bluuebunny/crossref_metadata_2025 huggingface dataset:
Study on Attitude Control of a Cabin-Suspended Catamaran by Using a Double-Loop Control System
Authors: Han Jialin , Kanno Sota , Mochizuki Akito , Kitazawa Daisuke , Maeda Teruo , Itakura Hiroshi
Published: November 2019

A series of cabin-suspended ships, named Wave Harmonizer, has been developed since 2008. The model ship consists of cabin part, hull part and conjunction part which is mounted in-between the cabin and the hull. The possibility and feasibility of introducing suspensions into small vessels are investigated. Effectiveness evaluations are made in two aspects: motion reduction of the cabin and wave energy harvesting through the oscillating cabin. According to the research results obtained in 2015 and 2016, it was found that the deck of the cabin may bear obvious inclination while weights were loading or unloading from it. Moreover, in relative long waves, the effectiveness of the motion reduction of the skyhook controller was insufficient. To solve those problems, an attitude control strategy is proposed. It is adopted as an outer-loop of the existing controller. Inclination sensors are employed to collect the rotational angle of the cabin in terms of pitch and roll. After finishing the design and construction of the double-loop control system, open loop tests are carried out in dry and wet conditions. The chain mechanism of the ship is investigated. Then a bench test is operated to validate the control concept and performance of the double-loop control system. Finally, tank tests are implemented to examine the inclination reduction of the cabin at regular head waves. In this paper, the development of the double-loop control system is described, experimental results are demonstrated with respect to the heave and pitch motion reduction of the cabin at the condition of with/without the outer control loop. It shows that the inclination reduction at the loading/unloading condition and the walking-on-deck condition are significant. However, in waves the effectiveness is not clearly verified. It suggests that the influence of the apparent gravity on the inclination measurement in waves should be investigated and solutions to accurately detect the inclination of the cabin should be sought.
Validation of the Motion Analysis Method of Floating Offshore Wind Turbines Using Observation Data Acquired by Full Scale Demonstration Project
Authors: Yoshimoto Haruki , Kamizawa Ken
Published: November 2019

In recent years, the social demands for the introduction of renewable energy are increasing, demonstration projects of floating offshore wind turbine are being implemented and planned around the world. In Japan, a demonstration test named Fukushima FORWARD (Fukushima Floating Offshore Wind Farm Demonstration Project) has been conducted since 2011. Fukushima FORWARD is a project carried out by the Ministry of Economy, Trade and Industry, the world’s first floating offshore windfarm with a total capacity of 14 MW, including three floating offshore wind facilities and one floating offshore substation. In Fukushima FORWARD, Japan Marine United Corporation is in charge of floater part EPCI (Engineering, Procurement, Construction and Installation) of one floating offshore wind facility and one floating offshore substation.

The floating offshore wind turbine (Ship Name: Fukushima Hamakaze) designed and built by Japan Marine United Corporation is equipped with a downwind 5 MW wind turbine. The floating structure adopts the advanced spar shape in order to reduce wave frequency motions and is moored by six spread catenary mooring lines.

In the design of floating offshore wind turbines, it is important to estimate motions with high accuracy. Especially floating offshore wind turbine equipped with horizontal axis wind turbine requires heavy RNA (Rotor Nacelle Assembly) to be installed on the tower, and the floater motion greatly influences the design of the tower base. The tower base is required to have sufficient reliability because it directly leads to collapse of the wind turbine if it is damaged. On the other hand, the tower base is generally constructed with a cylinder made of extremely thick steel plate which is difficult to bend and weld, and if it has excessive safety factor the cost has increased greatly. Also, estimating the motion is the basis for estimating the load on the floating structure.

In this paper, statistical analysis of long-term data measured by Fukushima FORWARD floating offshore wind turbine focusing on the motion and its features are introduced. In addition, we compare the motion obtained by potential theory and coupled analysis with actually measured motion using the measured wave and wind data and evaluate the validity of the analysis method.
Spatial and Seasonal Variability of Metocean Design Criteria in the Southern South China Sea From Covariate Extreme Value Analysis
Authors: Anokhin Vadim , Ross Emma , Randell David , Jonathan Philip
Published: November 2019

