bluuebunny/crossref_metadata_2025 huggingface dataset:
Assessment of Experimental Uncertainty for a Floating Wind Semisubmersible Under Hydrodynamic Loading
Authors: Robertson Amy N. , Bachynski Erin E. , Gueydon Sebastien , Wendt Fabian , Schünemann Paul , Jonkman Jason
Published: September 2018

The objective of this paper is to assess the sources of experimental uncertainty in an offshore wind validation campaign focused on better understanding the nonlinear hydrodynamic response behavior of a floating semisubmersible. The test specimen and conditions were simplified compared to other floating wind test campaigns to reduce potential sources of uncertainties and better focus on the hydrodynamic load attributes. Repeat tests were used to understand the repeatability of the test conditions and to assess the level of random uncertainty in the measurements. Attention was also given to understanding bias in all components of the test. The end goal of this work is to set uncertainty bounds on the response metrics of interest, which will be used in future work to evaluate the success of modeling tools in accurately calculating hydrodynamic loads and the associated motion responses of the system.
On the Handling of Moonpool Resonances by Green’s Function Methods
Authors: Vada Torgeir , Pan Zhi Yuan
Published: September 2018

In this paper, we will investigate the numerical treatment of irregular frequencies and moonpool resonance by using a Green’s function method. We will look at the interaction of these two resonances and how much the moonpool resonance will influence other results. We will also look at the importance of the meshing including the addition of a mesh in the moonpool. Such a mesh is normally only used when we want to include damping of the moonpool resonance, but it will be shown that including this mesh may be beneficial also without damping.

In addition to the wave elevation in the moonpool we will also investigate the effect of the moonpool resonance on the motions, sectional loads and mean drift forces.
Viscous Drift Forces on a Semi-Submersible in Combined Wave and Current
Authors: Kim Joo-Sung , Kim Hyun Joe , Lee Dong Yeon
Published: September 2018

A time domain simulation tool is used to setup a numerical simulation model in which the wave loads of the potential flow are combined with the viscous drag force computed by the Morison equation. The drag coefficients in the Morison equation are determined to provide the best numerical results compared to model test data.

The numerical simulations and analytic solutions have been used to examine the characteristics of mean viscous drift force on the semi-submersible for fixed condition. The results show that the mean viscous drift force is mainly produced by the column. The column is divided into splash zone and submerged zone because they give much different mean viscous drift force. In high waves, the splash zone leads to mean drift force much larger than the submerged zone for both wave-only and wave-current coexisting flow fields.

The present results clearly show that the drag coefficients can be expressed as a function of the Keulegan-Carpenter number, especially for high waves. There is strong correlation between regular and irregular waves for both wave-only and wave-current coexisting flow fields. If current is present, the drag coefficient decreases significantly.
Empirical Truncation Design of Deepwater Mooring System Using Supervised Learning Method
Authors: Handi Wei , Longfei Xiao , Xin Li , Yufeng Kou
Published: September 2018

Truncated mooring system is the foundation of hybrid model test of deepwater floating platform with spread mooring system. This paper presents supervised learning method using linear regression model as the learner to generate empirical formulas which can determine the properties of truncated mooring system given the properties of full-depth mooring system. Thousands of completed truncation tasks are used to train the leaner, and then empirical formulas determining the length, axial stiffness and wet weight of the truncated system are established. The formulas are tested using a set of new truncation tasks. The results indicate that the truncated mooring system can be properly predicted by simply using the formulas.
Pipeline and Cable Stability: Updated State of the Art
Authors: Griffiths Terry , Draper Scott , White David , Cheng Liang , An Hongwei , Tong Feifei , Fogliani Antonino
Published: September 2018

At OMAE in 2008 the ‘state of the art’ in pipeline on-bottom stability engineering was summarized, providing an overview of the current available knowledge for addressing pipeline stability. The aim of that work was to summarise key aspects of the pipeline stability design process and to include some historical perspective. The paper discusses the advantage and shortfalls of the different design approaches with a view to consolidate understanding, rather than to provide a ready-made solution to a complex design problem [1].

Since that time, a decade of research and further methodology refinement has extended the boundaries of the industry’s knowledge and understanding of the behaviour of subsea pipelines and cables, including geotechnics, hydrodynamics, oceanography and structural response modelling. In particular, progress has been made in:

• The response of pipelines to sediment transport and scour;

• Understanding the behaviour of small diameter pipelines and cables within wave and current boundary layers; and

• The behaviour of cables on rocky seabeds in high energy marine environments.

This paper summarises these innovations to enable the application of new paradigms in engineering practice and improved outcomes for initial project capital cost, reliability and operational integrity, as well as better models to predict the long-term behaviour where pipes are decommissioned in-situ.

