1.2 Marine Flexible Risers

Marine flexible risers are deep-water oil and gas development equipment constructed with multiple layers of anti-corrosion and pressure resistant materials. The inner lining of the riser will be made of materials with excellent corrosion resistance, such as polyethylene or polytetrafluoroethylene, to resist the corrosion that may be caused by the conveying medium, ensuring the long-term stable operation of the pipeline and the reliability of the system. The reinforcement layer is made of high-strength steel wire or fiber winding, providing compressive strength. The outer sheath layer protects the hose from external damage and corrosion, using materials such as polyurethane or polyethylene. Marine flexible risers can be divided into two main types based on the bonding state between the reinforcement layer, inner lining layer, and outer sheath layer. The first type is non adhesive flexible pipes, whose reinforcement layer is not bonded to the inner lining layer and outer sheath layer. This design allows for relative sliding between the layers of the riser during bending, thereby improving the flexibility of the pipeline and its ability to adapt to underwater variable loads. The second type is adhesive flexible pipes, whose reinforcement layer is tightly bonded to the inner lining layer and outer sheath layer, forming a whole. This structure enhances the compressive strength and fatigue resistance of the pipeline, significantly improving its service life and reliability.

In the manufacturing process of non adhesive flexible pipes, metal layers and polymer materials are composite, and relative displacement is allowed between each layer. This design endows the pipeline with excellent adaptability and can flexibly cope with complex seabed environments and dynamic changes. Relatively speaking, adhesive flexible pipes use special materials to directly bond the inner and outer layers into a sturdy pipe wall structure, providing higher strength and durability. This design is particularly suitable for engineering projects that require higher strength or are constructed in special environments. The advanced non adhesive flexible pipe technology developed by Kongsberg Offshore is specifically tailored for subsea oil and gas production. These pipelines are composed of multi-layer steel wire rope sleeves and high-performance anti-corrosion and compressive materials, with excellent design and the ability to operate stably in high-pressure and harsh marine environments in the deep sea.

Currently, research on non adhesive flexible pipes in the ocean focuses on the impact of connecting accessories on the lifespan and fatigue behavior of pipes in the overall system. Due to the fact that the structural layers inside the pipeline are composed of different materials, there are significant differences in their mechanical and thermodynamic properties. After prolonged operation, these structural layers may experience varying degrees of damage and failure, which can seriously affect the load-bearing capacity and expected service life of the pipe. TechnipFMC has developed the FRL17, FRL22, and FRL27 series of non adhesive flexible pipes. Their flexible pipe products have been applied to multiple deep-sea oil and gas projects, including providing 10 non adhesive flexible pipes for transporting oil and gas from subsea wellhead to floating production, storage, and unloading vessels in the Carabo oil field in Brazil. Twelve non adhesive flexible pipes were applied in the Cantarel oilfield in Mexico to transport oil and gas from subsea wellhead to onshore processing facilities.

To address the challenges of corrosion and fatigue failure that non adhesive flexible pipes may encounter in the development of deep and ultra deep water oil and gas fields, adhesive flexible pipes have emerged. Adhesive flexible pipes and non adhesive flexible pipes have similarities in structure, both of which are composed of multiple layers, including a skeleton layer, rubber buffer layer, reinforcement layer, and end connection structure. However, the fundamental difference between the two lies in the fact that during the manufacturing process of adhesive flexible pipes, physical extrusion and vulcanization processes are used to ensure tight adhesion between layers, thereby avoiding relative displacement between layers during operation. This not only improves the corrosion resistance of pipelines under dynamic loads and complex seabed environments, but also helps to reduce the risk of fatigue failure, making it more suitable for the harsh environment of deep water oil extraction. Strohm is a pioneer in adhesive flexible pipes, developing GL-PE, CL-PA12, and CL-PVDF pipes suitable for different needs, achieving the dual goals of production demand and cost control through flexible material combinations.

