bluuebunny/crossref_metadata_2025 huggingface dataset:
Probing the sRNA regulatory landscape of P. aeruginosa: post‐transcriptional control of determinants of pathogenicity and antibiotic susceptibility
Authors: Zhang Yi‐Fan , Han Kook , Chandler Courtney E. , Tjaden Brian , Ernst Robert K. , Lory Stephen
Published: October 2017

During environmental adaptation bacteria use small regulatory RNAs (sRNAs) to repress or activate expression of a large fraction of their proteome. We extended the use of the in vivo RNA proximity ligation method toward probing global sRNA interactions with their targets in Pseudomonas aeruginosa and verified the method with a known regulon controlled by the PrrF1 sRNA. We also identified two sRNAs (Sr0161 and ErsA) that interact with the mRNA encoding the major porin OprD responsible for the uptake of carbapenem antibiotics. These two sRNAs base pair with the 5′ UTR of oprD leading to increase in resistance of the bacteria to meropenem. Additional proximity ligation experiments and enrichment for Sr0161 targets identified the mRNA for the regulator of type III secretion system. Interaction between the exsA mRNA and Sr0161 leads to a block in the synthesis of a component of the T3SS apparatus and an effector. Another sRNA, Sr006, positively regulates, without Hfq, the expression of PagL, an enzyme responsible for deacylation of lipid A, reducing its pro‐inflammatory property and resulting in polymyxin resistance. Therefore, an analysis of global sRNA‐mRNA interactions can lead to discoveries of novel pathways controlling gene expression that are likely integrated into larger regulatory networks.
Meta‐analysis on shift work and risks of specific obesity types
Authors: Sun M. , Feng W. , Wang F. , Li P. , Li Z. , Li M. , Tse G. , Vlaanderen J. , Vermeulen R. , Tse L. A.
Published: October 2017

Aims

This systematic review and meta‐analysis evaluated the associations between shift work patterns and risks of specific types of obesity.

Methods

PubMed was searched until March 2017 for observational studies that examined the relationships between shift work patterns and obesity. Odds ratio for obesity was extracted using a fixed‐effects or random‐effects model. Subgroup meta‐analyses were carried out for study design, specific obesity types and characteristics of shift work pattern.

Results

A total of 28 studies were included in this meta‐analysis. The overall odds ratio of night shift work was 1.23 (95% confidence interval = 1.17–1.29) for risk of obesity/overweight. Cross‐sectional studies showed a higher risk of 1.26 than those with the cohort design (risk ratio = 1.10). Shift workers had a higher frequency of developing abdominal obesity (odds ratio = 1.35) than other obesity types. Permanent night workers demonstrated a 29% higher risk than rotating shift workers (odds ratio 1.43 vs. 1.14).

Conclusion

This meta‐analysis confirmed the risks of night shift work for the development of overweight and obesity with a potential gradient association suggested, especially for abdominal obesity. Modification of working schedules is recommended, particularly for prolonged permanent night work. More accurate and detailed measurements on shift work patterns should be conducted in future research.
Pollination and reproductive system of synchronopatric species of Cactaceae (Cactoideae) subject to interspecific flow of pollen: an example of ecological adaptation in the Brazilian Chaco
Authors: Ferreira B. H. S. , Gomes A. C. , Souza C. S. , Fabri J. R. , Sigrist M. R.
Published: October 2017

Three synchronopatric Cactaceae species, Echinopsis rhodotricha, Harrisia balansae and Praecereus saxicola, have mostly nocturnal anthesis and similar flowers, characteristics that motivated us to perform a comparative study of reproductive ecology.

Reproductive phenology was sampled monthly from December 2014 to November 2015. We describe floral biology, breeding system via pollination treatments and evaluate floral visitors from focal and filming observations. Pollen grains found on moth proboscis were compared among cactus species under light microscopy. We used fluorescent dye particles to test intra‐ and interspecific pollen flow.

These three species have extended flowering with greater intensity in the wet season, causing high overlap. They have white and hypocrateriformis flowers that open at twilight or nightfall and last about 15 h. H. balansae seems to be self‐incompatible, while E. rhodotricha presented self‐compatibility. P. saxicola presented self‐fertility, but most of the population seems to be self‐incompatible. We suggest sphingophily for the three species, but only P. saxicola was visited by Manduca rustica (Sphingidae). However, we observed pollen grains of all three species on the proboscis of moths, especially M. rustica and M. sexta. Prolonged anthesis allowed bees (herein considered as secondary pollinators) to visit flowers of E. rhodotricha and P. saxicola.

