bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of Acoustic Communication and Positioning System for Operation of Multiple AUVs
Authors: Watanabe Yoshitaka , Meguro Koji , Deguchi Mitsuyasu , Shimura Takuya
Published: September 2018

In underwater observation using an autonomous underwater vehicle (AUV), a support vessel typically monitors the AUV to support the observation. The AUV have to be positioned to be tracked by the vessel and informed the positioning result for navigation with acoustic communication. This process is significant especially in deep water. The AUV uplinks to inform its status and transmit some observation data in real time, and sometimes be commanded to change the observation plan by downlink from the vessel. Authors planned to use an autonomous surface vehicle (ASV) to track and monitor multiple AUVs for efficient observation. An acoustic multi-access communication and positioning system have developed as one of the elemental technologies for the observation system.

The developed acoustic system achieves multi-access with frequency division multiple access (FDMA) method, and the ASV can monitor up to three AUVs simultaneously. Positioning is performed with super short baseline (SSBL) method. The acoustic device has operation mode in which positioning and communication functions are integrated, called as auto mode. In auto mode, uplink packets can be very close and the AUV can uplinks efficiently, and timing of downlink is calculated from positioning result appropriately.

Results of two sea trials are shown. The precision of positioning was enough to track the AUV. However random error was much larger than that of SSBL of the vessel, hence some precise method is necessary to keep the quality of observation data high. Communication and auto mode sequence worked well in the sea trial.
Long Term Analysis of Steep and Breaking Wave Properties by Event Matching
Authors: Johannessen Thomas B. , Lande Øystein
Published: September 2018

Offshore structures are typically required to withstand extreme and abnormal load effects with annual probabilities of occurrence of 10−2 and 10−4 respectively. For linear or weakly nonlinear problems, the load effects with the prescribed annual probabilities of occurrence are typically estimated as a relatively rare occurrence in the short term distribution of 100 year and 10 000 year seastates. For strongly nonlinear load effects, it is not given that an extreme seastate can be used reliably to estimate the characteristic load effect. The governing load may occur as an extremely rare event in a much lower seastate. In attempting to model the load effect in an extreme seastate, the relevant short term probability level is not known nor is it known whether the physics of the wave loading is captured correctly in an extreme seastate. Examples of such strongly nonlinear load effects are slamming loads on large volume offshore structures or wave in deck loads on jacket structures subject to seabed subsidence.

The present paper is concerned with the long term distribution of strongly nonlinear load effects and a methodology is proposed which incorporates CFD analysis in a long term Monte Carlo analysis of crest elevations and wave kinematics. Based on a long term time domain simulation of a linear surface elevation, a selection of events is run in CFD in order to obtain a database of linear and corresponding fully nonlinear wave fields with the possibility of wave breaking included. In the subsequent long term analysis, a large linear event is then replaced by the closest matching event in the database. A technique is developed to Froude scale the database results and shift the origin in time and plane so that the database of typically only 100 events give a close match to all the events in the simulation.

The method is applied to the simple case of drag loading on a cylinder which is truncated above the still water level such that only the largest waves impact with the structure. It is observed that whereas the Event Matching method agree well with a second order model for return periods lower than 100 years, the loading on the cylinder is significantly larger for longer return periods. The deviation is caused by the increasing dominance of wave braking in the largest crest and illustrates the importance of incorporating wave breaking in the analysis of wave in deck loading problems.
Wave Diffraction on Arrays of Vertical Truncated Cylindrical Bodies
Authors: Mavrakos Spyros A. , Chatjigeorgiou Ioannis K. , Konispoliatis Dimitrios N.
Published: September 2018

Offshore Wave Energy Converter (WEC) farms are widely deployed as regards the individual isolated devices aiming at maximum wave energy absorption and facilitating installation and power transmission. This paper summarizes the theory behind the hydrodynamic interactions of diffracted waves by a large array of vertical cylinders. The latter exhibits some remarkable hydrodynamic interference effects — near resonant modes — in waves causing large loads in adjacent elements of the array. Numerical results concerning the exciting wave loads for a variety of different array configurations of truncated and bottomless cylinders are presented and the free surface elevation around the elements of the array is evaluated pointing out the near trapped modes.
Characterization of the Emergence of Rogue Waves From Given Spectra Through a Wigner Equation Approach
Authors: Athanassoulis Agissilaos G.
Published: September 2018

