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### Research Papers: Ocean Engineering

J. Offshore Mech. Arct. Eng. 2018;141(2):021101-021101-13. doi:10.1115/1.4041016.

Flow systems with highly nonlinear free/moving surface motion are common in engineering applications, such as wave impact and fluid-structure interaction (FSI) problems. In order to reveal the dynamics of such flows, as well as provide a reduced-order modeling (ROM) for large-scale applications, we propose a proper orthogonal decomposition (POD) technique that couples the velocity flow field and the level-set function field, as well as a proper normalization for the snapshots data so that the low-dimensional components of the flow can be retrieved with a priori knowledge of equal distribution of the total variance between velocity and level-set function data. Through numerical examples of a sloshing problem and a water entry problem, we show that the low-dimensional components obtained provide an efficient and accurate approximation of the flow field. Moreover, we show that the velocity contour and orbits projected on the space of the reduced basis greatly facilitate understanding of the intrinsic dynamics of the flow systems.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2018;141(2):021102-021102-9. doi:10.1115/1.4041533.

Due to the complexity of ocean environmental loading models and the nonlinearity and empirical parameters involved in hydrodynamic numerical modeling and model testing, many uncertainties still exist in the design and operation of floating platforms. On-site prototype measurements provide a valid strategy for obtaining accurate environmental loading parameters and floater motion responses. A prototype monitoring system was built as part of a joint industrial project in the South China Sea. Long-term ocean environmental loading parameter data and structural dynamic motion responses were collected from 2012 to the present. In this study, the dynamic motions of the platform structure were analyzed using an artificial neural network (ANN) and data obtained during a typhoon. Numerical modeling was performed to analyze the platform parameters using a radial basis function (RBF), and hydrodynamic modeling was conducted using ansys-aqwa. Five geometric parameters related to the platform design were selected for optimization and included the mass, moments of inertia of the three rotation degrees, and the position of the center of gravity (COG). The mean values of the surge and pitch and the standard deviations of the roll and pitch were used as the input parameters. The model validations showed that the proposed ANN-based method performed well for obtaining the optimal platform parameters. The maximum errors of the roll, pitch, surge, and sway motions were within 5%. The updated response amplitude operators (RAOs) and new design indices for a 100-year return period of a typhoon were determined to guide operations and evaluate platform designs.

Commentary by Dr. Valentin Fuster

### Research Papers: Polar and Arctic Engineering

J. Offshore Mech. Arct. Eng. 2018;141(2):021501-021501-15. doi:10.1115/1.4041015.

With increasing need to utilize inland waterways (IWW), the design of IWW vessels gains attention both from a transport efficiency and an emission control point of view. The primary challenge is to estimate the ice pressure acting on the ship hull for IWW. Ice information for Lake Mälaren is extracted and analyzed in this work. Since the ice properties have great influence on the impact load, they are studied based on empirical formulae and are calibrated by reference data. The ice impact is then predicted for an IWW barge. Probabilistic method is selected to derive the load based on available field test data. Several parent datasets are chosen, and different design strategies are implemented to evaluate the ice impact load and investigate the influence from exposure factors. The paper finds that the design curve of $α=0.265a−0.57$ can be used for Lake Mälaren. The approach itself introduces a possible way to investigate loads on ice-affected IWW.

Topics: Pressure , Design , Ice , Stress
Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2018;141(2):021502-021502-16. doi:10.1115/1.4041394.

In this paper, the ice load accumulated on a vertical plate of marine platforms during periodic spray icing in a cold room was investigated experimentally. The mass and thickness of ice formation on the plate along with several parameters such as relative humidity, the front and back surface temperatures of the vertical plate, initial temperature of water, and the spray mass flux impinging on the plate were measured and discussed. Analysis of variance (ANOVA), which is a statistical data analysis method, was utilized to interpret the contribution of the investigated parameters during the icing experiments, comparing the effect of each parameter and their interactions on the quantity of ice accumulated on the vertical plate. The primary analysis of the empirical results illustrates that the ambient temperature, airflow velocity, the distance between the fan and the plate, salinity and the timing of spray events have influences in the icing intensity and the amount of ice formation on the vertical plate. The errors between the average ice thicknesses obtained from two different experimental approaches were from 5 to 20%. For the saline ice formation, the temperature difference between the front and back of the vertical plate was greater than that of the pure ice formed during the spray icing event. The primary experimental results alongside the ANOVA analysis verified that airflow velocity is the most effective parameter, with a high level of interaction for time and temperature.

