bluuebunny/crossref_metadata_2025 huggingface dataset:
End Fitting Effect on Stress Evaluation of Tensile Armors in Unbonded Flexible Pipes Under Axial Tension
Authors: Dong Leilei , Zhang Qi , Huang Yi , Liu Gang , Li Zhiyuan
Published: October 2017

This paper deals with the effect of termination restraint due to end fitting on the stress evaluation of tensile armors in unbonded flexible pipes under axial tension. The problem is characterized by one single armoring tendon helically wound on a cylindrical supporting surface subjected to traction. The deviation from the initial helical angle is taken to describe the armor wire path as the pipe is stretched. The integral of this angle change gives lateral displacement of the wire, which is determined by minimization of the energy functional consists of the strain energy due to axial strain, local bending and torsion, and the energy dissipated by friction, leading to a variational problem with a variable endpoint. The governing differential equation of the wire lateral displacement, together with the supplementary condition, is derived using the variational method and solved analytically. The developed model is validated with a finite element simulation. Comparisons between the model predictions and the finite element results in terms of the change in helical angle and transverse bending stress show good correlations. The validated model is then applied to study the effects of imposed tension and friction coefficient on the maximum bending stress. The results show that the response to tension is linear and friction could significantly increase the stress at the end fitting compared with the frictionless case.
Impact Tests in an Air-Water Mixture
Authors: El Malki Alaoui Aboulghit
Published: October 2017

Experimental impact tests were performed using a shock machine and aerated water by means of an air-bubble generator. High speed shock test machine allows carrying out tests of impact on water (slamming). This machine permits to stabilise velocity with a maximal error equal to 10% during slamming tests. The air volume fraction in the bubble was measured by optical probe technique. The present work is aimed at quantifying the effects of the aeration on the hydrodynamic loads and pressures during the entry of a rigid body at constant speed in an air-water mixture. The impact tests were conducted with a rigid pyramid for an impact velocity equal to 15 m.s−1 and for two average void fractions, 0,46% and 0,84%. The reduction of the impact force and pressure due to aeration has been confirmed by these experiments.
Investigations on Oilwell Cement Strength Response to Ultrasonic Measurements in Presence of Additives
Authors: Romanowski Niklas , Ichim Adonis , Teodoriu Catalin
Published: October 2017

Well cements are an important aspect of wellbore integrity and recent investigations focus on describing the cement lifetime using, when possible, non-destructive tests like ultrasonic measurements. However, the original API and ASTM testing standards were based on destructive mechanical testing of cements, leading to the decision to investigate the backward and forward compatibility between ultrasonic measurements and mechanical testing, which makes the subject of this work.

Ultrasonic cement measurement became a very popular method to assess the mechanical properties of the cement in a non-destructive manner. Since various measurement systems exist on the market, the development of an accurate reference data base that can be used to calibrate such measurements becomes very important. Two major systems have therefore been compared: the ultrasonic compressive strength, using the ultrasonic pulse velocity (UPV) principle, and the unconfined compressive strength (UCS), using the standard testing frame according to API and ASTM standards. The tests have been performed at different curing times, using both devices, on class G API cements with bentonite and other additives.

This paper presents the results of over two hundred experiments that have displayed a different UPV response as a function of the additive content. These allow us to expand the existing UPV vs. UCS correlations. Moreover, it was observed that after a given curing time, depending on the additive and its concentration, the UPV response is not as sensitive as the results yielded by the UCS method. The outcomes are an important step forward to improve and understand the wellbore integrity.
Evaluation of the Effect of Different Mean Stress Levels on the Fatigue Resistance of OCTG Premium Threaded Connections
Authors: Ando Yoshinori , Oku Yosuke , Sugino Masaaki , Johnston Carol
Published: October 2017

Identification of the effect of mean stress for fatigue performance of the premium threaded connection for the OCTG pipes, was conducted via full-scale test. API standard material [grade L80-1] was used for the test. The nominal pipe outside diameter and wall thickness were 244.48 mm (9-5/8 inch) and 11.99 mm (0.472 inch), respectively. The fatigue life of the specimens tested with no mean stress was longer than that of the specimens tested with a tensile mean stress. Through-wall cracks were found at the imperfect thread area of the male embodiment, but the crack initiation site depended on the mean stress. However, the primary root cause of the failure on both mean stress levels can be regarded as the fretting fatigue. Fatigue life was also able to be estimated using modified Goodman relation.
Improved Laboratory Set-Up for Pressurized and Confined Cement Sheath Integrity Tests
Authors: Taghipour Ali , Opedal Nils , Skorpa Ragnhild , Vrålstad Torbjørn
Published: October 2017

The annular cement sheath is one of the most important well barrier elements, both during production and after well abandonment. It is however well-known that repeated pressure and temperature variations in the wellbore during production and injection can have a detrimental effect on the integrity of the cement sheath. Degradation of cement sheaths result in formation of cracks and microannuli, which leads to loss of zonal isolation and subsequent pressure build-up in the annulus.

