bluuebunny/crossref_metadata_2025 huggingface dataset:
Validation of a Quasi-Linear Numerical Model of a Pitching Wave Energy Converter in Close Proximity to a Fixed Structure
Authors: Heras Pilar , Thomas Sarah , Kramer Morten
Published: October 2017

Although linear theory is often used to analyse wave energy devices, it is in many cases too simplistic. Many wave energy converters (WECs) exceed the key linear theory assumption of small amplitudes of motion, and require the inclusion of non-linear forces. A common approach is to use a hybrid frequency-time domain model based on the Cummins equation with hydro-dynamic inputs coming from linear wave theory (Ref. [1]). Published experimental data is sparse (Ref. [2]) and the suitability for the broad variety of WEC technologies has yet to be proven.

This paper focuses on the challenges faced when attempting to validate a numerical model of a WEC using a variety of scaled physical tests in a waveflume. The technology used as a case study in this paper is a pitching WEC in close proximity to a fixed structure. Challenges are presented relating to waveflume effects and obtaining accurate physical input parameters to the numerical model.
Validation Exercises for the Calculation of the Flow Around a Squared Column With Rounded Corners at High Reynolds Numbers With the RANS Equations
Authors: Eça Luís , Vaz Guilherme , Koop Arjen , Pereira Filipe , Abreu Hugo
Published: October 2017

This paper presents Validation studies, i.e. evaluation of the modelling error, for the ensemble averaged Navier-Stokes (RANS) equations supplemented by eddy-viscosity models (SST k-ω and Spalart & Allmaras). The selected test case is the flow around a squared column with rounded corners at Reynolds number ranging from 105 to 107. Selected flow quantities include time-averaged quantities of drag and lift coefficients, base pressure coefficient, Strouhal number and the standard deviation of the lift coefficient. For this latter quantity, ensemble (phase) averaging must also be applied to the experimental data to obtain quantities with the same physical meaning, as illustrated in this work.

Even for the simple assumption of two-dimensional flow of the present simulations, the level of grid refinement and iterative convergence criteria required to obtain acceptable numerical uncertainties is more demanding than those typically observed in the open literature. However, the most important result obtained in this study is that the use of turbulence models developed for statistically steady flows may lead to flow fields that physically are at least questionable. From the three Reynolds numbers tested using three eddy-viscosity models, statistically periodic solutions with zero lift were only obtained with the SST k-ω at Re = 105. However, in that case, it is possible to numerically (including statistical, iterative and discretization errors) converge two different flow fields for the same space boundary conditions, but different solution strategies (start up of the vortex shedding).
The Application of Nonlinear Fourier Analysis to Soliton Quantification for Offshore Engineering
Authors: Jeans Gus , Xiao Wenting , Osborne Alfred R. , Jackson Christopher R. , Mitchell Douglas A.
Published: October 2017

This paper describes the first application of Nonlinear Fourier Analysis to the quantification of internal soliton current speeds in offshore engineering design. Large amplitude solitary internal waves produce strong, rapidly varying currents that may cause hazards to offshore operations in several regions of the world. These phenomena are commonly referred to in Industry as “solitons.” Soliton quantification was undertaken using the latest methodologies and software available from the Nonlinear Fourier Analysis Spectral Tools (NFAST) Joint Industry Project.

Solitons require rapidly sampled in-situ data for reliable quantification. Such measurements are typically of very short duration compared to the time scales needed for engineering quantification. Similarly, numerical models capable of representing solitons are computationally expensive, and thus have limited capabilities for efficiently developing the long-term simulations required to supplement in-situ data. NFAST aims to address these issues by enabling new Hyperfast Nonlinear Fourier Analysis computational techniques.

Interface displacements, derived from temperature measurements, were the primary input to soliton quantification. Associated current speeds were estimated from relevant theory and validated with available measured current data. In this particular case, the temperature measurements are considered to be more reliable than using the measured current data directly.

Application of NFAST codes produced a synthetic dataset of soliton amplitudes and speeds with an effective duration of approximately 100 years, a period that is considerably greater than the duration of available measured data. This provided extreme values consistent with extrapolation of the measured soliton data, but with a considerable reduction in uncertainty.
A Novel Measure to Reduce Ship Resistance in Waves
Authors: Guo Bingjie , Vartdal Bjørn-Johan , Steen Sverre
Published: October 2017

Ships travel in waves most of the time. The horsepower increase or speed loss in waves can become large in adverse sea. Speed loss needs to be compensated for by increasing the power. Moreover, the associated speed-loss can possibly not be compensated due to the limit of installed power. This will cause delays, and may even lead to safety concerns in maneuvering. Thus, there is a need for minimizing added resistance due to both economical and safety reasons. Ships with better performance in waves even with reduced power are desired. Latest researches on advanced ship optimization are taking added resistance into account. Ship bow optimization has been the main measure to reduce added resistance in waves.

