Call for Paper—Special Issue on “Fatigue life and integrity of ships and offshore structures”

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Arun Kamath1, Erlend Liav?g Grotle2, Hans Bihs1
Journal of Marine Science and Application,2021(2): 185-200
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Sloshing is relevant in several applications like ship tanks, space and automotive industry and seiching in harbours. Due to the relationship between ship and sloshing motions and possibility of structural damage, it is important to represent this phenomenon accurately. This paper investigates sloshing at shallow liquid depths in a rectangular container using experiments and RANS simulations. Free and forced sloshing, with and without baffles, are studied at frequencies chosen specifically in proximity to the first mode natural frequency. The numerically calculated free surface elevation is in close agreement with observations from experiments. The upper limit of the resonance zone, sloshing under different filling depths and roll amplitudes and sloshing with one, two and four baffles are also investigated. The results show that the extent of the resonance zone is reduced for higher filling depth and roll amplitude. It is also found that the inclusion of baffles moves the frequency at which the maximum free surface elevation occurs, away from the fundamental frequency. Finally, a submerged baffle is found to dissipate more energy compared to a surface piercing baffle and that the effect of several submerged baffles is similar to that of a single submerged baffle.

Akile Ne?e Halilbe?e1,2, Cong Zhang3, Osman Azmi Özsoysal1
Journal of Marine Science and Application,2021(2): 201-212
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In this study, the coupled torsional-transverse vibration of a propeller shaft system owing to the misalignment caused by the shaft rotation was investigated. The proposed numerical model is based on the modified version of the Jeffcott rotor model. The equation of motion describing the harmonic vibrations of the system was obtained using the Euler-Lagrange equations for the associated energy functional. Experiments considering different rotation speeds and axial loads acting on the propulsion shaft system were performed to verify the numerical model. The effects of system parameters such as shaft length and diameter, stiffness and damping coefficients, and cross-section eccentricity were also studied. The cross-section eccentricity increased the displacement response, yet coupled vibrations were not initially observed. With the increase in the eccentricity, the interaction between two vibration modes became apparent, and the agreement between numerical predictions and experimental measurements improved. Given the results, the modified version of the Jeffcott rotor model can represent the coupled torsional-transverse vibration of propulsion shaft systems.

Sertaç Bulut, Selma Ergin
Journal of Marine Science and Application,2021(2): 213-228
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The effects of the temperature, salinity, and fluid type on the acoustic characteristics of turbulent flow around a circular cylinder were numerically investigated for the Reynolds numbers of 2.25×104, 4.5×104, and 9.0×104. Various hybrid methods-Reynolds-averaged Navier-Stokes (RANS) with the Ffowcs Williams and Hawkings (FWH) model, detached-eddy simulation (DES) with FWH, and large-eddy simulation with FWH-were used for the acoustic analyses, and their performances were evaluated by comparing the predicted results with the experimental data. The DES-FWH hybrid method was found to be suitable for the aero- and hydro-acoustic analysis. The hydro-acoustic measurements were performed in a silent circulation channel for the Reynolds number of 2.25×104. The results showed that the fluid temperature caused an increase in the overall sound pressure levels (OASPLs) and the maximum sound pressure levels (SPLT) for the air medium; however, it caused a decrease for the water medium. The salinity had smaller effects on the OASPL and SPLT compared to the temperature. Moreover, the main peak frequency increased with the air temperature but decreased with the water temperature, and it was nearly constant with the change in the salinity ratio. The SPLT and OASPL for the water medium were quite higher than those for the air medium.

Domenico Flagiello1, Martina Esposito1, Francesco Di Natale1, Kent Salo2
Journal of Marine Science and Application,2021(2): 229-247
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Maritime shipping is a strategic sector with a strong international vocation and management. The need to define regulations valid for many different countries without generating disparities of treatment slowed down the formulation of environmental regulations, especially for atmospheric emissions. In particular, regulations pertaining to the reduction of sulphur compounds allowed two distinct approaches:the use of low-sulphur fuels or exhaust gas cleaning systems, the so-called Scrubbers. The actual implementation of these solutions presents specific concerns either related to the toxicity of atmospheric by-products and to the fuel cost or to the generation of polluting washwaters that may need treatment before discharge. In this paper we analyzed the potential environmental benefit deriving from the use of a distillate fuel, not compliant with current IMO Sulphur Regulations, together with a Scrubber. The pilot-scale experimental results indicated that a limited amount of water and/or scrubber volume is needed to reduce sulphur emissions below regulations on maritime shipping, especially with the addition of NaOH reaching a water-saving between 25%-33% compared to the use of pure seawater. Experiments indicated that scrubber washwater PAHs emissions are within the available water quality standards indicated by EU and USA guidelines. A bottom-up analysis on heavy metals concentration shed light on the prominent role of metal-parts corrosion on the washwater emissions. Taking into account for corrosion phenomena, the actual heavy metals concentration in the washwater deriving from scrubbing was normally below the water quality standards.