A unique laboratory set-up with downscaled samples of rock, cement and pipe has been constructed to study cement sheath failure mechanisms such as debonding and crack formation during thermal cycling. Cement integrity before and after thermal cycling is visualized in 3D by X-ray computed tomography (CT). However, this previous set-up had some significant limitations such as lack of direct physical confinement around the rock. This lack of direct confinement created unrealistic outer boundary conditions around the rocks during experiments as opposed to field conditions, which influenced the obtained experimental results.

This paper describes in detail an improved version of this laboratory set-up, where the set-up has been redesigned to include direct physical confinement around the rock as well as improved cement placement, while retaining all the advantages of the previous set-up.
Supporting the Engineering Analysis of Offshore Wind Turbines Through Advanced Soil-Structure 3D Modelling
Authors: Corciulo Simone , Zanoli Omar , Pisanò Federico
Published: October 2017

Monopiles are at present the most widespread foundation type for offshore wind turbines (OWTs), due to their simplicity and economic convenience. The current trend towards increasingly powerful OWTs in deeper waters is challenging the existing procedures for geotechnical design, requiring accurate assessment of transient soil-monopile interaction and, specifically, of the associated modal frequencies.

In this work, advanced 3D finite element (FE) modelling is applied to the dynamic analysis of soil-monopile-OWT systems under environmental service loads. Numerical results are presented to point out the interplay of soil non-linearity and cyclic hydro-mechanical (HM) coupling, and its impact on transient response of the system at increasing load magnitude. It is shown how the lesson learned from advanced modelling may directly inspire simplified, yet effective, spring models for the engineering dynamic analysis of OWTs.
Random Wave-Induced Burial and Scour of Short Cylinders and Truncated Cones on Mild Slopes
Authors: Myrhaug Dag , Ong Muk Chen
Published: October 2017

A stochastic approach calculating the random wave-induced burial and scour depth of short cylinders and truncated cones on mild slopes is provided. It assumes the waves to be a stationary narrow-band random process and a wave height distribution for mild slopes is adopted, also using formulae for the burial and scour depths for regular waves on horizontal beds for short cylinders and for truncated cones. Examples of results are also provided.
Value of Information-Based Inspection Planning for Offshore Structures
Authors: Agusta Arifian , Thöns Sebastian , Leira Bernt J.
Published: October 2017

Asset integrity and management is an important part of the oil and gas industry especially for existing offshore structures. With declining oil price, the production rate is an important factor to be maintained that makes integrity of the structures one of the main concerns. Reliability based and risk-based inspection (RRBI) constitutes an efficient method to optimize inspection planning. Basing the inspection planning on pre-posterior Bayesian decision analysis and especially a Value of Information analysis allows to explicitly quantify the expected benefits, costs and risks associated with each inspection strategy. A simplified and generic risk-based inspection planning utilizing pre-posterior Bayesian decision analysis had been proposed by Faber et al. [1] and Straub [2]. This paper provides considerations on the theoretical background and a Value of Information analysis-based inspection planning. The paper will start out with a review of the state-of-art RBI planning procedure based on Bayesian decision theory and its application in offshore structure integrity management. An example of the Value of Information approach is illustrated and it is pointed to further research challenges.
Wet Vibration of Low-Aspect-Ratio Aerofoil Wing: Semi-Analytical and Numerical Approach With Experimental Investigation
Authors: Datta N. , Kannamwar A. N. , Verma Y.
Published: October 2017

The free wet vibration characteristics of an idealized low-aspect-ratio cantilever wing are studied semi-analytically, numerically, and experimentally. The wing is modeled as a tapered, hollow Kirchhoff’s plate, with the chord-wise section as a symmetrical NACA0018 aerofoil. The chord length tapers from root to tip, over the span. The main aim is to set up a suitable radiation boundary problem for the vibrating cantilever wing, in order to semi-analytically generate the wet frequencies. The efficacy of the semi-analytical wet vibration approach is studied by comparing it with the other two approaches. The difficulties encountered are due to the hollow two-way tapered shape of the wing and the free edge boundary conditions on its three sides. The semi-analytical approach is based on Galerkin’s method, which includes the modal superposition of two orthogonal beam modeshapes (Free-Free beam in chord-wise and cantilever in span-wise directions). The plate modeshapes thus generated are further used in the 3D source distribution technique to calculate the fluid inertia, leading to a consistent drop in the natural frequencies. The cantilever wing has been fabricated and tested in-house. The underwater impact hammer test generates the wet natural frequencies. The free vibration frequencies are also verified numerically using ANSYS, and compared with experimental studies.
Finite Element Methods for the Structural Analysis of Tension Leg Platforms for Floating Wind Turbines
Authors: Starr Markus S. , Manjock Andreas , Arjes Christian , Nguyen Ngoc-Do , Faber Torsten
Published: October 2017

