bluuebunny/crossref_metadata_2025 huggingface dataset:
Identification of Key Elements for Compliance of the IMO Polar Code Requirement of Minimum 5 Days Survival Time
Authors: Solberg Knut Espen , Barane Endre , Gudmestad Ove Tobias
Published: October 2017

IMO has developed the functionally based Polar Code, which entered into force 01.01.2017. The code requires marine operators to provide lifesaving equipment that ensures a minimum of 5 days survival time. This requirement puts additional strain on the existing life-saving appliances.

To identify the key elements for compliance of the IMO Polar Code, a full-scale exercise (SARex, (April 2016)) was initiated and organized by University of Stavanger, the Norwegian Coast Guard and GMC Maritime. The object of the SARex full scale exercise was to identify the gaps between the functionality provided by existing SOLAS approved safety equipment and the functionality required by the Polar Code. The exercise was conducted with the help of the vessel KV Svalbard (Norwegian Coast Guard), and participated by leading experts from industry, governmental organizations and academia.

The following topics were specifically addressed in the exercise that took place in the marginal ice zone of the coast of Svalbard late April 2016:

1. Functionality of survival craft and PPE (Personal Protective Equipment) polar conditions.

2. Coast Guard’s search and rescue procedures, including handling of mass evacuations in Polar Regions.

3. Functionality of Personal/Group survival gear according to the requirements defined in the IMO Polar Code.

Survival for 5 days according to the requirements defined in the IMO Polar Code in a life raft represents a large challenge with regard to design of rescue craft, design of PPE (Personal Protective Equipment), rations and on-board organization. As the IMO Polar Code is functional based, a holistic approach has to be utilized when assessing the survival chain. For an assessment of the survival chain to be conducted in a sustainable manner, key elements essential for survival in a life raft have to be identified and prioritized.

Designing a survival chain applicable to maritime industry also involves considering commercial, economic and operational restrictions.

The paper will elaborate on the key elements essential for a minimum of 5 days survival in a life raft based on the findings identified in the Phase 1, Functionality of survival craft and PPE (Personal Protective Equipment) polar conditions, of the SARex exercise. The findings will be assessed and evaluated based on industry limitations.
Stochastic Dynamic Analysis of U-OWC Wave Energy Converters
Authors: Spanos Pol D. , Strati Federica M. , Malara Giovanni , Arena Felice
Published: October 2017

The paper presents a random vibration analysis of a U-Oscillating Water Column wave energy harvester (U-OWC). The U-OWC comprises a vertical duct on the wave beaten side, in addition to the elements of conventional OWCs. From a mathematical perspective, the U-OWC dynamic response is governed by a set of coupled non-linear differential equations with asymmetric matrices of mass, damping, and stiffness. In this work, an approximate analytical solution of the U-OWC equations of motion is sought by using the technique of statistical linearization. This technique allows pursuing rapid random vibration analyses via classical linear input-output relationships. The analysis is conducted by considering the case of the full-scale prototype in the port of Civitavecchia (Rome, Italy). The reliability of the proposed approach is assessed versus relevant Monte Carlo data. For this, realizations of sea states compatible with typical power spectral density functions of sea waves are employed. The performed analyses prove that the statistical linearization technique based approach is an efficient and reliable tool which may both circumvent the use of time-consuming Monte Carlo simulations, and be used for a variety of design optimization related parameter studies.
On the Adequacy of Existing Foundation Schemes for Offshore Wind Turbines Subjected to Extreme Loading
Authors: Kourkoulis Rallis , Gelagoti Fani , Georgiou Irene , Karamanos Spyros , Gazetas George
Published: October 2017

Dictated by the world’s escalating energy demands, offshore infrastructure is moving beyond the immediate continental shelf into deeper waters. Although the monopile solution has proven its reliability for many years, its feasibility in larger depths is questionable, or even limited, and multi-pod foundations, such as jacket structures, could be regarded as viable alternatives. Their main advantage, compared to the monopile alternative, is that they are able to sustain large lateral loads through axial stressing rather than bending at their supports (usually materialized using piles or suction caissons).

Recognizing this reality, the present study attempts to compare the performance of a conventional monopile system with that of a jacket foundation when taking into consideration extreme earthquake loading. Although safety fuses do exist to isolate the mechanical equipment from the direct effects of such loading, our focus in this study is on the irrecoverable deformation at the foundation level which, under circumstances, may render the turbine inoperable.

To this end, two foundation alternatives supporting an offshore wind turbine in the Mediterranean Sea are comparatively discussed: the conventional large diameter monopile and a jacket foundation supported by smaller piles or suction caissons. Results show that under expected operational loads the performance of the two systems is practically equivalent. However, extreme loading conditions may significantly alter the response and may, in some cases question the common practice.
On Maintainability of Winterised Plants Operating in Arctic Regions
Authors: Naseri Masoud
Published: October 2017

In Arctic regions, oil and gas (O&G) operations are adversely affected by harsh weather conditions and severe meteorological phenomena such as icing storms and, in certain regions, polar low pressures. Potential solutions, such as implementing winterisation concepts, are explored in the design and even operation phases in order to overcome such obstacles. Simply, the main aim of winterisation is to provide the crew and equipment units with a range of normal environmental and working conditions through, for instance, insulating equipment units, installing heat tracers, enclosing working areas, providing the crew with adequate clothing, etc.

