bluuebunny/crossref_metadata_2025_split huggingface dataset:
Numerical Simulation of an Oscillating Wave Surge Converter
Authors: Wei Yanji , Rafiee Ashkan , Elsaesser Bjoern , Dias Frederic 
Published: November 2013

It has been known that bottom hinged Oscillating Wave Surge Converters (OWSCs) are an efficient way of extracting power from ocean waves (Whittaker et al. 2007). OWSCs are in general large buoyant flaps, hinged at the bottom of the ocean and oscillating back and forth under the action of incoming incident waves (Schmitt et al. 2012, Renzi and Dias 2012). The oscillating motion is then converted into energy by pumping high-pressure water to drive a hydro–electric turbine.
This paper deals with numerical studies of wave loading on an OWSC using the FLUENT software. In numerical simulation of wave loading on an OWSC using mesh-based methods, the mesh around the flap is required to be updated frequently. This is due to the large amplitude rotation of the OWSC around the hinge. In this work, the remeshing was achieved by using the so-called dynamic mesh approach built in FLUENT. Furthermore, the motion of the OWSC is updated in time using a fourth order multi point time integration scheme coupled with the flow solver. The results for the flap motion and the excited torque on the hinged position were compared with experimental data obtained in a wave tank at Queen’s University of Belfast. The results showed the capability of the numerical model with a dynamic mesh approach in modeling large amplitude motions of the flap. In addition, the pressures at various locations on the flap were compared with the experimental measurements in order to demonstrate the accuracy of the proposed model in capturing local features of the flow as well as the global features.
Assessment of Significant Wave Height – Peak Period Distribution Considering the Wave Steepness Limit
Authors: Drago Michele , Giovanetti Giancarlo , Pizzigalli Claudia 
Published: November 2013

The physical limit of the significant wave steepness is generally exceeded when assessing the seastate climate and the extreme iso-probability contours, i.e. too short significant wave peak periods Tp are sometimes associated to a certain significant wave height Hs.
The occurrence of not physically consistent Tp is clearly due to a fault in the generally made assumption of a log-normal distribution of the Tp, where the physical limit for the period would be Tp > 0, i.e. the existence limit of the log-normal distribution, which is well below the real physical limit for significant wave steepness.
If this is not a problem for pipeline design, where stability and fatigue are dominated by longer peak periods associated at each significant wave height, loads overestimation could arise for near surface structures, e.g. riser, where the largest loads and fatigue, are caused by the shorter peak periods associated to a certain significant wave height. Hence, the possibility to define a Tp distribution which respects the physical lower bound of the limiting wave steepness has a significant relevance when dealing with design and installation of near surface structures.
The present paper proposes a new methodology for the assessment of the Hs-Tp distribution which respects an a-priori defined wave steepness limit.
This can be done basing on the definition of significant wave steepness Sp = 2πHs/gTp2 which, assessing a limiting steepness Sp, provide an physical lower bound Tplim for the peak period Defining a new variable Tp′ = Tp – Tplim and imposing that Tp′ follows a log-normal distribution, hence having a physical limit Tp′ > 0, is equivalent to assess a Tp distribution which respects the defined significant wave steepness limit Tp > Tplim.
A test case compares results obtained with the ‘old’ and ‘new’ methodologies and shows the implication on the design loads. Moreover, another test case has been investigated to verify the performance and characteristics of the new methodology.
Evaluation of Directional Analysis Methods for Low-Frequency Waves to Predict LNGC Motion Response in Nearshore Areas
Authors: van Essen Sanne , van der Hout Arne , Huijsmans René , Waals Olaf 
Published: November 2013