Based on analyzing the working principles and potential benefit of the different kinds of novel ship bows, a novel measure is proposed to reduce the ship resistance in waves is proposed. A novel measure to reduce the ship resistance in waves of existing ships by installing a simple structure at the ship bow. The structure is designed to sharpen the ship bow and therefore reduce added resistance due to wave reflection. Thus, it is suitable for the large tankers and bulk carriers, which have blunt bows. The volume above free surface was expected to reduce ship motions and the added resistance due to ship motions consequently. This measure does not change the original ship design. It can be retrofitted on existing ships fairly easily at low cost. In order to verify the benefit due to the retrofitted structure, CFD simulations are performed in both head and oblique waves, which can take 3D effect into account. Four different retrofitted structures are designed and the numerical simulations are performed with the same numerical and mesh settings in each wave condition. The CFD simulation results confirm that the novel measure can reduce ship added resistance efficiently and it also has benefits in oblique waves.
Second Order Wave Loads on TLP-TAD Multi-Body System
Authors: Ramirez Miguel A. M. , Fernandes Antonio Carlos
Published: October 2017

The motions of floating moored structures are affected by first order wave loads which are proportional to the wave amplitude and associated with the wave frequency. On the other hand, second order wave loads are proportional to the square of the wave amplitude and related to the sum or difference of a pair of frequencies of the irregular sea. Although the second order loads are usually smaller compared to the first order loads, these loads can excite resonance motions in frequencies that the system has very low damping. Therefore, second order wave loads have particular importance in the design of mooring systems.

The multi-body system composed by Tension Leg Platform (TLP) and Tender Assisted Drilling (TAD) is particularly susceptible to the second order effects, due to the very low natural frequencies of their horizontal modes and the very high natural frequencies of the vertical modes of the TLP.

This work presents a numerical study of second order wave loads on the TLP-TAD multi-body system. An extensive hydrodynamic analysis focus on the hydrodynamic interactions between the floaters and how these effects modify the second order loads on the platforms was performed. The second order quadratic transform functions (QTFs) were evaluated using the indirect and the direct method. Moreover, the importance of the free surface integral was checked. Finally, the accuracy of Newman approximation for the low-frequency QTF was evaluated.
An Engineering Tool for Predicting Corrosion-Fatigue Crack Growth Rates for Structural Steels in Sour Environments
Authors: Lu Baotong , Somerday Brian P. , Hudak Stephen J.
Published: October 2017

Laboratory testing has shown that sour brine environments can reduce the fatigue life of offshore steels by factors of 10× to 50× compared to fatigue lives measured in laboratory air. Thus, in order to ensure safe, reliable, and environmentally-friendly deepwater development, the effect of these sour service environments must be properly accounted for in riser and flowline design. However, to ensure that the environmental effect is fully captured, tests need to be conducted at cyclic loading frequencies representative of those experienced in service (typically 0.1 Hz or less), which makes corrosion-fatigue testing very time-consuming and costly. Consequently, there has been a need for predictive models that can reduce the dependence on extensive long-term testing, while at the same time enable existing data to be interpolated and/or extrapolated over a broad domain of relevant mechanical, environmental, and material variables. In response to this need, a Joint Industry Project (JIP) was organized by Southwest Research Institute® (SwRI®) with the objective of developing and validating an analytical model to predict corrosion-fatigue performance of structural steels in sour brine environments. The resulting model is based on the kinetics of hydrogen generation and transport to a fracture process zone (FPZ), where embrittlement occurs in the hydrostatic stress field ahead of the growing crack. The advantage of this kinetic model is that details of the embrittlement process, which are not presently well defined, need not be included since corrosion fatigue crack growth (CFCG) is governed by the rate-controlling process (RCP) in the elemental kinetic steps that supply hydrogen to the FPZ. A general outline of this model is provided here and its validation against independently generated experimental data is demonstrated. The validated model has been implemented in spreadsheet format for convenience as an engineering tool. Due to the fundamental concepts underpinning the model, the engineering tool is shown to be adaptable to predicting CFCG rates in steels exposed to a variety of other environments — including hydrated and dehydrated sour crude oil, moist H2S gas, sweet brine, and seawater — with and without cathodic polarization. An extension of this Phase 1 model from intermediate to lower CFCG rates is currently underway in Phase 2 of the JIP but will not be discussed in detail in the present paper. The primary objective of this paper is to introduce the engineering tool based on the Phase 1 analytical model and demonstrate its functionality in quantifying CFCG rates over wide ranges of mechanical variables (stress-intensity factor range (ΔK), load ratio (Rσ), and cyclic loading frequency), environmental variables (H2S partial pressure, pH, temperature, applied potential), and material variables (yield strength).
Experimental Investigation of Wellbore Fluid Displacement in Concentric and Eccentric Annulus
Authors: Ytrehus Jan David , Lund Bjørnar , Taghipour Ali , Divyankar Shreyansh , Saasen Arild
Published: October 2017

One of the most critical operations during well construction is the cementing procedure. Due to the curing nature of the cement slurry there will be only one opportunity to cement the well properly. Although one for top hole cases can fill cement in from the top in a remedial operation, this possibility cannot fully compensate for a non-optimal initial cement job. Furthermore, it cannot be applied to other well sections. In those sections, complex squeeze cementing operations may be necessary. Consequences of improper annular cement can be leakage during production phase and extensive costs when the well is to be plugged for abandonment after the production phase. To ensure that the risk of poor cement is minimised it is important to use the best procedures to place the cement properly. To be able to select the optimum procedures, it is necessary to improve the understanding of the displacement in the wellbore annulus. All wells will be cemented in several sections. Findings and improvements that can reduce risk of poor cementing results are thus highly relevant for a large number of operations every year.