While a relatively widely studied field of engineering, there remain areas of active ongoing research to improve our understanding and ability to model and predict subsea pipeline on-bottom behavior, with a summary of the anticipated future opportunities proposed.
Topology Optimization of Primary Support Members in Cargo Tank Region for Oil Tankers
Authors: Shen Xiaoxi , Feng Guoqing , Ren Huilong , Xu Huifen , Chang Qi
Published: September 2018

To meet the needs of weight-light of ships, topology optimization for oil tankers has been proven to be more effective and promising. Nevertheless, due to its characteristics of topology optimization, massive restriction conditions and load-cases, long-time calculation and differences of the strain energy between different load-cases, there are still problems during this process. The objective of this article is to discuss topology optimization design of primary support members in cargo tank region for oil tankers. Here we select SIMP algorithm as the topology algorithm to achieve optimization design of the cargo tank structures. Main control parameters of topology optimization such as optimization subjects, optimization variables, design regions, restrictions, weight of load-cases and volume fractions are all in consideration. The modeling methods and analytical methods of topology optimization are expounded based on HCSR. According to the study, we conclude a detail topology optimization process of primary support members for oil tankers under complex constraints and complex forces. A case is given of an oil tanker with a single longitudinal bulkhead to explain the algorithm and achieve clear topology configuration.
Investigation on Breaking Focused Wave-Induced Loads on a Monopile With CFD Models
Authors: Tomaselli Pietro D. , Aggarwal Ankit , Christensen Erik Damgaard , Bihs Hans
Published: September 2018

The design of new offshore structures requires the calculation of the wave-induced loads. In this regard, the Computational Fluid Dynamics (CFD) methodology has shown to be a reliable tool, in the case of breaking waves especially. In this paper, two CFD models are tested in the reproduction of an experimental spilling wave impacting a circular cylinder. The numerical results from the models are shown together with the experimental measurements.
Numerical Simulation of the Resistance and Self-Propulsion Model Tests
Authors: Lungu Adrian
Published: September 2018

The paper proposes a series of numerical investigations performed to test and demonstrate the capabilities of a RANS solver in the area of complex ship flow simulations. Focus is on a complete numerical model for hull, propeller and rudder that can account for the mutual interaction between these components. The paper presents the results of a complex investigation of the flow computations around the hull model of the 3600 TEU MOERI containership (KCS hereafter). The resistance for the hull equipped with rudder, the POW computations as well as the self-propulsion simulation are presented. Comparisons with the experimental data provided at the Tokyo 2015 Workshop on CFD in Ship Hydrodynamics are given to validate the numerical approach in terms of the total and wave resistance coefficients, sinkage and trim, thrust and torque coefficients, propeller efficiency and local flow features. Verification and validation based on the grid convergence tests are performed for each computational case. Discussions on the efficiency of the turbulence models used in the computations as well as on the main flow features are provided aimed at clarifying the complex structure of the flow around the stern.
6-DoF Hydrodynamic Modelling for Wind Tunnel Hybrid/HIL Tests of FOWT: The Real-Time Challenge
Authors: Bayati Ilmas , Facchinetti Alan , Fontanella Alessandro , Belloli Marco
Published: September 2018

This paper deals with the numerical approach and technical implementation of the 6-DoF hydrodynamic modelling, combined with the Politecnico di Milano HexaFloat robot, adopted for wind tunnel Hybrid/HIL tests floating offshore wind turbines. The hybrid testing methodology, along with its ocean-basin counterpart, is currently being considered as a valuable upgrade in the model scale experiments, for its capability to get rid of the typical scaling issues of such systems. The work reports an overview of the setup, the general testing methodology, presenting the main challenges about the deployment on the realtime hardware, summarizing the key solving choices. A set of results related to code-to-code comparison between the optimized HIL numerical model and the reference FAST computations are included, confirming the correctness of the approach.
Using Nonlinear Wave Kinematics to Estimate the Loads on Offshore Wind Turbines in 3-Hour Sea States
Authors: Bunnik Tim , de Ridder Erik-Jan
Published: September 2018

The effects of operational wave loads and wind loads on offshore mono pile wind turbines are well understood. For most sites, however, the water depth is such that breaking or near-breaking waves will occur causing impulsive excitation of the mono pile and consequently considerable stresses, displacements and accelerations in the monopile, tower and turbine. As has been shown in earlier, recent publications, Computational Fluid Dynamics (CFD) can be used to accurately analyze wave impacts on offshore wind turbines. However, it is not yet well suited to study the statistical variability of wave impact loads in long-duration sea states, and thus estimate the ULS and ALS loads for which a wind turbine has to be designed. An alternative, simplified approach, is the use of a Morison model in which the kinematics (water particle velocities and accelerations) from a nonlinear wave model are used. For long-crested waves the nonlinear wave model can be run in a 2D mode and is therefore relatively cheap. In this paper model tests for steep and breaking waves on an offshore wind turbine are compared with results from the Morison model. First, a deterministic comparison is made between the wave loads from the model tests and the simulation model (simulating the same 3-hour wave realization as in the basin), which turns out to be difficult because of differences between wave reflections in the wave basin (a physical beach) and the numerical wave model (absorbing boundary condition). Second, a statistical comparison is made by comparing with different wave realizations measured in the wave basin.
Uncertainty of Environmental Contours due to Sampling Variability
Authors: Gramstad Odin , Vanem Erik , Bitner-Gregersen Elzbieta
Published: September 2018

Environmental contours are commonly applied in design of marine structures to identify extreme environmental conditions that may give rise to extreme loads and responses. Recently, there has been some focus on the fact that different methods exist for establishing such contours, and that in some cases significant differences may be obtained from the different methods.

