bluuebunny/crossref_metadata_2025 huggingface dataset:
Preliminary Design of a Wind Driven Vessel Dedicated to Hydrogen Production
Authors: Gilloteaux Jean-Christophe , Babarit Aurélien
Published: October 2017

An innovative concept of harnessing wind energy is presented. The concept consists of a wind driven ship equipped with a hydro-generator that converts the kinetic energy of the water flow into electricity. The electricity is then converted into hydrogen by electrolysis. In the present study the use of a Flettner rotor is considered to propel the ship. A mathematical model of the hydrogen producing ship is developed based on existing data for high performance ship hulls and aerodynamic coefficients of existing Flettner rotors. The design is optimized with respect to the axial induction velocity through the water turbine disk. Results indicate that a 22m long vessel could produce 200 kW while a 80 m long vessel is able to generate 1 MW of mechanical power both for a true wind speed of 8 m/s.
On the Design Considerations of New Offloading Hose Applied on a Turret Moored FPSO
Authors: Yin Decao , Fylling Ivar , Lie Halvor , Baarholm Rolf J. , Kendon Timothy E.
Published: October 2017

Offloading hoses are used to transfer crude oil or liquid petroleum products from a fixed offshore production platform/floating production, storage and offloading (FPSO) unit to shuttle tankers. The hoses are subjected to environmental loads that are mainly waves, current, and vessel motions from both FPSO and the shuttle tanker.

New offloading hoses were planned to be applied in a FPSO in harsh environment, and a design analysis was done in this connection.

Numerical simulations were performed on ultimate limit state (ULS), serviceability limit state (SLS) and accidental limit state (ALS) by using the software RIFLEX [2]. Critical responses such as curvature and axial forces are checked.

The following conditions are checked:

1. Normal operation condition with oil filled hose

2. Connect operation condition, floating gas filled hose

3. Emergency disconnect condition

A SIMA [3] workflow was established to calculate accumulated fatigue damage of all the elements of the offloading hose model.

For the new offloading hose, it is important to have a combined bending-tension loading capacity check. A utilization factor is proposed that possibly may be generalized.

The results show that the specified hose has ample capacity for the considered operating conditions for the shuttle tanker to stay in any position within the 2nd emergency shut down sector (ESD2).
Effect of Weld Geometry on the Fatigue Behaviour of Umbilical Super Duplex Stainless Steel Tubes
Authors: Raji Hauwa , Fletcher Woods Jamie
Published: October 2017

The fatigue behavior of welded components is complicated by many factors intrinsic to the nature of welded joints. The mechanical properties of the material, the welding process and position, the type and geometry of the weld and the residual stress distribution across the weld are a few factors affecting fatigue behavior. Published studies [1, 2] have shown that weld geometry is significantly important in determining the fatigue strength of the weld. For a given weld geometry, the fatigue strength is determined by the severity of the stress concentration at the weld toe or at weld defects and by the soundness of the weld metal.

The effect of external weld geometry profile on the fatigue behavior of welded small bore super duplex umbilical steel tubes is investigated. Root cause analysis consisting of fractography, metallography and weld profile measurement is carried out on pairs of fatigue failure samples which were tested at the same stress range but failed at significantly different number of cycles. The samples are selected from Technip Umbilicals Ltd (TU) fatigue database. Following the failure analysis, weld geometric profile measurements are performed on fatigue test samples that were prepared for testing. The weld profile was measured in terms of the external weld cap height, weld width and external linear misalignment. Axial fatigue tests are carried out on these samples which are pre-strained before test to simulate the plastic bending cycles typically experienced during the manufacturing and installation processes prior to operational service. The fatigue tests results are interrogated together with the measured geometric data to identify trends and anomalies. Key weld geometric fatigue performance criteria are subsequently identified.

For the welded super duplex stainless steel (SDSS) tubes studied, the height of the weld and the weld toe angle provided the best correlation with fatigue life — shorter lives were obtained from specimens with the highest weld aspect ratio (weld height to width) and lowest weld toe angle.
One Step Installation of a TLP Substructure: Requirements, Assumptions, Issues
Authors: Hartmann Hauke , Walia Daniel , Adam Frank , Ritschel Uwe , Großmann Jochen
Published: October 2017

A crucial problem regarding the offshore electricity generation is the levelized costs of energy (LCOE). This is an even larger problem for floating substructures for offshore wind turbines. This paper highlights a substructure for floating offshore wind turbines (FOWT) for a one step installation process. It deals with the parametric study of the TLP’s structure to gain hydrostatic and hydrodynamic stability during the transport and installation process of the TLP equipped with a 6 MW wind turbine. At first a hydrostatic analysis with the software tool MOSES (V.7.06.062) has been performed. Hydrodynamic simulations with ANSYS AQWA (V.17.2), based on 2D potential flow theory, have been conducted afterwards to get information about the motion behavior of the TLP in wind, current and waves.
Two Phase Annular Flow Approximation Using 1-D Flow Equations Coupled With a Drift Flux Model for Concurrent Flow in Vertical or Near Vertical Channels
Authors: Gadgil Ashwin A. , Randall Robert E.
Published: October 2017

