bluuebunny/crossref_metadata_2025 huggingface dataset:
RANS Simulation on Wave Blocking and Breaking of Water Surface Waves Propagating on Current
Authors: Peng Bo , Ma Ning , Gu Xiechong
Published: September 2014

Wave-current interaction, a common phenomenon in the areas of estuary and gulf, is also notable in deep water sea area. When the current is strong enough, wave “blocking” accompanied by wave breaking takes place and leads to hazard sea conditions. On the other hand, wave countering current is also thought to be one of the main causes of occurrence of rogue waves in the ocean, which imperils offshore structures and is of much importance for research. Knowing this, viscous flow of wave generation on current is simulated by using Reynolds-Averaged-Navier-Stokes (RANS) equation for better understanding of its mechanism and flow field. In this study, two-dimensional waves propagating without current, along and against pre-existing large-scale current have been investigated. The validation of calculation has been demonstrated through comparing with preliminary experiments in Circulating Water Channel (CWC). When propagating in counter-current, wave is steepened and wave height increases dramatically, thus wave breaking occurs occasionally. Therefore, the variation of wave height and steepness with current velocity has been analyzed quantitatively. It has been confirmed that wave “blocking” occurs when current speed is comparable with group velocity of waves. And this is analyzed in detail in terms of kinematics, energy transfer and others. Accordingly, wave breaking is evaluated by the well-known classical wave breaking criterion. Furthermore, the proposed viscous model is compared with “simple bore dissipation model” to evaluate energy dissipation of wave breaking and improvement of numerical model for wave blocking is introduced. It has been shown that the present numerical model is applicable to reproduce the wave blocking and breaking phenomenon and it will be useful for future engineering purpose on predicting real sea conditions.
Towards a Lateral Line Sensor to Supplement Sonar in Shallow Water
Authors: Ziemer Tim
Published: September 2014

Sonar provides vessels with a sensory system to detect and identify still and moving obstacles. In shallow water both active and passive sonar meet their limits. Acoustical methods exist, aiming at supporting sonar systems by means of digital signal processing, or, coming from the field of biomimetics, imitating echolocation principles of marine animals. This paper introduces a sensor system combining these approaches by the use of a vector sensor array applying Near-field Acoustical Holography (NAH) imitating the Lateral Line organ (LL) of fish; a passive method to supplement active and passive sonar. LL is able to localize obstacles due to their dipole-like water displacement by comparing low-frequency water accelerations distributed along the whole body. In contrast to pressure, accelerations are highly evanescent and do not propagate into the far-field. Thus LL does not suffer under reverberation or scattering. The performance of the proposed NAH-based LL-sensor is tested by a computer simulation of a source in absence and in presence of a disturbing source. The LL-sensor has proven to be more robust than pressure detection methods like beamforming and conventional NAH.
Simulations of Air Cavity Dynamics During Water Entry and Slamming
Authors: Li Zhong , Jaiman Rajeev K. , Khoo Boo Cheong
Published: September 2014

An adaptive mesh refinement based Volume-of-Fluid (VOF) two-phase method has been employed to simulate the water entry and slamming problems of solid bodies with various geometries. The computations have been performed by using the GFS (Gerris Flow Solver) libraries. Solid boundaries or the interfaces between the solid and the fluid are tracked by a piece-wise linear volume-of-fluid method. The key ingredient in this simulation study is the capability of parallel adaptive refinement along the free-surface interface to capture the interface conditions accurately while using the Eulerian grid based VOF approach. The novelty of this work is that it is the first time to apply GFS libraries and adaptive VOF in solving complex water entry problems to the best of our knowledge. Before applying to a particular application, the adaptive VOF solver is validated by simulating the vertical water entry of a circular disk. The simulations of solid bodies with different geometries such as thin plates are then conducted to investigate the physics of air-cushioning effect during slamming.
Laboratory Scale Direct Shear Box Experiments on Ice Rubble: The Effect of Block to Box Size Ratio
Authors: Pustogvar Anna , Høyland Knut V. , Polojärvi Arttu , Bueide Ida M.
Published: September 2014

Direct shear box experiments on rubble ice were performed in the cold laboratory of Norwegian University of Science and Technology (NTNU) in order to estimate effect of the specimen scale on the resulting mechanical properties of the material. In total 23 tests were performed, including 18 tests on dry rubble and 5 tests with having the box and the rubble submerged in saline water for 30 minutes before testing. Experiments with two ice block sizes, different gradation and two different values of confining pressures were conducted. From the shear force records measured during the experiments, values for peak friction angles and cohesion of the ice rubble were derived. In addition to these, the dilation angle of the ice rubble was estimated using the video recordings from the tests. The results from the experiments and a review on earlier experiments also presented here suggest that there is a relation between the size of the ice blocks and material properties yielded by the experiments. The results further show that the rubble dilation and the load limiting mechanism in the experiment should be carefully studied and reported in the experiments in order to gain understanding on rubble behavior.
Direct Numerical Simulations of Wall Proximity Effect on Single and Tandem Cylinders
Authors: Khandelwal Prateek , Jaiman Rajeev K. , D’Souza James Evon , Mak Chan Keet
Published: September 2014