In this study, we address another uncertainty related to the calculation of environmental contours, namely the uncertainty due to sampling variability when constructing environmental contours based on metocean data of finite sample size. The uncertainty of environmental contours for joint distribution of significant wave height and wave period for different sample sizes (10, 20, 30 and 100 years of data) are investigated considering different underlying datasets. Both cases where samples are drawn from a known joint distribution of wave height and periods and cases where samples are drawn from a real hindcast dataset and fitted to the joint distribution are considered. The uncertainty of the estimated contours is quantified and discussed in light of differences that can be anticipated from the different methods used to calculate the contours.
Experimental Studies of Air-Lift Pump for Deep Sea Mining
Authors: Takano Satoru , Masanobu Sotaro , Kanada Shigeo , Ono Masao , Sasagawa Hiroki
Published: September 2018

The air-lift pumping system is one of the promising method for subsea mineral lifting. It is significant to figure out the relation between water flux and injected air volume or submergence ratio. In this paper, the authors conducted the experiment to obtain basic data for vertical air-lift pumping. As the results of the experiment, the water flux increased with increasing the submergence ratio. Then the water flux increased with increasing the injected air volume up to a value. In addition, the authors calculated the solid flux using large internal diameter of pipe. The calculated results are in good agreement with the experimental ones.
Calibration of Hydrodynamic Coefficients for a Semi-Submersible 10 MW Wind Turbine
Authors: Kvittem Marit I. , Berthelsen Petter Andreas , Eliassen Lene , Thys Maxime
Published: September 2018

Hydrodynamic model tests and numerical simulations may be combined in a complementary manner during the design and qualification of new offshore structures. In the EU H2020 project LIFES50+ (lifes50plus.eu), a model test campaign of floating offshore wind turbines using Real-Time Hybrid Model (ReaTHM) testing techniques was carried out at SINTEF Ocean in fall 2017. The present paper focuses on the process of calibrating a numerical model to the experimental results. The concepts tested in the experimental campaign was a 1:36 scale model of the public version of the 10MW OO-Star Wind Floater semi-submersible offshore wind turbine.

A time-domain numerical model was developed based on the as-built scale model. The hull was considered as rigid, while bar elements were used to model the mooring system and tower in a coupled finite element approach. First-order frequency-dependent added mass, potential damping, and excitation forces/moments were evaluated across a range of frequencies using a panel method. Distributed viscous forces on the hull and mooring lines were added to the numerical model according to Morison’s equation. Potential difference-frequency excitation forces were also included by applying Newman’s approximation.

The quasi static properties of the mooring system were assessed by comparing the restoring force and maximum line tension with the pull-out test. Drag coefficients for the line segments were estimated by imposing the measured fairlead motion from model tests as forced displacement and comparing the calculated and measured dynamic line tension.

The linear and viscous damping coefficients were first estimated based on the decay tests, and the tuned damping coefficients were compared to initial guesses based on the Reynolds and Keulegan-Carpenter number at model scale. The results were then applied in the numerical model, and simulations in extreme irregular waves were compared to the experiments. It was found that second order drift forces proved to be significant, particularly for the severe irregular seastate. These could not be modelled correctly applying the potential drift forces together with quadratic damping matrix tuned to the free decay test. And the model with viscous drag coefficients tuned to decay tests also underestimated the slow drift motions. Thus, new viscous drag coefficients were determined to match the low frequency platform response.

To inverstigate the performance of the tuned model, comparisons were made for a moderate seastate and for a simulation with both waves and wind on an operating turbine. In the end, possible further improvements to the modelling were suggested.
Field Experiments on Dielectric Elastomer Generators Integrated on a U-OWC Wave Energy Converter
Authors: Arena Felice , Daniele Luca , Fiamma Vincenzo , Fontana Marco , Malara Giovanni , Moretti Giacomo , Romolo Alessandra , Rosati Papini Gastone Pietro , Scialò Andrea , Vertechy Rocco
Published: September 2018

This paper describes the results of an experimental campaign conducted on a U-Oscillating Water Column (U-OWC) wave energy converter equipped with Dielectric Elastomer Generator (DEG) Power Take-Off (PTO) system. The considered PTO technology has the potential for overcoming some of the limitations associated with the use of traditional self-rectifying turbines. Experiments have been performed in the benign sea test site of the Natural Ocean Engineering Laboratory (NOEL), where the DEG/U-OWC was exposed to sea states with a significant wave height in the range of 0.15 m – 0.45 m and peak spectral periods in the range of 1.8 s – 3.3 s.