This paper represents the condensed version of an original Master Thesis written in 2016 [1]. The main aim lays in the structural analysis of Tension Leg Platforms (TLP) as support structures for Floating Wind Turbines (FWT). Special focus is given to a methodology study of different static analysis methods for components of a FWT support structure by Finite Element Method (FEM). The analysis considers Ultimate Limit States (ULS) only. Based on a given open source TLP design [2], the support structure is fragmented in its components. Each component is categorized, based on whether it can be analyzed by parametric equations or by computational methods. For the components, verifiable by parametric equations, the relevant standards are examined. The analysis of the decomposed support structure shows that existing standards can be applied for many of the components. One specific tubular joint of the support structure is selected and analyzed by computational methods. Considering the complexity of Finite Element Analysis (FEA) for FWT, the performed methodology study compares and evaluates various simulation methods on how loads are applied onto the structure. The methodology study results show that for ULS consideration, all load driven simulation methods are generally applicable. In the prospect of fatigue analyses further developments and improvements are considered crucial. This especially includes a better coherence of the stiffness representation between global load simulation model and local structural analysis model.
A Simplified Temperature Prediction of Circulation and Simulator Development Under Steady-State Heat Transfer in the Deepwater Wellbore
Authors: Feng Ming , Teodoriu Catalin
Published: October 2017

One of the critical factors in the deepwater exploration and development is the temperature during the drilling process. Temperature in the wellbore will induce the wellbore instability, which means the bottom hole pressure will change according to the accumulated thermal effects on the density, viscosity of the fluid during the circulation. Unpredicted bottom hole pressure will lead to unknown well control risks, which make the deepwater drilling is in great danger and unacceptable for the engineers.

In the casing design, the thermal effects were ignored in the past but with drilling deeper subsea wells, the thermal effects will exist inevitably. The extreme values observed at the seafloor could be as low as 40°F and as high as 150∼200°F at the wellbore annulus. Unfortunately, deepwater environments combine low temperatures, high pressures, gas and water that can induce hydrate formation. Hydrates can lead to drillpipe blockages and affect BOP operation. The low temperatures due to the hydrate or ocean current affect the properties of cement, which indicates to redesign cement slurry composition required.

Except the effects on the density and viscosity of fluid, the rising temperature in the annulus between the drill pipe and casing will make the trapped pressure increase in the annulus between outside casings. If the annulus build-up pressure can’t be released without rapture disks, otherwise the trapped pressure is beyond the initial design pressure, and the potential damage will happen.

This paper provides a simplified method to predict the circulation temperature under steady-state heat transfer in the deepwater rissssserswellbore, and a simulator is developed.
Estimating Zonal Flow Contributions in Deep Water Assets From Pressure and Temperature Data
Authors: Hasan A. Rashid , Sohel Rayhana N. , Wang Xiaowei
Published: October 2017

Producing hydrocarbon from deep water assets is extremely challenging and expensive. A good estimate of rates from multiple pay zones is essential for well monitoring, surveillance, and workover decisions. Such information can be gleaned from flowing fluid pressure and temperature; deep-water wells are often well instrumented that offers such data on a continuous basis. In this study a model is presented that estimates zonal flow contributions based on energy and momentum balances. Kinetic and heat energy coming from the reservoir fluid to the production tubing is accounted for in the model. The momentum balance for wellbore takes into account differing flow profile in laminar and turbulent flows.

In addition, when sandface temperature data are not available, a recently developed analytical model to estimate the effect of Joule-Thompson expansion on sandface temperature was used to estimate sandface temperature from reservoir temperature. The model developed can be applied to any reservoir with multiple pay zones and is especially useful for deep-water assets where production logging is practically impossible. Available field data for multiphase flow was used to validate the model. Sensitivity analyses were performed that showed accurate temperature data is essential for the model to estimate zonal contribution accurately.
Wave Drift Forces and Low Frequency Damping on the Exwave FPSO
Authors: Fonseca Nuno , Stansberg Carl Trygve
Published: October 2017

A method is followed in the present analysis to estimate realistic surge and sway wave drift force coefficients for the Exwave FPSO. Model test data is used to identify the difference frequency wave exciting force coefficients based on a second order signal analysis technique. First, the slowly varying excitation is estimated from the relationship between the incoming wave and the low frequency motion using a linear oscillator. Then, the full QTF of the difference frequency wave exciting forces is defined from the relationship between the incoming waves and the second order force response. The process identifies also the linearized low frequency damping.

The paper presents results from a few cases selected from the Exwave JIP test matrix. Empirical mean wave drift coefficients are compared to potential flow predictions. It is shown that the latter underestimate the wave drift forces, especially at the lower frequency range where severe seastates have most of the energy. The sources for the discrepancies are discussed.
Local Design Pressures During Ship Ram Events Modeling the Occurrence and Intensity of High Pressure Zones
Authors: Ralph Freeman , Jordaan Ian
Published: October 2017

During ship ice interaction events (i.e. rams with multi-year ice), the occurrence of local pressures vary in time and space. A link between local pressures and global forces is the sum of the local forces from n High Pressure Zones (HPZs) across the interaction face equals the total force transmitted into the structure.