Because LNG terminals are located increasingly close to shore, the importance of shallow-water effects associated with low-frequency (LF) waves increases as well. The LF wave spectrum in these areas is generally complex, with multiple frequency peaks and/or directional peaks due to LF wave interaction with the shore. Both free and bound LF waves at the same frequency can be present. Since LF waves are potentially very significant for moored vessel motions, it is important to include their effect in an early stage of the terminal design. This requires an efficient and relatively simple tool able to estimate the LF wave spectrum in nearshore areas. The benefit of such a procedure with respect to state-of-the-art response methods is the ability to include the LF free wave distribution in a local wave field in the vessel response calculation.
The objectives of the present study are to identify such a tool, and to evaluate the use of its output as input for a vessel motion calculation. Three methods, designed for the determination of wave spectra of free wave-frequency (WF) waves, were applied to artificial LF wave fields for comparison of their performance. Two stochastic methods, EMEP (Hashimoto et al., 1994) and BDM (Hashimoto et al., 1987) and one deterministic method, r-DPRA (De Jong and Borsboom, 2012) were selected for this comparison. The foreseen application is beyond the formal capabilities for which these three methods were intended. However, in this study we have investigated how far we can take these existing methods for the determination of directional LF wave spectra.
Sensitivity analyses showed that the EMEP method is the most suitable method of the three for a range of LF wave fields. The reconstructed LF wave spectra using EMEP resembled the input spectra most closely over the whole range of water depths and frequencies, although its performance deteriorated with increasing water depth and wave frequency. Subsequently, a first effort was made to use the information in the reconstructed EMEP LF wave spectrum of a representative shallow-water wave field for a first estimate of the motions of a moored LNG carrier. The results were acceptable. This is a first indication that EMEP output might be used to calculate the motions of an LNG carrier moored in shallow water.
DCV Aegir Pipelay Installation Analyses and Capabilities
Authors: Smienk Henk , Karjadi Erwan , Vazquez Gabriel , Doherty Peter , Dooley Patrick 
Published: November 2013

Heerema Marine Contractors (HMC) is entering a new era of pipe laying with the new Deep water Construction Vessel (DCV) Aegir, designed to be able to reel/J-lay pipelines for a broad range of pipe dimension and water depth combinations. On the one side this is governed by equipment/vessel limitations (moonpool size, high top tension capacity, stinger component capabilities) and on the other side limited by pipeline design code (e.g. DNV, API) acceptance criteria for reel-lay and J-lay installation.
This paper outlines the pipelay capabilities of DCV Aegir and details the J-lay (with quad joints) and reel-lay installation analyses performed to aid in the design of the vessel pipelay equipment. DCV Aegir has two modes for J-lay installation, which are light J-lay with friction clamps and heavy J-lay with collar clamps in combination with collars in the pipeline. DCV Aegir reel-lay installation from the pipeline in the tensioners down to the seabed will also be explained.
Light J-lay, heavy J-lay and reel-lay have maximum top tension capacities (related to the equipment) of 600 mT, 2000 mT and 800 mT, respectively. The top tension capacity also depends on pipe OD, coating type and thickness. J-lay and reel-lay installation analyses are performed with the non-linear finite element software package Flexcom from MCS Kenny to determine installation capabilities with respect to pipe OD, wall thickness and water depth combinations. Together with that the pipelay equipment design is validated by pipeline installation analyses.
Shallow and deep water normal pipeline installation for all three pipelay options will be discussed. DCV Aegir pipelay equipment includes a retractable hang off module/stinger for deployment of pipelines. The usage and benefits of the hang off module will be documented. For the J-lay installation modes the procedure for lowering a quad joint is analysed in order to optimise equipment usage. DCV Aegir possesses a high capacity abandonment and recovery system (up to 2000mT). Abandonment and recovery analyses description and design review aspects will be discussed. Finally, the pipeline in-line structure installation analyses, together with design review considerations will be documented.
Real-Time Hybrid Testing for Marine Structures: Challenges and Strategies
Authors: Chabaud Valentin , Steen Sverre , Skjetne Roger 
Published: November 2013

Within the field of hydrodynamics, it is fairly easy to find examples of model tests whose performance is impaired by only a subpart of the whole system, which may not be the one of interest.
Real-time hybrid testing (RTHT) overcomes this issue by performing scale model testing only on a subpart of the whole structure, the remainder being simulated numerically. The loads acting on the virtual substructure are calculated from online-measured motions of the physical substructure and actuated back on the latter in real-time. RTHT involves data measurement, filtering, force estimation, motion observing and force actuation. The main challenge is to fit all of those items into one time step.
A simple case study is suggested. It consists in a linearized one degree of freedom floating wind turbine, whose floating substructure is physically tested while wind loads are numerically simulated and actuated. Design rules to build the corresponding RTHT set up are then presented.
Development of Turbine Access System
Authors: ter Brake Erik , Clegg Andy , Perdrix Frederic 
Published: November 2013

