Advanced Research in Flexible Risers and Pipelines

1. Introduction

Over the past few decades, with the continuous expansion of offshore oil and gas resource development into deepwater and ultra-deepwater environments, slender structures such as flexible risers, subsea pipelines, mooring cables, and underwater cables have served as core links connecting offshore platforms, subsea production systems, and onshore terminals. They have been widely applied in the fluid transportation and energy transmission between various offshore production units, becoming indispensable key equipment for deep-sea resource development [1]. Benefiting from prominent characteristics such as convenient installation, excellent fatigue resistance, and high adaptability, such slender structures have demonstrated significant advantages in both technical feasibility and economic efficiency and are regarded as efficient solutions to promote the advancement of marine resource development towards the far-reaching sea [2].

However, multiple loads in the marine environment, such as strong currents, massive waves, complex terrain, corrosive media, extreme temperature and pressure, and internal flow coupling effects, combined with extreme operating conditions including typhoons, internal waves, and subsea geological hazards, pose severe challenges to the mechanical response characteristics, structural integrity, and long-term service safety of slender structures such as flexible risers and subsea pipelines [3,4,5,6]. This series of technical challenges not only imposes higher requirements on structural design theories, numerical simulation methods, and engineering test technologies but also provides important opportunities for technological innovation and disciplinary development in the field of marine engineering. They continuously drive researchers and engineers to conduct systematic exploration and breakthrough research in aspects such as material modification, structural configuration optimization, intelligent monitoring and operation, and maintenance technologies.

At present, research on slender structures such as flexible risers and subsea pipelines has formed a complete system covering theoretical research, numerical simulation, and model testing, achieving systematic progress. In terms of theoretical research, the Morison equation is generally adopted to calculate the hydrodynamic loads on slender structures, and improved finite element methods are combined with S-N curve approaches to conduct in-depth analysis on the global dynamic response and fatigue damage of marine slender structures under the combined action of wind, waves, and currents [7,8,9,10,11]. In the research on local failure mechanisms, based on elastoplastic mechanics and shell theory with nonlinear buckling theory as the core analytical framework, energy principles are widely applied to establish governing equations, thereby revealing the local buckling failure mechanisms of flexible risers, subsea pipelines, and other structures [12,13].

Numerical simulation technology is characterized by equal emphasis on global and local analysis. At the level of global dynamic response analysis, professional marine engineering software such as OrcaFlex (version 11.6b), relying on its powerful nonlinear time-domain finite element analysis capabilities, is widely used to simulate the global dynamic response of structures in complex marine environments [14,15,16] and can achieve coupling and integration with hydrodynamic calculation software such as ANSYS AQWA (version 2025 R2) and MOSES (version 2025). At the level of local refined analysis, general finite element software such as ABAQUS (version 2025) and ANSYS (version 2025 R2) plays a key role. They are mainly employed to construct local refined models of multi-layer structures such as unbonded flexible risers/pipes so as to explore the stress distribution, contact behavior, and local failure modes (e.g., critical collapse pressure, tensile stiffness, minimum bending radius, etc.) of each layer under various load conditions [17].

Despite significant advancements in theoretical research and numerical simulation, both often involve numerous assumptions, and the reliability of their results still needs verification. Therefore, model testing remains an important approach for the marine engineering community to obtain accurate results. However, offshore platforms and their riser/mooring systems typically have large spatial scales. Even when models are constructed using conventional scale ratios (e.g., 1/50 to 1/100), their sizes often exceed the carrying capacity of existing test facilities (such as the water depth and flow velocity conditions of test tanks), and the test costs are high. This makes it difficult to carry out full-model tests on the global dynamic response of flexible risers/moorings. Therefore, existing model tests focus more on the research of local phenomena, such as local buckling failure [18] and vortex-induced vibration (VIV) response [19,20]. Such tests involve relatively small spatial scales and are more feasible to implement in laboratories.

