Umbilicals, which link top floaters and subsea devices, provide control functions through electrical cables and hydraulic remote transmission. These cables are considered the “lifeline” of subsea production systems for offshore oil and gas exploitation. Umbilicals should undertake self-weight and periodic loading during operation because of the severe conditions of the ocean environment. Heat is released to the umbilical body during power transmission in electric cables, which influences the mechanical properties and optical transmission in the cable. However, several sectional arrangements can be applied to a number of umbilical components. Thus, sectional layout design with multiple components should be treated as a multidisciplinary optimization problem. From the mechanical point of view, the umbilical structure should be designed with compact and symmetric layout to obtain an even probability of resistance to loads and reduce structural stress, thereby improving fatigue performance. In terms of thermal effect, these electric cables should be arranged to dissipate heat easily and avoid influence on functional and structural components. This study quantifies compactness, symmetry, and temperature distribution by introducing corresponding indices. A multidisciplinary optimization framework is then established. Particle swarm optimization (PSO) intelligent algorithm is adopted to perform optimization and obtain the optimal solution, which is superior to the initial design. The optimization design strategy is proven effective and efficient by a case study, which provides a reference for umbilical design.

A design method is presented, which aims at the optimal cross-sectional layout of the multilayer steel tube umbilical. The merit of the method is to improve the tension stiffness and the fatigue life of umbilical. An optimization model is developed for arranging each component layer of the layout by considering the symmetry of mechanical performances of components. Finite element analysis (FEA) is carried out to check the contact force and the radial deformation of different layout types and to confirm the optimal layout design.

Umbilical is an important equipment in the subsea production to supply a connection between the floater and the subsea well. Analyzing strength and fatigue behaviors under bending is a key requirement to assure safety. An analytical model is proposed for predicting the bending behavior of a steel tube wounded helically around a frictionless cylinder. A full three-dimensional (3D) finite element (FE) model of an umbilical is developed by considering the frictions and contacts among its components. The numerical results of the bending stress of a steel tube were validated against that of the analytical model. The impacts of friction coefficients on the bending stress, contact pressure, and friction stress have been further investigated by the established FE model.

Recently, the flexible cryogenic hose has been preferred as an alternative to exploit offshore liquefied natural gas (LNG), in which helical corrugated steel pipe is the crucial component with C-shaped corrugation. Parametric finite element models of the LNG cryogenic helical corrugated pipe are established using a three-dimensional shell element in this paper. Considering the nonlinearity of cryogenic material and large geometric structural deformation, mechanical behaviors are simulated under axial tension, bending, and internal pressure loads. In addition, design parameters are determined to optimize the shape of flexible cryogenic hose structures through sectional dimension analysis, and sensitivity analysis is performed with changing geometric parameters. A multi-objective optimization to minimize stiffness and stress is formulated under operation conditions. Full factorial experiment and radial basis function (RBF) neural network are applied to establish the approximated model for structure analysis. The set of Pareto optimal solutions and value range of parameters are obtained through nondominated sorting genetic algorithm II (NSGA-II) under manufacturing and stiffness constraints, thereby providing a feasible optimal approach for the structural design of LNG cryogenic corrugated hose.

A new type of umbilical cable named “high-current composite umbilical cable” is composed of electronic cables, optical cables, steel tubes, and structural strengthening components. It can be regarded as a key piece of industrial equipment in subsea production systems that provide control functions, high electric current, and hydraulic remote transmission. When it is oriented at a power supply with a relatively high rated current, power transmission will produce a lot of heat. Then, the cross-sectional temperature increases, which affects the performances of its material and mechanical responses. Therefore, electro–thermal–mechanical coupled analysis is critical for the cross-sectional design of the high-current composite umbilical cable. Accordingly, a multi-physics coupled analysis was performed based on two typical umbilical cable cross sections. Finite element models were established and subjected to electro–thermal analysis to obtain a temperature distribution of the two sections at different current capacities. Based on results of temperature field analysis, the section models were subjected to thermo–mechanical analysis. The results of the two types of analyses are compared and differences are discussed, which illustrate the multi-physics coupled effect cannot be neglected. The armored layers will relatively reduce the heat dissipation performance, but compared with the umbilical cable model without the armored layers, the model with double-armored layers is less affected by temperature, so its capacity of resistance external pressure is relatively better. The proposed coupled analysis methodology provides a new guidance for the design of the high-current composite umbilical cables.

Flexible pipes are distinctive multi-layer structures that are designed to resist different loads when they are utilized in severe deep-water environments. However, they lack a special structural layer to withstand torsion. Tensile armors mainly resist torque although they are designed to bear only tension with the consideration of torque balance. Especially, when a flexible pipe is loaded out from the cargo vessel into the installation vessel, twist angle could be accumulated at high level so that some failure modes will occur due to the large torsion. However, the failure mechanism is very complicated owing to the interaction effect between the different layers. First, the interaction mechanism between the layers of flexible pipes is analyzed under large torsion, and a few potential failure modes are identified, such as the tensile armors strength failure and inner structural layers collapse failure. In addition, to offer a quantitative prediction of the maximum allowable twist angle for flexible pipes, a mechanical model is set up to analyze their torsion behavior. The theoretical descriptions of the involved failure behaviors are investigated, and the theoretical methodology of the failure criteria for predicting the torsion resistance capacity is proposed. Finally, a numerical model is established through experimental verification. The numerical results illustrate that the theoretical prediction methodology is conservative, which can be used to predict the torsion resistance capacity of flexible pipes and to guide their operation and installation in engineering.