It can be concluded that the studied species share nocturnal and diurnal pollinators, suggesting interspecific pollen flow, which, however, could not be detected with fluorescent dye particles. In view of the low frequency of primary pollinators, it appears that these three species have different reproductive strategies, ensuring the fruiting and production of viable seeds.
Platelet immunobiology: platelets as prey and predator
Authors: Kapur R. , Semple J. W.
Published: October 2017

The immune response against platelets is a complex process involving many aspects of the innate and adaptive immune system. Immune destruction of platelets and megakaryocytes and the thrombocytopenia that ensues can cause different clinically significant haematological disorders such as foetal and neonatal alloimmune thrombocytopenia (FNAIT), transfusion‐induced platelet refractoriness or immune thrombocytopenia (ITP). These conditions are frequently difficult to manage so an understanding of the biological nature of these adverse conditions is critical for an understanding of how to potentially reduce them. Superimposed on these immune platelet attack mechanisms are the immune characteristics of the platelets themselves. This paper will give an overview of how the immune system recognizes platelet antigens and mounts efficient effector mechanism to mediate thrombocytopenia. It will also present evidence to suggest that platelets also play a role in stimulating these responses and suppressing them: a scenario where the ‘prey’ can become the ‘predator’.
Design Approach for Turret Moored Vessels in Highly Variable Squall Conditions
Authors: Brown Alison , Gorter Ward , Vanderschuren Luc , Tromans Peter , Jonathan Phil , Verlaan Paul
Published: October 2017

This paper focuses on examining the response of a large turret-moored FPSO or FLNG vessel in squall conditions and presents a novel and statistically robust response-based approach for the derivation of squall governed design loads. Turret mooring arrangements are typically used as a permanent mooring for FPSO and FLNG vessels, usually in deeper waters and in remote areas where storage capacity is required. The weather-vaning capability makes this type of mooring suitable for many types of environment; however, in tropical environments where metocean conditions are otherwise relatively benign, squalls can dominate aspects of design. Squalls are mesoscale convective systems that cause rapid increases in wind speed and are often associated with large changes in wind direction Squalls are highly variable in both their wind speed and direction profiles and typically uncorrelated with the wave and current conditions; hence the impact of squalls can be difficult to predict and a statistically robust industry standard design approach does not currently exist. Where squall events are the design drivers for mooring arrangements they require particular focus due to the industry’s imperfect knowledge of squall intensity and frequency coupled with the particularly high inter-annual variation of squalls at most locations. To understand the design criteria for a squall environment the horizontal motions of the turret-moored system are modelled using a simplified time-domain approach, which makes considerable simplifying assumptions. This significantly reduces computational time, allowing the analysis to be conducted efficiently for a large range of both squall conditions and associated environmental (wave and current) conditions, including by season and direction. An extreme value analysis approach is then applied to all of the maximum values of the desired response for all combinations of squall and associated conditions, by selecting a number of bootstrap resamples, each the size of the number of squall events. The bootstrap resamples are selected on the probability of occurrence of the associated conditions. A generalised Pareto model can be automatically fitted to all bootstrap resamples and for every return period a user defined number of simulations is performed to estimate the most probable or percentile estimates of the desired return period. Finally, a back calculation of the transient design conditions at the desired return period can then be made and applied into the standard design process. This ensures statistically robust design conditions for squall design loads and deriving a few statistically robust design squalls and associated conditions minimises later analysis. Importantly the approach allows the users to propagate the remaining uncertainty estimates into subsequent robustness analyses.
Numerical Study of Bottom Shape Effect on Pressure Performance in a Sloshing FLNG Membrane Tank
Authors: Yan Yan , Ni Zhonghua , Liu Xiaojun , Yin Jinsong
Published: October 2017

This paper presents a numerical study on the structure optimization of a full-sized floating liquefied natural gas (FLNG) membrane tank. When floating with partially filled liquid, impact pressure on boundaries plays an important role in the sloshing membrane tank. Modification of bottom structure can lead to various pressure distributions on the tank bottom and sidewalls. The optimization is studied by carrying out the numerical simulations and particularly focusing on the structure of sloped-bottom. The applied CFD package, FLOW-3D 10.1, solves the Navier-Stokes equations with the standard k-ε turbulence model and Volume-of-Fluid method to describe the free surface movement. The numerical result is validated against the experimental data and numerical result from the Smoothed Particle Hydrodynamics method. Liquid behavior is investigated in four different tanks. A modified method using average value of fluid peak depth is applied to locate the natural sloshing frequency. The greatest local pressure was approximately 65 KPa near the intersection of slope and bottom. The comparison of peak pressure indicates that tank with 15% width sloped-bottom has smaller affected area of high pressure and lowest value of peak pressures, which is the optimized sloped-bottom tank in pressure performance. The outcomes from the numerical simulation are expected to be a reference data for the design of FLNG cargo containment system.
Improved Sustainable Speed due to Thrusters With Ducted Propellers
Authors: Bulten Norbert , Stoltenkamp Petra
Published: October 2017

The selection of the propulsion system is made in the concept design phase, based on a trial design point. Evaluation of the vessel design and the propulsion units has been based on model scale measurements for decades. Recent developments of CFD simulations have brought these numerical simulations to a sufficient mature level, to be considered as a good alternative for the model scale measurements. Comparisons of CFD results for model scale and full scale have revealed new interesting insights in the Reynolds scaling effects.

