Efficacy of Analysis Techniques in Assessing Broken Wave Loading on a Cylinder Upon a Shoal

Estimation of impulsive wave loading on a cylinder upon a shoal, such as an offshore rock lighthouse constructed on a partially emerged offshore rock with a steep foreshore, poses a unique challenge to marine structural engineers. Large and small scale experimental investigations are the most feasible methods to study this phenomena. However, any experimental investigation with impulsive loading faces challenges in the data analysis, when it is necessary to separate the total impact forces from the force response time series, and to separate the impulsive and quasi-static force components of the total force measurements. This paper is an attempt to re-visit several data analysis techniques with a unique experimental data set to study the advantages and limitations of those techniques, and also to learn how different techniques can be combined into a data analysis methodology to harness maximum information.

Influence of Mixed Flows on Ship Hydrodynamics in Dredged Channels

Although there is a significant body of research devoted to the shallow water hydrodynamic aspects of ships, several unexamined topics remain. Among these is that of critical outer flow in a dredged channel and its influence on parameters of interest. While empirical methods can be used with ease to resolve this, they can provide results with reliability sufficient only for an early design stage. On the other hand, more sophisticated potential flow theories are either inapplicable or do not perform well at the critical limit. However, RANS (Reynolds Averaged Navier-Stokes) – based tools can accurately capture all underlying phenomena without relying on limiting assumptions. This paper presents an attempt at comparing some results obtained via a CFD-based RANS solver and the slender body theory for critical outer flow in a dredged channel.

Experimental and Numerical Study of Oil Whip in Propeller Shaft

An experimental study and theoretical study is carried out to understand the vibration signature of a propeller shaft. A test rig consists of a rotor shaft and three-disc supported on hydrodynamic bearing was analyzed. Presence of hydrodynamic bearing makes the systems natural frequency speed dependent. A theoretical model of the rotor disc system was developed using FEM. The rotor was formulated on Euler–Bernoulli beam theory. Proportional damping was assumed for the shaft. The stiffness and damping coefficients of the bearing are calculated by short bearing assumption. A Campbell diagram was plotted to observe the variation in natural frequencies with rotational speed. There was an indication of mode approaching each other with a speed which could result in the self-excited phenomena such as “Oil whip”. The hydrodynamic forces in the fluid film produce Oil whip. The presence of Oil whip was ascertained by carrying out the experimental study. The time-frequency plot during the run-up indicated the presence of a whip. The study indicated the influence of modes on the whip phenomena. This can be used in forming guidelines for the safe operating regime for the propeller shaft.

System Identification of Abkowitz Model for Ship Maneuvering Motion Based on ε-Support Vector Regression

System identification is crucial to predict the maneuverability of the ship. In this work, ε-support vector regression (ε-SVR) is implemented to identify hydrodynamic derivatives of Abkowitz maneuver model. A proposed technique, batch learning, is implemented with the addition of Gaussian white noise to reconstruct the samples and alleviate the parameter drift in the system identification of the ship maneuvering model. The predicted results are compared with results obtained from Planar Motion Mechanism (PMM) test. Standard maneuvers, 35° turning circle, 10°/10° and 20°/20° zigzags, are simulated and compared with the predicted model by ε-SVR. The presented results show that the proposed batch learning technique with Gaussian white noise is an effective technique, which improves the accuracy and robustness of ε-SVR in system identification. The results obtained from the predicted model match well with the those obtained from PMM results, which shows its excellent generalization performance. The developed model is applied to understand control requirements for vessels under different conditions.

Sensitivity Analysis of Different Parameters of Taut Mooring System of a Truss Spar

The taut mooring system is widely used for some advantages, such as smaller mooring radius, lighter total weight and better anti-corrosion performance. In this paper, the taut mooring system of a Truss Spar platform which was taken as the research object was investigated under the condition of 2000 m water depth in South China Sea. Firstly, the main body of the platform was analyzed in frequency domain based on the 3-d potential theory, and then the nonlinear solutions of platform displacement and mooring line force were obtained by using coupling analysis method in time domain, which determined the preliminary design parameters of mooring system. The sensitivity of the taut system is studied by changing several design parameters such as the top angle of mooring line, cable hole position and method of mooring disposal. In summary, the variation of the motion and dynamic response of the platform and mooring system has been explored and summarized by studying the design process and influential parameter of dynamic characteristics of mooring system and optimizing ideas of relevant parameters, which can further provide technical support and engineering reference for the design and application of the taut mooring system of deepwater Truss Spar platforms.

Impact of the Uncertainties of the RAOs of a Semisubmersible Platform on the Performance of a Motion-Based Wave Inference Method

Motion based wave inference allows the estimation of the directional sea spectrum from the measured motions of a vessel. Solving the resulting inverse problem is challenging as it is often ill-posed; as a matter of fact, statistical errors of the estimated platform response functions (RAOs) may lead to misleading estimations of the sea states as many noise values are severely amplified in the mathematical process. Hence, in order to obtain reliable estimations of the sea conditions some hypothesis must be included by means of regularization parameters. This work discusses how these errors affect the regularization parameters and the accuracy of the sea state estimations. For this purpose, a statistical quantification of the errors associated to the estimated transfer functions has been included in an expanded Bayesian inference approach. Then, the resulting statistical inference model has been verified by means of a comparison between the outputs of this approach and those obtained without considering the statistical errors in the Bayesian inference. The assessment of the impact on the accuracy of the estimations is based on the results of a dedicated model-scale experimental campaign, which includes more than 150 different test conditions.

