bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculation of VIV Fatigue of Multi-Pipe Systems
Authors: Williams Dara , Kenny Feargal
Published: October 2017

In various regions of the world offshore oil and gas production and exploration is met with the challenges of operating in extreme currents. These extreme current loads increase the risk of vortex induced vibration (VIV) of the riser system and the associated fatigue damage induced by these vibrations. Regions of particular concern are the Gulf of Mexico where strong loop currents occur, East Africa where the Benguela current dominates and regions offshore South America where strong eddy currents can occur. In order to ensure robust riser design for these regions it is necessary to perform detailed evaluations of the expected VIV response of the global riser system. The assessment of VIV response of steel riser systems is commonplace in the industry and there are well established industry practices in relation to this. In addition there are a number of commercially available VIV analysis tools. Whilst commercially available VIV analysis tools have proved effective in the analysis of long slender structures such as risers there are also a number of limitations in the level of complexity of the model that can be achieved. The analysis approach utilized by these models is based on simplified geometries and linearization of complex non-linear interactions. As a result of these simplifications these analysis tools are not suited to the analysis of non-linear multi-pipe models. Many offshore riser configurations consist of one or more pipe or casing strings enclosed by an outer pipe which is exposed to the environment. In a scenario where VIV of the outer pipe occurs then there will also be a corresponding deflection and curvature transmitted to the inner pipe(s). The displacements and associated stresses of the inner pipe(s) are largely dictated by the response of the outer pipe. Thus for multi-pipe configurations where the inner pipe is not constrained by cement or centralisers (e.g. landing string inside marine riser) the issue arises as to how to calculate the fatigue of the inner strings due to VIV oscillation of the outer string. At the time of writing no standard industry approach exists to address this issue and therefore this paper will outline and demonstrate a proposed approach to address this issue.

The objective of this paper is to outline a novel analysis approach for the calculation of inner string fatigue resulting from VIV of the outer pipe. This proposed methodology combines the benefit of simplified VIV analysis tools with more detailed non-linear global finite element modelling techniques to enable a more comprehensive and accurate assessment of fatigue life of the complete system. The benefits and effectiveness of the proposed method are demonstrated through the analysis of a series of case studies which include landing string fatigue and riser fatigue for deep water applications.
Casing Centralization in Irregular Wellbores
Authors: Skadsem Hans Joakim , Saasen Arild , Håvardstein Stein
Published: October 2017

Wellbore irregularities can cause excessive mechanical friction forces while attempting to land casing strings, and unfavorable conditions for primary cementing of casing strings. In an eccentric annulus, the fluid velocity is largest in the wide section of the annulus. The drilling fluid on the narrow side of the annulus may be immobilized due to the lower wall shear stress in this section of the annulus. A direct consequence for the primary cementing operation is the potential for having residual drilling fluid between casing and formation, and thereby failing to achieve zonal isolation and adequate mechanical support for the casing.

Casing strings are usually fitted with centralizers at predetermined intervals in order to achieve a minimum degree of centralization in the wellbore and efficient fluid displacement during primary cementing. Centralizer distribution and design are based on assumptions of regular wellbore geometries and often analytical models for estimating lateral casing string displacement in the well. The latter assumption implies that bending moments are not transmitted across centralizers, and may lead to nonconservative centralizer designs. To investigate the effect of irregularity and casing string stiffness, we consider a stiff string model that approximates the casing string as finite beam elements with bending and axial degrees of freedom at each end, thereby accounting for transmission of both axial and bending stresses between elements. In this work, we evaluate wellbore irregularity by inspecting a six-arm caliper log and estimate the cross-sectional shape of the wellbore by cubic spline interpolation between the arms of the caliper tool.

Analyses of the caliper logs indicate that long, continuous strecthes conform to the nominal wellbore size, and that local hole enlargements may be significant. Irregularities are found to be largely symmetric about the wellbore axis, although some examples exhibit elliptic or oval shapes that may conform with local in-situ stress directions. We detail the stiff string model assumptions and implementation for evaluating casing centralization, and demonstrate the approach on model irregularities and on selected caliper log sections. Calculations suggest that bow spring centralizers result in better casing centralization in vertical parts of the wellbore, while large bow spring compression favour rigid centralizers in more inclined parts of the well. Axial compression close to the bottom of the wellbore section leads to a geometric softening effect of the casing, which affects transverse displacement and centralization between centralizers. Higher in the well where the casing is in tension, a geometric stiffening effect reduces transverse displacement. In proximity of washed out and irregular sections, centralization is affected both by placement of the centralizers and a reduction in the restoring capability of bow spring centralizers.
Wellbore Dynamics of Kick Evolution Considering Hydrate Phase Transition on Gas Bubbles Surface During Deepwater Drilling
Authors: Sun Xiaohui , Sun Baojiang , Wang Zhiyuan
Published: October 2017