There are, however, some concerns about the efficiency of such winterisation measures and potential changes in operation risk level, of which the changes in plant downtime, production loss, and plant maintainability are the focus of present study. The issue of complex effects of winterisation measures on maintainability analysis of O&G plants operating in the Arctic offshore has gained little attention in the literature. In this study, different aspects of winterisation from the viewpoint of equipment maintainability are discussed. Further, a mathematical framework for maintainability analysis of equipment units subjected to winterisation measures is proposed. The impact of winterisation-related downtimes on plant downtime is analysed as well by employing a Monte Carlo system simulation technique. The application of the proposed framework is illustrated by a case study. The results are further compared with those for a non-winterised system designed for normal-climate regions.
A Study of 3D Flexible Caudal Fin for Fish Propulsion
Authors: Shi Guangyu , Xiao Qing , Zhu Qiang
Published: October 2017

This paper is inspired by a recent numerical study (Shoele and Zhu, 2012, “Leading edge strengthening and the propulsion performance of flexible ray fins,” Journal of Fluid Mechanics, Vol. 693, pp. 402–432), which shows that, for a 2D flexible ray replicating the pectoral fins of live fish, undergoing a flapping motion in a viscous fluid, the performance can be significantly improved via the flexibility distribution on the rays. In present study, we investigate the propulsion capability of a 3D caudal fin undergoing a flapping motion. The embedded rays are modeled as linear springs and the soft membrane is modeled as a flexible plate being able to deform in span-wise direction. A finite-volume method based Navier-Stokes solver is used to solve the fluid-structure interaction problem. The present paper focuses on the effects of various distributions of the ray and the ray flexibilities, which can lead to different fin deformations. It is shown that the detailed ray distribution has significant influence on the propulsion performance. By distributing fin rays at the tips rather than the middle of fin, a less power expenditure is observed, leading to higher propulsion efficiency. However, larger thrust force is obtained through distributing the rays at the middle, which is attributed to larger effective flapping amplitude. Additionally, ray flexibilities also play a pivotal role in the thrust generation of the fin.
Development of Control Strategies for Interconnected Pneumatic Wave Energy Converters
Authors: Thacher Eric , Bailey Helen , Robertson Bryson , Beatty Scott , Goldsworthy Jason , Crawford Curran , Buckham Bradley
Published: October 2017

In the field of wave energy converter control, high fidelity numerical models have become the predominant tool for the development of accurate and comprehensive control strategies. In this study, a numerical model of a novel wave energy converter, employing a pneumatic power take-off, is created to provide a low-cost method for the development of a power-maximizing control strategy. Device components and associated architectures are developed in the time domain solvers Proteus DS and MATLAB/Simulink. These two codes are dynamically coupled at run time to produce a complete six degree of freedom, time domain simulation of the converter. Utilizing this numerical framework, a genetic algorithm optimization procedure is implemented to optimally select eight independent parameters governing the PTO geometry. Optimality is measured in terms of estimated annual energy production at a specific deployment location off the West Coast of Canada. The optimization exercise is one layer of PTO force control — the parameters selected are seen to provide significant improvements in the annual power output, while also smoothing the WEC power output on both a sea-state by sea-state and wave-by-wave basis.
Numerical Simulation of JCO Pipe Forming Process and its Effect on the External Pressure Capacity of the Pipe
Authors: Chatzopoulou Giannoula , Antoniou Konstantinos , Karamanos Spyros A.
Published: October 2017

Large-diameter thick-walled steel pipes during their installation in deep-water are subjected to external pressure, which may trigger structural instability due to excessive pipe ovalization with catastrophic effects. The resistance of offshore pipes against this instability mode strongly depends on imperfections and residual stresses introduced by the line pipe manufacturing process. In the present paper, the JCO pipe manufacturing process, a commonly adopted process for producing large-diameter pipes of significant thickness, is examined. The study examines the effect of JCO line pipe manufacturing process on the structural response and resistance of offshore pipes during the installation process using nonlinear finite element simulation tools. At first, the cold bending induced by the JCO process is simulated rigorously, and subsequently, the application of external pressure is modeled until structural instability is detected. For the simulation of the JCO manufacturing process and the structured response of the pipe a two dimensional generalized plane strain model is used. Furthermore, a numerical analysis is also conducted on the effects of line pipe expansion on the structural capacity of the JCO pipe.
Structural Reliability Analysis for Offshore Drilling Riser Deployment Operability
Authors: Zhang Xiaodong , Yang Hezhen , Adaikalaraj Peter Francis Bernad , Low Ying Min , Koh Chan Ghee
Published: October 2017

Drilling riser system provides a short-term connection between subsea oil well and drilling vessel or platform. The analysis of different operability envelopes are required for drilling riser analysis, for example, connected drilling, connected non-drilling and hang-off analysis. The operability envelope analysis could provide operators statistical information for riser operational management.