The actual operating conditions of offshore vessels are significantly different from the calm water trial condition. The optimized propulsion units for the trial condition are not per se the optimum solution for the actual operating conditions. A comparison has been made of two azimuth thruster types: pulling thruster with open propeller and a pushing thruster with ducted propeller. It has been shown that the performance in calm water is comparable for both type of thrusters. However, when the vessel is operating at increased loading, the benefits of the ducted propeller units become more pronounced. This is in the end reflected in an increased capability of keep a certain transit sailing speed in more harsh conditions.
Evaluation of CFD Analysis to Gather the Open-Water Characteristics of a Specific B-Series Propeller With Verification and Validation Assessments
Authors: Machado Lucas do Vale , Fernandes Antonio Carlos
Published: October 2017

A very interesting topic in naval architecture is the choice of propeller. The engineer team should pay attention to several characteristics of the vessel to choose properly the most effective propeller for that specific application.

At the beginning of the Second World War, the diagrams of a well-known propeller design were published, the B-Series Propellers. They were tested at the Netherlands Ship Model Basin in Wageningen and analyzed with multiple polynomial regression analysis. Until today this propeller is studied because it possesses satisfactory efficiency and adequate cavitation properties. Besides that, open-water characteristics (KT, KQ and η0) are easy to get.

The purpose of this work is to evaluate the usefulness of CFD analysis to get the main characteristics of a propeller in order to make this selected process more effective, as long as with a lower cost is possible to analyze much more designs of propellers at a lower cost. A tool using Microsoft Excel was developed to draw any B-Series propeller. For this work was used a specific B-Series propeller, the B3.80. The CFD results are verified using Richardson Extrapolation and then validated against the propeller model results. The open-water characteristics are used for this purpose and the numerical results showed a fairly good agreement with the experimental data. Other interesting results of this propeller are shown too, e.g. the pressure distribution, streamlines and wall y+.
Rogue Waves in Wind Seas: An Experimental Model in an Annular Wind-Wave Flume
Authors: Toffoli A. , Proment D. , Salman H. , Monbaliu J. , Stramignoni E. , Forza R. , Manfrin M. , Onorato M.
Published: October 2017

The probability of occurrence of rogue waves in wind-generated fields is investigated experimentally in an annular wind-wave flume. Unlike many experiments on rogue waves, where waves are mechanically generated, here the wave field is forced naturally by wind as it is in the ocean. The peculiar geometry of the flume makes waves propagating circularly in an unlimited-fetch condition. Water surface elevation was measured at specific cross-sections under the effect of different wind speeds to monitor the temporal evolution of the wave field. Results show that the kurtosis of the surface elevation, the fourth order moment of the probability density function and a measure of the percentage of extremes in a wave record, gradually increases in time with the evolution of the wave field. Deviations from Normal statistics are observed to be a function of wind speed. The maximum departure from Normality resembled the one induced by quasi-resonant wave-wave interactions and it is observed at the final stage of wave growth and immediately before reaching the fully development regime.
Numerical Studies on Slosh-Induced Loads Using Coupled Algorithm for Sloshing and 3D Ship Motions
Authors: Sen Debabrata , Saripilli Jai Ram
Published: October 2017

This paper reports the development of an efficient algorithm coupling ship motions with interior tank sloshing. The algorithm integrates a potential flow based solver for the ship motions with an open source viscous flow solver (OpenFOAM) for the interior sloshing. The ship motion problem is solved using a three dimensional forward speed transient Greens function method in the time domain. The internal tank sloshing problem is solved using a standard Finite Volume Method (FVM) based incompressible multiphase interface capturing Volume of Fluid (VoF) technique. The objective is to determine the influence of slosh motion on ship responses and in turn the modifications in the slosh-induced interior pressures and loads due to this coupling. The reason for influence of coupling on roll response is investigated. The maximum pressure at the interior of the tank considering uncoupled and coupled sloshing and ship motions is studied. Further numerical simulations are carried out considering simultaneous sloshing in two tanks.
Vortex-Induced Vibration of Two Side-by-Side Cylinders With a Small Gap in Uniform Flow
Authors: Munir Adnan , Zhao Ming , Wu Helen
Published: October 2017