It is of high potential and risk to form gas hydrate along the wellbore in deepwater drilled-kick scenarios. Considering the transient mass transfer process that appears as the hydrate shell renewal at gas-liquid interface, we build a fully coupled hydrodynamic-hydrate model to describe the interaction of hydrate phase transition characteristics and wellbore multiphase flow behaviors. Through comparison with experimental data, the performance of proposed model is validated and evaluated. The simulation results show that the hydrate formation region is mainly near the seafloor affected by the fluid temperature and pressure distributions along the wellbore. The volume change and mass transfer over a hydrate coated moving bubble, vary complicatedly, because of the hydrate formation, hydrate decomposition and bubble dissolution (both gas and hydrate). Overall, hydrate phase transition can significantly alter the void fraction and migration velocity of free gas in two aspects: (1) when gas enters the hydrate stability field, a solid hydrate shell will form around the gas bubble, and thereby the velocity and void fraction of free gas can be considerably decreased; (2) the free gas will separate from solid hydrate and expand rapidly near the sea surface (out of hydrate stability field), which can lead to an abrupt hydrostatic pressure loss and explosive development of kick accident. These two phenomena generated by hydrate phase transition can make deepwater gas kick to be “hidden” and “abrupt” successively, and present challenges to early kick detection and wellbore pressure management.
Poro-Elastoplastic Modelling of Uplift Resistance of Shallowly-Buried Pipelines
Authors: Qi Wen-gang , Shi Yu-min , Gao Fu-ping
Published: October 2017

During operational cycles of heating and cooling of submarine pipelines, variations of temperature and internal pressure may induce excessive axial compressive force along the pipeline and lead to global buckling of the pipeline. Reliable design against upheaval buckling of a buried pipeline requires the uplift response to be reasonably predicted. Under wave loading, the effective stress of soil could be reduced significantly in the seabed under wave troughs. To investigate the effects of wave-induced pore-pressure on the soil resistance to an uplifted buried pipeline, a poro-elastoplastic model is proposed, which is capable of simulating the wave-induced pore-pressure response in a porous seabed and the development of plastic zones while uplifting a shallowly-buried pipeline. The uplift force on the buried pipeline under wave troughs can be generated by the pore-pressure nonuniformly distributed along the pipe periphery. Numerical results show that the value of uplift force generally increases linearly with the wave-induced mudline pressure under troughs. Parametric study indicates that the peak soil resistance (under wave troughs) decreases with increasing wave height and wave period, respectively. The ratio of peak soil resistance under wave action to that without waves is mainly dependent on the normalized wave-induced mudline pressure, but influenced slightly by the internal friction angle of soil.
Analysis of Failure Mechanisms in Silica and Carbonate Sands Beneath a Strip Foundation Under Vertical Loading
Authors: Teng Yining , Stanier Sam , Gourvenec Susan
Published: October 2017

This paper reports the deformation behavior of silica and carbonate sands under a strip foundation subjected to uniaxial vertical load. Small-scale physical modelling tests of a strip surface foundation under vertical load were conducted in a geotechnical centrifuge and Particle Image Velocimetry/Digital Image Correlation (PIV/DIC) was used to analyze images of an exposed plane of the model beneath the foundation to visualize the failure mechanisms. The observed mechanisms are interpreted in conjunction with load-settlement response and cone penetrometer resistance profiles. The failure mechanisms are illustrated through normalized vertical and horizontal displacement fields and shear and volumetric strain fields derived from the PIV analysis. Different soil deformation mechanisms and load-settlement responses were observed in the different sands. Soil resistance profiles measured using a miniature cone penetrometer do not correlate with the measured foundation bearing resistance and an interpretation of particle shape effect is introduced to explain the differing behaviors. The results presented improve understanding of the different responses in carbonate sands and silica sand beneath a shallow foundation under vertical load.
Stresses in Tensile Armour Layers of Unbounded Flexible Risers Loaded With External Pressure: Application to Lateral Buckling Mode
Authors: Caleyron Fabien , Leroy Jean-Marc , Guiton Martin , Duchêne Pascal , Estrier Pascal , Vraniskoski Kristof , Damiens Alexandre
Published: October 2017

Life6 software, developed by IFP Energies nouvelles, is the local model used by Technip to determine stresses in tensile armour layers of unbounded flexible risers. These stresses and their variations are then used to predict fatigue limits of the dynamically loaded risers. Life6 is based on periodic conditions to reduce the model length, with the assumption that all the tensile armour wires of a same layer share the same kinematics.

This paper firstly presents recent improvements to obtain a better modeling of tensile armour wires kinematics, when the flexible riser loading includes external pressure. New models of the external sheath and the anti-buckling tapes have been developed and implemented in Life6. The results are successfully compared to a Finite Element periodic model.