Annular flow is a flow regime of two-phase gas-liquid flow dominated by high gas flowrate moving through the center of the pipe (gas core). In this paper we have developed and studied an innovative phenomenological model which combines the Zuber & Findlay’s Drift Flux Model’s weighted mean value approach [1] with the 1-D flow approximation equations. The flow is described in terms of a distribution parameter and an averaged local velocity difference between the phases across the pipe cross-section. The average void fraction is calculated as a function of the ratio of weighted mean gas velocity to the weighted mean liquid velocity (Slip ratio) and the drift flux velocity. The void fraction thus estimated is then applied to the 1-D continuity, momentum and energy equations. The equations are solved simultaneously to obtain the pressure gradient. Lastly, we obtain the liquid film thickness using the triangular hydrodynamic relationship between the liquid flow rate, pressure gradient and the liquid film thickness. The thickness of layer obtained, is then used to verify the original estimate of the void fraction. An iterative procedure is used to match the original estimate to the final value. The results from this study were validated against PipeSIM© software and two field measurements conducted on a wet-gas field in Brazil. As opposed to conventional drift flux models which are based on four simultaneous equations, this model relies on three, thereby significantly reducing the computational resources necessary and is more accurate as we account for variable velocities and void fractions across the pipe cross-section.
Geotechnical Design of Vertically Loaded Hybrid Suction-Gravity Anchors
Authors: Hesar Majid , Maciel Raquel
Published: October 2017

The Sapinhoá and Lula North-East fields, 300km offshore Rio de Janeiro, Brazil are the two pilot fields of the Pre-salt development strategy. Field architecture consists of satellite wells connected to a spread moored FPSO in each of the fields. Through a design competition Petrobras and Partners selected the de-coupled riser system developed by Subsea 7. In each field two BSRs (Buoy Supporting Risers) anchored at 250m below waterline support the rigid risers on one side and flexible flowlines running to the FPSO on the other. Each BSR displaces close to 10,000t of water and provides a nominal net up thrust of 3,250t. This Paper highlights the engineering challenges and the solution developed for the large-scale foundation anchors that support these massive BSRs in the harsh environment of the Santos Basin in 2140m water depth.

Vertically below each corner of the BSR tank an 8m diameter by 18m penetration suction anchor houses the receptacles for the pair of tethers. Tension in the tethers was tuned to optimise the system stiffness (to minimise lateral BSR excursion orbits and avoid clash of risers and FPSO mooring lines) while requiring minimum anchor capacity. Four ballast modules of 150t each sit on top of each suction anchor to provide the remainder of the required uplift resistance. Soil conditions across both fields consist of soft silty clay. A geotechnical FE model of the suction anchor in Abaqus was used to evaluate the interaction of the structure, surrounding soil and trapped water beneath the top cap. The model was used to develop the complete load-displacement curves of the system during the undrained design current events.

A consolidation FE model using the Soft Soil model in PLAXIS showed that the hybrid anchor/ballast system under sustained uplift loading is stable throughout the design life for the level of sustained design load.

Although the LRFD verification format of suction anchors and gravity anchors are well covered in the design codes, there seems to be a gap in the coverage of this particular type of hybrid anchor. The governing condition was found to be the long term drained pull-out capacity under sustained loading with the assumption of slowly leaking suction port and air evacuation port at the top.
Environmental Description in the Design of Fish Farms at Exposed Locations
Authors: Kristiansen David , Aksnes Vegard , Su Biao , Lader Pål , Bjelland Hans V.
Published: October 2017

This paper addresses the description of exposure from waves and currents in coastal regions for design of marine fish farms. Representative descriptions of environmental conditions are important inputs to the design and dimensioning of reliable fish farm structures. A trend with moving production to more exposed sites and introduction of new and novel fish farm structures increase the need for more precise descriptions of the marine environment to keep control of uncertainties in design. Dedicated field measurements at two exposed aquaculture sites from February to December 2016 are presented. Results from statistical analyses of the measurement data demonstrate that common practice for characterization of exposure in design of fish farms has several deficiencies that should be improved to reduce uncertainties in design.
A Risk Assessment of a Novel Bulk Cargo Ship-to-Ship Transfer Operation Using the Functional Resonance Analysis Method
Authors: Clarke Lauchlan J. , Macfarlane Gregor J. , Penesis Irene , Duffy Jonathan T. , Matsubara Shinsuke , Ballantyne Ross J.
Published: October 2017

Risk assessments underpin a maritime operation’s safety management system. When applied to an untested concept a risk assessment can also assist with overcoming resistance to new technology. This paper proposes the functional resonance analysis method (FRAM) as a tool for developing design recommendations and fulfilling the safety management objectives of the ISM Code. The FRAM is applied to benefit the floating harbour transhipper (FHT), a novel concept for the transhipment of bulk commodities. The FHT acts as a large floating warehouse with an aft well dock that provides shelter for a feeder vessel. The FHT’s materials handling equipment transfers bulk cargo from the feeder vessel onto its own stockpile or directly to an export vessel moored alongside, or from its stockpile to the export vessel.