Vortex-induced vibrations of two elastically mounted and rigidly coupled circular cylinders in side-by-side arrangement in steady flow are investigated numerically. The vibration of the cylinders is limited to the cross-flow direction only. The three-dimensional Navier-Stokes equations are solved using the Petrov-Galerkin Finite element method and the equation of motion is solved using the fourth order Runge Kutta method. It is well known that when the gap between two stationary side-by-side cylinders is very small, the flow between the two cylinders is biased towards one cylinder and the lift force on each cylinder is significantly smaller than that of an isolated single cylinder. The aim of this study is to investigate the effect of a small gap ratio of 0.5 between the two cylinders on the lock-in regime and the amplitude of the vibration of two side-by-side cylinders in a fluid flow. Simulations are carried out for a constant mass ratio of 2, a constant Reynolds number of 1000 and a range of reduced velocities. It is found that in the lock-in range of the reduced velocity, the two cylinders vibrate about their balance position with high amplitudes. Outside the lock-in regime the flow from the gap becomes biased towards one cylinder, which is similar to that from the gap between stationary cylinders.
Analysis of Unsteady Hydrodynamics Related to Vortex Induced Vibrations on Bluff-Bodied Offshore Structure
Authors: Tabib Mandar , Rasheed Adil , Fuchs Franz Georg
Published: October 2017

Flows around a fixed cylinder with uniform and pulsating inflow conditions at different Reynolds numbers are simulated using Large Eddy Simulation (LES). For pulsating inflow, a sinusoidal profile, with an amplitude ΔU and a pulsation frequency fe, is superimposed onto the mean velocity U∞ at the inlet plane. The current study reveals that the pulsation can influence flow-physics in three possible ways as compared to uniform inflow conditions: (a) The vortex shedding pattern is seen to be more asymmetric for pulsating inflow than for uniform inflow. This needs to be validated with an experimental campaign devoted to the study of flow-asymmetricity due to pulsatile and uniform flow condition. (b) The dominant shedding frequency fd gets locked with respect to the frequency of the pulsating inflow fe, (for both the turbulent and transition regime) at a ratio of fe/fs0 equivalent to 0.65 – 0.75 (where fs0 is the vortex shedding frequency for uniform inflow) and ε = ΔU / (2πfeD) ≈ 0.2, where D is the diameter of the cylinder. This numerical observation is validated using the experimentally observed turbulent vortex regime work ( [1])in this range. For conditions with fe/fs0 > 0.75 the lock-in may happen at fe/2. (c) Compared to uniform inflow, the pulsating inflow leads to a larger drag coefficient. The drag coefficient is influenced by the ratios fe/fs0 and ΔU / U∞.
Long Term Analysis of TLP Extreme Tendon Tensions Using a Coupled Model and Comparison With the Contour Line Approach
Authors: Jimenez Puente Isabel , Lian Gunnar
Published: October 2017

This paper presents a full long term analysis of a TLP extreme tendon tensions using the all seas approach, and its comparison to the results estimated by the contour line approach.

The analysis of the TLP tendon tension response is performed in the time domain using a coupled model where the floater is modelled in the software SIMO, while the tendon system is represented by a Finite Element Model in RIFLEX, including therefore the effects of non-linear restoring from the tendon system and bending and deformations of the tendons. The characteristic tendon tensions with q-annual probability of exceedance are estimated from a full long term analysis where both the short and long term variability are considered. These results are then compared to those obtained through the long term estimate from the contour line approach when assuming the 90th percentile for the worst sea state with q-annual probability of exceedance.

The results from the full long term analysis will allow us to verify the adequate percentile level to be used with a contour line approach when estimating extreme TLP tendon tensions.
A Non-Iterative Method for Vortex Induced Vibration Prediction of Marine Risers
Authors: Lu Ziqi , Fu Shixiao , Zhang Mengmeng , Ren Haojie , Song Leijian
Published: October 2017

A non-iterative method in frequency domain for vortex induced vibration (VIV) predictions of marine risers is proposed in this paper. A solving model is established in modal space for predicting risers’ VIV responses, which consists of a hydrodynamic force equation and a dynamic response equation. By utilizing a non-iterative solving process in the modal space, the equations are solved without power-balance iterations. And through comparisons between this method and conventional prediction methods, the validity and applicability of this non-iterative method is verified. This method completely gets rid of the difficulty of getting convergent predictions, which is conducive to practical engineering applications and provides fresh ideas for understanding conventional empirical model methods for VIV prediction.
Semi-Empirical Crest Distributions of Long-Crest Nonlinear Waves of Three-Hour Duration
Authors: Huang Zhenjia (Jerry) , Guo Qiuchen
Published: October 2017

In wave basin model test of an offshore structure, waves that represent the given sea states have to be generated, qualified and accepted for the model test. For seakeeping and stationkeeping model tests, we normally accept waves in wave calibration tests if the significant wave height, spectral peak period and spectrum match the specified target values. However, for model tests where the responses depend highly on the local wave motions (wave elevation and kinematics) such as wave impact, green water impact on deck and air gap tests, additional qualification checks may be required. For instance, we may need to check wave crest probability distributions to avoid unrealistic wave crest in the test. To date, acceptance criteria of wave crest distribution calibration tests of large and steep waves of three-hour duration (full scale) have not been established.