The Wigner transform can be used to derive equations directly for the evolution of the autocorrelation of the sea elevation. This has been known in the literature as the derivation of the Alber equation, and applies to envelope equations. Wigner-Alber equations have been used to characterise spectra as either stable or unstable, and to predict Fermi-Pasta-Ulam recurrent dynamics for the unstable ones. Here we show that a systematic study of Wigner equations can improve this analysis in several respects, including: (i) the incorporation of accurate dispersion and (simple) wave breaking effects; and (ii) the characterization of the space and time scales over which localized extreme events emerge. More broadly this approach can be seen as a full modulation instability analysis for any measured spectrum. This work builds upon recent joint work with G. Athanassoulis and T. Sapsis.
Limit State Analyses in Design of Thin-Walled Marine Structures: Some Aspects on Length-Scales
Authors: Romanoff Jani , Remes Heikki , Varsta Petri , Goncalves Bruno Reinaldo , Lillemäe-Avi Ingrit , Körgesaar Mihkel , Jelovica Jasmin , Liinalampi Sami
Published: September 2018

Present paper gives an overview of the factors that affect the strength and structural design of advanced thin-walled marine structures with reduced plate thickness or alternative topologies to those used today in marine industry. Due to production-induced initial deformations and resulting geometrical non-linearity, the classical division between primary, secondary and tertiary responses becomes strongly coupled. Volume-averaged, non-linear response of structural element can be used to define the structural stress strain relation that enables analysis at the next, larger, length scale. This, today’s standard homogenization process needs to be complemented with localization, where the stresses are assessed at the details, such as welds for fatigue analysis. Due to this, the production-induced initial distortions need to be considered with high accuracy. Another key question is the length-scale interaction in terms of continuum description. Non-classical continuum mechanics are needed when consequtive scales are close. Strain-gradients are used to increase the accuracy of the kinematical description of beams, plates and shells. The paper presents examples of stiffened and sandwich panels covering limit states such as fatigue, non-linear buckling and fracture.
Potential Utilization for a Rock-Based Geopolymer in Oil Well Cementing
Authors: Khalifeh Mahmoud , Motra Hem Bahadur , Saasen Arild , Hodne Helge
Published: September 2018

In this work, we have studied rheological behavior and mechanical properties of selected modified rock-based geopolymer, which are suggested for zonal isolation and permanent abandonment of hydrocarbon wells. Our rheological measurements of the geopolymeric slurries shows a very small Yield Stress and a constant plastic viscosity. Even though the yield stress is small, a non-Newtonian flow behavior is distinguished. Mechanical vibration measurements showed that vibrations have surprisingly little effect on the shear stress changes. Consistency of the geopolymeric slurry was measured by utilization of atmospheric and pressurized consistometers to find the impact of pressure and temperature on pumpability. A temperature decrease of 10°C postponed the geopolymerization by 4 hours and a temperature increase by 10°C accelerated the geopolymerization by 30 minutes. Static-fluid-loss test showed only 1.6 (ml) of fluid loss after 30 minutes which is another advantage of the geopolymeric slurry. The elastic shear wave and compressional wave velocities and velocity anisotropies of the samples were tried to be determined experimentally. However, our efforts showed that geopolymeric structure of the samples requires different frequencies as it damps the high frequency signal.
Experimental Study of the Influence of the Pore Water Pressure Evolution and the Shear Band Formation on the Extraction Resistance of Submerged Anchor Plates
Authors: Kanitz Manuela , Grabe Juergen
Published: September 2018

Floating offshore structures used to generate wind energy are founded on submerged foundations such as anchor plates. Their extraction resistance is of major importance during and at the end of the lifetime cycle of these offshore structures. During their lifetime cycle, the foundation is suspended to complex loading conditions due to waves, tidal currents and wind loads. To guarantee a stable structure, the extraction resistance of the anchor plates has to be known. At the end of the lifetime cycle of the offshore structures, the extraction resistance is mainly influencing the removal of the anchor plates. This resistance is a lot higher than the sum of its self-weight and hydrostatic and earth pressure acting on the structure. With initiation of a motion of the anchor plate, the volume underneath this structure is increased leading to negative pore water pressure until inflowing pore water is filling the newly created volume. In order to investigate this effect, an extensive experimental study at model scale with a displacement-driven extraction is performed. Pore pressure measurements are carried out at various locations in the soil body and underneath the plate. The soil movement is tracked with a high-speed camera to investigate the shear band formation with the particle image velocimetry method (PIV). The experiments will be conducted considering different packing densities of the soil body and at different extraction velocities to investigate their effect on the extraction resistance of anchor plates.
Experimental Study of Two-Phase Flow in an Oscillating Vertical Pipe
Authors: Silva Elinaldo Santos , Bordalo Sergio N.
Published: September 2018

