bluuebunny/crossref_metadata_2025 huggingface dataset:
A CFD Study of Focused Extreme Wave Impact on Decks of Offshore Structures
Authors: Lu Xin , Kumar Pankaj , Bahuguni Anand , Wu Yanling
Published: September 2014

The design of offshore structures for extreme/abnormal waves assumes that there is sufficient air gap such that waves will not hit the platform deck. Due to inaccuracies in the predictions of extreme wave crests in addition to settlement or sea-level increases, the required air gap between the crest of the extreme wave and the deck is often inadequate in existing platforms and therefore wave-in-deck loads need to be considered when assessing the integrity of such platforms.

The problem of wave-in-deck loading involves very complex physics and demands intensive study. In the Computational Fluid Mechanics (CFD) approach, two critical issues must be addressed, namely the efficient, realistic numerical wave maker and the accurate free surface capturing methodology. Most reported CFD research on wave-in-deck loads consider regular waves only, for instance the Stokes fifth-order waves. They are, however, recognized by designers as approximate approaches since “real world” sea states consist of random irregular waves. In our work, we report a recently developed focused extreme wave maker based on the NewWave theory. This model can better approximate the “real world” conditions, and is more efficient than conventional random wave makers. It is able to efficiently generate targeted waves at a prescribed time and location. The work is implemented and integrated with OpenFOAM, an open source platform that receives more and more attention in a wide range of industrial applications.

We will describe the developed numerical method of predicting highly non-linear wave-in-deck loads in the time domain. The model’s capability is firstly demonstrated against 3D model testing experiments on a fixed block with various deck orientations under random waves. A detailed loading analysis is conducted and compared with available numerical and measurement data. It is then applied to an extreme wave loading test on a selected bridge with multiple under-deck girders. The waves are focused extreme irregular waves derived from NewWave theory and JONSWAP spectra.
Deep Water Installation, Hydrodynamic Aspects and Practical Considerations
Authors: van Dijk Radboud , van Zandwijk Kees , Harmsen Eelco
Published: September 2014

The past decade has shown a fast growing subsea production market tending to complete factories on the sea floor involving ever more complicated infrastructure and increasing size and weight components to be installed. The water depth of this market typically ranges between 1000 and 3000m. This new subsea market triggers a need for increased deep water lowering capacity to enable safe, fast and efficient installation of this infrastructure.

The authors’ company has gained extensive experience in the field of deep water installation over the past 15 years, ranging from complex mooring foundations to the installation of large subsea production units in up to 3000m water depth. This experience is used and further deployed in the specification of new and higher capacity deep water lowering systems for recent and upcoming investments.

The paper will share a number of the contractor’s considerations with respect to a state-of-the-art deep water lowering system meeting the needs of today’s subsea production market. The paper will address the challenges associated with the various stages of deep water installation, choices made in type of ropes (steel versus fiber), pros and cons of various floater types that can be used as an installation platform and the need for and effect of passive or active heave compensation.
Lifting Analysis of Subsea Framework Structures
Authors: Søfteland Tor E. , Skrunes Odd V. , Karunakaran Daniel
Published: September 2014

Installation of subsea structures and equipment involves a lifting operation where the objects are exposed to large hydrodynamic forces when entering the oscillating sea-surface. During deployment, as the structure is lifted through the splash zone, snap forces due to slack or overload due to dynamic forces contribute to the maximum load experienced by the structure over the course of its design lifetime. This paper presents a method of how to verify the structural integrity of a subsea framework including determination of the maximum allowable sea-state in which the structure is safely installed. As well as describing an overall methodology for a subsea lifting analysis, hydrodynamic coefficients for cylinders in the splash zone is provided.
Reduction of the Non-Causal Horizon of the Optimal Wave Energy Converter Control
Authors: Genest Romain , Clément Alain H.
Published: September 2014

In our days, wave energy still remains an important resource of renewable energy that has not been yet completely exploited and fully understood. Various prototypes of point absorbers have already been tested numerically and experimentally in wave-tanks or real sea, but only a few of them has reach the full scale prototype stage. For the family of wave absorbers based on oscillating bodies principle, the energy production may be enhanced by motion control. The choice of a particular mode of control remains decisive in the design of point absorbers and is closely linked to the mechanism architecture. It has been shown [1] that the theoretical maximum absorption can be reached by bringing the system into resonance applying a so called “complex-conjugate” control. Several sub optimal control strategies have been derived from this observation, trying to overcome the draw-backs of this method, mainly the non-causality of the optimal control [3]. Non-causality implies that one needs to predict the excitation signal in the near future to optimize the control command. The aim of the present study is to propose a new methodology to reduce the prediction horizon needed to apply a complex-conjugate control. Afterwards, a simplification is made leading to a causal non-adaptive control. In this study, a cylindrical buoy constrained to move in heave only is employed to test numerically the aforementioned control. Numerical comparisons are made under regular and irregular waves with the performance of control based on the classical complex-conjugate method. The new method shows a good energy absorption capacity for a broad range of frequency without having to adapt the control regulator unit to the incident waves.
Shaft Response as a Propulsion Machinery Design Load
Authors: Polić Dražen , Ehlers Sören , Æsøy Vilmar , Pedersen Eilif
Published: September 2014