Subsea pipelines placed at or near the seabed are exposed to waves and underwater currents. The unstable seabed, irregular terrain, erosion of sediments or installation procedures may lead to free span in sections along the length of pipelines. This problem can be explained in simplified terms as a cylinder near an impermeable wall subjected to boundary layer based shear flow. It is common practice to use fixed cylinder hydrodynamic coefficients (drag, lift, and interia) to calculate the pipeline stability with the assumption that the pipeline is either trenched or lying on the seabed. However, due to the gap between the pipe and the seabed, the force coefficients have a strong dependence on the relative gap.

In this paper, a moving wall is considered rather than a fixed wall to avoid the confusing interaction of wall boundary layer and thus focus completely on wall proximity effects to evaluate the lift and drag forces. In particular, the wall-induced lift force is due to two competing mechanisms in subsea pipelines. First, the vorticity generated at the surface of the cylinder cross-section advects and diffuses downstream. The presence of a nearby wall breaks the axisymmetry of the wake vorticity distribution. The resulting induced velocity also breaks the symmetry and results in an effective lift force that tends to move the pipeline away from the wall. Second, from inviscid theory, one can argue that the flow relative to the cylinder will accelerate faster in the gap between the cylinder and the wall. The resulting low pressure in the gap will induce a lift force directed toward the wall. In the first part, we focus on the wall-induced lift force in the finite Reynolds number regime using a Spectral Element method to perform direct numerical simulations (DNS) with high accuracy. The effects of gap ratio will be investigated in detail to characterize the oscillating forces on the cylinder. In the second part, this paper will focus on studying how seabed can affect two tandem cylinder arrangements as a function of spacing and the interaction of wall-induced forces with the wake dynamics between the two cylinders.
Methodology to Obtain the Life Cycle Mooring Loads on a Semisubmersible Wind Platform
Authors: Guanche Raúl , Meneses Lucía , Sarmiento Javier , Vidal César , Losada Íñigo
Published: September 2014

Nowadays there are few methodologies related with the design of mooring systems for floating offshore wind platforms. The ones used until the moment are inherited from the oil and gas industry. Because of that, mooring loads may be incorrectly estimated. This study presents a validated methodology in order to estimate the loads of the moorings of offshore floating platforms along the life cycle of the structure.

The methodology is based on an extensive laboratory test data base carried out in a wave basin of the University of Cantabria. The proposed methodology has been applied to a floating semisubmersible platform (similar to the one in Agucadoira by Principle Power).

The methodology is composed by a few steps. The first step consist on the selection of the most representative sea states of a long term met-ocean data base through a selection technique named MDA (Maximum dissimilitude algorithm). Afterwards, mooring system loads and platform motion are numerically simulated. SESAM (DNV) numerical model has been used in this particular application. SESAM numerical model was previously calibrated based on the laboratory tests. Finally, based on a multidimensional interpolation technique named Radial Basis Function life cycle mooring system loads were reconstructed.

A sensitivity analysis of the methodology were carried out. Based on it, it can be concluded that selecting 1000 sea states with the MaxDiss technique, life cycle mooring loads can be accurately predicted.
Slamming Loads Calculation for an Oil Tanker
Authors: Chen Chi-Chuan , Huang Chien-Hua , Chen Kuan-Chen , Wang Po-Wen
Published: September 2014

The International Association of Classification Societies (IACS) Harmonised Common Structural Rules (CSR-H) have been released and will take effect on July 1, 2015. The local loads under extreme motion conditions, defined in the CSR-H, were investigated for an oil tanker by using computational fluid dynamics (CFD) software, STAR-CCM+, and included bow flare slamming pressure and bottom slamming pressure at the bottom of the bow or stern bottom. The extreme motion defined in the CSR-H was investigated using the potential code HydroSTAR. To benchmark the numerical results, resistance and seakeeping model tests were performed on the oil tanker. The two test results were simulated using the two software packages, respectively, and the numerical results were in good agreement with the test results. Finally, this study demonstrated that the slamming pressures defined by the CSR-H are safe and conservative with regard to the structural design.
Hydrodynamic Response of Multiple Fish Cages Under Wave Loads
Authors: Hu Ke , Fu Shixiao , Xu Yuwang , Ma Leixin , Chen Yifan
Published: September 2014

