bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural Dynamic Analysis of a Tidal Current Turbine Using MBDyn
Authors: Leng Jun , Li Ye
Published: September 2018

In recent years, tidal current energy has gained wide attention for its abundant resource and environmentally friendly production. This study focuses on analyzing dynamic behavior of a three-bladed vertical axis tidal current turbine. The multibody dynamics code MBDyn is used in the numerical simulation. It performs the integrated simulation and analysis of nonlinear mechanical, aeroelastic, hydraulic and control problems by numerical integration. In this study, tidal current turbine is idealized as an assembly of flexible beams including axis of rotation, arms and blades. We firstly conduct a modal analysis on the tidal current turbine and validate the model with the results obtained by ANSYS. The natural frequencies of blades with different size parameters are compared and the corresponding mode shapes are presented. Next, a parametric study was performed to investigate the effect of internal force on the dynamic response. It is concluded that the proposed method is accurate and efficient for structural analysis of tidal current turbine and this flexible multibody model can be used in the fluid-structure-interaction analysis in the future.
Uncertainties of Ship Speed Loss Evaluation Under Real Weather Conditions
Authors: Prpić-Oršić Jasna , Sasa Kenji , Valčić Marko , Faltinsen Odd Magnus
Published: September 2018

A correct assessment of the ship speed loss in conditions of exploitation is becoming increasingly important for ship owners as well as ship designers. We are witnessing the increasing concern for the environment and awareness of the necessity to preserve it as much as we could. The ship speed drop in the real environmental conditions can cause the increased fuel consumption as well as increased emissions of CO2 and other GHG (greenhouse gases) from ships. Decrease of the ship speed in real conditions is a consequence of the added resistance due to the impact of weather conditions, i.e. waves and wind, and due to aggravated working conditions of propeller, i.e. engine system. Moreover, the solution estimation of this problem is very affected by human factors. Ship master, concerning for safety, can make a judgment that, under certain adverse weather loads, it is necessary to slow down or change ship’s course to moderate or bypass the worst condition. In addition, the loading condition of the ship is constantly changing which govern the basic parameters of the ship: the mass and mass moment of inertia, draft and trim and, consequently, the ship behavior at sea.

All these parameters affect the assessment of ship speed and it is necessary to be conscious of the intensity of their impact on the final value. At the same time, they cannot be predicted with absolute certainty so the purpose of this analysis is to estimate the impact of weather and operational uncertainties on the actual speed of the ship in real operating conditions.
A Projection-Based Reduced Order Model (PBROM) for Coupled Physical-Numerical Simulations
Authors: Dao My Ha
Published: September 2018

This paper presents the construction and solution algorithm for a projection-based reduced model emphasising on integration with sensing data from a physical modelling for a coupled physical-numerical simulation. The reduced model is constructed based on the well-known Galerkin projection of the equations governing the physical processes on reduced subspaces obtained by the Proper Orthogonal Decomposition method. The projection is done using numerical discretisation in the existing OpenFOAM® platform. A simple yet efficient way to integrate physical sensing data into the reduced model to enhance the model accuracy is presented. The reduced model is demonstrated for classical incompressible flows at low and intermediate Reynolds numbers driven by various boundary conditions. Model results show excellent agreement with the full CFD simulations while only a fraction of computational resource is required.
Control Signal Optimization for Non-Linear Wave Generation
Authors: Hicks J. B. H. , Bingham H. B. , Read R.
Published: September 2018

This paper investigates the use of optimization for numerical-physical wave generation in wave tanks. Control signals for a wedge-shaped plunger-type wave generator are developed to produce stable non-linear, deep-water waves in both numerical and physical wave tanks. A fully non-linear potential flow solver developed at DTU is used for the numerical work. Numerical optimization proceeds by a defect correction scheme, resulting in optimized control signals for wavelengths of 0.7–2 m (corresponding to non-dimensional wave numbers kh = 2–5.5) and steepnesses of 3–11%.
A Study on Fluid-Structure Interaction Performance of a Flexible Hydrofoil
Authors: Huang Zheng , Xiong Ying , Xu Ye , Li Shancheng
Published: September 2018

To research the flexible hydrofoils’ hydroelastic response, the fluid-structure interaction (FSI) characteristic investigation is conducted on the basis of the analysis of a rigid hydrofoil’s hydrodynamic performance. For a rigid cantilevered rectangular hydrofoil, the pitching hydrodynamic performance is calculated using boundary motion with remeshing strategy. The Laminar Separation Bubble (LSB) and turbulent transition are captured. Numerical flow analysis revealed that the LSB occurs at 0.8c when pitching at initial angle of attack. As the angle increases to 5.1°, the laminar to turbulent transition occurs and the lift presents an inflection. For a geometric equivalent flexible hydrofoil, the static FSI characteristic is researched using oneway and two-way FSI method. The lift decreases and the drag increases using two-way compared to one-way FSI. The center of pressure and the maximum deformation move from trailing edge to leading edge as the angle of attack increases, showing the necessary of two-way FSI calculation. The transient FSI characteristic of the flexible hydrofoil is then studied using LES model. The lift fluctuation at 8° in frequency domain is calculated . The dry mode and wet mode natural frequency of the flexible hydrofoil are calculated to simulate the vibration performance, which meet the experiment data quite well, laying foundation for further research on the hydroelastic vibration response.
How to Estimate Performances of Cryogenic Spillage Protection Materials?
Authors: Viale Sebastien
Published: September 2018

The planned operation of natural gas liquefaction units offshore has raised many new safety issues. Cryogenic spillage is among the most critical as load bearing structures cannot be designed to resist such accidents.