The aim of this work is to analyze the dynamic response of the coupled DEG-PTO and U-OWC system. The analysis of the experimental data shows that the presence of the DEG determines a slight decrease in the natural period of the water column oscillations. Through the tests, we also demonstrate that a relief valve can be successfully used to actively tune the dynamic response of the system to ensure the safety of the DEG in severe sea-states.
Basin Tests and Numerical Simulations of Wake-Induced Oscillations of In-Line Risers in Subcritical/Critical Regime
Authors: Levasseur Vincent , Leca Charles , Rousse Benjamin , Pétrié Francois
Published: September 2018

This paper adresses Wake Induced Oscillations in the transition between subcritical and critical regime. Both experimental and numerical approaches are proposed and compared here to model tandem risers motion. The main purpose is to enlarge our insights of the behavior of different arrangements of risers in this very tricky range of incoming flow (Re ∈ [134,000; 300,000]) and to assess the CFD ability as an industrial design tool.
Numerical Modeling of Sand Production Potential Estimation and Passive Control Optimization: A Case Study
Authors: Lopez-Puiggene Eva , Gonzalez-Molano Nubia Aurora , Alvarellos-Iglesias Jose , Segura Jose M. , Lakshmikantha M. R.
Published: September 2018

Solids/sand production is an unintended byproduct of the hydrocarbon production that, from an operational point of view, might potentially lead to undesirable consequences. This paper focuses on a study centered in the geomechanical perspective for solids production. An integrated workflow is presented to analyze the combined effect of reservoir pore-pressure, drawdown, in-situ stress, rock properties and well/perforations orientation on the onset of solid production. This workflow incorporates analyses at multiple scales: rock constitutive modeling at lab scale, 1D geomechanical models at wellbore scale along well trajectories, a 3D geomechanical model at the reservoir scale and 3D/4D high resolution reservoir - geomechanical coupled models at the well and perforation scale.

1D geomechanical models were built using log and field data, drilling experience and laboratory tests in order to characterize in situ stresses, pore pressure and rock mechanics properties (stiffness and strength) profiles for several wells. Rock shear failure mechanism was also analyzed in order to build a pre-drill model and evaluate the wellbore stability from a geomechanical point of view.

Pre-production stress modeling was simulated to obtain a representative initial stress state integrating 1D geomechanics well results, 3D dynamic model and seismic interpretations. Mechanical properties were distributed considering properties calculated in the 1D geomechanical models as input. 3D stress field was validated with in-situ stress profiles from 1D modeling results. This simulated pre-production stress state was then used as an initial condition for the reservoir - geomechanical coupled simulations. Effective stress changes and deformations associated to pore pressure changes were calculated including the coupling between reservoir and geomechanical modeling.

Finally, a 3D/4D high resolution well scale reservoir - geomechanical coupled numerical model was built in order to determine the threshold of sand production. A limit of plastic strain was obtained based on numerical simulations of available production data, DST and ATWC tests. This critical plastic strain limit was used as a criterion (strain-based) for rock failure to define the onset of sand production as a function of pore pressure, perforation orientation and rock strength. Conclusions regarding the perforation orientations related to the possibility of producing solids can support operational decisions in order to avoid undesirable solid production and therefore optimize the production facilities capacity and design to handle large amounts of solids and/or the clogging of the well.
Analysis of Rogue Waves in North-Sea In-Situ Surface Wave Data
Authors: Gramstad Odin , Bitner-Gregersen Elzbieta , Breivik Øyvind , Magnusson Anne Karin , Reistad Magnar , Aarnes Ole Johan
Published: September 2018

The statistical properties of individual wave heights and wave crests from time series of recorded surface elevation are analyzed with a particular focus on the occurrence of extreme and rogue waves in the datasets. The datasets include surface elevation measurements from three different sensors: a wave buoy, a wave laser and a Saab wave radar — all situated at the Ekofisk field in the North-Sea and providing sea surface elevation measurements at 2Hz temporal resolution.

The resulting statistical properties of wave heights and wave crests are compared with common reference statistical distributions such as Rayleigh, Tayfun (1980) and Forristall (1978, 2000) distributions for wave heights and crest heights. In particular, the occurrence of rogue waves (H > 2.2Hs or C > 1.25Hs) in the datasets is investigated. Possible relations between the occurrence of rogue waves and spectral characteristics of the corresponding sea states are briefly discussed.
The Quantification of Soliton Current Profiles for Offshore Engineering
Authors: Jeans Gus , Osborne Alfred R. , Jackson Christopher R.
Published: September 2018

In widely used metocean terminology, solitons are large amplitude, often highly nonlinear, internal waves. They are responsible for complex vertical profiles of rapidly varying ocean currents. These current profiles need to be reliably quantified for a wide range of offshore engineering applications, often with very limited suitable data.

