3.3 Fatigue Life Testing and Analysis Techniques

Both marine flexible risers and LNG low-temperature hoses are subjected to the effects of upper floating bodies and marine environmental loads during service, making them dynamic pipelines that may experience fatigue damage during long-term service. The service life requirement of flexible pipelines is usually long, and the impact of various failure modes that may occur throughout the entire life cycle must be considered in the design phase.

The fatigue life of marine flexible risers is generally determined by the fatigue life of the tensile armor layer, while small molecules such as H₂O, CO₂, H₂S, CH₄ will penetrate into the annulus through the polymer layer. At the same time, seawater may also enter the annulus due to the rupture of the outer protective sleeve. Therefore, it is necessary to study the stress life (S-N) curve of carbon steel materials in air, seawater, and acidic environments through experimental methods.At the same time, it is necessary to conduct pressure holding and dynamic fatigue testing on full-scale flexible risers under internal medium corrosion environment, analyze the degree of fatigue damage, especially considering the stress concentration phenomenon caused by pitting corrosion or other local damage on the surface of carbon steel, in order to improve the reliability and accuracy of fatigue life assessment of flexible risers.In addition, it is necessary to develop a slip model that considers factors such as interlayer friction coefficient, axial deformation, and pipeline curvature to better describe the slip behavior between the tensile armor layers of flexible risers.

Due to the fact that LNG low-temperature hoses are coiled into reels in non conveying states, fatigue issues need to be considered only during the conveying process. To test its fatigue life, it is necessary to pressurize the inside of the hose with liquid nitrogen and repeatedly apply positive and negative angular displacement at the end of the hose for over a million load cycles.The specific conditions for testing cycle, curvature, and tension need to be obtained through sea condition data of the service environment and hydrodynamic response analysis of the transfer vessel.In addition, thermal fatigue performance also needs to be considered, as repeated cooling heating cycles will not cause hose leakage. At the same time, it is necessary to improve the fatigue resistance of the thin film material inside the hose body and the sealing area of the joint.In terms of fatigue damage prediction models, it is necessary to quantitatively describe the influencing factors such as nonlinearity, damping, hysteresis behavior, and local damage to improve the accuracy of fatigue behavior prediction.

3.4 Mobile Security Protection Technology

In deepwater oil and gas development, the application of ocean flexible pipes faces challenges such as low temperature environments, seabed conditions, and complex composition of flowing media. It is particularly important to develop reliable flow safety assurance technologies to ensure the safe operation of marine flexible pipes under these conditions.For the insulation layer of flexible pipelines, thermoplastic synthetic polypropylene material reinforced with glass microbeads can be used, and its thermal conductivity, thickness, specific heat, density, mechanical properties and other parameters can be designed and wrapped between the tensile armor layer and the outer sheath; Active electric heating and other temperature control systems can also be developed to form a current loop between the skeleton layer and the tensile armor layer, generate heat through the resistance loss of the metal layer, and heat the pipeline as a whole or locally based on pipeline temperature monitoring data and solid sediment prediction results to ensure flow safety. In addition, the internal temperature of the pipe can also be maintained by circulating hot water. The insulation design method of flexible pipes is a prerequisite for the selection of insulation methods and engineering implementation, which needs to consider various factors such as interlayer thermal resistance, cross-sectional configuration, annular heat transfer characteristics, internal medium pressure and temperature changes. The key to ensuring the flow of oil and gas lies in prevention, which can be achieved by winding optical fibers in flexible pipes to monitor changes in pressure and temperature inside pipelines, thereby determining the rate and location of hydrate or paraffin accumulation, in order to detect and remove them early using pipeline cleaning techniques.

The interior of ocean flexible pipes often has a skeleton layer, which is accompanied by flow induced vibration and pressure loss along the way when high-speed gas passes through.We need to develop analytical models to predict the initial flow velocity and vortex frequency of flow induced vibration, in order to provide rational engineering recommendations.At the same time, a flexible pipe solution with a smooth inner surface can be developed by designing the anti crushing layer to the outer side of the internal sealing layer, or adding a molding tape to the inner layer of the skeleton layer to naturally form a smooth inner surface after the winding process.