A vessel navigating in ice-covered waters experiences an additional ice-related load that acts on the propulsion machinery. Thus, the influence of this load on the propulsion machinery must be known, because it affects the safety and efficiency of the design and operation of the ice-going vessel. The ice-related load, in full-scale trials and model-scale tests, is typically measured at the shaft line between the propeller and diesel engine. Hence, the measured load includes a dynamic response of all elements in the mechanical transmission line. Therefore, this paper describes an approach to link the measured ice-related response of the shaft with the ice-propeller load. Therein, the rule-based DNV ice-propeller load is implemented in a bond graph model of the propulsion machinery system, which is used to obtain the dynamic response of all elements of the machinery system from the propeller to a diesel engine. The obtained shaft response is collected, filtered with different sampling frequency and converted to the ice-propeller load using a developed reverse model of the propulsion machinery. Furthermore, two different models of the shaft line: a rigid and a first order flexible are developed and verified. As a result, the converted ice-propeller load is compared with the rule-based DNV ice-propeller load and the influence of the sampling frequency and shaft models on the converted ice-propeller load is presented.
Estimation of Failure Parameters for Finite Element Simulations Based on a Single State of Stress and Arbitrary Stress-Strain Relation
Authors: Voormeeren Lars O. , Walters Carey L. , Tang Lisa , Vredeveldt Alex W.
Published: September 2014

Crash analysis in the maritime and offshore industry typically relies on failure criteria that are based only on uniaxial states of stress. However, it is well known that the failure strain depends on the state of stress, and the state of stress in ship collisions is not necessarily uniaxial. Although more detailed failure models are routine in other industries, the cost of finding the parameters for these models remains a barrier within the maritime and offshore industry. This paper explores an approach of applying the Modified Mohr-Coulomb (MMC) failure criterion based on a single tension test. This is based on the observation that there is only a single independent calibration parameters when a von Mises yield locus and plane stress are assumed. The MMC yield condition is also generalized by applying it to an arbitrary stress versus strain relationship, which does not follow the power hardening law.

Results of impact tests on small-scale cryogenic storage tanks filled with water and air are presented and are used to validate the adjustments made above. This validation test also shows how the above findings can be applied to storage tanks for LNG fuel on inland waterway ships.
Hull Structural Analysis of Turret-Moored FPSOs Considering Hull–Turret Interaction
Authors: Lee Ki-Myung , Choi Won-Hyuk , Kim Hyun Soo , Moon Seung Han , Kim Jin Tae
Published: September 2014

For turret-moored ship-type offshore structures such as floating production storage and offloading (FPSO) units, the hull structure is affected by mooring and riser loads that are transferred through turret systems, in addition to environmental loads on the hull itself. Moreover, the existence of turret structures has an influence on the structural behavior of the hull around the turret system. In the structural design of FPSOs, the turret structure and its loads are considered in a direct analysis of hull structure for a realistic strength assessment of FPSOs.

This paper investigates several specific techniques for hull structural analysis considering the interaction with the turret system. The linear gap function is utilized to represent the nonlinear contact behavior between the hull and turret structures. The linear superposition of structural responses is also adapted, and its validity is demonstrated in the case of hull–turret contact problems. These studies conclude that the hull structures with turret systems that involve contact nonlinearity in their interface can be assessed using the conventional hull strength assessment based on linear analysis. Moreover, by including the turret model directly in hull structural analysis, the uncertainty arising from hull–turret interface loads can be reduced, and a robust and adaptive design procedure can be set up in the detailed engineering stages.
Modelling Installation and Construction of Offshore Wind Farms
Authors: Stempinski Florian , Wenzel Sebastian , Lüking Jan , Martens Luigi , Hortamani Mahboubeh
Published: September 2014

While the offshore climate in the North Sea bears a great potential for the exploitation of reliable and powerful wind energy it poses a challenge for the constructors of offshore wind farms. Large heavy lift jack-up vessels (HLJV) are employed to transport the components of the wind energy converters to the offshore location. After a preloading and jacking procedure, subsea lifts of tripod foundations weighing up to 950tons as well as tower and nacelle installations at large heights need to be undertaken. As typical offshore wind farms consist of 80 or more separate wind turbines the installation works are conducted in a serial manner — often through the winter season. Thus, many critical offshore operations are conducted consecutively on the basis of daily or weekly weather reports. These operations cannot rely on optimal weather conditions therefore planning and engineering has to cover appropriate wind and wave conditions taking into account contingencies for uncertainties in the reliability of weather windows as well as in the soil conditions.