Applications to lateral buckling prediction of tensile armour layers are secondly presented in the paper. Indeed, in the design of flexible pipelines for offshore field developments, lateral buckling is one of the critical failure modes for the tensile armour wires, being associated with external pressure and flexible pipe cyclic bending. The latest developments made on the modeling of the external kernel of the flexible pipe allow to use Life6 as the basis of the enhancement of the lateral buckling engineering model used by Technip. It has been calibrated and validated against an extensive full scale test data base resulting in a physical, reliable and fast engineering model to predict lateral buckling mode. In accordance with standards, Technip methodology for flexible pipe lateral buckling determination ensures a robust and competitive design.
A Study on the Operation Strategies for Bottom Otter Trawling in Ise Bay
Authors: Nakamura Kenta , Suzuki Shota , Tabeta Shigeru
Published: October 2017

In order to simulate the bottom trawl fishery in Ise Bay, a typical semi-enclosed bay in Japan, a fishery simulator has been developed that consists of a fish behavioral model and a fishing operations model. The fish behavioral models are developed for two major target species, conger eel and mantis shrimp, which consider the fish migration as well as the growth and the population change. The fish behavioral models are combined with a fishing operation model, which predicts the behavior of trawling boats based on economic conditions and resource distribution as well as the fish catch amount by each boat. The model is applied to evaluate the effects of fishery management, and it is investigated that how the control of fishing efforts or number of owned fishing boats affects the profit of the target fishery. The mesh size of the trawling net which maximizes the total catch is estimated considering both conger eel and mantis shrimp. At the target ports, reduction of boats number will increase the profit because the effect of fixed cost change exceeds the impact of fish catch decrease. However, the optimum reduction rate is different in each port and interactive effect should be also considered.
Fatigue Design Recommendations for Conical Connections in Tubular Structures
Authors: Lotsberg Inge
Published: October 2017

Conical connections are important structural members for the integrity of most types of welded tubular structures. They are for example used in traditional jacket structures for oil and gas production and in monopiles for support of wind turbines where an optimal design is aimed for. From contact with the industry it is noted that there is uncertainty about the basis for the stress concentration factors for conical connections in design standards for fatigue assessment. This is related to how fabrication tolerances are accounted for and how a transition in thickness from the cone to the tubular or the cylinder should be made to minimise stresses due to thickness transitions and fabrication tolerances.

Analytical expressions for stress concentrations at conical transitions are outlined in this paper to get a better understanding of the thickness of the cone and the cylinder. By a proper basis for fatigue design it is possible to control additional stresses from thickness transitions and fabrication tolerances at these connections.
Idealized Structural Unit Method for Dynamic Collapse Analysis of Plates
Authors: Oksina Anna , Lindemann Thomas , Kaeding Patrick
Published: October 2017

Marine and offshore structures are subjected to dynamic loads during the lifetime. The values or directions of dynamic loads rapidly change in time, causing a significant rise of inertial forces in structural elements. Dynamic loads appear as result of ship’s movement at sea, wind and wave acting, machinery operation, hull vibration and sometimes even as result of collision or explosion. The corresponding dynamic forces and moments act on the ship hull provoking the appearance of stresses, often leading to buckling, plastic deformations or fatigue cracks of the structural members. To ensure the safety and reliability of structures under dynamic loading it is necessary to estimate the transient effects on the collapse behavior of plate panels. According to the Common Structural Rules (CSR) the safety of marine structures must be proved performing the hull girder ultimate strength check. As a possible tool for the ultimate strength analysis, the Finite Element Method (FEM) is widely spread among engineers. In spite of the great effectiveness, the transient Finite Element Analysis (FEA) remains very time consuming and sometimes difficult to accomplish well. Therefore, the formulation of the Idealized Structural Unit Method (ISUM) is extended for the dynamic collapse analysis of marine structures.
The Influence of Hold-Back Vessels on the Operation of a DP Drilling Rig: Control System and Stability Analysis
Authors: Huang Alex S. , Tannuri Eduardo Aoun , Queiroz Filho Asdrubal N. , Ianagui André S. S. , Yuba Douglas G. T. , Nogueira Sérgio , Abdalla Thiago C.
Published: October 2017

Certain maritime operations require the accurate positioning of the vessel, and in order to accomplish that DP (dynamic positioning) systems were developed. It combines the information obtained from sensors with the expected dynamic of the ship to better estimate its actual position and the external forces, and with those information the controller allocates the forces among the available actuators so the vessel keeps a desired position.

In situations where drift of the vessel could cause great harm (human, material or environmental losses) it might be necessary to provide additional safeguards. One possible solution is to connect an AHTS (anchor handling tug supply) to the original DP vessel, in order to complement the forces generated by its thrusters. However as shown by Jensen (2008) and IMCA M 185 (2012), this connection could actually degrade the position keeping ability of the vessel, nullifying the purpose of improving the safety of the operation.

The objective of the present paper is to confirm the hypothesis that the use of hold-back vessels to support DP drilling rigs may degrade the performance of the DP system, causing dynamic instability, and to determine the boundaries of operation under which this phenomenon occurs: sea state, parameters of the vessels and force transmitted by the hold-back vessel.