Two-phase vertical flows are of utmost importance for petroleum production, since underground petroleum reservoirs produce oil and gas simultaneously, which must flow together upward to the sea floor through wells, and from these to the production floating units through suspended pipes (risers). Along the pipelines, the mixture of oil and gas may develop several flow patterns — such as bubble, slug, churn and annular flow. These physical configurations present specific characteristics that demand distinct modeling of the head loss as a function of the flow rate. The correct characterization of the flow patterns, under given operational conditions, is fundamental to the modeling of the dynamics of the flow and to the relation between head loss and flow rate. In the literature, most studies on the establishment of the flow patterns have been carried with water and air, and have been restricted to the case of static pipes, while production risers are in constant movement due to the action of waves, sea currents and the displacement of the floating production unit. In the present work, an experimental study of vertical two-phase flow of water and air is conducted with the oscillatory movement of a long and slender flexible vertical pipe of 8,0 m of length and 25,4 mm of diameter. The head loss is measured for different frequencies of oscillation. Comparisons are made between the static and oscillating pipe, with regard to the flow patterns and head losses. The effect of the frequency of oscillation is detected.
A 2D Experimental and Numerical Study of Moonpools With Recess
Authors: Ravinthrakumar Senthuran , Kristiansen Trygve , Ommani Babak
Published: September 2018

Moonpool resonance is investigated in a two-dimensional setting in terms of regular, forced heave motions of a model with moonpool with different rectangular-shaped recess configurations. A recess is a reduced draft zone in the moonpool. Dedicated experiments were carried out. The model consisted of two boxes of 40 cm width each, with a distance of 20 cm between them. Recess configurations varying between 5 cm to 10 cm in length and 5 cm in height were tested. Different drafts were also tested. The free-surface elevation inside the moonpool was measured at eight locations.

A large number of forcing periods, and five forcing amplitudes were tested. A time-domain Boundary Element Method (BEM) code based on linear potential flow theory was implemented to investigate the resonance periods, mode shapes as well as the moonpool response as predicted by (linear) potential flow theory. Dominant physical effects were discussed, in particular damping due to flow separation from the sharp corners of the moonpool inlet and recess. The effect of the recess on the piston-mode behavior is discussed. BEM simulations where the effect of flow separation is empirically modelled were also conducted.

The non-dimensional moonpool response suggests strong viscous damping at piston-mode resonance. The viscous BEM simulations demonstrate improvement over inviscid BEM, although further improvement of the method is needed. The piston mode shapes are clearly different from the near flat free-surface elevation for a moonpool without recess, consistent with recently published theory.
Quantification of Multiple Factors and Interaction Effects on Drilling Fluid Invasion
Authors: Ezeakacha Chinedum Peter , Salehi Saeed , Kiran Raj
Published: September 2018

Water-based drilling mud is one of the commonly used fluid systems for drilling operations. The loss of drilling fluid in porous media and fractured formations have been one of the industry’s focus in the past decades. However, the dynamics and constantly changing wellbore conditions push the boundaries for more research into accurate quantification and mitigation methods for fluid loss. In the design and development of drilling fluids, most test conditions are kept constant during fluids property testing. Drilling fluid loss and rheological parameters are determined experimentally at constant test conditions, and according to the combination of mud additives, rather than a comprehensive approach. In addition, conventional methods of quantifying drilling fluid loss properties for field application can be is time-consuming, considering that multiple factors impact fluid loss.

This study presents a statistical engineering approach for pore-scale characterization of water-based mud (WBM) invasion. The methods used in this research are: special case of factorial design of experiment (DoE), analysis of variance (ANOVA), and regression. Important field parameters based on previous studies and industry recommendations were carefully integrated in the DoE and result analyses. These parameters include but not limited to: porous media type, temperature, type of lost circulation material (LCM), concentration of LCM, drilling string rotary speed, and eccentricity. Ceramic filter tubes were used for the first set of experiments and Upper Grey sandstone rock samples were used for the second set of experiments. The statistical analyses performed in this study were based on a 95% confidence interval (CI). The results show that for single factor interpretation, increase in temperature and rotary speed increased dynamic fluid invasion significantly. Increase in LCM concentration resulted to a significant decrease in fluid invasion. LCM concentration and rotary speed interaction revealed a significant decrease in fluid invasion. LCM concentration and temperature interaction significantly increased fluid invasion. Rotary speed and temperature interaction also increased fluid invasion significantly. The three-factor interaction effect of LCM concentration, rotary speed, and temperature was not significant in reducing fluid invasion. For the conditions used in this study, the regression analysis showed that dynamic fluid invasion in Upper Grey sandstone can be explained from variation in LCM concentration and rotary speed. The results and methods from this study can provide reliable information for drilling fluids design and selecting operating conditions for field application.
Laboratory Test on Cement Plug Integrity
Authors: Opedal Nils , Corina Anisa Noor , Vrålstad Torbjørn
Published: September 2018

A recurring issue in the petroleum industry is the performance of cement in relation to its primary role of providing zonal isolation. Enhanced understanding of this subject offers the possibility to improve the planning and design of the cementing job to minimizing the risk of poor bonding of cement and loss of well integrity. The design and execution of the cement job is by no means an easy task, mainly due to the complexity of the material and process, and the variety in conditions one can encounter downhole. Thus, screening of different materials and conditions is necessary to optimize the success of a cement operation.