Commentary by Dr. Valentin Fuster

### Research Papers: Structures and Safety Reliability

J. Offshore Mech. Arct. Eng. 2018;141(2):021601-021601-7. doi:10.1115/1.4041301.

Scientific studies dealing with mechanical vibration attenuation by means of piezoelectric actuators are mostly focused on special details of structure modeling through finite elements and the amount of attenuation that can be achieved. However, a little explored issue in the scientific literature is the size of the actuator in relation to the size of the vibrating structure and the voltage applied to the piezoelectric actuator in order to achieve optimum vibration attenuation. This paper presents a theoretical and experimental study of mechanical vibration control of an aluminum plate with attached piezoelectric actuator. The aluminum plate was clamped at all four sides and a piezoelectric actuator based on lead zirconium titanate (PZT) was positioned at its center. Its natural frequency was close to 50 Hz, which is a frequency being constantly present on oil drilling platforms, producing annoying sound. The contribution of this paper is the determination of the relationship between the areas of the aluminum plate and the PZT actuator associated with the voltage value applied to the piezo-actuator for the purpose of vibration attenuation. The work demonstrates the possibility of the development of open-loop control using finite elements, to attenuate the vibration via piezoelectric actuator plates. This method makes it possible to vary the electric voltage across the piezoelectric actuator and/or the actuator dimensions involved, leading to the best attenuation condition. Numerical simulations and experimental results show the relation between size of the PZT actuator and the electric field which must be applied for best attenuation.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2018;141(2):021602-021602-16. doi:10.1115/1.4041069.

The probability distributions of extreme responses of a flexible riser connected to a weather-vaning floating production storage and offloading (FPSO) are developed and investigated numerically for two tropical cyclones. Statistical properties of riser responses provide the foundation for response based analysis (RBA), a comprehensive approach for the prediction of extreme responses and design metocean conditions of offshore systems. The storm-based probabilistic analysis is applied to responses of flexible risers with the objective to develop their distributions in a storm and to determine their most probable maximum (MPM) values. An asymptotic form of the response distribution in a storm is formulated, which can be used in the random event, method of Tromans and Vandersohuren (1995, “Response Based Design Conditions in the North Sea: Application of a New Method,” Offshore Technology Conference, Houston, TX, May 1–4). The methodology is illustrated by two case studies for an FPSO in cyclonic storms at a location offshore Australia. Time domain simulations are employed to predict the FPSO motions, critical riser responses, and their probability distributions. It is shown that the maximum storm responses can be reproduced by governing “equivalent” metocean intervals with increased percentiles or inflated durations. Effects of different environmental excitation upon the risers and their impact on the statistical properties of responses are discussed, providing important insights for extension toward a multistorm RBA approach. The study also discusses issues with practices such as the analysis for a 3 h design event and presents observations on the variability of several types of responses, which reveal their environmental sensitivities.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2018;141(2):021603-021603-10. doi:10.1115/1.4041395.

In the construction of thin plate steel structures, including ships, welding is widely used to join parts. Welding inevitably causes deformation in thin plate structures, which may cause various problems. In the present study, an analysis method is developed to realize the prediction of deformation during the construction of large-scale structures based on the thermal elastic plastic analysis method. The developed method uses the idealized explicit finite element method (IEFEM), which is a high-speed thermal elastic plastic analysis method, and an algebraic multigrid method (AMG) is also introduced to the IEFEM in order to realize an efficient analysis of large-scale thin plate structures. In order to investigate the analysis accuracy and the performance of the developed method, the developed method is applied to the analysis of deformation on the welding of a simple stiffened structure. The developed method is then applied to the prediction of welding deformation in the construction of a ship block. The obtained results indicate that the developed method has approximately the same analysis accuracy as the conventional method, and the computational speed of the developed method is dramatically faster than that of the conventional method. The developed method can analyze the welding deformation in the construction of the ship block structure which consists of more than 10 million degrees-of-freedom and is difficult to solve by the conventional method.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2018;141(2):021604-021604-9. doi:10.1115/1.4041534.