Firstly, an analytical study of the system was done. It was considered a simplified model of two vessels connected by a cable with two degrees of freedom (one for each vessel), since the force applied by a cable is unidirectional. Using control theory, the limiting stiffness of the cable was determined by analyzing the poles of the system. Considering a catenary model for the connecting cable, it was possible to determine the maximum force that could be transmitted between the vessels without the system becoming unstable. The influence of the Kalman Filter in the stability of the system was also studied.

Those results were then compared and confirmed with fast time dynamic simulations of the system, in which the influence of different environmental conditions were also added to the analysis. To complete the study, real time simulations were done on a full mission simulator, equipped with the original Kongsberg DP system for the drilling rig.

The simplified model showed consistent results, validated by the simulations, demonstrating it can be a useful tool when analyzing the stability of two connected vessels.
Wind Loads Simulator for Free-Running Model Ship Test
Authors: Tsukada Yoshiaki , Suzuki Ryosuke , Ueno Michio
Published: October 2017

The authors have developed a wind loads simulator (WiLS) that enables us to carry out free-running model tests for investigating wind effects on ship performance. WiLS provides a free-running model ship with simulated wind loads taking account of supposed true wind speed and direction, and instantaneous model ship speed, drift angle, and heading angle. It does not generate environmental wind but exerts forces and moment on a model ship using three pairs of duct fans. A control PC calculates time varying longitudinal and lateral wind forces and yaw moment using wind loads coefficients estimated beforehand and ship motion data, and distribute them to the three pairs of duct fans. Feedback control ensures the intended wind loads using data from load cells on which the duct fans are mounted and those from accelerometers for correcting inertia forces of the duct fans. This paper reports the concept, configuration, and control procedure of WiLS, and presents tank test data, which confirms effectiveness and usefulness of WiLS as a device in free-running model tests to evaluate ship performance at actual seas.
A Bézier Curve Based Ship Trajectory Optimization for Close-Range Maritime Operations
Authors: Li Guoyuan , Zhang Houxiang
Published: October 2017

Ship maneuvering in close-range maritime operations is challenging for pilots, since they have to not only prevent the ship from collisions and compensate environmental impacts, but also steer it close to the target towards a proper heading. This paper presents a path planner to assist the pilots to foresee the optimal trajectory in the scenario. The path planning is formatted as an optimizing problem to minimize the turning variation fluctuation and the fuel consumption of the ship through ocean current while satisfying the constraint of orientations at the start and the end positions. Taking advantages of Bézier curves’ smoothness and adjustability, feasible trajectories are divided into two categories based on the location of the intersection between the start and end directions, and are designed as a set of parameterized Bézier curves. The variables in the Bézier curves become the state space. By searching the space using an evolutionary technique, the candidate of the Bézier curve that has the best turning and the minimized fuel consumption can be obtained. Through two case studies, the feasibility and effectiveness of the proposed planner is verified.
Residual Stresses Redistribution in Girth Weld Pipe After Reduction of the Wall Thickness
Authors: Ficquet Xavier , Romac Remi , Serasli Karim , Kingston Ed J.
Published: October 2017

Girth welded pipes, such as those located offshore on platforms in the North Sea, are subjected to highly corrosive environment. The need to consider welding residual stresses in the assessment of the fitness for service and damages to these pipes when investigating local corrosion damages across a welded region is therefore important for the operators of the platforms and the manufacturers of the pipes. This paper presents a review of work carried out to ascertain the welding residual stresses present within a thin-walled girth welded pipe mock-up made from steel API 5LX Grade 52 before and after reduction of the wall thickness. The mock-up was manufactured to replicate typical pipes used to convey gas, oil and water through the platforms. The mock-up was of diameter 30” and of thickness 19mm. The incremental deep hole drilling (iDHD), contour, hole drilling, XRD, and ultrasonic technique were applied to characterise the residual stresses in the weld and heat affected zone of the specimen. The residual stresses were then measured during the manufacture of a groove located on the weld at the ID and were compared to an FE prediction. Ultrasonic measurements were then carried out on the outer surface of the pipe and show a significant increase in the residual stress and could be used to monitor the changes in the residual stress caused by internal corrosion.
Comparing Different Approaches for Calculating Wave Impacts on a Monopile Turbine Foundation
Authors: Burmester Simon , de Ridder Erik-Jan , Wehmeyer Christof , Asp Erik , Gujer Philipp
Published: October 2017