The purpose of the work presented in the paper is to provide a semi-empirical nonlinear wave crest distribution of three-hour duration for practical use, i.e. as an acceptance criterion for wave calibration tests. The semi-empirical formulas proposed in this paper were developed through regression analysis of a large number of fully nonlinear wave crest distributions. Wave time series from potential flow simulations, computational fluid dynamics (CFD) simulations and model test results were used to establish the probability distribution. The wave simulations were performed for three-hour duration assuming that they were long-crested. The sea states are assumed to be represented by JONSWAP spectrum, where a wide range of significant wave height, peak period, spectral peak parameter, and water depth were considered. Coefficients of the proposed semi-empirical formulas, comparisons among crest distributions from wave calibration tests, numerical simulations and the semi-empirical formulas are presented in this paper.
Time Varying Hydrodynamics Identification of a Flexible Riser Under Multi-Frequency Vortex-Induced Vibrations
Authors: Liu Chang , Fu Shixiao , Zhang Mengmeng , Ren Haojie
Published: October 2017

In this paper, Forgetting Factor Recursive Least Squares (FF-LS) method is proposed to identify time-varying vortex-induced force coefficients of the flexible riser under multi-frequency VIV. FF-LS method introduces the forgetting factor, which gives more weight to the data closer to the present. This modification improves the method’s sensitivity to the time-varying parameters, and enables it to identify the time-varying parameters under multi-frequency coupling. In this paper, the mass-spring-dashpot model is used to verify FF-LS method’s ability to accurately identify time-varying parameters under multi-frequency coupling. Then, this methodology is used to identify the vortex-induced force coefficients of flexible riser at the basic frequency when vortex-induced vibration occurs. Identified coefficients are consistent with the result obtained from Least Squares method, which indicates that the proposed FF-LS method retrogrades to the Least Squares method when VIV response and the vortex-induced force contains only one single frequency. Finally, this methodology is used to identify the time-varying vortex-induced force coefficients of a flexible riser under VIV considering the coupling effect between the basic frequency and high frequency. The vortex-induced force reconstructed from identified time-varying coefficients is consistent with real vortex-induced force, which verifies the validity and applicability of this methodology in identifying time-varying vortex-induced force coefficients considering multi-frequency coupling effect. The results show that when the flexible riser is subjected to multi-frequency VIV, its vortex-induced force coefficients change periodically, the time-averaged values of these time-varying coefficients are different from the vortex-induced force coefficients at the basic frequency, which results from the coupling effect between the basic frequency and high frequency. Linear superposition of vortex-induced force coefficient at basic frequency and high frequency are different from time-varying coefficients considering coupling effect of multi-frequency and cannot reconstruct vortex-induced force correctly.
Hydrodynamics of Flexible Pipe With Staggered Buoyancy Elements Undergoing Vortex-Induced Vibrations
Authors: Zhang Mengmeng , Fu Shixiao , Song Leijian , Wu Jie , Lie Halvor , Ren Haojie
Published: October 2017

Flexible riser with staggered buoyancy elements has been widely used in ocean engineering, such as steel lazy wave riser, drilling riser, etc. Both the buoyancy elements and the riser may experience vortex induced vibrations (VIV), subject to sea current. However, hydrodynamic characteristics of the buoyancy elements undergoing VIV and influence of buoyancy elements on hydrodynamic force of the bare section are still under discussion. The purpose of this paper is to reveal the hydrodynamic characteristics of flexible riser with staggered buoyancy elements, both for buoyancy element and bare pipe section. The cross flow hydrodynamic coefficients of the flexible riser with 25%, 50% and 100% staggered buoyancy covered are obtained from VIV model tests, using hydrodynamic forces and coefficients identification method. Distribution of the added mass coefficients and excitation coefficients along the flexible riser were investigated, and compared with those on the bare flexible pipe and rigid cylinders under forced oscillations. In addition, the relationship between added-mass coefficients of buoyancy element and that of bare section were obtained.
Reducing Relative Horizontal Motion Between Cargo and HTV During Offshore Loading and Discharge
Authors: Peters Onno A. J. , Huijsmans René H. M.
Published: October 2017

In Heavy Marine Transport it is common practice to dry-transport large and heavy floating offshore structures. In general, loading and discharge of these floating cargoes on- and from heavy transport vessels is done at sheltered locations like harbors where sea-state and swell conditions are insignificant. Often these locations are at large distance from operating fields of the offshore structures, which means that the structures need to be towed from- or to these fields. To save time and costs, it is beneficial to perform the loading and discharge operations in the field. This necessitates a reconsideration of the maximum allowable wave condition such as to perform the loading- and discharge operations within specified time frame whilst ensuring safety of crew, cargo and heavy transport vessel. Since precise positioning of the cargo on the HTV cribbing beams is of importance to support the cargo on its structural strong points, the allowed relative horizontal motion during loading or discharge operations is limited to a fraction of the width of these cribbing beams. When increasing the maximum allowable wave conditions, relative horizontal motions between heavy transport vessel and cargo easily exceed these limits if only the standard handling equipment is used. Also, the loads in the handling equipment may exceed safe limits.

