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### Research Papers: Ocean Engineering

J. Offshore Mech. Arct. Eng. 2017;139(6):061101-061101-12. doi:10.1115/1.4037247.

The flow around an oscillating cylinder close to a horizontal solid boundary is studied to gather information about the load acting on pipelines, while they are laid on the sea bottom. The problem is simplified assuming that the pipeline section is rigid and oscillates harmonically only in the normal-to-seabed direction so that the problem can be tackled in two dimensions. A computational fluid dynamics (CFD) solver is used to take into account viscous effects in the hypothesis of laminar flow conditions. This best suits the conditions of pipeline layering when the Reynolds number, $Re=Um·D/ν$, ranges in order of 450–120,000, while the Keulegan–Carpenter number, $KC=Um·D/T$, ranges in order of 0.45–2. Nonetheless, boundary layer separation and vortex shedding are considered. Focus is on the determination of the lift force for which a novel analytical approximate expression is proposed. Such an analytical result can provide useful support to the studies related with the structural analysis of the pipe laying.

Commentary by Dr. Valentin Fuster

### Research Papers: Offshore Technology

J. Offshore Mech. Arct. Eng. 2017;139(6):061301-061301-7. doi:10.1115/1.4036911.

Mechanical properties and corrosion resistance of UNS S32205 duplex stainless steel (DSS) welds obtained under the application of controlled magnetic fields were evaluated in the context of offshore pipelines and flowlines applications. Tensile tests, impact toughness, and hardness measurements were performed. Corrosion behavior was evaluated by linear polarization resistance (LPR) and potentiodynamic polarization curves (PCs) using a synthetic seawater solution at different temperatures. An improvement in tensile strength, impact toughness, and corrosion resistance was observed with the application of magnetic fields during welding. This effect is attributed to the refinement in the microstructure of the weld metal along with the suppression of detrimental intermetallic tertiary phases. Applying an axial magnetic field of 3 mT during DSS welding by the gas metal arc welding (GMAW) process is a potential technique for improving the performance of offshore pipeline welds and may be implemented in both, double-sided single pass and single-sided multipass butt joints.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(6):061302-061302-8. doi:10.1115/1.4037065.

Umbilical is an important equipment in the subsea production to supply a connection between the floater and the subsea well. Analyzing strength and fatigue behaviors under bending is a key requirement to assure safety. An analytical model is proposed for predicting the bending behavior of a steel tube wounded helically around a frictionless cylinder. A full three-dimensional (3D) finite element (FE) model of an umbilical is developed by considering the frictions and contacts among its components. The numerical results of the bending stress of a steel tube were validated against that of the analytical model. The impacts of friction coefficients on the bending stress, contact pressure, and friction stress have been further investigated by the established FE model.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(6):061303-061303-8. doi:10.1115/1.4037173.

A finite difference method (FDM) solving the coupled partial differential equations governing three-dimensional (3D) motions of a towed underwater cable has been implemented in a combined ensemble Kalman filter (EnKF) and ensemble Kalman smoother (EnKS), as a new approach to combined state and parameter estimation for towed underwater cables. A simulation study of the method applied to a seismic streamer has been performed. Cable state variables as well as model parameters are estimated. Parameters estimated are crossline ocean current varying with time as well as cable tangential drag coefficient. The presented results indicate that the method is able to estimate state as well as parameters for seismic streamers.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(6):061304-061304-8. doi:10.1115/1.4037293.

A framework is presented for data mining in multivariate time series collected over hours of ship operation to extract vessel states from the data. The measurements made by a ship monitoring system lead to a collection of time-organized in-service data. Usually, these time series datasets are big, complicated, and highly dimensional. The purpose of time-series data mining is to bridge the gap between a massive database and meaningful information hidden behind the data. An important aspect of the framework proposed is selecting relevant variables, eliminating unnecessary information or noises, and extracting the essential features of the problem so that the vessel behavior can be identified reliably. Principal component analysis (PCA) is employed to address the issues of multicollinearity in the data and dimensionality reduction. The data mining approach itself is established on unsupervised data clustering using self-organizing map (SOM) and k-means, and k-nearest neighbors search (k-NNS) for searching and recovering specific information from the database. As a case study, the results are based on onboard monitoring data of the Norwegian University of Science and Technology (NTNU) research vessel, “Gunnerus.” The scope of this work is limited to detecting ship maneuvers. However, it is extendable to a wide range of smart marine applications. As illustrated in the results, this approach is effective in identifying the prior unknown states of the ship with acceptable accuracy.