This Special Issue, entitled “Advanced Research in Flexible Risers and Pipelines”, aims to promote international academic development and technical exchange in the field of marine slender structures such as flexible risers, subsea pipelines, mooring cables, and underwater cables. The Special Issue collects cutting-edge research achievements and practical reviews from experts in different countries and regions around the world, and presents an in-depth discussion on the key scientific issues and engineering challenges currently existing in this field. Despite significant progress in research, such marine slender structures still face multiple challenges in practical applications: first, the high computational cost of numerical models makes it difficult to meet the needs of real-time monitoring; second, the scarcity of experimental data, especially the lack of full-scale test data; third, the insufficient simulation of multi-physics coupling mechanisms (such as thermo-fluid–structure coupling), which affects the accurate prediction of the global structural behavior; and fourth, the long-term impact mechanisms caused by material aging, fatigue damage, and climate change all urgently need in-depth research. Future research directions should include the development of efficient reduced-order models and AI-aided tools, strengthening long-term on-site monitoring, exploring the structural response in extreme environments (such as the Arctic region), integrating sustainable materials (such as environmentally friendly polymers), and promoting the application of digital twin technology for full-life cycle management.

2. Published Papers

Wu et al. (Contribution 1) systematically investigated the cyclic tensile shakedown behavior of flexible risers by incorporating armor wire winding-induced residual stresses into an ABAQUS numerical model. Simulations accurately characterized the spatially inhomogeneous residual stress distribution across wire cross-sections. The team quantified the influence of initial residual stresses on the riser’s cyclic tensile response. It reveals that within the lower load range (0–16.1 kN), the armor wire cross-section tends to attain a shakedown state with increasing loading cycles; in contrast, within the higher range (0–30.2 kN), the cross-sectional strain accumulates progressively per loading–unloading cycle, creating a ratcheting effect. This highlights the necessity of incorporating such shakedown behavior into the riser design criteria.

Saneian et al. (Contribution 2) developed and compared three ABAQUS-based finite element models (a simplified two-dimensional (2D) axisymmetric model, a fully detailed three-dimensional (3D) helical model, and an efficient periodic boundary condition (PBC) model) to address internal pressure-induced bursting of flexible risers’ pressure armor layer. The results demonstrate that the PBC model, which enforces structural periodicity to simulate an infinitely long pressure armor layer, achieves an optimal trade-off between predictive accuracy and computational efficiency—yielding burst pressure predictions within 1% of analytical solutions while reducing computational expense by more than 15-fold compared to a full 3D helical model. By effectively eliminating end effects, the PBC model can deliver high-fidelity stress distributions that closely reflect real structural behavior. This study further underscores the importance of capturing helix-induced stiffness and contact interactions, as evidenced by the significant underestimation of collapse pressure in the oversimplified 2D axisymmetric model.

To resolve traditional numerical model inaccuracy in predicting the bending stiffness of flexible pipes, Costa et al. (Contribution 3) developed a finite-difference model integrated with a return-mapping algorithm to study the response of a 2.5″ flexible pipe under combined axisymmetric/cyclic bending loads, validated against non-degraded samples and samples degraded for 18 months (flooded with CO₂-saturated water at 1 bar and 40 °C). The model aligned well with experiments, capturing hysteretic behavior driven by initial adhesion (0.9 MPa) and a friction coefficient (0.05). The model reveals that higher adhesion and friction increase bending forces until the no-slip condition occurs, while energy dissipation peaks at partial slip; degraded samples retain an unchanged global bending response but require updated fatigue curves. The 2D axisymmetric model’s limitations in ignoring interlayer interactions are highlighted, underscoring the crucial role of interlayer mechanics in flexible pipe performance.

Liu et al. (Contribution 4) proposed a comprehensive analytical–numerical framework to clarify the coupled failure mechanism of pressure- and tensile-armor layers in unbonded flexible risers. They established a tension–pressure coupled failure boundary for ultimate-limit-state design with theoretical verification. The research shows that pre-applied internal pressure barely affects axial stiffness, but initial axial tension notably boosts burst resistance (35% higher failure pressure at 500 kN tensile load). During tensile failure, both inner and outer tensile-armor layers assume primary load-bearing responsibility; however, the internal tensile-armor layer experiences higher stresses compared to the external counterpart. Under combined loading, the pressure-armor layer carries most of the load, which first undergoes a stress drop then re-loads until burst; after burst, the inner tensile-armor layer immediately takes over as the primary pressure-bearing component. This work provides a novel tension–pressure coupled failure boundary for ultimate-limit-state design, enabling more accurate prediction of failure thresholds and contributing to safer, more cost-effective deepwater riser engineering.