This paper presents two methods including complementary equipment to reduce- and limit the relative horizontal motions. The first method is based on increasing the stiffness of the connection between cargo and heavy transport vessel. This means that there is a transition from a soft (standard handling) system with a low natural frequency to a stiff (clamping) system with high natural frequency. During this transition the system natural frequency will coincide with the wave frequent excitation force. Resonant behavior during the transition is avoided as the complementary equipment also employs a damping force. The second method is based on a closed-loop controller applied to the desired relative horizontal position. The resulting desired load to control the relative horizontal motion is then allocated to several line tension actuators. Contradictory to well-known Dynamic Positioning systems which control low frequent motions, motion control during offshore loading and discharge is performed on wave frequent behavior. This implies that the line tension actuators also need to deliver loads within a wave frequent time-frame. In fact, the peak tension needs to be obtained within a quarter of a wave period.

System design and simulation results are presented. Depending on the cargo type, different solutions and operational aspects are discussed. Simulations are done for a typical cargo where both methods to reduce the relative horizontal motions are utilized.
Concrete Modeling for Extreme Wave Slam Events
Authors: Wåsjø Kasper , Stavang Terje P. , Søreide Tore H.
Published: October 2017

Experience from model tests has initiated a growing attention towards extreme wave slam as a critical load situation for offshore large volume structures. Most of the problem is related to the local slam pressure, which may go up to several MPa’s for 100-year and 10 000-year waves.

The paper deals with modeling techniques for marine concrete structures under extreme slam loading from waves where dynamic effects together with material softening play a major role for the response. Different analysis approaches for ultimate limit state (ULS) and accidental limit state (ALS) controls are discussed in view of reliability philosophy as basis for conventional design approach.

The present paper is devoted to the local impact scenario and the alternative approaches for response and capacity control involving non-linear time domain analyses. Conventional design schemes as based on linear elastic models for response calculation together with code specified capacity control often come out more conservative than non-linear approach. The paper demonstrates by case studies how softening of the structure in general reduces the response in terms of section forces.

A key issue when going from conventional linear approaches into non-linear techniques is to still keep an acceptable reliability level on the capacity control. Load and material factors are normally based on structures with limited non-linearity where linear response modeling is representative. Implementing non-linear material model in time domain analysis has a major challenge in limiting the sensitivity in response and capacity calculation. The paper demonstrates the way material model of concrete affects the section forces to go into local capacity control, and concludes on needed sensitivity analyses.

Practical approaches on the concrete slam problem together with resulting utilizations from the control are demonstrated. The full non-linear technique by response and capacity control in one analysis is also handled, using average material parameters and justifying safety factors for the effect of implementing characteristic lower strength of concrete in the capacity.

The paper ends up in a recommendation on non-linear time domain analysis procedure for typically slam problems. A discussion is also given on applicable design codes with attention to non-linear analysis.
Theoretical and Experimental Study of Heading Stability and Heading Control of a Turret-Moored FPSO
Authors: Kaasen Karl E. , Ludvigsen Halgeir , Nygaard Ivar , Aas Kristian
Published: October 2017

The behaviour and characteristics of a turret-moored FPSO subjected to loading from waves, wind and current are investigated. Of particular importance is to find out if fishtailing instabilities may occur, and if such instability can be disclosed by simple criteria involving basic parameters of the system’s mathematical model. Eight cases from model tests are chosen for theoretical study and time domain simulation. Four of the cases involve heading control with thrusters.

For the stability study, a simplified linear model in sway and yaw is formulated. It is shown that the inherent characteristics of the model depend on the strengths and relative directions of the metocean processes. The eigenvalues of the sway-yaw model are computed for the eight selected cases to check the stability. A simple approximate criterion for heading stability is derived from the sway-yaw model. It is assumed, but not proven, that the criterion is a sufficiency criterion for stability.

Both the experiments and the simulations show that the eight cases are stable. This is also confirmed by the eigenvalues of the sway-yaw model, while the simple criterion wrongly deems several cases unstable. The simple stability criterion is therefore probably conservative, at least when there is significant damping in the system. In one additional hypothetical case with only wave excitation and weak or lacking stability, the simplified criterion agrees well with the model test and simulation.

