In this study, effects of damage levels of fiber ropes on the performance of a hybrid taut-wire mooring system are investigated. The analysis is performed using a numerical floating production storage and offloading (FPSO) model with a hybrid mooring system installed in 3000 m of water depth. An in-depth study was conducted using the numerical model, the dynamic stiffness equation of damaged fiber ropes, the time-domain dynamic theory, the rainflow cycle counting method, and the linear damage accumulation rule of Palmgren-Miner. Results indicate that, in a mooring line with an increasing damage level, the maximum tension decreases, while the offset of the FPSO increases. Particularly, when a windward mooring line failure occurs, in addition to the significant increase in the offset of the FPSO, the maximum tension, tension range, and annual fatigue damage levels of the remaining lines adjacent to the failed also increase significantly. The present work can be of great benefit to the evaluation of the offset of the floating platform, the tension response, and the service life of the hybrid mooring systems.

Vortex-induced vibrations (VIV) can lead to fast accumulation of fatigue damage and increased drag loads for slender marine structures. VIV responses mainly occur at the vortex shedding frequency, while higher harmonics can also be excited. Recent VIV model tests with flexible pipes have shown that higher harmonics in the crossflow (CF) direction can contribute to the fatigue damage significantly due to its higher frequency. Rigid cylinder experiments show that the CF third-order harmonics are more pronounced when the motion orbit is close to a “figure 8” shape and the cylinder is moving against the flow at its largest CF motion. However, there is still lack of understanding of when and where higher harmonics occur for a flexible pipe. Therefore, significant uncertainty remains on how to account for fatigue damage due to higher harmonics in VIV prediction. In the present paper, representative VIV data from various riser model test campaigns are carefully studied and analyzed. The key parameters that influence the magnitude of the third-order harmonic stress are found to be the bending stiffness, the reduced velocity, and the orbit stability. The experimental data are analyzed in order to assess the impact of each parameter on the third-order harmonic stress. A preliminary empirical response model to estimate the maximum CF third-order harmonic stress based on these identified structural and hydrodynamic parameters has been proposed. The results of this study will contribute to reduce the uncertainty and unnecessary conservatism in VIV prediction.

This paper presents a study of traditional netting materials subjected to disinfecting chemicals during fish farming and treatment of net cages. A series of tests were performed in order to study the effect of various concentrations of disinfecting chemicals on the tensile strength of Raschel knitted Nylon netting materials. Simulated spill of diluted hydrogen peroxide (HP) to the jump fence during de-lousing did not affect the strength of the applied new and used knotless nylon netting samples. Hydrogen peroxide reacted with biofouling forming gas bubbles, but this did not result in reduced netting strength. The performed tests did not indicate any effect on netting strength from a simulated single, traditional bath disinfection as performed at service stations applying the disinfectant Aqua Des (AD) containing peracetic acid (PAA). However, increasing the AD concentration from 1 to 10% resulted in a strength reduction of 3–6%. Simulated spill of concentrated AD on the jump fence of a net with copper coating residuals resulted in a severe reduction in strength of 45%. This strength loss was probably a consequence of chemical reaction between copper and Aqua Des, and uncoated netting did not experience any loss in strength subjected to the same chemical exposure. These findings from application of AD should also apply to other PAA disinfection chemicals with trade names as, for example, Perfectoxid and Addi Aqua.

Different from the fixed-based wind turbines, the floating type wind turbines are regarded as under a free–free end operating condition. The tower structure of a floating offshore wind turbine is an integrated part connecting the nacelle and support platform. An analytic solution is presented in this technical brief for the free-vibration of the tower structure of a spar-type offshore wind turbine. The tower structure is modeled as a free–free beam based on Euler–Bernoulli beam-column theory. The platform and the nacelle are considered as two large mass components connected by torsion springs at two tower ends with different stiffness. The effects of system parameters on the natural frequencies are investigated under a range of variables, including the tower structure parameters, platform and nacelle parameters, and the connection types. Nonlinear relationships between those variables and the natural frequency of the tower structure are numerically found and some design issues are discussed for the spar-type floating wind turbines.

