Submarine power cables in offshore wind farm operate within a complex multiphysics environment. Despite being designed to be both flexible and robust though, their mechanical characteristics are susceptible to variations of thermal field. Bending studies of submarine power cables present challenges rooted in geometry complexity, component contact, and material non-linearity, compounded by the intricate stick–slip mechanism. The difficulty is further intensified when incorporating the thermal impact on material and contact properties. This paper presents a three-dimensional Representative Volume Element (RVE) model for predicting the nonlinear bending stiffness of three-core submarine power cables. The RVE model, developed with constant curvature and periodic boundary conditions, incorporates dashpots to address the stick–slip challenges associated with cable bending. This modeling approach minimizes the required cable length for bending analysis, significantly reducing computational costs. Validation against the bending test of a three-core cable at room temperature, alongside comparison with a 3D full-scale finite element (FE) model, demonstrates the efficiency and accuracy of the proposed RVE approach. Furthermore, the study explores the thermal effect on cable bending, highlighting the capabilities of the proposed RVE model in facilitating thermal–mechanical coupled flexural analysis of submarine power cables. This research contributes to advancing understanding and optimization of submarine power cable design for offshore applications.

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Test on a small scaled model is an effective approach to predict the dynamic response of full scale structure under blast loadings. However, the geometric dimensions of specimens cannot simply comply with complete geometrical similarity due to manufacture or test restrictions. It would result in the difference structural performance between the full and small scaled models. This paper proposed a corrected similarity relationship of the dynamic behaviour between prototype and replica of stiffened plates subjected to blast load, in which both the thickness of the plate and the configuration (cross-sectional shape) of stiffeners are distortedly scaled-down (double distorted geometric scaling factors). Firstly, based on the mesh convergence study and comparing with results from experimental tests, a numerical method in predicting the confined blast load and dynamic response of structure was verified, which provides a reliable means to determine the dynamic behaviour of stiffened plate designed by the corrected similarity criterion of this paper. Then, the influence of altering the stiffener configuration on the dynamic response of stiffened plates was analysed and on the basis of it, a criterion for scaling the stiffener is proposed to help design a stiffener-distorted model from prototype structure. In addition, a method for scaling the double-parameter distortedly small scaled model is proposed to predict the dynamic response of the prototype. Finally, two sets of examples of both the small size and prototype stiffened structures subjected to blast load were analysed by using the presented method. It is shown that the replica developed by applying the present method is able to accurately predict the behaviour of the full-size stiffened plates, even when the thickness of the plate and the configuration of the stiffeners are distortedly scaling down with different factors.

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A flexible riser is a multi-layered pipe device which enables deep-water production by connecting seabed facilities to floating vessels. To withstand huge hydro-static pressure, it is required to have strong collapse capacities. At present, the collapse capacity of a flexible riser is designed based on a "wet collapse'' concept, in which the outer sheath is damaged and the seawater has flooded the annulus. For a given water depth, the hydro-static collapse design of a flexible riser needs to be confirmed by a wet collapse calculation. Calculating the wet collapse pressure of flexible risers is always challenging since the layers within risers are different in geometries and materials. Such a complex cross-sectional configuration makes the numerical simulation become the main approach for collapse analysis, which is quite time-consuming for the design stage. As the production is moving towards ultra-deep water fields, the design of new riser product is being required to achieve a balance between collapse resistance and self-weight. Therefore, there is a demand to develop an efficient tool to facilitate the collapse design. In view of this, this thesis presents an integrated analytical model, which addresses three challenges in the wet collapse analysis of flexible risers: the interlocked layer profiles, the geometric imperfections and the pipe curvature. The integrated analytical model contributes to the hydro-static collapse design of flexible risers, which can provide the designers a rapid feedback for their designed cross-section configuration. In our research work, the whole collapse analysis conducted by the proposed analytical model takes less than one hour to finish the prediction. Most of the time is spent on modeling the metallic layers for obtaining their equivalent properties. The actually wet collapse calculation given by this analytical model takes only a few seconds. By contrast, the numerical simulation requires 8-12 hours for modeling and consumes 2-3 days on average to complete one job. For companies that are developing new riser product to enable the ultra-deep water production, this proposed integrated analytical model can effectively facilitate the collapse design of new riser products. @en