Most risk assessment tools focus on identifying and addressing system components that can potentially fail. With the FRAM however, the scope, direction and recommendations are guided by a practical understanding of the variability of work undertaken rather than preconceived notions of potential failure modes. Adopting a method based on maximising resilience rather than minimising the causes of accidents promotes a shift from a blame culture to a safety culture. Applying the FRAM generated a deeper, broader and more transparent understanding of the FHT transfer operation than what would have been achievable using traditional risk assessment tools. This understanding was used to develop recommendations designed to improve the resilience of the FHT operation.
Wave Attenuation due to Ice Cover: An Experimental Model in a Wave-Ice Flume
Authors: Dolatshah A. , Nelli F. , Alberello A. , Bruneau L. , Bennetts L. G. , Meylan M. H. , Monty J. P. , Toffoli A.
Published: October 2017

Waves penetrate deep into the ice covered seas, inducing breakup of the ice cover. Concomitantly, the ice cover attenuates the wave energy over distance, so that wave impacts die out eventually. Observations of wave attenuation and concurrent wave-induced breakup in the literature are serendipitous due to difficulties in making measurements in ice covered seas. Hence understanding of wave-ice interactions remain uncertain. Here we present measurements of wave propagation through ice covered waters in the new experimental wave-ice facility at the University of Melbourne. The facility comprises of a 14m long and 0.76m wide flume in a refrigerated chamber, where temperatures can be lowered down to −12 degrees Celsius to generate a continuous ice cover on the water surface. A wave maker, installed at one end, is used to generate regular waves, ranging from gently-sloping to storm-like conditions. Wave attenuation rates are determined from video-camera images of the displacements of markers embedded in the ice cover. The experiments investigated wave propagation through the continuous ice cover, breakup, and propagation through the broken ice cover. Spatial evolution of the breakup and geometrical properties of floes are monitored and correlated with incident wave properties. Wave attenuation over broken ice is investigated and compared against the continuous ice case.
Fracture Mechanics Based Fatigue Assessment for a Spar-Type Floating Wind Turbine
Authors: Wu Junyi , Chen Nian-Zhong
Published: October 2017

Floating wind turbines (FWTs) are exposed to dynamic and cyclic environmental loads during their service life. Fatigue assessment has become an important aspect in the design phase of a FWT. A fracture mechanics (FM) based fatigue assessment was performed for the 12 points around the tower base of a 5 MW floating wind turbine supported on a spar platform. The aligned wind and waves are selected as environmental conditions for the fatigue assessment. The stress ranges on the wind turbine tower base are achieved through a rainflow counting method based on the results from the time-domain analysis using the FAST software. A comparison between fatigue lives predicted by the FM and S-N curves based approaches is made. The impact of the variation of initial crack depth, critical crack depth and stress concentration factors (SCFs) on the ratio of the fatigue life predicted by two approaches is investigated. The study shows that the fatigue life predicted by the FM based approach is more conservative than that predicted by the S-N curves based approach and also the fatigue life is highly sensitive to the material constant of Paris Law C and SCFs.
A Probability Analysis of Atomization Rate for Fully Developed Annular Flow in Vertical Pipes
Authors: Zhang Ri , Dong Sheng
Published: October 2017

The phase distributions and mechanical properties of annular flow are constantly fluctuating, so they can be regarded as random states. The probability analysis of annular flow is an appropriate method to research the formation, development and evolution of the flow pattern. In the present work, the atomization and deposition rates of fully developed annular flow are investigated in detail by the method of a probability analysis. First, the basic equations of the probability model are applied to solve some important intermediate parameters of annular flow. Second, the atomization and deposition rates of any size droplets are closely related to the probabilities of droplet generation and disappearance. Third, the interchange rate of the whole liquid phase can be obtained by summing the generation and disappearance probabilities of arbitrary size droplets. The predictions of atomization rate are well verified by comparing with the experimental date of 71 cases from three sets of tests. It is demonstrated that the probability model can accurately calculate the atomization rate of the fully developed annular flow for most cases. The predicted deviation for some cases may be caused by the neglect of droplet breakup process. Furthermore, the effects on the atomization rate of seven parameters of annular flow are discussed in detail.
Simulation of Ice Force and Breaking Pattern for Icebreaking Ship in Level Ice
Authors: Sawamura Junji , Yamauchi Yutaka , Anzai Keisuke
Published: October 2017