In this paper, a model for HPZ density, and force during ship ram events is presented. The occurrence and intensity of HPZs on panel areas were simulated using a Poisson process and an exponential distribution for HPZ force. The model is extended to consider HPZ occurrence in time through a ramming event, modeling HPZ rate. Such a model allows the designer to determine baseline ‘parent’ local pressure design parameters based on vessel size and expected operational speed. The faster a ship operates through an ice regime, the greater the HPZ rate. Larger and faster ships will penetrate further, having longer interaction durations and hence a greater number of HPZs forming (unless, for example, the ship passes through a ridge). Rates too will vary along the vessel being greater on the bow and least from mid-body to stern. For design, we are interested in the maximum local pressure on a single panel area through the ram duration.

The results are compared with previous local pressure models for design.
Wave-Current Interaction Effects on Airgap Calculations
Authors: Hermundstad Elin Marita , Hoff Jan Roger , Fonseca Nuno , Bjørkli Rune
Published: October 2017

The importance of wave-current interaction effects on the determination of mean drift forces on floating offshore structures is well documented. Wave-current interaction effects will also influence the first-order motions and loads as well as the diffracted and radiated waves around the structure. One of the significant contributions to the influence of wave-current interaction effects on the motion responses is the additional coupling between motion modes due to the current. These effects are well known from seakeeping calculations of ships with forward speed. A structure with fore-aft symmetry will have no hydrodynamic coupling between heave and pitch in regular waves only. Due to the presence of a current, the symmetry of the flow around the body is lost, resulting in hydrodynamic coupling between the modes. This will also occur for a moored structure with slowly varying motions in the horizontal plane. The most important couplings are from the heave motion into pitch and surge and from heave to roll and sway. These couplings are otherwise present only for asymmetric structures. Due to the presence of the heave resonance and cancellation periods, the motion responses in roll and pitch for a semi-submersible will be influenced by the wave-current interaction effects. Due to the differences in phase between the different motion modes, the hydrodynamic coupling may have significant influence on the rotational motions roll and pitch and thus significant influence on the prediction of airgap. This coupling between the heave and roll/pitch modes due to the current adds complexity to the numerical simulations since the structure responses are more sensitive to the actual orientation of the structure, mooring configuration etc.

A three-dimensional linear potential flow code, MULDIF, has been developed by SINTEF Ocean. This code accounts for hydrodynamic interaction between waves and current from arbitrary directions. The code can be applied to single or multiple bodies in infinite or finite water depth. Verification studies have previously shown good agreement with other numerical codes, Hermundstad et.al. [1], Zhiyuan et.al [2].

Validation studies with emphasis on airgap and comparison with experimental results are presented and numerical results for airgap and upwell are visualized and discussed. It is demonstrated how MULDIF can be used in airgap studies.
Bending Mechanics of Cable Cores and Fillers in a Dynamic Submarine Cable
Authors: Tjahjanto Denny D. , Tyrberg Andreas , Mullins Jonathan
Published: October 2017

Dynamic submarine cables are used to provide electrical power to floating oil/gas production platforms and to export power from marine renewable energy systems such as floating wind turbines. During installation and life-time operation, the dynamic power cable is subjected to various loads, e.g. axial tension, bending and torsion. A typical three-phase AC cable consist of three helically-interwound power cores with three polymeric fillers to accomplish a circular circumference over which sheaths and tensile armour layers are applied. The present paper focuses on the mechanical behaviour of a dynamic submarine cable subjected to a combined axial tension, radial pressure and bending load. Particular interest is emphasized on the stick-slip behaviour of the helically-interwound power cores. 3D finite element (FE) model has been developed, where the interactions on all layers of the cable are extrinsically taken into account. The stick-slip behaviour of the power cores and the resulting friction stress induced during cable bending can have a large impact on the fatigue life of the dynamic cable. Kinematics of slips between the cable core components resulting from FE simulations will be analyzed and compared to the assumption of fully loxodromic slips typically used in the formulation of analytical model for helically-wound cables. The output of the present analyses will provide a better understanding on the mechanical behaviour of the power cores of dynamic submarine power cables in response to cable bending and provide a basis to verify and develop analytical models that can be used for, e.g. fatigue life assessments.
Numerical Modelling and Analysis of a Hybrid-Spar Floating Wind Turbine
Authors: Utsunomiya Tomoaki , Sato Iku , Kobayashi Osamu , Shiraishi Takashi , Harada Takashi
Published: October 2017

