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Research Papers: Offshore Technology

J. Offshore Mech. Arct. Eng. 2017;140(2):021301-021301-14. doi:10.1115/1.4037829.

Wave breaking is one of the major concerns for offshore structures installed in shallow waters. Impulsive breaking wave forces sometimes govern the design of such structures, particularly in areas with a sloping sea bottom. Most of the existing offshore wind turbines were installed in shallow water regions. Among fixed-type support structures for offshore wind turbines, jacket structures have become popular in recent times as the water depth for fixed offshore wind structures increases. However, there are many uncertainties in estimating breaking wave forces on a jacket structure, as only a limited number of past studies have estimated these forces. Present study is based on the WaveSlam experiment carried out in 2013, in which a jacket structure of 1:8 scale was tested for several breaking wave conditions. The total and local wave slamming forces are obtained from the experimental measured forces, using two different filtering methods. The total wave slamming forces are filtered from the measured forces using the empirical mode decomposition (EMD) method, and local slamming forces are obtained by the frequency response function (FRF) method. From these results, the peak slamming forces and slamming coefficients on the jacket members are estimated. The breaking wave forces are found to be dependent on various breaking wave parameters such as breaking wave height, wave period, wave front asymmetry, and wave-breaking positions. These wave parameters are estimated from the wave gauge measurements taken during the experiment. The dependency of the wave slamming forces on these estimated wave parameters is also investigated.

Topics: Waves
Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;140(2):021302-021302-12. doi:10.1115/1.4038031.

Free vibration analysis of functionally graded (FG) rectangular plates on two-parameter elastic foundation and vertically coupled with fluid is the objective of this work. The fluid domain is considered to be infinite in length, but it is bounded in depth and width directions, and the effects of hydrostatic pressure and free surface waves are not taken into account. The mechanical properties of the FG plates are assumed to vary continuously through the thickness direction according to a power-law distribution of the volume fraction of the constituents. The accuracy and applicability of the formulation is illustrated by comparison studies with those reported in the open literature. At the end, parametric studies are carried out to examine the impact of different parameters on the natural frequencies.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;140(2):021303-021303-9. doi:10.1115/1.4038343.

Floating breakwaters (FBWs) are widely used in moderate wave climatic conditions for coastal protection against erosion and for wave reduction around offshore loading terminals and open ocean construction sites. Literature shows that the width of a pontoon-type FBW is about 50% of the incident wavelength in order to achieve 50% wave height reduction at the lee side of the FBW. Hence, for a typical wavelength of 40 m, the width needed for pontoon FBW is about 20 m. Such an FBW may not be cost competitive. Is it possible to reduce the width of the pontoon FBW significantly by adding skirt walls (single, twin, triple, or five) at its keel. What will be the effect on mooring forces? In order to find solutions for these problems, experimental investigations were carried out on a typical pontoon-type FBW as well as pontoon with skirt walls. Both opaque and porous skirt walls were used. Wave transmission, reflection, and mooring forces, both on the sea side and lee side, were measured. It was found from this study that it is possible to reduce the width by 20 to 40% by introducing three or five skirt walls. However, introducing skirt walls increased the mooring forces by 10 to 30%. The results of this study are expected to be useful for cost-effective design of FBWs.

Commentary by Dr. Valentin Fuster

Research Papers: Materials Technology

J. Offshore Mech. Arct. Eng. 2017;140(2):021401-021401-8. doi:10.1115/1.4038345.

Creep tests of high modulus polyethylene (HMPE) samples are performed to obtain the corresponding creep and creep-rupture curves. The results show that the secondary creep stage is especially pronounced, and its creep rate is nearly constant. Based on this fact, the creep rate at the secondary stage has been utilized as the representative value of each case. Therefore, the similarity criterion of creep rate at secondary stage is proposed to analyze the corresponding relationship between engineering creep rate and that in laboratory tests. Besides, an empirical expression of HMPE yarns is proposed to account for the effects of both loading level and test temperature on the creep rate at the secondary stage. Moreover, an equation for calculating the creep lifetime of HMPE samples is derived and verified by test data. This study will help to improve the understanding of creep behaviors of HMPE ropes and provide a significant reference for the application of HMPE ropes in mooring engineering.