The current practice to calculate the drilling operability envelope is to use deterministic approach. However, deterministic approach could not take the randomness from environmental loadings and structures into consideration. Structural reliability method is an analysis tool to quantify probability of failure of components or systems accounting for uncertainties in environmental conditions and system parameters. It is particularly useful in cases where limited experience exists or a risk-based evaluation of design is required. It is gaining increasing popularity in the offshore and marine industry to predict failure probability.

In this paper, structural reliability analysis is adopted to analyze the offshore drilling riser deployment. The uncertainties are mainly from wave and current loadings. The significant wave height HS is modeled by a Weibull probability density function, the zero-crossing wave period TZ conditional on HS is modeled by a lognormal distribution, and the surface current speed is modeled as Weibull distribution. Multiple simulations are performed by software Flexcom [1] and efficient structural reliability methods are adopted to get the failure probabilities. The deployment operability will be calculated based on structural reliability methods and the results will be compared with those calculated based on deterministic approach.
An Automated Approach for Designing Monopiles Subjected to Lateral Loads
Authors: Doherty James P. , Lehane Barry M.
Published: October 2017

This paper describes an automated algorithm for determining the length and diameter of monopile foundations subject to lateral loads with the aim of minimising the pile weight, whilst satisfying both ultimate and serviceability limit states. The algorithm works by wrapping an optimisation routine around a finite element p - y model for laterally loaded piles. The objective function is expressed as a function representing the pile volume, while the ultimate limit state and serviceability limit states are expressed as optimisation constraints. The approach was found to be accurate and near instantaneous when compared to manual design procedures and may improve design outcomes and reduce design time and costs.
Experimental Study on Dynamic Control of Oscillation Characteristics of a Spar-Buoy
Authors: Iseki Toshio
Published: October 2017

Experimental studies were carried out to investigate the possibility of utilization of auto-parametrically excited oscillation based on the Mathieu instability. For the simplicity, the subject is limited to the heaving motion of a spar-buoy type point absorber. The device consists of an inner cylinder and 12 outer cylinders. Each outer cylinder is equipped a movable floating column controlled by a ball screw mechanism and the buoyancy of the outer cylinders can be dynamically changed to induce the Mathieu instability. Based on results of the numerical simulation and the model tests, the possibility of utilization of auto-parametrically excited oscillation for WEC was shown.
Shape Optimization Design of Brackets Connecting Girders of an Internal Bulkhead and Pressure Hull Under External Pressure
Authors: Jiang Chengtao , Cheng Yuansheng , Xiao Wei , He Qijian , Gao Shangdi
Published: October 2017

In order to decrease the local high stress in the brackets which connect to the horizontal and vertical girders of an internal bulkhead and submersible pressure shell, the mathematical models for the shape optimization of the brackets are proposed. In the study, stress analysis of the pressure hull structure including an internal bulkhead and brackets with coarse mesh is firstly conducted, then the submodeling technique is further employed to analyze the refinement stress distribution of the brackets with refined mesh. The boundary shapes of the brackets are assumed as the design variables while the maximum stress of the bracket is treated as objective function to be minimized in the shape optimization problem. The proposed mathematical model is solved by using analysis code Hyperworks/Optistruct and optimal shapes of the brackets are obtained. Results of the shape optimization show that the optimized bracket types can effectively reduce the level of stress. Therefore, the proposed method can be referred to similar structure designs.
Assessment of the Motion of Wave Power Generation by Water Tank Test
Authors: Yoshida Takero , Kitazawa Daisuke , Mizukami Yoichi
Published: October 2017

Installing devices of marine renewable energy is planned off Kamaishi city, Iwate prefecture, Japan. A device of wave power generation was settled at this site as a test case. It is important to coexist the power generation device and local fishermen. Since Kamaishi is known as a good fishing area, acceptance of fishermen is required to run the wave power generation at the test site. Especially, local fishermen in Kamaishi have a concern about how wave power generation moves along with ocean waves and currents. To acquire an acceptance from local fishermen in terms of setting wave power generation, a model test was conducted to understand the device of wave power generation. It consists of float, spar, middle float, gimbal mechanism and anchor. Middle float, gimbal mechanism and anchor are mooring. The submerged weights of the model almost satisfy the targeted values calculated by the scale ratio and actual submerged weights. The experiment was conducted for 1/125 scale model at a water tank in marine environmental and ecosystem laboratory, the university of Tokyo. We monitored the motion of the model under several conditions of regular waves and currents. The motions of the model were recorded by a video camera and were analyzed. The model was moved with respect to the surface waves and currents. We assessed the motions of the float based on the experiment. The experimental results will be used to explain local fishermen.
Effect of Reefs Spacing on Flow Field Around Artificial Reef Based on the Hydrogen Bubble Experiment
Authors: Tang Yanli , Long Xiangyu , Wang Xinxin , Zhao Fenfang , Huang Liuyi
Published: October 2017