Flexible risers are designed with strong anti-collapse capacities which enable them to operate in deep-water reservoirs. However, this anti-collapse capacity is susceptible to the pipe curvature in the flooded annulus condition. For the curved riser sections within the touch-down zone, significant reduction of collapse capacity can occur once their external sheaths are worn out by the seabed, resulting in the so-called “wet collapse”. Mostly, wet collapse studies of curved flexible risers are performed through costly numerical simulations since there are no alternative analytical approaches. In view of it, this work presents an analytical model for predicting the wet collapse pressure of curved flexible risers. The analytical model is developed based on a spring-supported arch model that from our previous work, which is able to take the curvature-induced factors into account. With the stability theories of arched structures, the wet collapse pressure of curved flexible risers can be solved. To verify this analytical model, 3D full FE models are employed. The critical collapse pressures predicted by these two kinds of approach are in good agreement, indicating this proposed analytical model can be an useful tool to facilitate the collapse analysis in pipe design stage.

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As offshore hydrocarbon production moves towards ultra-deep water, flexible risers have to withstand the huge hydro-static pressure without collapse. They are designed with strong collapse capacities, allowing them to operate under the condition where their annuli are flooded by the seawater. However, initial imperfections can weaken the collapse capacity under such a flooded condition, triggering the so-called “wet collapse”. Two common initial imperfections, the carcass ovality and the radial gap between the carcass and pressure armor, would reduce the collapse strength of flexible risers significantly. Mostly, collapse analyses are performed through numerical simulations, which are less feasible for the design stage of flexible risers comparing with analytical models. To date, there are few analytical models available in public literature to predict the wet collapse pressure of flexible risers accounting for initial ovality and gap. To meet this demand, an analytical model is established in this paper to address these issues. This model is developed as a spring-supported arch, solving the collapse pressure with stability theories of ring and arched structures. This analytical model is verified by numerical simulations, which gives prediction results that correlate well with the numerical ones.

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Flexible risers that for deep-water production are designed with strong collapse capacity. However, this collapse capacity is susceptible to pipe curvature in the flooded annulus condition. For the curved riser sections within the touchdown zone, significant reduction of collapse capacity can occur once the external sheaths are worn out, resulting in the so-called ‘wet collapse’. Therefore, the collapse capacity of flexible risers should be well-determined during design stage. Mostly, curved collapse analyses are performed through costly numerical simulations since there is no well-established analytical models . This study focuses on the wet collapse mechanism of curved flexible risers, aiming to pave the way for the development of analytical solutions. 3D full FE models were adopted to gain insight into the wet collapse mechanism of curved risers, which had been verified by the data from public literature. The investigation indicates that the deformed cross-sectional shape of the carcass is the dominant factor of the curvature effect.@en

A flexible riser is a flexible pipe that transports materials between seafloor and topside structures. As oil and gas production heads to water depths greater than 3000 meters, huge hydrostatic pressure may cause the collapse failure of flexible risers. Generally, the collapse strength of a flexible riser is designed by considering the effects of initial imperfections, e.g., ovality of the carcass, and radial gap between the carcass/liner and pressure armor. These two imperfections may cause a significant reduction in the collapse strength of flexible risers under the flooded annulus condition. However, there are few analytical models available in the public literature that could take those factors into account. In this paper, an analytical approach is presented to predict the critical collapse pressure of the flexible risers with initial imperfections. The analytical results were compared with the numerical simulation, which showed reasonably good agreement.@en

Flexible risers are being required to be installed in a water depth of over 3000 m for fewer remaining easy-to-access oil fields nowadays. Their innermost carcass layers are designed for external pressure resistance since the hydrostatic pressure at such a water depth may cause the collapse failure of flexible risers. Determining a critical collapse pressure for the carcass is of great importance to the whole structural safety of flexible risers. However, the complexity of the carcass profile always makes FE analysis computational intensive. To overcome that problem, the treatment of the interlocked carcass as an equivalent layer is adopted by researchers to accelerate the anti-collapse analyses. This paper presents an equivalent layer method to enable that treatment, which obtains the equivalent properties for the layer through strain energy and membrane stiffness equivalences. The strain energy of the carcass was obtained through FE models and then used in a derived equation set to calculate the geometric and material properties for the equivalent layer. After all the equivalent properties have been determined, the FE model of the equivalent layer was developed to predict the critical pressure of the carcass. The result of prediction was compared with that of the full 3D carcass model as well as the equivalent models that built based on other existing equivalent methods, which showed that the proposed equivalent layer method performs better on predicting the critical pressure of the carcass.