This work focused on experimentally testing cement plugs to be able to understand the sealing ability of cement to a casing at relevant temperatures and pressures. A built-for-purpose test setup was designed and assembled, and the goal of this work was to test this new setup and to establish a proper baseline for future test on various cement systems.

The setup consists of a test cell containing the cement plug, an automated pressure regulator used for generating a pressure differential across the cement plug and flow meters to measure the flow rate through the cement plug. The output data from the tests is the differential pressure needed to have breakthrough of gas, and the connection between the flow rate and differential pressure across the cement plug. The possible manipulated variables for the test setup is the cement type and casing surface properties.
Investigating a Simplified Model for Moonpool Piston Mode Response in Irregular Waves
Authors: Ommani Babak , Kristiansen Trygve , Berget Kjetil
Published: September 2018

Estimating moonpool piston mode response at resonance is important for operation safety. This is a difficult task, in particular, due to nonlinear nature of the moonpool response connected to the damping imposed by the flow separation at the moonpool’s inlet. In the present work, the applicability of a simplified model, based on decomposing the problem into potential and viscous components, is investigated. The moonpool piston mode response is modeled as an additional degree of freedom. The coupling terms between this new degree of freedom and other vessel’s modes of motions are calculated based on potential flow calculations. Radiation and diffraction problems are considered separately. A finite volume solver with linearized boundary conditions is used to obtain the moonpool response under forced vertical motions. A quadratic damping model is fitted to the obtained responses and added to the free-surface condition of the potential flow formulation. The problem is solved both in frequency and time-domain. The validity of the obtained model is investigated by model test comparison for a dummy vessel with moonpool undergoing regular and irregular forced oscillations, as well as an offshore operation vessel with moonpool exposed to irregular waves. The benefits and shortcomings of the model are discussed. It is suggested that this method can be used as a practical tool to address moonpool piston mode response in irregular waves.
An Assessment of Load and Response for Horizontal Slamming Loads From Model Scale Experiments
Authors: Storheim Martin , Lian Gunnar
Published: September 2018

Steep breaking waves can result in high impact loads on offshore structures, and several model test campaigns have been conducted to assess the effect of horizontal wave slamming. High loads have been measured, and they can be challenging to withstand without significant deformation.

For wave slamming problems it is common to estimate the characteristic slamming load and assume that this will give an equivalent characteristic response. One challenge related to the slamming load is that it has a large variability in load level, the duration of the load and the shape of the overall load pulse. This variability can have a large impact on the estimated response to the characteristic load, causing a similar or larger variability in response. Due to the sensitivity to the structural response, it may be difficult to interpret large amounts of such data to arrive at a relevant design load without making overly conservative assumptions.

This paper investigates the sensitivity of the structural response to assumptions made in the material modelling and how the short term variability is affected if we instead of load use response indicators such as plastic strain and max deformation to arrive at a characteristic load. For this purpose, a simplified dynamic response model is created, and the recorded wave impact events can then be evaluated based on the predicted structural response from the simplified model.

It was found that the structural response is sensitive to the structural configuration. The assumed material behavior and hydro-elastoplastic effects were identified to greatly affect the structural response. A reasonable approach to arrive at the q-annual response seems to be to first estimate the q-annual extreme slamming load, and then run the structural analysis on several of the measured slamming time series with the estimated q-annual extreme pressure.
Experimental Research on Vortex-Induced Vibration of Flexible Catenary Riser Model
Authors: Zhao Lin , Tan Zhimin , Hou Yucheng , Yin Yanju , Meng Weihao
Published: September 2018