In this paper, numerical modelling and analysis of a hybrid-spar floating wind turbine is presented. The hybrid-spar consists of steel at the upper part and the precast prestressed concrete (PC) at the lower part. Such a configuration is referred to as a hybrid-spar in this paper. The hybrid spar was successfully installed offshore of Kabashima Island, Goto city, Nagasaki prefecture, Japan on October 18, 2013 (see OMAE2015-41544 [1] for details). In this paper, some details on numerical modelling of the hybrid-spar for design load analysis are presented. Then, the validation of the numerical analysis model is presented for a full-scale hybrid-spar model with 2-MW wind turbine.
Development of a Novel Floater to Power Take-Off Connection for Wave Energy Converters Based on a Belt-Pulley System
Authors: Kusch Robin , Rahmati Mohammad , Peckolt Jan P. , Pütz Jan , Schay Julius
Published: October 2017

While renewable energy is generally considered to be a well-researched field, wave energy converters (WECs) are still in early industrial stages, for example due to high costs, even though the potential of WECs in countries such as the UK is very high. Apart from the power plant location, the amount of power generated by a wave energy converter is also highly influenced by the efficiency of both the energy transfer from the wave to the plant’s generator and the power take-off (PTO) itself. Improving on any of these aspects therefore increases the power output and economic attractiveness. Based on a commercial development project by the NEMOS GmbH in Germany, this paper presents a more efficient means of connecting a floater and a rotary PTO based on a free traction mechanism consisting of a custom belt and matching pulley. In addition to regular longitudinal forces, the belt system can transfer transversal forces of up to 14 % of its pulling force and allowing run-in angles up to 8°. First tests show an average efficiency to 99.6 % in wet conditions. The paper lays out the theoretical background of the new design and discusses existing alternatives, before detailing the taken approach to design and optimization. The results are validated and compared to an existing rope design and a benchmark flat belt.
Study on Influence of Vortex Induced Loads on the Motion of Spar-Type Wind Turbine Based on Aero-Hydro-Vortex-Mooring Coupled Model
Authors: Tang Yougang , Li Yan , Liu Liqin , Jin Weichen , Qu Xiaoqi
Published: October 2017

The coupled nonlinear motion and the influence of vortex induced loads on the motion of Spar type FOWT is studied based on the aero-hydro-vortex-mooring coupled model. The first order and second order difference-frequency wave loads are calculated based on 3D potential theory; the aerodynamic loads on the rotor are calculated based on blade element momentum theory; the vortex induced loads are simulated with CFD approach; the mooring forces are solved by the catenary theory and the nonlinearity of restoring forces are also considered. The nonlinear coupled model is set up and a numerical code is developed for solving the motion of OC3-Hywind Spar-type FOWT in time domain. The responses of FOWT are calculated under different load cases. It is shown that the amplitudes of sway and roll are excited by vortex excited force, and the roll amplitudes can reach to pitch value order in some circumstances. Due to the coupling effects, the heave motion is also influenced by vortex-induced forces. When vortex-shedding frequency is close to roll natural frequency, not only the roll but also the motions in other DOFs are increased and the super-harmonic resonance response occurs in heave motion.
New Combined CFD and Model Testing Technique for Identification of Wave Impact Loads on a Semisubmersible
Authors: Pakozdi Csaba , Östman Anders , Abrahamsen Bjørn C. , Økland Ole D. , Vestbøstad Tone M. , Lian Gunnar , Stansberg Carl T.
Published: October 2017

A procedure is presented describing how to estimate realistic loads using combined numerical and model test data. Measured platform motions are imposed on the structure during the CFD analysis. The combination of the wave matching procedure with the imposed measured platform motion gives a very good numerical reproduction of the observed extreme event. The numerical reproduction of model test events provide all necessary information on the hydrodynamic loads for further structure analysis. This represents an improvement in industry design applications.

Imposing the measured motion from regular wave model test into CFD simulation is validated by comparison of relative wave height time series. This comparison shows a very good agreement between the measured and the simulated time series. Existing model test data from irregular model test and CFD generated numerical wave are compared. A wave matching procedure has been developed, which shows very promising results with respect to reproducing critical hydrodynamic conditions observed during the model tests. This paper presents a case study how CFD can be used to enhance model test data in an efficient way to provide the critical hydrodynamic conditions for structure analysis. Comparison of the measured free surface elevation of the calibrated waves with the time series of the numerical waves, as well as the measured and simulated relative wave probes time series and the slamming load time series show that the applied numerical wave events show similar physical conditions as those observed in the model test. The effect of the platform motion on the impact force is identified by comparison of the impact force time series of the simulation with and without platform motion against model test time series. The results demonstrate that the approach provides a clear improvement compared to numerical or model testing alone. The observed steep wave events are numerically reproduced in a simplified manner, instead of trying to reproduce measured events directly. This approach significantly reduces the computational time, as well as computational costs, to an industrially acceptable level. Traditional load estimation is not able to provide such reliable detailed local load history for structural design purpose at areas exposed to wave impacts. This new procedure, where CFD simulates realistic breaking waves with coupling to measured vessel motion, offers new possibilities for the design of structures subject to risk of wave impact loading.
Technical Challenge on VLFS in Japan After Mega-Float Project
Authors: Suzuki Hideyuki , Harada Hidetoshi , Natsume Takumi , Maeda Katsuya , Iijima Kazuhiro , Hayashi Tatsuya
Published: October 2017

Intensive research and development of Mega-float was implemented by Technological Research Association of Mega-Float (TRAM) in late 1990s. Mega-float is a concept of a pontoon type VLFS installed in protected calm water typically large bay, and the concept was promoted by TRAM after relatively long concept development phase in Japan.