The article is based on analysing experimental results that illustrates a drilling fluid being displaced by a cement slurry. These fluids are represented by realistic model fluids and circulated through a transparent annular section. The geometry used is a 6,5″outer diameter with an inner string of 5″that also can rotate. The selected pipe sizes may normally be found in the lower parts of a well and often in deviated sections where the inner pipe cannot be assumed concentric at all times. Both concentric and eccentric inner pipe positions have therefore been selected. The test section was run both in horizontal and in inclined position. The test section was 10 meters long and instrumented with conductivity probes in an array around the perimeter at 4 separate positions along the pipe. Together with cameras along the test section the fluid interphases was observed along the test section.

Results presented in the article show that inner string rotation provides a steeper displacement front, On the other hand such rotation will also cause more mixing at the interphase. Results also show that the displacement front in a concentric annulus is significantly affected by gravity. While for an eccentric annulus, with the low side at the bottom, the narrow gap is poorly displaced when realistic fluids are applied. It was also observed that the displacement front in concentric annulus was more stable when the test section was inclined than in horizontal position.
Drag on Nets Fouled With Blue Mussel (Mytilus Edulis) and Sugar Kelp (Saccharina Latissima) and Parameterization of Fouling
Authors: Gansel Lars C. , Endresen Per Christian , Steinhovden Kristine Braaten , Dahle Stine Wiborg , Svendsen Eirik , Forbord Silje , Jensen Østen
Published: October 2017

Biofouling is a serious problem in marine finfish aquaculture with a number of negative impacts. Marine growth obstructs net openings, thereby reducing water exchange through the net and affecting fish welfare and health, as well as the spreading of dissolved nutrients, particles and pathogens. Furthermore, additional water blockage leads to increased hydrodynamic forces on fish cages, which potentially threaten the structural integrity of the fish farm. However, detailed knowledge about the effects of biofouling on the flow past, and the resulting forces on fish cages, is limited and systematic investigations of the effects of different types of fouling have been called for.

This study investigates the effects of different amounts and sizes of two important fouling organisms in Norwegian aquaculture, blue mussel (Mytilus edulis) and kelp (Saccharina latissima) on the drag on net panels. Drag forces on a number of clean and fouled nets were measured in a flume tank at a flow speed of 0.1 m/s. Net solidity was calculated from images acquired of all nets in the current. The relationship between net solidity and drag was then found for clean nets and for each type of fouling, and biofouling was parameterized by comparing clean and fouled net results: for a given fouled net, a clean net can be found that experiences the same drag. The latter can then be used in numerical models to estimate the effect of fouling on net drag. That means existing models can be used to model the drag effect of fouling. This study found a solidity increase due to mussel and kelp fouling to affect drag roughly at the same rate as an increase in clean net solidity at a flow speed of 0.1 ms−1 and within the tested fouling size range for two net types. Therefore, existing models, describing the relationship between net solidity and drag, can be used directly or with minor alterations (especially at high solidities) to estimate effects of additional mussel and kelp fouling on drag. In contrast, wet weight seems to be unsuitable as a measure to estimate drag on nets fouled with seaweed or mussels. It should be noted that these findings are only valid under similar conditions, and that other fouling types and sizes, as well as test parameters and tank size can affect the relationship between solidity and drag.
The First Full Operative U-OWC Plants in the Port of Civitavecchia
Authors: Arena Felice , Romolo Alessandra , Malara Giovanni , Fiamma Vincenzo , Laface Valentina
Published: October 2017

The Resonant Wave Energy Converter 3 (REWEC3) is a wave energy converter belonging to the family of Oscillating Water Columns (OWCs). It comprises an oscillating water column and an air pocket connected to a turbine, as for traditional OWCs. In addition, it has a small vertical U-shaped duct used for connecting the water column to the open wave field. Because of this particular geometrical configuration, it is also known as U-Oscillating Water Column (U-OWC). During the past decade, small scale field experiments and theoretical analyses proved its potential for full scale applications. Currently, a full-scale prototype has been operating in the Port of Civitavecchia (Rome, Italy), where a REWEC3 was constructed within the context of a major port enlargement. This paper shows some results of the monitoring activity on a single chamber equipped with pressure gauges. The results show some initial energetic performances of the REWEC3 in wind-generated seas.
Low Cycle Fatigue Analysis of Offshore Wind Turbines Subjected to Hurricane
Authors: Amirinia Gholamreza , Jung Sungmoon
Published: October 2017