Topics: Creep , Ropes , Rupture , Stress
Commentary by Dr. Valentin Fuster

Research Papers: Structures and Safety Reliability

J. Offshore Mech. Arct. Eng. 2017;140(2):021601-021601-9. doi:10.1115/1.4038346.

This paper presents an object-oriented modeling (OOM) approach to model development of marine operation systems, specifically the hydraulic systems of marine cranes. Benefited from the rapid development of computation technology, many modeling and simulation techniques and software tools have proved to be very useful during the product and system development process. However, due to the increasing complexity of the physical systems, many challenges still exist regarding model flexibility, model integration, simulation accuracy, stability, and efficiency. The goal of introducing OOM to complex dynamic systems is to provide flexible, effective, and efficient models for different simulation applications. Previous work presented a virtual prototyping (VP) framework based on the functional mock-up interface (FMI) standard. The advantage of using FMI co-simulation is that modeling and simulation of stiff and strongly coupled systems can be distributed. As a result, the modeling tradeoffs between simulation accuracy and efficiency can be evaluated. The essential features of OOM and its application within dynamic operation system domain are highlighted through a case study. These features include model causality, model encapsulation, and inheritance that facilitate the decomposition and coupling of complex system models for co-simulation. The simulation results based on the proposed VP framework showed speedups in the computation efficiency at the cost of moderate accuracy loss.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;140(2):021602-021602-14. doi:10.1115/1.4038347.

A time domain code based on strip theory is applied to calculate the probability distributions of relative motions and bending moments of a cruise ship in a set of extreme seas. The code includes two levels of complexity. The simpler one combines linear radiation and diffraction forces with nonlinear Froude–Krylov forces, hydrostatic forces, and shipping of green water on the bow. Cummins formulation is used to represent the radiation forces. The second approach is a generalization of the first one and, although the formulation is based on the linear assumption (of the radiation forces), the effects of body nonlinearity are considered by a simplified method: the memory functions, infinite frequency added masses, and the radiation restoring coefficients are assessed at each time instant as function of the instantaneous wetted surface. A similar procedure is used to calculate the diffraction forces. The code is used to analyze the responses of a cruise ship in a set of extreme sea conditions. The short-term nonlinear responses are represented by empirical probability distributions, obtained from the nonlinear time domain simulations, and the quality of the predictions is assessed by comparing with model tests experimental data. Finally, the long-term value of the bending moment is calculated from the short-term distribution of the nonlinear loads in a few extreme sea states selected based on coefficient of contribution method, and the results are compared with the International Association of Classification Societies (IACS) rule bending moment.

Commentary by Dr. Valentin Fuster

Research Papers: Piper and Riser Technology

J. Offshore Mech. Arct. Eng. 2017;140(2):021701-021701-9. doi:10.1115/1.4038325.

The industry consensus would appear that the effect of currents on wave-induced fatigue damage accumulation is assumed as insignificant and can be ignored. Only when dealing with stability, ultimate limit state design, and vortex-induced vibration (VIV), is the recommended industry practice to consider both currents and waves simultaneously, except for fatigue design. This paper presents a study on how environmental loads should be considered in terms of currents and waves for the fatigue life design of offshore pipelines and risers. The study is intended as a spur to redress the misapprehension by focusing on the coupling effect of direct waves and currents in the context of fatigue damage assessment. It is demonstrated unequivocally that waves and currents cannot be decoupled for fatigue design assessments. Wave-induced fatigue with the inclusion of currents is manifested twofold, not only the increased mean stress correction effect but also higher total damage accumulation due to elevated stress ranges. The practice of using wave histograms while ignoring currents is shown to result in an unacceptable nonconservative fatigue design. Both effects should be accounted for in the engineering assessment. A first-order correction factor involving the ratio of current and wave velocities is introduced to evaluating the environmental load coupling effect. It is recognized that fatigue associated specifically with VIV phenomena is well understood and documented elsewhere, its discussion is thus out with the aims of this paper.