This paper describes spatial and seasonal variability of metocean design criteria in the southern South China Sea. Non-stationary extreme value analysis was performed using the CEVA approach (Covariate Extreme Value Analysis,[1]) for a 59-year long SEAFINE hindcast of winds and waves, estimating metocean design criteria up to 10,000-year return period. Wind design criteria are mostly driven by large-scale monsoonal events; at higher return periods infrequent cyclonic events have strong influence on the tail of the extreme value distribution but confined to a limited geographical area. The CEVA analysis of waves showed much less dependence on the tropical cyclone events; the spatial metocean design criteria were smoother, mostly influenced by the monsoonal wind strength, fetch and local bathymetry. Return value estimates illustrate the strong seasonality of metocean design criteria, with boreal winter (December-February, Northeasterly monsoon) contributing most to the extremes, while April and May are the mildest months. Estimates for the ratio of 10,000/100-year return values are also presented, both for winds and waves. There is empirical evidence that the range of “typical” values of generalised Pareto shape parameter observed for Hs is different to that observed for wind speed. For this reason, an upper bound of +0.2 for generalised Pareto shape was specified for wind speed analysis, compared to 0.0 for Hs. In some cases, increase of upper bound for waves to 0.1 is justified, leading to slightly more conservative Hs values. We confirmed that the upper end point constraint was not too influential on the distributions of generalised Pareto shape parameter estimated. Nevertheless, it is apparent that specification of bounds for generalised Pareto shape is a critical, but problematic choice in metocean applications.
Mechanical Modeling of the Polymeric Coating on a Subsea Pipeline
Authors: Vestrum Ole , Langseth Magnus , Børvik Tore
Published: November 2019

Offshore pipelines may be exposed to a range of extreme loading situations during operation on the seabed such as impact by trawl gear or anchors. While not primarily being designed for it, thermal insulating polymeric coatings are experienced to provide beneficial contributions to the structural integrity of subsea pipeline designs. In recent editions, the prevailing standards and design guidelines are allowing for the inclusion of external coating products in the mechanical design evaluation. This secondary functionality of insulating coatings presents a great potential in terms of more optimized pipeline designs. However, due to the lack of reliable and versatile mechanical models, any beneficial effects from these complex polymeric insulating coatings are often omitted in simulations. This work presents a finite element based approach for assessing the mechanical response of polymeric coatings on offshore pipelines with different porous structures imaged using X-ray micro computed tomography. The modeling approach is also compared with experimental results.
Validation of Hydrodynamic Loads on a Large-Diameter Monopile in Regular Waves
Authors: H. Dadmarzi Fatemeh , Thys Maxime , Bachynski Erin E.
Published: November 2019

Validated hydrodynamic load models for large-diameter support structures are increasingly important as the industry moves towards larger offshore wind turbines. Experiments at 1:50 scale with stiff, vertical, bottom-fixed, extra-large (9m and 11m diameter full-scale) monopiles in steep waves are conducted. The tests are carried out at two water depths, 27 m and 33 m. A range of regular waves, with varying period and amplitude, are used. The first, second, and third harmonics of the total wave loads, where measurements are available, are calculated with different methods. For the first harmonic of the force (and consequently the mudline moment), MacCamy-Fuchs gives the best agreement with experiments, especially for the larger diameter model. For the second harmonic, for the shortest waves the generalized FNV theory and Morison equation overpredict the forces, while for the longest (and largest) waves, the opposite is observed. The third harmonic of the force is generally overpredicted by the calculations.
Experimental Study on Coupled Motions of a Spar-Buoy Under Mathieu Instability
Authors: Iseki Toshio , Xu Peng
Published: November 2019

A series of experiments were carried out to investigate the occurrence of the Mathieu-type instability. The main objective of this study is utilization of an auto-parametrically excited oscillation for wave energy converters. In this paper, the subject is the auto-parametrically excited oscillation of a spar-buoy type point absorber with two degrees of freedom. A small spar buoy model with a ballast controlling system was made and the model experiments were conducted to realize the large oscillating motion based on the Mathieu-type instability. The ballast controlling system is installed in the buoy model and the vertical movement of the ballast produces a certain change of the pitching natural period. Using the controlling system, the pitching motion in regular waves under the heave resonant period was measured. In some experiments, it was observed that the large pitching motion occurred suddenly, and the time histories showed different excitation pattern from the theoretical Mathieu-type instability. Based on the model experiments and considerations of the theory of Mathieu-type instability, the occurrence of the large pitching motion is discussed.
Data Assimilation of the Stereo Reconstructed Wave Fields to a Nonlinear Phase Resolved Wave Model
Authors: Watanabe Shogo , Fujimoto Wataru , Nose Tak , Kodaira Tsubasa , Davies Graham , Lechner Daniel , Waseda Takuji
Published: November 2019

A stereo camera system was installed facing Southeast at the observational tower owned by the University of Tokyo in the Sagami Bay, Japan. The three-dimensional wave fields were reconstructed from the stereo images, which were successfully captured from April 2017 until now, by using an open source software WASS (Waves Acquisition Stereo System). The significant wave heights and periods calculated from the stereo images covering an area of 80 m by 80 m were compared against those derived from the ultra-sonic wave gauge at the tower. Overall, a reasonable agreement is achieved, but the accuracy of the stereo reconstruction degrades with the distance from the camera. Also, the significant wave period derived from stereo imaging tends to be shorter, likely related to the error at high frequencies. The reconstructed wave field will be assimilated into a phase-resolved nonlinear wave model. The ensemble Higher Order Spectral simulations and the implementation of the a4DVAR data assimilation scheme, allowed us to substantially extend the estimated wave field beyond the stereo imaging domain. A field campaign with an ADCP in the field of view of the stereo camera and two surface wave buoys outside of the view were conducted for validation.
Hybrid Renewable Energy Systems Sizing for Offshore Multi-Purpose Platforms
Authors: Recalde Luis , Yue Hong , Leithead William , Anaya-Lara Olimpo , Liu Hongda , You Jiang
Published: November 2019