Some recent advances in this field of applied research were described at OMAE2017 by Jeans et al (2017) [1]. Vertical displacements, derived from temperature measurements, were the primary input for soliton quantification. Associated current speeds were estimated from relevant theory and validated using available measured current data. This represents a notable development, because soliton current profiles are traditionally quantified via direct measurements of velocity. However, reliable current measurements can be a challenge, so the new approach is considered more reliable in some circumstances.

Jeans et al (2017) [1] applied one simple and elegant theory for relating vertical displacement to velocity. This theory performed well, considering its limitations. This paper further evaluates different theoretical options, using a new dataset with much larger amplitude solitons. Theories with higher order nonlinearity are required for estimation of soliton current profiles in such challenging conditions.
Time Domain Coupled Analysis and Load Transfer for Floating Wind Turbine Structures
Authors: Han Kaijia , Vada Torgeir , Wang QingQing
Published: September 2018

A time domain coupled analysis and load transfer for floating wind turbine type structures is demonstrated in this paper. Hydrodynamic coefficients are first sent to a time domain solver for coupled analysis including mooring lines and wind turbine. The wave, motions of the structure and auxiliary forces from mooring lines and wind turbine are sent back to a time domain Rankine source solver to transfer the loads to structural model before the time domain structural analysis and fatigue analysis. A converter program is applied between two solvers to get correct transformations of the wave, motions and forces. Special emphasis has been put on investigation of the consistency of the data transfer. The whole analysis procedure is straightforward and can be applied for other similar structures.
Machine Learning Approaches to Anomaly Detection of Top Drive Torque Causing Drill Pipe Failure
Authors: Inoue Tomoya , Sugiyama Daisuke , Shimotomai Takayuki
Published: September 2018

The scientific drillship Chikyu, operated by JAMSTEC has faced the problems with drill pipe damage and failure. After a failure, we examined the surface-measured drilling data, including Fourier transform analysis and spectrogram, and conducted an outlier investigation with an autoregressive model. We found some possible indications of top drive torque anomalies among the surface-measured drilling data. The purpose of this study is to attempt anomaly detection of top drive torque by applying machine learning approaches to such pre-examinations.

Data acquired during the drilling operations of the Chikyu are first analyzed to observe the operations and determine the characteristics of the drilling data. Several hundred sets of surface drilling data are extracted and prepared as labeled learning data. Machine learning is then performed using several algorithms to build predictive models for anomaly detection of top drive torque. Simulations using these predictive models are then conducted for other sets of drilling data, including the anomaly conditions acquired during the operations of the Chikyu. The results demonstrate the validity of the predictive models.

This study applies machine learning technique to the anomaly detection of the top drive torque with aim of preventing the drill pipe damage and failure. This paper describes our approaches in detail and discuss the results.
Progress on the Development of a Holistic Coupled Model of Dynamics for Offshore Wind Farms: Phase I — Aero-Hydro-Servo-Elastic Model, With Drive Train Model, for a Single Wind Turbine
Authors: Lin Z. , Cevasco D. , Collu M.
Published: September 2018

Currently, around 1500 offshore wind turbines are operating in the UK, for a total of 5.4GW, with further 3GW under construction, and 13GW consented. Until now, the focus of the research on offshore wind turbines has been mainly on how to minimise the CAPEX, but Operation and maintenance (O&M) can represent up to 39% of the lifetime costs of an offshore wind farm, due mainly to the high cost of the assets and the harsh environment, limiting the access to these assets in a safe mode.

The present work is a part of a larger project, called HOME Offshore (www.homeoffshore.org), and it has as aim an advanced interpretation of the fault mechanisms through a holistic multiphysics modelling of the wind farm.

The first step (presented here) toward achieving this aim consists of two main tasks: first of all, to identify and rank the most relevant failure modes within a wind farm, identifying the component, its mode of failure, and the relative environmental conditions. Then, to assess (for each failure mode) how the full-order, nonlinear model of dynamics used to represent the dynamics of the wind turbine can be reduced in order, such that is less computationally expensive (and therefore more suitable to be scaled up to represent multiple wind turbines), but still able to capture and represent the relevant dynamics linked with the inception of the chosen failure mode.