When an ocean current encounters a vertical structure, such as an artificial reef (AR), it can create a plankton-rich upwelling that provides a reliable feeding spot for fish. The fish attracted into the AR zones depends largely on the size, structure, and layout of the ARs. Thus, this study was conducted to demonstrate the effect of different layouts on the flow field around cubic ARs, a common type of AR in China. Water tank tests and numerical simulations were used to study the flow field around the cubic ARs. Experiment of hydrogen bubble was conducted in a water tank to observe the effect of the reef spacing on the flow field around the reefs. Using computational fluid dynamics, a three-dimensional numerical model was established based on the Navier-Stokes equation. The standard k-ϵ model was adopted to simulate the flow field around the ARs. On comparing the streamlines of the flow field around the ARs, it was found that the numerical results were in good agreement with the hydrogen bubble results. Using the numerical model, the flow around the ARs with different transverse or longitudinal spacing was simulated. When the transverse spacing distance is equal to the width of the AR, the upwelling becomes stronger. When the distance increases to 7L, the interaction between the reefs can be ignored. When the longitudinal spacing distance is equal to 1L, the effect of the interaction of the reefs on the back eddy is quite significant. However, as the distance increases, the interaction between two reefs gradually weakens.
Observing Fish Using Underwater Camera at the Test Site Before Installing Ocean Power Generation
Authors: Yoshida Takero , Kitazawa Daisuke , Mizukami Yoichi
Published: October 2017

Assessment of local ocean environment is significant when devices of marine renewable energy are installed at a demonstration site. A concern about how these devices affect the existence of fish is remained among local fishermen. To confirm variation of fish before and after installing the devices of marine renewable energy, we investigated fish using an underwater camera before installation as a main objective of this paper. First, fish-eye video camera was tested to monitor in a fish cage off Kamaishi city, Iwate prefecture, Japan. We aimed to observe the abundance of fish and to specify the species of fish. The fish-eye video camera is composed of a dome-shaped lens, a container, and a digital video equipped into the container. The recorded video showed outlines of fish, consequently it may be possible to know the abundance of fish. However, specifying the species of fish has a difficulty if we use fish-eye camera since the recorded image is unclear enough to identify correctly. In addition, we examined horizontal transparency of fish-eye camera using a fish-shaped target. Before recording by the video camera, vertical transparency was measured using a white plate. Then we recorded the images by varying the water depth and the distance between the camera and target. When the target was close to the camera, it can be displayed clearly, nevertheless the outline of the target was smeared with increasing the distances between them. Our measurement suggested that the horizontal transparency might be relating to the parameters of water depth and vertical transparency. The investigation was conducted under the one condition, thus we need to obtain further data to understand horizontal transparency qualitatively. Next time, we will conduct another test using additional underwater optical video camera, which is attached on the fish-eye video camera. The combination of these two types of video cameras may enable to observe the abundance of fish as well as to specify the species of fish.
Experimental Determination of Water Motions Inside a Moonpool
Authors: Abeil Bastien
Published: October 2017

Model tests of a drillship with a rectangular moonpool opening were conducted in regular and irregular waves from the bow and bow-quarter. Most tests were conducted at zero speed, the rest was performed with the model towed to a speed of 10 kn. From the video-recordings and transfer functions of the measured relative water elevation inside the moonpool, the typical piston and first sloshing modes are well captured, for wave frequencies that agree relatively well with relevant formulations. A few tests conducted at varying wave amplitudes show that the water elevation is non-linear by nature, while repeat tests conducted with the moonpool fitted with two layers of side wall flanges shows that these can reduce the water motions by nearly 40 %.
A 3D Wave-Structure-Seabed Interaction Analysis of a Gravity-Based Wind Turbine Foundation
Authors: Li Yuzhu , Tang Tian , Ong Muk Chen
Published: October 2017

In order to prevent the future risk of soil and structural failures, it is essential to evaluate the dynamic seabed soil behaviors in the vicinity of the offshore foundations under dynamic wave loadings. Three-dimensional (3D) numerical analysis is conducted on the interaction between waves, seabed soil and a gravity-based wind turbine foundation. An OpenFOAM based numerical code developed by Tang [1]for wave-structure-seabed interaction is applied. The nonlinear waves are modeled by solving the Navier-Stokes equations for incompressible flow. The dynamic structural response of the foundation is computed using a linear elasticity solver. The transient responses of the seabed are solved by an anisotropic poro-elastic soil solver. The dynamic interaction between different physical domains is implemented by boundary condition coupling and updating in the integrated FVM based framework.

