Due to the interaction between elastic structural deformations and fluid motions, the responses of floating bridges should be analyzed by hydroelasticity methods. We firstly employed a linear time-domain approach, based on the discrete-module-based hydroelastic method to investigate the responses of two surface bridge concepts under first order wave forces, a straight bridge with mooring system and an end-anchored curved bridge. The results show that the displacement of the curved bridge is smaller, compared with the straight bridge. However, the reaction force and the vertical bending moment of the curved bridge are larger. Furthermore, a nonlinear time-domain hydroelasticity approach for analysis of the floating bridge under nonlinear wave forces within inhomogeneous wave conditions is established. Finally, a comparison between the linear and nonlinear responses of the moored straight bridge is made. The results show that the nonlinearity of the wave excitation forces has a significant influence on the mooring forces and horizontal displacement.

A new numerical method, which is based on three-dimensional (3D) potential flow theory and finite element method (FEM), is used to predict the wave-induced hydroelastic responses of flexible floating bridges. The floating bridge is discretized into several modules based on the positions of the pontoons which are connected by elastic beams. The motion equations of the entire floating structure are established according to the six degrees of freedom (6DOF) motions of each rigid module coupled with the dynamics of the elastic beams. The hydrodynamics loads on each module are considered as external loads and simultaneously applied. The method is extended to take into account the shore side effect, which is obtained from the 3D potential flow theory and considered as a hydrodynamic boundary condition. The effects of inclination of shore side on the responses of the bending moment, horizontal and vertical displacements of the pontoon and their distribution along the bridge are investigated. The results show that the displacement response increase with an increasing steepness of the shore side.

Prediction of vortex induced vibration (VIV) for a long-flexible pipe has always been an important concern for the design of risers. Currently, VIV prediction methods are mainly based on the linear beam theory, where the axial tension is treated as time-independent, and the couples between VIV and axial tension are totally ignored. However, experimental results have illustrated strong couples between the axial tension and VIV [1–2]. The purpose of this paper is to develop a time domain VIV prediction model. This model consists of pipe’s structural non-linearity, couplings between axial force, cross-flow/in-line (CF/IL) VIV responses, and the hydrodynamic forces. The hydrodynamic forces are further divided into vortex-induced force in CF and IL directions, and drag force in IL direction. The former one is determined via empirical force model based on forced oscillation test of rigid cylinders. The IL drag coefficients model considering the effects of VIV developed by Song [3] is adopted. VIV responses under these hydrodynamic forces at each time step are solved by Newton-Raphson method. Comparison between present method and the experimental results under uniform flows and shear flows are conducted, which verified the feasibility and reliability of the proposed method. In addition, by comparing the results under constant tension and time-varying tension, it is proved that the time-varying tension has a significant effect on VIV responses, especially under the case of high flow velocity and high vibration mode.

The helical strakes are now widely being used in offshore riser design for the suppression of vortex induced vibrations (VIV). The purpose of this paper is to investigate the responses and suppression effectiveness of a straked pipe in a more real working conditions of the risers which will endure a kind of “oscillatory flow” due to the relative motions between the fluid around and the risers induced by the top platform motions. Experiments are performed on a flexible straked pipe with pitch length/height of 15D/0.25D in ocean basin. The pipes are forced to harmonically oscillate in various combinations of amplitude and period with Keulegan-Carpenter (KC) number varying between 5 and 165, and maximum reduced velocities from 4 to 12. Responses in both in direction are firstly investigated. Inverse analysis method and the Least Square method are adopted to identify drag coefficients and added mass coefficients. The results show that strakes can reduce the higher frequency responses in both CF and IL direction. Suppression efficiencies of the strakes and are not ideal as expected in oscillatory flow. Moreover, the hydrodynamic coefficients change dramatically under the small KC number and stabilize under the large KC number. The drag coefficients obviously magnify at KC ∼ 20.

A 38m long flexible pipe with staggered buoyancy modules and strakes has been tested in the ocean basin of SINTEF Ocean (former Marintek) for VIV investigation of a lazy wave riser. In this paper the inverse analysis method was presented and applied into the investigation of the hydrodynamic force coefficients along this tested flexible pipe with the measured responses as inputs. The feasibility of the inverse analysis method is firstly validated by numerical simulations. The distributions of the added mass and excitation coefficients along the flexible pipe with staggered buoyancy modules and strakes are then investigated. The identified coefficients are validated by check of the natural frequencies and responses of the model, and are finally compared against those from the forced oscillation tests.