A methodology to rank the failure modes is presented, followed by an approach to reduce the order of the Aero-Hydro-Servo-Elastic (AHSE) model of dynamics adopted. The results of the proposed reduced-order models are discussed, comparing it against the full-order coupled model, and taking as case study a fixed offshore wind turbine (monopile) in gearbox failure condition.
Study on the Effect of Climate Change on Ship Responses Based on Nonlinear Simulations
Authors: Guo Bingjie , Gramstad Odin , Vanem Erik , Bitner-Gregersen Elzbieta
Published: September 2018

Hull girder ultimate strength, which governs sagging and hogging failures, is one of the most critical failure modes for a ship hull. The Structural Reliability Analysis (SRA) methodology has been used to develop Common Structural Rules (CRS) for tankers and bulk carriers. A linear model for bending moment in extreme weather with a nonlinear correction factor has been adopted in the analysis. It is difficult to conclude on the model uncertainty associated with nonlinear effects of bending moment as, until now, there are very limited studies addressing this topic. In this paper, the nonlinear effect on ship responses will be analyzed, and the potential effect of climate change on ship responses will be investigated with the improved 3D Rankine Panel method using nonlinear wave input. The nonlinear wave input is generated by the HOSM (Higher order Spectral Method) wave model incorporating higher order nonlinear effects, including nonlinear free-wave modulation as well as higher order bound harmonics. The North Atlantic past and projected future wave climates are considered.
Wave Force on a Partially Submerged Fixed Horizontal Circular Cylinder
Authors: Silva Monica C. , Vitola Marcelo A. , Esperança Paulo de Tarso T. , Sphaier Sergio H.
Published: September 2018

Circular cylinders are one of the most common geometries used in many structures, such as fixed platforms, risers, umbilical cables, offshore fish farms, floating offshore wind turbines, wave energy devices, plastic cleanup booms, and oil containment booms. Although partially submerged horizontal circular cylinders can be found in many offshore and marine structures, few works have investigated the influence of their positions beneath the free surface and the resulting wave force. The present work aimed to numerically study the wave force acting on a fixed horizontal circular cylinder near the free surface for different depths. The wave flow was modeled using a viscous model available in the StarCCM+ software using a two-dimensional numerical wave tank. The governing equations were solved using the finite volume in an unstructured mesh. A circular cylinder with a diameter (D) of 0.10 m and a regular wave with a steepness (H/L) of 0.025 were used in the present study. In this case, the cylinder diameter is much smaller than the wave length. Three different submerged depths were investigated, and the numerical results were compared with experimental data extracted from Dixon [1]. Good agreement was found for the first two cylinder positions (zc/D = 0 and −0.3). For the last case (zc/D = −0.5), a phase shift was observed. However, by correcting it, the agreement between the numerical and experimental data was also good.
Remnant Life Assessment of Bonga FPSO and SPM Mooring Chains
Authors: Ibekwe Achinike , Sorensen Kjeld , Fernández Jonathan , Arredondo Alberto , Mena Alex
Published: September 2018

Delivering full service life performance for mooring systems of Floating Production Storage & Offloading assets (FPSO) has been a frustrating challenge to operators across the industry.

Remaining strength and fatigue assessment on degraded top mooring chains of the Bonga FPSO and Single Point Mooring (SPM) loading Buoy has been investigated as part of an in-house Bonga Asset Preservation Program. Both facilities are located approximately 120 km off the coast of Nigeria in the Gulf of Guinea operating in tropical waters just North of the Equator, where top chain links have been subjected to accelerated deterioration from Sun Corals and other forms of Microbiologically Induced Corrosion (MIC). These phenomena have led to overall corrosion rates being slightly above general design requirements, but more importantly to formations of large pitting on several sections of the top chain links.

Remotely Operated Vehicles (ROV) assisted inspections, chain link cleaning and underwater 3D photogrammetry have allowed capturing the surface geometry of representative degraded chain links of the mooring lines to provide detailed input data for further analyses.

Reverse engineering has been performed via Finite Element Analysis and fracture mechanics methodologies using the scanned geometry of selected highly exposed critical links to estimate the residual strength and fatigue life performance of the degraded links relative to their original design criteria.

To evaluate the potential impact of cracks on the capacity of degraded chains relative to a reference link, crack tip Stress Intensity Factors have been computed at worst case stress-raising pits and parametric analyses using varying initial crack sizes have been performed to calculate the number of years for the cracks to propagate to critical sizes.

A baseline for benchmarking the strength, fatigue and crack growth behaviour of the degraded links investigated has been provided by analysing non-degraded and uniformly corroded links after 12 years of service with projection to end of service life capacity.

The paper provides a comprehensive application of numerical methods for assessing the fitness-for-service of the chains and recommendations on in-situ performance integrity management by circumventing the need to retrieve chain samples for testing.
Energy Consumption of a Sailing Quad-Maran Automated Vessel
Authors: Zhang Chenfang , Srinivasamurthy Sharath , Kitamura Shinichi , Masuda Norikazu , Park Choong Sik , Nihei Yasunori
Published: September 2018

An innovative electrically operated automated Quad-maran is developed in this study. The vessel consists of 4 hulls and each hull is capable of yawing independently around its axis. The vessel is expected to sail and carry out environmental measurements automatically in waters like oyster farms, with an outstanding efficiency and brilliant energy-saving characteristics. An elemental research was first carried out to investigate the vessel’s resistance and propulsion performance as well as its electric energy consumption during sailing. Water tank tests were implemented on the actual hulls. Later, sea trials using the whole vessel were carried out in Ishikawa Prefecture, Japan, where the vessel sailed under remote control from a PC far away. Pool trials were also carried out. In this paper, detailed information is given on the design and working features of the Quad-maran automated vessel. Results of the tests and trials are evaluated, and the interference between hulls observed during tests is discussed. The electric energy consumption and as well as the relating efficiency are calculated and discussed.
Effect of Tensile Force for Wear Performance of Mooring Chain
Authors: Gotoh Koji , Nakagawa Masataka , Murakami Koji , Utsunomiya Tomoaki
Published: September 2018