The main objective of this paper is to investigate the effects of hurricanes on low cycle fatigue of tower and blades in offshore wind turbines. For this purpose, first, recent observations on hurricane turbulence models were discussed. Second, the buffeting wind loads on the wind turbine structure were introduced. A new formulation was used to address unsteady wind forces on the tower. This new formulation was later used to modify NREL-FAST (Fatigue, Aerodynamics, Structures, and Turbulence) for the analysis. In the next step, according to importance of recent findings about hurricanes, hurricane wind and wave fields were simulated based on the Saffir-Simpson hurricane wind scale. Then, to investigate the effects of various turbulence models on the wind turbine structures, the modified NREL-FAST was used to analyze structure-wind-wave-soil interaction of the NREL-5 MW monopile wind turbine. Finally, the low cycle fatigue analysis was presented and discussed. Results for various hurricane turbulence models showed that by using quasi-steady analysis of the tower, the spectrum Model A and Model B resulted in average 53% lower and 12% higher damage index compared to the conventional Kaimal spectrum model respectively; however, by considering unsteady formulation on the tower, spectrum Model A and Model B resulted in average 96% and 24% lower blade root damage indices compared to the conventional Kaimal spectrum model respectively.
Summary of the Joint Industry Project Wave Impact on Fixed Foundations (WiFi JIP)
Authors: de Ridder Erik Jan , Bunnik Tim , Peeringa Johan M. , Paulsen Bo Terp , Wehmeyer Christof , Gujer Philipp , Asp Erik
Published: October 2017

The objective of the Joint Industry Project Wave impact on Fixed foundations (WiFi JIP) was to increase the understanding of breaking and steep wave impact’s on fixed foundations of offshore wind turbines (OWT). The project was set-up as a Joint Industry Project (JIP) and in total 20 companies and research institutes participated in the project.

In this paper a summary of the complete WiFi JIP project will be presented. At the start of the project the state of the art design methods and guidelines were reviewed (WP1). Thereafter a jacket and a monopile foundation were designed using these state-of-the-art tools that were available at the start of the project. This effort has been reported in WP2 , where design computations were carried out using the embedded stream function approach for several sea states. In this WP Siemens, ECN and Ramboll also calculated the impact response of the monopile to surging and spilling type wave breakers with their engineering tools. In the next phase the designed foundations were tested in MARIN’s shallow water model basin. The foundation for the monopile was modelled as a rigid and flexible foundation. The foundations were tested in regular waves, irregular sea states and so called focused waves. During the model tests the wave heights, wave run-up, accelerations, impact pressures and loads on the foundation and boat landing were measured. The model test results were reported in WP3 and 7 and used as validation for WP9 and 10. WP4 delivered more understanding of realistic design conditions for areas typical for OWT, like the North Sea. Particular attention was paid to the probability of occurrence of breaking and steep waves and the associated slamming load. For this an extensive 5 week experimental program was performed from September to October 2013 in the wide wave-current flume at Deltares (Atlantic Basin). During these tests both waves and current were simulated and two bathymetries.

WP8 provided analyses of the performed full scale measurements on the response of a OWT. The full-scale measurements were done for a Vestas V90 3MW wind turbine in the Belwind windfarm which is located 46 km off the coast of Zeebrugge on the Bligh Bank. The CFD simulations performed in WP 9 showed that a good agreement is obtained between the CFD simulations and the model and full scale measurement.

In work package 10, an improved methodology was developed based on the outcome of the previous WP’s to model the breaking wave impact of plunging type breakers. In WP11 and 12 this new approach is applied on different case study’s by ECN.
Reliability Based Stiffness Analysis for Application During Installation of Suction Caissons
Authors: Sturm Hendrik , Mirdamadi Alireza
Published: October 2017

The design of suction caissons for offshore wind turbines is generally performed deterministically, using load and resistance factors taken from relevant codes and standards. This approach has been widely accepted for the geotechnical design of suction caissons in well-characterised soil conditions. However, soil layering and properties often vary considerably from location to location within an offshore wind farm. Furthermore, the installation process can also cause changes in the soil state in the vicinity of the skirts, which will affect noticeably the response of the suction caisson during operation. While this can be considered implicitly in the geotechnical design by means of assumptions, the inherent uncertainty of the soil state will impact the overall performance of the structure considerably. On the other hand, an optimized design for the system requires an accurate prediction of the foundation stiffness.

The authors present a reliability-based framework for the assessment of foundation stiffness, taking into account the most important input parameters and their expected variability. The framework can be applied both in foundation design as well as during actual installation in order to provide immediate feedback to permit adjustment or mitigation before the installation is finalized. Using a reliability-based approach in design allows an assessment of the probability of reaching the limiting design criteria and to quantify the related risk.

