bluuebunny/crossref_metadata_2025 huggingface dataset:
Challenges due to Welds Fabricated at a Close Proximity on Offshore Structures, Pipelines, and Piping: State of the Art
Authors: Bhardwaj Sachin , Ratnayake R. M. Chandima
Published: December 2020

In offshore structures, pipelines and piping, there are various configurations where welds are often in close proximity to each other. TKY joints in offshore structures, welds of nozzles, tees, elbows and circumferential welds of piping in compact layouts are a few examples. Depending on their application, different codes (ASME B31.3, ISO 19902, etc.) either recommend different criteria or provide no explanation for deciding a minimum distance between proximity welds. The guidance provided in these codes is inadequate for a field inspector to make an informed decision on site. Some international standards specify a minimum distance between welds, based upon some factor regarding the diameter or thickness of the parts under welding or they give no explanation, which creates an ambiguity among contractors and inspectors on site. The overlapping of two adjacent heat affected zones (HAZ) can have a deleterious effect on the material properties of the metals, eventually leading to catastrophic failures in the life cycle of high-risk components like offshore structures, process piping, etc. The different thermal cycles of adjacent welds with varying cooling times can drastically change the microstructure/mechanical properties of the metals, where locked in residual stress can amplify the effect towards failure. Since the aforementioned codes are deficient in providing technical justification regarding noncompliance of maintaining proximity distances, an approach for understanding close proximity effects on structural integrity is needed. This manuscript presents a brief assessment of international standards for specifying the minimum distance between close proximity welds for structural and pressure vessels’ codes. Challenges like residual stresses’ impact on fatigue strength, fatigue crack growth and fitness for service for fracture assessment are discussed briefly for proximity welds. Finally, building on the literature review, a framework for future research is presented.
Numerical Modeling of Wave Damping Induced by Emerged Moving Vegetation
Authors: Gupta Aditya , Behera Manasa R. , Heidarpour Amin
Published: December 2020

The unprecedented risk of global warming has put the coastal population at greater risk from coastal hazards due to an increase in sea level and other storm-related activities. Coastal vegetations are one of the soft solutions that can be implemented for wave mitigation. This study aims to investigate the wave damping effect of a regular wave by emergent moving coastal vegetation. Smoothed Particle Hydrodynamics (SPH), a particle-based method is used for generating fluid particles and Differential Variational Inequality (DVI) is coupled with SPH to deal with the dynamics of moving vegetation. The 3-D numerical model is simulated using an open-source tool DualSPHysics 4.4. The model is tested for regular wave height (H) of 0.08 m, wave period (T) of 2 seconds in a water depth (d) of 0.40 and 0.45 m for two relative vegetation height (h/d) of 1.25 and 1.11 respectively. The results are validated with the experimental study for the rigid vegetation and then the model is extended for moving vegetation. The results indicate that the wave damping is overestimated in the case of rigid vegetation. Further, the application of this study can be extended for studying the tsunami hazard mitigation in the presence of coastal forest.
Fretting Fatigue Design of Subsea Connectors: A Generalised Approach to Testing
Authors: Andresen H. , Hills D. A. , Wormsen Anders , Macdonald K. A.
Published: December 2020

In this paper fretting fatigue is addressed as a potential design consideration for wellhead connectors. The study of near-edge relative motion for frictional contacts under constant normal load is described using analytical, numerical and asymptotic methods. Based on published fretting fatigue experimental data an argument is drawn for a generalised fretting fatigue test design. We do this by reducing the parameters responsible for crack nucleation to the smallest number possible and thereby revealing the fretting fatigue strength as a material property independent of geometrical features. Easy to apply recipes are described and thoughts on a potential apparatus are shared with the reader. Commercial potential lies in the wide-ranging applicability of experimental results across many prototypes and loadings once an appropriate amount of fretting fatigue data has been generated for the material in question.
On the Correlation Between Floating Wind Turbine Accelerations, Rotor and Tower Loads
Authors: Choisnet Thomas , Percher Yohan , Adam Raphaël , Favré Mathieu , Harries Robert
Published: December 2020

Research in floating wind turbine resulted in the publication of a number of studies comparing wind turbine loads on a variety of floaters and fixed foundations. Some of them concluded that the blade and drivetrain loads would only be marginally increased -if increased at all — when a turbine would be installed on a floater [1] [2] and [3]. This paper proposes to evaluate how rotor and tower loads are correlated to nacelle acceleration, wind and wave conditions. It can somehow be considered as an extension to a barge-type floater and onsite measurements, of published work applied to fixed offshore wind turbines [4] and a spar-type floating wind turbine [5].

