Research Papers: Ocean Engineering

J. Offshore Mech. Arct. Eng. 2019;141(3):031101-031101-11. doi:10.1115/1.4041997.

In this paper, the effect of weather routing and ship operations on the extreme vertical bending moment (VBM) in a 6000TEU class large container ship which is operated in North Atlantic Ocean is addressed. A direct time-domain nonlinear response simulation method taking account of the wave-induced vibrations is combined with a voyage simulation based on 10 years of meteorological data in the area. The probability distribution of the ship's operational parameters conditional upon the meteorological conditions is considered. It is clarified that the most severe wave condition with the significant wave height over 16 m in the area may not be encountered by the ship due to the weather routing and the extreme value is determined mostly by the wave condition much milder than the most severe in the area. It is also found out that the ship speed assumed in the most contributing sea state strongly affects the extreme value of the total VBM. It is explained by the fact that the wave-induced vibrations in the ship tend to be excited at faster speed.

Topics: Waves , Vibration , Ships , Seas
Commentary by Dr. Valentin Fuster

Research Papers: Offshore Technology

J. Offshore Mech. Arct. Eng. 2019;141(3):031301-031301-10. doi:10.1115/1.4041993.

Fatigue design standards for offshore structures became needed with development of offshore structures in harsh environments like the North Sea during the 1970s. The need for fatigue design of ship structures became increased as more high strength steel was being used in these structures during the 1970s. New types of structures and structural components have been developed such as tension leg platforms and floating production platforms and support structures for wind turbines. These structures are subjected to significant dynamic loading such that fatigue design becomes the main issue and relevant fatigue design standards are needed. This paper gives an overview of the development of fatigue design standards for marine structures over the last 40 years.

Commentary by Dr. Valentin Fuster

Research Papers: Polar and Arctic Engineering

J. Offshore Mech. Arct. Eng. 2019;141(3):031501-031501-15. doi:10.1115/1.4042263.

This paper presents a numerical model intended to simulate the mooring force and the dynamic response of a moored structure in drifting ice. The mooring lines were explicitly modeled by using a generic cable model with a set of constraint equations providing desired structural properties such as the axial, bending, and torsional stiffness. The six degrees-of-freedom (DOF) rigid body motions of the structure were simulated by considering its interactions with the mooring lines and the drifting ice. In this simulation, a fragmented ice field of broken ice pieces could be considered under the effects of current and wave. The ice–ice and ice–structure interaction forces were calculated based on a viscoelastic-plastic rheological model. The hydrodynamic forces acting on the floating structure, mooring line, and drifting ice were simplified and calculated appropriately. The present study, in general, demonstrates the potential of developing an integrated numerical model for the coupled analysis of a moored structure in a broken ice field with current and wave.

Commentary by Dr. Valentin Fuster

Research Papers: Structures and Safety Reliability

J. Offshore Mech. Arct. Eng. 2019;141(3):031601-031601-12. doi:10.1115/1.4041992.

The Norwegian Public Roads Administration is running a project “Ferry Free Coastal Route E39” to replace existing ferry crossings by bridges across eight fjords in western Norway. Since most of the fjords are wide and deep, construction of traditional bridges with fixed foundations is not possible. Therefore, floating bridge concepts are proposed for the fjord-crossing project. Since the floating foundations of the bridges are close to the water surface, the concern of accidental ship collisions is raised. Considering the displacement and speed of the passing ships and the significant compliance of the bridge, interaction between the bridge and the ship can be significant should a collision occur. Many studies have been conducted on ship collision with bridge structures with a special focus on bridge piers. However, the research on ship collision with bridge girders is quite limited. The purpose of this study is to investigate the collision response of a floating bridge for ship–girder collision events. Both the local structural damage and the global dynamic response of the bridge are assessed. Local structural deformation and damage are first investigated by numerical simulations with detailed finite element (FE) models in ls-dyna. Subsequently, the bridge global response to the collision loads is analyzed in usfos using the force–deformation curves from the local analysis. By combining the local and global analysis results, a comprehensive overview of the bridge response during ship–girder collisions can be obtained.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(3):031602-031602-6. doi:10.1115/1.4041991.

The objective of the present study is to identify the most suitable corrosion degradation model, fitted with real corrosion depth measurement data sets and to reproduce the corroded steel plate surface as a function of time and spatial distribution using advanced statistical methods. An approach for adequately identifying the best-fitted model to real corrosion depth measurement data sets is employed. Two distinct statistical methods for generating a statistical representation of a corroded plate surface in the case of significant and insignificant correlation of the corrosion degradation are provided. A sequence-dependent data analysis is performed based on the fast Fourier transform, which is used as an input for a random field modeling of corroded steel plate surfaces. The output of this study represents very important information about the corroded plate surface topology that can be used in any advanced finite element analyses of structural integrity assessment. The formulations can be adapted to any structural components and corrosion environments.