The layers of unbounded flexible pipes have relative movement, enhancing its capabilities to handle curvatures and moment loads. In a simplified approach, those pipes can be described using bonded elements; but to really capture this behavior, a frictional contact is utterly needed. In general, dealing with contact problems in computational mechanics is complicated, since it involves the constant evaluation of its status and can lead to convergence problems or simulation failure, due to intrinsically problematic and inefficient contact models or due to contact models that are insufficient to capture the desired details. The macroelement formulation, which was created to deal with flexible pipes in a simplified way, needed a frictional contact element to enhance the quality of results and closeness to real behavior. The major drawback for developing such element is the different nature of the nodal displacements descriptions. The first approach possible is the simplest contact model: it involves only the nodes in each contacting elements. The gap information and distances are evaluated using exclusively the nodal information. This kind of model provides good results with minimum computational effort, especially when considering small displacements. This paper proposes such element: a node-to-node contact formulation for macroelements. It considers that the nodal displacements of both nodes are in cylindrical coordinates with one of them using Fourier series to describe the displacements. To show model effectiveness, a case study with a cylinder using Fourier series and multiple helical elements connected with the contact element is done and shows great results.

In hydroelastic model tests, segmented ship models are usually used to make sure that the model scale and the full size ship satisfy the similarity law of structural natural frequency and distribution of ship bending stiffness. However, springing barely occurs in those tests because the natural frequency of segmented ship models is too high for the regular waves required to be generated in a tank. In order to investigate the springing effect, three sets of backbone of variable cross section are adopted in the test. One set of backbones satisfies the similarity law of natural frequency, and two extra sets of low stiffness backbones are used so that the springing effect can appear and be measured. Experimental results show that the springing occurs when the wave encounter frequency coincides with the first elastic natural frequency of the ship, or with half or one-third of it. A good agreement has also been obtained between the experimental and the numerical results by a three-dimensional (3D) hydroelasticity method. Based on these results, the contribution of the springing responses to the fatigue damage of the ship is estimated and analyzed.

Composite flexible pipe is used in the offshore oil and gas industry for the transport of hydrocarbons, jumpers connecting subsea infrastructure, and risers with surface platforms and facilities. Although the material fabrication costs are high, there are technical advantages with respect to installation and performance envelope (e.g., fatigue). Flexible pipe has a complex, composite section with each layer addressing a specific function (e.g., pressure containment, and axial load). Continuum finite element modeling (FEM) procedures are developed to examine the mechanical response of an unbonded flexible pipe subject to axisymmetric loading conditions. A parameter study examined the effects of: (1) pure torsion, (2) interlayer friction factor, (3) axial tension, and (4) external and internal pressure on the pipe mechanical response. The results demonstrated a coupled global-local mechanism with a bifurcation path for positive angles of twist relative to the tensile armor wire pitch angle. These results indicated that idealized analytical- and structural-based numerical models may be incomplete or may provide an accurate prediction of the pipe mechanical response. The importance of using an implicit solver to predict the bifurcation response and simulate contact mechanics between layers was highlighted.

Flow-induced vibration (FIV), primarily vortex-induced vibrations (VIV), and galloping have been used effectively to convert hydrokinetic energy to electricity in model-tests and field-tests by the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan. It is known that the response of cylinders with passive turbulence control (PTC) undergoing vortex shedding differs from the oscillation of smooth cylinders in a similar configuration. Additional investigation on the FIV of two elastically mounted circular cylinders in a staggered arrangement with low mass ratio in the TrSL3 flow-regime is required and is contributed by this paper. The two PTC-cylinders were allowed to oscillate in the transverse direction to the oncoming fluid flow in a recirculating water channel. The cylinder model with a length of 0.895 m and a diameter of 8.89 cm, a mass ratio of 1.343 was used in the tests. The Reynolds number was in the range of 2.5 × 10 4 < Re < 1.2 × 10 5 , which is a subset of the TrSL3 flow-regime. The center-to-center longitudinal and transverse spacing distances were T/D = 2.57 and S/D = 1.0, respectively. The spring stiffness values were in the range of 400 < K (N/m) <1200. The values of harnessing damping ratio tested were ζ harness = 0.04, 0.12 and 0.24. For the values tested, the experimental results indicate that the response of the upstream cylinder is similar to the single cylinder. The downstream cylinder exhibits more complicated vibrations. In addition, the oscillation system of two cylinders with stiffer spring and higher ζ harness could initiate total power harness at a higher flow velocity and obtain more power.