The body of data used for this exercise includes results from full-scale prototype measurements and simulations for a variety of turbine ratings. It can be concluded that in power production cases, blade and main shaft loads are only weakly correlated to nacelle low frequency accelerations and hence wave conditions, hence aerodynamic loads are still the main driver for rotor and tower top loads. In rotor-idling conditions, the situation is mainly dependent on the wind speed range, but aerodynamics are the largest contributor of blade and main shaft loads in severe wind conditions. These results can help understand where design uncertainties lie in floating wind turbine loads.
On Normalized Fatigue Crack Growth Modeling
Authors: Glavind Sebastian T. , Brüske Henning , Faber Michael H.
Published: December 2020

Modeling of fatigue crack growth plays a key role in risk informed inspection and maintenance planning for fatigue sensitive structural details. Probabilistic models must be available for observable fatigue performances such as crack length and depth, as a function of time. To this end, probabilistic fracture mechanical models are generally formulated and calibrated to provide the same probabilistic characteristics of the fatigue life as the relevant SN fatigue life model. Despite this calibration, it is recognized that the rather complex fracture mechanical models suffer from the fact that several of their parameters are assessed experimentally on an individual basis. Thus, the probabilistic models derived for these parameters in general omit possible mutual dependencies, and this in turn is likely to increase the uncertainty associated with modeled fatigue lives. Motivated by the possibility to reduce the uncertainty associated with complex multi-parameter probabilistic fracture mechanical models, a so-called normalized fatigue crack growth model was suggested by Tychsen (2017). In this model, the main uncertainty associated with the fatigue crack growth is captured in only one parameter. In the present contribution, we address this new approach for the modeling of fatigue crack growth from the perspective of how to best estimate its parameters based on experimental evidence. To this end, parametric Bayesian hierarchical models are formulated taking basis in modern big data analysis techniques. The proposed probabilistic modeling scheme is presented and discussed through an example considering fatigue crack growth of welds in K-joints. Finally, it is shown how the developed probabilistic crack growth model may be applied as basis for risk-based inspection and maintenance planning.
A Suggested Shaping Exponent in Wheeler Approach Under Overload Condition for High Manganese Steel
Authors: Park Jeong-Yeol , Kim Myung-Hyun
Published: December 2020

Recently, International Maritime Organization (IMO) reinforced the regulations for nitrogen oxide (NOx) and sulfur oxides (SOx) toward the reduced emission. To avoid serious environmental issues, the need for liquefied natural gas (LNG) fuel tanks arose for new constructed vessels. LNG fuel tanks are typically fabricated with low temperature materials that exhibit excellent fatigue performance at cryogenic temperature. In manufacturing of LNG fuel tanks, high manganese steel is known to be cost effective compared to other low temperature materials such as 9 wt.% nickel alloy steel, SUS304L and INVAR alloy. Therefore, extensive efforts are carried out to investigate the fatigue performance of high manganese steel. However, such investigations are yet limited to the constant amplitude load case only.

In order to reflect loading or unloading condition of LNG vessels in terms of fatigue performance, this study investigates the fatigue crack growth characteristics of high manganese steel under overload condition. The main parameters considered are the overload ratio (OLR) and the mean stress. We employ the Wheeler approach for the estimation of fatigue crack growth characteristics. In general, the Wheeler approach consists of crack length, plastic zone size and shaping exponent. Among three parameters in Wheeler approach, we assess the relationship between OLR and shaping exponent and suggest a modified equation for the shaping exponent considering the mean stress effect. In order to predict the effect of overload, the estimated fatigue life based on the Wheeler approach with the modified equation was compared to that with a constant amplitude load.
Method for Fatigue Testing of Subsea Wellhead Connector Segments
Authors: Barros Pedro , Horn Agnes Marie , Wormsen Anders , Osen Per , Macdonald Kenneth A.
Published: December 2020

For subsea well drilling, the drilling rig is connected to the subsea well by a marine riser and subsea BOP equipped with a remotely controlled wellhead connector latched onto the subsea wellhead profile.