This paper shows how the weather criteria derived from numerical seakeeping and structural simulations are taken into a project simulation model covering 90 separate serial installations. Based on hindcast re-analyses installation simulations are conducted in multi-seasonal weather scenarios. This enables the quantification of the suitability of a particular marine spread and its associated installation processes in combination. The risk profile of weather related delays are derived.
Reliable and Efficient Injury Assessment for Free-Fall Lifeboat Occupants During Water Entry: Correlation Study Between Lifeboat Acceleration Indicators and Simulated Human Injury Responses
Authors: van Dam E. , Uittenbogaard J. , Reinholdtsen S. A. , Fouques S. , Sauder T.
Published: September 2014

The evacuation of personnel from an offshore installation in severe weather conditions is generally ensured by free-fall lifeboats. During the water entry phase of the launch, the lifeboat may be subject to large acceleration loads that may cause harmful acceleration-induced loads on the occupants.

The present/common methodology for assessing the occupant safety of free-fall lifeboats uses one single characteristic launch to perform injury risk analysis for a given free-fall lifeboat launch condition that includes e.g. weather conditions, lifeboat and host installation loading conditions.

This paper describes an alternative methodology to fully assess the risk of injury for lifeboat occupants during water entry by introducing a correlation model between acceleration load indicators and injury responses. The results are presented in terms of seating matrices showing critical seat rows, in which the probability of being injured exceeds a pre-defined threshold.
A Hybrid Bayesian Belief Network Model for Risk Modeling of Arctic Marine Operations
Authors: Farid Farzad , Lubbad Raed , Eik Kenneth
Published: September 2014

Optimizing the design of offshore structures to withstand ice loads is a challenging task and various efforts are under way to develop robust concepts with acceptable structural safety. To ameliorate the deficiencies of structural design, as well as to reduce the costs of such Arctic offshore field developments, ice management operations may be considered to reduce the ice severity. Ice management in sea ice will typically involve use of 1 to 4 icebreakers depending on the operating environment. The ice management fleet is aimed at protecting the offshore installation by breaking the incoming ice into smaller pieces and by reducing the confinement in the ice cover. Failure of such a marine operation in the demanding Arctic environment can threaten the integrity of the offshore platforms or drilling units, e.g. by increasing the chances of failure of mooring lines in the occurrence of extreme events. Therefore, understanding the causes of such potential failure, as well as the factors influencing it is of crucial importance in order to plan for and mitigate the risks. Factors with an influence on the risk are called risk influencing factors (RIFs) and can be technical, organizational and human. RIFs are identified and structured in this study in a way that they affect the basic events of a conventional fault tree analysis and consequently the total risk. In this study, the RIFs are treated as uncertain variables. The established model is called a hybrid model because it is a merger of a Bayesian belief network (BBN) for the RIF structure and a conventional fault tree model. The Bayesian framework provides the opportunity for updating of the model constituents as more evidence becomes available over time. Case studies are defined to illustrate the methodology. Results show how the improvement in the status of the RIFs (better practices) can improve the reliability of the mooring lines of a floating unit and how the precision in data and other model parameters affect the results. At the end, an investment priority measure is proposed that can help in determining where among the various influencing factors the available limited resources should be spent in a way that it results in maximum gain in ultimate reliability.
Semi-Submersible Gravity Based Hybrid Structure: An Alternative to Jacket and Topside Platforms
Authors: Sjögren Peter , Fagerström Björn , Bellgran Monica , Sandeberg Peter
Published: September 2014

The aim of this research is to present the result from a case study comparing a semi-submersible gravity based hybrid structure and a jacket topside structure for two offshore wind farm converter station projects. The cases are analyzed from a transportation and installation perspective. Converter stations enable the conversion from alternating to direct current, more efficiently bringing generated wind energy to shore. Out of the project process phases e.g. planning, engineering, fabrication, transport, installation and commissioning; transportation and installation are two of the most demanding during offshore platform projects. The weight and size of an offshore platform require specialized equipment, vessels and marine contractors’ expertise to be transported and installed. The risks associated with transportation and installation (T&I) operations are related to health, safety and environment, and T&I operations are also sensitive to any disturbance e.g. accidents and delays. Many interdependencies between disciplines in the project execution process may result in consequential and immediate impact, should an activity not follow the plan and thus the risk of cost overruns increases. In an attempt to circumvent risks related to heavy lifts offshore and consequently mitigate the corresponding risks, a new platform concept has been developed a Norwegian EPC–company and a Swiss converter station manufacturer, a semi-submersible gravity based hybrid, SSGBH. The subjects presented in this paper include the general principles of the SSGBH concept and in what way risk associated with T&I operations are reduced. This paper present the platform concepts, data gathered from interviews, archival data and direct observations. Their pros and cons are presented in an objective way, while concluding that platform concepts of this kind are and should be highly customized, as should the concept selection.
Type Classification for Horizontal Axis Tidal Turbine (HATT): An Important Step Towards Commercialisation
Authors: Bittencourt Claudio , Argyriadis Kimon , Steiniger Michael
Published: September 2014

One key aspect of serial production is to define a set of parameters that can define the limits and conditions that product can be used. Type certification is applied to serial production such as wind turbines when the matching of the site characteristics and the turbine design conditions is summarised in a short description of the site parameters that fundamentally describes the wind conditions (energy and turbulence). This easy matching is used by all stakeholders in assessing the suitability of different turbines to a specific project development. However, type classification is also important to optimise the design, manufacturing, installation and maintenance costs by setting key parameters to production of turbines with same characteristics (scale factor).