Heading control is necessary when a heading different from the natural weathervaning heading is wanted. The controller used in the experiments and simulations is of simple SISO PID type. With control, the heading variations are reduced significantly.
Numerical Simulations of Regular and Irregular Wave Forces on a Horizontal Semi-Submerged Cylinder
Authors: Liu Shengnan , Ong Muk Chen , Obhrai Charlotte , Seng Sopheak
Published: October 2017

Two-dimensional (2D) numerical simulations have been performed using OpenFOAM (an open source CFD software package [1]) and waves2Foam (an OpenFOAM based add-on library for wave generations and absorption [2]) to investigate free surface waves past one fixed horizontally semi-submerged cylinder. The 2-D simulations are carried out by solving Navier-Stokes equations which are discretized based on finite volume method (FVM). Volume of Fluid (VOF) method is employed to capture the free surface in the numerical wave tank. Validation studies have been performed by comparing the numerical results of Stokes first-order wave past a semi-submerged circular cylinder with the published experimental data at different incident wave properties. The numerical results are in good agreement with the experimental data. Subsequently, regular and irregular waves past semi-submerged cylinder at different wave heights and the wave lengths are computed numerically to investigate the effect of the wave height and wave length on wave-structure interaction. The numerical results for irregular waves are compared with those induced by regular waves.
Long-Term Extreme Response Analysis of Marine Structures Using Inverse SORM
Authors: Giske Finn-Idar G. , Leira Bernt Johan , Øiseth Ole
Published: October 2017

In this paper the first order reliability method (FORM) found in connection with structural reliability analysis is first used in an inverse manner to efficiently obtain an approximate solution of the full long-term extreme response of marine structures. A new method is then proposed where the second order reliability method (SORM) is used to improve the accuracy of the approximation. This method is compared with exact results obtained using full numerical integration. The new method is seen to achieve improved accuracy for large return periods, yet keep the number of required short-term response analyses within acceptable levels.
Lateral Resistance of Pipes on Rocky Seabeds: Comparison Between Measurements and Models Based on Synthetic Seabeds
Authors: Griffiths Terry , White David , Draper Scott , Leighton Adam , Fogliani Antonino
Published: October 2017

The behavior of pipelines, cables and umbilicals on rocky seabeds has to date received little research attention. This is despite the marine renewable energy and oil and gas industries relying on these ‘pipes’ to cross a variety of rocky seabed types in the presence of extreme metocean conditions. Present design solutions are challenging and costly, yet there remains a track record of in-service failures.

This paper forms part of a wider research effort being undertaken by the University of Western Australia (UWA) into pipe behavior on rocky seabeds. This work includes the effects intermittent gaps have on hydrodynamic forces, the effect of seabed roughness on enhanced boundary layer thickness and the validity of existing hydrodynamic force models for small diameter cables.

In this paper, the lateral resistance of pipes on rocky seabeds is investigated using both physical and numerical testing of model pipes over artificially-created rocky seabeds. Four model pipes of varying diameter have been displaced laterally over 1 m square model rocky seabeds, with a range of pipe to rock diameters. The lateral resistance of the physical pipe tests were recorded using load cells and a digital data-logger. Analysis of the physical test results has enabled comparison to (and refinement of) numerical models as well as improved understanding of the importance of different parameters. Our results show peak frictions above 6 arising under conditions where interface friction is only about 0.3, which contrasts dramatically with the friction value of 0.6 nominated in F109. This work contributes towards generation of new design methods suitable for application to field conditions.
Experimental Study on Hydroelastic Impact of One Wedge With Stiffened Panels
Authors: Dong Chuanrui , Ren Huilong , Liu Ning , Wang Qiang
Published: October 2017

In this paper, two wedges with deadrise angle 45 degrees made of steel and aluminum alloy respectively was designed and a series of free-drop model tests were carried out, we recorded time histories of water-entry velocity of models, slamming pressures on outer surface of stiffened panels and stress response of the structures. The time space distribution of slamming load and characteristic of structure dynamic response were studied. And through comparison analysis of slamming loads between two model’s corresponding measuring points, the effect of hydroelastic on slamming loads were discussed. Finally, an equivalent static analysis method to determine the structural response of hull structures under slamming loads was presented by analyzed the slamming loads and structure dynamic response that got from experiment combined with finite element method.
Time Domain Simulation of Jack-Up Platform in Second-Order Irregular Seas
Authors: Lubis Michael Binsar , Haver Sverre , Amdahl Jørgen
Published: October 2017

This paper reviews the significance of applying second-order irregular waves for assessing hydrodynamic loads on a jack-up platform. The study is based on a realistic jack-up model. The wave load is determined utilizing Morison equation. The study focuses on extreme wave elevations when the drag load component dominates and the magnitude of wave particle horizontal velocity is crucial. Both extreme surface elevation and wave particle kinematics are observed. For wave particle kinematic, two different stretching methods are compared. The static response of a pile structure using the second-order model and the 5th Stokes wave are compared. In the end, a dynamic analysis of the jack-up utilizing a second-order irregular wave model is performed.
Vortex-Induced Vibrations of a Cylinder With a Control Rod in its Wake
Authors: Huera-Huarte Francisco J. , Jiménez-González José I.
Published: October 2017