Vortex-induced vibration (VIV) is one of the most important factors accounted for the fatigue damage of long flexible risers in deep water. In this paper, the VIV response characteristics of flexible catenary riser model with the slenderness ratio of 578 were firstly studied by means of scale physical model experiments. In the experiment, the riser model was installed on a towing carriage, which might move horizontally above a wave basin with constant speed to simulate the working condition of the riser model under uniform current. The tension sensor was used to measure the time-history variation of the top tension. The acceleration sensor was used to measure the accelerations of the riser model in cross-flow (CF) and in-line (IL) directions. And the top tension, vibration spectrum, amplitude and vibration locus of the riser were analyzed in accordance with the flexible riser model experiment, and the VIV law of the model experiment working condition was analyzed; then the hydrodynamic software Orcaflex was used to verify the finite element analysis (FEA) of the experiment. The experimental results show that the flexible catenary riser with lower mass ratio and lower bending stiffness has more complex hydrodynamic characteristics than the steel catenary riser (SCR). When the suspension angle remains the same, the top tension of the riser increases with the increase of flow velocity, and the higher the flow velocity, the faster the growth rate; the natural frequency of the riser increases with the increase of flow velocity; the VIV of the riser is the second-order vibration mode when flow velocities range from 0.2m/s to 0.4m/s. The vibration frequencies corresponding to acceleration sensors at different measuring points along the axis of the riser tend to be the same and increase with the increase of flow velocity, which results in “lock-in” near a certain natural frequency close to the vortex shedding frequency. When V = 0.2m/s, the VIV responses of some measuring points of the flexible riser present a positive “8”-shape or oblique “8”-shape vibration, when the amplitude tends to be the maximum. As the flow velocity approaches to 0.8m/s, the “8”-shape vibration disappears and the VIV vibration locus begins to become cluttered, and the variation rate of the VIV phase angle is faster than that at low speeds. The experimental results show that the physical model experimental results is well matched with that of Orcaflex numerical model. The physical model experimental results can reflect the vibration law of flexible risers under actual working conditions and can be used to predict the actual vibration law and characteristics of the VIV of flexible risers.
Phase-Resolved Reconstruction Algorithm and Deterministic Prediction of Nonlinear Ocean Waves From Spatio-Temporal Optical Measurements
Authors: Desmars Nicolas , Pérignon Yves , Ducrozet Guillaume , Guérin Charles-Antoine , Grilli Stephan T. , Ferrant Pierre
Published: September 2018

We investigate a nonlinear phase-resolved reconstruction algorithm and models for the deterministic prediction of ocean waves based on a large number of spatio-temporal optical measurements of surface elevations. We consider a single sensor (e.g., LIDAR, stereo-video, etc.) mounted on a fixed offshore structure and remotely measuring fields of free surface elevations. Assuming a uniform distribution of measurement points over the sensor aperture angles, the density of free surface observation points geometrically decreases with the distance from the sensor. Additionally, wave shadowing effects occur, which become more important at small viewing angles (i.e., grazing incidence on the surface). These effects result in observations of surface elevation that are sparsely distributed. Here, based on earlier work by [1], we present and discuss the characteristics of an algorithm, aimed at assimilating such sparse data and able to deterministically reconstruct and propagate ocean surface elevations for their prediction in time and space. This algorithm could assist in the automatic steering and control of a variety of surface vehicles. Specifically, we compare prediction results using linear wave theory and the weakly nonlinear Choppy Wave Model [2, 3], extended here to an “improved” second order formulation. The latter model is based on an efficient Lagrangian formulation of the free surface and was shown to be able to model wave properties that are important to the proper representation of nonlinear free surfaces, namely wave shape and celerity. Synthetic datasets from highly nonlinear High Order Spectral simulations are used as reference oceanic surfaces. Predicted results are analyzed over an area that evolves in time, using the theoretical amount of information assimilated during the reconstruction of the wave field. For typical horizons of prediction, we discuss the capabilities of our assimilation process for each wave model considered.
GPU Based Acceleration of MPS Method for Three-Dimensional Dam-Break Flows
Authors: Chen Xiang , Wan Decheng
Published: September 2018

The Moving Particle Semi-implicit (MPS) method has been proven effective to simulate violent flows such as dam-break flow, liquid sloshing and so on. But the low computational efficiency is one disadvantage of MPS. In the field of scientific computations, GPU based acceleration technique is widely applied to reduce the computation time of various numerical methods. In this paper, an in-house solver MPSGPU-SJTU is developed based on modified MPS method and GPU acceleration technique. A three-dimensional (3-D) dam-break flow is simulated by present solver and the validity and accuracy of GPU code are investigated by comparing the results with those by other researches. By comparisons, the flow field of GPU-based calculation is in better agreement with the experiment. In addition, the computation times of GPU and CPU solvers are compared to demonstrate the effect of GPU acceleration technique on the computational efficiency of MPS method.
Kriging-Based Surrogate Model Combined With Weighted Expected Improvement for Ship Hull Form Optimization
Authors: Liu Xinwang , Wan Decheng , Chen Gang
Published: September 2018

The paper explores Kriging-based surrogate model combined with Weighted Expected Improvement approach and for the ship hull form optimization. The training dataset of the Kriging-based surrogate model is obtained by sampling the design space (Design of Experiments, DOE) and performing expensive high-fidelity computations on the selected points. Expected Improvement (EI) is used as a criterion to select one additional sample point in each iteration. The Weighted Expected Improvement (WEI) is derived from EI by adding a tunable parameter which can adjust the weights on exploration and exploitation in the Efficient Global Optimization (EGO).

The proposed method selects more than one new sample point by changing the weight parameter for each optimization iteration, thus it can be performed by parallel computation or multi-computer runs which improves the computational efficiency distinctly. This makes it possible not only to improve the accuracy of the surrogate model, but also to explore the global optimum much more quickly. The present method is applied to mathematical test function and a ship hull form optimization design in order to find the optimal hull form with best resistance performance in calm water in different speeds.