Mohua Das, M. Rafiqul Islam, Tariqul Islam Shazeeb
Journal of Marine Science and Application,2021(2): 248-267
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Saint Martin Island is the only coral island and one of the well-known tourist spots in Bangladesh. Because of its geographic location, electricity cannot be supplied from the mainland through the electricity grid. Diesel generators and solar power are the only means of electricity generation presently available there. Surrounded by the sea, Saint Martin Island has the ideal conditions for wave energy extraction. In this research, numerical models have been developed using the Delft3D simulation software to determine the wave characteristics of different locations around Saint Martin Island. The results have been calibrated and validated against the data obtained from well-known data sources. The wave power densities have been calculated using the data obtained from the simulation models. The findings of the research show that the wave power density increases significantly from shallow water to deep water and a large amount of wave energy can be extracted during the summer and rainy monsoon seasons. The maximum hourly average value of wave power in 2016 has been determined to be 6.90 kW/m at location with a water depth of 27.80 m. Wave energy resources are also observed to be sufficiently stable with the coefficients of variation of wave power density less than 0.62, except for December, January, and May of that particular year. Moreover, the annual effective energies have been determined to be within the range of 36.57 to 57.28 MWh/m, which will be sufficient to meet the electricity requirement of the island communities.

Bingqi Liu1, Carlos Levi1, Segen F. Estefen1, Zhijia Wu2, Menglan Duan3
Journal of Marine Science and Application,2021(2): 268-283
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Lower efficiencies induce higher energy costs and pose a barrier to wave energy devices’ commercial applications. Therefore, the efficiency enhancement of wave energy converters has received much attention in recent decades. The reported research presents the double snap-through mechanism applied to a hemispheric point absorber type wave energy converter (WEC) to improve the energy absorption performance. The double snap-through mechanism comprises four oblique springs mounted in an X-configuration. This provides the WEC with different dynamic stability behaviors depending on the particular geometric and physical parameters employed. The efficiency of these different WEC behaviors (linear, bistable, and tristable) was initially evaluated under the action of regular waves. The results for bistable or tristable responses indicated significant improvements in the WEC’s energy capture efficiency. Furthermore, the WEC frequency bandwidth was shown to be significantly enlarged when the tristable mode was in operation. However, the corresponding tristable trajectory showed intra-well behavior in the middle potential well, which induced a more severe low-energy absorption when a small wave amplitude acted on the WEC compared to when the bistable WEC was employed. Nevertheless, positive effects were observed when appropriate initial conditions were imposed. The results also showed that for bistable or tristable responses, a suitable spring stiffness may cause the buoy to oscillate in high energy modes.

Vishakh S. Kumar, Prabhu Rajagopal
Journal of Marine Science and Application,2021(2): 284-301
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There is much need for autonomous underwater vehicles (AUVs) for inspection and mapping purposes. Most conventional AUVs use torpedo-shaped single-rigid hull, because of which their manoeuvrability is limited. Moreover, any increase in payload results in a larger hull size and the turning diameter, limiting its operation in constrained areas. As a solution to this problem, we develop M-Hull, a subsurface mapping AUV with a modular-split hull design that provides better manoeuvrability than a conventional torpedo-shaped vehicle. At the same time, it has more agility than an unconventional bio-inspired snake-like vehicle though their designs look similar. This approach makes it a hybrid solution between conventional torpedo-shaped AUVs and unconventional bio-inspired vehicles. We focus on improving the turning diameter during the mapping operation, and hence this paper concentrates on the dynamic aspects of the 2D turning motion of the vehicle. It will provide the relationship between turning speed, thrust, and joint torque requirements for the multi-hull underwater vehicle. Different turning modes are compared to choose an optimum turning configuration, and the critical speed is calculated for the vehicle’s safe operation. In the end, the modelling is verified using the experimental data. One can follow the method followed here for the 2D motion analysis of similar underwater vehicles.