A 2D numerical model was proposed to predict the repetitive icebreaking pattern and ice force of an advancing ship in level ice are presented. The numerical model focuses on the icebreaking at the waterline and neglects the broken ice rotating and sliding underwater hull. The repeated ship-ice contact and bending failure of a floating ice along the waterline are evaluated numerically. The computed ice channel width and icebreaking resistance are compared with measured values in the model test. Numerical results show moderately good agreement with the model test data. The effects of ice thickness and ship speed on the icebreaking resistance are investigated numerically. The icebreaking resistance depends on both the ice thickness and ship speed. The ice channel, however, depends on ice thickness, but there is little difference in ship speed.
The Influence of an Arrangement of an Array of Semi-Submersible Type FOWTs to Their Hydrodynamic Responses
Authors: Murai Motohiko , Takahashi Kensaku
Published: October 2017

The progress of the global warming is one of our key issue should be overcome in the 21st century. We should balance the energy consumption and reduction of exhausting greenhouse effect gas. The wind energy is expected as one of the key technology for utilizing renewable energy. It is said that as for the efficiency of the electric generation, it is better the wider wind farm with larger wind mill. But, as the land space for the extensive wind farm is quite limited, it will be a candidate how to use the vast ocean space for human activity. So, the construction of an offshore wind farm is one of the solution now. To extend the wind farm widely in the ocean the floating wind turbine system becomes an essential technology. Under such background, there are many researches on the floating wind turbine in waves and in wind. And hydrodynamic response of them in waves is one of the major topics. Many of the researches on the hydrodynamic response, however, deal with the motion of a single floating wind turbine. This report deals with the hydrodynamic response of a wind farm which is consisted of semi-submersible wind turbines. In addition, the investigation of how does the arrangement of the array impact on the hydrodynamic response including the motion of the nacelle is also reported. In the last the example of the change of the expected efficiency of the generator by change of the array arrangement in each sea area around Japan are shown.
Research on Optimum Function and Utilization of Oceanic Architectures for Marine Space Use: Case Study on Kyushu Region, Japan
Authors: Sugahara Ryo , Kuroyanagi Akio
Published: October 2017

Recently, the demanding for effective utilization of marine spaces in forms of oceanic architectures has been rising high by new social requirements as well as other constructions, and the like. Consequently, architecutural characteristics and structural effectiveness of them should be reevaluated accordingly.

Objects of this study are four architectures located in the shallow area of sea around Kyushu Region,namely Marizon, Genkai Sea Hot Spring Palea, Marine Aquarium Sea Doughnut, and Akasaki Elementary School.

Their functions, structures, setting forms, then conditions for situating buidings, processes of construction at sea, and challenges after construction had been clarified by our literature survey, the hearing investigation and the field research.

Our results are as following. Functionally, each above architectures has respective facilities to fully utlize regional marine resources and environment, etc.; Structurally, all of them had premeditated both the bad conditions of coastal environment and providing spaces for water activities. In terms of their construction processes at sea, factors blow had been carefully considered before being sited: limitations of local spatial scale, full utilization of local environmental conditions, measures against disadvantages of sites, the particularities of regional nature, environment, and society.

In conclusion, our findings suggest that planning of the oceanic architecuture should combine the usage, the fuction, and the understructures well with its marine conditions.
Models and Methods for Efficiency Estimation of a Marine Electric Power Grid
Authors: Bø Torstein I. , Pedersen Eilif
Published: October 2017

Diesel electric propulsion has become industry standard for many marine applications. Typically, a significant part of the operations of vessels with diesel electric propulsion is done with low loads on the motors and generators. However, the efficiency of a drive train is typically only calculated for full load conditions. This underestimates the losses during low load conditions. This article presents modeling methods for the electric drive train, which can be used to estimate the efficiencies, also at low load. The models are established with limited parameter sets, as detailed information about of the components are seldom available. This article compares the estimated efficiency of the generator and motor with the given data from datasheets.
Numerical and Experimental Damping of Piston and Sloshing Motions in Moonpools
Authors: Löhrmann Jan , Carrica Pablo M. , Cura-Hochbaum Andrés
Published: October 2017

Damping of waves in moonpools has been investigated. Several different geometries of the moonpool have been considered, which are being mainly characterised by a piston motion of the free surface in the moonpools, while some sloshing effects are visible. A numerical damping approach has been implemented into a viscous solver to simulate the effect of perforated bulkheads without the necessity to create a complex grid. The simulations have been compared with respective model tests. Two different porosities of the bulkhead have been experimentally investigated. The simulations have been conducted for a variety of Froude numbers, moonpool layouts and in calm water as well as waves. A dependency of the surface elevation natural frequency on the ship speed has been found. Furthermore, the numerically determined surface elevation inside the moonpool has been compared with respective experimental data. Based on the surface elevation, the resulting damping for the different layouts is determined and discussed.
A Particle Filter SLAM Approach to Online Iceberg Drift Estimation From an AUV
Authors: Norgren Petter , Skjetne Roger
Published: October 2017