The result shows that the criterion of WEI can be applied in EGO for optimization design and can be easily extended to other hull form optimization design problems based on computational fluid dynamics.
Optimal Damping for Energy Extraction From Drag-Aided Vortex-Induced Motions
Authors: Malefaki Iro , Konstantinidis Efstathios
Published: September 2018

In this study, we investigate the efficiency of a novel energy converter that exploits vortex-induced motions of a cylindrical body. The converter comprises a rigid circular cylinder supported by an arm such that it can perform angular oscillations with respect to a pivot point while held perpendicular to an incident fluid current. Two configurations are considered in which the pivot point is located either upstream or downstream of the oscillating cylinder. We simulate the angular response of the cylinder via the torque balance using an analytical hydrodynamic model that includes fluid damping due to drag, fluid inertia due to added mass, and fluid excitation due to vortex shedding in the cylinder wake based on the instantaneous relative velocity between the moving cylinder and the uniform fluid current. A simplified version of the equation of motion with linearized terms indicates that the drag force on the cylinder modifies the stiffness and thereby the natural frequency of the system. We performed simulations of the vortex-induced motion of the cylinder by numerically integrating the full nonlinear equation of motion in order to study the effects of the mechanical parameters. The amplitudes of angular response obtained from simulations with the present model compare satisfactorily with experimental data from the literature. The results for a structure-to-fluid density ratio of 5 show that there is an optimal value of the damping ratio of 0.1 that maximizes the power-extraction efficiency, which reaches approximately 11%. It is also shown that the peak efficiency is attained at different arm lengths for the two configurations studied.
Risk Analysis and Management of Moored Ships in Ports
Authors: Pinheiro Liliana V. , Fortes Conceição J. E. M. , Santos João A.
Published: September 2018

The risks associated with mooring of ships are a major concern for port and maritime authorities.

Sea waves and extreme weather conditions can lead to excessive movements of vessels and mooring loads affecting the safety of ships, cargo, passengers, crew or port infrastructures.

Normally, port activities such as ships’ approach manoeuvres and loading/unloading operations, are conditioned or suspended based solely on weather or wave forecasts, causing large economic losses. Nevertheless, it has been shown that some of the most hazardous events with moored ships happen on days with mild sea and wind conditions, being the culprit long waves and resonance phenomena. Bad weather conditions can be managed with an appropriate or reinforced mooring arrangement.

A correct risk assessment must be based on the movements of the ship and on the mooring loads, taking into account all the moored ship’s system.

In this paper, the development of a forecast and warning system based on the assessment of risks associated with moored ships in port areas, SWAMS ALERT, is detailed.

This modular system can be scaled and adapted to any port, providing decision-makers with accurate and complete information on the behaviour of moored ships, movements and mooring loads, allowing a better planning and integrated management of port areas.
Numerical Modeling and Experimental Comparison of the Response of Elastically Connected Barges
Authors: Dessi Daniele , Faiella Edoardo
Published: September 2018

This paper presents a numerical model for the investigation of some hydroelastic issues related to floating systems used for the transportation and installation of components of offshore platforms, e.g., topsides. The system consists of two symmetric barges, linked together by the carried structure to form a catamaran-like layout. The flexibility of the transported structure may allow for proper excitation of the float-over system under oblique wave conditions at certain wave lengths, both during transportation and installation phases. The developed numerical code, based on multi-body dynamics and linear potential theory regarding the calculation of hydrodynamic loads, allows for a robust and fast description of the flexible floating system dynamics, including also the effect of mooring-line dynamics in the time-domain. Comparison with experimental data from a previous experimental campaign [1] shows that, notwithstanding the simplifying assumptions in the numerical model development, the amplitude of the relative pitch rotation between the barges due to the system flexibility can be properly described with the present approach.
Efficient Computation of Irregular Wave Wire Stresses in Flexible Risers
Authors: Rombado Gabriel , Cooke Nathan , Pasala Dharma , Ni Xianglei , Low Andrew , Majed Arya
Published: September 2018

Accurate computation of tensile armor wire stresses remains a major challenge in flexible riser fatigue life predictions and integrity management. Accuracy of the results relies heavily on capturing the kinematics of the flexible’s helically contra-wound tensile armor layers and their interaction with the other metallic and thermo-plastic layers in a dynamic simulation. The standard industry practice to assess the fatigue life of flexibles is to use high fidelity 3D Finite Element Models (FEMs) to capture the complex kinematics and produce accurate stresses. However, direct simulation of flexible riser detailed FEMs is limited to regular wave analyses and computation of wire stress time-histories subjected to irregular waves have been computationally infeasible. This is due to the complexity of the nonlinear FEM and the long simulation time of the irregular wave environment coupled with large number of fatigue sea states. As a result, simplified approaches which do not directly simulate the local model and instead assume that wire stresses can be interpolated based on static stress versus curvature material curves within a pre-defined tension /pressure envelope have been utilized.