Floating wind turbine facilities, which are installed in the deep sea area, plays an essential role to promote the green energy application. One of the problems associated with the commercialization of facilities installed in the deep sea is the reduction of the maintenance cost of mooring chain, because breaking of the mooring chain caused by the wear between links leads to enormous economic losses. Therefore, it is necessary to establish a quantitative wear evaluation method for mooring chains.

Experimental facility to reproduce the wear caused by sliding between links in actual scale applied for floating wind turbine, which had been proposed by the authors, was updated and the wear tests was conducted by setting some tensile force conditions between the links. Besides, procedure of the nonlinear finite element analysis was improved to estimate the behaviour of wearing between links.

From experiments and numerical analysis, it has been confirmed that the tensile force between links is an important factor of the wear amount between links.
Pipeline Fracture due to Compression-Tension Loading Caused by Foreign Object Impact
Authors: Kristoffersen Martin , Børvik Tore , Olovsson Lars
Published: September 2018

In areas frequented by fishing vessels, trawl equipment or anchors may interfere with pipelines and cause damage through impact, potential hooking, and ensuing release of the pipeline. This load sequence of denting followed by global bending and springback results in a complex stress and strain history. Experiments have shown that fracture in an impacted pipe typically arises along the bottom of the dent, where the material suffers high compressive strains in the impact and hooking phase, and a rapid change to tension during the rebound phase. High compressive strains may reduce the strain to failure significantly for a succeeding tensile phase. A common trait of ductile damage models is to account for damage through nucleation, growth and coalescence of voids, which traditionally is thought to occur during tension. In this study, an uncoupled phenomenological Cockcroft-Latham-type fracture model accounting for anisotropic damage is used. The fracture model is implemented in the explicit finite element programme IMPETUS Afea Solver, and calibrated using material tests. Simulations show that the proposed fracture model is able to account for the observed behaviour.
Estimating the Rate of Scour Propagation Along a Submarine Pipeline in Time-Varying Currents and in Fine Grained Sediment
Authors: Draper Scott , Yao Weidong , Cheng Liang , Tom Joe , An Hongwei
Published: September 2018

Estimating the horizontal rate of scour propagation along a submarine pipeline is a key step in estimating changes in the pipelines burial state and on-bottom stability due to sediment transport. However, whilst recent work has been undertaken to estimate the horizontal rate for non-cohesive uniform sands in steady current, in field conditions currents can vary in time and the sediment can be fine grained and exhibit very different erosion properties to sand. As a first attempt to account for these complications, in this paper we present results from a series of experiments designed to measure the rate of scour along a model pipeline in time varying currents and in a fine grained sediment. The scour experiments are also supplemented by erosion testing, which indicate that the erosion resistance of the fine grained sediment is larger than that predicted by the well-known Shields curve. Based on the scour experiments, it is found that in time-varying currents the scour rate can be predicted using an amalgamation of the results obtained for steady current conditions; this is a convenient result because theoretical predictions already exist for the scour rate in steady current conditions. In the fine grained sediment experiments, it is found that the horizontal rate of scour is much lower than that predicted by existing theoretical models that assume a non-cohesive sandy seabed. To provide an improved estimate of the horizontal rate of scour, a new theoretical model is introduced that relates the horizontal rate of scour to the measured erosion properties of the sediment. This new model is found to agree well with the experimental measurements. Although further experimental testing is recommended, in combination, it appears that these results may be used to better estimate the horizontal rate of scour in both time-varying and fine grained sediment.
Comparisons of Turning Abilities of Submarine With Different Rudder Configurations
Authors: Feng Dakui , Chen Xuanshu , Liu Hao , Zhang Zhiguo , Wang Xianzhou
Published: September 2018

Submarine is usually equipped with two different control device arrangements, namely a cruciform and a X rudder configuration. In this paper, numerical simulations of the DARPA Suboff submarine and its retrofitted submarine with a X rudder configuration are presented. Turning simulations in model scale were studied to compare the turning abilities of the two different control device arrangements. The computations were performed with a house viscous CFD solver based on the conservative finite difference method. In the solver, RANS equation are solved coupled with six degrees of freedom (6DOF) solid body motion equations of the submarine in real time. The structured dynamic overlapping grids were used to simulate the real-time changes of the attitude of the submarine and the rotation of the rudder. The volume force method was used to replace the real propeller to realize the self-propelled movement of submarine. In the free running maneuvering simulations, the submarines move at the same initial velocity and rudder angle, restricted to the horizontal plane with four degrees of freedom (4DOF). Comparisons of the trajectory and kinematic parameters including relative turning radius and turning period between the two cases were presented in this paper. The results show that, compared with the cruciform rudder configuration, the X rudder configuration has obvious advantages for submarine in the turning abilities.
Mastering High Product Variety of an Underwater Vehicle Class in the Concept Design Stage
Authors: Wehner Willem Hendrik , Richter Nicolas , Schiemann Marc , Haselberger Pia-Maria , Ritz Sebastian , Golz Matthias , Boeck Florin
Published: September 2018