Commentary by Dr. Valentin Fuster

### Research Papers: Materials Technology

J. Offshore Mech. Arct. Eng. 2017;139(6):061401-061401-10. doi:10.1115/1.4037290.

This paper examines the applicability of the different surrogate-models (SMs) to predict the stress intensity factor (SIF) of a crack propagating in topside piping, as an inexpensive alternative to the finite element methods (FEM). Six different SMs, namely, multilinear regression (MLR), polynomial regression (PR) of order two, three, and four (with interaction), Gaussian process regression (GPR), neural networks (NN), relevance vector regression (RVR), and support vector regression (SVR) have been tested. Seventy data points (consisting of load (L), crack depth (a), half crack length (c) and SIF values obtained by FEM) are used to train the aforementioned SMs, while 30 data points are used for testing. In order to compare the accuracy of the SMs, four metrics, namely, root-mean-square error (RMSE), average absolute error (AAE), maximum absolute error (MAE), and coefficient of determination (R2) are used. A case study illustrating the comparison of the prediction capability of various SMs is presented. python and matlab are used to train and test the SMs. Although PR emerged as the best fit, GPR was selected as the best SM for SIF determination due to its capability of calculating the uncertainty related to the prediction values. The aforementioned uncertainty representation is quite valuable, as it is used to adaptively train the GPR model (GPRM), which further improves its prediction accuracy and makes it an accurate, faster, and alternative method to FEM for predicting SIF.

Commentary by Dr. Valentin Fuster

### Research Papers: Polar and Arctic Engineering

J. Offshore Mech. Arct. Eng. 2017;139(6):061501-061501-9. doi:10.1115/1.4036478.

Sloping structures are widely used in ice-infested waters because of their ability to reduce ice loading by inducing a bending failure in ice sheets. From model test data, a significant velocity effect on the breaking load of ice sheets has been reported. In this paper, the ice–fluid interaction process is investigated by adopting the Euler–Bernoulli beam theory for the ice sheet and the potential theory for the underlying fluid domain. Accounting for the inertia effect of the ice sheet and the hydrodynamics of sea water beneath the ice sheet, the results demonstrate a velocity effect on the ice breaking loads in-plane deformation, which compare well with the available model test data. Moreover, our model formulation and implementation is such that the solutions for different ice velocities can be obtained rapidly from the reference solution, which facilitates the development of a real-time simulator. It is also shown that the velocity effect depends on the ice compressive strength and the angle of sloping structure.

Topics: Stress , Ice , Failure , Fluids
Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(6):061502-061502-7. doi:10.1115/1.4037141.

Shipping in ice-covered regions has gained high attention within recent years. Analogous to weather routing, the occurrence of ice in a seaway affects the selection of the optimal route with respect to the travel time or fuel consumption. The shorter, direct path between two points—which may lead through an ice-covered area—may require a reduction of speed and an increase in fuel consumption. A longer, indirect route, could be more efficient by avoiding the ice-covered region. Certain regions may have to be avoided completely, if the ice thickness exceeds the ice-capability of the ship. The objective of this study is to develop a computational method that combines coastline maps, route cost information (e.g., ice thickness), transport task, and ship properties to find the optimal route between port of departure, A, and port of destination, B. The development approach for this tool is to formulate the transport task in the form of a potential problem, solve this equation with a finite element method (FEM), and apply line integration and optimization to determine the best route. The functionality of the method is first evaluated with simple test problems and then applied to realistic transport scenarios.

Commentary by Dr. Valentin Fuster

### Research Papers: Structures and Safety Reliability

J. Offshore Mech. Arct. Eng. 2017;139(6):061601-061601-12. doi:10.1115/1.4037175.

Slamming loads from plunging breaking waves feature a high impulsive force and a very short duration. It is difficult to measure these loads directly in experiments due to the dynamics of the structures. In this study, inverse approaches are investigated to estimate the local slamming loads on a jacket structure using hammer test and wave test data from a model scale experiment. First, a state-of-the-art approach is considered. It uses two deconvolution techniques to first determine the impulse response functions and then to reconstruct the wave impact forces. Second, an easier applicable approach is proposed. It uses linear regression with the ordinary least square technique for the force estimation. The results calculated with these two approaches are highly identical. The linear regression approach can be extended to account for the loads transferred among different locations. This leads to lower and theoretically more accurate estimation of the loads compared to the previous two approaches. For the investigated case, the total impulse due to the wave is 22% lower. The estimated forces by the extended approach have a resolution at the millisecond level, which provides detailed information on the shape of the forces. The approach is an important tool for statistical investigations into the local slamming forces, and further on for the development of a reliable engineering model of the forces.