In order to propose better protection at manageable cost, to enhance structural integrity and to lower the risks associated with cryogenic spillage on marine structures, TechnipFMC as FLNG market leader, has run an extensive physical test program to support the development of tailor made numerical modeling tools.

In a first step, numerous protective materials were screened and then tested to assess their performances. We found that all products currently available in the market display limitations. During the test campaign, alternate product designs were investigated to improve structural design and avoid the need for human intervention “Inspection Maintenance and Repair” during service lifetime.

In this paper, a summary of the test campaign and a complete introduction of Series of standard of ISO 20088 will be provided.
An Experimental Study of Mooring Line Damping in Shallow Water
Authors: Hsu Wen-Yang , Chuang Tzu-Ching , Yang Ray-Yeng , Hsu Wei-Ting , Thiagarajan Krish P.
Published: September 2018

The aim of this paper is to establish a simple approach to experimentally study the mooring line damping in shallow water. Experimental measurements were conducted in a wave basin at a scale of 1:50, which corresponds to a full scale of 28 m water depth. A chain made by stainless steel was used and the tension force at the fairlead was measured by tension gauges. Moreover, the mooring line geometry and velocity were computed from image processing technique. Series of horizontal motions at fairlead were driven from a programmable wavemaker. Regular surges with different frequency and pretensions were tested in this system to investigate the quasi-static and dynamic behaviors of the mooring chain. In the quasi-static test, mooring line keeps a typical catenary shape and its indicator diagram exhibits a smooth-closed curve. In the dynamic test, the mooring line is fully lifted from the seabed and it cyclically goes through the stage of semi-taut and fully taut. We successfully reproduced a snap event in the laboratory scale and the resulting mooring line damping can considerably increase in this manner. The mechanism of snap load is investigated by the measured tension, surge and mooring line velocities.
Investigations on the Mud Pumps Valves Repair Using Hard Metal Depositing by Welding
Authors: Badicioiu Marius , Caltaru Mihaela , Teodoriu Catalin , Minescu Mihail
Published: September 2018

Mud pumps are essential components of the drilling rig, since they support the drilling process, and are required to operate 24/7. Their ability to generate high pressure at relatively high flow rates led to a wide application of reciprocal pumps, from mud pumps to fracking pumps, within the oil and gas business. The elements of the mud pump, especially the mud pump valve, are exposed to erosion-corrosion processes during the pumping operations because of the solids and the corrosive components of the pumped fluid which leads to a drastically reduction of their durability. The downtime of such a pump may add high costs to the drilling or fracking operations, and any improvement of the components durability helps. The mud circulation system is of main interest during the drilling operation, with 7% of the personnel time being spent on mud pumps and 26% on monitoring from the total time spent in the mud area. The present paper shows the experimental results regarding the hard metal deposition of high erosion/abrasion resistance materials on the valve seat’s working surfaces, and the use of the welding process in order to increase their durability, and eventually, to establish the possibility of reconditioning the mud pump valve. The experiments were performed on used valves as part of their reconditioning process, but can also be performed on new elements, as means to increase the durability and erosion resistance. The results are very promising and show a good and strong hard metal deposition with very good mechanical properties. The paper describes the steps and the specific pre and post welding treatment in order to achieve the optimum hard-facing parameters.
A Case Study for an Offshore Structure for Aquaculture: Comparison of Analysis With Model Testing
Authors: Berstad Are Johan , Aarsnes Jan Vidar
Published: September 2018

This paper presents a case study of a structure of a novel fish farm cage. The “AquaTraz” cage is more similar to structures known to the the oil and gas industry. The cage has the same shape as the normal cages used in the aquaculture industry and can be introduced to existing grid-moorings.