In this paper two different approaches to calculate the wave impacts on a monopile are introduced and compared to model test results. Within the scope of the WiFi JIP (Joint Industry Project Wave Impacts on Fixed turbines) model tests at MARIN (Maritime Research Institute Netherlands) and at Deltares were conducted to investigate the effects of steep (and breaking) waves on the support structure of bottom fixed offshore wind turbines. Three different ways to generate breaking waves were used. At MARIN focused waves were generated to obtain breaking wave events. Two different methods to calculate slamming loads on monopiles are compared to the wave impacts measured. The results are analyzed in time history and load maxima plots. In addition the obtained curling factors of the focused waves are compared to irregular sea states recorded on a flat seabed and a sandwave. A good representation of the loads calculated by Wienke’s approach was observed for all waves investigated. However, to estimate the kinematics of focused breaking wave events more accurate methods than stream function are needed.
Offshore Wind Turbine Nonlinear Wave Loads and Their Statistics
Authors: Sclavounos Paul D. , Zhang Yu , Ma Yu , Larson David F.
Published: October 2017

The development is presented of an analytical model for the prediction of the stochastic nonlinear wave loads on the support structure of bottom mounted and floating offshore wind turbines. Explicit expressions are derived for the time-domain nonlinear exciting forces in a seastate with significant wave height comparable to the diameter of the support structure based on the fluid impulse theory. The method is validated against experimental measurements with good agreement. The higher order moments of the nonlinear load are evaluated from simulated force records and the derivation of analytical expressions for the nonlinear load statistics for their efficient use in design is addressed. The identification of the inertia and drag coefficients of a generalized nonlinear wave load model trained against experiments using Support Vector Machine learning algorithms is discussed.
Kill Line Model Cross Flow Inline Coupled Vortex-Induced Vibration
Authors: Wu Baiheng , Le Garrec Jorlyn , Fan Dixia , Triantafyllou Michael S.
Published: October 2017

Currents and waves cause flow-structure interaction problems in systems installed in the ocean. Particularly for bluff bodies, vortices form in the body wake, which can cause strong structural vibrations (Vortex-Induced Vibrations, VIV). The magnitude and frequency content of VIV is determined by the shape, material properties, and size of the bluff body, and the nature and velocity of the oncoming flow. Riser systems are extensively used in the ocean to drill for oil wells, or produce oil and gas from the bottom of the ocean. Risers often consist of a central pipe, surrounded by several smaller cylinders, including the kill and choke lines. We present a series of experiments involving forced in-line and cross flow motions of short rigid sections of a riser containing 6 symmetrically arranged kill and choke lines. The experiments were carried out at the MIT Towing Tank. We present a systematic database of the hydrodynamic coefficients, consisting of the forces in phase with velocity and the added mass coefficients that are also suitable to be used with semi-empirical VIV predicting codes.
A Robust Three-Phase Isenthalpic Flash Algorithm Based on Free-Water Assumption
Authors: Li Ruixue , Li Huazhou Andy
Published: October 2017

Multiphase isenthalpic flash calculations are often required in compositional simulations of steam-based enhanced oil recovery methods. These flash calculations are challenging in the narrow-boiling regions and in the determination of the correct number of existing phases. Based on the free-water assumption that the aqueous phase is pure water, a robust and efficient algorithm is proposed to perform isenthalpic three-phase flash calculations in this work. Multiphase equilibria can be considered by this algorithm, including single-phase equilibria, two-phase equilibria, and three-phase vapor-liquid-aqueous equilibria.

Isenthalpic flash is a type of flash calculation conducted at given pressure and enthalpy for a feed mixture. In the proposed algorithm, assuming the feed is stable, the temperature is first determined by solving the energy conservation equation. Then the stability test on the feed mixture is conducted at the calculated temperature and the given pressure. If the mixture is found unstable, the two-phase and three-phase vapor-liquid-aqueous isenthalpic flash calculations can be simultaneously initiated without resorting to stability tests. To achieve simultaneous flashes, the outer loop is used to update the temperature by solving the energy conservation equation. The inner loop is used to obtain phase fractions and compositions by performing a three-phase free-water isothermal flash. Note that a two-phase isothermal flash will be initiated if an open feasible region in the phase fractions appears in any iteration during the three-phase isothermal flash or any of the ultimately calculated phase fractions from the three-phase flash do not belong to [0,1]. Negative flash is allowed in the three-phase free-water isothermal flash.

A number of example calculations for water/hydrocarbon mixtures are carried out to test the robustness of the proposed algorithm. At low to medium pressures, a good agreement can be achieved between the results obtained by this algorithm and those obtained by the conventional algorithm. This algorithm performs well for the narrow-boiling regions, for example, the three-phase vapor-liquid-aqueous equilibrium region encountered for the water/hydrocarbon mixtures. During the iteration, the new algorithm can readily handle the appearance and disappearance of phases in the inner loop as temperature updates in the outer loop. The number of stability tests involved in the new algorithm is significantly reduced, helping to boost its computational efficiency.
Near Field Expression of Ship Wave Resistance by Yeung’s Method
Authors: Tsubogo Takashi
Published: October 2017

The ship wave resistance can be evaluated by two alternative methods after solving the boundary value problem. One is the far field method e.g. Havelock’s formula, and another is the near field method based on direct pressure integration over the wetted hull surface. As is well known, there exist considerable discrepancies between wave resistance results by far field method and by near field method.