Effect of the Wind Drag Estimation Methods on Numerical Storm Surge Modeling

Tropical cyclones have always proved the extent of its catastrophe on several occurrences over the years. In particular, the Bay of Bengal (BoB) basin in the Northern Indian Ocean has produced such historic devastating events, thereby mandating accurate real-time predictions. Numerical modeling of storm surge has always been an arduous task, as it is integrated with various uncertain factors. Among those, the major governing component being the wind forcing or the wind stress — that signifies, the computational accuracy of simulated surge and wave parameters. The present study is aimed at analysing the most suited wind drag evaluation method for real-time predictions of storm surge along the BoB. Cyclone Phailin (2013) was considered for the numerical simulations. To evaluate the wind drag coefficient, three most extensively used linear empirical relations along with the enhanced Wave Boundary Layer Model (e_WBLM) were used. The surge was subsequently simulated (using the coupled hydrodynamic circulation and wave model: ADCIRC and SWAN, respectively), individually for each of the above wind stress methods to obtain the corresponding storm surge (residual) and the storm wave features. The modeled values were further validated with the in-situ data obtained from tide gauge station and buoys respectively. It was quite intuitively observed that, e_WBLM based results correlated well with the in-situ values than its linear counterparts since, the former pragmatically includes the effects of air-sea interaction at high wind speeds in the model. The e_WBLM-based computation of significant wave heights (Hs) in deep as well as shallow water, nevertheless enabled efficient and reasonably-reliable estimations of the peak incidents.

Designing Coastal Structures for Tsunami Loads per ASCE 7-16

The 2016 edition of ASCE 7, Minimum Loads and Associated Criteria for Buildings and Other Structures, contains a brand new Chapter 6 on Tsunami Loads and Effects. This new chapter applies to the tsunami design of all Risk Category III (high occupancy) and IV (essential) buildings, and potentially many taller Risk Category II (regular) buildings, in coastal communities in Alaska, Washington, Oregon, California and Hawaii. These provisions can also be applied to other communities exposed to tsunami hazard, including Guam, American Samoa, Puerto Rico, and communities outside the US. This paper shows an example of how the new tsunami design provisions would apply to the design of prototypical multi-story coastal reinforced concrete buildings at different locations on the US Pacific Coast. The prototypical Risk Category II buildings are located in Seaside OR, Monterey CA, Waikiki HI and Hilo HI. Economic consequences of including tsunami design for mid- to high-rise Risk Category II buildings are discussed.

A Broadband Array Signal Processing Method Based on Joint Sparsity of Signal Spatial Domain

Broadband underwater acoustic signal direction of arrival (DOA) estimation method is an important part of underwater array signal processing. The commonly used array signal DOA estimation algorithms due to the restriction of algorithm principles, are unable to process broadband array signal effectively, at the case of the arriving signals have strong correlation, small sampling snapshots or small arrival angle. Therefore, we need a new efficient algorithm to meet the increasing demand of broadband under water acoustic signal processing method. This paper makes use of the broadband acoustic signal similarity of joint sparsity in signal spatial domain received by underwater sonar arrays, establishes the whole space grid covering all broadband frequency domain slices. On the basis, the global sparsity of each frequency domain slice is combined with sparse element extraction class algorithm. By integrating the energy of signal on each slice, the spatial sparsity of each slice is obtained, from which we can get the directions of the arriving broadband wave signals. Through the simulation analysis and experimental verification on lake, we can be see that: The SDJS algorithm improves the performance and signal processing capability of the algorithm compared with the traditional algorithms. Therefore SDJS algorithm has a widely range of research value and application space.

Dynamics of an Array of Submersible Mussel Rafts in Waves and Current

A three-by-three grid of submersible mussel rafts was analyzed using an experimentally validated dynamic numerical modeling approach. When submerged, the rafts’ pontoons are flooded, and they are held vertically by lines attached to surface floats and horizontally by a mooring grid. The rafts’ decreased waterplane area and increased inertia reduce the heave and pitch natural frequencies so that they are below the frequencies associated with the greatest wave energy. This has been found to significantly reduce the motion of the rafts compared to the surfaced configuration. The nine submersible rafts were anchored with 16 anchors and mooring lines. These mooring lines were connected to a grid of adjacent rectangular bays, with each corner (node) supported by a grid float. Each bay contains a raft connected to the submerged nodes of the grid by four bridle lines. The dynamics of the full system were modeled using a combined multibody and Finite Element Analysis (FEA) approach with dynamic loads computed using a modified Morison formulation. This model was implemented in the commercial code OrcaFlex. A similar model for a single submersible raft was previously validated with full-scale field experiments. The full dynamic system was simulated in the maximum expected waves and currents. Mean and maximum tensions in each grid line were quantified. Accelerations and velocities at the mussel rope attachment points were also examined, since these relate to mussel drop-off.