The same applies to tidal turbines. Although, at the present phase of development, serial production is not yet a main driver and designs are mainly carried out with specific sites in mind, it is useful to develop a type classification now to support generic design parameters and that can be tested for future serial production and matching sites with serial products.

While for wind turbines, the set of key parameters is reasonably simple [1], for tidal there are a number of environmental conditions (for example waves, astronomical tide, current turbulence, water depth, etc), and parameters (such as hub height related to the seabed and sea surface) which must be considered within the design of a HATT, parameters which may also be utilised in the construction of generic type classes for Tidal Turbines.

Thus, the identification of generic type classes for HATTs is dependent upon the ability (by conjunction of assumptions, formulations, etc.) to distil the existing extensive list of site / HATT parameters down to a minimum number of key parameters upon which a simple but robust set of generic type classes could be based.

Whilst it is recognised that adjustments of type classes may be required in the future, a first approach will be presented in the DNV GL Standard for Certification of HATT developed under the ReDAPT project and it is described in detail in this paper. The ReDAPT project is commissioned and co-funded by the ETI (Energy Technologies Institute).
On the Axisymmetric Response of a Damaged Flexible Pipe
Authors: de Sousa José Renato M. , Magluta Carlos , Roitman Ney , Campello George C.
Published: September 2014

This work focuses on the structural analysis of a damaged 9.13″ flexible pipe to pure and combined axisymmetric loads. A set of experimental tests was carried out considering one up to ten broken wires in the outer tensile armor of the pipe and the results obtained are compared to those provided by a previously presented finite element (FE) model and a traditional analytical model. In the experimental tests, the pipe was firstly subjected to pure tension and, then, the responses to clockwise and anti-clockwise torsion superimposed with tension were investigated. In these tests, the induced strains in the outer armor were measured. Moreover, the axial elongation of the pipe was monitored when the pipe is subjected to tension, whilst the twist of the pipe was measured when torsion is imposed. The experimental results pointed to a slight decrease in the stiffness of the pipe with the increasing number of broken wires and, furthermore, a redistribution of forces among the intact wires of the damaged layer with high stress concentration in the wires close to the damaged ones. Both theoretical models captured these features, but, while the results obtained with the FE model agreed well with the experimental measurements, the traditional analytical model presented non-conservative results. Finally, the results obtained are employed to estimate the load capacity of the pipe.
Comparison of Hydrodynamic Load Predictions Between Reduced Order Engineering Models and Computational Fluid Dynamics for the OC4-DeepCwind Semi-Submersible
Authors: Benitz Maija A. , Schmidt David P. , Lackner Matthew A. , Stewart Gordon M. , Jonkman Jason , Robertson Amy
Published: September 2014

Hydrodynamic loads on the platforms of floating offshore wind turbines are often predicted with computer-aided engineering tools that employ Morison’s equation and/or potential-flow theory. This work compares results from one such tool, FAST, the National Renewable Energy Laboratory’s wind turbine computer-aided engineering tool, and the high-fidelity computational fluid dynamics (CFD) package, OpenFOAM, for the OC4-DeepCwind semi-submersible analyzed in the International Energy Agency Wind Task 30 project. Load predictions from HydroDyn, the offshore hydrodynamics module of FAST, are compared with results from OpenFOAM. HydroDyn uses a combination of Morison’s equation and potential-flow theory to predict the hydrodynamic forces on the structure, at a small computational cost compared to CFD. The implications of the assumptions in HydroDyn are evaluated based on this code-to-code comparison.
Implementation of Computational Methods to Obtain Accurate Induction Factors for Offshore Wind Turbines
Authors: Bahuguni Anand , Sivalingam Krishnamoorthi , Davies Peter , Gullman-Strand Johan , Nguyen Vinh Tan
Published: September 2014

Most of the wind turbine analysis softwares widely being used in the market are based on the Blade Element Momentum method (BEM). The two important parameters that the BEM codes calculate are the axial and the tangential induction factors. These factors are calculated based on the empirical blade lift coefficient Cl and drag coefficient Cd along with some loss/correction functions to account for the losses near the blade tip and the hub. The current study focusses on verifying the values of induction factors using Computational Fluid Dynamics (CFD) simulations for floating offshore wind turbines at a selected sea state. The study includes steady state calculations as well as transient calculations for pitching motions of the turbine due to waves. The NREL FAST software is used to set the simulation scenarios according to OC3 Phase IV cases. The blades are divided a number of elements in CFD calculations and the data are extracted at individual elements to have an exact comparison with the BEM based calculations.
System-Level Approach to the Design, Development, Testing, and Validation of Wave Energy Converters at Ocean Power Technologies
Authors: Mekhiche Mike , Edwards Kathleen , Bretl James
Published: September 2014