Integrating marine renewables and aquaculture is a complex task. The generated power of each renewable technology depends on its source cycle (wind, wave, solar PV), leading to periods of zero power production. On the other side, aquaculture farms require smooth and stable power supply since any power shortage can lead to the loss of the entire farm production. This paper illustrates the sizing of a hybrid energy system (wind,solar PV, energy storage) to power up the aquaculture farm. The sizing is based on available commercial technology and the system is mounted on a single multi-purpose platform. Reliability is improved by considering device redundancies. Such hybrid system has not been considered before for aquaculture farms. System rough sizing, based on simple online renewable energy calculators, is used to select existing renewable technologies and HOMER Pro simulation software is used to evaluate the technical and economic feasibility of the microgrid for all possible combinations of the technology selected and perform sensitivity analysis on wind turbine tower height, battery state of charge and solar PV panels reflectance. The optimisation is subject to combined dispatch strategy and net present cost.
On Motion and Hydroelastic Analysis of a Floating Offshore Wind Turbine
Authors: Lamei Azin , Hayatdavoodi Masoud , Wong Carlos , Tang Bin
Published: November 2019

This study is concerned with motion analysis and hydroelastic response of a floating offshore wind turbine to wave loads. The novel floating structure, made of prestressed concrete, is designed to support multiple wind turbines, and it rotates according to the environmental loads to face the incoming wind. The floating structure is attached to a mooring line that allows the rotation of the structure in response to the environmental loads. The floating structure is an equilateral triangular platform. The wind turbines are located at the vertices. Due to the dimensional characteristics of the structure, elasticity of the floating platform plays an important role in its dynamics. While the dynamic response of the structure is driven by both aerodynamic and hydrodynamic loads, this study focuses on the motion and elastic response of the novel floating structure to the hydrodynamic loads only. The three dimensional hydrodynamic loads on the floating structure are obtained by use of the constant panel approach of the Green function method, subject to linear mooring loads. A finite element analysis is carried out for the calculation of the elastic response of the structure. Computations of the integrated linear structure-fluid-structure interaction problem are performed in frequency domain using HYDRAN, a computer program written for the linear dynamic analysis of rigid and flexible bodies. Results presented here include the response amplitude operators of both the rigid and flexible bodies to incoming waves of various frequencies and directions. Also presented are the wave-induced stresses on the floating body, and the elastic deformations.
A Numerical Method for Ice Resistance Calculation of Polar Ships Navigating in Floating Ice Region
Authors: Li Hui , Qian Yuan , Feng Yan , Sheng Weijia , Li Haojin
Published: November 2019

This paper presents a numerical method for forecasting the ice resistance of polar ships navigating in floating ice region. The finite element method is adopted to analyze the interaction between ship and floating ice including the coupling effect between ice and water. Based on the comparison between the sea ice test data and the numerical simulation of the ice model, it is verified that the numerical method can successfully simulate the characteristics of sea ice materials. The accuracy of the numerical method is validated against ice tank model test. By using this method, the resistances of a polar ship navigating in floating ice region with different ice coverage rate (60%, 70%, 80% and 90%) at different speeds are calculated. The results show that the ice coverage rate has a great influence on the ice resistance, and the ice resistance varies with ship speeds.
A PDE Model for Estimating the Life Time of a Riser
Authors: Gustad Halvor Snersrud , Moe Per Thomas , Celledoni Elena
Published: November 2019

Subsea intervention operations may expose wellheads to very high structural loads. Traditionally, structural fatigue was not regarded as a challenge for wellheads, and design codes did not include verification with respect to structural loads. However, development in mathematical modeling and numerical techniques as well as monitoring systems have revealed that semi-submersible rigs with marine riser systems and BOP (Blowout Preventer) can cause quite significant loads on permanently installed equipment.

Being able to describe a risers movement under the action of external forces is important in order to calculate the effects on the wellhead. This may be used to predict the expected remaining lifetime of the system. In this article, the existing literature on numerical modeling of a riser system is discussed. Different mathematical models of a riser system were approximated numerically by a finite element method. The models are compared and their correctness is discussed. The error bounds for the finite element solutions are derived. Boundary conditions are further developed to improve realism and ensure compliance with existing standards and guidelines. The sensitivity of such changes are assessed and discussed. A discussion on how to use the numerical simulated data to train a machine learning algorithm to estimate well loading is presented and key parameters affecting the cyclic wellhead loads are identified.
The Ocean Cleanup System 001 Performance During Towing and Seakeeping Tests
Authors: Sterenborg Joost , Grasso Nicola , Schouten Rogier , Tjallema Arjen
Published: November 2019