The offshore wind industry is growing rapidly and the number of personnel transfers between turbines and work boats is increasing. As this is considered an operation that carries a significant risk, a number of improvements have been developed in the industry. In order to provide a cost effective solution optimised for the safety and convenience of operations, a Turbine Access System (TAS) was developed. The functionality of TAS is based on the principle that the motions of the vessel are actively compensated in heave, roll and pitch in order to provide a stable platform for personnel transfer.
This paper presents the technical design of TAS, including the development of the global design, the hydraulic system and the control systems. This covers deriving the general arrangement of the unit, obtaining the required inverse motions of the gangway, the means of hydraulic cylinder activation and developing the control system.
The control system comprises cascade control with feed-forward and nonlinear compensation in order to minimise trajectory tracking errors. The methodology to derive high degrees of accuracy and smooth operations is discussed alongside the operational logic, monitoring, fault actions and safety features which are accessed through two touch-panel computers.
The first TAS came into operation during the summer of 2012 where it was mounted on a 24m catamaran. The sea trials, offshore commissioning and control system optimisation provided extensive amounts of data on the performance of TAS in waves. The results of the performance in waves has been investigated and documented in order to validate the claims of the functionality of TAS.
Further optimisation of the motion compensation principle is discussed where the main focus lies on the increase of the operability range beyond the current standards. By changing the bow fenders of the work boat to rollers that provide hydraulic energy dissipation, the additional imposed pitch damping is expected to reduce the boat motions and increase the TAS operability. The paper discusses the methodology and logic behind the active pitch damping devices and the effect on the operation of TAS.
The paper discusses the main technical challenges and solutions to achieve a safe, user friendly and comfortable transfer system for the offshore industry.
Importance of Second-Order Difference-Frequency Wave-Diffraction Forces in the Validation of a FAST Semi-Submersible Floating Wind Turbine Model
Authors: Coulling Alexander J. , Goupee Andrew J. , Robertson Amy N. , Jonkman Jason M. 
Published: November 2013

To better access the abundant offshore wind resource, efforts are being made across the world to develop and improve floating offshore wind turbine technologies. A critical aspect of creating reliable, mature floating wind turbine technology is the development, verification, and validation of efficient computer-aided-engineering (CAE) tools. The National Renewable Energy Laboratory (NREL) has created FAST, a comprehensive, coupled analysis CAE tool for floating wind turbines, which has been verified and utilized in numerous floating wind turbine studies. Several efforts are underway to validate the floating platform functionality of FAST to complement its already validated aerodynamic and structural simulation capabilities. The research employs the 1/50th-scale DeepCwind wind/wave basin model test dataset, which was obtained at the Maritime Research Institute Netherlands (MARIN) in 2011. This paper describes further work being undertaken to continue this validation. These efforts focus on FAST’s ability to replicate global response behaviors associated with dynamic wind forces and second-order difference-frequency wave-diffraction forces separately and simultaneously.
The first step is the construction of a FAST numerical model of the DeepCwind semi-submersible floating wind turbine that includes alterations for the addition of second-order difference-frequency wave-diffraction forces. The implementation of these second-order wave forces, which are not currently standard in FAST, are outlined and discussed. After construction of the FAST model, the calibration of the FAST model’s wind turbine aerodynamics, tower-bending dynamics, and platform hydrodynamic damping using select test data is discussed. Subsequently, select cases with coupled dynamic wind and irregular wave loading are simulated in FAST, and these results are compared to test data. Particular attention is paid to global motion and load responses associated with the interaction of the wind and wave environmental loads. These loads are most prevalent in the vicinity of the rigid-body motion natural frequencies for the DeepCwind semi-submersible, with dynamic wind forces and the second-order difference-frequency wave-diffraction forces driving the global system response at these low frequencies. Studies are also performed to investigate the impact of neglecting the second-order wave forces on the predictive capabilities of the FAST model. The comparisons of the simulation and test results highlight the ability of FAST to accurately capture many of the important coupled global response behaviors of the DeepCwind semi-submersible floating wind turbine.
Guidelines for CFD Simulations of Spar VIM
Authors: Lefevre Charles , Constantinides Yiannis , Kim Jang Whan , Henneke Mike , Gordon Robert , Jang Hyunchul , Wu Guangyu 
Published: November 2013