The proposed reliability-based framework can be applied to other design aspects. This is exemplified using the example of coupled foundation installation and capacity assessment. The authors further discuss how the framework can be extended to more complex design procedures.
Further Experimental Investigations on Vortex Self-Induced Vibrations (VSIV) With a Small-Scale Catenary Riser Model
Authors: Pesce Celso P. , Franzini Guilherme R. , Fujarra André L. C. , Gonçalves Rodolfo T. , Salles Rafael , Mendes Pedro
Published: October 2017

This paper presents further experimental results of the dynamic response of a small-scale catenary riser model subjected to sinusoidal vertical motion imposed to the top, as a continuation of a previous one, presented at OMAE’2013. In that paper, a general view of an innovative experimental methodology using underwater optical techniques was given, together with some experimental results on VSIV - Vortex Self-Induced Vibrations, also referred to as Heave-Induced Lateral Motion, or Vessel Motion Induced VIV. It was then shown that such a behavior recovered similar ones reported in the technical literature by other authors and resembled fundamental studies, by Sumer and Fredsøe. In the present paper, new experimental tests are reported and analyzed. A similar catenary configuration is assessed. The analysis of VSIV trajectories is made via space-frequency amplitude spectra and space-time amplitude scalograms, revealing rich dynamic responses. The results are meant to serve as an experimental basis, contributing to the understanding of the VSIV phenomenology and to the benchmarking of numerical models.
On the Feasibility of Using Underwater Acoustic Data Transmission for Subsea Equipment Monitoring
Authors: Ribeiro Bessie A. , Rodrigues Viviane , Ferreira Viviane , Xavier Fabio C. , Netto Theodoro A.
Published: October 2017

The present work uses the BELLHOP ray tracing model to simulate an acoustic propagation channel in a deep water environment in order to analyze its viability to provide data transmission for monitoring submarine equipment. The simulated scenario is located in the Campos Basin, Rio de Janeiro, on the Brazilian coast, responsible for more than 80% of Brazilian oil and gas production. Temperature and salinity data from five stations were used to calculate the sound speed profiles required to the transmission loss simulations of the acoustic propagation channel. In order to estimate the signal detection capacity according to the medium characteristics, a characterization of the parameters that influence the physical propagation channel was performed. The parameters of three modem models with different operation frequencies were selected and analyzed in order to obtain the Signal to Noise Ratio (SNR) of the transmission signal.
Impact of Viscoelastic Characteristics of Oil Based Muds/Synthetic Based Muds on Cuttings Settling Velocities
Authors: Altindal M. C. , Ozbayoglu E. , Miska S. , Yu M. , Takach N. , May R.
Published: October 2017

Inaccurate calculation of settling and slip velocities of cuttings leads to inaccurate determination of cuttings concentration and, hence, borehole pressure, as well as inaccurate lag times. To minimize these problems, an understanding of the relation between drilling fluid characteristics and the cuttings transport process is essential. It is desirable for drilling fluids to form a gel structure to help cuttings transportation and suspension of solids. The gel structure development is proportional to increase in aging time. The increase in aging time yields higher shear stress responses at a constant rate of deformation to the drilling fluid sample. The gel structure development helps keep cuttings in suspension and shows a viscoelastic response to small deformations. Understanding these viscoelastic responses is important in rheological characterization and settling velocity prediction. Thus, viscoelastic drilling fluid characteristics should be investigated in depth to better estimate settling and slip velocities of cuttings and to increase cutting transport efficiency.

The main focus of this project is to work on viscoelastic and time-dependent fluid characterization to identify the relation between rheological properties and settling velocities of cuttings. Rheological experiments were conducted using an Anton Paar Physica MCR 301 Rheometer. Three different drilling fluids, Water Based Mud (WBM), Oil Based Mud (OBM) and Synthetic Based Mud (SBM), are used for rheological and settling velocity experiments. Stress Overshoot Tests (SOTs) and Steady-Shear experiments were performed to investigate viscoelastic properties and gel structure of the fluids, and to examine time and temperature dependence of WBM, OBM and SBM. Information obtained from the viscoelastic and time-dependent fluid characterization tests was coupled with settling velocity data using both arbitrary shape of cuttings and spherical particles. A mathematical model that considers viscoelastic properties and time dependency of drilling fluids was developed to estimate settling and slip velocities of the cuttings. Comparisons between the proposed models and existing models based on standard rheological measurements were also done. The results show that the proposed model has good agreement with the experimental data.
Two Tandem Cylinders With Passive Turbulence Control in Flow Induced Vibration: Relation of Oscillation Patterns to Frequency Response
Authors: Lan Kai , Sun Hai , Bernitsas Michael M.
Published: October 2017