Model testing has been carried out and results from the model test are compared with results from numerical simulations using the tool AquaSim. The response deviating from classic fish farms is shown and what extra considerations that should be carried out for this type of systems compared to classic fish farms is discussed.
Performance Simulation of Solar-Ocean Thermal Energy Conversion
Authors: Juan Yue , Dashu Li , Zhichuan Li , Gang Xiao , Li Zhang , Yonghu Wu , Ting Yu
Published: September 2018

Compared to the restriction of intermittency of solar power generation, ocean thermal energy conversion (OTEC) is not only 24/7 base-load, but also comprehensive utilization of fresh water production, air-conditioning, mariculture etc. However, limited temperature difference between warm surface seawater and the cold deep seawater is a crucial factor that restricts the thermal efficiency of OTEC. But today, with the appliance of solar collector in OTEC net power output and the net thermal efficiency have been significantly improved. In this study theoretical analysis and performance simulation of 1MW solar-ocean thermal energy conversion (SOTEC) in South China Sea area is conducted. Net power output and net thermal efficiency of SOTEC with solar-boosted temperature of 20K and OTEC under the condition of weather conditions in South China Sea are compared and analyzed. The results show that the net power output and net thermal efficiency of SOTEC have been significantly improved by combining the solar collector. This study is practical for autonomous supply of islands and coastal areas, and instructive for the comprehensive utilization of renewable energy.
Station-Keeping Trials in Ice: Data Acquisition, Post-Processing, Storage and Visualization
Authors: Almkvist Erik , Kjøl Andreas , Tisell Claes , Lindvall Joakim K. , Ingers Claes , Hansson Anders , Dupuy Maxime
Published: September 2018

In March 2017, Statoil performed station-keeping trials in drifting ice in the Bay of Bothnia with the two AHTS (anchor handling tug supply) vessels, Magne Viking and Tor Viking. The main objective of the SKT project was to collect full scale data for calibration of numerical models. This paper describes the design and setup of the tailor-made data acquisition system, including description of the individual sensors in the system as well as how the data was handled during the different project phases. A description of how all data was recorded, time synced and stored is given. Data post-processing, storage and visualization done within the scope is also described.
Surface Waves Generated by a Translating and Oscillating Source Atop Realistic Shear Flows
Authors: Li Yan , Ellingsen Simen Å.
Published: September 2018

We analyze surface waves generated by a translating, oscillating surface disturbance atop a horizontal background flow of arbitrary depth dependence, with a focus on determining the Doppler resonance. For a critical value of the dimensionless frequency τ = ωV/g (ω: oscillation frequency, V: source velocity, g: gravitational acceleration) at which generated waves cannot escape. In the absence of shear the resonant value is famously 1/4; the presence of a shear current modifies this. We derive the theoretical and numerical tools for studying this problem, and present the first calculation of the Doppler resonance for a source atop a real, measured shear current to our knowledge. Studying graphical solutions to the (numerically obtained) dispersion relation allows derivation of criteria determining the number of far-field waves that exist in different sectors of propagation directions, from which the criteria for Doppler resonance follow. As example flows we study a typical wind-driven current, and a current measured in the Columbia River estuary. We show that modeling these currents as uniform or with a linear depth dependence based on surface measures may lead to large discrepancies, in particular for long and moderate wavelengths.
Experimental and Numerical Study for Gap Resonance of Drillship Moonpool in Waves With/Without Forward Speed
Authors: Magee Allan R. , Feng Aichun , Karthikeyan Kandasamy , Liu Xiang , Yan Deguang
Published: September 2018

Experimental and numerical studies are carried out to examine the moonpool gap resonance for a drillship at both stationary position and forward speed conditions. The moonpool size and draft are also changed to study their effects for the gap resonance phenomenon. An OpenFOAM based CFD model is developed and the numerical results show good agreement with model tests. Both piston and sloshing modes gap resonances are clearly observed. The study shows that the resonance frequency and RAO of the wave elevation inside the moonpool are subject to the effects of moonpool length, drill ship draft and ship forward speed. The model test shows that moonpool elevation RAO generally significantly increases in head seas and noticeably decreases in following seas condition. It is interesting to notice that the wave flume sidewall significantly depresses the moonpool elevation RAO at a certain frequency regardless of moonpool length and draft. Further study shows that the presence of the flume sidewall results in a trapped mode that coincides with the moonpool piston mode resonance at zero speed. This depresses the peak of the moonpool resonance, which occurs at the same frequency.
An Overview of Risk-Based Inspection Planning for Flexible Pipeline
Authors: Hameed Hamad , Sun Guowei , Bai Yong
Published: September 2018

There are many failure mechanisms that may occur in flexible pipes during their service life, such as external sheath damage, damage of tensile armours due to fatigue, kinks due to corrosion, and over-bending. The above mentioned defects are very common and can lead to the failure of flexible pipes. Therefore, huge operational costs have to be spent to ensure the integrity of the flexible pipeline system. Risk-Based inspection (RBI) methodology can establish a cost-effective strategy for inspection and monitoring, while maintaining the expected level of safety. However, there are some factors that can hamper the development of an optimized RBI for the integrity management of flexible pipelines, such as the complexity and variety of flexible pipes’ multi-layer structures with their interacting failure mechanisms, the somewhat poor defect capability of the existing inspection techniques and the lack of broad [1]. In this paper, RBI planning is described thoroughly to ensure the integrity management of flexible pipelines. All risk terms have been described in detail with respect to integrity management strategy. The working process of RBI, including initial assessment and detailed assessment, is introduced. Estimation and evaluation of risk level, including the development of optimized inspection planning, have been described completely.
An Artificial Neural Network Based Method for Explosion Risk Analysis of Floating Offshore Platform
Authors: Shi Jihao , Chen Guoming , Zhu Yuan
Published: September 2018