The dynamic wave pressure on the structure and the seabed, the elastic responses of the structure and the changes of the pore pressure, shear stress and seepage flow structure in the seabed are investigated. Highest wave-induced shear stress along the foundation is predicted by solving the deformable structure model. For the seabed soil in the vicinity of the foundation, it is found that the presence of the foundation affects the soil responses by amplifying the wave induced shearing effect on the underlying seabed. Vertical distributions of the pore pressure in the seabed beneath the foundation are investigated with different angles relative to the wave propagation direction. A parametric study of isotropic and anisotropic soil permeability is performed and demonstrates that for the simulated soil in this work, the consideration of the anisotropic permeability is suggested.
Model Test and Simulation Comparison for an Inclined-Leg TLP Dedicated to Floating Wind
Authors: Caillé François , Bozonnet Pauline , Perdrizet Timothée , Poirette Yann , Melis Cécile
Published: October 2017

A new floating foundation for multi-MW wind turbine is being developed within a collaboration between SBM Offshore and IFP Energies nouvelles. This inclined leg TLP, is made up of four immersed buoys and a bracing structure, making the floater transparent to wave excitation. The particular mooring arrangement gives the floater interesting motion properties since it creates a fixed point close to the nacelle, strongly reducing the motion at this elevation.

In order to validate the concept and the simulation strategy, a model test campaign has been carried out during three weeks in 2015 at MARIN’s offshore basin. The downscaling is performed according to the Froude law of similitude to maintain the hydrodynamic loadings and behavior. The tower bending natural period, the mooring stiffness, and the turbine rotation speed are also maintained in order to reproduce the relevant structural modes and check that no unexpected phenomena occur in the system during production or parked conditions.

The scale 1/50 was initially selected so that the MARIN Stock Wind Turbine (MSWT) can be used. This model wind turbine was designed by MARIN with low Reynolds blade airfoils to mimic the NREL 5 MW wind turbine, especially the thrust force. However because of mass distribution issues, the scale has to be changed from 1/50 to 1/40, at this scale only the thrust force and the rotation speed can be replicated.

First, a set of calibration tests are performed in the basin and simulated with Orcaflex™ and DeepLinesWind™ for a better understanding of the system and to validate independently the various components of the numerical models. Secondly, design parked and operational cases are conducted with wind, wave and current loadings for two floater orientations and two water depths.

The objective of this campaign is to validate the concept behavior as well as the simulation tools and methodologies. Hydrodynamic and structural models are very similar in both software and are checked with the calibration tests from the basin, whereas two strategies are implemented to model the aerodynamic contribution. The Simplified Coupled Simulations (SCS), performed with Orcaflex, use the aerodynamic forces recorded during the model tests to be imposed at tower top; the Fully Coupled Simulations (FCS), run with DeepLinesWind, use the aerodynamic loading computed with the BEM theory from the measured wind.
De-Risking Marine Energy Project Development Through Improved Financial Uncertainty Analysis
Authors: Shah Sunny , Buckland Hannah , Thies Philipp R. , Cohen Claire , Bruce Tom
Published: October 2017

The financial performance of a marine energy project is based on assumptions with significant uncertainty. To fully appraise the risk, potential investors require an understanding of the likelihood of deviations from the assumed most likely case for a project’s financial performance. A Monte Carlo Analysis (MCA) model with flexible user defined uncertainty definitions for all inputs is developed for this study. A realistic tidal energy project is used as a case study to compare the central, optimistic and pessimistic Levelised Cost of Energy (LCOE) and Internal Rate of Return (IRR) values derived using commonly used deterministic methods and the probabilistic MCA model. The improvement in decision support due to the probabilistic analysis is shown and the possibility for misinterpreting the deterministic results in highlighted. Two sensitivity analysis methods are employed to identify key risks and emphasise the need to use the most appropriate method for the type of analysis being conducted. Finally, the significance of some commonly ignored parameters is tested and shown to be important for accurately appraising the investment risk in a real project. Thus this paper provides guidance and tools to help investors make informed decisions with confidence.
Combined Experimental and Numerical Studies of Multi-Channel Inlet Design for Ocean Basin
Authors: Zheng Yingying , Magee Allan R. , Tuyen Le Quang , Dao My Ha
Published: October 2017