The present study investigates the environmental conditions in the Sulafjord in Norway, where a floating bridge is being considered for construction. Fifteen months of wave and wind measurement data in the fjord are compared to the hindcast data at a relevant offshore site and a good overall correlation between the two is found. Furthermore, a quantitative relationship between the wave conditions offshore and in the fjord is established based on the storm event analysis. Accordingly, the identified relationship and the 60-year of offshore hindcast data enable the estimation of the design environmental conditions in the fjord, by adapting the fitted marginal and joint distribution of the wave conditions at the offshore site. The present study illustrates the possibility of using more data from the hindcast model for the design when the measurement data are limited.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2018;141(2):021605-021605-10. doi:10.1115/1.4041510.

Operational modal analysis (OMA) has been widely used for large structures. However, measured signals are inevitably contaminated with noise and may not be clean enough for identifying the modal parameters with proper accuracy. The traditional methods to estimate modal parameters in noisy situation are usually absorbing the “noise modes” first, and then using the stability diagrams to distinguish the true modes from the “noise modes.” However, it is still difficult to sort out true modes because the “noise modes” will also tend to be stable as the model order increases. This study develops a noise reduction procedure for polyreference complex exponential (PRCE) modal analysis based on ambient vibration responses. In the procedure, natural excitation technique (NExT) is first applied to get free decay responses from measured (noisy) ambient vibration data, and then the noise reduction method based on solving the partially described inverse singular value problem (PDISVP) is implemented to reconstruct a filtered data matrix from the measured data matrix. In our case, the measured data matrix is block Hankel structured, which is constructed based on the free decay responses. The filtered data matrix should maintain the block Hankel structure and be lowered in rank. When the filtered data matrix is obtained, the PRCE method is applied to estimate the modal parameters. The proposed NExT-PDISVP-PRCE scheme is applied to field test of a jacket type offshore platform. Results indicate that the proposed method can improve the accuracy of OMA.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2018;141(2):021606-021606-11. doi:10.1115/1.4041457.

Currently, in the oil and gas industry, finite element method (FEM)-based commercial software (such as ANSYS and abaqus) is commonly employed for determining the stress intensity factor (SIF). In their earlier work, the authors proposed an adaptive Gaussian process regression model (AGPRM) for the SIF prediction of a crack propagating in topside piping, as an inexpensive alternative to FEM. This paper is the continuation of the earlier work, as it focuses on the experimental validation of the proposed AGPRM. For validation purposes, the values of SIF obtained from experiments available in the literature are used. The experimental validation of AGPRM also consists of the comparison of the prediction accuracy of AGPRM and FEM relative to the experimentally derived SIF values. Five metrics, namely, root-mean-square error (RMSE), average absolute error (AAE), mean absolute percentage error (MAPE), maximum absolute error (MAE), and coefficient of determination ($R2$), are used to compare the accuracy. A case study illustrating the development and experimental validation of the AGPRM is presented. Results indicate that the prediction accuracy of AGPRM is comparable with and even higher than FEM, provided the training points of AGPRM are chosen aptly. Good prediction accuracy coupled with less time consumption favors AGPRM as an alternative to FEM for SIF prediction.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(2):021607-021607-7. doi:10.1115/1.4042538.

Data scarcity has always been a significant challenge in the domain of human reliability analysis (HRA). The advancement of simulation technologies provides opportunities to collect human performance data that can facilitate both the development and validation paradigms of HRA. The potential of simulator data to improve HRA can be tapped through the use of advanced machine learning tools like Bayesian methods. Except for Bayesian networks, Bayesian methods have not been widely used in the HRA community. This paper uses a Bayesian method to enhance human error probability (HEP) assessment in offshore emergency situations using data generated in a simulator. Assessment begins by using constrained noninformative priors to define the HEPs in emergency situations. An experiment is then conducted in a simulator to collect human performance data in a set of emergency scenarios. Data collected during the experiment are used to update the priors and obtain informed posteriors. Use of the informed posteriors enables better understanding of the performance, and a more reliable and objective assessment of human reliability, compared to traditional assessment using expert judgment.