You et al. (Contribution 5) explored the impacts of internal fluid conditions (closed or constant pressure) and fluid types (incompressible or compressible) on the axial stiffness of fiber-reinforced flexible pipes via numerical simulation and experimental validation. Pioneering the fluid cavity method, they developed a fluid–structure coupling model to overcome the limitations of the traditional uniformly distributed load (UDL) assumption and adopted a hyperelastic constitutive model for an accurate description of rubber’s nonlinear behavior. With only a 1.3% discrepancy in experimental validation, the UDL assumption is applicable to gas-filled pipes (both in the closed and constant pressure states) and liquid-filled pipes in the constant pressure state; however, the incompressibility of the filled liquid significantly enhances the pipe’s axial stiffness, rendering the UDL approximation invalid for liquid-filled pipes in the closed state.

To address the limitations of iterative simulation/experiential tuning-based conventional methods, Fu et al. (Contribution 6) proposed an empirical model to predict optimal valve opening for offshore riser severe slugging elimination. Its core innovation integrates two empirical closures (valve resistance factor equivalence between risers and simple vertical pipes under slugging elimination; control target quantification as dual-frequency fluctuations) into a framework linking measurable parameters (e.g., pressure) to valve characteristics (e.g., flow coefficient). Using a non-slip gas fraction model and real valve data, it achieved high predictive accuracy (average absolute error of +0.01%), significantly outperforming industry-standard tools like OLGA software (+4.91% before tuning and +0.08% after tuning). Validated via laboratory experiments and an S-shaped riser field case (deviation < ±2%), it enables valve selection/operational optimization in the design stage without complex transient simulations.

Unlike most prior studies focusing on small-diameter pipes (<12 inches) or simplified boundary conditions, Wang et al. (Contribution 7) investigated the mechanical properties of 24-inch X65+Alloy625 metallurgically clad pipes via combined experimental and numerical approaches. This study adopted an integrated multi-scale, multi-physics methodology, correlating macroscopic mechanical properties with microstructural characteristics and quantifying the influence of initial ovality and residual stresses on bending performance. Digital image correlation (DIC) was employed to measure full-field strain, accurately capturing the complete sequential failure process—from uniform deformation between the base and clad layers to interfacial debonding (at an average strain threshold of 34.17%), followed by clad layer fracture and ultimately base layer fracture. The proposed finite element model incorporated measured initial ovality (0.407%) and manufacturing-induced residual stresses (54.58 MPa), ensuring simulation alignment with engineering reality.

Traditional methods predict the free-span fatigue damage of submarine pipelines assuming a constant span length, yet in practice, span length and position undergo spatiotemporal variations induced by local scour and sand wave migration. Jiang et al. (Contribution 8) proposed a probabilistic approach to account for such variations and predict the pipeline free-span fatigue life. A validated VIV numerical model based on Euler–Bernoulli beam theory and nonlinear seabed–pipeline contact was developed. The approach’s core lies in establishing a mathematical transformation between span length and fatigue life: span length uncertainty from local scour is characterized via truncated Gaussian, Rayleigh, and uniform distributions to derive the probability density function of fatigue life. The findings demonstrate that compared with the fatigue life of a fixed span, the proposed approach leads to an increase in the fatigue life by about ten times.

To investigate the overbending-induced unlocking of the pressure armor layer, Wei et al. (Contribution 9) developed a three-layer finite element model implemented with uniform bending via pre-defined displacement and rotation loads and boundary constraints. This model accurately reproduced a true Zeta cross-sectional shape and helical structure of the pressure armor wire. In addition, by simplifying the complex carcass layer into an analytical cylindrical surface and adopting sophisticated meshing and coupling constraint strategies, it achieved an effective simulation of the complex contact nonlinearity unlocking problem. They pointed out that the thickness and internal diameter of the armor wire profile may be crucial variables for the analytical model of unlocking curvature radius, and internal pressurization reduces the difficulty of unlocking to a limited extent.