Nowadays, marine aquaculture is playing an important role in meeting people’s ever increasing need for sea foods high in protein. The multiple fish cages are commonly used in large fish farms currently. Mooring systems in these fish cages should be strong enough to withstand extreme environmental loads. Thus, a reliable mooring system is required in designing safe structures. In this paper, a finite element model is built. The floating collar is simulated by the beam element, the twine of the net and the mooring lines are simulated by the truss elements. The geometric nonlinearity of the net model and the material nonlinearity of the mooring line are considered. In this analysis, the hydrodynamic forces estimated by Morison equations are applied to the model. The hydrodynamic response of multiple fish cages under the wave loads by considering the pretension of mooring line is carefully studied. The maximum tension in mooring lines under different wave propagation direction is also analyzed in this study.
Investigation of Coalescing and Bouncing of Rising Bubbles Under the Wake Influences Using SPH Method
Authors: Sun Pengnan , Ming Furen , Zhang Aman , Yao Xiongliang
Published: September 2014

In the exploiting and processing of submarine energy, such as natural gas, petroleum and combustible ice, it is always accompanied with multi-phase flow of large density and viscosity ratio, like bubbly flows. The essential mentioned subjects are bubbles rising in the viscous medium, the coupling effects among rising bubbles. In this paper, SPH method is used to simulate the interaction between two bubbles, where the focusing problem is the interface between gas and liquid. The multiphase flow characteristics are greatly influenced by surface tension and viscous force especially when the characteristic length scales are relatively small. As many experiments in previous literatures indicate, the rising bubbles are often followed by a long tail which greatly affects the shape and motion path of a single bubble and bubble groups. Though Boundary Element Method (BEM) may be well used to simulate the movement and deformation of a single bubble, there are still many challenges in simulating the bubble interactions like coalescing and bouncing. The traditional Smoothed Particle Hydrodynamics (SPH) method was well employed in simulating moving boundary and large deformation problems in single-phase problems, but in the ocean engineering, the density and viscosity ratio at the gas-liquid interface may be up to nearly 1000 and 100 respectively, which will always cause unphysical penetrations and pressure fluctuations at the gas-liquid interface. The present improved SPH algorithm based on volume approximation can guarantee the continuous conditions at the gas-liquid interface. Through a staggered particle distribution and an appropriate re-mesh, the process of rising, pulsing and jet of a single bubble is simulated, which agree well with that of experiments in the existing literatures. Besides, the trails of the rising bubble and interactions among bubbles are studied. On these bases, the coalescing and bouncing of two bubbles posited at different directions are simulated, which are consistent well with the experiment carried out in previous literatures. The present studies aims to provide a reference for the industrialized productions.
Nonlinear Motion Analysis of the Wavestar Wave Energy Converter With a Focus on the Structural Responses
Authors: Zurkinden Andrew S. , Jepsen Michael S. , Sichani Mahdi T. , Damkilde Lars
Published: September 2014

The scope of this paper is to connect a nonlinear WEC numerical model with a structural response model. The numerical WEC model takes into account the nonlinear hydrostatic restoring moment of the Wavestar float. A parameterized structural model of the Wavestar arm is developed in ANSYS APDL. Based on the assumption that the structural displacements remain small, linear first order theory is used to calculate the structural response. The section moments and forces are thus superimposed according to the superposition law. The effect of the nonlinear hydrostatic restoring moment on the structural response is investigated. Moreover, an analysis is carried out which shows that reactive control, applied as a closed loop control, increases the section moments and shear forces.
Study of the Volumetric Behaviour of Ice Rubble Based on Bi-Axial Compression Data
Authors: Kulyakhtin Sergey , Høyland Knut V.
Published: September 2014

Rubble ice in unconsolidated part of ice ridges or rubble fields for some engineering applications can be considered as a granular material. The common practice is to assume that the ice rubble obeys Mohr-Coulomb failure criterion which requires cohesion and internal friction angle as material parameters (see e.g. [1]). In order to obtain friction angle of ice rubble in small scale a bi-axial compression apparatus was proposed by Timco et al. [2]. Later, analyzing test results obtained from this apparatus Timco and Cornett [3] made a rather surprising conclusion that the ice rubble angle of friction is not a constant but has different values for different ratios of strains applied to the sample. This paper gives an overview of biaxial compression data available in the literature. It also shows that if volumetric behavior is included (which is not the case for Mohr-Coulomb criterion) then tests performed on the same types of ice rubble gives the same constant describing shearing resistance independent of applied boundary conditions.
Path Following of Underactuated Marine Underwater Vehicles in the Presence of Unknown Ocean Currents
Authors: Moe Signe , Caharija Walter , Pettersen Kristin Y. , Schjølberg Ingrid
Published: September 2014

