A rigid jumper is an important part of a subsea production system, and it may experience significant vortex-induced vibrations (VIVs) if subjected to current. It has normally a non-straight geometry shape in three-dimensional space. Consequently, the response of a rigid jumper under VIVs is much more complicated compared with straight pipeline structures. Currently, there are very limited studies and design guidelines including methods on how to assess the fatigue damage of rigid jumpers under VIVs. The methodology used for straight pipelines is often applied by ignoring the non-straight geometry characteristics and the multiaxial stress states. However, both experimental and numerical results show that the torsional stress does exist besides the flexural stress for rigid jumpers under VIVs. The objective of this study is to do a fatigue assessment practice based on the state-of-the-art calculation methods to a rigid jumper on model scale. The VIV response is obtained from experimental tests and numerical calculations by either force or response model methods. The influence of torsional stress on fatigue assessment is studied. Two approaches have been investigated. In the first approach, the flexural and torsional stresses are evaluated separately. The second approach uses the first principal stress to calculate the fatigue damage; thus, the flexural and torsional stresses are evaluated together. It appears that the use of the first principal stress gives higher fatigue damage if the torsional stress contribution is significant. Furthermore, the principal stress method is also less time-consuming in processing the results.

A three-dimensional (3D) numerical two-phase flow model based on solving unsteady Reynolds-averaged Navier–Stokes (URANS) equations has been used to simulate breaking waves and steep waves past a vertical cylinder on a 1:10 slope. The volume of fluid (VOF) method is employed to capture the free surface and the k– ω shear–stress transport (k– ω SST) turbulence model is used to simulate the turbulence effects. Mesh and time-step refinement studies have been conducted. The numerical results of wave forces on the structure are compared with the experimental data (Irschik et al., 2004, “Breaking Wave Loads on a Slender Pile in Shallow Water,” Coastal Engineering, Vol. 4, World Scientific, Singapore, pp. 568–581) to validate the numerical model, and the numerical results are in good agreement with the measured data. The wave forces on the structure at different Keulegan–Carpenter (KC) numbers are discussed in terms of the slamming force. The secondary load cycles are observed after the wave front past the structure. The dynamic pressure and velocity distribution, as well as the characteristics of the vortices around the structure at four important time instants, are studied.

Recently, the concept of a vessel-shaped fish farm was proposed for open sea applications. The fish farm comprises a vessel-shaped floater, five fish cages, and a single-point mooring system. Such a system weathervanes, and this feature increases the spread area of fish waste. Still, the downstream cages may experience decreased exchange of water flow when the vessel heading is aligned with the current direction, and fish welfare may be jeopardized. To ameliorate the flow conditions, a dynamic positioning (DP) system may be required, and its power consumption should relate to the heading misalignment. This paper proposes an integrated method for predicting the heading misalignment between the vessel-shaped fish farm and the currents under combined waves and currents. A numerical model is first established for the fish farm system with flexible nets. Current reduction factors are included to address the reduction in flow velocity between net panels. The vessel heading is obtained by finding the equilibrium condition of the whole system under each combined wave and current condition. Then, the Kriging metamodel is applied to capture the relation between the misalignment angle and environmental variables, and the probability distribution of this misalignment angle is estimated for a reference site. Finally, the requirement for the DP system to improve the flow condition in the fish cages is discussed.

A free-hanging riser (FHR) is a typical riser configuration seen in the disconnected drilling riser, the water-intake riser, and the deep-sea mining riser. In offshore productions, these marine risers will move back and forth in water and further generate an equivalent oscillatory current around themselves, due to the vessel motions. Both in full-scale marine operations and model tests, it has been reported that such oscillatory current leads to riser vortex-induced vibration (VIV) and therefore causes structural fatigue damage. Recently, there have been some attempts to numerically predict vessel motion-induced VIV on the compliant production risers, with emphasize on relatively large Keulegan–Carpenter (KC) numbers. In the real marine operations, the risers experience small KC number scenarios during most of their service life. Therefore, the investigation of vessel motion-induced VIV under small KC number is of great significance, especially considering its contribution to the fatigue damage. In this paper, numerical investigation of VIV of a FHR attached to a floating vessel is carried out. A new response frequency model for vessel motion-induced VIV under small KC numbers is proposed and implemented in vivana . Validation of the proposed numerical methodology is performed against the published experimental results, where a good agreement is achieved.