Ahmed Hammad, Yehia Abdel-Nasser, Mohamed Shama
Journal of Marine Science and Application,2021(2): 302-316
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The main configuration of ship construction consists of standard and fabricated stiffening members, such as T-sections, which are commonly used in shipbuilding. During the welding process, the nonuniform heating and rapid cooling lead to welding imperfections such as out-of-plane distortion and residual stresses. Owing to these imperfections, the fabricated structural members may not attain their design load, and removing these imperfections will require extra man-hours. The present work investigated controlling these imperfections at both the design and fabrication stages. A typical fabricated T-girder was selected to investigate the problem of these imperfections using double-sided welding. A numerical simulation based on finite element modeling (FEM) was used to investigate the effects of geometrical properties and welding sequence on the magnitude of the welding imperfections of the T-girder. The FEM results were validated with the experimental measurements of a double-sided fillet weld. Regarding the design stage, the optimum geometry of the fabricated T-girder was determined based on the minimum steel weight and out-of-plane distortion. Furthermore, regarding the fabrication stage, a parametric study with two variables (geometrical properties and welding sequence) was conducted to determine the optimum geometry and welding sequence based on the minimum welding out-of-plane distortion. Increasing the flange thickness and reducing the breadth while keeping the T-girder section modulus constant reduced the T-girder weight and out-of-plane distortion. Noncontinuous welding produced a significant reduction in the out-of-plane distortion, while an insignificant increase in the compressive residual stress occurred.

Thiago Henrique1, Walnorio Ferreira1, Cláudio Martins2
Journal of Marine Science and Application,2021(2): 317-324
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Operational modal analysis is a non-destructive structural investigation that considers only the loads resulting from service conditions. This approach allows the measurement of vibrations on a given structure with no need to interrupt its use. The present work aims to develop a numerical model to represent the global structural behavior of a vessel breasting dolphin using a technique that is simple and cheap in order to obtain a fast answer about the stiffness of a pier after the collision of ships with capacity up to 400,000t. To determine the modes of vibration, one accelerometer was installed on the breasting dolphin located on the pier and a frequency domain technic was conducted over recorded data to obtain modal parameters of the structure. In situ measurements were compared to data from a finite element model based on the original structural design in order to adapt the model to accurately represent the actual behavior of the system. This allowed a reliable structural analysis that accounted for existing structural damage and imperfections. The results of the experiment presented herein are the numerical characterization of the structure, along with the structural analysis to assess the degree of damage currently observed on the system. It is noted that the dolphin subjected to ship impacts presents a reduction in stiffness of approximately 10% and its global damage level can be monitored from now after new accidents.

Hamede Karami, Reza Ghasemi
Journal of Marine Science and Application,2021(2): 325-332
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Designing a controller to stabilize maneuvering hovercrafts is an important challenge in amphibious vehicles. Hovercrafts are implemented in several applications, such as military missions, transportation, and scientific tasks. Thus, to improve their performance, it is crucial to control the system and compensate uncertainties and disruptions. In this paper, both classic and intelligent approaches are combined to design an observer-based controller. The system is assumed to be both controllable and observable. An adaptive neural network observer with guaranteed stability is derived for the nonlinear dynamics of a hovercraft, which is controlled via a nonsingular super-twisting terminal sliding-mode method. The main merits of the proposed method are as follows:(1) the Lyapunov stability of the overall closed-loop system, (2) the convergence of the tracking and observer errors to zero, (3) the robustness against uncertainties and disturbances, and (4) the reduction of the chattering phenomena. The simulation results validate the excellent performance of the derived method.

Karan Sotoodeh
Journal of Marine Science and Application,2021(2): 333-342
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Hydraulic systems provide a clean and stable supply of hydraulic fluid for subsea valves and actuators installed on the subsea bed in subsea production systems. Subsea control systems are used for contemporary subsea fields instead of installing the control system on topside. Although all-electric subsea systems are state-of-the-art with benefits such as health, safety, and environment improvement, as well as efficiency and lower cost, hydraulic systems are still used for the development of many subsea fields. One of the main questions in the selection of a subsea hydraulic field is whether to choose an open or closed loop hydraulic system. The main characteristic of an open loop hydraulic system is that the hydraulic fluid is discharged into the marine environment during the actuation of the subsea valves. Conversely, the hydraulic fluid is returned to the topside facilities through an umbilical system in a closed loop system. Given that closed loop systems are more eco-friendly, the main question in this research is to examine the effect of the actuator connection of the closed loop system on actuator design. Two cases of actuated valves connected to a closed loop system are analyzed in this paper. The first is a 71/16-in. subsea slab gate valve in the pressure class of 517 bar with a linear spring return fail-safe close (FSC) actuator located on a manifold branch. The data indicates that the piston rod and cylinder diameter of the FSC linear actuator should be increased by some millimeters due to the accumulation of hydraulic oil at the bottom of the actuator. The hydraulic oil in the closed loop system helps in closing the actuator and spring force, so the spring constant and torque should be reduced as a result. The second case involves a 16-in. subsea ball valve in the pressure class of 517 bar with a double-acting fail-as-is rack and pinion actuator. The conclusion in this case is to avoid making any change in the design of double-acting actuator in connection to the closed loop system.