The use of autonomous marine vehicles, and especially autonomous underwater vehicles, is rapidly increasing within several fields of study. In particular, such vehicles can be applied for sea floor mapping, oceanography, environmental monitoring, inspection and maintenance of underwater structures (for instance within the oil and gas industry) and military purposes. They are also highly suitable for operations below ice-covered areas in the Arctic. However, there are still many challenges related to making such underwater vehicles autonomous. A fundamental task of an autonomous underwater vehicle vessel is to follow a general path in the presence of unknown ocean currents. There exist several results for underwater vehicles to follow a general path when no ocean currents are present [1] and to follow a geometrically simple path such as a straight line when ocean currents affect the vehicle [2, 3], but the problem of general path following in the presence of unknown ocean currents has not been solved yet. This paper presents a method to achieve this. The results are an extension of the results in [1], and introduce a virtual Serret-Frenet reference frame that is anchored in and propagates along the desired path. The closed-loop system consists of an ocean current observer, a guidance law, a controller and an update law to drive the Serret-Frenet frame along the path, and is shown to be asymptotically stable given that certain assumptions are fulfilled. This guarantees that the autonomous underwater vehicle will converge to the desired path and move along it with the desired velocity. Simulation results are presented to verify and illustrate the theoretical results.
Alternative Method for Estimation of Design Extremes for Moored Floaters
Authors: Bonnaffoux Guillaume , Melis Cécile
Published: September 2014

After having relied for decades totally on small scale model tests, the design of anchoring systems for moored floaters like FPSOs is now widely performed numerically. Estimation of design maxima during mooring analyses requires calculating system response for a large number of sea states in order to screen all possible scenarios between wind, waves and current parameters. In addition, the slow-drift response motions of the system constituted by the floater and the anchoring system are highly dependent of the wave elevation realization, which is not an input parameter of the simulations and can lead to extremely variable responses.

This is generally addressed by designers by performing the analysis for the same sea state and varying the wave group spectrum (or wave components phases) a large number of times N (20–50 realizations is a typical range, see Refs [4], [5]). For these N realizations, N response maxima are extracted, and a distribution of response extremes is derived, from which the response level is extracted. In terms of computational cost, performing N 3-hour simulations to derive N values of extreme response is extremely expensive.

The paper will focus on methods that can be employed to reduce the computational cost of analyses. In a first step, the rapidity of statistical convergence of response estimates depending on the system will be investigated. This will allow pre-determining the number of sea states realizations required to reach a satisfactory convergence of response. In a second step, a mean of improving the computational efficiency of calculations carried-out to reach the statistical convergence will be proposed.
Effects of Top-End Vessel Heave on Submarine Riser VIV of Deep Water Platform
Authors: Chen Weimin , Li Min , Guo Shuangxi , Gan Kuan
Published: September 2014

The dynamic coupling between moving top-end vessel and submarine riser becomes more remarkable for a floating platform in deeper water due to the larger top-end motion amplitude, compared with the fixed platform in shallow water. In this study the impacts of top-end heave on the riser undergoing vortex-induced vibration (VIV) are explored in terms of the parametric excitation and the consequent dynamic behaviors. By using finite element simulations based on a coupled hydrodynamic force approach, the dynamic responses of the integrated system including both a floating top-end and the riser experiencing VIV are examined.

Our numerical results show that the riser displacement becomes several times larger than the displacement for the case without top-end motion, and the impact of heave on riser VIV response gets larger as the modal order number dropping. Riser VIV amplitude becomes, almost linearly, more profound when the tension ratio, as one of critical parameters that influence the riser dynamic response, gets larger. Moreover, an interesting phenomenon called mode transition is observed, particularly at lower frequency, during modal dynamics response.
Research on the Coupling Effects Between Ship Motions and Sloshing
Authors: Zhao Wenhua , Yang Jianmin , Tao Longbin , White David
Published: September 2014

Floating Liquefied Natural Gas (FLNG) facilities, which are a new type of floating platform, have been developed as an alternative to long pipelines for the exploitation of stranded offshore fields. FLNG vessels will be subjected to very complicated and onerous sea states in some regions and very severe motion responses must be avoided. The vessel motion can induce internal tank sloshing, which can in turn affect the global motion response of the vessel. This coupling response is of great importance for the safe design and operation of FLNG facilities in real sea states.