This paper reviews the research and development of VLFS in Japan. The research and development of Mega-float is briefly reviewed firstly and two research and developments after Mega-float are reviewed.

Concerning development of Oil-Gas development, logistic hub concept has been developed by J-DeEP, a consortium comprised of major Japanese heavy industries, shipping company, ClassNK and NMRI, and proposed for presalt oil field development offshore Brazil.

Another research is pontoon type coal storage and offloading facility to be installed offshore South Sumatra, Indonesia. The coal storage and offloading facility was investigated by JIP comprised of JMU, ClassNK, Osaka University and University of Tokyo.
Wave Energy Prize Experimental Sea State Selection
Authors: Bull Diana , Dallman Ann
Published: October 2017

A detailed methodology was used to select the sea states tested in the final stage of the Wave Energy Prize (WEPrize), a public prize challenge sponsored by the U.S. Department of Energy [1]. The winner was selected based on two metrics: a threshold value expressing the benefit to effort ratio (ACE metric) and a second metric which included hydrodynamic performance-related quantities (HPQ). HPQ required additional sea states to query aspects of the techno-economic performance not addressed by ACE. Due to the nature of the WEPrize, limited time was allotted to each contestant for testing and thus a limitation on the total sea states was required. However, the applicability of these sea states was required to encompass seven deployment locations representative of the United States West Coast and Hawaii. A cluster analysis was applied to scatter diagrams in order to determine a subset of sea states that could be scaled to find the average annual power flux at each wave climate for the ACE metric. Four additional sea states were selected, including two highly energetic sea states and two bimodal sea states, to evaluate HPQ. These sea states offer a common experimental testing platform for performance in United States deployment climates.
Hydroelastic Responses of VLFS Deployed Near Islands and Reefs
Authors: Wu Yousheng , Ding Jun , Li Zhiwei , Ni Xinyun , Wu Xiaofeng , Tian Chao
Published: October 2017

The rapid expansion of world population, the exhausting of inland resources and the requirement of sustainable development of world economy have strengthened the efforts of mankind to increase the capability of resource exploitation and space utilization in the ocean. Very Large Floating Structures (VLFS) are among those marine structures that have attracted long lasting attention in ocean utilization for several decades. The applications of different size VLFS as floating piers, floating airports, floating hotels, floating fuel facilities and even floating cities have triggered extensive researches. Several projects including the conceptual design and construction of VLFS have been launched, for instance, Mega-Float in Tokyo Bay, Floating oil storage bases in Kamigoto and Shirashima islands, Floating emergency rescue bases in Yokohoma, Floating performance stage in Singapore, Large floating bridge in Norway, Mobile offshore base (MOB) in USA and Multi-purpose floating base near islands in China.

Due to much larger dimensions, relatively smaller global rigidities and lower natural frequencies than an ordinary ship, a VLFS has apparent flexible body responses rather than rigid body motions in waves. Hence hydroelastic analyses are of great importance in design and safety assessment of a VLFS. Extensive researches have been carried out during the past decades in the development of prediction methods of hydroelastic responses of VLFSs. However, most publications in this field were for VLFSs in open sea. If a VLFS is deployed near islands and reefs in complicated geographical environment, the wave conditions, wave loads and the hydroelastic responses of a VLFS will be quite different than in open sea.

In this paper the three-dimensional hydroelasticity theories that have been widely used in the analysis of a VLFS in deep or shallow open sea with constant water depth are briefly introduced. Based on these theories the numerical approaches of hydroelastic analyses of a VLFS near island and reefs in shallow sea, developed recently by CSSRC, are described. Some important technical problems, including description of wave environment, design scheme, connectors between modules, hydroelastic responses, coupled responses with mooring system and safety analysis of a VLFS deployed near islands and reefs are also discussed.
Sea Test and Numerical Simulations on the Vortex Induced Vibration of the Unbonded Composite Riser
Authors: Zhao Lin , Li Qian , Xiong Xiangqian , Yin Yanju , Yin Xiao
Published: October 2017