Using autonomous underwater vehicles (AUVs) for mapping the underwater topography of sea-ice and icebergs, or detecting keels of ice ridges, is foreseen as an enabling technology in future Arctic offshore operations. This paper presents a method for online iceberg drift estimation using a Simultaneous Localization and Mapping (SLAM) approach using an AUV with a multi-beam echosounder (MBE) during such survey/monitoring operations. Iceberg drift is affected by wind, current, and Coriolis forces. This can be hard to predict, making automated mapping of icebergs difficult. The method proposed in this paper estimates the iceberg’s pose using a particle filter, where each particle uses extended information filters to estimate the topography of the iceberg. A grid map is used to store the iceberg topography, and distributed particle mapping is used to avoid expensive copy operations during particle resampling. The proposed method is verified through a simulation study, using a 6 DOF AUV model, an MBE sensor model, and an iceberg topography taken from the PERD iceberg sightings database. The method is able to provide a georeferenced iceberg position, thus, estimating the iceberg’s drift trajectory. A topography estimate of the iceberg, corrected for iceberg drift, is also generated. Furthermore, the algorithm estimates the iceberg drift velocity, as well as the relative iceberg-AUV pose, for use in future iceberg mapping guidance algorithms. The simulation study illustrates the performance of the method, and a short execution time analysis is presented to illustrate the method’s real-time potential.
Numerical Simulations of Turbulent Flow Over Two Surface-Mounted Tandem Square Cylinders
Authors: Dai Yukun , Wang Hong , Tian Cai
Published: October 2017

Numerical simulations of wake flow generated by a surface-mounted square cylinder over a subsequent square cylinder are presented for incompressible low-Re turbulent flow. The behaviors of the square cylinder in the wake flow are investigated. A developed numerical method — Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) is adopted to solve the governing equations in steady state. Turbulence effect is modelled by Reynolds Averaged Navier Stokes (RANS) equations with a standard k-ω turbulence model. The k-ω SIMPLE method is validated by comparison with experimental data. The results show that in wake flow, the square cylinder produces a much weaker wake effect and is subjected to an inverse drag force.
Strength Assessment on Support System of LNG Independent Type B Tank Under Sloshing Loads
Authors: Dong Wen , Zhang Zhengyi , Liu Jingxi , Xie De
Published: October 2017

A Liquefied Natural Gas (LNG) tank usually contains cryogenic cargo with free liquid surface and requires high level of safety. In this study, a two-step procedure has been developed for strength evaluation of support structures in LNG carriers with independent type B tank, which is partly loaded and under sloshing impact. First, an integral model containing a rigid tank and connectors is used to investigate the displacement and force response of supports in time domain. Then, several local models are used for the fine mesh stress analyses of support structures. The nodal displacements of connectors are extracted from integral model and applied to the local model as boundary conditions. The finite element method has been implemented in both steps. The arbitrary Lagrangian-Eulerian technique is applied for kinematical descriptions of the fluid domain in the direct sloshing analysis of type B tank in first step. The present numerical results are compared with previous experimental values, results of other numerical method and analytical solutions, which validates the reliability and accuracy of this method. It is noted that the proposed procedure can be utilized for strength analysis of other types of LNG tanks and helps to optimize their structural design straightforwardly.
An Optimization Method for the Configuration of Inter Array Cables for Floating Offshore Wind Farm
Authors: Poirette Yann , Guiton Martin , Huwart Guillaume , Sinoquet Delphine , Leroy Jean Marc
Published: October 2017

IFP Energies nouvelles (IFPEN) is involved for many years in various projects for the development of floating offshore wind turbines. The commercial deployment of such technologies is planned for 2020.

The present paper proposes a methodology for the numerical optimization of the inter array cable configuration. To illustrate the potential of such an optimization, results are presented for a case study with a specific floating foundation concept [1].

The optimization study performed aims to define the least expensive configuration satisfying mechanical constraints under extreme environmental conditions. The parameters to be optimized are the total length, the armoring, the stiffener geometry and the buoyancy modules. The insulated electrical conductors and overall sheath are not concerned by this optimization. The simulations are carried out using DeepLines™, a Finite Element software dedicated to simulate offshore floating structures in their marine environment. The optimization problem is solved using an IFPEN in-house tool, which integrates a state of the art derivative-free trust region optimization method extended to nonlinear constrained problems. The latter functionality is essential for this type of optimization problem where nonlinear constraints are introduced such as maximum tension, no compression, maximum curvature and elongation, and the aero-hydrodynamic simulation solver does not provide any gradient information.