This paper investigates the coupling effect between FLNG roll motions and sloshing. Model tests are carried out for a section of an FLNG vessel containing a tank excited in roll by band-limited white noise waves. During the model tests, the FLNG model is ballasted with fresh water and equivalent steel ballast weights respectively, to quantify the coupling effects due to the internal sloshing. Time histories of the global motions and the internal sloshing oscillations have been measured. Statistical and spectral analyses have been carried out on the measured data. The response amplitude operators are obtained using measured motion spectra divided by the excitation wave spectrum. The influence of the internal sloshing on the global motions has been illustrated through the comparison of the experimental results between the liquid and solid (steel) ballast weight cases. Based on the experimental results, some conclusions regarding the coupling mechanism between FLNG motions and sloshing are drawn.
Numerical Simulation of Wave in Deck Loading on Offshore Structures
Authors: Chen Yu , Wu Yanling , Stewart Graham , Gullman-Strand Johan , Lu Xin
Published: September 2014

Extreme wave impacts on the decks of offshore structures with insufficient air gap may cause damage or even collapse with safety, economic, and pollution consequences. In this study, the impact loads on a fixed platform deck have been predicted numerically by employing a Navier-Stokes solver with the free-surface captured by the volume of fluid (VOF) method. 3D numerical simulations of wave-deck interactions for long-crested extreme waves were performed. The simulations successfully captured the evolution of impact loads and free surface of the waves during the interaction with the platform deck. A detailed parametric analysis of wave-deck interactions showed significant differences in loads under various situations and confirmed the large magnitudes of the loads to be expected during impact. The results presented include a solid box and a more realistic case of under-deck beams. These provide a useful benchmark for predicting wave loadings on platform decks and through this research programme the longer term aim is to establish improved guidelines for assessing the risk of existing structures.
Prediction of Corrosion Condition Considering Effect of Maintenance
Authors: Yamamoto Norio
Published: September 2014

Corrosion prediction model which takes an effect of maintenance and repair into account is developed based on the full probability corrosion model. Over-all coating repair, touch-up coating repair and renewing repair of worn member are considered. Change in the corrosion condition due to the maintenance and repair work is modeled and possibility of such change is evaluated.

Based on the developed model, the effects of these maintenance and repair methods to the corrosion condition are numerically examined and discussed. In these numerical examinations, a conventional paint coating and a coating which meets the PSPC (IMO’s Performance Standard for Protective Coatings) specification are examined. Change in corrosion wastage condition due to the maintenance and repair works is evaluated and compared. And the probability that the corrosion wastage exceeds the permissible level is also evaluated.
Experimental Study on High Frequency Effect on Fatigue Strength of Welded Joints by Using Plate-Bending-Vibration Type Fatigue Testing Machines
Authors: Osawa Naoki , Nakamura Tetsuya , Yamamoto Norio , Sawamura Junji
Published: September 2014

Fatigue strength of out-of-plane gusset welded joints subject to springing and whipping superimposed wave loadings is examined by using Plate-Bending-Vibration (PBV) type fatigue testing machines developed in the previous reports [Osawa, N. et al. (2013) Proc. OMAE2013, Paper OMAE2013-11582, Osawa N. et al. (2013) Proc. PRADS2013, pp.550–556]. Springing vibration is superimposed by attaching an additional vibrator to the test specimen, and whipping vibration is superimposed by intermittent hammering. ‘Enlargement counting’ method, in which the stress history is approximated by a waveform with the low frequency component’s period and the enlarged total amplitude, is proposed. Fatigue damages and equivalent stress ranges are calculated by enlargement and rainflow cycle counting methods. It is found that the fatigue life under high frequency superimposed loads can be predicted with acceptable accuracy by the modified Miner rule when enlargement or rainflow stress counting is performed and the S-N curve is modified so that it fits the equivalent stress range’s Ps = 50% curve. Based on test results, a simplified assessment method for high frequency effect on fatigue strength of ship’s welded joints is proposed. The validity of the proposed assessment method should be further examined by carrying out fatigue tests with realistic stress histories which emulate intermittent occurrence of springing and whipping in ship structure.
Symmetric Response of a Hydroelastic Scaled Container Ship Model in Regular and Irregular Waves
Authors: Peng Sheng , Temarel Pandeli , Bennett S. S. , Wu Weiguo , Liu Zhengguo , Wang Yiwen
Published: September 2014

Wave-induced vibrations, such as whipping and springing, of container carriers have been attracting much attention because of their effects on hull-girder bending moments and fatigue damage. An investigation has been carried out comparing experimental measurements and numerical predictions of symmetric wave-induced loads (i.e. vertical bending moment) of the latest River-sea link container ship design, LPP = 130 m. The dual mission characteristics, namely rivers and open seas, make this type of ship an extremely interesting type of container carrier, particularly in terms of springing and whipping.

A backbone beam segmented model is used in the experiments with the focus on springing- and whipping-induced vertical bending moments, for the model travelling at Fn = 0.21 in regular and long-crested irregular head waves, of 2.5m full-scale height or significant wave height. In addition higher order (harmonics) vertical bending moments (VBM) are also extracted from the experiments. The measurements are taken at amidships and the fore and aft quarters. Numerical predictions, for both the full-scale vessel and segmented model, are obtained using the two-dimensional linear hydroelasticity theories, where the hull structure is idealized as a non-uniform beam and the fluid actions evaluated using strip theory.