Multiple-channel inlet design is commonly used in artificial ocean basins for improving the uniformity of current flows in horizontal direction, as well as for the generation of uniform or sheared currents in vertical direction. However, boundary layers developing along the walls between the channels result in lower velocities after the fluid leaves the ducts and enters the basin, which is undesired. To reduce the effects of the boundary layers and increase the flow uniformity at the basin inlet, the present work aims to improve the inlet design. Experimental study are performed in a wind tunnel at wind velocity of 20 m/s. To simulate the walls, a Perspex plate with thickness of 20mm is fixed at the center of wind tunnel test section. A triangle end tip with tip angle of 7° is attached to the training edge of the plate. Four configurations of honeycombs are applied to study the effects of honeycombs on the flow uniformity. Among the four configurations, honeycomb with thickness t = 50 mm and cell size dg = 5 mm is used as bench mark case. In the second and third configurations, the thickness of vertical central 80mm region is reduced to be 25 mm. In the fourth configuration, the central region is then replaced by honeycomb with thickness of 50mm, but with cell size of 10mm. The experimental results show the possibility to eliminate the lower velocity region by using shaped honeycomb or honeycomb with various cell sizes. With the experimental results as validation, the honeycomb configuration is then optimized using numerical simulation with OpenFoam.
Development of a CFD Model to Simulate Three-Dimensional Gap Resonance Applicable to FLNG Side-by-Side Offloading
Authors: Wang Hongchao , Draper Scott , Zhao Wenhua , Wolgamot Hugh , Cheng Liang
Published: October 2017

This paper addresses the process of establishing a numerical model to accurately reproduce experimental results presented by Zhao et al. (2017) of three-dimensional (3D) gap resonance between two fixed ship-shaped boxes. The ship-shaped boxes were arranged in a side-by-side configuration to represent FLNG offloading and were subjected to NewWave-type transient wave groups. To develop the numerical model we employ the open-source Computational Fluid Dynamics (CFD) package OpenFOAM and systematically optimize mesh topology and size, domain size and boundary conditions. CFD is necessary for this problem to accurately reproduce the viscous losses and non-linear free surface effects that are observed in the experiments. The incident transient wave group used in the experiment is regenerated using various iterative schemes. The results show satisfactory agreements between the target and regenerated waves.
Experimental and Numerical Statistics of Storm Wave Forces on a Monopile in Uni- and Multidirectional Seas
Authors: Schløer Signe , Bredmose Henrik , Ghadirian Amin
Published: October 2017

Experiments with both uni- and multidirectional wave realizations with a stiff pile subjected to extreme wave forces are considered. Differences in crest heights and force peaks resulting from directional spread waves are analysed. The wave realizations are reproduced numerically in the fully nonlinear wave model OceanWave3D. The numerical reproductions compare well to the experiments. Only for the largest forces significant differences are seen, which is due to a very simple breaking filter applied in OceanWave3D. In the wave spectra, the higher harmonics occur for smaller frequencies than the straight multiples of the peak frequency. Further, the higher harmonics of the multidirectional wave spectra contain less energy. Both effects can be explained by the second order wave theory. Finally, the computed wave kinematics are used to investigate the dynamic response of an offshore wind turbine. The excitation of the first natural frequency is largest for the unidirectional wave realizations, as the higher harmonics are largest for these realizations.
Numerical Analysis of Submersible Mussel Raft for Exposed Marine Environment
Authors: Wang Xinxin , Zhao Fenfang , Tang Yanli , Huang Liuyi , Wan Rong , Cheng Hui
Published: October 2017

To study the hydrodynamic characteristics of the submersible mussel raft in waves and currents, the numerical model of the submersible raft was established based on the finite element method and kinematics theory. The finite element program Aqua-FE™ was applied to simulate the impacts of waves and currents on the hydrodynamic responses of the surface and submerged rafts, respectively. Morison Equation was applied to compute the tension of the mooring lines. Apart from the wave condition, the flow has a significant effect on the mooring line tension of the submersible raft. The submerged raft is useful for reducing the mooring loads. The submergence depth of the mussel raft can be adjusted depending on the marine environment. The results show that the submerged raft wave response was found to be reduced relative to the surface raft. The vertical motion of mussel rope connection points was significantly reduced by submergence, resulting in reduced potential for mussel drop-off. Compared the performance of the submerged raft in the same condition, the motion amplitude of the framework of the raft decreased significantly while increasing the submergence depth. At the same period, the trend of the decrease followed by levelling off with an increasing wave height. However, the submergence depth had no significant effect on the mooring line tension.
Comparison of Breaking Wave Kinematics From Numerical Simulations With PIV Measurements
Authors: Düz Bülent , Lindeboom Rene , Scharnke Jule , Helder Joop , Bandringa Henry
Published: October 2017

Breaking waves have been a popular research area among scientists and engineers since they present a strongly nonlinear and turbulent phenomenon. When these waves encounter an offshore or coastal structure, they exert significant amount of loads and stresses, which may result in a catastrophic consequence. Therefore, it is of utmost importance to study breaking waves and associated phenomena. Inspired by this need, in a recent MARIN experiment kinematics of breaking waves were measured with Particle Image Velocimetry (PIV). Among different types of breaking waves, spilling breakers were selected in this initial campaign. First, a summary of the measurement results will be given. These results will then be used for validation of a Computational Fluid Dynamics (CFD) tool. In numerical simulations two methods were followed in order to reproduce the focused wave: in the first method, the CFD tool was coupled to a nonlinear wave model, and in the second method an iterative scheme was used with the CFD tool. Results from these methods were then compared with the measurements.
Impact of Sea Ice Compression on Navigation Performance
Authors: Sagawa Genki
Published: October 2017