We study the dynamics of a flexibly-mounted cylinder having a control rod in its near wake, when subject to a uniform cross-flow. The objective is to investigate the effect of a localized perturbation introduced in the near wake of the cylinder. The work consists of a parametric study with a rigid cylinder mounted in an air bearing rig, with its motion constrained to move in the direction transverse to the flow. The cylinder system is characterised by a low combined mass-damping parameter. During the experiments, the location of the rod will be fixed, but its size and the free stream velocity will be varied. The dynamic response of the cylinder will be measured using non-intrusive techniques. The vortex dynamics in the near wake of the system will also be described after measurements obtained using Digital Image Particle Velocimetry (DPIV). The objective is to understand the physical mechanisms of the fluid-structure interaction phenomena involved in the changes of the dynamic response of the cylinder when using the control rod.
Wave Impact Experiment of a GBS Model in Large Waves
Authors: Huang Zhenjia (Jerry) , Spencer Don , Oberlies Robert , Watts Gracie , Xiao Wenting
Published: October 2017

For the design of offshore structures in harsh wave environments, model testing continues to be the recommended industry practice for determining wave impact forces on offshore structures. Accurate measurements of wave impacts in model tests have been a challenge for several decades. Transducers are required to accurately capture the short duration, high magnitude, and dynamic nature of impact loads. The structural model, transducers, and the transducer mountings need to be designed such that mechanical vibrations in the integrated transducer-mounting-structural model system do not contaminate the wave impact measurements. In this work, the dynamic oscillations in the measurements were controlled through the design and fabrication of transducers, their mounting and the GBS model. Wave crest probability distributions were developed that included fully nonlinear effects. These distributions were used as a benchmark to qualify the waves in the wave calibration tests. The highly stochastic nature of impact loads makes it challenging to obtain converged probability distributions of the maximum impact loads (i.e. forces or pressures) from model tests. To increase the confidence in the statistical values of wave impact loads, a large number of realizations were used for a given sea state. Variability of the maximum pressure due to wave basin effects (such as wait-time between tests) was examined with fifteen repeat tests using the same wave maker control signal. These tests provided insights into the random behavior of the impact loads.
Optimization of Mooring Systems for Floating Offshore Platforms Considering Seabed Obstacles
Authors: Monteiro Bruno da Fonseca , Albrecht Carl Horst , Lima Beatriz Souza L. Pires de , Jacob Breno Pinheiro
Published: October 2017

With the constant increase of known oil reserves in deep and ultra-deep waters, floating production systems (FPS) have been established as the main option for offshore production activities. This work presents recent advances regarding an innovative application of evolutionary optimization methods to the design of mooring systems for FPS. Here the focus in in the consideration of actual scenarios with seabed obstacles and other restricted areas where anchors cannot be positioned. The optimization tool employs the Particle Swarm Optimization (PSO) method. The constraints are treated by specialized constraint-handling techniques, and each candidate solution is evaluated through nonlinear analyses coupling the hydrodynamic model of the platform hull with a Finite Element model of the mooring lines and risers. A case study illustrates the application of the optimization for a real-world scenario.
Determining Thruster-Hull Interaction for a Drill-Ship Using CFD
Authors: Koop Arjen , Cozijn Hans , Schrijvers Patrick , Vaz Guilherme
Published: October 2017

In this paper CFD results are presented for the thruster-hull interaction effects for a drillship with 6 azimuthing thrusters. The results using different approaches to model or simulate the propeller are compared and their advantages and disadvantages are discussed. The approaches investigated are the so-called Frozen Rotor approach, where the propeller rotation is modeled, the Actuator Disk approach with prescribed body forces and the unsteady Sliding Interface approach where the motion of the propeller is simulated in time.

First, open-water calculations for a tilted thruster are carried out using the Frozen Rotor approach. The open-water calculations are repeated using the Actuator Disk prescribing the propeller thrust and torque distribution obtained from the Frozen Rotor calculations. The results with Actuator Disk are very similar for the unit thrust and nozzle thrust compared to the results using the Frozen Rotor approach. Furthermore, the results using the Frozen Rotor or the Actuator Disk are very close to the experimental results for the nozzle thrust.

The thruster-hull interaction of one active thruster under the drillship is investigated using the Actuator Disk approach, the Frozen Rotor Approach and the Sliding Interface approach. A comparison to experimental results is presented for the thruster-hull interaction coefficients. Using the Actuator Disk a good agreement with the experiments is obtained. The results using the Actuator Disk and Sliding Interface are very similar to each other, but the computational costs for the Sliding Interface method are at least a factor 20 higher. The results using the Frozen Rotor deviate due to an unphysical wake behind the thruster.

Based on the results presented in this paper we conclude that, using the steady-state approach with the Actuator Disk, CFD can be a cost-efficient and accurate method to determine the thruster-hull interaction effects at bollard pull conditions for a typical offshore vessel.