Motion responses of moored very large floating structures (VLFSs) in coastal regions are remarkably influenced by shallow water, seabed topography, and mooring system, which were given particular focus in this paper. A three-dimensional (3D) numerical model of a moored semisubmersible single module (SMOD) was described, and time domain simulated and experimentally validated. A catenary-taut-hybrid mooring system was adopted considering coastal space limitations. Large-scale catenary mooring lines were deployed on the deep water side, while taut chains were used on the shore side to decrease the anchor radius. Although the mooring system may induce a stiffness difference between the two sides, the effectiveness of the mooring system was demonstrated by time-domain simulation and model tests. The moored semisubmersible SMOD in shallow water exhibits significant low frequency characteristics. Water depth, asymmetric stiffness, and bottom topography effects were investigated by a series of sensitivity studies. The results show that these factors play an important role in motion responses of the moored SMOD, which can further conduce to better understandings on the hydrodynamic of the semisubmersible-type VLFSs.

This study presents an analytical model of flexible riser and implements it into finite-element software abaqus to investigate the fatigue damage of helical wires near touchdown point (TDP). In the analytical model, the interlayer contact pressure is simulated by setting up springs between adjacent interlayers. The spring stiffness is iteratively updated based on the interlayer penetration and separation conditions in the axisymmetric analysis. During the bending behavior, the axial stress of helical wire along the circumferential direction is traced to determine whether the axial force overcomes the interlayer friction force and thus lead to sliding. Based on the experimental data in the literature, the model is verified. The present study implements this model into abaqus to carry out the global analysis of the catenary flexible riser. In the global analysis, the riser–seabed interaction is simulated by using a hysteretic seabed model in the literature. The effect of the seabed stiffness and interlayer friction on the fatigue damage of helical wire near touchdown point is parametrically studied, and the results indicate that these two aspects significantly affect the helical wire fatigue damage, and the sliding of helical wires should be taken into account in the global analysis for accurate prediction of fatigue damage. Meanwhile, different from the steel catenary riser, high seabed stiffness may not correspond to high fatigue damage of helical wires.

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.

To achieve a reliable structural model for vortex-induced vibration (VIV) the prediction of flexible risers, this paper employs structural systems with different geometrical nonlinearities (including a linear structure, a nonlinear one, a coupled cross-flow, and axial nonlinear one) and a classical oscillator to simulate cross-flow VIV. By comparing the experimental and simulation results, it is found that when the drag coefficient is assumed to be a fixed constant along the cylinder (i.e., the damping model is linear function of current velocity), it can affect the vibration amplitude considerably and may alter the dominant modes. When the excited mode of VIV is bending-stiffness dominant, the cross-flow structural nonlinearities can have a profound stiffening effect on vibration response. Although the introduction of axial deformation can reduce this function, the coupled cross-flow and axial nonlinearities still have the effect of decreasing the VIV amplitude.

In recent years, wave energy harvesting systems have received considerable attention as an alternative energy source. Within this class of systems, single-point harvesters are popular at least for preliminary studies and proof-of-concept analyses in particular locations. Unfortunately, the large displacements of a single-point wave energy harvester are described by a set of nonlinear equations. Further, the excitation is often characterized statistically and in terms of a relevant power spectral density (PSD) function. In the context of this complex problem, the development of efficient techniques for the calculation of reliable harvester response statistics is quite desirable, since traditional Monte Carlo techniques involve nontrivial computational cost. The paper proposes a statistical linearization technique for conducting expeditiously random vibration analyses of single-point harvesters. The technique is developed by relying on the determination of a surrogate linear system identified by minimizing the mean square error between the linear system and the nonlinear one. It is shown that the technique can be implemented via an iterative procedure, which allows calculating statistics, PSDs, and probability density functions (PDFs) of the response components. The reliability of the statistical linearization solution is assessed vis-à-vis data from relevant Monte Carlo simulations. This novel approach can be a basis for constructing computationally expeditious assessments of various design alternatives.