Ship navigation performance in the Arctic ice-covered sea was investigated from various kinds of satellite data and a numerical model of sea ice. The effect of dynamical processes of ice on the performance was especially examined, for it was not focused enough in previous studies. As a result, it was found that ice stress can explain some parts of the navigation when high amount of speed reduction occurred in thin ice area, and vice versa. The result indicates an importance of considering dynamical processes of ice in addition to static condition of ice, to improve an accuracy of an ice navigation performance estimation.
Extreme Wave Impacts on Monopiles: Re-Analysis of Experimental Data by a Coupled CFD Solver
Authors: Ghadirian Amin , Bredmose Henrik , Schløer Signe , Dixen Martin
Published: October 2017

Two different numerical models, OceanWave3D and a coupled solver, OceanWave3D-OpenFOAM (Waves2Foam), are used to reproduce extreme events in one sea state. The events are chosen as, the measured event that generates the largest peak moment (exceedance probability of 0.05%) and one event with a slightly smaller peak moment (exceedance probability of 0.3%). Time series of free surface elevation, depth integrated forces, bending moment at the sea bed and pressure time series at 5 different heights on the cylinder are compared for two events between the measurements and the numerical models. The numerical pressure field on the monopile at impact is analyzed and stagnation pressures at the back side of the cylinder, in addition to the main impact pressure at the front side are observed. There is a good agreement between the OceanWave3D results and the measurements in the reproduction of the first selected event. However, for the larger selected event, OceanWave3D results in the peaks of time series are smaller than the measurements. This illustrates the sensitivity of the strong impact loads to the state of wave breaking. For small values of the inline force, the OpeanFOAM results provided good agreement with the measurements. The secondary load cycles are observed in the measured force and bending moment time series and the reproduced times series using OpenFOAM.
Improvement on Mechanical Properties of Cu-Containing Low Alloy Steel of Long Part Forging for Offshore Applications by Manufacturing Process
Authors: Honma Yuta , Sasaki Gen , Hashi Kunihiko
Published: October 2017

Effects by intercritical quenching, which is quenching from dual-phase of ferrite (α) and austenite (γ) region from 953 to 1068 K, on mechanical properties and microstructures of Cu-containing low alloy steel based on ASTM A707 5L grade (hereafter called A707 modified steel) were investigated using 50 kg test ingots. The mechanical properties of the A707 modified steel, i.e. strength at room temperature and fracture toughness at low temperature, were significantly improved by intercritical quenching. This is probably because its effective grain size decreased by intercritical quenching. Then, the optimum temperature of intercritical quenching for A707 modified steel was 1068 K near the AC3 point.

Based on the experimental results of the test ingot, we applied intercritical quenching to a trial full-size forging production of about 20,000 mm in length, and researched the tensile, Charpy impact, crack tip opening displacement (CTOD) and drop weight test (DWT) properties across whole length of the trial production. It was found that the trial production has good mechanical properties across whole length. From the present work, an appropriate intercritical quenching is considered to apply for improvement method of the mechanical properties in A707 modified steel forgings.
Assessment of Impact Damage Caused by Dropped Objects on Glass Reinforced Plastic (GRP) Covers
Authors: Tauqeer Muhammad Ahmad , Ong Muk Chen
Published: October 2017

Glass reinforced plastic (GRP) subsea protection covers are used to protect offshore pipelines, umbilicals and structures from dropped objects released from offshore oil and gas and fishing activities. The aim of the present study is to perform assessment of impact damage caused by various dropped objects on GRP covers with different geometries. Impact energies carried by different selected offshore dropped objects are calculated by using the impact energy method given by NORSOK N-004 [1]. Computed results show a good agreement with the published dropped object impact energy results reported by DROPS [2]. The present study shows that the impact energies carried by dropped objects in oil and gas activities can range from 28 kJ for a small equipment to 2627 kJ for a typical subsea tree. Generally, the impact energies carried by the dropped objects in fishing activities range from 3 kJ to 14 kJ which is less significant than those in oil and gas activities. The impact absorption capacities of three different GRP cover geometries (i.e. square, triangular and semi-circular shapes) are computed based on finite element analysis (FEA). The results show that the triangular GRP cover geometry has the highest impact absorption capacity among the three investigated GRP cover geometries.
Underwater Positioning Using Near Surface Long Baseline Transponder’s Induced by Wave Motion
Authors: Sandøy Stian S. , Schjølberg Ingrid
Published: October 2017