This study aims to introduce the Artificial Neuron Network (ANN) technique, namely Bayesian Regularization Artificial Neuron Network (BRANN) to Explosion Risk Analysis (ERA) of floating offshore platform and eventually develop the ANN-based ERA procedure. In order to verify the feasibility of this developed procedure, a case study of floating offshore platform is conducted. Firstly, several dispersion simulations and explosion simulations are performed by FLACS. With those simulation results, the corresponding BRANN models are subsequently developed. Furthermore, comparison between BRANN model and widely-used RSM model is conducted. Eventually, the exceedance curve of maximum overpressure is determined. All the results illustrate the more robustness and efficiency of this developed procedure.
Flow-Riser Interaction in Deep-Sea Mining: An Analytic Approach for Multi-Layered FRP Risers
Authors: Pavlou Dimitrios G.
Published: September 2018

Fiber reinforced polymeric laminated materials are suitable for risers in deep-sea applications due to their superior strength, corrosion and fatigue resistance, light weight, low maintenance cost, low transportation cost, and ability for continuous manufacturing. However, due to their anisotropic material properties, the modeling of the dynamic response due to interaction with the internal flow and the sea water is more complicated. In the present work a model for flow induced instability analysis of long, multi-layered, fiber reinforced risers is performed. The motion equations take into account the elastic flexural restoring force of the anisotropic material, the centrifugal force of the fluid flowing in curved portions of the pipe, the Corriolis force, the inertia force of the mass of pump, pipe, and fluid, and the effect of the surrounding water. Combination of the motion equations yields a fourth order partial differential equation in terms of flexural displacements. The transfer matrix method is implemented to the above equation for the critical flow velocities calculation. The “global stiffness matrix” of the pipe-pump system containing the boundary conditions, the anisotropic material properties and the flow parameters, is derived. The condition for non-trivial solution is solved numerically yielding the values of the critical flow velocity, i.e. the internal flow velocity causing flow induced pipeline instability. The results are affected by the anisotropic properties of the material, the mass of the hanged pump, the drag coefficient, and the flow parameters. The results are commented and discussed.
Decommissioning of Offshore Monopiles, Occuring Problems and Alternative Solutions
Authors: Hinzmann Nils , Stein Philipp , Gattermann Jörg
Published: September 2018

The German offshore wind industry has historically grown since the first offshore wind farm (OWF) “alpha ventus” was completed in 2010. Since the end of 2017 a total number of 18 OWF with a capacity of about 5 GW have been operating in the German Exclusive Economic Zone (EEZ) [1]. While the majority of the population and the industry focus on new projects, it appears the life cycle observation and especially the decommissioning phase remain largely unattended. This narrow view can lead to unexpected and expensive consequences in the future. The decommissioning of a complex structure as an offshore wind turbine (OWT) needs to be planned well in advance. There are numerous aspects that make the decommissioning a challenge, such as the federal regulations, the marine environment and the technical limitations of offshore operations.

This article gives an overview of the problematic matter of dealing with monopiles after the predicted lifetime, the geotechnical condition, analyses of the current decommissioning options and identification of issues in regards to the decommissioning method. Furthermore, other promising decommissioning methods for a complete removal of offshore monopiles, such as vibratory extraction, internal dredging, external jet drilling and the use of buoyancy force, are presented and compared concerning a possible combination. Some of the presented methods are highly experimental, others are commonly used in other industries.
Design Wave Analysis for the Extreme Horizontal Slow-Drift Motion of Moored Floating Platforms
Authors: Lim Dong-Hyun , Kim Yonghwan
Published: September 2018

In this study, a new design wave analysis method for estimating the extreme slow-drift motion of floating offshore structures is introduced. Here, the design wave refers to the irregular incident wave of short duration that induces the extreme response of desired return period. The present method is composed of following four steps: linearization of the dynamic system, probabilistic analysis of the Volterra series, generation of the design waves, and the fully-coupled nonlinear time-domain simulations. In generating the design waves, the conditioning of the most likely extreme response profile is suggested. The method was applied to a deep-water semi-submersible platform, and the results appeared to be promising compared to the full-length nonlinear simulations.
Interaction of Nomadic Bergy Bits With an Offshore Gravity Based Structure in Waves
Authors: Seo Dong Cheol , Zaman M. Hasanat , Akinturk Ayhan
Published: September 2018