Vortex-Induced Motion (VIM), which occurs as a consequence of exposure to strong current such as Loop Current eddies in the Gulf of Mexico, is one of the critical factors in the design of the mooring and riser systems for deepwater offshore structures such as Spars and multi-column Deep Draft Floaters (DDFs). The VIM response can have a significant impact on the fatigue life of mooring and riser components. In particular, Steel Catenary Risers (SCRs) suspended from the floater can be sensitive to VIM-induced fatigue at their mudline touchdown points.
Industry currently relies on scaled model testing to determine VIM for design. However, scaled model tests are limited in their ability to represent VIM for the full scale structure since they are generally not able to represent the full scale Reynolds number and also cannot fully represent waves effects, nonlinear mooring system behavior or sheared and unsteady currents. The use of Computational Fluid Dynamics (CFD) to simulate VIM can more realistically represent the full scale Reynolds number, waves effects, mooring system, and ocean currents than scaled physical model tests.
This paper describes a set of VIM CFD simulations for a Spar hard tank with appurtenances and their comparison against a high quality scaled model test. The test data showed considerable sensitivity to heading angle relative to the incident flow as well as to reduced velocity. The simulated VIM-induced sway motion was compared against the model test data for different reduced velocities (Vm) and Spar headings. Agreement between CFD and model test VIM-induced sway motion was within 9% over the full range of Vm and headings. Use of the Improved Delayed Detached Eddy Simulation (IDDES, Shur et al 2008) turbulence model gives the best agreement with the model test measurements. Guidelines are provided for meshing and time step/solver setting selection.
Investigation of the Mighty Servant 3 Accident by a Progressive Flooding Method
Authors: Dankowski Hendrik , Dilger Hendrik 
Published: November 2013

The semi-submersible heavy-lift vessel Mighty Servant 3 sank off the port of Luanda, Angola in the morning of December 6th, 2006 during a ballast operation to offload the drilling platform Aleutian Key. The official investigations carried out after the accident identified an error in the control of the submerging ballast operations as the direct cause of the sinking. However, the detailed phenomenons and reasons for the sudden excessive trim development has not been investigated further. This paper intends to identify the most likely sceneario which lead to the hydrostatic stability failure during the discharge operation by computing the flooding process during the ballast operation in the time domain. A numerical progressive flooding simulation method is presented for applications like accident investigations or damage stability assessments. This method is modified to fit the special requirements of simulating the operational procedures of semi-submersible vessels in the time domain. Extensions like the inclusion of pump elements but also the multi-body interaction of the cargo and the vessel with regard to the hydrostatics is presented. The direct flooding simulation computes the flux between the compartments based on the Bernoulli equation and the current pressure heads at each intermediate step. Large and partly flooded holes are taken into account as well as optional air compression and flooding through completely filled rooms. Pressure losses due to viscous effects are taken into account by applying semi-empirical discharge coefficients to each opening. The flooding paths are modeled by directed graphs.
A detailed investigation of the Mighty Servant 3 accident and an identification of the possible failure modes leading to the sinking of the vessel is presented. This will help to better understand the phenomenons leading to critical situations during the submerging procedure of semi-submersible heavy-lift vessels and to avoid such accidents in the future. Applying time domain flooding simulations allows to predict the ship behavior during ballast operations to identify critical situations and to better schedule the different steps of such an operation in advance.
Modelling Thruster-Hull Interaction With CFD
Authors: Maciel P. , Koop A. , Vaz G. 
Published: November 2013

The application of CFD for the analysis of thruster interactions effects (or degradation effects) is still a largely unexplored area. Quantifying these effects is essential to the design of dynamic positioning (DP) vessels. Presently, thruster interactions can be quantified using data available from literature, which unfortunately is often scarce, too general or not applicable to specific designs. Dedicated model tests, on the other hand, do provide detailed results but are relatively expensive and tend to become available relatively late in the design process. Alternatively, with CFD different vessel designs and thruster layouts can be investigated for thruster interaction effects in a cost-efficient manner early in the design process.
This paper presents a practical method for modelling thrusters, describing the propeller blades as an actuator disk. Numerical choices for the calculations are discussed and steady state calculations are performed. This approach is evaluated on thruster-hull interaction cases in both bollard pull and under current conditions, for a series of configurations with increasing complexity: a thruster in open water, a thruster positioned under a flat plate and a thruster under a barge.
A comparison to experimental results is presented for the flow field (against PIV experimental data) and forces on the thrusters and hull of the vessel. A modern Verification and Validation procedure is used to quantify uncertainties. The results are encouraging, both in a qualitative and quantitative sense. In this context, the modelling approach employed proves to be a robust and accurate solution, with low computational effort, and therefore appropriate to be used for much more complex real applications.