Matteo Schiaretti1, Jie Cai2, Xiaoli Jiang1, Shengming Zhang3, Dingena Schott1
Journal of Marine Science and Application,2021(2): 343-353
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Industry design standards such as BS 7910 deployed some empirical formulas for the prediction of stress intensity factor (SIF) based on simulation results from traditional finite element method (FEM). However, such FEM simulation occasionally failed to convince people due to the large discrepancies compared with engineering practice. As a consequence, inaccuracy predictions via such formulas in engineering standards inevitably occur, which will compromise the safety of structures. In our previous research work, an abnormal phenomenon of SIF in a cracked T-butt joint accounting for welding effect has been observed. Compared with BS 7910, the calculation results of SIF at the surface points of welded specimens cannot be well predicted, with a large discrepancy appearing. In order to explore such problem with an abnormal increase at the surface points of cracked welded specimens, a numerical investigation in terms of SIF among BS 7910, XFEM, and FEM is performed in this paper. Numerical models on both a simple cracked plate without welding effect and a cracked T-butt joint with welding effect are developed through ABAQUS. Parametric studies in terms of the effects of varied crack depth to thickness ratio (a/T) and the effects of crack depth to crack half-length ratio (a/c) are carried out. Empirical solutions from BS 7910 are used for comparison. It is found that the XFEM can provide predictions of SIF at both the crack deepest point and crack surface point of a simple cracked plate as accurate as FEM. For a T-butt joint with a transverse stiffener, a large discrepancy in terms of the weld magnification factors (Mk) occurs at the crack surface point compared with empirical predictions. An exceptional increase of von Mises stress gradient in regions close to the weld-toe is found through the simulation of FEM, whereas a constant stress gradient is obtained through XFEM. The comparison results indicate an inappropriate prediction of SIF by the utilization of the empirical formulas in BS 7910. A more reasonable prediction of the SIF at the surface point of a crack is obtained by the XFEM. Therefore, further updating of the empirical solutions in BS 7910 for SIF accounting for welding effect is recommended.

Peng Liu1, Qianqian Chen1, Chao Zheng2, Guofa Sun1
Journal of Marine Science and Application,2021(2): 354-370
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The subsea all-electric Christmas tree (XT) is a key equipment in subsea production systems. Once it fails, the marine environment will be seriously polluted. Therefore, strict reliability analysis and measures to improve reliability must be performed before a subsea all-electric XT is launched; such measures are crucial to subsea safe production. A fault-tolerant control system was developed in this paper to improve the reliability of XT. A dual-factor degradation model for electrical control system components was proposed to improve the evaluation accuracy, and the reliability of the control system was analyzed based on the Markov model. The influences of the common cause failure and the failure rate in key components on the reliability and availability of the control system were studied. The impacts of mean time to repair and incomplete repair strategy on the availability of the control system were also investigated. Research results show the key factors that affect system reliability, and a specific method to improve the reliability and availability of the control system was given. This reliability analysis method for the control system could be applied to general all-electric subsea control systems to guide their safe production.

Shuo Yang
Journal of Marine Science and Application,2021(2): 371-380
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Because the applications of single-anchor leg mooring yoke systems (SYSs) are rarely studied in the offshore industry, the design of such systems features some uncertainties. This paper investigated the effect of eccentricity on the wear of the topside axial bearing of a SYS. The eccentricity of the topside was verified by on-site inspection, and the axial bearing wear was found to be far more serious than the original design. The contact status between the axial bearing and flange surface was studied on the basis of the actual topside load by using nonlinear finite element analysis. Wear tests of the topside bearing under uniform and eccentric loads were also performed to study the effect of eccentric loads on the wear rate. The key parameters obtained from numerical simulations and experimentation were used to calculate the wear depth via a simplified linear wear model based on the product of the pressure and sliding distance. Results showed that eccentric loads are the main factor responsible for the excessive wear of topside axial bearings.

Haipeng Zhang1, Zichen Zhou2, Miao Chen1, Difei Yi3
Journal of Marine Science and Application,2021(2): 381-392
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The living area of an offshore platform is the main living place for operators in offshore oil and gas fields. Fire risk assessment plays an important role in the safety of personnel in offshore platforms. In this paper, a fire risk assessment mathematical model for offshore platforms is proposed based on a comprehensive safety assessment method. The concept of danger time is presented according to the evaluation criteria of safe evacuation. The fire risk of offshore platforms is assessed by combining probability statistics with numerical simulation. The fire risk is quantitatively assessed by using an N500 deep water semi-submersible support platform as an example. According to the FN curve, fire frequency, fire escalation probability, and casualty probability, the rationality of marine general layout is analyzed, and the general layout design could be optimized to reduce the fire risk.