The paper provides considerations for a novel unmanned underwater vehicle class that offers new options to the offshore industries and marine science in matters of endurance, payload capacity, development time and economic viability. Today, different mission scenarios require different underwater vehicles. By applying modularization approach to the development of modular product classes, another way to design such vehicles is shown. Radical modularization of the vehicle enables collaborative as well as independent development of payload modules by industry or science. The design idea allows the combination of proven basic modules with novel mission modules. This allows assigning development activities of mission modules to diverse 3rd-party developers or customers. Topics covered in this paper are related to potential missions and the requirements they make on the vehicle. An evaluation of application scenarios considering the technical challenges vs. their economical relevance is made. The requirements for the MUM system are identified by analyzing the mission procedures regarding specific scenarios. The modular design method and challenges to validate feasibility of an extreme number of possible vehicle variants follow. Examples of variant drivers like diving depth or vehicle range as well as possible solutions will be discussed. The topics covered are the basis for further work within the three year research project MUM – Large Modifiable Underwater Mothership.
Seakeeping Behavior of an Innovative, Twin-Hull LNG-FPSO Design During Side by Side Offloading Operations: A Numerical and Experimental Investigation
Authors: Gkikas Georgios
Published: September 2018

A numerical and experimental investigation is performed with respect to the seakeeping behavior of a prototype, twin-hull, LNG-FPSO and its hydromechanic interactions with the offtake (LNG) shuttle carrier during Side-by-Side offloading. The numerical model describing the SbS mooring system as well as the assumptions and calibration steps made towards the development of a robust numerical realization are presented herewith.

Following calibration, the numerical model was tested against dedicated seakeeping experiments in order to assess the effectiveness of the proposed approach as well as the hydrodynamic performance of the overall offloading system. For a more realistic offloading-scenario case, three-component, i.e., wave, wind and current, offloading environments were used for validation purposes.

For the time domain simulation, as far as the hydrodynamic (diffraction) database is concerned, the multibody response amplitude operators are implemented directly, instead of employing retardation functions, in order to observe whether such an approach can still yield robust results for such complicated hydromechanic system.

Statistics of the relative motions between the LNG-FPSO and LNGC manifolds, fender loads and lines’ tensions are obtained and presented, illustrating the good agreement between numerical and experimental results.

Lessons learned and further recommendations are subsequently summarized and stated.
Validation of a Deterministic Wave and Ship Motion Prediction System
Authors: Naaijen Peter , van Oosten Kees , Roozen Karel , van 't Veer Riaan
Published: September 2018

The workability of various types of operations offshore are largely affected by waves and wave induced motions. Examples are crew transfer from crew transfer vessels or service operation vessels to offshore wind turbines for maintenance, landing of helicopters in (navy) vessels and various crane operations. Over the recent years quite some effort has been put in technology aiming to provide a real time on-board prediction of approaching waves and wave induced vessel motions some minutes in advance. Enabling crew to anticipate, thus enhancing the safety and operability of these operations. This paper addresses the performance during a field test of the system as being under development by Next Ocean enabling such predictions, based on using an off-the-shelve (non-coherent) navigation radar system as a remote wave observer. Briefly summarizing (earlier publications on) the technical approach, focus will be on results obtained from a field test where the system was validated. Good agreements between ship motions as measured by an on-board motion reference unit and predictions obtained by the wave and motion prediction system during a field test on the North Sea near the Dutch coast on a 42 m patrol vessel will be shown in the results section, from which the usefulness of the system for operational decision support can be concluded.
Body Force Propeller Model for Unsteady Surge Motion
Authors: Knight Bradford G. , Maki Kevin J.
Published: September 2018

Accurately modelling a self-propelled vessel in a large amplitude seaway with CFD is very expensive and practically out of reach. The expense is due to the very small numerical time-step required for the propeller rotation and the large mesh size. A method for accurately modelling a propeller while reducing computational cost is desirable. This paper describes the first step towards developing a body force propeller model for unsteady conditions. The purpose of this study is to train a semi-empirical algorithm to accurately prescribe the unsteady body force to model the propeller. The MOERI Container Ship propeller is analyzed with RANS CFD. Open water test data is compared to the RANS CFD results of a steady Moving Reference Frame approach. Harmonic surge is applied to a transient rotating mesh model in open water and the behind condition.