Although adhesive flexible pipes are relatively simple in structure, when considering key factors such as their applicable environment, safety performance, and intelligent systems, it can be found that the core technology behind them still needs to be continuously developed and mastered. At present, there are several enterprises in China, such as Wudi Haizhong Flexible Pipe Manufacturing Co., Ltd. and Hebei Hengantai Oil Pipe Co., Ltd., which have the manufacturing capability for adhesive flexible pipes. However, currently the application depth of domestically produced marine flexible pipes is limited to below 500 meters, with an external pressure resistance of 4.0 MPa and a tensile strength of 60 tons. They are mainly used in shallow sea areas, and their strength and structural design have not yet met the standards required for deepwater oil and gas export.

1.3 LNG Low-temperature Hose

With the increasing global demand for clean energy, LNG, as an efficient and environmentally friendly form of energy, its stability and safety during transportation and distribution have become particularly critical. Therefore, hoses with excellent flexibility and low temperature resistance are crucial to ensure the safe and stable transportation of LNG under extreme low temperature conditions of -162℃. LNG low-temperature hoses are not only an important technological foundation for the widespread application of clean energy, but also a key area for promoting the transformation and upgrading of China's energy industry and the development of offshore oil and gas. However, the design and construction of LNG low-temperature hoses are very complex, as they are subjected to long-term effects from low-temperature media and multiple loads such as wind and waves at sea. Once the hose is structurally damaged, it may lead to serious leakage accidents. Therefore, the mechanical performance of LNG hoses in ultra-low temperature environments is crucial for their safe operation. According to the requirements of different application scenarios, LNG low-temperature hoses are mainly divided into two types: suspended and floating (Figure 3), each form has been optimized for specific usage conditions to meet the needs of safe and efficient LNG transportation.

Suspended span LNG low-temperature hoses are mainly used for ship to ship LNG transfer operations and nearshore LNG transmission systems. With the promotion of the "green shipping" policy, the shipbuilding industry is undergoing a transformation from fuel to natural gas, which has promoted the widespread application of suspended LNG low-temperature hoses. This hose demonstrates significant flexibility in terms of fuel filling volume and filling speed, meeting the needs of practical engineering. In order to solve the shaking problem that LNG low-temperature hoses may encounter during offshore operations and extend their service life, Guttering B.V. has adopted innovative solutions. The company has introduced ultra-high relative molecular weight polyethylene (UHMWPE) film into the inner lining of the hose. This material is known for its excellent mechanical strength and wear resistance, effectively improving the overall performance and durability of hoses. Through this design, the suspended LNG low-temperature hose can better adapt to the offshore operating environment and provide safer and more efficient LNG refueling services for ships. As of the end of 2022, the suspended span LNG low-temperature hoses carried by relevant ships are all produced by foreign companies.China's suspended span LNG low-temperature hose technology is in the early stage of rapid development and has made significant research progress. China National Offshore Oil Corporation (CNOOC) has achieved breakthrough results in this field, successfully developing a 203.2 mm (8 inch) suspended LNG low-temperature hose and its supporting equipment.This achievement marks a solid step for China in the field of LNG low-temperature hose technology. In 2023, China National Offshore Oil Corporation (CNOOC) successfully configured this independently developed hose system on the offshore oil 301 refueling vessel. This is not only the first engineering application of domestically produced LNG low-temperature hoses, but also an important milestone in China's path towards LNG refueling technology autonomy. This progress not only demonstrates China's technological strength in the field of LNG low-temperature hoses, but also makes positive contributions to promoting the widespread application of clean energy in China and the transformation and upgrading of the energy industry.