One of the aims of The Ocean Cleanup is to develop technologies to extract plastic pollution from the world’s oceans. Several concepts of passive floating systems were considered that are supposed to confine plastics to ease their collection. Such concepts consist of a floating member and a submerged flexible skirt and have in common that their span is generally more than 500 meters. Consequently, fluid-structure interaction plays an important role in the response of such a floating system. To support numerical simulations, MARIN carried out extensive model tests on a 120 meter system section of the final concept, with focus on the fluid-structure interaction (FSI) of the submerged skirt in operating conditions and in towing configuration. The ability to capture plastics was not investigated in these model tests. Novel for wave-basin tests were non-intrusive measurements using underwater Digital Image Correlation (DIC) to obtain the displacements and deformations of the flexible skirt. DIC proved to be a capable measurement technique for this type of structure in combination with a wave basin. Detailed quantitative data on skirt motions and deformations were delivered and the last concept of the cleanup system was tested in the towing configuration and operational configuration.
Multi-Pronged Approach for the Design of HP/HT Deepwater Steel Catenary Risers
Authors: Bangalore Gurudutt , Cheng Yongming , Banumurthy Surya
Published: November 2019

As the industry plans to move forward with the development of high pressure and high temperature (HP/HT) fields, challenges exist for the design of complaint risers in deepwater applications. The conventional wet tree application risers such as Steel Catenary Risers (SCRs) have proven to be mature technologies in the deepwater applications around the world. HP/HT application leads to many challenges in deepwater risers. This paper investigates ways to overcome the design challenges of deepwater SCRs using a multi-pronged approach.

The paper first addresses the challenge in riser pipe wall thickness. The HP/HT applications require higher pipe wall thicknesses mainly governed by the burst pressure design requirements. The pipe wall thicknesses required for the HP/HT applications will result in very high installation loads, reeling issues with low D/t, fabrication (welding) challenges, high payloads to the host vessel, etc. Any optimization to the wall thickness design in terms of using higher grade material, advancements in welding technology and advantages to be gained in the use of design codes can potentially help in overcoming of these challenges.

The pipe sizes used in the HP/HT applications can pose significant challenges to the touchdown zone (TDZ) of standard steel catenary riser configurations, in terms of experiencing large TDZ compressions and having fatigue life concerns. Use of different SCR problem-solving approaches — TDZ clump weights, Lazy Wave Steel Riser (SLWR) and weight optimized risers will be reviewed in this paper.

Vessel motion characteristics can significantly affect the riser performance. Vessels with improved motions, particularly from the riser response viewpoint, can play a significant role in the selection of optimum riser configurations. This has to be achieved in the early stages of the project. Some of the work done recently in development of the optimized vessel performance particularly in the case of semi-submersibles and their implications on the riser design are studied in this paper.

Design of the risers in HP/HT fields in deepwater applications presents significant challenges and requires a multi-dimensional problem solving approach. An example is provided in this paper for the investigation. A multi-pronged approach has to be adopted for the successful design of riser system for HP/HT deepwater applications.
Concept Design of Very Large Floating Structures and Laboratory-Scale Physical Modelling
Authors: Cappietti Lorenzo , Simonetti Irene , Crema Ilaria
Published: November 2019

The use of Very Large Floating Structures, VLFS, may represent a strategic approach in order to cope with some of the future societal challenges arising from the impressive growth of the world population. In this article, the motivations of this perspective are briefly discussed, the main issues for the development of VLFS are summarized and a concept structural design based on building-blocks technology is proposed. A small-scale physical model was manufactured and tested in the wave-current flume of the Laboratory of Maritime Engineering, LABIMA, of the Florence University, Italy. The aim of this study is the assessment of the structural feasibility and the effectiveness of the proposed VLFS concept, in terms of resistance to wave loads and control of floating behavior. The experimental measurements provide a first contribution to the necessary knowledge, about load magnitudes and floating behavior, for sizing some of the key structural components. The results appear to support the feasibility of the system in terms of usage of structural materials, technical components and building technologies, available at present, that can withstand the measured loads. Moreover, the acquired experimental database is fundamental in order to validate numerical models, in the perspective of using also such tools as complementary methodology for further improvement of the knowledge of design issues.
Recent Developments on Welding, NDT and ECA of Clad and Lined Offshore Pipelines
Authors: Zumpano Petrônio , Garmbis Alexandre G. , Moraes Diogo O. , Hirata Fausto , Cunha Bruno R. M. , Hippert Eduardo , Paes Marcelo T. P.
Published: November 2019