Flow Induced Vibrations (FIV) are conventionally destructive and should be suppressed. Since 2006, the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan has been studying FIV of multiple cylinders to enhance their response for harnessing hydrokinetic power from ocean, river, and tidal currents. Interactions between multiple cylinders in FIV enable high power-to-volume ratio in a converter consisting of multiple oscillators of cylinders. This paper investigates experimentally the relation between oscillation patterns and frequency response of two cylinders in tandem. All experiments are conducted in the recirculating channel of the MRELab for 30,000<Re<120,000. Phase analysis reveals three dominant patterns of oscillation of two tandem cylinders by calculating the instantaneous phase difference between the two cylinders. This phase difference characterizes each major pattern. One is characterized by nearly 180° out of phase oscillations and one by small lead or lag of one cylinder over the other. In the third pattern, the instantaneous phase difference changes continuously from −180° to +180°. Using frequency spectra, oscillation characteristics of each cylinder are revealed in each flow speed range. Comparison of oscillation patterns and frequency spectra reveals that each oscillation pattern is related to a distinctly different frequency response.
Analysis of a Wave-Powered, Reverse-Osmosis System and its Economic Availability in the United States
Authors: Yu Yi-Hsiang , Jenne Dale
Published: October 2017

A wave energy converter (WEC) system has the potential to convert the wave energy resource directly into the high-pressure flow that is needed by the desalination system to permeate saltwater through the reverse-osmosis membrane to generate clean water. In this study, a wave-to-water numerical model was developed to investigate the potential use of a wave-powered desalination system (WPDS) for water production in the United States. The model was developed by coupling a time-domain radiation-and-diffraction-method-based numerical tool (WEC-Sim) for predicting the hydrodynamic performance of WECs with a solution-diffusion model that was used to simulate the reverse-osmosis process. To evaluate the feasibility of the WPDS, the wave-to-water numerical model was applied to simulate a desalination system that used an oscillating surge WEC device to pump seawater through the system. The annual water production was estimated based on the wave resource at a reference site on the coast of northern California to investigate the potential cost of water in that area, where the cost of water and electricity is high compared to other regions. In the scenario evaluated, for a 100-unit utility-scale array, the estimated levelized cost of energy for these WECs is about 3–6 times the U.S.’s current, unsubsidized electricity rates. However, with clean water as an end product and by directly producing pressurized water with WECs, rather than electricity as an intermediary, it is presently only 12% greater than typical water cost in California. This study suggests that a WEC array that produces water may be a viable, near-term solution to the nation’s water supply, and the niche application of the WPDS may also provide developers with new opportunities to further develop technologies that benefit both the electric and drinking water markets.
Structural Loads Analysis for Wave Energy Converters
Authors: van Rij Jennifer , Yu Yi-Hsiang , Guo Yi
Published: October 2017

This study explores and verifies the generalized body-modes method for evaluating the structural loads on a wave energy converter (WEC). Historically, WEC design methodologies have focused primarily on accurately evaluating hydrodynamic loads, while methodologies for evaluating structural loads have yet to be fully considered and incorporated into the WEC design process. As wave energy technologies continue to advance, however, it has become increasingly evident that an accurate evaluation of the structural loads will enable an optimized structural design, as well as the potential utilization of composites and flexible materials, and hence reduce WEC costs. Although there are many computational fluid dynamics, structural analyses and fluid-structure-interaction (FSI) codes available, the application of these codes is typically too computationally intensive to be practical in the early stages of the WEC design process. The generalized body-modes method, however, is a reduced order, linearized, frequency-domain FSI approach, performed in conjunction with the linear hydrodynamic analysis, with computation times that could realistically be incorporated into the WEC design process.

The objective of this study is to verify the generalized body-modes approach in comparison to high-fidelity FSI simulations to accurately predict structural deflections and stress loads in a WEC. Two verification cases are considered, a free-floating barge and a fixed-bottom column. Details for both the generalized body-modes models and FSI models are first provided. Results for each of the models are then compared and discussed. Finally, based on the verification results obtained, future plans for incorporating the generalized body-modes method into the WEC simulation tool, WEC-Sim, and the overall WEC design process are considered.
Hybridization of Theory and Experiment in Optimizing Di-Hull Configuration With Respect to Wave Resistance
Authors: Yu Dongchi , Yeung Ronald W.
Published: October 2017

A fast method for optimizing the configuration of a di-hull system is to take advantage of the wave-cut signatures of each hull and evaluate the combined resistance of the hull system using analytical expressions that portray the interference effects of the hull-generated waves. This interference formula is available in Yeung et al. [1] and can be used in conjunction with the wave-cut signatures. The Longitudinal Wave-cut Method (LCM) is utilized to acquire the wave-making spectrum for each monohull. Then the di-hull interference wave resistance is deduced by substituting these experimentally-acquired information into analytical expressions for resistance computation. The pre-acquired wave-spectrum information can be stored and used for a combination of any component hulls, identical or not. This hybridization procedure of theory and experiments is tested and evaluated. Its merits and deficiencies are discussed.
The Fracture Resistance Approach to Rationalize Overall Temperature and Wall Thickness Effects on Fracture Toughness for Design of Offshore Structures Under Arctic Conditions
Authors: Horn Agnes Marie , Østby Erling , Akselsen Odd , Hauge Mons
Published: October 2017