The optimization tool is able to find various local feasible extrema thanks to a multi-start approach, which leads to several solutions of the cable configuration. The sensitivity to the choice of the initial point is demonstrated, illustrating the complexity of the feasible domain and the resulting difficulty in finding the global optimum configuration.
Study on the Mechanism of Bird-Cage Buckling of Armor Wires Based on Experiment
Authors: Lu Qingzhen , Yang Zhixun , Yang Yucheng , Yan Jun , Yue Qianjin
Published: October 2017

Armor wires which are typical spiral wound structures are widely applied to flexible structures in ocean engineering, such as flexible pipes and umbilical cables. Birdcage buckling phenomenon would appear under the condition of damage of the cover sheath and strength tapes when armor wires are subjected to high axial compression which exceeding its critical load. In this paper a special experiment device is designed to simulate the armor wires winding a cylinder. Based on the experiment, the birdcage buckling mechanism of spiral wound steel wire is analyzed. The global configuration time-history of spirally wound steel wire is recorded to find the critical buckling load with the increasing compression displacement. At the same time, strain state at the critical position is analyzed, which could provide a reference for curved beam’s buckling research. Moreover, sensitivity analysis of critical buckling load is investigated with consideration of some influencing factors such as spiral wound radius, length and pitch. Finally, the relationships between them and critical buckling compression load are quantified. The experimental results could guide the anti-buckling design of armor wires for flexible pipe or umbilical cables.
A Novel Approach to the Development of Squall Database for Mooring Response Based Analysis
Authors: Mariani Alessio , Elliott Grant , Wake Geoff , Whelan James , Jeans Gus
Published: October 2017

This paper describes a novel approach to the characterisation of the winds associated with squall events. Squalls are short duration events during which wind speed increases rapidly and is often characterised by a substantial change in direction. Squalls are an important input for the design of offshore structures, particularly weather vaning facilities such as turret moored FPSO’s for which squalls may induce substantial mooring loads.

This paper presents the development of a synthetic squall database covering 63 years and combining direct high resolution wind measurements together with weather balloon observations of the lower atmosphere and historical daily maximum wind gusts. The squall database was a key input into a mooring Response Based Analysis (RBA) of a turret moored FPSO located offshore North Western Australia.

Details of the synthetic squall database development are presented including: regional squall regime characterization, direct measurements, identification of peak squall events from long term measured datasets, characterisation based on weather balloon observations, generation of representative synthetic time histories of squall events.

The squall database, coupled with a conventional hindcast metocean database including associated wave and currents parameters, allowed characterization of the weather vaning performance of the FPSO and identification of design loads for mooring design purposes.
Large Eddy Simulation of the Wake Behind an Ellipsoid at 45° Incidence Angle
Authors: Yu Zongxin , Liu Hao , Wang Xianzhou , Zhang Zhiguo , Zhang Fanchen
Published: October 2017

For the well-defined bodies of revolution, the viscous flow past Prolate spheroids has fascinated scientists in fluid mechanics, marine hydrodynamics and aero-dynamical communities for decades. Previous experiment in different yaw and pitch angles of a prolate-spheroid like submarine mode suggests that the asymmetry is a feature of the high Reynolds wake of the symmetric body of revolution with incline angle. The objective of this paper is to examine the similar phenomenon — the asymmetric wake flow — in the relatively low Reynolds number. The present paper focuses on flow field with LES model for a 45° inclined 6:1 prolate spheroid. LESs of the flow and wake have been conducted at Reynolds numbers ReD = 3000, and 10000 (ReD based on the free-stream velocity U0 and the minor axes length D). The maximum grid points reaches 30 million. Results from simulation show the dominant structure of the wake to be a pair of counter-rotating vortices. Detailed observations and analyses show strong leeward side axial flow. Simulation results from LES were compared with the DNS results for ReD = 3000. The wake at ReD = 3000 and 10000 shown was complex with numerous disordered vortical structures. Strong asymmetric wake and large side-force were surprisingly observed and the asymmetry in near field and far field were analyzed and compared.
Predicting the Changing Soil Response for Vertical Pipe-Seabed Interaction Accounting for Remoulding, Reconsolidation and Maintained Load
Authors: Zhou Zefeng , White David J. , O’Loughlin Conleth D.
Published: October 2017

Steel catenary risers (SCRs) are subjected to fatigue in the touchdown zone (TDZ) where the pipe interacts with the seabed. In this zone the seabed is subjected to intermittent episodes of cyclic loading and reconsolidation during long-term operation. Cyclic loading, reconsolidation and maintained load can cause variations in the soil strength and stiffness, which has a significant influence on the fatigue life of the riser in the TDZ.

The weakening effect of cyclic loading on soil strength is well recognized throughout design practice, and methodologies for determining the cyclic ‘fatigue’ of clay during undrained cyclic loading are well established (e.g. Andersen et al. 1988; Andersen 2015). However, traditional undrained assessments neglect the effects of drainage and consolidation that inevitably occur in pipe-seabed interaction during long-term operational stages, and can lead to changes in stiffness by a factor of up to 5 or 10. This overlooked effect of consolidation on soil resistance and stiffness can be very important for SCR fatigue analysis.