This paper presents a Lagally expression in consistency with Havelock’s formula. In order to derive the Lagally expression, the symmetry of Havelock’s Green function is used in the same manner as Yeung et al (2004). Another expression to examine the relation with water pressure integrations or to ensure physical consistency is also derived by slightly deforming that expression. Some numerical comparisons of wave resistance of Wigley, KCS and KVLCC2 models among by Havelock’s formula, some direct pressure integration methods and present two new near field expressions, are shown to demonstrate consistency numerically.
Modeling the Fatigue Damage Evolution in Welded Joints
Authors: Mikulski Zbigniew , Hellum Vidar , Lassen Tom
Published: October 2017

The present paper presents a two-phase model for the fatigue damage evolution in welded steel joints. The argument for choosing a two-phase model is that crack initiation and subsequent crack propagation involve different damage mechanisms and should be treated separately. The crack initiation phase is defined as the number of cycles to reach a crack depth of 0.1 mm. This phase is modelled based on the Dang Van multiaxial stress approach. Both a multiaxial stress situation introduced by the acting loads and the presence of the multiaxial welding residual stresses are accounted for. The local notch effect at the weld toe becomes very important and the irregular weld toe geometry is characterized by extreme value statistics for the weld toe angle and radius. The subsequent crack growth is based in classical fracture based on the Paris law including the effect of the Stress Intensity Factor Range (SIFR) threshold value. The unique fatigue crack growth rate curve suggested by Huang, Moan and Cui is adopted. This approach keeps the growth rate parameters C and m constant whereas an effective SIFR is calculated for the actual stress range and loading ratio. The model is developed and verified based on fatigue crack growth data from fillet welded joints where cracks are emanating from the weld toe. For this test series measured crack depths below 0.1 mm are available. The two-phase model was in addition calibrated to fit the life prediction in the rule based S-N curve designated category 71 (or class F). A supplementary S-N curve is obtained by the Random Fatigue Limit Method (RFLM). The test results and the fitted model demonstrated that the crack initiation phase in welded joins is significant and cannot be ignored. The results obtained by the Dang Van approach for the initiation phase are promising but the modelling is not yet completed. The fracture mechanics model for the propagation phase gives good agreement with measured crack growth. However, it seems that the prediction of crack retardation based on a threshold value for the SIFR gives a fatigue limit that is overly optimistic for small cracks at the weld toe. The threshold value has been determined based on tests with rather large central cracks in plates. The validity for applying this threshold value for small cracks at the weld toe is questioned. As the present two-phase model is based on applied mechanics for both phases the parameters that have an influence on the fatigue damage evolution are directly entering into the model. Any change in these parameters can then be explicitly taken into account in logical and rational manner for fatigue life predictions. This not the case with the rule based S-N curves that are based on pure statistical treatment of the bulk fatigue life.
Development of a Simulation Tool Coupling Hydrodynamics and Unsteady Aerodynamics to Study Floating Wind Turbines
Authors: Leroy Vincent , Gilloteaux Jean-Christophe , Philippe Maxime , Babarit Aurélien , Ferrant Pierre
Published: October 2017

Depending on the environmental conditions, floating Horizontal Axis Wind Turbines (FHAWTs) may have a very unsteady behaviour. The wind inflow is unsteady and fluctuating in space and time. The floating platform has six Degrees of Freedom (DoFs) of movement. The aerodynamics of the rotor is subjected to many unsteady phenomena: dynamic inflow, stall, tower shadow and rotor/wake interactions. State-of-the-art aerodynamic models used for the design of wind turbines may not be accurate enough to model such systems at sea. For HAWTs, methods such as Blade Element Momentum (BEM) [1] have been widely used and validated for bottom fixed turbines. However, the motions of a floating system induce unsteady phenomena and interactions with its wake that are not accounted for in BEM codes [2]. Several research projects such as the OC3 [3], OC4 [4] and OC5 [5] projects focus on the simulation of FHAWTs.

To study the seakeeping of Floating Offshore Wind Turbines (FOWTs), it has been chosen to couple an unsteady free vortex wake aerodynamic solver (CACTUS) to a seakeeping code (InWave [6]). The free vortex wake theory assumes a potential flow but inherently models rotor/wake interactions and skewed rotor configurations. It shows a good compromise between accuracy and computational time.