The level of cyclic loading on subsea wellheads is steadily increasing due to use of increasingly larger drilling rigs with larger BOPs, the drilling of wells in harsher environments characterized by strong high waves.

The remotely controlled wellhead connector forces a series of locking dogs into an externally machined profile on the wellhead. This external profile is generally referred to as a wellhead profile. The fatigue resistance of this safety-critical connection is typically estimated by FE analysis. Due to the large size of the equipment, and high cost of testing, very limited fatigue testing, if any, has been carried out. A test method has therefore been developed, where a special test fixture is used to apply realistic boundary conditions and variable tensile loads to a small sector or segment of a wellhead connector. A primary objective is to generate fatigue-critical stress fields in the segments under tensile test load that closely replicates the stress fields in a full-scale connector subject to bending loads. A secondary objective is to support the introduction of the practice of testing several segments cut from a single wellhead connector. The testing of narrow sector segments allows the use of readily available test apparatus. It is thereby envisaged that the total cost of testing (specimens and test laboratory costs) can be substantially reduced in comparison with full-scale connector fatigue testing.

This paper describes the text fixture, the connector locking dog, and wellhead segments designed to replicate the stress fields in a full-scale wellhead connector.

The test fixture and test specimens are designed to match conditions and fatigue stress of the full-scale connector. The test specimens are instrumented with strain gauges at fatigue hotspots. Digital image correlation (DIC) is used to measure the relative motion between the wellhead segment and the locking dog. The measured strains are compared with corresponding values from finite element analysis of the test. The DIC results are also used for estimating the coefficient of friction between wellhead profile and locking dog.

Very good agreement is found between measured hotspot strains and strains from the FE analyses for consistent load conditions. The test fixture is therefore considered suitable for segment fatigue testing, where the test results can be used to estimate the bending fatigue capacity of a full-scale wellhead connector.

Results from fatigue testing by this test method are presented in a separate OMAE2020 paper.
Horizontal and Torsional Modes of an Ultra Large Container Ship (ULCS)
Authors: Vijith P. P. , Rajendran Suresh
Published: December 2020

The hydro elastic responses of flexible structures under fluid loading is an important concern during the design of large ocean structures. The two-way coupling between the structural responses and the hydrodynamic loads is a complex problem in large flexible floating structures since the structures can vibrate in longitudinal, vertical, horizontal, or torsional modes. The antisymmetric distortion modes may be coupled depending on the location of the centroid and the shear centre. In the case of thin walled open structures, horizontal and torsional vibrations are usually coupled due to the asymmetry of cross section as well as eccentricity between centroid of the section and shear deformation centres. The acurate estimation of dry natural frequency and modes shapes of structure is indispensable since it helps to validate the accuracy of the structural modelling. A numerical method available from one of the existing literatures is used for the estimation of dry and wet natural frequencies, and mode shapes of horizontal and torsional vibrations of an ULCS. The natural frequency and modes are essential parameters for the analysis of interaction between structural responses and hydrodynamic loads. The numerical method is based on a 1D FEM beam model. Distortion due to warping is included in the numerical model since it is well known that containerships with large hatch opening are susceptible to warping. The numerical model is subdivided into 50 stations and the mass distribution and the sectional properties are calculated in order to match the bending, shear, torsion and warping moduli of the experimental model.

The dry and wet natural frequency and mode shapes for the horizontal and torsional vibrations of the ULCS is numerically calculated and compared with the experimental results.
Addressing Uncertainties Within Flow Induced Vibrations in Subsea Piping
Authors: Macchion Olivier , Stachyra Leszek , Hørte Torfinn , Renaud-Bezot Charles
Published: December 2020

The assessment of the risk arising from multiphase flow induced vibrations in subsea production systems is strongly affected by the accuracy of data and modelling for the production conditions, flow induced loading and estimate of the damping of the structure, both structural and flow induced. These uncertainties in the flow conditions and fluid structure interaction analyses are further compounded by uncertainties in the capacity of the structure to withstand vibration and fatigue damage.

