2.2 Marine Flexible Riser

1) High risk of corrosion and infiltration in ultra deep water marine flexible risers

At present, many traditional composite materials are extremely sensitive to H₂S and CO₂, and their performance in corrosion resistance and hydrogen embrittlement resistance is not ideal. It is urgent to develop new corrosion-resistant materials. These materials not only need to be able to resist the erosion of H₂S and CO₂, but also have sufficient strength and toughness.In terms of numerical simulation, the current models for predicting the corrosion rate of flexible marine risers are not yet perfect, and the effects of gas permeation and microbial corrosion have not been fully considered. It is urgent to develop more advanced modeling tools to more accurately predict corrosion behavior, optimize the design and operating conditions of risers, and effectively reduce corrosion risks.

2) Difficulty in designing the line shape of flexible marine risers in ultra deep water service environments

In ultra deep water environments, the weight of flexible risers significantly increases, and fatigue damage at the top and bottom contact points of the seabed intensifies. It is necessary to reduce the degree of damage through reasonable configuration design. The linear design of flexible risers is a complex and interdisciplinary process that requires comprehensive consideration of various loads and environmental factors that the risers will bear during installation and service.These factors, including material and geometric nonlinearity, wave and ocean current effects, internal multiphase medium flow, and interactions with the seabed and floating platforms, all pose challenges to the accuracy of finite element analysis models.In addition, configuration design not only needs to meet technical requirements, but also needs to be economically feasible, find a balance between cost and performance, and consider actual construction and maintenance feasibility.

3) Difficulty in monitoring the operational status of marine flexible risers

The ocean flexible riser monitoring system needs to have real-time capability, which can capture and transmit the dynamic changes of the riser in a timely manner, in order to provide timely warning and take measures. Therefore, it is necessary to develop reliable warning algorithms that can predict possible risks based on monitoring data, such as fatigue damage, excessive vibration, etc. The length of the ocean flexible riser determines that sensors need to be arranged at multiple key locations to capture comprehensive dynamic information, which requires precise layout schemes and installation techniques.Sensors in marine environments need to have high reliability and durability, be able to work in harsh conditions for a long time, and be difficult to maintain and replace. Underwater data transmission faces issues of signal attenuation and interference, especially in deep-sea environments where stable and efficient solutions are needed for long-distance data transmission.In addition, real-time monitoring of large amounts of data requires efficient data processing and analysis methods to timely identify the dynamic behavior and potential risks of marine flexible risers.

2.3 LNG Low Temperature Hose

1) Difficulty in Nonlinear Buckling Failure Analysis of LNG Low Temperature Hoses under Multiple Load Coupling

Buckling instability is one of the main failure modes of LNG low-temperature hoses. Once the pipeline structure undergoes buckling instability, it will directly affect the safety of the LNG transportation system. Therefore, it is necessary to focus on the buckling instability analysis of the thin-walled structure of LNG low-temperature hoses.However, LNG low-temperature hoses need to withstand harsh external environmental conditions in practical engineering applications, such as strong winds, waves, ocean currents, and multiple loads such as ship motion. To ensure the safety of LNG low-temperature hoses in practical engineering applications, further research is needed on nonlinear buckling failure analysis based on multi load coupling.By constructing a low-temperature internal flow environment, multiple load coupled extreme condition loading tests were conducted on the prototype tube to verify the accuracy and stability of the structural design and processing technology.At present, China National Offshore Oil Corporation (CNOOC) has partnered with universities to build the first comprehensive performance testing platform with full performance and full size under extreme conditions in China. However, the exploration of ultimate load testing and performance verification tests for multi load coupling is still ongoing.

2) Insufficient experience in overall hydrodynamic optimization of LNG low-temperature hoses

When LNG low-temperature hoses are in service in marine environments, they are affected by factors such as ocean currents, wind and waves, and relative motion between two ships, resulting in sway, heave, and roll movements.These sports will have an impact on the stability of the static balance position of the hose, leading to hose vibration. This vibration not only affects the transmission efficiency of LNG, but may also cause looseness at the end connection of the hose, increasing the risk of LNG leakage.Therefore, in-depth analysis of the dynamic characteristics of LNG low-temperature hoses is crucial for guiding the optimization of hose structure design.The overall length design of the hose must take into account the maximum movement distance between the export points and the bearing capacity of the hose itself. The overall hydrodynamic results should be combined with the buckling failure analysis results under multi load coupling to obtain the overall linear layout of the hose, ensuring the stability and safety of the transportation process in complex sea conditions.