Offshore oil and gas operations are being conducted in harsh environments such as offshore fields around Newfoundland and Labrador. The platforms used in these fields may be exposed to additional risks due the presence of drifting ice pieces in various sizes. To avoid a dangerous collision with a large floating ice, ice management and risk mitigation plans are required firstly. In case of smaller ice pieces (or bergy bits), if they collide with an offshore platform, the combined load of bergy bits and waves may be larger than the typical design wave loads. Hence, it is necessary to predict how these bergy bits may interact with a platform in a given sea state to estimate the combined load for developing a reliable design. In this paper, dynamic interaction between a moving bergy bit and a fixed offshore platform is simulated using CFD (Computation Fluid Dynamics) method. Trajectories and velocities of ice are simulated and compared to understand the hydrodynamic interaction by applying a series of different wave conditions such as wave period, height and relative incident peak location. The simulation outcomes show the importance and possibilities to apply it to the design process of such structures in the ocean.
Design and Make of a Segmented Ship Model for Vertical Bending Moment Measurement
Authors: Yue Jingxia , Guo Yulong , Peng Lihua
Published: September 2018

With the development of the large-scale ship, the hull becomes more and more “soft” and “elastic”. Accurate simulation of ship’s hydro-elastic performance through scaled model test plays an important role in structural safety assessment. This paper presents the detail preparation of a segmented model which is used to investigate the vertical bending moment (VBM) for a 260m TEU container ship.

Some innovative concepts were involved in the scaled model design. Firstly, the segmentation of the ship model was based on the hull’s vertical vibration mode for better simulation of the hull’s rigidity distribution. Secondly, the section of the backbone beam was varied by polishing along ship length in order to simulate the varied section modulus of ship hull. Thirdly, new backbone fixed type was carried out by two flange plates for a better wave load transmission.

Besides, some useful techniques were provided, including the model making technique, calibration technique, and backbone system technique. It increases the feasibility of test, at a certain extent. Finally, an overview of the ongoing large scale model test plan and its future development directions is prospected.
Model Scale Testing of the Tupperwave Device With Comparison to a Conventional OWC
Authors: Benreguig Pierre , Murphy Jimmy , Sheng Wanan
Published: September 2018

A model testing campaign of the Tupperwave device was carried out to prove the working principal and validate a numerical modelling that had previously been developed. An appropriate and challenging scaling method was applied to the floating device to correctly model the air compressibility in the Tupperwave PTO. In parallel, a model scale conventional OWC was also built using the same axisymmetric structure geometry and both devices were tested and compared. The testing showed that the Tupperwave device produced less average useful pneumatic power than the conventional OWC. The primary losses were attributed to pneumatic power dissipation through the valves. The pneumatic power delivered by the Tupperwave device was however significantly smoother.

The paper describes the experimental set-up and the methods used to assess the devices performance. The results provide a direct comparison between the two physical models pneumatic power performances and an in-depth analysis of the valves behaviour is shown.
Numerical Investigation Into the Keying Process of a Plate Anchor Vertically Installed in Cohesionless Soil
Authors: Al Hakeem Nabil , Aubeny Charles
Published: September 2018

Vertically driven plate anchors offer an attractive anchoring solution for floating offshore structures, as they are both highly efficient and suitable for a wide range of soil conditions. Since they are oriented vertically after installation, keying is required to orient the anchor into the direction of applied loading. Simulation of the keying process has not been extensively investigated by previous research, especially for cohesionless soil. Reliable prediction of irrecoverable embedment loss during keying is needed, since such loss can lead to significant reduction in the uplift capacity of the plate anchors. Large deformation finite element analyses LDFE method using RITSS (Remeshing and Interpolation Technique with Small Strain) were used to simulate the keying process of strip plate anchor embedded in uniform cohesionless soil. LDFE showed that the loss in embedment depth of plate anchor during rotation is inversely proportional to the loading eccentricity e/B. It was also found that the maximum pullout capacity occurs before the end of keying process at orientations between 60° to 85° degrees for vertical loading. Also, the LDFE study showed that reduced elastic soil stiffness leading to increased levels of displacement at which the peak load is approached.
Nonlinear Lid Technique: Application to Gap and Moonpool Resonance
Authors: Lécuyer B. , Rouault M.-C.
Published: September 2018

This paper focuses on the implementation of a nonlinear Lid Technique, as done in the software Diodore™. Several dummy plates are used and modelled as mechanical structures with their own degrees of freedom, capable to account for nonlinear damping.