This paper utilizes Nonlinear Dynamic Substructuring (NDS), a simulation-based approach that that extends the framework of dynamic substructuring to nonlinear problems. NDS enables the efficient nonlinear dynamic simulation of multiple pitch lengths of detailed flexible riser FEM subjected to irregular wave inputs and the computation of wire stress time-histories at any location on the local model.

In this paper, a 14-inch diameter flexible riser under consideration by ExxonMobil is subjected to vessel motion and wave load in irregular wave environments and is modeled using a detailed 3D FEM and simulated via NDS. The flexible riser design features four tensile armor layers to mitigate localized lateral buckling of the wires near the touch down point. Tension and curvature time-histories of the riser near the hang-off, calculated from a conventional beam model global analysis, is used to drive a 5.1m long local model. Irregular wave wire stress time-histories extracted at the corners of the tensile armor wires are used to compute the fatigue life of the flexible. To demonstrate the inaccuracies associated with the regular wave approach, fatigue life is computed via the regular wave approach and compared against the irregular wave approach. It is shown that the NDS capability to efficiently compute irregular waves mitigates over- and under-predictions due to environment idealizations leading to a more accurate and reliable flexible riser life prediction and structural integrity assessment.
A Data-Driven Approach for the Stability Analysis of Vortex-Induced Vibration
Authors: Reddy Sandeep B. , Magee Allan Ross , Jaiman Rajeev K.
Published: September 2018

In this paper, a general data-driven approach to construct a reduced-order model (ROM) for the coupled fluid-structure interaction (FSI) problem of a transversely vibrating bluff body in an incompressible flow is presented. The proposed data-driven approach relies on the Eigensystem Realization Algorithm (ERA) to design ROM models in a state-space format. The stability boundaries of the coupled FSI system are obtained by examining the eigenvalue trajectories of the ERA-based ROM. These stability boundaries provide us valuable quantitative insights into the lock-in phenomenon of the bluff-body vibration. We demonstrate the present ERA-based ROM technique for various configurations of bluff bodies such as an isolated single cylinder, the side-by-side and the tandem cylinder arrangements. A comparative study on the effect of different appendages to suppress the VIV of a cylinder is also presented using the ERA-based stability analysis. The validity of the proposed method for the FSI stability analysis on such variety of configurations has not been presented before and is the novel contribution of this paper. Overall, the proposed data-driven framework is found to be much more effective in terms of computational costs and the predicted lock-in regions are comparable to high-fidelity full-order simulations. This work has a potential for a profound impact on the design optimization and control of bluff body structures used in offshore industry.
Numerical and Experimental Investigation of the Wave Loading on a Three-Legged Offshore Wind Turbine Jacket Platform
Authors: Chatjigeorgiou Ioannis K. , Chatziioannou Konstantinos , Katsardi Vanessa , Koukouselis Apostolos , Mistakidis Euripidis
Published: September 2018

The purpose of this work is to examine a three-legged jacket tower support system subjected to wave loading. To this end, linear as well as nonlinear wave scenarios are investigated. The structure was designed for offshore wind turbines installed in intermediate water depths. The phenomenon of the wave-structure interaction is examined experimentally with a 1:18 scaled model as well as numerically with the use of Finite Element Model (FEM). The structural calculations were performed using the structural analysis software SAP2000, which was enhanced by a special programming interface that was developed to calculate the wave loading and to directly apply the wave loads on the structural members. The FEM model in combination with the key parameters that are taken into account, provides a good correlation with the experimental results. The wave theories of Airy and Stokes 5th are employed for the calculation of the wave particle kinematics. The resulting wave forces are examined both in the frequency and in the time domain.
Data-Driven Computing With Convolutional Neural Networks for Two-Phase Flows: Application to Wave-Structure Interaction
Authors: Mao X. , Joshi V. , Miyanawala T. P. , Jaiman Rajeev K.
Published: September 2018