In this paper, a new analytical framework considering these effects has been used to analyze vertical pipe-seabed interaction. This framework is developed using a critical-state concept with effective stresses, and by discretizing the soil domain as a one-dimensional column of soil elements. The model can accurately capture the changing soil resistance and stiffness to account for the effects of remoulding, reconsolidation and maintained load. The framework is used to back-analysis the pipe-soil interaction response during small and large amplitude vertical cycles. The simulation prediction compares well with the measured results from the laboratory (Aubeny et al., 2008), and can accurately capture the observed changes in stiffness of up to a factor of 5.
A Hydrodynamic Analysis of Motion Coupling Effect of Floating Storage Tank Supported by Marine Fenders
Authors: Han Mengmeng , Magee Allan Ross , Wan Ling , Jin Jingzhe , Wang Chien Ming
Published: October 2017

This study concerns a new concept of floating oil storage facility, to be deployed in coastal waters, in which separate oil storage tanks float in an array, separated by a mooring fender system. In this paper, hydrodynamic properties of a single module are investigated numerically. The effects of different mooring fender parameters including fender stiffness and fender position on the coupled motions are studied. Design criteria and a design approach for the marine fender selection are proposed. Next, time-domain simulations under random waves are performed. Finite water depth effects are taken into consideration. Then a brief parametric study on sloshing phenomenon in fender-supported tanks is conducted. Results show that a carefully designed marine fender will help reduce the roll and pitch motions of the storage tank, and thus function as a stabilizer. This analysis is the basis of a global hydrodynamic response analysis for multiple tanks, including the effects of multibody hydrodynamic interactions between tanks in the future.
URANS Prediction of Berthed Ship–Passing Ship Interactions
Authors: Kok Zhen , Jin Yuting , Chai Shuhong , Denehy Shaun , Duffy Jonathan
Published: October 2017

In this paper, the unsteady Reynolds-Averaged Navier-Stokes computational method has been employed for investigating the hydrodynamic interactions between berthed and passing ships. Initially, simulations in model-scale were performed for validating the numerical modelling technique using available benchmark experimental test cases. A formal study of verification and validation was carried out for quantifying the numerical uncertainties. Based on the validated numerical setup, systematic computations were conducted for further investigations on the influence of varying passing ship speeds and lateral separations on the interaction forces and moments. The same conditions were repeated in full scale to quantify possible scale effects. The numerical results demonstrate that the interaction forces and moments are proportional to the square of the passing ship speed and inversely proportional to the lateral separation between the two ships, which agrees well with the findings by Remery (1974) and Kriebel (1995) respectively. In addition, when comparing model and full scale results, the overall differences are not very significant and are within the simulation uncertainty for most cases.
Wind Tunnel 2-DoF Hybrid/HIL Tests on the OC5 Floating Offshore Wind Turbine
Authors: Bayati Ilmas , Belloli Marco , Facchinetti Alan
Published: October 2017

This paper presents the numerical and experimental implementation of a 2 degrees-of-freedom (DoF) setup for simulating the surge and pitch motion of OC5 semi submersible floating offshore wind turbine, through the “hardware-in-the-loop” (HIL) approach during wind tunnel tests. This approach is hybrid since a real-time combination of computations and measurements are carried out during the experiments. This allows to separate the model tests of floating wind turbines into wave/ocean basin and wind tunnel tests, as it is currently done within the H2020/LIFES50+ project respectively at Marintek (Norway) and Politecnico di Milano (Italy), with the possibility of exploiting the advantages of each facility and overcoming the scaling issues and conflicts (e.g. Froude-Reynolds) that are emphasized when it comes to testing both wind and wave in a single test facility. In this paper the modelling approach and experimental implementation are presented, with a special focus on signals and data handling in the real-time HIL control system aimed at minimizing the effect of model/full scale discrepancies. Results are shown for free decays, regular and irregular sea states, showing promising results for the next 6-DoF system being finalized.
Prediction of Combined IL and CF VIV Response of Deepwater Risers
Authors: Wu Jie , Lekkala Malakonda Reddy , Ong Muk Chen , Passano Elizabeth , Voie Per Erlend
Published: October 2017

Deepwater risers are susceptible to Vortex Induced Vibrations (VIV) when subjected to currents. When responding at high modes, fatigue damage the in in-line (IL) direction may become equally important as the cross-flow (CF) components. If a riser experiences directional currents, fatigue damage must be evaluated at several locations on the cross-section’s circumference. Accurate calculation of both IL and CF responses are therefore needed.

Empirical VIV prediction programs, such as VIVANA, SHEAR7 and VIVA, are the most common tools used by the offshore industry to design against VIV loads. Progress has been seen in the prediction of CF responses. Efforts have also been made to include an IL load model in VIVANA. A set of excitation coefficient parameters were obtained from rigid cylinder test and adjusted using measured responses of one of the flexible cylinder VIV tests. This set of excitation coefficient parameters is still considered preliminary and further validation is required. Without an accurate IL response prediction, a conservative approach in VIV analysis has to be followed, i.e. all current profiles have to assumed to be uni-directional or acting in the same direction.