Due to the level of contaminants of Brazilian pre-salt fields, alloy 625, e.g. UNS N06625, clad or lined steel linepipes have been chosen in order to resist such environmental conditions. Recent advances on welding, non-destructive testing (NDT) and Engineering Critical Assessment (ECA) approaches for bimetallic materials have significantly changed since discussed in OMAE-2012. Regarding welding of alloy 625 clad and lined pipes, maximum welding interpass temperature, back purge gas control, root discoloration, visual inspection, root profile for fatigue performance and other issues are discussed herein. The Fatigue Crack Growth Rate (FCGR) to be adopted for alloy 625 is still one of the issues, since curves for alloy 625 in specific environment are normally not available. New appendix C of DNVGL-RP-F108 gives orientations regarding how to conduct tests in sour environment. DNVGL JIP on clad and lined material has made an extensive approaching of the undermatching condition of alloy 625 weld metal under different design strain levels. Regarding NDT, developments and improvements in Automated Ultrasonic Testing (AUT) methods have been obtained for inspection of Corrosion Resistant Alloy (CRA) girth welds and weld overlay. Film and digital radiography are still used for the inspection of the pipe end of the mechanically lined pipe, but ultrasonic solutions are under development. Welding and NDT challenges of alloy 625 reported in 2012 have been overcame by state-of-art technologies used in offshore rigid risers construction and installation. Alternative CRA materials are under research and development, but alloy 625 still is the most reliable option.
Performance of a Passive Tuned Liquid Column Damper for Floating Wind Turbines
Authors: Yu Wei , Lemmer Frank , Cheng Po Wen
Published: November 2019

The motivation of the present paper is to show the proof-of-concept of a passive Tuned Liquid Column Damper (TLCD) for floating wind turbines, which increases the platform pitch damping and power production under wind and wave excitations. As the first step, a reliable TLCD model is implemented and coupled with a reduced order floating wind turbine model. Here, the TLCD is modelled as a second order system which is known for ships, whereas the structural model is a coupled aero-hydro-servo-elastic model with five degrees of freedom. The results show that the TLCD is able to damp the platform resonances but to a limited extent, which is inline the findings of previous research. However, the improved platform pitch stability allows a larger blade pitch control bandwidth, which is normally limited by the underdamped soft support platform. Therefore, by introducing the passive TLCD into the floating wind turbine system, a better power production is achieved.
The Effect of Aspect Ratio on the Drag of Bare Cylinders
Authors: Potts Douglas A. , Binns Jonathan R. , Potts Andrew E. , Marcollo Hayden
Published: November 2019

The drag coefficient for long-slender structures that is typically provided in design guidance has been determined from test specimens of sufficient length that they are unaffected by the aspect ratio (L/D), whereby they are considered to be of “infinite” length. However, it is apparent from the literature that aspect ratio does have a significant non-linear effect at short L/D ratios. DNV provides guidance on the aspect ratio effect on the drag coefficient of a cylinder, for which no experimental source data has been cited. The DNV design guidance has wide usage in the offshore industry and merits critical evaluation. This paper critically reviews the literature and presents the results of a series of tow tank experiments performed by the authors. A series of tow tank tests of a surface-piercing cylinder has been undertaken using a range of aspect ratios as well as testing the effect of various end conditions, where the effects of VIV and ventilation has been deemed insignificant. Correlation of the various data sets of the literature and the experimental test programme provides the basis for developing an alternate design guidance curve for the effect of aspect ratio on the drag coefficient of cylinders.
Evaluations of Ductile and Cleavage Fracture Using Coupled GTN and Beremin Model in API X70 Pipelines Steel
Authors: Jang Youn-Young , Moon Ji-Hee , Huh Nam-Su , Kim Ki-Seok , Cho Woo-Yeon , Lee Myeong-Woo , Kim Yun-Jae
Published: November 2019

This paper is aimed to characterize ductile and cleavage fracture behavior of API X70 pipeline steel and investigate applicability of a micro-damage mechanics model to simulate static and dynamic crack propagation of single-edge notched tension (SENT) and drop-weight tear test (DWTT) specimens, as well as a local approach to describe cleavage fracture behavior. Gurson-Tvergaard-Needleman (GTN) model was applied to simulate ductile fracture behavior of SENT and DWTT specimens, where GTN model has been widely known for well-established model to characterize micro-damage process of void nucleation, growth and coalescence. As for a local approach, Beremin model was considered to estimate probability of cleavage fracture. In this regard, this study was especially focused on abnormal fracture appearance of DWTT specimen.

In the present study, firstly, experiment data from tensile specimen test was used to obtain plastic flow curve (i.e. stress and strain curve). And load-CMOD and J-integral/CTOD resistance curves obtained from SENT test were used to characterize static ductile fracture and calibrate GTN model parameters for X70 pipeline steel. And the calibrated GTN model parameters were verified by comparing experiment data from DWTT test such as load-displacement and crack length-time curves with those from FE analysis. To accommodate dynamic effect on material properties, rate-dependent stress-strain curves were considered in FE analyses. To describe cleavage fracture, the Weibull stress was calculated from FE analyses of DWTT and Weibull parameters were calibrated by comparing with probability distribution of cleavage fracture from experiment data of DWTT specimen. Using Weibull parameters, the whole of cleavage fracture probability can be estimated as ductile shear area of DWTT specimen increases.
Hydrodynamic Investigation of a Novel Concept of OWC Type Wave Energy Converter Device
Authors: Rezanejad Kourosh , Guedes Soares Carlos
Published: November 2019