3) The fatigue failure model of LNG low-temperature hoses urgently needs to be broken through

As a key equipment for marine natural gas extraction and transportation, LNG low-temperature hoses undergo nearly a thousand cyclic load changes such as temperature cycling, pressure cycling, and retraction operations during their service life due to the requirements of their application scenarios and operating conditions. At the same time, boundary conditions such as marine environment and ship motion must also be considered. Therefore, the fatigue failure model of LNG low-temperature hoses is extremely complex.Firstly, based on the layout of LNG low-temperature pipelines, it is necessary to consider the complex load conditions under boundary conditions and environmental conditions such as wind, waves, and currents, analyze the overall pipeline shape, and obtain the load time history curve of LNG low-temperature hoses under various marine environment combinations;Secondly, it is necessary to combine ultra-low temperature material specimen experiments, convert the stress time history curve of LNG low-temperature hoses through local theoretical models, calculate the fatigue life of low-temperature hoses based on cumulative damage criteria, and verify the results of prototype fatigue tests to obtain the fatigue failure model.Therefore, how to accurately predict the fatigue life of LNG low-temperature hoses, and how to extend the life of LNG low-temperature hoses through material selection, design optimization, processing adjustment, and line layout, are still important issues that urgently need to be overcome for industrial application.In the future, through comprehensive research and development work, LNG low-temperature hoses that can adapt to various extreme marine environmental challenges will be designed and manufactured, providing solid technical support for the safe and efficient transportation of LNG.

3. Development Trend of Ocean Flexible Pipe Application Technology

3.1 Buckling Theory Research and Prevention Techniques

Due to low bending stiffness and high axial expansion coefficient, submarine flexibel pipes are prone to buckling failure. Once the submarine flexible pipe buckles and fails, it may cause pipeline rupture or deformation, leading to serious environmental pollution and economic losses.At present, the design methods and principles to avoid buckling are mostly targeted at underwater steel pipes, and it is urgent to establish appropriate buckling theories and prevention techniques to effectively predict and prevent the occurrence of this failure mode.

Submarine flexible pipes have strong flexibility and are susceptible to buckling under compressive loads due to initial defects in non straightness.Their mechanical properties are highly nonlinear and depend on interlayer interactions. Before interlayer slip, the bending stiffness of submarine flexible pipes is mainly determined by the tensile armor steel wire; After sliding, the bending stiffness is mainly affected by the bending stiffness of the outer sheath, and is also related to internal pressure, water depth, and annular conditions.We need to develop a nonlinear finite element analysis tool for underwater pipes to simulate the buckling behavior of pipelines. For the radial buckling problem of submarine flexible pipes, anti buckling bands can be wrapped around the outer side of the tensile armor layer to resist the radial expansion load of the tensile armor layer during installation or service.For anti buckling bands, it is necessary to select suitable polyester fiber reinforcement materials and determine their application range through aging, static load, bending resistance, temperature resistance, and durability tests. For the current lack of prediction models for submarine flexible pipe buckling, a combination of numerical models and experimental methods can be used to obtain failure loads, and machine learning algorithms can be used to obtain simple and practical empirical design formulas.

3.2 Marine Flexible Pipe Structure Design and Optimization Technology

The improvement of design methods, analysis tools, and on-site practical experience has promoted the development of marine flexible pipe structure design and optimization technology, shortened the design cycle, reduced uncertainty and risks in project implementation, and effectively reduced investment costs.

The infiltration and corrosion of dissolved acidic gases in deep-sea oil and gas reservoirs on pipelines have been widely concerned. In order to control the permeation of H₂S and CO₂ gases in flexible marine risers, it is urgent to develop new corrosion-resistant materials and establish a permeation model for acidic gases in the water tight layer inside the flexible riser.In addition, as the water depth increases, the weight of the riser increases sharply. Carbon fiber reinforced plastic (CFRP) can be used to replace the anti tensile armor layer steel, which has a density of only 20% of traditional steel and has good strength weight ratio, high corrosion resistance, and fatigue resistance.Faced with the influence of natural factors such as wind, waves, ocean currents, and ship motion, full-scale experiments, theoretical models, and numerical simulations can be used to comprehensively study the response of marine flexible pipes in terms of mechanical properties such as tension, bending, torsion, and fatigue.These studies contribute to optimizing the overall structural design of ocean pipes to meet the requirements of various extreme working conditions. For LNG hoses, the manufacturing process must consider the effects of low temperature, high pressure, and complex marine environments to ensure the stable structure and performance of marine flexible pipes during long-term operation.In material selection, priority should be given to materials that can maintain good mechanical properties and stability even under extreme low temperature conditions, including good low-temperature impact resistance, corrosion resistance, and high strength.