Topics: Corrosion , Steel , Modeling
Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(3):031603-031603-9. doi:10.1115/1.4042420.

Long floating bridges supported by pontoons with span-widths between 100 m and 200 m are discrete hydro-elastic structures with many critical eigenmodes. The response of the bridge girder is dominated by vertical eigenmodes and coupled horizontal modes (lateral) and rotational modes (about the longitudinal axis of the bridge girder). This paper explores the design principles used to reduce the response with regards to these eigenmodes. It is shown for a floating bridge with 200 m span-width that by inserting a bottom flange the vertical eigenmodes can be lifted out of wind-driven wave regime. It is also shown that selecting a pontoon length that leads to cancelation of horizontal excitation forces is beneficial, and that the geometrical shaping of the pontoon can be efficient to decrease the bridge response.

Commentary by Dr. Valentin Fuster

Research Papers: Ocean Renewable Energy

J. Offshore Mech. Arct. Eng. 2019;141(3):031901-031901-7. doi:10.1115/1.4041996.

Common industrial practice for designing floating wind turbines is to set an operational limit for the tower-top axial acceleration, normally in the range of 0.2–0.3 g, which is typically understood to be related to the safety of turbine components. This paper investigates the rationality of the tower-top acceleration limit by evaluating the correlation between acceleration and drivetrain responses. A 5-MW reference drivetrain is selected and modeled on a spar-type floating wind turbine in 320 m water depth. A range of environmental conditions are selected based on the long-term distribution of wind speed, significant wave height, and peak period from hindcast data for the Northern North Sea. For each condition, global analysis using an aero-hydro-servo-elastic tool is carried out for six one-hour realizations. The global analysis results provide useful information on their own—regarding the correlation between environmental condition and tower top acceleration, and the correlation between tower top acceleration and other responses of interest—which are used as input in a decoupled analysis approach. The load effects and motions from the global analysis are applied on a detailed drivetrain model in a multibody system (MBS) analysis tool. The local responses on bearings are then obtained from MBS analysis and postprocessed for the correlation study. Although the maximum acceleration provides a good indication of the wave-induced loads, it is not seen to be a good predictor for significant fatigue damage on the main bearings in this case.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(3):031902-031902-10. doi:10.1115/1.4041995.

Real-time hybrid testing of floating wind turbines is conducted at model scale. The semisubmersible, triangular platform, similar to the WindFloat platform, is built instead to support two, counter-rotating vertical-axis wind turbines (VAWTs). On account of incongruous scaling issues between the aerodynamic and the hydrodynamic loading, the wind turbines are not constructed at the same scale as the floater support. Instead, remote-controlled plane motors and propellers are used as actuators to mimic only the tangential forces on the wind-turbine blades, which are attached to the physical (floater-support) model. The application of tangential forces on the VAWTs is used to mimic the power production stage of the turbine. A control algorithm is implemented using the wind-turbine generators to optimize the platform heading and hence, the theoretical power absorbed by the wind turbines. This experimental approach only seeks to recreate the aerodynamic force, which contributes to the power production. In doing so, the generator control algorithm can thus be validated. The advantages and drawbacks of this hybrid simulation technique are discussed, including the need for low inertia actuators, which can quickly respond to control signals.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(3):031903-031903-5. doi:10.1115/1.4041994.

In this paper, numerical modeling and analysis of a hybrid-spar floating wind turbine is presented. The hybrid-spar consists of steel at the upper part and precast prestressed concrete at the lower part. Such a configuration is referred to as a hybrid-spar in this paper. The hybrid spar was successfully installed offshore of Kabashima island, Goto city, Nagasaki prefecture, Japan, on Oct. 18, 2013 (see Utsunomiya et al., 2015, “Design and Installation of a Hybrid-Spar Floating Wind Turbine Platform,” ASME Paper No. OMAE2015-41544 for details). In this paper, some details on numerical modeling of the hybrid-spar for design load analysis are presented. Then, the validation of the numerical analysis model is presented for a full-scale hybrid-spar model with 2-MW wind turbine. The comparison has been made for the natural periods and the response during rated power production test. Basically, both comparisons have shown good agreement between the measured values and the simulation, showing reliability of the developed code and the numerical model.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(3):031904-031904-8. doi:10.1115/1.4042264.

The development of an analytical model for the prediction of the stochastic nonlinear wave loads on the support structure of bottom mounted and floating offshore wind turbines is presented. Explicit expressions are derived for the time-domain nonlinear exciting forces in a sea state with significant wave height comparable to the diameter of the support structure based on the fluid impulse theory (FIT). The method is validated against experimental measurements with good agreement. The higher order moments of the nonlinear load are evaluated from simulated force records and the derivation of analytical expressions for the nonlinear load statistics for their efficient use in design is addressed. The identification of the inertia and drag coefficients of a generalized nonlinear wave load model trained against experiments using support vector machine learning algorithms is discussed.

Commentary by Dr. Valentin Fuster

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