The paper describes first the general equations governing the multi-body sea-keeping model including dummy plate structures, and the expression of the relative quadratic damping. In a second step, examples of applications with gap and moonpool are exposed, demonstrating the capability of the nonlinear Lid Technique with multiple plates to render not only the piston mode, but also the other modes of deformation of the free surface.
Impact of Hydrodynamic Interaction Between Pontoons on Global Responses of a Long Floating Bridge Under Wind Waves
Authors: Xiang Xu , Viuff Thomas , Leira Bernt , Øiseth Ole
Published: September 2018

The hydrodynamic interaction between floating bridge pontoons and its impact on the bridge global responses are investigated in the current study. The global model for end-anchored floating bridge for Bjørnefjord crossing is modelled in ORCINA-OrcaFlex. Forty-six pontoons are used to support the bridge, with a centerline distance of 100m between two pontoons. Two models are setup for comparison: (1) The OrcaFlex model with hydrodynamic coefficients of pontoons without hydrodynamic interaction; (2) The OrcaFlex model with hydrodynamic interaction coefficients, which were calculated by ANSYS-AQWA. Firstly, a case study of hydrodynamic interaction effects on added mass, potential damping and diffraction force is given, showing that the piston and sloshing modes have strong correlation with the resonances. Then two sea states were run on the two models with and without hydrodynamic interaction effects. The first order wave effects are included in the analysis. The observed extremes of the time-domain bridge girder bending moments and motions were compared between two models. The comparison shows both reduction and increase of bridge responses depending on the wave directions. A sensitivity test of drag coefficients applied on the pontoon vertical motion is carried out as a rough examination of the neglected viscous damping on the hydrodynamic resonances. The viscous damping effects on the resonances should be further quantified.
Deterministic Decomposition of the Unsteady Flow in a Unidirectional Axial Turbine for OWC Plant
Authors: Alvarez Bertrand Nuria , Fernández Oro Jesús Manuel , Pereiras García Bruno , García Díaz Manuel
Published: September 2018

The “twin-turbine” configuration has recently emerged as a feasible possibility for unidirectional turbines to be introduced in Oscillating Water Column wave energy plants without requiring auxiliary rectifying systems.

Previous investigations by the authors have been focused on the development of a numerical CFD model to analyze the performance of a unidirectional axial turbine for twin turbine configuration in an OWC system.

In this paper, all these numerical databases are further post-processed using a deterministic framework to give more insight about the flow patterns within the turbine. The final objective is the analysis of the unsteady features of the flow and the stator-rotor interactions using a deterministic decomposition. The present study reveals that levels of deterministic unsteadiness in the inter-row region are moderate, being more intense as the flow rate is decreased. Turbulence intensities are also observed to be clearly prominent in case of lower flow rates. Although these findings appear to be contradictory with the high-efficiency low flow rates of the turbine, the major levels of stator-rotor unsteadiness at higher flow rates (shown by the deterministic decomposition) justify the serious penalty in the aerodynamic efficiency as the turbine flow rate is increased. Finally, some advices with respect the design of the vane row in the turbine stage are given to control the generation of turbulence and stator-rotor interaction.
Study on Deformation and Energy Absorption of Liquid Cabin Under Ballistic Impact
Authors: Lei Zhang , Pengduo Zhao
Published: September 2018

Liquid cabin is the most significance part protecting the ship from damage due to high-speed fragments. In order to guide the design of liquid cabin, response regularity of the impact wave in water and the speed of projectile were studied by simulation method. The deformation and energy conversion of liquid cabin in different design were also investigated. The results show that, the existence of a liquid medium can change the energy absorption model of a cabin. The thickness of water layer put significance influence on the deformation and energy absorption model of liquid cabin. Lower the level of the water lead to the free surface truncation effect of impact wave. The asymmetry and locality of the deformation of bulkhead occur to the free surface truncation effect, but it put little effect on energy absorption of liquid cabin.
The Investigation on Cabin Noise Control of Ship Structure Based on SEA Graph Method
Authors: Shi Zeyu , Yao Xiongliang , Wu Guoxun , Tian Yue
Published: September 2018

The noise of ship structure is mainly transmitted by two types which are air sound and structural sound. As a kind of mechanical energy, sound is produced by the sound source and goes through various transmission paths to the recipient. This process is a process of constant loss of energy. Therefore, according to aspects of noise generation, output, transmission and reception, the principle of cabin noise control can be divided into four aspects which are cabin structure acoustics design, noise source control, noise transfer path and individual protection at the end of cabin. In order to determine the best noise reduction measures, noise control measures should be considered on the basis of three principles of science, advanced nature and economy. Statistical energy analysis (SEA) graph method is compared a series of adjacent loss factor matrices in the SEA model with the data structure of graphs in graph theory, a plurality of transmission path of SEA model can be obtained by giving different weights to adjacent matrix loss factor matrices in SEA model. The problem of finding maximum energy transfer path in the SEA model is actually equivalent to the issue of seeking shortest path in the graph theory. In order to reduce the cabin noise of the ship structure, it is necessary to know the main source and the main energy conduction path of the noise cabin. The problem is translated into K shortest path problem in graph theory. In this paper, acoustical sensitivity analysis of noise reduction design parameter is developed according to sound energy transmission of two layer cavities structure, which can guide the noise reduction design of the ship cabin. The proposed cabin noise control method is applied to the problem of overproof cabin noise, and the optimal noise control scheme is obtained.
Technique for Early Consideration of Human Reliability (TECHR): Applying a Generic Model in an Oil Tanker Operation to Study Scenarios of Collision
Authors: Maturana Marcos Coelho , Martins Marcelo Ramos
Published: September 2018

In the oil sector, analysis, evaluation, and management of risk are vital, considering the accidents potential severity with respect to human life, environment and property. Since the greater part of the accidents in this sector is caused by human factors, and given the lack of suitable tools for its consideration along the systems life cycle, especially in the design phase, the development of models and processes dedicated to human factors in risk analysis is essential.