A first code-to-code validation has been done with results from FAST [7]on the FHAWT OC3 test case [3] considering the NREL 5MW wind turbine on the OC3Hywind SPAR platform. The code-to-code validation includes hydrodynamics, moorings and control (in torque and blade pitch). It shows good agreement between the two codes for small amplitude motions, discrepancies arise for rougher sea conditions due to differences in the used aerodynamic models.
Green Water on a Fixed Model in a Large Wave Basin: Flow Velocity, Void Fraction, and Impact Pressure Distributions
Authors: Chuang Wei-Liang , Chang Kuang-An , Mercier Richard
Published: October 2017

Green water impact due to extreme waves impinging on a fixed, rectangular shaped model structure was investigated experimentally. The experiment was carried out in the large wave basin of the Offshore Technology Research Center at Texas A&M University. In the study, two wave conditions were considered: a plunging breaking wave impinging on the frontal vertical wall (referred as wall impingement) and a breaking wave directly impinging on the deck surface (referred as deck impingement). The aerated flow velocity was measured by employing the bubble image velocimetry (BIV) technique with high speed cameras. The pressure distribution on the deck surface was measured by four differential pressure sensors. The fiber optic reflectometer (FOR) technique was employed to measure the void fraction in front of each pressure sensor end face. The flow velocity, void fraction, and impact pressure, were synchronized and simultaneously measured. Comparisons between an earlier study by Ryu et al. (2007) and the present study were performed to examine the scale effect. Results between Song et al. (2015) and the present results were also compared to investigate the influence of structure geometry on green water flow and impact pressure. To examine the role of air bubbles during the impact, the velocity, pressure, and void fraction were correlated. Correlation between the peak pressure and the aeration level shows a negative trend before the wave impingement but a positive linear relationship after the impingement.
Effect on Doppler Resonance From a Near-Surface Shear Layer
Authors: Smeltzer Benjamin K. , Li Yan , Ellingsen Simen Å.
Published: October 2017

For waves generated by a wave source which is simultaneously moving and oscillating at a constant frequency ω, a resonance is well known to occur at a particular value τres of the nondimensional frequency τ = ωV/g (V: source velocity relative to the surface, g: gravitational acceleration). For quiescent, deep water, it is well known that τres = 1/4. We study the effect on τres from the presence of a shear flow in a layer near the surface, such as may be generated by wind or tidal currents. Assuming the vorticity is constant within the shear layer, we find that the effects on the resonant frequency can be significant even for sources corresponding to moderate shear and relatively long waves, while for stronger shear and shorter waves the effect is stronger. Even for a situation where the resonant waves have wavelengths about 20 times the width of the shear layer, the resonance frequency can change by ∼ 25% for even a moderately strong shear VS/g = 0.3 (S: vorticity in surface shear layer). Intuition for the problem is built by first considering two simpler geometries: uniform current with finite depth, and Couette flow of finite depth.
Experimental Studies of Pressure Loss for Large Particle Slurry Transport in Oscillated Pipe for Subsea Mining
Authors: Takano Satoru , Masanobu Sotaro , Kanada Shigeo , Ono Masao , Araki Motoki , Sasagawa Hiroki
Published: October 2017

Subsea minerals exist in the deep water within Japanese exclusive economic zone. Development of slurry pump passing large particles is required for lifting ore. In design of slurry pump, it is significant to estimate the pressure loss in a riser pipe for large particle slurry transport. Therefore the authors have been studied the slurry flow model for large particle slurry transport. In addition, the authors developed the model for the static pipe including the inclined configurations. Since the lifting pipe will be oscillated due to the connected ship motion and VIV (Vortex Induced Vibration), the authors conducted the scaled model experiment to investigate the effects of pipe oscillation on the pressure loss. The model scale was 1/8. Alumina beads and glass beads were used as solid particles in the experiment. The pipe was vertical, and oscillated in horizontal or vertical direction. The experimental results showed that the horizontal and vertical oscillation had little influence on the static pressure loss in most of the experimental conditions. However the influence was observed for the horizontally oscillating pipe in the low slurry velocity and short oscillation period condition. On the other hand, the significant fluctuation components of pressure loss and flow rate were observed in vertically oscillating pipe. The results also indicated that the density of slurry and amplitude of oscillation had influence on the fluctuation components of pressure loss and flow rate but the particle diameters had little influence on them.
Numerical Study of a Moored Structure in Moving Broken Ice Driven by Current and Wave
Authors: Su Biao , Aarsæther Karl Gunnar , Kristiansen David
Published: October 2017

This paper presents a numerical model intended to simulate the mooring load and the dynamic response of a moored structure in drifting ice. The mooring lines were explicitly modelled by using a generic cable model with a set of constraint equations providing desired structural properties such as the axial, bending and torsional stiffness. The 6 degrees-of-freedom (DOF) rigid body motions of the structure were simulated by considering its interactions with the mooring lines and the drifting ice. In this simulation, a fragmented ice field of broken ice pieces can be considered under the effects of current and wave. The ice-ice and ice-structure interaction forces were calculated based on a viscoelastic-plastic rheological model. The hydrodynamic forces acting on the floating structure, mooring line and drifting ice were simplified and calculated appropriately. The present study, in general, demonstrates the potential of developing a full numerical model for the coupled analysis of a moored structure in a broken ice field with current and wave.
Deep Water XLPE Cable With Aluminum Conductor: Risk of Stress Induced Electrochemical Degradation (SIED)
Authors: Bengtsson Magnus , Floden Randi , Clasen Torunn Lund
Published: October 2017

One of the challenges with cables in deep waters is the high topside tension caused by the weight of the cable and the risk of overloading the conductor. Keeping the cable weight as low as possible is therefore a key parameter to reach deep waters. One way to reduce the cable weight is to replace the copper conductor in the cable with an aluminium conductor.