3.5 Operation Status Monitoring and Detection Technology

For ocean flexible pipes, real-time monitoring can provide a continuous data stream, helping operators to timely understand the health status of the pipes and avoid sudden failures. In addition, modern detection techniques can also be used to evaluate different potential failure modes without damaging the pipe, in order to improve detection efficiency and accuracy.

For the monitoring of fatigue damage in flexible marine risers, acceleration sensors can be placed along the axial direction of the riser. By measuring the components of gravity acceleration on each axis, the inclination angle of the sensor can be calculated, and the curvature can be obtained to calculate the fatigue life and structural health status.Electromagnetic stress induction technology can be used to detect the fracture failure of the tensile armor steel strip near the upper end of the flexible riser.Firstly, magnetize the tensile armor layer, and then utilize the principle that the magnetic permeability of ferritic steel changes with mechanical stress. If the internal medium pressure changes, the tensile armor tape should have stress changes;If there is no change in the internal medium pressure, it indicates that the steel strip may have broken. With the increasingly widespread application of artificial intelligence algorithms such as deep learning in the field of ocean engineering, digital twin technology based on finite element analysis and artificial neural network methods can also be used for monitoring fatigue damage of flexible riser tensile armor layers. It takes ocean environmental conditions, ship motion, and riser motion monitoring data as input data, and trains the model directly to predict cumulative fatigue damage, which can significantly improve analysis efficiency.

For the monitoring of the bearing layer of ocean low-temperature flexible pipes with different structural forms, a combination of overall hydrodynamic analysis of the transfer process and local finite element analysis of the layer can be used to verify the effective range of the mechanical model control equation, and thus propose monitoring parameters to obtain real-time results such as pipeline curvature, axial force, and stress hotspots.

4. Development Suggestions

(1).  Conduct in-depth research on the structure and construction design of ocean flexible pipes to promote technological innovation. Develop an overall plan for the basic research and technological development of ocean pipe design based on the national strategy of building a strong maritime and technological nation. Make full use of the advantages of universities and research institutions in theoretical and basic research, cooperate with key enterprises with rich engineering practice experience, promote the innovation chain of industry academia research application of marine flexible pipes, and achieve comprehensive progress in the design theory and technology of marine flexible pipes.

(2).  Develop an overall plan for the construction of ocean flexible pipe demonstration lines and promote the construction of domestically produced ultra deep water ocean flexible pipe demonstration projects. Select enterprises with good design and application foundations for marine flexible pipes in various engineering backgrounds as pilot projects for demonstration engineering construction, build domestically produced marine flexible pipe demonstration projects, continuously promote the construction of marine flexible pipe application technology test platforms, real-time dynamic monitoring technology, and systematic marine flexible pipe construction guidelines, and create the world's most complete marine flexible pipe full scene systematic dynamic test chain.

(3).  Promote the construction of a standard system for the design and construction of marine flexible pipes, and grasp the discourse power of the international marine flexible pipe application technology index system. Strengthen the cross integration of ocean pipe dynamics and control engineering, conduct multi-level research from the perspective of deepwater service environment, and propose a technical standard system for the design of ocean pipes, including dynamic testing techniques, performance evaluation indicators, and key parameters.

5. Conclusions

With the rapid growth of the Chinese economy and the continuous promotion of the strategy of building a maritime power, China's marine flexible pipe application technology has entered the "deep water zone" of technological breakthroughs.In order to achieve domestic production and application of marine flexible pipes in the development of oil and gas resources in deeper waters, it is necessary to overcome the current development bottlenecks. Based on the existing theoretical and technological research on marine flexible pipe applications, a series of policy measures must be introduced to promote the domestic design, production, processing, application, export, and operation of marine flexible pipes. At the same time, domestic enterprises and manufacturers are encouraged to establish a positive design system for the performance and parameter matching of marine flexible pipes throughout the entire process and scenario.In addition, it is necessary to break the boundaries between research and development, production, and engineering applications in the field of ocean pipes, and strengthen connections and cooperation among all parties through orderly industrial chain transformation. This will provide solid support for the application of ocean flexible pipe technology in the development of deep and offshore oil and gas resources, and promote the development of China's ocean pipe application technology to a higher level.