In the present study, the hydrodynamic performance of a novel and efficient concept of a floating Oscillating Water Column device has been investigated. The new concept consists of two chambers that are placed in the upstream (fore chamber) and in the downstream (rear chamber) with respect to the incident wave direction. The rear chamber acts mainly similar to a Backward Bent Duct Buoy system, while the design of the fore chamber follows conventional types of Oscillating Water Column systems with the harbour plates (bottom plate as well as side plates) elongated outside of the fore chamber. The primary efficiency of the devised concept has been investigated in the frequency domain. In this context, to solve the corresponding diffraction and radiation problems due to the influence of the air pressure inside the chambers as well as motions of the body, an in-house code has been developed in 2D using the Boundary Element Method based on linear wave theory. The obtained numerical results show that the introduced concept has advanced hydrodynamic efficiency in a broad range of waves.
A Critical Examination of the Hysteresis in Wells Turbines Using CFD and Lumped Parameter Models
Authors: Ghisu T. , Cambuli F. , Puddu P. , Virdis I. , Carta M. , Licheri F.
Published: November 2019

The hysteretic behavior of OWC-installed Wells turbines has been known for decades. The common explanation invokes the presence of unsteady aerodynamics due to the continuously varying incidence of the flow on the turbine blades. This phenomenon is neither new nor unique to Wells turbines, as an aerodynamic hysteresis is present in rapidly oscillating airfoils and wings, as well as in different types of turbomachinery, such as wind turbines and helicopter rotors, which share significant similarities with a Wells turbine. An important difference is the non-dimensional frequency: the hysteresis appears in oscillating airfoils only at frequencies orders of magnitude larger than the ones Wells turbines operate at. This work contains a reexamination of the phenomenon, using both CFD and a lumped parameter model, and shows how the aerodynamic hysteresis in Wells turbines is negligible, and how the often measured differences in performance between acceleration and deceleration are caused by the capacitive behavior of the OWC system.
An Iterative Updating Method for Dynamic Responses of a Floating Platform Under Action of Internal Solitary Waves
Authors: Junrong Wang , Junfeng Du , Min Zhang , Anteng Chang
Published: November 2019

The internal solitary waves, have properties of two-way shear profile, significant velocity and acceleration, etc., which threaten the safety of deepwater floating systems in South China Sea (SCS), for the frequent occurrence and the high intensity of internal solitary waves in SCS. In recent years, offshore oil companies of China encountered many strong internal waves during its deepwater oil and gas exploration and development activities in South China Sea. However, the action mechanism of ISWs to floating structures is not understood clearly, and the internal solitary waves are classified as ocean current in API RP 2SK (3rd edition), therefore engineers ignore the velocity variance and long period “wave” characteristics in the design of floating structures. Furthermore, the offshore floating structures is oscillating under the action of environmental forces, due to the horizontal velocity of the platform is comparative to that of ISWs, thus fluid-body coupling is significant that one cannot analyse it by simply adding a specified ISW force time history to the floater. This paper proposes a new iterative updating method for ISW loading calculation considering the fluid-body coupling, and applied this method to a semi MODU, numerical study shows the iteration is efficient and the result is more reasonable compared to conventional method, and it is found that the maximal offset decreases significantly.
Global Assessments of Surface Winds and Waves From an Ensemble Forecast System Using Satellite Data
Authors: Campos Ricardo Martins , Guedes Soares C.
Published: November 2019

This paper evaluates the 10-m wind intensities and significant wave heights from the NCEP Ensemble Forecast System using altimeter data. A total of 20 perturbed members plus a control member (deterministic run) compose the ensemble. The assessment is focused on the comparison between the control run and the ensemble mean, in terms of benefits presented by four error metrics. Four satellite missions are selected for the assessments, obtained from AVISO and NESDIS/NOAA databases. Results show that the scatter components of the errors strongly depends on the latitude, were extra-tropical locations at longer forecast times present large errors. A significant improvement using the ensemble forecast compared to deterministic runs was verified at these locations, where the RMSE of day 10 was reduced from 5 to 3.5 m/s for U10 and from 1.8 to 1.3 meters for Hs.
Direct Time Domain Simulations for a FPSO Tandem Offloading Operation
Authors: Koo Bonjun , Jegannathan Manoj , Kyoung Johyun , Lim Ho-Joon
Published: November 2019

In this study, direct time domain offloading simulations are conducted without condensing the metocean data using High Performance Computing (HPC). With rapidly growing computing power, from increased CPU speeds and parallel processing capability, the direct time domain simulation for offloading analyses has become a practical option. For instance, 3-hour time domain simulations, covering the entire service life (e.g. 100,000 simulations for 35 years) of a floating platform, can now be conducted within a day. The simulation results provide realistic offloading operational time windows which consider both offloading operation sequence (i.e. berthing, connection, offloading duration and disconnection) and required criteria (i.e. relative responses, loads on hawser and flow line, etc.). The direct time domain offloading analyses improve the prediction of offloading operability, the sizing of the FPSO tank capacity, and the shuttle tanker selection. In addition, this method enables accurate evaluations of the economic feasibility for field development using FPSOs.
System Identification of Abkowitz Model for Ship Maneuvering Motion Based on ε-Support Vector Regression
Authors: Liu B. , Jin Y. , Magee A. R. , Yiew L. J. , Zhang S.
Published: November 2019