It is well known that when the temperature decreases, steel becomes more brittle. In order to prevent brittle fracture in the Arctic, the structure needs adequate toughness for the loading seen at these low temperatures. None of the common offshore design codes today consistently addresses low temperature applications. Generally the design codes and steel construction material with reference to EN 10225 is applied down to −10°C, while below this temperature it is up to the designer to show fit for purpose of the selected material. This paper is concerned with discussion regarding model to potentially rationalize overall temperature and wall thickness effects on fracture toughness for low temperature application. The proposed model is linked up to results in the Arctic Material research project (2008–2017) and used for support of guideline formulations. This paper is a continuation of the work presented in by the authors in 2016 [1] with the aim to support a more design friendly approach addressing both temperature effect and material utilization. In addition, the paper will sum up findings and conclusions from the overall research work carried out in this project with focus on the implication on design for Arctic applications.
Experimental and Theoretical Quantification of Non-Equilibrium Phase Behaviour and Physical Properties of Foamy Oil Under Reservoir Conditions
Authors: Shi Yu , Yang Daoyong
Published: October 2017

A novel and pragmatic technique has been proposed to quantify the non-equilibrium phase behaviour together with physical properties of foamy oil under reservoir conditions. Experimentally, constant-composition expansion (CCE) experiments at various constant pressure decline rates are conducted to examine the non-equilibrium phase behaviour of solvent-CO2-heavy oil systems. Theoretically, the amount of evolved gas is firstly formulated as a function of time, and then incorporated into the real gas equation to quantify the non-equilibrium phase behaviour of the aforementioned systems. Meanwhile, theoretical models have been developed to determine the time-dependent compressibility and density of foamy oil. Good agreements between the experimentally measured volume-pressure profiles and calculated ones have been achieved, while both amounts of evolved gas and entrained gas as well as compressibility and density of foamy oil were determined. The time-dependent effects of entrained gas on physical properties of oleic phase were quantitatively analyzed and evaluated. A larger pressure decline rate and a lower temperature are found to result in a lower pseudo-bubblepoint pressure and a higher expansion rate of the evolved gas volume in the solvent-CO2-heavy oil systems. Apparent critical supersaturation pressure increases with either an increase in pressure decline rate or a decrease in system temperature. Physical properties of the oleic phase under non-equilibrium conditions follow the same trends as those of conventionally undersaturated oil under equilibrium conditions when pressure is higher than the pseudo-bubblepoint pressure. However, there is an abrupt increase of compressibility and decrease of density associated with pseudo-bubblepoint pressure instead of bubblepoint pressure due to the initialization of gas bubble growth. The amount of dispersed gas in the oleic phase is found to impose a dominant impact on physical properties of the foamy oil. Compared with CCE experiment at constant volume expansion rate, a rebound pressure and its corresponding effects on physical properties cannot be observed in the CCE experiments at constant pressure decline rate.
Nonlinear Dynamics of a Free-Hanging Riser
Authors: Jung DongHo , Kwon YongJu
Published: October 2017

The characteristics of the dynamic behavior of a free-hanging riser is analyzed under top excitation conditions by using a numerical method. A top excited riser that is free hanging presents strong nonlinear behavior characteristics. We verified that the top tension of a free-hanging riser under top-end forced excitation dominantly vibrates in the double with the top excitation frequency. The non-linear response of a top tension is disclosed from analysis for the local velocities at nodes along the riser length. The top tension of a free-hanging riser shows to respond in double with the top excitation frequency under no current. On the other hand, the top tension of a top-end forcibly excited riser has its double component as well as the excitation frequency component under a current. We proved that the axial vibration of a free-hanging riser dominates the riser tension under top-end forcibly excitation conditions.
Drilling Riser Case Studies Comparing the Drag Performance of LGS Technology to Conventional Buoyancy Units and Fairings
Authors: Johnstone Daniel R. , Potts Andrew E. , Marcollo Hayden , Kurts Phillip
Published: October 2017

Deepwater drilling risers have a substantial portion (>70%) of buoyancy modules covering their entire length. The circular cross-section of these modules acts as a bluff body exhibiting significant Vortex-Induced Vibration (VIV) response, resulting in fatigue in the riser/wellhead and drag force amplification of the riser.

Drilling operations can be suspended when the drag force on the riser adversely affects the top and bottom angles, a response which is exacerbated by drag force amplification due to VIV.

In 2015 and 2016, Longitudinally Grooved Suppression (LGS)1 technology was developed to field qualified Technology Readiness Level (TRL of 6) [1] offering a technical advantage to drilling operators over existing options.