Topics: Stress , Waves , Fatigue , Currents
Commentary by Dr. Valentin Fuster

Research Papers: Ocean Renewable Energy

J. Offshore Mech. Arct. Eng. 2017;140(2):021901-021901-8. doi:10.1115/1.4037826.

Environmental conditions created by winds blowing oblique to the direction of the waves are necessary to conduct some survivability tests of offshore wind turbines. However, some facilities lack the capability to generate quality waves at a wide range of angles. Thus, having a wind generation system that can be rotated makes generating winds that blow oblique to the waves possible during survivability tests. Rotating the wind generation system may disrupt the flow generated by the fans because of the effect of adjacent walls. Closed or semiclosed wind tunnels may eliminate the issue of wall effects, but these types of wind tunnels could be difficult to position within a wave basin. In this work, a prototype wind generation system that can be adapted for offshore wind turbine testing is investigated. The wind generation system presented in this work has a return that minimizes the effect that the walls could potentially have on the fans. This study characterizes the configuration of a wind generation system using measurements of the velocity field, detailing mean velocities, flow directionality, and turbulence intensities. Measurements were taken downstream to evaluate the expected area of turbine operation and the shear zone. The dataset has aided in the identification of conditions that could potentially prevent the production of the desired flows. Therefore, this work provides a useful dataset that could be used in the design of wind generation systems and in the evaluation of the benefits of recirculating wind generation systems for offshore wind turbine research.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;140(2):021902-021902-13. doi:10.1115/1.4038249.

In this paper, numerical and experimental investigations are presented on the hydrodynamic performance of a horizontal tidal current turbine (TCT) designed and made by our Dalian University of Technology (DUT) research group. Thus, it is given the acronym: DUTTCT. An open-source computational fluid dynamics (CFD) solver, called pimpledymfoam, is employed to perform numerical simulations for design analysis, while experimental tests are conducted in a DUT towing tank. The important factors, including self-starting velocity, tip speed ratio (TSR), and yaw angle, which play important roles in the turbine output power, are studied in the investigations. Results obtained show that the maximum power efficiency of the newly developed turbine (DUTTCT) could reach up to 47.6%, and all its power efficiency is over 40% in the TSR range from 3.5 to 6; the self-starting velocity of DUTTCT is about 0.745 m/s; and the yaw angle has negligible influence on its efficiency as it is less than 10 deg.

Commentary by Dr. Valentin Fuster

Research Papers: Offshore Geotechnics

J. Offshore Mech. Arct. Eng. 2017;140(2):022001-022001-9. doi:10.1115/1.4038032.

Artificial frozen soils (AFS) have been used widely as temporary retaining walls in strata with soft and water-saturated soil deposits. After excavations, frozen soils thaw, and the lateral earth pressure penetrates through the soils subjected to freeze–thaw, and acts on man-made facilities. Therefore, it is important to investigate the lateral pressure (coefficient) responses of soils subjected to freeze–thaw to perform structure calculations and stability assessments of man-made facilities. A cubical testing apparatus was developed, and tests were performed on susceptible soils under conditions of freezing to a stable thermal gradient and then thawing with a uniform temperature (Fnonuni–Tuni). The experimental results indicated a lack of notable anisotropy for the maximum lateral preconsolidated pressures induced by the specimen’s compaction and freeze–thaw. However, the freeze–thaw led to a decrement of lateral earth pressure coefficient  K0, and  K0 decrement under the horizontal Fnonuni–Tuni was greater than that under the vertical Fnonuni–Tuni. The measured  K0 for normally consolidated and over-consolidated soil specimens exhibited anisotropic characteristics under the vertical Fnonuni–Tuni and horizontal Fnonuni–Tuni treatments. The anisotropies of  K0 under the horizontal Fnonuni–Tuni were greater than that under the vertical Fnonuni–Tuni, and the anisotropies were more noticeable in the unloading path than that in the loading path. These observations have potential significances to the economical and practical design of permanent retaining walls in soft and water-saturated soil deposits.

Commentary by Dr. Valentin Fuster

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