the static equipment will impose additional displacement on the connected pipeline due to thermal expansion and contraction, and we need to take this feature into account in the process of flexible design. Second, dynamic equipment will be affected by physical factors and thermal expansion and contraction, if added displacement at the pipeline connection, it can effectively reduce the negative impact caused by pipeline stress. Third, equipment such as storage tanks will produce additional displacement of large magnitude at the connection orifice when the foundation settles.

4. Conclusion

From a comprehensive point of view, petrochemical pipeline design is the main design focus of marine petrochemical engineering at this stage, which has a pivotal role in the development of modern petrochemical enterprises and is the main direction in the future development. In the process of practical application, we should fully recognize the importance of pipeline design in marine petroleum engineering and fully realize the influence of stress conditions on pipeline design, so that the design and analysis of pipelines in petrochemical enterprises can be better improved and petrochemical enterprises have better development prospects.
Author: Nan Yang
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