The floating LNG low-temperature hose is suitable for the LNG string unloading method, and is in a floating state between two hulls. It is a key core equipment for long-distance transportation between FLNG and LNG ships.Since 2000, a few foreign companies such as TechnipFMC, Nexans, Trelleborg, and Dunlop have started developing this type of low-temperature hose and have long monopolized related core technologies. To meet the requirements of LNG unloading under harsh sea conditions, the LNG floating low-temperature composite hose (COOLTM) produced by SBM Offshore in the Netherlands has proposed the concept of "pipe in pipe", greatly enhancing the overall insulation and fatigue resistance of the hose. Although there are currently no gas fields with harsh sea conditions internationally that require FLNG string unloading, the environmental adaptability and operational flexibility of floating LNG low-temperature hoses can greatly improve the operating window and FLNG working days for the development of marginal gas fields in the South China Sea, achieving cost reduction and efficiency increase in offshore oil and gas field development.In order to solve the technological blockade imposed by foreign companies on China, CNOOC has established a multi-scale analysis and optimization design theory for the multi-layer structure and overall linear integration of ultra-low temperature composite hoses, and completed the trial production and industrial testing of 8-inch floating LNG low-temperature hoses.This marks that China has the ability to develop a complete set of LNG low-temperature hose transportation systems, laying the foundation for the assembly and engineering application of low-temperature dynamic flexible composite pipeline equipment systems, and breaking the technological monopoly of foreign countries.

2. Challenges faced by the Application Technology of Marine Flexible Pipes

2.1 Submarine Flexible Pipes

1) Insufficient accuracy of buckling analysis and difficulty in prevention and control

In the transportation of deepwater oil and gas, high-temperature and high-pressure methods are commonly used, with a maximum temperature of 177℃ and a maximum pressure of 70 MPa. In addition, due to the usually low underwater water temperature (3-5℃), the thermal load caused by temperature changes along the pipeline can lead to the accumulation of temperature stress.The existing calculation methods are difficult to achieve temperature stress calculation at any connection spacing, which poses significant challenges to the design of anchoring connection spacing for inner and outer pipes. In addition to the difficulty in calculating the temperature stress of high-temperature pipelines, the non straightness and nonlinear characteristics of lateral constraints during pipeline laying further increase the complexity of buckling analysis of high-temperature pipelines.Many anti buckling designs and solutions are committed to releasing axial forces within pipelines by inducing numerous, long wavelength, and low amplitude minor buckling, in order to prevent severe and destructive buckling of local pipelines.Although these methods are theoretically feasible, in the actual implementation process of pipeline engineering, determining the acceptable degree of buckling of the pipeline, the appropriate release of axial force after buckling, and ensuring the stability between various buckling parts of the pipeline to avoid buckling convergence and mode transition are still challenges that need to be overcome when preventing buckling.

2) The theory of pipe-soil interaction is not yet perfect.

There are still many issues to be studied in the field of pipe-soil interaction, such as the constitutive model of soil mechanical behavior, the coupling characteristics of soil and pipelines, the influence of pore water pressure changes on pipeline stability, and the failure mechanism of soil entering a plastic state, which need to be further solved. Due to the complex mechanical processes such as soil failure and soil arching effect that may be caused by pipe-soil interaction, the establishment of pipe-soil interaction models faces considerable challenges. Under different conditions such as pipe weight, pipe diameter, and burial depth, the behavior of soil cannot be simply regarded as a rigid body, nor can it be described solely by the Coulomb friction model. It is necessary to comprehensively consider the influence of multiple factors such as the failure mode of soil, the evolution of soil arching effect, consolidation effect, and changes in shear strength.

3) Pipeline insulation methods face technical bottlenecks in ultra deep water environments.

For deep-water oil pipelines with long-distance reconnection, especially when the wax content in the transported oil is high, it is inevitable that wax will precipitate on the inner wall of the pipeline under the influence of external low-temperature conditions.If natural gas or oil gas two-phase flow is being transported, there may also be the formation of hydrates, which can lead to pipeline blockage.At present, various pipeline insulation methods have been adopted in engineering. Experience has shown that the effect of passive insulation measures is not ideal, and some active heating measures are needed to prevent wax precipitation and the formation of hydrates.However, traditional thermal fluid tracing technology has the problem of low heating efficiency, which makes it difficult to meet the heating needs of long-distance pipelines; Indirect electric heating technology requires the installation of heating equipment before pipeline laying, which is not applicable to pipelines that have already been put into production.At present, China's independent development of deep-sea oil and gas fields is still in its early stages, and research on the electric heating technology of submarine long-distance pipelines is not sufficient, especially considering the possible heat sources inside the pipelines, the study of their heat transfer characteristics is particularly important.