To understand the evolution of dynamic stiffness of damaged synthetic fiber mooring ropes, experimental investigations of polyester and high modulus polyethylene (HMPE) ropes are systematically performed utilizing a specially designed experimental system. An experimental procedure is proposed and test results show that the dynamic stiffness increases with increasing mean load and loading cycles, while decreases with increasing strain amplitude and damage level. The similarity criterion of dynamic stiffness is derived from the dimensional analysis for damaged ropes and verified by experiments. An empirical expression that accounts for the damage, mean load, strain amplitude, and loading cycles is proposed to describe the damage effect upon dynamic stiffness of synthetic fiber mooring ropes.

Extreme waves have led to many accidents and losses of ships at sea. In this paper, a two-dimensional (2D) hydroelastoplasticity method is proposed as a means of studying the nonlinear dynamic response of a container ship when traversing extreme waves, while considering the ultimate strength of the ship. On one hand, traditional ultimate strength evaluations are undertaken by making a quasi-static assumption and the dynamic wave effect is not considered. On the other hand, the dynamic response of a ship as induced by a wave is studied on the basis of the hydroelasticity theory so that the nonlinear structural response of the ship cannot be obtained for large waves. Therefore, a 2D hydroelastoplasticity method, which takes the coupling between time-domain waves and the nonlinear ship beam into account, is proposed. This method is based on an hydroelasticity method and a simplified progressive collapse method that combines the wave load and the structural nonlinearity. A simplified progressive collapse method, which considers the plastic nonlinearity and buckling effect of stiffened, is used to calculate the ultimate strength and nonlinear relationship between the bending moment and curvature, so that the nonlinear relationship between the rigidity and curvature is also obtained. A dynamic reduction in rigidity related to deformation could influence the strength and curvature of a ship's beam; therefore, it is input into a dynamic hydrodynamic formula rather than being regarded as a constant structural rigidity in a hydroelastic equation. A number of numerical extreme wave models are selected for computing the hydroelastoplasticity, such that large deformations occur and nonlinear dynamic vertical bending moment (VBM) is generated when the ship traverses these extreme waves. As the height and Froude number of these extreme waves are increased, a number of hydroelastoplasticity results including VBM and deformational curvature are computed and compared with results obtained with the hydroelasticity method, and then, some differences are observed and conclusions are drawn.

The results of a four points bending test on a box girder are presented. The experiment is part of series of tests with similar configuration but with different thickness and span between frames. The present work refers to the slenderest plate box girder with a plate's thickness of 2 mm but with a short span between frames. The experiment includes initial loading cycles allowing for partial relief of residual stresses. The moment curvature relationship is established for a large range of curvature. The ultimate bending moment (UM) of the box is evaluated and compared with the first yield moment and the plastic moment allowing the evaluation of the efficiency of the structure. The postbuckling behavior and collapse mode are characterized. Comparison of the experiment with a progressive collapse analysis method is made taking into consideration the effect of residual stresses on envelop of the moment curvature curve of the structure.

The tremendous growth of the fish farming industry in Norway over the past decades was supported by new designs and materials for fish farms, enabling bigger fish cages to be positioned in more exposed sea areas. Today, the nets of most fish cages in Norway are made from nylon, but also new net materials are proposed to better prevent escapes, protect fish from predator attacks, improve the stability of fish cages, and reduce biofouling. Some of these materials are stiff in at least one direction, and there still is a lack of knowledge about the behavior of nets with bending stiffness in currents and waves. The aim of this study was to determine how nets with bending stiffness deform in different currents and how the deformation influences the drag on the nets and to compare the results with predictions from a numerical model. Three types of net (PET, copper, and steel) were clamped to a solid steel bar on the top side but were otherwise unrestricted. The nets were subjected to several flow speeds between 0.1 and 0.9 m/s. The net deformation was determined with an optical tracking system, and the forces on the net panels were measured with a multi-axis force/torque sensor system. It is shown that bending stiffness and density of nets affect net deformation, as both parameters impact the balance between drag and gravitational forces on the nets. Net deformation leads to a decrease of the projected net area. As the rate of deformation with current speed varies greatly between different net types, the discrepancy between measured drag and drag values normalized to the projected net area at different current speeds follows different relationships for different nets. A numerical model, FhSim, was able to predict net deformation of nets with bending stiffness well, and it is shown that FhSim could not only account for the effect of bending stiffness on net deformation, but also that the model captures the structural dynamics of nets with bending stiffness in a current.