With the development of offshore oil and gas technology, the development activities have moved into deeper waters. More challenges are posed by the increasingly complex in the real applications that facing a more adverse environment with more complicated structures adopted. All these results in the pursuit of qualified deeper and lighter alternatives of the steel riser. In this research, a sea test study of the unbonded composite riser is performed. The objective of this research is to investigate the feasibility and stability of the tested riser that under the effect of VIV. Details of the sea test implementation are described. Analysis and discussions focus on the structural mechanical characteristics and dynamic responses are demonstrated. Furthermore, the numerical simulation of the riser is conducted as well. A numerical simulation model is built in OrcaFlex with both static and dynamic analyses are calculated. Finally, more insights for the further development of numerical model are provided through the comparison between the full–scale experimental results and the numerical results.
Flow-Induced Vibration (FIV) and Hydrokinetic Power Conversion of Two Staggered, Low Mass-Ratio Cylinders, With Passive Turbulence Control in the TrSL3 Flow Regime (2.5×104<Re<1.2×105)
Authors: Xu Wanhai , Ji Chunning , Sun Hai , Ding Wenjun , Bernitsas Michael M.
Published: October 2017

Flow-induced vibration (FIV), primarily vortex-induced vibrations (VIV) and galloping have been used effectively to convert hydrokinetic energy to electricity in model-tests and field-tests by the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan. The developed device, called VIVACE (VIV for Aquatic Clean Energy), harnesses hydrokinetic energy from river and ocean flows. One of the methods used to improve its efficiency of harnessed power efficiency is Passive Turbulence Control (PTC). It is a turbulence stimulation method that has been used to alter FIV of a cylinder in a steady flow. FIV of elastically mounted cylinders with PTC differs from the oscillation of smooth cylinders in a similar configuration. Additional investigation of the FIV of two elastically mounted circular cylinders in staggered arrangement with a low mass ratio in the TrSL3 flow-regime is required and is contributed by this paper. A series of experimental studies on FIV of two PTC cylinders in staggered arrangement were carried out in the recirculating water channel of MRELab. The two cylinders were allowed to oscillate in the transverse direction to the oncoming fluid flow. Cylinders tested have, diameter D = 8.89cm, length L = 0.895m and mass ratio m* = 1.343. The Reynolds number was in the range of 2.5×104<Re<1.2×105, which is a subset of the TrSL3 flow-regime. The center-to-center longitudinal and transverse spacing distances were T/D = 2.57 and S/D = 1.0, respectively. The spring stiffness values were in the range of 400<K<1200N/m. The values of harnessing damping ratio tested were ζharness = 0.04, 0.12, 0.24. For the values tested, the experimental results indicate that the response of the 1st cylinder is similar to a single cylinder; however more complicated vibration of the 2nd cylinder is observed. In addition, the oscillation system of two cylinders with stiffer spring and higher ζharness could initiate total power harness at a larger flow velocity and harness much higher power. These findings are very meaningful and important for hydrokinetic energy conversion.
Hydrodynamical Aspects of Pontoon Optimization for a Side-Anchored Floating Bridge
Authors: Fredriksen Arnt G. , Heiervang Mads F. , Larsen Per N. , Sandnes Pål G. , Sørby Bernt , Bonnemaire Basile , Nesteby Anders , Nedrebø Øyvind
Published: October 2017

Long floating bridges supported by pontoons with span-widths between 100m and 200m are discrete hydro-elastic structures with many critical eigenmodes. The response of the bridge girder is dominated by vertical eigenmodes and coupled horizontal modes (lateral) and rotational modes (about the longitudinal axis of the bridge girder). In this paper it is focused on design principles to reduce the response with regards to these eigenmodes.

It is shown for a floating bridge with 200m span-width that by inserting a bottom flange the vertical eigenmodes can be lifted out of wind driven wave regime. It is also shown that selecting a pontoon length that give cancellation of excitation forces is beneficial, and that the geometrical shaping of the pontoon can be efficient to decrease the bridge response.
Flexible Risers Lifetime Extension: Riser In-Service Monitoring and Advanced Analysis Techniques
Authors: Elosta Hany , Gavouyere Thierry , Garnier Pierrick
Published: October 2017