The purpose of the present paper is to provide a reliable combined IL and CF load model for the empirical VIV prediction programs. VIV prediction using the existing combined IL and CF load model in VIVANA is validated against selected flexible cylinder test data. A case study of a deepwater top tension riser (TTR) has been carried out. The results indicate VIV fatigue damage 1 using 2D directional current profiles is less conservative compared to the traditional way of using unidirectional current profiles.
Cyber Security Issues in Navigation Systems of Marine Vessels From a Control Perspective
Authors: Hassani Vahid , Crasta Naveena , Pascoal António M.
Published: October 2017

Autonomous marine vessels are the way forward to revolutionize maritime operations. However, the safety and success of autonomous missions depend critically on the availability of a reliable positioning system and time information generated using global positioning system (GPS) data. GPS data are further used for guidance, navigation, and control (GNC) of vehicles. At a mission planning level GPS data are commonly assumed to be reliable. From this perspective, this article aims to highlight the perils of maritime navigation attacks, showing the need for the enhancement of standards and security measures to intercept any serious threats to marine vessels emanating from cyber attacks and GPS spoofing. To this end, we consider a case where a cyber attacker blocks the real GPS signals and dupes the GPS antennas on board the marine vehicle with fake signals. Using the Nomoto model for the steering dynamics of a marine vessel and exploiting tools from linear control theory we show analytically, and verify using numerical simulations, that it is possible to influence the state variables of the marine vessel by manipulating the compromised GPS data.
Learning From Riser Analyses and Predicting Results With Artificial Neural Networks
Authors: Authén Kristian
Published: October 2017

Subsea wellheads are subjected to fatigue loading from semi-submersible drilling vessels in harsh and relatively shallow waters like in the North Sea. Dynamic finite element riser simulations are run to ensure safe operations. These simulations calculate loads and fatigue damage on the subsea wellheads, and are run in large numbers, often with only small alterations in input. Building and running all these models for each new well are both time consuming and costly.

By storing and structuring the results from such analyses, machine learning algorithms can be trained, and used to predict new results, without the need of running simulations for every new well. If there are insufficient simulation data available, data with inconsistent modeling, or little variety in the input of the data, it is also possible to use model builders to generate a sparse but sufficient set of simulation data to train the model.

A trained model can predict simulation results instantly, and gains accuracy as more analysis data becomes available for training.

The trained model is efficient for estimating fatigue status on oil fields with large numbers of subsea wells, since it is unnecessary with separate simulations for each well. It can also be used for early phase concept studies and as a QA tool for verifying results of other simulations.
Characteristics of the Wave Slamming Forces on Jacket Structures Under Plunging Breaking Waves Based on Experimental Data
Authors: Jose Jithin , Podrażka Olga , Gudmestad Ove Tobias , Cieślikiewicz Witold
Published: October 2017

Due to increased energy demand and thrive for clean energy, offshore wind energy has become popular these days. A large number of offshore wind turbines supported by fixed type substructures have been installed, among which jacket structures are getting popular in recent times. The forces from breaking waves are a major concern in the design of offshore structures installed in shallow waters. However, there are only limited studies available regarding breaking wave forces on jacket structures and still there exist many uncertainties in this area. During the WaveSlam experiment carried out in 2013, a jacket structure of 1:8 scale was tested on a large number of breaking wave conditions. Wave properties and the forces on the structure were measured during the experiment. The total wave slamming forces are being filtered from the experimental measured force using the Empirical Mode Decomposition method and local slamming forces are obtained by the Frequency Response Function method. Based on these results, the peak slamming force and slamming coefficients on the jacket members are estimated. The wave parameters (wave height and period) and wave front asymmetry are obtained from measured wave properties. The variation of slamming forces and slamming coefficients with respect to these parameters are also investigated.
Projected Changes in the Occurrence of Extreme and Rogue Waves in Future Climate in the North Atlantic
Authors: Gramstad Odin , Bitner-Gregersen Elzbieta , Vanem Erik
Published: October 2017

We investigate the future wave climate in the North Atlantic with respect to extreme events as well as on wave parameters that have previously not been considered in much details in the perspective of wave climate change, such as those associated with occurrence of rogue waves. A number of future wave projections is obtained by running the third generation wave model WAM with wind input derived from several global circulation models. In each case the wave model has been run for the 30-year historical period 1971–2000 and the future period 2071–2100 assuming the two different future climate scenarios RCP 4.5 and RCP 8.5. The wave model runs have been carried out by the Norwegian Meteorological Institute in Bergen, and the climate model result are taken from The Coupled Model Intercomparison Project phase 5 - CMIP5.

In addition to the standard wave parameters such as significant wave height and peak period the wave model runs provided the full two-dimensional wave spectrum. This has enabled the study of a larger set of wave parameters. The focus of the present study is the projected future changes in occurrence of extreme sea states and extreme and rogue waves. The investigations are limited to parameters related to this in a few selected locations in the North Atlantic.

Our results show that there are large uncertainties in many of the parameters considered in this study, and in many cases the different climate models and different model scenarios provide contradicting results with respect to the predicted change from past to future climate. There are, however, some situations for which a clearer tendency is observed.