In this context, a technique for early consideration of human reliability (TECHR) was designed for developing a prospective human performance model which can be exploited in the system design phase and which can be updated along the system life cycle. TECHR is based on the use of expert opinion in relation to systems that operate or have operated in recent years to obtain estimates of the probabilities of the various types of human error which may occur during the performance of a specific action.

This paper briefly presents the application of a prospective human performance model — obtained by the TECHR — in the study of an oil tanker operation, focusing the human factor quantification in scenarios of collision. In this work, the actions presented in a previous fault tree — for the vessel operation — are quantified considering the mentioned model, and the results are discussed in view of the previous results of this fault tree.
Calculation of Friction Factors and Downhole Weight on Bit Using Analytical Model of Torque and Drag
Authors: Wu Zebing , Guo Longlong , El Mokhtari Adnane , Pan Yujie , Wang Yongyong , Zhang Shuai , Wang Wenjuan , Lv Lantao
Published: September 2018

To maintain high rate of penetration (ROP) and to realize automatic drilling, it is necessary to monitor and control the weight on bit (WOB). Generally, the WOB is measured at the wellhead, and it is called surface WOB (SWOB). During directional drilling, there is big difference between the SWOB and downhole WOB (DWOB). In this paper, an analytical model for torque and drag of the drillstring is developed to calculate the DWOB. When building the model, the well profiles are separated to two categories, including straight section and curved section. Moreover, for the curved section, the azimuth and inclination of a well are considered while modeling, adequately. Based on a set of field tests, the friction factors and DWOB are calculated. The results indicate that there is a good match in value and trend between the data calculated and the one measured. In practice, the model can be used to predict and adjust the DWOB in autodriller system for real-time drilling monitoring. In addition, the model can also be applied to estimate downhole conditions and diagnose drilling status.
Analysis of Catenary Mooring Systems Based on Truncated Mooring Experiments
Authors: Xu Sheng , Ji Chunyan , Guedes Soares C.
Published: September 2018

Considering the activities of floating systems pushing to the ultra-deepwater, the full scale mooring system needs to be truncated in order to carry out model tests in wave basin with reasonable model scale. In this paper, a Semi-Submersible moored by a catenary mooring system, which operates in 1500m water depth is truncated at 200m water depth. The mooring induced damping of truncated mooring system and prototype mooring system in 100 year typhoon condition in South China Sea are studied experimentally and numerically. The de-couple technique is applied to obtain truncated mooring damping, which is implemented by inputting the vessel six DOF motion responses and solving the mooring tensions, then decompose mooring tension in x, y and z directions. The vessel six DOF motion responses were measured in the model tests. For the full depth mooring system, the fully coupled dynamic analysis is carried out to study mooring induced damping. The results of truncated mooring damping will be compared with full depth mooring damping.
Numerical Modeling of Neutrally-Stable and Sustainable Atmospheric Boundary Layer for the Wind Load Estimation on an Offshore Platform
Authors: Kim Jang Whan , Jang Hyunchul , Xu Wei , Shen Zhirong , Kara Mustafa , Yeon Seongmo , Yan Hongmei
Published: September 2018

This paper summarizes a joint effort among operators, class societies, research institutes, shipyards, and engineering companies to verify a common modeling practice to generate a sustainable wind profile in CFD simulations for wind tunnel tests. The same numerical guideline is applied to various CFD software commonly used in the offshore industry. This is part of the effort from the CFD Task Force under the SNAME OC-8 Panel, “Guidance on Wind Technologies.” The verification results show that the sustainable wind profile can be generated within 1% tolerance from the target wind profile, for all five CFD software participated. The uncertainties in the wind load are also contained within 4% for the semisubmersible hull tested in this study with a traditional turbulence model and the 1%-tolerance wind profile, regardless of the CFD software.
Effect of Negative Damping on Offshore Structures
Authors: Amare Gizat Derebe , Ayele Yonas Zewdu
Published: September 2018

Offshore structures are inevitably exposed to flow induced loads and flow-induced vibrations. The effect of these loads will affect the responses of structures, and the combined of two together on the response can lead the structures to induce different phenomena. The effect of damping is to counteract any dynamic response; however, “negative damping” increases the response amplitude. For example, the response amplitude may increase and can lead to structure instabilities, and it might cause damage in the short time. In order to achieve the best possible structural design, it is then relevant to study conditions under which structure instabilities occur. The purpose of this paper is to discuss the conditions under which offshore structures could induce “negative damping” and different structural phenomena that have been caused by “negative damping”. The discussion suggests a damping model with linear and time-varying terms, and shows theoretically that the model is negative under certain wave conditions.
The 2010 Oil Spill in the Gulf of Mexico: Flow-Rate Estimation Based on Satellite-Images Analysis
Authors: Giraldo Diego Garcia , Yeung Ronald W.
Published: September 2018