This paper describes the multi-discipline, multi-physics systems-based development of PowerBuoy® products at Ocean Power Technologies (OPT). The PowerBuoy is a moored system which extracts useful electrical power from waves. Different PowerBuoy systems have been developed for different markets (grid-connected systems vs. smaller autonomous systems) and/or different customer applications. Here the overall approach of PowerBuoy solution development will be described along with examples of system solutions recently developed and tested at OPT. Specifics focusing on developing initial concepts, thorough modeling and initial testing to eliminate risk, and down-selection of a final solution will be discussed. Further, final system design and validation and test results will be provided.
Harmonic Analysis on Low Quality Tidal Data
Authors: den Haan Joost
Published: September 2014

The aim of the study is to devise a method to conservatively predict a tidal power generation based on relatively short current profile measurement data sets. Harmonic analysis on a low quality tidal current profile measurement data set only allowed for the reliable estimation of a limited number of constituents leading to a poor prediction of tidal energy yield. Two novel, but very different approaches were taken: firstly a quasi response function is formulated which combines the currents profiles into a single current. Secondly, a three dimensional vectorial tidal forcing model was developed aiming to support the harmonic analysis with upfront knowledge of the actual constituents. The response based approach allowed for a reasonable prediction. The vectorial tidal forcing model proved to be a viable start for a full featuring numerical model; even in its initial simplified form it could provide more insight than the conventional tidal potential models.
Numerical and Experimental Research on a Podded Propulsor
Authors: Islam Mohammed , Ryan Ron , Molynuex David
Published: September 2014

This paper presents methodologies and some results of a numerical and experimental program to evaluate the effects of static azimuthing conditions on the propulsive characteristics of a puller podded propulsor in open water. In the experimental effort, the model propulsor was instrumented to measure thrust, torque and rotational speed of the propeller, and three orthogonal forces and moments, and azimuthing angle of the pod. The experimental results included the bare propeller (ahead only) and the combined propeller and pod over a range of advance coefficients at various static azimuthing angles in the range of −180° to 180°.

A complementary numerical study is being carried out to predict the hydrodynamic forces of podded propulsor in static azimuthing conditions. A Reynolds-Averaged Navier Stokes solver is used to predict the propulsive performance of the bare propeller as well as the podded propulsor system. The thrust and torque for the bare propeller were compared to the corresponding measurements. The propeller thrust and torque as well as the loads on the pod in straight-ahead condition and at static azimuthing angles were then compared with the measurements. Preliminary analysis demonstrates that the RANS solver could predict the performance coefficients of the bare propeller as well as the podded propulsor in straight-ahead and static azimuthing angles in puller configurations.
Thruster-Interaction Effects on a DP Shuttle Tanker - Wake Flow Measurements of the Main Propeller and Bow Tunnel Thrusters
Authors: Cozijn J. L. , Hallmann R.
Published: September 2014

This paper discusses thruster interaction effects for a DP shuttle tanker, equipped with two main propellers and rudders, as well as two bow tunnel thrusters. Thruster-interaction model tests were carried out in MARIN’s Deepwater Towing Tank. Detailed PIV measurements were taken of the wake flow behind the main propellers and rudders. Furthermore, PIV measurements were taken of the wake flow of one of the two bow tunnel thrusters. The flow velocities were measured in a large number of cross sections at different down-stream positions. The PIV measurements provide a detailed image of the velocities in the thruster wake, showing axial velocities, as well as transverse and vertical velocity components.

The results of the first set of measurements showed in detail the wake flow behind the main propeller of the DP shuttle tanker. The wake flow pattern was determined at rudder angles of 0 deg and 10 deg. Since the research is related to DP performance, bollard pull conditions (zero forward speed) were considered in the model tests.

The results of the second set of measurements showed in detail the wake flow of one of the bow tunnel thrusters. The wake flow pattern was investigated in zero speed conditions, as well as for the vessel at forward speed. The observed flow patterns helped to explain the reduced bow tunnel performance at forward speed.