The demand for the lifetime extension of flexible pipes is increasing due to the need to extend the lifetime of the existing production fields. There have been many challenges with the lifetime extension of flexible pipes after the end of the initial design service life due to the inherent conservatism with the common analysis approach, safety factors and operation beyond the design limits. A lifetime assessment should be performed on flexible risers for re-qualification during the original design life if the design envelope is exceeded or there is a need for lifetime extension. Hence, a systematic approach for lifetime assessment execution is established to determine the integrity level of the flexible risers and define the recommended actions required, such as mitigations, repairs or monitoring to maintain an acceptable risk for the required extended service life based on consistent methodology. The primary objective of this paper is to present a riser integrity management field-proven technology to monitor the riser’s behaviour in-service in addition to the advanced analyses guidelines to form a basis for the lifetime extension of flexible risers. The primary objective for the integrity management is to manage and control the risk of failure by detecting failure at an earlier stage when preventive action can be taken to avoid failure propagation. In addition, it is demonstrated that the primary hot-spots for the dynamic behaviour and fatigue life assessments of the flexible risers are primarily in bend stiffener regions and the touchdown zone (TDZ) due to large tension fluctuations caused by vessel motions and cyclic movement in the TDZ. Therefore, analysis techniques have been developed in two primary areas: advanced bend stiffener modelling using pipe-in-pipe (PIP) to model the sliding friction and the bend stiffener/flexible pipe’s annular space and flexible pipe–seabed interaction modelling using a non-linear seabed model. Therefore, the flexible riser’s lifetime extension assessment will be based on more reliable models that reflect the realistic and dynamic behaviour of the flexible risers. Consequently, these advanced analysis techniques can be used for new designs or lifetime extension of flexible pipes.
Prediction of Vortex Shedding Control by Means of Splitter Plates
Authors: Dai Shaoshi , Zhang Rongyu , Younis Bassam A. , Zhang Hongyang
Published: October 2017

The paper presents preliminary results of a computational study aimed at quantifying the effectiveness of splitter plates in reducing the strength of vortex shedding from circular cylinders at high Reynolds number. The principal interest is in determining the optimal ratio of splitter plate width to cylinder diameter to achieve maximum reduction in the magnitude of the fluctuating lift and drag forces relative to their original values. The computations were performed using the URANS approach. The effects of turbulence were accounted for using a turbulence closure that has been adapted to account for the effects of organized vortex shedding on the random turbulent motions. Comparisons with experimental data show that this approach is successful in capturing the main effects that arise from the attachment of splitter plates to the cylinder base.
Safety of Marine Operations Involving Dynamically Positioned Vessels
Authors: Chen Haibo
Published: October 2017

Dynamically positioned (DP) vessels are widely used in marine operations in the offshore oil & gas industry. The risk caused by DP vessels’ position loss is not negligible, and managing DP safety in marine operations is indispensable. This paper presents an overall framework for DP safety management in offshore marine operations. It consists of the following three key steps, i.e. 1) Identification of position loss scenarios, 2) Risk analysis in terms of position loss frequency and consequences, and risk evaluation, 3) Mitigation of risk via measures to eliminate the risk, or to reduce the likelihood of position loss, as well as to mitigate the consequence in marine operations given DP vessel position losses. Case studies from DP shuttle tankers and DP mobile offshore drilling units are presented to illustrate key principles in each of the above three steps. Quantitative risk analysis and evaluation approach is highlighted, and the associated challenges are described. Recommendations to further improve DP safety management in offshore marine operations are proposed.
Safety Barriers in Structural and Marine Engineering
Authors: Ersdal Gerhard
Published: October 2017

Safety regulation and standards have developed intensively since the startup of the petroleum industry in Norway in the early 1970’ties. Prescriptive standards where used in the early days. In the 1980’ties risk analysis and the 10−4 criteria were introduced, along with accidental loads and the progressive collapse limit state. Since 2002 barriers have been a general part of the safety regulation in Norway. This paper aims at reviewing the development of safety regulation of structures and marine systems in Norway, present the barrier regulation as it is described in the Norwegian regulations, and finally evaluate the relevance of the barrier regulation for structures and marine systems.
Overview of Floating Bridge Projects in Norway
Authors: Eidem Mathias Egeland
Published: October 2017

This paper describe the development of floating bridge projects along the coast of Norway. Over the decades, the need to replace existing ferry connections with fixed linkes has beed rapidly increasing. However, the Norwegian nature with its deep and wide fjords, together with low traffic numbers have given the engineers big challenges in designing cost effective bridges for many of the crossings. One solution that was developed in the late 80s and early 90s by a combination of offshore technology and bridge technology was the end-anchored floating bridge concept. So far, two bridges of this design have been completed, the Bergsøysund Bridge and the Nordhordland Bridge. Prof. Moan was a key contributer in the developing work, enabling a safe design of the bridges. As the remaining ferry crossings are even deeper and wider, there is a need to further develop this concept, as well as developing new concepts, so that we are able to replace even further ferries with fixed links. The paper will look back at the two already buildt floating bridges, as well as the floating bridge concepts that might be used to cross the Bjørnafjord, on the west coast of Norway.
Torsion in Flexible Pipes, Umbilicals and Cables Under Loadout to Installation Vessels
Authors: Mainçon Philippe
Published: October 2017

Operations where a flexible pipe, umbilical or cable is loaded out from an on-shore spool to the hold of an installation vessel can cause a build-up of torsion. In unfavourable cases, the torsion has been known to cause spiralling or various forms of damage to the tensile armour.

This paper presents a vocabulary for the description of torsion. It then gives a short review of design codes, enumerates known failure modes (some of which have not always been identified as torsion related), and discusses the mechanisms of torsion generation, with an emphasis on the effect of internal friction. It concludes with some ideas on torsion prevention.