This paper presents a filter for underwater positioning in an aquaculture environment with demanding weather conditions. The positioning system is based on acoustic transponders mounted at a net pen on the sea surface. The transponders are exposed to oscillations due to wave disturbance. This will have an impact on the accuracy of the positioning system. An extended Kalman filter (EKF) solution has been proposed including a wave motion model integrated with the pseudo-range measurements from the transponders. Simulations show that the proposed filter compensates well for the disturbances.
Variability of Extreme Riser Responses due to Wave Frequency Motions of a Weather-Vaning FPSO
Authors: Armstrong C. , Drobyshevski Y. , Chin C. , Penesis I.
Published: October 2017

The variability of extreme responses of a flexible riser due to wave frequency motions of weather-vaning FPSO is investigated numerically. The objective of this study is to examine such variability in isolation from that caused by the low frequency (slow drift) vessel motions and vessel offsets.

Investigation of the extreme value distributions of flexible risers provides the statistical foundation for flexible riser Response Based Analysis (RBA) for use in system design; the determination of the statistical properties of extreme flexible riser responses provides a method for the prediction of extreme responses of offshore systems in cyclonic conditions.

A case study conducted in OrcaFlex included an FPSO vessel with a Lazy-S configured riser system. Five riser responses were selected in critical locations including tension, heave, and curvature responses. The environmental cases included two cyclonic storms consisting of multiple half-hour intervals. For each interval, time domain simulations included 40 wave realizations in order to provide a dataset for robust fitting of the extreme value distributions in the Gumbel format.

Once the short term interval distributions were established, response distributions in a storm were generated by multiplying the short term distributions and the most probable maximum (MPM) response in a storm computed. A comparison of maximum interval, storm and 3-hour MPMs is presented, which indicates to what extent the MPM response in a storm exceeds the corresponding maximum interval response. Differences between the tension and heave responses are compared with those observed in the curvature responses.

This study was limited to riser excitation by waves, current and wave frequency motions of a turret moored FPSO and it is expected that further inclusion of low frequency motions would contribute to the response variability. The inclusion of such variability will ultimately enable the storm-based statistical approach to be used for the development of long-term distribution of the riser responses.
On Long Term Statistics of Ocean Storms Starting From Partitioned Sea States
Authors: Laface Valentina , Arena Felice , Maisondieu Christophe , Romolo Alessandra
Published: October 2017

The paper proposes an analysis of ocean storms carried out starting from significant wave height time series of HOMERE sea-states hindcast database based on WAVEWATCH III model. Considering that wave spectra often exhibit multiple peaks due to the coexistence of wind waves and swells, here sea states are described by partitioned sea states that can be interpreted physically as representing independent wave systems. The analysis presented here in the paper deals with the contribution of swells to the storm peaks and on how they influence the long term statistics. The sensitivity of return values of significant wave height to swell contribution is investigated via an application of the Equivalent Triangular Storm Model (ETS).

The ETS model provides analytical solution for the calculation of the return period R(Hs>h) of a sea storm whose maximum significant wave height exceeds a given threshold h. The approach of ETS consists in substituting each actual storm with an ETS described by two parameters: the storm intensity, that is the triangle height and it is equal to the maximum significant wave height during the actual storm; the storm duration, that is achieved imposing the equality between the maximum expected wave height of actual and equivalent storms. It has been experimentally proved that the actual storm and associated ETS are statistically equivalent because they have the same maximum significant wave height and the same probability P(Hmax>H) that the maximum wave height exceeds a given threshold H. The sequence of ETSs obtained in this way represents the equivalent sea, while the sequence of actual storms is the actual sea. The equivalent and actual seas present the same wave risk because they are characterized by the same number of storm events, each of them with the same intensity and the same P(Hmax>H).

For the proposed analysis a set of four points from open sea to the coast is considered in area of the Gulf of Biscay (France). The results show that the contribution of swells is more significant for the storms of small and medium intensity and decreases for increasing storm intensities. Further return values variability neglecting swell is less than 7% at any point for return periods up to 100 years.
A Fundamental Study on the Optimal Design of the Floating Tsunami Protection Wharf: About the Floating Body Length
Authors: Masuda Mitsuhiro , Minami Kiyokazu , Masuda Koichi
Published: October 2017

On the vessels moored at the wharf, the situation such as drifting and wash up on the wharf due to the breaking of the mooring lines is occurred by the tsunami. The authors are clarified for applicability of the proposed floating tsunami protection wharf (FTPW). FTPW is the floating pier for tsunami disaster. The effect of FTPW is most promising as disaster prevention and mitigation measures for the moored vessels. The authors examined large scale FTPW until now. However, when floating body length was longer than a ship breadth, the possibility that disaster prevention performance of FTPW decreased was confirmed. In this study, the examination about the influence that floating body length gives in disaster prevention performance of FTPW is performed using numerical simulation. Therefore, in the range of length of FTPW/breadth of vessel = 1.0 to 8.0, it was confirmed that tsunami protection performance of FTPW was shown enough.