Based on the three dimensional potential theory and finite element method (FEM), this paper presented a method for time-domain hydroelastic analysis of a floating bridge in inhomogeneous waves. A floating bridge in both regular and irregular waves, is taken as a numerical example. This method is firstly validated by the comparisons of the results between frequency domain method and presented time domain method under regular wave condition. Then the hydroeleastic responses of the floating bridge in waves with spatially varying significant wave height/peak period are presented, with the purpose to illustrate the feasibility of the proposed method. The primary results at this stage indicate that the inhomogeneity of the waves might affect the structure dynamic responses of the floating bridge in waves.

In this paper, Forgetting Factor Recursive Least Squares (FF-LS) method is proposed to identify time-varying vortex-induced force coefficients of the flexible riser under multi-frequency VIV. FF-LS method introduces the forgetting factor, which gives more weight to the data closer to the present. This modification improves the method’s sensitivity to the time-varying parameters, and enables it to identify the time-varying parameters under multi-frequency coupling. In this paper, the mass-spring-dashpot model is used to verify FF-LS method’s ability to accurately identify time-varying parameters under multi-frequency coupling. Then, this methodology is used to identify the vortex-induced force coefficients of flexible riser at the basic frequency when vortex-induced vibration occurs. Identified coefficients are consistent with the result obtained from Least Squares method, which indicates that the proposed FF-LS method retrogrades to the Least Squares method when VIV response and the vortex-induced force contains only one single frequency. Finally, this methodology is used to identify the time-varying vortex-induced force coefficients of a flexible riser under VIV considering the coupling effect between the basic frequency and high frequency. The vortex-induced force reconstructed from identified time-varying coefficients is consistent with real vortex-induced force, which verifies the validity and applicability of this methodology in identifying time-varying vortex-induced force coefficients considering multi-frequency coupling effect. The results show that when the flexible riser is subjected to multi-frequency VIV, its vortex-induced force coefficients change periodically, the time-averaged values of these time-varying coefficients are different from the vortex-induced force coefficients at the basic frequency, which results from the coupling effect between the basic frequency and high frequency. Linear superposition of vortex-induced force coefficient at basic frequency and high frequency are different from time-varying coefficients considering coupling effect of multi-frequency and cannot reconstruct vortex-induced force correctly.

Steel Lazy Wave Riser (SLWR) is an attractive deep water riser concept. When subjected to vortex induced vibrations (VIV), the vortex shedding process of the buoyancy element and the bare riser section will be different due to the difference in diameter. VIV responses can be strongly influenced by the dimension of the buoyancy element and its arrangement. Empirical VIV prediction programs, such as VIVANA, SHEAR7 and VIVA, are widely used by the industry for design against VIV loads. However, there is lack of hydrodynamic data to be used in these programs when buoyancy elements are present. Experiment to obtain hydrodynamic data for riser with staggered buoyancy elements was carried out in the towing tank in SINTEF Ocean. A rigid cylinder section with three staggered buoyancy elements was subjected to harmonic forced cross-flow (CF) motions. Hydrodynamic forces on one of the buoyancy elements were directly measured in addition to the measured forces at both ends of the test section. Two buoyancy element configurations were tested and the corresponding hydrodynamic data are compared with that of a bare cylinder. The obtained hydrodynamic data was also used in VIV prediction software and good prediction against existing flexible cylinder staggered buoyancy element VIV test data was achieved. A roadmap to achieve an optimal SLWR design by combining different experimental and numerical methods is suggested.

Flexible riser with staggered buoyancy elements has been widely used in ocean engineering, such as steel lazy wave riser, drilling riser, etc. Both the buoyancy elements and the riser may experience vortex induced vibrations (VIV), subject to sea current. However, hydrodynamic characteristics of the buoyancy elements undergoing VIV and influence of buoyancy elements on hydrodynamic force of the bare section are still under discussion. The purpose of this paper is to reveal the hydrodynamic characteristics of flexible riser with staggered buoyancy elements, both for buoyancy element and bare pipe section. The cross flow hydrodynamic coefficients of the flexible riser with 25%, 50% and 100% staggered buoyancy covered are obtained from VIV model tests, using hydrodynamic forces and coefficients identification method. Distribution of the added mass coefficients and excitation coefficients along the flexible riser were investigated, and compared with those on the bare flexible pipe and rigid cylinders under forced oscillations. In addition, the relationship between added-mass coefficients of buoyancy element and that of bare section were obtained.

A non-iterative method in frequency domain for vortex induced vibration (VIV) predictions of marine risers is proposed in this paper. A solving model is established in modal space for predicting risers’ VIV responses, which consists of a hydrodynamic force equation and a dynamic response equation. By utilizing a non-iterative solving process in the modal space, the equations are solved without power-balance iterations. And through comparisons between this method and conventional prediction methods, the validity and applicability of this non-iterative method is verified. This method completely gets rid of the difficulty of getting convergent predictions, which is conducive to practical engineering applications and provides fresh ideas for understanding conventional empirical model methods for VIV prediction.