This study investigates the effects of changes in flow speed and angle of attack on drag and lift forces on nets with bending stiffness. Today most fish cage nets are made from nylon, but new cage materials are proposed in order to improve the stability of cages in currents and waves, to reduce biofouling, prevent escapes, and to secure fish from predator attacks. The use of some of these materials leads to nets with bending stiffness in at least one direction. However, not much is known about the performance of such nets in currents and waves. In this study, three different nets with bending stiffness were tested together with nylon nets. Net panels were subjected to different flow speeds at different angles between flow direction and net plane, and the forces on the nets were measured with a multi-axis force/torque sensor system. Based on the experiments, drag, and lift coefficients were determined for the different net materials and compared to existing theory with which they are in reasonably good agreement for the nets with low solidity. However, for nets with higher solidity the results are significantly lower than the drag and lift coefficients provided other authors. Also, the change of drag coefficient with changing flow speed and angle of attack was different for a monofilament and a multifilament net with similar solidity and aperture form and size. These differences may partly be due to differences in twine structures and net construction between the monofilament and multifilament net and between nets used by other authors and in the present study.

The structural behavior of flexible pipes and umbilical cables is difficult to model due to their complex construction that includes components of different materials, shapes, and functions. Also, it is difficult to model due to the nonlinear interaction between those components, which includes contacts, gaps, and friction. To model a flexible pipe or umbilical cable, one can rely on analytical or numerical approaches. Analytical models need a large set of simplifying hypotheses. Numerical models, like classical finite elements models, require large meshes and have great difficulties to converge. But one can take profit of the particular characteristics of a specific component and develop a custom-made finite element that represents its structural behavior, a so-called finite macro-element. Adopting this approach, in a previous work, it was developed a cylindrical macro-element with orthotropic behavior, to model the plastic layers of a flexible pipe or umbilical cable. This paper presents a three-dimensional (3D) curved beam element, built to model a helical metallic component, which takes into account the effects of curvature and tortuosity of that kind of component. This is accomplished by using a strong coupling between displacements and assuming that the twist and shear strains vary linearly within the element, to avoid the shear lock phenomenon. The complete formulation of this element is presented. Results obtained with this formulation are also presented and compared to those obtained by a classical finite element modeling tool, with good agreement.

A steel catenary riser (SCR) attached to a floating platform at its upper end encounters oscillations in and near its touchdown zone (TDZ), which results in interaction with the seabed. Field observations and design analysis of SCRs show that the highest stress and greatest fatigue damage occurred near the touchdown point where the SCR first touches the seabed soil. The challenges regarding the fatigue damage assessment of an SCR in the TDZ are primarily because of the nonlinear behavior of SCR–seabed interaction and considerable uncertainty in seabed interaction modeling and geotechnical parameters. Analysis techniques have been developed in the two main areas: SCR–seabed interaction modeling and the influence of the uncertainty in the geotechnical parameters on the dynamic response and fatigue performance of SCRs in the TDZ. Initially, this study discusses the significance of SCR–seabed interaction on the response of an SCR for deepwater applications when subjected to random waves on soft clay using the commercial code OrcaFlex for nonlinear time domain simulation. In the next step, this study investigates the sensitivity of fatigue performance to geotechnical parameters through a parametric study. It is proven that employing the improved lateral SCR–seabed interaction model with accurate prediction of soil stiffness and riser penetration with cyclic loading enables us to obtain dynamic global riser performance in the TDZ with better accuracy. The fatigue analyses results prove that the confounding results indicated by the previous research studies on the SCR in the TDZ are due to different geotechnical parameters imposed with the seabed interaction model. The main benefit of employing nonlinear seabed approach is to capture the entity of realistic soil interaction behavior in modeling and analysis and to predict the likelihood of the fatigue damage of the SCR with seabed interaction, thereby minimizing the risk of the loss of the containment with the associated environmental impact.