Deepwater risers are susceptible to vortex-induced vibrations (VIV) when subjected to currents. When responding at high modes, fatigue damage in the inline (IL) direction may become equally important as the crossflow (CF) components. Accurate calculation of both IL and CF responses is therefore needed. Empirical VIV prediction programs, such as VIVANA “Passano et al. (2016, “VIVANA—Theory Manual Version 4.8,” Trondheim, Norway),” SHEAR7 “(Vandiver, J. K., and Li, L., 2007, “Shear7 v4.5 Program Theoretical Manual,” Department of Ocean Engineering, Massachusetts Institute of Technology, Cambridge, MA),” and VIVA “Triantafyllou et al. (1999, “Pragmatic Riser VIV Analysis,” Offshore Technology Conference, Houston, TX, May 3–6, Paper No. OTC-10931-MS.)” are the most common tools used by the offshore industry. Progress has been seen in the prediction of CF responses. Efforts have also been made to include an IL load model in VIVANA. A set of excitation coefficient parameters were obtained from rigid cylinder test and adjusted using measured responses of one of the flexible cylinder VIV tests. This set of excitation coefficient parameters is still considered preliminary and further validation is required. Without an accurate IL response prediction, a conservative approach in VIV analysis has to be followed, i.e., all current profiles have to be assumed to be unidirectional or acting in the same direction. The purpose of this paper is to provide a reliable combined IL and CF load model for the empirical VIV prediction programs. VIV prediction using the existing combined IL and CF load model in VIVANA is validated against selected flexible cylinder test data. A case study of a deepwater top tension riser (TTR) has been carried out. The results indicate that VIV fatigue damage using two-dimensional directional current profiles is less conservative compared to the traditional way of using unidirectional current profiles.

This paper presents an engineering approach to study the effects of soil profile variation and scour on structural response of an offshore monopile wind turbine. A wind-wave model for finite water depth is proposed to obtain the corresponding sea-state based on the incident wind. Different wind, wave, and current loads on the wind turbine for the operational conditions are considered. The interaction between the foundation and the soil is simulated by nonlinear springs, for which stiffness properties are obtained from the axial load transfer curve, the tip load–displacement curve, and the lateral load–deflection curve. Four types of soil conditions are considered, i.e., 100% sand layer, 50% sand layer (top) and 50% clay layer (bottom), 50% clay layer (top), and 50% sand layer (bottom), as well as 100% clay layer. For a given current speed, the variations of the structural response of the wind turbine due to the effects of different wind–wave load combinations, soil conditions and scour have been investigated. Different wind–wave load combinations directly affect the mean internal bending moment and mean displacement vertically along the support structure. Different soil conditions change the eigenfrequency of the structure significantly. The top layer of the soil appears to have a strong influence on the mean internal bending moment and the mean shear force distribution along the foundation. Moreover, the effect of scour alters the eigenfrequency of the structure significantly. The maximum mean bending moment and displacement increase for the cases with a scour hole as compared to the cases with scour protection.

The aquaculture industry is aiming to move fish farms from nearshore areas to open seas because of many attractive advantages in the open water. However, one major challenge is to design the structure to withstand the environmental loads due to wind, waves, and currents. The purpose of this paper is to study a vessel-shaped fish farm concept for open sea applications. The structure includes a vessel-shaped hull, a mooring system, and fish cages. The shape of the hull minimizes the wave loads coming from the bow, and the single-point mooring system is connected to the turret at the vessel bow. Such a system allows the whole fish farm to rotate freely about the turret, reduces the environmental loads on the structure and increases the spread area of fish wastes. A basic geometry of the vessel hull was considered and the hydrodynamic properties were obtained from the frequency-domain (FD) analysis. A mooring system with six mooring lines was designed to avoid possible interactions with the fish cages. Time-domain (TD) simulations were performed by coupling the hull with the mooring system. A simplified rigid model of the fish cages was considered. The global responses of the system and the mooring line loads were compared under various wave and current conditions. The effects due to misalignment of wave and current directions on the responses were discussed. Finally, the responses using flexible and rigid net models were compared under steady current conditions.

This paper provides a practical stochastic method by which the burial and scour depths of short cylinders and truncated cones exposed to long-crested (two-dimensional (2D)) and short-crested (three-dimensional (3D)) nonlinear random waves plus currents can be derived. The approach is based on assuming the waves to be a stationary narrow-band random process, adopting the Forristall second-order wave crest height distribution representing both 2D and 3D nonlinear random waves. Moreover, the formulas for the burial and the scour depths for regular waves plus currents presented by previous published work for short cylinders and truncated cones are used.

Two-dimensional (2D) numerical simulations have been performed to investigate both regular and irregular waves past a fixed horizontally semisubmerged circular cylinder. The 2D simulations are carried out by solving Navier–Stokes equations discretized by finite volume method. Volume of fluid (VOF) method is employed to capture the free surface in the numerical wave tank (NWT). Validation studies have been performed by comparing the numerical results of free surface waves past the cylinder with the published experimental and numerical data. The present numerical results are in good agreement with both the experimental and the other numerical results in terms of hydrodynamic forces and free surface elevation. Subsequently, the effects of the wave height and the wavelength on wave–structure interaction are investigated by conducting numerical simulations on the regular and the irregular waves past a semisubmerged cylinder at different wave heights and the wavelengths. The averaged and maximum vertical wave forces on the cylinder increase with the increasing wave height. The numerical results for the irregular waves are compared with those induced by the regular waves in terms of the maximum and averaged vertical wave forces. When the significant wave height and the spectral peak period of the irregular waves are equal to the wave height and the wave period of the regular waves, the maximum vertical wave force induced by the irregular waves is larger than that induced by the regular waves, meanwhile, the average vertical wave forces have the contrary relationship.