System identification is crucial to predict the maneuverability of the ship. In this work, ε-support vector regression (ε-SVR) is implemented to identify hydrodynamic derivatives of Abkowitz maneuver model. A proposed technique, batch learning, is implemented with the addition of Gaussian white noise to reconstruct the samples and alleviate the parameter drift in the system identification of the ship maneuvering model. The predicted results are compared with results obtained from Planar Motion Mechanism (PMM) test. Standard maneuvers, 35° turning circle, 10°/10° and 20°/20° zigzags, are simulated and compared with the predicted model by ε-SVR. The presented results show that the proposed batch learning technique with Gaussian white noise is an effective technique, which improves the accuracy and robustness of ε-SVR in system identification. The results obtained from the predicted model match well with the those obtained from PMM results, which shows its excellent generalization performance. The developed model is applied to understand control requirements for vessels under different conditions.
Rheological Properties of Water-Based Drilling Fluids in Deep Offshore Conditions
Authors: Ding Qian , Sun Baojiang , Wang Zhiyuan , Gao Yonghai , Gao Yu , Liao Yongqiang , Wang Di , Xia Andi
Published: November 2019

In deep-water drilling, the drilling fluid is affected by the alternating temperature field derived from the low temperature of the seawater and the high temperature of the formation. The complicated wellbore temperature and pressure environments make the prediction of rheological properties of the drilling fluid difficult.

In this study, the rheological properties of water-based drilling fluid in full temperature and pressure range of deep-water conditions were tested from 2 to 150 °C (35.6 to 302 °F) and 0.1 to 70 MPa (14.5 to 10000psi). The experiment was carried out by the OFI130-77 high temperature and high pressure rheometer. The experimental data were processed by multiple regression analysis method, and the mathematical model for predicting the apparent viscosity, plastic viscosity and yield point of water-based drilling fluid under high temperature and high pressure conditions was established.

The experimental results show that when the temperature is lower than 65 °C (149 °F), the apparent viscosity and plastic viscosity of the water-based drilling fluid decrease significantly with increasing temperature. When the temperature is higher than 65 °C (149 °F), the apparent viscosity and plastic viscosity decrease slowly. Under low temperature conditions, the effect of pressure on the apparent viscosity and plastic viscosity of water-based drilling fluids is relatively significant. The calculated values of the prediction model have a good agreement with the experimental measurements. Compared with the traditional model, this prediction model has a significant improvement in the prediction accuracy in the low temperature section, which can provide a calculation basis for on-site application of deepwater drilling fluid.
Effect of Rock on Cement Sheath Integrity: Shale vs. Sandstone
Authors: Skorpa Ragnhild , Werner Benjamin , Vrålstad Torbjørn
Published: November 2019

It is crucial to understand cement sheath degradation mechanisms, since the cement sheath is an important well barrier element. Repeated pressure cycling is known to cause radial cracks and microannuli in the cement sheath, and the stiffness of the surrounding rock determines how much pressure the cement withstands before failure. However, experimental data on the effect of surrounding rock (shale vs. sandstone) on cement sheath integrity are scarce.

In this paper we present experimental studies on how different surrounding rocks influence cement sheath integrity. We have used our unique downscaled experimental set-up to perform pressure cycling tests with both shale and sandstones, where cement sheath integrity is visualized in 3D by X-ray computer tomography (CT). The obtained results confirm that a cement sheath surrounded by a rock with a relative higher Young’s modulus can withstand higher casing pressure compared to a cement sheath surrounded by rock with relative lower Young modulus. All cracks were initially observed as small defects in the cement sheath prior to expanding to full radial cracks and propagation into the surrounding formation.
Stability Analysis of Free Hanging Riser Conveying Fluid for Ocean Thermal Energy Conversion (OTEC) Utilization
Authors: Adiputra Ristiyanto , Utsunomiya Tomoaki
Published: November 2019

This paper examines the stability of a riser conveying fluid for Ocean Thermal Energy Conversion (OTEC) application by analytical and numerical approaches. Initially, the analytical solution for free hanging riser with fixed end connection is governed by including the components of the riser dynamics. Considering the boundary conditions, the general solution can be obtained using power series expansion. The imaginary and real parts of the solutions are plotted in an Argand diagram which is then used to predict the occurrence of instability. To verify the analytical solution, the dynamic behavior observed in the analytical solution is compared with the one predicted using numerical analysis. The numerical analysis here refers to a coupled analysis between riser and the fluid. After being verified, the analytical model is used to determine the acceptance of the proposed riser designs for 100 MW-Net OTEC power plant. In this paper, there are three materials being investigated which are steel, aluminum and FRP. For the given conditions, the results show that the critical velocity for steel-made riser is only about 0.9 m/s, for aluminum-made riser is about 1.7 m/s and the critical velocity for riser made of FRP is about 2.1 m/s.
FEA Based Simplified Integrated Analysis for Mudmat Design
Authors: Srigiriraju Srikanth , Chakraborty Arindam , Ozturk Burak , Rathore Devvrat
Published: November 2019

