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research-article  
Alfredo Gay Neto, Clovis de Arruda Martins, Eduardo Malta, Rafael L. Tanaka and Carlos Godinho
J. Offshore Mech. Arct. Eng   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 restrict three-dimensional finite element models, are presented in this work. They are compared to full three-dimensional 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.
TOPICS: Pipes, Finite element model, Collapse, External pressure, Failure, Floods, Seawater, Three-dimensional models, Damage, Annulus, Pressure, Stress
research-article  
Liqin Liu, Weichen Jin and Ying Guo
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4037292
This paper studies the dynamic characteristic of the truss Spar type floating foundation used to support the offshore VAWT (vertical axis wind turbine). 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 2P (twice-per-revolution) response are far smaller than the amplitudes of the wave response.
TOPICS: Trusses (Building), Dynamic analysis, Spar platforms, Vertical axis wind turbines, Wing spars, Stress, Waves, Buoyancy, Wave frequency, Center of mass, Ocean engineering, Rotors, Wind turbines
research-article  
Swagata Bisoi and Sumanta Haldar
J. Offshore Mech. Arct. Eng   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 are forecasted using statistical downscaling model at the regional level using the General Circulation Model 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 high-resolution data and station data. Responses of OWT is obtained from dynamic analysis in a time domain using finite element. 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 require modification in the design.
TOPICS: Design, Offshore wind turbines, Soil, Climate change, Shorelines, American Petroleum Institute, Simulation, Waves, Maintainability, Ocean engineering, Resolution (Optics), Earth resistance, Stress, Safety, Wind velocity, Springs, Time series, Wind, Dynamic analysis, Finite element analysis, Wind energy, Fatigue life, Emissions, Fatigue limit
research-article  
Afshin Abbasi Hoseini and Sverre Steen
J. Offshore Mech. Arct. Eng   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 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.
TOPICS: Data mining, Databases, Ships, Time series, Vessels, Bridges (Structures), Noise (Sound), Monitoring systems, Principal component analysis, Self-organization
research-article  
Arvind Keprate, R.M. Chandima Ratnayake and Shankar Sankararaman
J. Offshore Mech. Arct. Eng   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, multi-linear regression (MLR), polynomial regression 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 thirty 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 (AAE), and Coefficient of Determination (R^2) 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, which further improves its prediction accuracy and makes it an accurate, faster and alternative method to FEM for predicting SIF.
TOPICS: Fracture (Materials), Underwater pipelines, Stress, Finite element methods, Trains, Errors, Uncertainty, Matlab, Polynomials, Support vector machines, Pipes, Testing, Artificial neural networks
research-article  
Alessio Pierro, Enrico Tinti, Stefano Lenci, Maurizio Brocchini and Giuseppina Colicchio
J. Offshore Mech. Arct. Eng   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=U_m·D/v, ranges in order of 450-120000, while the Keulegan-Carpenter number, KC=U_m·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.
TOPICS: Viscosity, Stress, Circular cylinders, Seabed, Pipelines, Computational fluid dynamics, Vortex shedding, Pipes, Cylinders, Seas, Boundary layers, Flow (Dynamics), Separation (Technology), Structural analysis, Dimensions, Lift (Fluid dynamics), Laminar flow, Reynolds number
research-article  
Jan Vidar Grindheim, Inge Revhaug and Egil Pedersen
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4037173
A finite difference method solving the coupled partial differential equations governing 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 are estimated as well as model parameters. 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.
TOPICS: Kalman filters, Parameter estimation, Cables, Simulation, Finite difference methods, Drag (Fluid dynamics), Partial differential equations, Ocean currents
research-article  
Wilson Guachamin Acero, Zhen Gao and Torgeir Moan
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4037174
In this paper, a methodology suitable for assessing the allowable sea states for installation 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 are 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 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 non-stationary) 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.
TOPICS: Offshore wind turbines, Seas, Dynamic response, Motion compensation, Simulation, Engineering simulation, Numerical analysis, Damage, Time-domain analysis
research-article  
Ying Tu, Thorvald C. Grindstad and Michael Muskulus
J. Offshore Mech. Arct. Eng   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, 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 of the local slamming forces, and further on for the development of a reliable engineering model of the forces.
TOPICS: Stress, Waves, Impulse (Physics), Shapes, Resolution (Optics), Hammers, Dynamics (Mechanics), Engineering models
research-article  
Henry Piehl, Aleksandar-Sasa Milakovic and S⊘ren Ehlers
J. Offshore Mech. Arct. Eng   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 form of a potential problem, solve this equation with a finite element method and apply edge detection methods and line integration to determine the optimal route. The functionality of the method is first evaluated with simple test problems and then applied to realistic transport scenarios.
TOPICS: Potential theory (Physics), Finite element methods, Ice, Ships, Fuel consumption, Computational methods, Shorelines, Edge detection

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