Commentary by Dr. Valentin Fuster

### Research Papers: Piper and Riser Technology

J. Offshore Mech. Arct. Eng. 2017;139(6):061701-061701-10. doi:10.1115/1.4037291.

When the external sheath of flexible pipes experiences damage, seawater floods the annulus. Then, the external pressure is applied directly on the internal polymeric layer, and the load is transferred to the interlocked carcass, the innermost layer. In this situation, the so-called wet collapse failure of the interlocked carcass can occur. Simplified methodologies to address such a scenario, using restricted three-dimensional (3D) finite element models, are presented in this work. They are compared with full 3D models, studying both straight and curved flexible pipes scenarios. The curvature of the flexible pipe is shown to be important for wet collapse pressure predictions.

Commentary by Dr. Valentin Fuster

### Research Papers: Ocean Renewable Energy

J. Offshore Mech. Arct. Eng. 2017;139(6):061901-061901-16. doi:10.1115/1.4037174.

In this paper, a methodology suitable for assessing the allowable sea states for installation of a transition piece (TP) onto a monopile (MP) foundation with focus on the docking operation is proposed. The TP installation procedure together with numerical analyses is used to identify critical and restricting events and their corresponding limiting parameters. For critical installation phases, existing numerical solutions based on frequency and time domain (TD) analyses of stationary processes are combined to quickly assess characteristic values of dynamic responses of limiting parameters for any given sea state. These results are compared against (nonlinear and nonstationary) time domain simulations of the actual docking operations. It is found that a critical event is the structural damage of the TP's bracket supports due to the potential large impact forces or velocities, and a restricting installation event (not critical) is the unsuccessful mating operation due to large horizontal motions of the TP bottom. By comparing characteristic values of dynamic responses with their allowable limits, the allowable sea states are established. Contact–impact problems are addressed in terms of assumed allowable impact velocities of the colliding objects. A possible automatic motion compensation system and human actions are not modeled. This methodology can also be used in connection with other mating operations such as float-over and topside installation.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(6):061902-061902-9. doi:10.1115/1.4037292.

This paper studies the dynamic characteristic of the truss Spar-type floating foundation used to support the offshore vertical-axis wind turbine (VAWT). The effects of changes in foundation structural parameters on its motions were evaluated. The results show that radius of the buoyancy tank, radius of the upper mechanical tank, interval of the center of gravity and center of buoyancy, and height of the upper mechanical tank have important effects on the heave and pitch motions of the foundation. Two sets of foundation parameters (FS-1 and FS-2) were selected to support the 5 MW Darrieus wind turbine. The motion performances of the two floating VAWTs, S-1 (the VAWT supported by FS-1) and S-2 (the VAWT supported by FS-2), were analyzed and compared. It was observed that the amplitudes of the heave and pitch motions of the floating VAWT depend on the wave loads; the mean values of the heave and pitch motions depend on the aerodynamic loads. The floating VAWT S-2 had better motion performance; its heave and pitch motions were all small. The heave frequencies of the floating VAWT were equal to the wave frequencies. For the pitch frequencies, there is a component of the rotor rotational frequency (0.175 Hz) for cases LC1 to LC4, while the amplitudes of the twice-per-revolution (2P) response are far smaller than the amplitudes of the wave response.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(6):061903-061903-11. doi:10.1115/1.4037294.

This study assesses the serviceability and fatigue limit states of the offshore wind turbine (OWT) founded in clay incorporating the impact of climate change. Two different offshore locations at east and west coasts in India are chosen. The ensemble of future time series of wind speed, wave height, and period is forecasted using statistical downscaling model (SDSM) at the regional level using the general circulation model (GCM) corresponding to the A1B, A2, and B1 emission scenarios. The downscaling model is calibrated by comparing simulations driven by the National Centers for Environmental Prediction (NCEP) high-resolution data and station data. Responses of OWT are obtained from dynamic analysis in a time domain using finite element (FE). The tower and monopile are modeled as Euler–Bernoulli beam, and soil resistance is modeled as American Petroleum Institute (API)-based p–y springs. The study shows future wind and wave loads are site specific, and it increases in the west coast and decreases in the east coast of India due to climate change. The simulation shows a substantial increase in future wind energy production at west coast compared to that of the east coast; however, safety margin considering serviceability and fatigue life decreases which requires modification in the design.

Commentary by Dr. Valentin Fuster