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Flexible riser is a key enabler for the oil and gas production in ultra-deep water which transports production fluids between floating production systems and subsea wells. As oil and production heads to water depths in excess of 3000 m, high hydrostatic pressure has been one primary challenge facing the riser operators. Excessive hydrostatic pressure may cause collapse failure of flexible risers and thus predicting the critical collapse pressure is of significant importance to their anti-collapse design. Collapse is a complex phenomenon related to the material properties, the geometry of the pipe and its overall surface topography and, therefore, makes the prediction of critical pressure challenging. Related prediction approaches of flexible risers have been developed for decades, yet a comprehensive review of their predictive capabilities, efficiency and drawbacks is lacking. This paper reviews the recent advances on collapse studies of flexible risers and highlights the gaps in existing prediction methods, aiming to facilitate the current anti-collapse design and be a baseline for future utilization of flexible risers in deeper water expansion.

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Flexible risers are one kind of flexible pipes that transport fluid between subsea facilities and topside structures. This pipelike structure consists of multiple layers and its innermost carcass layer is designed for external hydrostatic pressure resistance. For the flexible risers used in ultra-deep water fields, the critical collapse pressure of the carcass layers is one of the dominant factors in their safety design. However, the complexity of the interlocked carcass design introduces significant difficulties and constraints into the engineering analysis. To facilitate the anti-collapse analysis, equivalent layer methods are demanded to help construct an equivalent pipe that performs a similar collapse behavior of the carcass. This paper proposes a strain energy based equivalent layer method which trying to bridge the equivalence between those two structures by considering equivalent geometric and material properties for the equivalent layer. Those properties are determined through strain energy equivalence and membrane stiffness equivalence. The strain energy of the carcass is obtained through numerical models and is then used in a derived equation set to calculate the equivalent properties for the equivalent layer. After all the equivalent properties have been determined, an equivalent layer FE model is built and used to predict the critical pressure of the carcass. The prediction result is compared to that of the full 3D carcass model as well as the equivalent models that built based on other existing equivalent methods, which shows that the proposed equivalent layer method gives a better performance on predicting the critical pressure of the carcass.@en

Flexible riser is a device which transports production fluids between floating vessels and subsea wells. With fewer remaining easy-to-access oil fields nowadays, flexible risers are being required to be installed in a water depth of over 3000m. However, the hydrostatic pressure at such a water depth may cause the collapse of flexible risers, and therefore predicting the critical collapse pressure is of great importance to their design. Riser collapse is a complex phenomenon related to material properties, geometry of the pipe and its overall surface topography and, therefore, makes the prediction of critical pressure challenging. Collapse prediction approaches of flexible risers have been developed for decades, yet a comprehensive review on their predictive capabilities, efficiency and drawbacks is lacking. In this paper, the recent advances on collapse studies of flexible risers are reviewed, which summarizes the methods developed for critical pressure prediction and highlights the related gaps in current research. This review aims to facilitate the current anti-collapse design and be a baseline for future utilization of flexible risers in deeper water expansion.

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In this paper, the recent advances on collapse studies of flexible risers are reviewed. Collapse refers to radial buckling of flexible riser caused by excessive hydrostatic pressure. Since there is a constant push from oil & gas industry to extend deepwater exploitation beyond 3000m, hydrostatic collapse has been one primary challenge facing the riser operators. Riser collapse is a complex phenomenon related to material properties, geometry of the pipe and its overall surface topography and, therefore, leads to inherent unpredictability of the exact collapse pressure. Collapse prediction approaches of flexible risers have been developed for decades, yet a comprehensive review on their predictive capabilities, efficiency and drawbacks is lacking. This paper concentrates on these collapse predictive techniques of flexible riser in ultra-deepwater fields and the current gaps that exist in offshore technology, aiming to help develop the current anti-collapse design and be a baseline for future utilization of flexible risers in deeper water expansion.@en