The Deepwater Horizon Mobile Offshore Drilling Unit (MODU) was one of several classes of floatable drilling machines. The explosion on April 20, 2010 led to the worst ecological disaster with regard to oil spills in the USA. The objective of this paper is to develop a logical and independent estimate of the oil flow rate into the Gulf of Mexico produced by the rupture in this rig. We employed the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) satellite photographs [1] starting from the days immediately following the disaster to determine the size and intensity of the oil spill. From these images, we obtained the surface area of the oil spill and calculated the oil flow rate by two different methods based on contrasting luminance within the area. The first assumes a constant thickness for the total area with upper and lower bounds for the thickness. The second method separates the spill area into different patches, based on the luminance levels of each. It was found that the probability density function (PDF) of the luminance plots typically showed some natural grouping, allowing patches to be defined. Each patch maps to a specific thickness and the result of the addition of all the patches provides a more accurate average thickness of the spill. With the assumption that evaporation and other loss amounted to 40% of the spill, we obtained, as a result of this analysis procedure, a minimum flow rate of 9,300 barrels per day and a maximum of 93,000 barrels per day using the first method. A value of 51,200 barrels per day was obtained using the method based on patch separation. This latter estimate was a reasonable value obtained based on this relatively simple method but with no details presented in an Extended Abstract in OMAE2012 [4]. It is remarkably consistent with the “official US-Govt. estimates” of [2, 3].
Flare Structure Heat-Up During Depressurization
Authors: Gasmi Rami , Cahay Marc
Published: September 2018

Despite regulations becoming more and more stringent, significant quantities of gas are still flared around the world every year. Indeed, for safety reasons, flaring remains a usual practice in oil and gas production in cases of process upset. For instance, emergency shutdown, when the unit must be depressurized in a short period of time, most of the gas inventories are flared to limit as much as possible the potential consequences of fire and explosion within the facility.

With the increase of the global demand for energy and especially in Liquefied Natural Gas (LNG), the recent development of Floating Liquefied Natural Gas unit (FLNG) has raised new challenges concerning flare stack design. Since FLNG facilities handle large flammable gas quantities the flare stack needs to be designed considering much more stringent cases. It results in an increased length of flare stack, to reduce the radiation effects on personnel and equipment. The thermal response of the flare structure needs also to be accounted for in the design, in addition to other load cases such as piping and structural weight or vessel accelerations.

To accomplish the structural design of the flare stack, the engineers will have to convert the radiative heat fluxes from the flame into the resulting temperature of the structure exposed.

Indeed, temperature is the parameter that can be used as a thermal load case in any finite element analysis calculation code. Current temperature mapping methodologies applied on projects are not exhaustive and are often based on a simplified approach which is now challenged by operators and certification bodies who require more detailed verifications on flare structure heating during continuous or emergency flaring. Moreover, such simplified modelling approaches tends to overestimate thermal protection to mitigate the heat radiation impact.

The proposed approach described in this paper will address these points through a multidisciplinary workflow to form a flexible, simple and robust technical methodology to be applied during project execution. The proposed approach will assess heat radiation and temperature calculations in a spatial-temporal reference including the dynamic response.

This transient approach is more attractive as computed temperatures will be lower than steady-state approach results which are the usual engineering practice, especially for accidental loading cases, such as emergency depressurization, where the flare release can decrease quickly.
An Improved Body-Exact Method to Predict Large Amplitude Ship Roll Responses
Authors: Subramanian Rahul , Rakesh Naga Venkata , Beck Robert F.
Published: September 2018

Accurate prediction of the roll response is of significant practical relevance not only for ships but also ship type offshore structures such as FPSOs, FLNGs and FSRUs. This paper presents a new body-exact scheme that is introduced into a nonlinear direct time-domain based strip theory formulation to study the roll response of a vessel subjected to moderately large amplitude incident waves. The free surface boundary conditions are transferred onto a representative incident wave surface at each station. The body boundary condition is satisfied on the instantaneous wetted surface of the body below this surface. This new scheme allows capturing nonlinear higher order fluid loads arising from the radiated and wave diffraction components. The Froude-Krylov and hydrostatic loads are computed on the intersection surface of the exact body position and incident wave field. The key advantage of the methodology is that it improves prediction of nonlinear hydrodynamic loads while keeping the additional computational cost small. Physical model tests have been carried out to validate the computational model. Fairly good agreement is seen. Comparisons of the force components with fully linear and body-nonlinear models help in bringing out the improvements due to the new formulation.