Vieira and de Sousa (Contribution 10) developed 2D and 3D finite element permeation models in ANSYS (Release 2023 R1) to address oversimplified pipeline geometry in existing permeation models. These models use fugacity (instead of concentration) to describe mass transfer, incorporate radial temperature gradients, and adopt a decoupled analysis approach for heat and mass transfer. Compared to traditional concentration-based models, fugacity can more accurately characterize non-ideal gas behavior, preserve continuity at material interfaces, and eliminate computational complexity from concentration discontinuities. Results show that under dry annulus conditions, annulus fugacity distribution is uniform, and 2D/3D model results are highly consistent (complex geometric effects are insignificant). In flooded annulus conditions, water causes uneven fugacity distribution; notably, the 3D model captures the critical influence of the metallic armor’s helical geometry—an effect the 2D axisymmetric model misses—leading to substantial discrepancies between the two models.

Malta et al. (Contribution 11) proposed a decoupled numerical approach to analyze the mechanical response of multi-layer flexible pipelines buried in clay under large lateral soil displacement. Traditional methods, limited by computational complexity, over-simplify pipelines (as rigid sections) or soil (as elastic springs), resulting in insufficient calculation accuracy. This proposed approach decomposes the pipe–soil interaction analysis into two core steps: first, using the coupled Eulerian–Lagrangian (CEL) ABAQUS (version 2016) model to simulate the soil large deformation and obtain the soil pressure distribution on the pipe section resulting from the pipeline’s interaction with the seabed; second, importing the pressure data into an ANSYS-built refined pipeline model to quantify cross-sectional deformation and local stress. Additionally, parametric studies were conducted to explore the influence of soil strength, the pipeline’s internal diameter, and burial depth on the pipeline failure risk. A collapse analysis was performed based on the deformed cross-section to determine the critical loads at the maximum operational depth. The study demonstrates that soil strength and internal diameter critically govern pipe failure and the risk of collapse.

Through a systematic review, Schnepf and Gudmestad (Contribution 12) systematically expounded the core elements in the configuration design of dynamic inter-array power cables for floating offshore wind turbines (FOWTs). This study pointed out that the primary objective of the design is to achieve optimal performance and cost minimization by optimizing power cable selection, overall configuration design, and the integration of specialized components and ancillaries on the premise of ensuring safety and reliability. The authors emphasized that design constraints (such as tension/compression limits, minimum bend radius, and required minimum fatigue life) and environmental conditions (especially wind, waves, currents, and the impacts of marine growth and climate change) are the key drivers of the optimization process, while local specific conditions determine which constraints take the highest priority.

A comprehensive review by Qi et al. (Contribution 13) provides valuable insights into cables and towed objects within ocean engineering towing systems. These towing operations involve a complex multi-body system comprising tugboats, cables, and towed objects that are subjected to environmental loads such as wind, waves, and currents. Current studies primarily employ numerical simulations (e.g., OrcaFlex software), theoretical models, and experimental methods (both reduced-scale and full-scale) to analyze motion responses, yet they often overlook the integration of mechanical response and structural strength with actual towing conditions. The authors identified key gaps, such as insufficient attention to cable behavior during initial towing stages, and proposed future directions, including mechanical response during the initial stage of towing, multi-tug towing experiments, composite fiber cable studies using finite element methods, and the structural optimization of towing-related components to enhance safety and efficiency.

Ruan et al. (Contribution 14) presented a systematic review of the latest research progress on lazy wave risers (LWRs), focusing on key issues including their structural characteristics, hydrodynamic loads, global responses, fatigue damage assessment, and structural optimization. This study elaborated on the applications of numerical simulation (e.g., CFD and finite element analysis), theoretical models (e.g., nonlinear large deformation beam theory), and experimental methods in the research on LWR global responses. It emphasized the importance of multi-physics coupling effects such as vortex-induced vibration (VIV) and wave–current interaction and identified deficiencies in current research regarding model accuracy, experimental validation, and the integration of multiple environmental factors. Finally, the authors called for the use of advanced algorithms (e.g., genetic algorithms) for fatigue life assessment and structural optimization to achieve the safe and efficient application of LWRs in deepwater oil and gas development, providing directional guidance for future research.