The measured model test results, in relatively moderate conditions based on a particular area of operation for this low-draught vessel, indicate that nonlinear springing accounts for a significant portion of the total wave-induced bending moments in regular and, to an extent, irregular waves and slamming effects are small due to the operational area selected. The numerical predictions in regular waves show that linear hydroelasticity analysis can only predict similar trends in the variation of the VBM and the resonance peak. On the other hand, in long crested irregular waves the linear hydroelasticity analysis provides peak statistics that are commensurate with the measurements. The numerical predictions were obtained for two variants, having L = LPP and L = 0.9 LPP, the latter corresponding to the length of the backbone.
Cost-Effective Load Monitoring Methods for Fatigue Life Estimation of Offshore Platform
Authors: Perišić Nevena , Tygesen Ulf T.
Published: September 2014

The lifetime of offshore structures are ruled by accumulated fatigue damage so structural load monitoring is of special importance for re-assessment of offshore platforms. However, direct measuring of the actual loading is usually not feasible due to sensor limitations. An alternative approach to direct load measuring is load estimation from the limited number of measurements of dynamic responses of the structure. This paper presents two, conceptually different, model based, methods for load estimation in time domain: modal expansion method and Kalman filter based method. The Kalman filter based method uses the reduced FEM and provides estimations of limited number of signals, where the modal expansion method expands the mode shape vectors to estimate the displacements and stresses at all unmeasured locations. This paper is a part of ongoing development of methods for linear and nonlinear system identification, FEM updating, wave load calibration and lifetime prediction of offshore structures.
Estimation of Weight and Torque on Bit: Assessment of Uncertainties, Correction and Calibration Methods
Authors: Cayeux Eric , Skadsem Hans Joakim
Published: September 2014

The efficiency of a drilling operation is to a great extent governed by how well one is able to optimize the rate of penetration (ROP) throughout each stage of the operation. ROP optimization normally involves balancing drilling speed on the one hand with acceptable wear to the drill bit on the other. The bit lifetime is largely determined by the mechanical conditions at the bit-formation rock interface and the weight on bit (WOB) and torque on bit (TOB) provide important information related to the working condition of the bit. The accuracy of WOB and TOB measurements can thus become a determining factor for the overall drilling efficiency.

Due to the low bandwidth of downhole mud pulse-based telemetry systems, the WOB and the TOB are generally derived from surface measurements, i.e. from the hook load and the top-drive torque. Field experience indicates that a WOB derived from surface measurements can be of limited accuracy, such as when surface measurements suggest a negative WOB even though the ROP is positive, or when high sampling rate and high precision downhole measurements confirm a large discrepancy between the memory recorded downhole data and the estimated values based on the measurements made at the level of the hoisting equipment and the top-drive.

The reason for these inconsistencies is simply that there are numerous physical processes taking place between the bit and the surface measurements that are normally not accounted for when WOB and TOB are estimated. This paper reviews and analyses the sources of these deviations and models the physical processes in order to quantify the precision for which the WOB and the TOB can be ascertained using solely surface measurements. Methods are also proposed that compensate for certain side effects by utilizing real-time torque and drag and hydraulic calculations.
Analysis of the Fatigue Life of Offshore Wind Turbine Generators Under Combined Ice- and Aerodynamic Loading
Authors: Hendrikse Hayo , Renting Frank W. , Metrikine Andrei V.
Published: September 2014

A modelled wind turbine generator subjected to combined ice- and aerodynamic loading is analyzed with the focus on its fatigue lifetime. A comparison is made between the prediction of a combined analysis, taking both ice- and wind loads into account simultaneously, and a superposition analysis, computing the response of the structure as a result of ice and wind loading separately. The accumulated fatigue damage is computed considering different descriptions of the ice load. Prescribed ice load curves from current design standards, as well as phenomenological models for the prediction of dynamic ice-structure interaction are employed. Results show that the superposition method underpredicts the accumulated fatigue damage in the range of frequency lock-in, but only when phenomenological models, which are more advanced than those recommended by the design standards, are used to model the ice load. Furthermore the predicted fatigue damage computed using the design standards for the description of the ice load is found to be much larger than that resulting from the application of the phenomenological models. It is concluded that the combined analysis is desired when phenomenological models are applied. Furthermore, improvement of the predictive capabilities of such models might ultimately lead to a reduction of the predicted fatigue damage accumulation of the combined ice- and aerodynamic load case, as compared to the current prescribed methods in standards.
The Geotechnical Challenges of Tidal Turbine Projects
Authors: Small Andrew A. , Cook Greg K. , Brown Michael J.
Published: September 2014

Tidal energy projects offer a predictable renewable energy source. Therefore several sites around the coast of the United Kingdom (UK) have been identified to have the potential to host multiple array turbine developments that could be used to generate upwards of 100MW each.