The results of the present research are used to further improve the understanding of the physics of thruster interaction effects. Furthermore, the results will serve as validation material for CFD calculations that are currently being performed.
A Study of LES Models for the Simulation of a Turbulent Flow Around Supercritical Tandem Cylinders
Authors: Sainte-Rose B. , Allain O. , Leca C. , Dervieux A.
Published: September 2014

A comparison between two Large Eddy Simulation (LES) approaches is proposed to reproduce the flow around a tandem cylinder arrangement in highly turbulent conditions (Reynolds number of 166,000) at a supercritical distance (L/D = 3.7 with L the distance between the cylinder centers and D their diameter). A LES - Variational Multi-Scale (VMS) and a classic LES methods are employed to model turbulence on the same unstructured grid. The results of the computations are compared to a set of experimental database built by NASA which includes averaged and fluctuating velocities, steady and unsteady pressure sensors. The top-end purpose of the study is to further demonstrate the suitability of the LES-VMS approach for turbulent flows around slender bodies with high aspect ratio.
A Method for Determining Quasi-Static and Dynamic Riser Inclination Using Collocated Accelerometers and Angular Rate Sensors
Authors: McNeill Scot , Angehr Paul , Kluk Dan , Saruhashi Tomokazu , Sawada Ikuo , Kyo Masanori , Miyazaki Eigo , Yamazaki Yasuyuki
Published: September 2014

A method is described for determining quasi-static and dynamic riser angles using measured data typically found in a riser fatigue monitoring system, specifically acceleration and angular rate data. Quasi-static riser inclination and orientation of the inclination plane are determined from the low frequency triaxial accelerations, containing measurement of the gravitational body force. Components of the gravitational body force along the accelerometer axes vary slowly with the riser quasi-static response. The slowly varying Euler angles are determined from the components of gravity along the three axes.

Dynamic riser inclination along and transverse to the quasi-static inclination plane are determined by integration of the angular rates, followed by transformation into a coordinate system aligned with the quasi-static inclination plane. The quasi-static and dynamic inclination angles are combined to arrive at the time trace of riser inclination angles.

Following implementation of the method in Matlab®, the procedure was validated and the program verified using laboratory test data. A double-gimbaled platform was constructed, on which were mounted a triaxial accelerometer, biaxial angular rate and biaxial inclinometer (reference sensor). A battery of static and dynamic tests was carried out on the platform. Machinists’ levels and angle gauges were used to set the inclination in the various tests. The angles derived from the acceleration and angular rate data were compared to those of the reference inclinometer. Angle estimates were shown to match the reference angles with negligible error.

The method was then implemented into the real-time Riser Fatigue Monitoring System (RFMS) aboard the Chikyu drillship. The algorithm was run using data from an emergency disconnect event that occurred in November, 2012. Quasi-static riser inclination angles were quite large due to high currents near the sea surface. Dynamic riser inclination angles proved to be significant due to Vortex Induced Vibration of the lower portion of the riser that immediately followed the disconnect event.

It is important to note that the method uses data that is typically already included in real-time riser monitoring systems. Therefore only a software update is required to provide real-time riser angle information. If the method is built into data processing routines for real-time riser monitoring systems, the need for additional instrumentation, such as inclinometers near flex joints, may be circumvented. On the other hand, if inclinometers already exist, the method serves as an independent source of riser angle information at several locations on the riser. The method can also be used to calculate riser and Blow out Preventer (BOP) stack angles from data recorded using stand-alone, battery-powered loggers.
Warm Model Ice and Propeller Interaction, Capturing the Driving Mechanics?
Authors: Hagesteijn Gerco , Brouwer Joris
Published: September 2014

In the drive to design robust and more efficient propellers, the need to understand propeller-ice interaction is increasing. Traditionally MARIN has built up this kind of propeller design knowledge by developing and testing large systematic propeller series, such as the classic Wageningen B-series and the recent C and D-series. With systematic variations of blade number, area, pitch and profiles it can be investigated how these parameters influence the design and how the most efficient designs for operation in open water and in ice can be reached. This is becoming especially important for the lower ice class propellers, which are rarely sailing in ice covered areas. Apart from this, these designs should be optimized for low noise emission as well, to ensure application in the sensitive Arctic environment. When both milling and crushing loads can be captured using a warm model ice, the testing of such a large systematic propeller series becomes possible without the use of an ice basin.

In the present paper an evaluation is presented, showing the results of a first series of testing that was carried out with a warm model ice. The test set-up used is one that has also been used for the same kind of tests in a real ice basin. The warm model ice was designed to resemble the correct crushing strength at model scale. The warm ice samples are fed into the propeller, while at the same time the load on one of the propeller blades is recorded with a 6C-load transducer. At the same time the impact is recorded with a high speed video camera. To enable a detailed analysis, the force vector is plotted on the high speed recording. The force vector can be derived using a centre of pressure method. In this way an evaluation can be made if the captured phenomena are indeed similar to those that can be expected from real (model-)ice.
Non-Linear Air Gap Analyses of a Semi-Submersible Compared With Linear Analyses and Model Tests
Authors: Kvaleid Jørgen , Oosterlaak Volkert , Kvillum Tor
Published: September 2014

For semi-submersible units, the magnitude of air gap or local wave impact in the survival condition is a key design driver. Linear analyses are widely used in the industry to predict survival air gap for semi-subs. Large relative motions, leading to large changes in shape of the submerged hull and large changes in water plane area make this approach questionable.