Commentary by Dr. Valentin Fuster

### Research Papers: Piper and Riser Technology

J. Offshore Mech. Arct. Eng. 2018;141(2):021701-021701-11. doi:10.1115/1.4041458.

In recent years, there has been unprecedented interest shown in the Arctic region by the industry as it has become increasingly accessible for oil and gas exploration. This paper reviews existing literature on heat transfer coefficients and presents a comprehensive study of the heat transfer phenomenon in horizontal pipes (single/multiple pipe configurations) subjected to cross-flow wind besides the test methodology used to determine heat transfer coefficients through experiments. In this study, cross-flow winds of 5 m/s, 10 m/s, and 15 m/s blowing over several single pipe and multiple pipe configurations of diameter 25 mm and 50 mm steel pipes with insulation are examined. Based on the findings, the best correlation for use by the industry for single and multiple pipe configurations was found to be Churchill–Bernstein correlation. The deviation from the theoretical calculations and the experimental data for this correlation was found to be in the range of 0.40–1.61% for a 50 mm insulated pipe and −3.86% to −2.79% for a 25 mm insulated pipe. In the case of a multiple pipe configurations, the deviation was in the range 0.5–2.82% for 50 mm insulated pipe and 12–14% for 25 mm insulated pipes.

Commentary by Dr. Valentin Fuster

### Research Papers: CFD and VIV

J. Offshore Mech. Arct. Eng. 2018;141(2):021801-021801-10. doi:10.1115/1.4040980.

Fluid structure interaction (FSI) simulations of the NREL 5 MW wind turbine are performed using a combination of two separate computational codes: abaqus for the finite element analysis (FEA) of turbine structures and STAR-CCM+ for the unsteady Reynolds-averaged Navier–Stokes analysis of flow around the turbine. The main aim of this study is to demonstrate the feasibility of using two-way coupled FSI simulations to predict the oscillation of the tower, as well as the rotor blades, of a full-scale wind turbine. Although the magnitude of the oscillation of the tower is much smaller than that of the blades, this oscillation is crucial for the assessment of the fatigue life of the tower. In this first part of the paper, the aerodynamic characteristics of the turbine predicted by the two-way coupled FSI simulations are discussed in comparison with those predicted by Reynolds-averaged Navier–Stokes simulations of a rigid turbine. Also, two different computational domains with a cross-sectional size of 2D × 2D and 4D × 4D (where D is the rotor diameter) are employed to investigate the blockage effect. The fatigue life assessment of the turbine is planned to be reported in the second part of the paper in the near future.

Commentary by Dr. Valentin Fuster

### Research Papers: Ocean Renewable Energy

J. Offshore Mech. Arct. Eng. 2018;141(2):021901-021901-8. doi:10.1115/1.4041459.

Marine current energy is a reliable and clean source of energy. Many marine current turbines have been designed and developed over the years. Placement of an appropriately designed duct or shroud around the turbine significantly improves the turbine performance. In the present work, a ducted Savonius turbine (DST) is designed and optimized and its performance analysis carried out. The components of DSTs are simple and easily available and can be manufactured in developing countries like Fiji. A scaled-down model of 1/20 of a DST was fabricated and tested in a water stream at a velocity of 0.6 m/s and the results were used to validate the results from a commercial computational fluid dynamics (CFD) code ANSYS-cfx. Finally, a full-scale DST was modeled to study the flow characteristics in the turbine and the performance characteristics. The maximum efficiency of the turbine is around 50% at the tip speed ratio (TSR) of 3.5 and the maximum shaft power obtained is 10 kW at the rated speed of 1.15 m/s and around 65 kW at a freestream velocity of 2.15 m/s. The stress distribution on the ducted turbine was also obtained.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2018;141(2):021902-021902-8. doi:10.1115/1.4041203.

This paper theoretically and experimentally investigates the inertial effects of the flap body on the performance of a bottom-hinged oscillating wave surge converter (BH-OWSC). A two-dimensional (2D) hydrodynamic theory for a BH-OWSC based on the assumption of potential flow is developed to show that one simple but critical parameter, i.e., the square of sum of three mechanical-impedance terms associated with the inertial effects, can precisely characterize the performance trend of a BH-OWSC. Model testing in a small-scaled wave basin follows to validate the theoretical formulations with a flap body consisting of multiple hollow cylinders into which water can be filled individually to alter the values of flap's inertial parameters. The performance of each inertial specification of the flap model is evaluated based on the measurement of the mean water discharge from the hydraulic pump (or the power take-off). Finally, the “near resonant condition” has been validated experimentally by altering the inertial parameters of the flap. Thus, the aforementioned parameter is shown to be capable of characterizing the inertial effects on the performance of a BH-OWSC, and the minimization of it will maximize the power capturing performance of a BH-OWSC. Consequently, the parameter can be used for design guidelines of the flap body in its inertial aspect, such as locating the center of mass and determining the geometric dimensions of a flap body.

Topics: Waves , Surges , Water
Commentary by Dr. Valentin Fuster