The objective of this study is to explore the opportunity to improve the design and sizing of mudmat for subsea structures, such as Pipeline End Termination (PLET). This is done by comparing the traditional approach following the limit equilibrium methods in API RP 2GEO with a more rigorous simplified integrated analysis approach that involves a single finite element analysis (FEA) model that includes both the pipeline and jumpers together along with the soil-mudmat interaction modeled as non-linear springs, and to quantify any conservatism inherent in the traditional approach.

A mudmat design with aspect ratio of 1:2 was considered for detailed analysis. Initially, jumper and pipeline loads were determined by imposing artificial boundary conditions at the hubs and end terminals. Using analytical methods and considering a total dead (submerged) weight of the mudmat and superstructure, a mudmat size was determined per the American Petroleum Institute (API) approach. Factor of Safety (FOS) for bearing and sliding loads were also determined. Thereon, using this mudmat size, the FOS for bearing and sliding were determined using the simplified integrated approach with nonlinear springs representing soil-mudmat interactions.

The FOS values using the simplified approach were observed to be higher than those obtained using the traditional approach. This provides an opportunity for a “leaner” design, especially as new high pressure, high temperature (HPHT) fields are made feasible where the mudmat size, if designed with conservatism in API RP 2GEO, may be impractically large for installation.
Comparative Study on Fatigue Damage Assessment of a Structure Member in a Bulk Carrier Using Various Environmental Conditions
Authors: Prasetyo Fredhi Agung , Osawa Naoki , Kurniawan Mohammad Arif , Komariyah Siti
Published: November 2019

Specific design life could be identified by using fatigue damage assessment in the structure engineering field as well as in the maritime sector. Fatigue assessment is one of the assessments to be conducted during review of ship structure design. Fatigue assessment of ship structural member is mainly conducted based on specific environmental condition. In general, specific environmental condition, which is provided by Classification Society rules, is a long term sea-state data of North Atlantic Ocean. The wave scatter diagram presents the tabulation of a long term data of sea state history in the specific ocean. Therefore, a realistic encounter of wave scatter diagram is essential to simulate the variation of wave loadings applied on the ship structure in determination of fatigue design life.

Since the application of North Atlantic ocean environmental condition is commonly used by major Classification societies, this condition might give the substantial deterioration on the fatigue design life of the ship that specially operate only in specific ocean area, i.e. South East Asia area. In this work, the wave scatter diagram of various environmental conditions is chosen and the statistical characteristic is compared. The wave load sequence that is used on the fatigue damage assessment are generated by using the concept of storm model, so that the changing nature of sea state could be emulated as in real ocean. Fatigue damage of a structure member of 220 meter Bulk Carriers is calculated based on various environmental conditions.
Hydrodynamic Load Modeling for Offshore Free-Floating Macroalgal Aquaculture Under Extreme Environmental Conditions
Authors: Chen Ming , Yim Solomon C. , Cox Daniel , Wang Taiping , Huesemann Michael , Yang Zhaoqing , Mumford Thomas , Wood Geoffrey
Published: November 2019

This article describes a preliminary study of an on-going ARPA-E (Advanced Research Projects Agency-Energy) MARINER Phase I project. The hydrodynamic load and dynamic response of an innovative offshore macroalgae cultivation system, Nautical Offshore Macroalgal Autonomous Device (NOMAD), under extreme environmental conditions is examined. The high strength, extremely durable, recyclable carbon fiber (rCF) free-floating long-line is applied with polyculture (Nereocystis luetkeana (bull kelp) and Saccharina latissima (sugar kelp)) in the NOMAD system. This novel macroalgal farming system is designed to free float from Washington State to California along the west coast of the US to avoid anchoring costs and the failure of earlier offshore growth trials. In this study, we expect to identify possible failure modes for the preliminarily design of NOMAD free-floating long-line macroalgal farming system based on the preliminary numerical predictions. We developed a 1km system-scale NOMAD free-floating long-line numerical model and performed a dynamic response analysis on the long-line to determine the behaviors of the long-line under extreme environmental conditions. The 1km free-floating rCF long-line responses very flexible due to wave and current activities even for large bending stiffness. Therefore, the potential entanglement of free-floating long-line on a global scale may cause the system failure even when the tensions and bending moments are in the safe range. Three cases include 10m NOMAD free-floating long-line with sugar kelp, bull kelp, and polyculture numerical models are developed, and the simulation results are analyzed. The tensions at the holdfast of the kelps in these cases are found to be below the breakage limit approximately. However, the severe clumping of the kelps and potential entanglement of adjacent lines may result in damage to the farming system.