In this paper, the GG5000 [1], a twin pontoon four legged semi-sub is considered. Both linear analyses and model tests had been performed, but the results were diverging. It was decided to investigate further, using non-linear hydrodynamic analyses.

Initially, the model test setup is reproduced in the numerical model. The simulation model is verified for both response power spectra and extreme response distributions. In the non-linear simulations, the wetted surface of the hull is updated for each time step. Both excitation and restoring forces are based on the instantaneous wetted surface. This proves essential for the prediction of large motions. Later, the verified simulation model is run with realistic full scale setup including elastic catenary moorings with coupled cable dynamics, thruster assist, irregular waves and irregular wind.

Highly non-linear effects proven to be vital to accurate air gap prediction are investigated and their representation in the non-linear analyses is validated against model tests.
Development of a Fatigue Life Assessment Tool for Pipelines With Local Wrinkling Through Physical Testing and Numerical Modelling
Authors: Bakhtyar Fahad , Kenny S.
Published: September 2014

Pipelines are one of the primary modes to transport hydrocarbon products throughout the world. A majority of these pipeline systems are buried, which are susceptible to ground movement that imposes axial forces and bending moments on the pipeline section. As a result of these forces and moments pipeline section may experience localized deformation that result in wrinkling, buckling and other damage mechanisms. This may impair the pipeline mechanical performance with respect to local stress concentration, lower strength and reduced fatigue life. Cyclic operational loads or ground movement events may result in strain reversals within the pipeline section that may lead to fracture through low cycle fatigue process.

In this study, physical testing was conducted to examine the strength and fatigue life performance characteristics of hot rolled steel (44-W). The effects of the test sample bend radius, wall thickness and imposed displacement stroke range on fatigue performance were examined. The data was used to develop and refine continuum finite element modeling procedures that can be used to assess the influence of pipeline damage on fatigue life in a more detailed numerical simulation framework.
A Comparative Study of Hole Cleaning Performance — Water Versus Drag Reducing Fluid
Authors: Rodriguez Corredor Fabio Ernesto , Bizhani Majid , Kuru Ergun
Published: September 2014

Effective hole cleaning in horizontal and extended reach wells (ERD) often requires use of high circulation rates, which may not be always achievable due to the risk of circulating bottom hole pressure reaching the fracture limit of the rock. Achieving good hole cleaning while keeping the circulating bottom hole pressure within the safe operational window is very often the major engineering challenge. A drag reducing fluid with good hole cleaning ability could be a potential solution in this case.

In order to see if it is possible to use a drag reducing fluid and still achieve a good hole cleaning, an experimental program was designed and conducted. The main objective of this experimental study was to compare the hole cleaning performances of water and a drag reducing fluid.

The hole cleaning experiments were conducted using a 9m long horizontal flow loop with concentric annular geometry (Outer Pipe ID = 95 mm, Inner Pipe OD = 38 mm, ID/OD ratio = 0.4). The drag reducing additive was a commercially available partially hydrolyzed polyacrylamide (PHPA). Water and two drag reducing fluids with 0.07% V/V and 0.1% V/V PHPA concentrations were used. Critical velocities for the initiation of cuttings movement with rolling, saltation/dunes, and suspension modes were determined and compared when using water and drag reducing fluids as a carrier fluid. Critical velocities for the initiation of cuttings movement were found to be lower with water than that of drag reducing fluid in all transport modes.
Warm Pre-Stressing and Leaks in Pipelines
Authors: Cosham Andrew , Hopkins Phil , Jones David G. , Barnett Julian
Published: September 2014

Line pipe steel is a carbon manganese steel. The toughness of line pipe steel undergoes a transition from high toughness (on the upper shelf) to low toughness (on the lower shelf) as the temperature decreases.

A fluid will cool significantly as it expands through a leak in a pipeline. This has led to the suggestion that localised cooling of the material surrounding the leak might be sufficient to cool the material down to below the ductile to brittle transition temperature and cause a brittle fracture.

Warm pre-stressing occurs when a load is applied to a structure containing a defect and then the temperature of the structure is reduced. Warm pre-stressing causes the defect in the structure to fail at a higher load at the lower temperature than if it had not experienced this prior loading at the previously higher temperature.

A programme of single edge notch bend tests has been conducted on behalf of National Grid Carbon to demonstrate the beneficial effect of warm pre-stressing in a line pipe steel. The material tested was a sample of 914.4 mm outside diameter, 19.1 mm wall thickness, Grade API 5L X60 line pipe. Single edge notch bend specimens were subject to the ‘load-cool-fail’ cycle and the ‘load-unload-cool-fail’ cycle. The effect of different levels of stable ductile crack growth during the pre-load was also investigated.