In order to investigate the relative performance and quantify the benefits of LGS over conventional cylindrical Drilling Riser Buoyancy Modules (DRBMs) with and without fairings, a series of case studies encompassing typical operating conditions at drilling locations in several regions around the world (including the Gulf of Mexico, Brazil, the North Sea, and Australia) have been carried out to examine the performance in terms of drag (affecting operability due to top angle) and fatigue damage rate of the riser. The results indicate that LGS outperforms conventional buoyancy risers and fairings-equipped risers with regard to riser operability limits and fatigue.
The Principle of an Integrated Generation Unit for Offshore Wind Power and Ocean Wave Energy
Authors: Chen Weixing , Gao Feng
Published: October 2017

Energy resources of offshore wind and ocean wave are clean, renewable and abundant. Various technologies have been developed to utilize the two kinds of energy separately. This paper presents the principle of an integrated generation unit for offshore wind power and ocean wave energy. The principle of the unit includes that: The wind rotor with retractable blades and the 3-DOF (degrees of freedom) mechanism with the hemispherical oscillating body are used to collect the irregular wind and wave power, respectively; The energy conversion devices (ECDs) are utilized to convert mechanical energy from both the wind rotor and the 3-DOF mechanism into hydraulic energy; The hydraulic energy is used to drive the hydraulic motors and electrical generators to produce electricity. Some analyses and experiments of the unit is conducted.
The Effects of Surface Waves and Submergence on the Performance and Loading of a Tidal Turbine
Authors: Guo Xiaoxian , Gao Zhen , Yang Jianmin , Moan Torgeir , Lu Haining , Li Xin , Lu Wenyue
Published: October 2017

Tidal energy has the advantages of high predictability, high energy density, and limited environmental impacts. As tidal turbines are expected to be used in the most energetic waters where there might be significant waves, the assessment of unsteady hydrodynamic load due to surface waves is of great concern. The objective of this paper is to assess the effects of surface waves and submergence of the turbine on the power performance and loads of a tidal turbine by experimental approach. The experiments on a 1 : 25th model tidal turbine were carried out in a towing tank. A wide range of regular waves with periods from 1.0 s to 3.0 s at model scale were generated. Different submergence conditions were considered to investigate the effects of the presence of free surface. The cases with blade tip partly going out of water were also performed. The regular waves did not have significant influences on average loads and power output in the present experiments, but caused large amplitude of the cyclic variation of the loads. The amplitudes of the cyclic load were proved to be proportional to the incident wave height, and to be sensitive to the wave frequency and submergence of the rotor. As the tidal turbine getting close to free surface, significant waves were induced by the underwater rotating blade. The effects of surface waves and submergence need to be taken into account in design.
Dynamic Modelling of a Spar Buoy Wind Turbine
Authors: Tomasicchio Giuseppe Roberto , Avossa Alberto Maria , Riefolo Luigia , Ricciardelli Francesco , Musci Elena , D’Alessandro Felice , Vicinanza Diego
Published: October 2017

In the present paper, the dynamic response of a spar buoy wind turbine under different wind and wave conditions is discussed. Physical model tests were performed at the Danish Hydraulic Institute (DHI) off-shore wave basin within the EU-Hydralab IV Integrated Infrastructure Initiative.

The OC3-Hywind spar buoy was taken as reference prototype. A spar buoy model, 1:40 Froude-scaled, was tested using long crested regular and irregular waves, orthogonal (0 degrees) and oblique (20 degrees) to the structure. Here the results concerning regular waves, with incidence orthogonal to the structure, are presented; the selected tests considered rotating and non-rotating blades. Measurements of displacements, rotations, accelerations, forces response of the floating structure and at the mooring lines were carried out. Based on the observed data, FAST wind turbine simulation tool, developed and maintained by the U.S. Department of Energy’s (DOE’s), National Renewable Energy Laboratory (NREL), was calibrated and verified. The numerical model takes into account the wave induced response and the effects of the mooring lines on the overall system.

The adopted spar buoy has three equally spaced mooring lines that were modelled as quasi-static taut or catenary lines through MAP++ (static module) and MoorDyn (dynamic module) in the FAST simulation tool. The tensions along the fairleads of the three mooring lines were examined.

At the end of the calibration procedure, the numerical model was successfully used to simulate the dynamic motions of the floating wind turbine under combinations of wind and sea states for the selected wave attacks.

All data from the DHI tests were converted to full scale using Froude scaling before being analyzed.
Operation of the Under Water TV on Chikyu
Authors: Namba Yasuhiro , Ishiwata Junya
Published: October 2017

The Under-Water TV (UWTV) is used as a reentry support system for the scientific drilling by the drilling vessel “Chikyu”. The “Chikyu” UWTV performs scientific operations cost-effectively where the normal Remotely Operated Vehicles (ROVs) in the market can’t be available. The UWTV in the 2nd generation has been tested in a sea trial and operated in the subsequent scientific drilling project. The shape of the UWTV frame has been modified after the trial and the behavior of the UWTV during its dives has been improved. The tension data in the operations has been measured and the effect of the dynamic behavior is extracted from the data. Then the extracted dynamic effect is used as the indicator to see the UWTV situation under the sea surface and to prevent the UWTV from the operational failure.