However, the high tidal energy environment required to provide the input to tidal power devices present a combination of design and installation challenges to developers and engineers. Whilst the water depths are comparable to offshore oil and gas projects in the southern North Sea, tidal energy projects present significantly different seabed and associated geotechnical challenges that require novel solutions.

Concerns have been expressed regarding the design of turbine base structures, which is often based upon typical nearshore/offshore soil sampling and geotechnical laboratory testing techniques that are often inappropriate for the seabed materials encountered. This issue is further compounded by the design constraints imposed on the base structure foundations (e.g. tripod gravity base structures). These include strict leveling tolerances required for efficient turbine operation, and which require a detailed understanding of seabed bathymetry at each turbine location where multiple seabed contact points may be required.

In addition to the tidal energy turbines, inter-array cables that connect the turbines and export cable routes which distribute the power to shore also present their own design and installation challenges. It is often overlooked, but significant uncertainty lies in the definition of cable corridors with respect to the seabed materials. In many cases burial may be considered as a means of product protection where seabed conditions allow.

This paper aims to examine the geotechnical issues associated with the difficult seabed and environmental conditions present at typical tidal turbine sites in the UK and globally in an attempt to offer some potential solutions. These issues typically include; difficulties in performing site investigation operations, coarse mobile seabed sediments, challenging seabed topography, exposed bedrock and hard substrates, derivation of appropriate geotechnical parameters, difficulties in foundation design and installation, high cyclic environmental loading, array and export cable lay, on-bottom stability and mechanical protection, amongst others.
Study on the Probabilistic Scenario Based Fire Risk Analysis of FPSO Topside Module
Authors: Jin Yanlin , Jang Beom Seon
Published: September 2014

Many offshore platforms are normally exposed to the flammable oil & gas circumstances; hence hydrocarbon fire is a big threat to the offshore platform. Fire accidents on the offshore platform usually occupy a major part of the total risks. Nevertheless, existing methods of fire risk analysis used in the industry are still not intact enough to advisably evaluate the risks related to fire accident especially for supporting structures of process modules. For this reason a new procedure of fire risk analysis is established in this paper to overcome those deficiencies in the existing methods. The new method is constructed through a scenario based probabilistic approach.

In the new FRA (Fire Risk Analysis) procedure, the key parameters used for constructing fire scenarios are cautiously considered and discussed in case of causing a conservative judgment. Also, dynamic effect of transient fire accident is properly considered by applying an effective method called ‘snapshot’ during the CFD fire simulation. Furthermore, for investigating the structural failure under the fire, thermal elasto-plastic structural analysis is included in the new FRA.

In the former FRAs, the traditional exceedance curve with certain physical variable is used in the frequency analysis to combine the results of fire simulation and fire frequency calculation. The main purpose of using exceedance curve is for deciding a DAL (Design Accidental Load), which is used for structure design against fire accident or even determining of the proper risk mitigation measures e.g. PFP (Passive Fire Protection). However, different from former FRAs, structure response analysis is carried out in the new procedure and the results are taken into consideration through a newly proposed concept i.e. structural cumulative failure frequency. The purpose of proposing cumulative failure frequency in the new FRA can be summarized as following two aspects. One is to avoid the difficulties of applying the identified DAL to the structure analysis in the former FRAs, and other one is to determine the optimized PFP application area for topside structure. For showing the details of new FRA procedure a demo is also demonstrated in this paper.
Green Water on FPSO Analyzed by a Coupled Potential-Flow-NS-VOF Method
Authors: Pakozdi Csaba , Östman Anders , Stansberg Carl-Trygve , de Carvalho e Silva Daniel Fonseca
Published: September 2014