Warm pre-stressing is shown to have a beneficial effect. The load at failure in the specimens that had been subject to warm pre-stressing was higher than those that had not been subject to warm pre-stressing, and, in most cases, it was higher than the pre-load. The fracture toughness (in terms of the stress intensity factor) of the specimens that had been subject to warm pre-stressing was 1.4 to 1.7 times higher than that of those that had not been subject to warm pre-stressing. The results of the tests were conservatively predicted using the theoretical models. Also, the results are consistent with previous tests on structural steels.

Therefore, localised cooling of the material around a leak in a pipeline is not predicted to result in a failure.
From Single Point Gauge to Spatio-Temporal Measurement of Ocean Waves: Prospects and Perspectives
Authors: Liu Paul C. , Nieto Borge J. C. , Rodriguez German , MacHutchon Keith R. , Chen Hsuan S.
Published: September 2014

With the recent advancement of spatial measurements of ocean waves, we are clearly facing new challenges regarding how to handle an expanded new data system when it becomes widely available. In this paper we wish to present a preliminary attempt at confronting these prospects. Because the data is still very limited at present and also conceptually new, it’s a new, unfamiliar, and unrelenting world to pursue. We need a paradigm shift away from our familiar single-point conceptualization in order to effective approach the new world of truly spatial ocean waves.
Next-Generation Hydrokinetic Power Take-Off via a Novel Variable-Stroke Hydraulic System
Authors: Plagge Amanda M. , Jestings Lee , Epps Brenden P.
Published: September 2014

Hydrokinetic power generation has the potential to supply nearly ten percent of the United States annual energy demand. However, the hydrokinetic generation has lagged behind other renewable energy technologies, and many engineering challenges remain. Here, we consider the impacts of using a hydraulic power transfer system for hydrokinetic power generation. The incorporation of hydraulic power transfer into hydrokinetic systems has the potential to increase durability, reduce required maintenance, and increase power-to-weight ratio, all of which would lower the overall levelized cost of energy (LCOE). In the proposed system, patented low friction, variable-stroke hydraulic pump and motor pairs would allow energy to be harvested efficiently throughout the full range of water velocities in either tidal or riverine flows and with any type of rotary prime mover. A full system characterization is provided, along with a calculation of expected LCOE and a considered analysis of the applicability of hydraulic PTO systems as a way to advance commercial hydrokinetic power generation.
Floating Concrete Barge Assessment and Inspection Plan: N’Kossa Case Study, a Large Pre-Stressed Concrete Floating Production Unit in Operation in Congo
Authors: Collet Pascal , Vaucquelin Nicolas , Bury Arnaud
Published: September 2014

The paper describes the way a large pre-stressed concrete Floating Production Unit (FPU) located offshore Congo on N’KOSSA field has been inspected and assessed in order to meet both Bureau Veritas Class and Floating Units Integrity Management System (FUIMS) requirements.

This FPU is the largest existing pre-stressed concrete barge. She was built with high performance concrete, installed offshore in 1996 in 170 m water depth. She has now accumulated 18 years of production.

A significant part of the methodology is based on close inspection for the concrete structure, a graphic assessment of the defects and an implementation of monitoring.

This assessment is done a long time after construction and we had to cope with difficulties relative to the structure size, loss of data, barge in operations off-shore. It gives a good feedback of what we should implement during the design and the construction to keep a clear view of the concrete barge integrity.
Numerical Simulation of Oblique Wave Breaking and Wave-Induced Currents in the Surf Zone
Authors: Kolokythas Gerasimos A. , Dimas Athanassios A.
Published: September 2014

In the present study, the three-dimensional, incompressible, turbulent, free-surface flow, developing by the propagation and breaking of nonlinear gravity waves over a constant-slope beach, is numerically simulated. The main objective is to investigate the flow structure in the surf zone as a result of the interaction between the longshore and the undertow current, induced by spilling wave breaking, oblique to the shoreline. The simulations are performed employing the so-called large-wave simulation (LWS) method coupled with a numerical solver for the Navier-Stokes equations. According to the employed LWS methodology, large velocity and free-surface scales are fully resolved, while the effect of subgrid scales is modeled by eddy-viscosity stresses, similar to large-eddy simulation (LES) methodology. In order to validate our model, the case of incoming Stokes waves with wavelength to inflow depth ratio λ/dI ≈ 6.6 and wave steepness H/λ ≈ 0.025, propagating normal to the shore over a bed of constant slope 1/35, is investigated. Our results are compared to published experimental measurements, and it is found that the LWS model predicts adequately the wave breaking parameters — breaking height and depth — and the distribution of the undertow current in the surf zone. Two cases of oblique breaking waves, with inflow angles φI = 20° and 30°, and all other parameters identical to that of the validation case, are considered. The gradual breaking of the refracted waves is captured, as well as the three-dimensional structure of the flow in the surf zone. LWS-predicted profiles of the undertow and the longshore current at several positions in the surf zone, are presented. It is indicated that the undertow prevails in the outer surf zone, while the longshore current becomes stronger in the inner surf zone and reaches its maximum magnitude close to the shore.