Fluctuating wave force on a bluff body is of great significance in many offshore and marine engineering applications. We present a Convolutional Neural Network (CNN) based data-driven computing to predict the unsteady wave forces on bluff bodies due to the free-surface wave motion. For the full-order modeling and high-fidelity data generation, the air-water interface for such wave-body problems must be captured accurately for a broad range of physical and geometric parameters. Originated from the thermodynamically consistent theories, the physically motivated Allen-Cahn phase-field method has many advantages over other interface capturing techniques such as level-set and volume-of-fluid methods. The Allen-Cahn equation is solved in the mass-conservative form by imposing a Lagrange multiplier technique. While a tremendous amount of wave-body interaction data is generated in offshore engineering via both CFD simulations and experiments, the results are generally underutilized. Design space exploration and flow control of such practical scenarios are still time-consuming and expensive. An alternative to semi-analytical modeling, CNN is a class of deep neural network for solving inverse problems which is efficient in parametric data-driven computation and can use the domain knowledge. It establishes a model with arbitrarily generated model parameters, makes predictions using the model and existing input parametric settings, and adjusts the model parameters according to the error between the predictions and existing results. The computational cost of this prediction process, compared with high-fidelity CFD simulation, is significantly reduced, which makes CNN an accessible tool in design and optimization problems. In this study, CNN-based data-driven computing is utilized to predict the wave forces on bluff bodies with different geometries and distances to the free surface. The discrete convolution process with a non-linear rectification is employed to approximate the mapping between the bluff-body shape, the distance to the free-surface and the fluid forces. The wave-induced fluid forces on bluff bodies of different shapes and submergences are predicted by the trained CNN. Finally, a convergence study is performed to identify the effective hyper-parameters of the CNN such as the convolution kernel size, the number of kernels and the learning rate. Overall, the proposed CNN-based approximation procedure has a profound impact on the parametric design of bluff bodies experiencing wave loads.
Effect of Secondary Motion on Hydrodynamics of a Cylinder Oscillating in Still Fluid
Authors: Baranyi L. , Konstantinidis E.
Published: September 2018

In this study, we conducted numerical simulations to compute the hydrodynamic forces acting on a circular cylinder undergoing bidirectional oscillations in still fluid. The simulations correspond to the regime of attached laminar two-dimensional flow at low values of the Keulegan-Carpenter number (KC ≤ 5) and Reynolds numbers from 35 to 1000 based on the primary motion of the cylinder. The effect of a secondary motion transverse to the primary motion having twice the frequency and a fifth of the amplitude of the latter is investigated and the results are compared with the corresponding case of unidirectional motion and theoretical predictions from Stokes–Wang theory. The results for unidirectional motion show that the computed force in-line with the motion agree well with theory for KC < 1 and KCRe > 100. The agreement between computations and theory improves as KC decreases and Re increases. The addition of a secondary motion with different phase angles with respect to the primary motion did not have any observable effect on the force acting along the direction of the primary motion compared to that for the same unidirectional motion, although it had a marked effect on the distribution of vorticity around the cylinder. The forces on the cylinder undergoing bidirectional oscillations could be well predicted from Stokes–Wang theory applied in each individual direction for the range of parameters examined in this study. The present study provides insight into the relationship between the generation of vorticity around an oscillating cylinder and the fluid forces acting on it.
Experimental Study on Flexible Bare Pipe Arrays Undergoing Vortex-Induced Vibrations
Authors: Liu Y. , Shi C. , Liu Z. , Wang J. , Bao X.
Published: September 2018

Vortex-induced vibration (VIV) excited by current is a major contributor to the fatigue accumulation of marine risers. For deepwater operations, several risers are often arranged together in an array configuration. In this study, a set of four identical flexible pipes of a rectangular arrangement were tested in a water tunnel. By comparing the dynamic responses of a pipe in an array with that of a single isolated pipe, the effects of the current speed and the center-to-center distance between the up-stream and downstream pipes on their dynamic responses were investigated. Fatigue damages accumulated on each pipe in an array was calculated and a factor, termed “fatigue damage amplification factor”, was defined as a ratio between the fatigue damage rate of pipe in an array and the fatigue damage rate of a single pipe at a same current condition. The results showed that for bare pipes (i.e., without helical strakes), the downstream pipes in an array configuration may have larger dynamic responses and fatigue damage rates than those of a single pipe; and, it is not always conservative to assume that the fatigue damage rate estimated for a single pipe can be used to represent the fatigue damage rates of pipes in an array. This preliminary study provided some meaningful results for the design, analysis and operation of marine riser arrays.
Design of a Flexible-Base Hinged Connector for Very Large Floating Structures
Authors: Shi Qijia , Xu Daolin , Zhang Haicheng
Published: September 2018

Connector of multi-modular VLFS is a key component which determines the connection load and the dynamic behavior of the system. This paper presents a new design of the flexible-base hinged connector (FBHC) for VLFS in order to reduce the connection load. The connector consists of a hinged joint and two flexible bases. A finite element model of the connector is established for the deformation analysis to match up the requirement on the optimal stiffness combination of the connector [1]. Further the strength analysis is also carried out to check the safety of the structure design according to the maximum design loads. By applying the connector model to a three-modular floating platform, the dynamic behavior of the platform is analyzed. The results show that the responses of the modules are within the tolerable range and the connector meets the strength requirements.