The paper presents numerical simulations and analysis of experimental results obtained from a model scale test on steel lazy wave riser conducted by the DeepStar ® consortium. The study focuses on the hydrodynamic response of a steel lazy wave riser under forced oscillatory and random motions in a wave basin. The riser pipe deformations were measured by Fiber Bragg Grating (FBG) strain sensors, as well as force transducers. Both in-plane and out-of-plane signals were recorded. Numerical simulations were performed, focusing on in-plane riser response, and compared with the experiments. The typical hydrodynamic coefficients (added mass, and drag) for the riser design are used in analysis and the calculated riser reponses are compared with measurements. In addition, some coefficients are adjusted to improve the comparison and the results are promising. The test data post-processing approach and modeling of steel lazy wave riser are also discussed in the paper.

The Steel Lazy Wave Risers (SLWR) is a newly utilized riser concept with increasing popularity due to its improved structural performance in harsh environment. However, SLWR technology is still new, and has very limited qualification test data and field experience to establish a consistent technology readiness and maturity level through industry. The present study, championed by the DeepStar ® consortium, planned and executed a comprehensive test campaign of a SLWR in an offshore model basin. The objective was to better understand the global dynamic behavior of SLWR due to vessel induced motions and currents to advance the concept maturity and quantify the design margins. The paper describes the experiment and highlights key findings from the tests that are essential in addressing the challenges around SLWR design. The findings of the study progress the technology readiness level of the riser concept and provide essential guidelines for the design of SLWRs.

Typically, in some side-by-side offshore operations, the speed of vessels is very low or even 0 and the headings are manually maneuvered. In this paper, the hydrodynamic responses of a two-body system in such operations under irregular seas are investigated. The numerical model includes two identical PSVs (Platform Supply Vessel) as well as the fenders and connection lines between them. A horizontal mooring system constraining the low frequency motions is set on one of the ships to simulate maneuver system. Accounting for the hydrodynamic interactions between two bodies, 3D potential theory is applied for the analysis of their hydrodynamic coefficients. With wind and current effects included, these coefficients are further applied in the time domain simulations in irregular waves. The relevant coefficients are estimated by experiential formulas. Time-varying loads on fenders and connection lines are analyzed. Meanwhile, the relative motions as well as the effects of the hydrodynamic interactions between ships are further discussed, and finally an optimal operation scheme in which operation can be safely performed is summarized.

We employed maturely developed methods and software, RIFLEX/SIMO/SIMA to look into the feasibilities of different floating tunnel/bridge design concepts. The global hydroelastic responses of two concepts, i.e. tether/pontoon supported hybrid tunnel concept and floating foundation supported girder concept and have been investigated. The distributions of maximum values of the deflection, bending moment and stress along the bridges under different sea conditions are presented.

The increasing scale of the ship structure calls for hydroelastic analysis due to the growing flexibility compared with the regular ships which are regarded as rigid bodies traditionally. In the berthing stage of a barge, the motion responses and bending/torsional forces of the barge will be strongly affected by the flexible deformations of the barge itself and the wave elevation, which will be further affected by both the neighboring floating mobile harbor’s rigid motions and flexible deformations. In this paper, based on 3D potential theory, the coupling analysis model of a flexible barge adjoining a mobile harbor for the predictions of the motion responses and forces of the flexible barge is built. The effects of the flexibility of the barge, the distance between two adjacent structures, and the angles of incoming waves are numerically investigated. Wave elevation in the gap is also analyzed in order to explain the resonant peaks of the motion response.

A model test of a free-hanging riser under vessel motion was performed in the ocean basin at Shanghai Jiao Tong University to confirm whether vortex-induced vibration (VIV) can happen due to pure vessel motion, to investigate the equivalent current velocity and Keulegan–Carpenter (KC) number effect on the VIV responses and to obtain the correlations for free-hanging riser VIV under vessel motion with VIV for other compliant risers. Top end of the riser was forced to oscillate at given vessel motion trajectories. Fiber Brag Grating (FBG) strain sensors were used to measure the riser dynamic responses. Experimental results confirmed that the free-hanging riser would experience significant out-of-plane VIV. Meanwhile, VIV responses in terms of response amplitude, response frequency and cross-section trajectories under different test cases were further compared and discussed. Most importantly, the correlation among VIV response frequency, vortex shedding pairs and maximum KC number KC max was revealed. The presented work is supposed to provide useful references for gaining a better understanding on VIV induced by vessel motion, and for the development of future prediction models.