The nowadays frequent use of FPSOs for offshore oil production in areas prone to green water events has increased the industrys focus on wave-induced impact loads as an important design parameter. This is a complex hydrodynamic problem where simplified engineering methods are often used in connection with model testing. Various efforts have been presented during the recent 10–15 years to establish reasonably good industry design tools, while the use of fully nonlinear methods and CFD is still in its development. The main focus of this paper is to investigate the potential of a simplified coupled method between a potential theory based Green Water engineer tool (Kinema3) and the commercial CFD tool Star-CCM+ based on its Navier-Stokes Solver (NS) and the Volume of Fluid (VOF) method. Results from a case study application on a large FPSO are validated against model test data. The case study contains analyses of the FPSO in long crested regular seas, both in fixed and in moored conditions. Three different heading directions are included. The approach for modeling green water events uses a Finite-Volume-VOF method with a complex velocity inlet boundary condition. Thus the Kinema3 engineering tool is used to generate simplified spatio-temporal inlet conditions from the relative wave elevation and wave kinematics at the bulwark, based on linear potential theory combined with nonlinear random wave kinematics. The VOF method is then used to model the detailed flow on deck, including impact forces on deck structures. Kinema3 can also generate simplified estimates for the peak water height, velocity as well as impact force values assuming an extended dam-break approach together with a simplified, local 2D deck layout, and comparisons to the CFD results show an overall fairly good agreement although flow details on deck can of course not be expected to be modeled that well. Comparisons of the above results to model test data show good agreement both for the relative wave height, water height and impact force level, in regular and irregular waves. Detailed time histories, including force rise time, from the coupled Kinema3 - Star-CCM+ CFD simulation analysis are quite similar to the measured ones. The CPU time consumption for the coupled simulation is moderate compared to a full CFD simulation of the FPSO in waves. Hence the achieved calculation time and the simplicity of the simulation setup of the numerical simulation makes this method an interesting candidate for industrial use. This work is a part of the research project “Green Water and Wave Impact on FPSO” carried out for and in cooperation with PETROBRAS.
Consequences of Using the Plane Stress Assumption for Damage Calculations in Crash Analyses
Authors: Walters Carey L. , Voormeeren Lars O.
Published: September 2014

Simulation of failure in plate materials (represented as shell elements) is critical for the correct determination of crash performance of ships and offshore structures. This need has traditionally been filled with failure loci that give the failure strain in terms of stress triaxiality. In recent years, a third dimension (Lode parameter) has been introduced in the form of the Modified Mohr Coulomb failure criterion and Lode parameter adjusted Gurson-type models. This development introduces ambiguity for shell structures, in which only two dimensions are represented. The typical way of addressing this is to assume that shell structures fail in plane stress, thus reducing the problem back to 2-D. However, the assumption of plane stress is violated as soon as necking begins, causing different stress triaxialities and Lode parameters than would be expected from the planar case. More importantly, the inhomogenous necked region is then homogenized over the entire shell element. In this paper, the consequences of the through-thickness plane stress assumption are assessed through a finite element model of a plate that is subjected to a far-field stress.
Relating Structural Loading Rate to Testing Rate for Fracture Mechanics Specimens
Authors: Walters Carey L. , Przydatek Jan
Published: September 2014

It is very well-known that fracture toughness depends on loading rate. Higher strain rates can shift the ductile to brittle transition curve to higher temperatures, resulting in a more brittle structure at the same temperature. However, there is little effort to relate the testing rate to the loading rate within the offshore and maritime industry. For example, BS 7448-1 requires that the stress intensity factor loading rate be 0.5 MPa√m/s to 3.0 MPa√m/s. The loading rates of BS 7448-1 are very far away from the vibrational modes of the specimen, so these limitations are not necessary in order to assure a quasi-static test. In comparison, SSC 275 indicates that normal ship loading rates can be of the order of 220–440MPa√m/s. The results of SSC 275 are consistent with results obtained from a Dutch offshore equipment supplier, who indicates a time to maximum loading of 0.25–1.3 seconds. In general, a conservative loading scenario for the maritime and offshore industry is on the order of 200 times faster than the loading rate that is recommended by BS 7448-1. Testing at the standard rate has the consequence of artificially lowering the ductile to brittle transition temperature by 8–35°C in comparison to a real loading scenario, thus possibly giving a false impression of safety. This means that a CTOD measured as 0.2 mm for static testing conditions could be 0.08–0.15 mm for actual loading. The analysis is shown to be consistent with CTOD test data on a Quenched and Tempered (QT) and a Thermo-Mechanically Controlled Processed (TMCP) S690 grade steel.
A Methodology to Predict the Remaining Fatigue Life of a Flexible Pipe With Broken Tensile Armor Wires
Authors: de Sousa José Renato M. , de Sousa Fernando Jorge M. , de Siqueira Marcos Q. , Sagrilo Luís Volnei S. , Campello George , de Lemos Carlos Alberto D.
Published: September 2014

In this work, an approach to predict the remaining fatigue life of flexible pipes with damages in their tensile armor wires is proposed. This approach relies on a previous proposed approach to calculate the fatigue life of intact flexible pipes. By relying on results from theoretical and experimental investigations, the previous proposed expressions were modified in order to account for damages in the tensile armor wires of these structures. Furthermore, the computation of the fatigue life was also modified in order to account for results from inspections in these pipes thus allowing the estimative of the remaining fatigue life of the pipe. The use of this methodology is illustrated in the analysis of a 9.13″ flexible pipe considering different conditions in its outer tensile armor wires: intact and with one up to ten wires broken along time. The results obtained indicate that the rupture of the tensile armor wires may significantly reduce the fatigue life of flexible pipes and, consequently, may lead to the premature failure of the pipe.