When using aluminium conductor in a wet design XLPE cable (no water barrier) there is a risk of local corrosion of aluminium that forms porous structures in the inner semiconductor, leading to accelerated ageing of the XLPE insulation. This phenomenon is known as Stress-Induced Electrochemical Degradation (SIED). SIED has so far only been seen in service-aged cables, which had been exposed to external mechanical damage causing water penetration in the conductor. It has also been found that the deterioration of the inner semicon is more severe when the cable is mechanically stressed. In order to learn more about the risk of SIED in a wet XLPE cable an extensive ageing test program has been carried out. This paper summarizes the results from this testing.
Less=Moor: A Time-Efficient Computational Tool to Assess the Behavior of Moored Ships in Waves
Authors: Wang Yijun , van Deyzen Alex , Beimers Benno
Published: October 2017

In the field of port design there is a need for a reliable but time-efficient method to assess the behavior of moored ships in order to determine if further detailed analysis of the behavior is required. The response of moored ships induced by gusting wind and/or waves is dynamic. Excessive motion response may cause interruption of the (un)loading operation. High line tension may cause lines to snap, introducing dangerous situations. A (detailed) Dynamic Mooring Analysis (DMA), however, is often a time-consuming and expensive exercise, especially when responses in many different environmental conditions need to be assessed.

Royal HaskoningDHV has developed a time-efficient computational tool in-house to assess the wave (sea or swell) induced dynamic response of ships moored to exposed berths. The mooring line characteristics are linearized and the equations of motion are solved in the frequency domain with both the 1st and 2nd wave forces taken into account. This tool has been termed Less=Moor.

The accuracy and reliability of the computational tool has been illustrated by comparing motions and mooring line forces to results obtained with software that solves the nonlinear equations of motion in the time domain (aNySIM). The calculated response of a Floating Storage and Regasification Unit (FSRU) moored to dolphins located offshore has been presented.

The results show a good comparison. The computational tool can therefore be used to indicate whether the wave induced response of ships moored at exposed berths proves to be critical. The next step is to make this tool suitable to assess the dynamic response of moored ships with large wind areas, e.g. container ships, cruise vessels, RoRo or car carriers, to gusting wind. In addition, assessment of ship responses in a complicated wave field (e.g. with reflected infra-gravity waves) also requires more research effort.
Comprehensive Instrumentation of Two Offshore Rigs for Wellhead Fatigue Monitoring
Authors: Nilsen Svein Herman , Russo Massimiliano , Grytøyr Guttorm
Published: October 2017

Over the last decades, the complexity and duration of offshore drilling operations have steadily increased. The size and weight of the risers and BOP stack has grown significantly. These factors have led to an increase in fatigue loads imposed on the wellhead structures during drilling and completion operations. Wellhead fatigue might ultimately lead to loss of well structural integrity and pressure containment and therefore safe and reliable drilling of subsea wellheads has gained high priority in the global oil and gas industry. This paper presents two of the most complex real time instrumentation campaigns for drilling operations.

Analyses of a connected drilling riser system including the well structure are complex and involve several engineering disciplines. In addition, there are many unknowns going into the equations when accumulated fatigue damage of the wellhead is estimated. Therefore, assumptions need to be made, very often on the conservative side. A typical example are the global drilling riser analyses where the environmental conditions, actual rig motion and riser / BOP behavior are uncertain.

With the duplex scope of accurately documenting the wellhead fatigue status during drilling operations and of achieving a better understanding of the actual risk level of wellhead fatigue, Statoil decided to start a very comprehensive monitoring campaign. Two MODU representing very different generations of rigs in terms of weights and types of equipment were instrumented from topside to BOP connector. Strain gauges were installed around the BOP connector as close as possible to the wellhead in order to capture wellhead response as accurately as possible. Due to the large number of sensors, high accuracy requirement and high sampling frequency of data to be shown live, a cabled solution was selected vs remote battery operated sensors transmitting via acoustic. Double set of cables, sensors and topside equipment were installed in order to make the instrumentation system fully redundant and suited for permanent installation. All data were additionally made available real time onshore to allow the full overview of the operation. To author’s knowledge, these two instrumentation systems are the most comprehensive and complex of this type installed on a drilling riser as of today. The first of the two system was installed approximately three years ago and it is still in operation. This paper describes the instrumentation systems installed and gives an extract of the quality data extracted and already used in already published studies [1, 2, 3].