As most calculation codes rely on deterministic analysis, these uncertainties are often addressed by their characteristic input and through safety factors, which are intended to ensure sufficient safety level. Subjective judgements, with unknown degree of conservatism, may be difficult to avoid. A need to address the associated uncertainties and provide a prediction of the accumulated probability of fatigue failure, using a consistent and sound methodology, has been expressed throughout the industry.

Structural Reliability Analysis (SRA) offers a framework to take into account, based on the same modelling basis as the deterministic approach, the uncertainties in all relevant parameters. Moreover, the framework allows for simultaneous consideration of these identified uncertainties when modelled by probability distributions and enables calculation of the probability of fatigue failure.

This article describes a general approach to implement reliability principles in the analysis of multiphase flow induced vibrations, discusses the use of the methodology, both in design and in operations. It also concludes on the feasibility of using Bayesian analysis, based on the results of subsea inspection, to update the probability assessment and provide updated input to the risk evaluations for future operations. The methodology may also be used to determine operational limitations for future production.
Numerical Investigation of the Physics of Higher Order Effects Generated by Wave Paddles
Authors: Law Yun Zhi , Liang Hui , Santo Harrif , Lim Kian Yew , Chan Eng Soon
Published: December 2020

When free-surface waves are generated using wave paddles to produce the desired waves, higher order effects might be inevitable for some cases. These can be due to the mismatch in the wave paddle displacement and non-linear free-surface wave kinematics, as well as the moving boundary of wave paddles. Such higher order effects are often manifested as higher harmonic waves, which can propagate independently (or free waves). The presence of such waves will contaminate the quality of the tank test, and together with effects due to scaling and finite size of tank, it is important to reduce or mitigate such effects as much as possible in a wave tank in order to simulate a more realistic scenario.

This study investigates the above problem in a systematic manner by using a fully-nonlinear numerical wave tank based on the three-dimensional time-domain Harmonic Polynomial Cell (HPC) method. Wave is generated by flap-type wave paddles on one end of the tank, and is damped on the other end. The paddle boundary conditions are satisfied on the instantaneous paddles surfaces, and the free surface is tracked by the generalized semi-Lagrangian scheme. In this study, first order paddle signal is used to generate regular waves, and the focus is on characterising the behaviour of the generated free higher harmonic waves.

We first look into a rectangular wave tank where the paddles are distributed at one side of the tank. Upon the generation of an oblique regular wave (primary wave), it is observed that the generated free waves propagate at a different angle/direction. An explicit analytical expression is derived for the direction of the free waves, which agrees with the numerical observation. Besides propagating at a different direction, the free waves also interact with the primary waves resulting in additional bound waves of the first and third harmonics. Next, we consider a circular wave tank, where paddles along half of the circumference are used to generate planar regular wave, while paddles at the other half are assumed to be able to fully absorb the wave. The generated free waves are observed to focus at a particular region in the tank due to constructive interference.

To eliminate or at least mitigate such undesired waves, correction to first order paddle signal is required. Second order correction scheme based on Schaffer (1996) is implemented for such purpose. Preliminary results seem to suggest that second order correction to the paddle signal can only mitigate but cannot completely eliminate the existence of free higher harmonic waves.
Case Study ROP Modeling Using Random Forest Regression and Gradient Boosting in the Hanover Region in Germany
Authors: Höhn Patrick , Odebrett Felix , Paz Carlos , Oppelt Joachim
Published: December 2020

Reduction of drilling costs in the oil and gas industry and the geothermal energy sector is the main driver for major investments in drilling optimization research. The best way to reduce drilling costs is to minimize the overall time needed for drilling a well. This can be accomplished by optimizing the non-productive time during an operation, and through increasing the rate of penetration (ROP) while actively drilling. ROP has already been modeled in the past using empirical correlations. However, nowadays, methods from data science can be applied to the large data sets obtained during drilling operations, both for real-time prediction of drilling performance and for analysis of historical data sets during the evaluation of previous drilling activities. In the current study, data from a geothermal well in the Hanover region in Lower Saxony (Germany) were used to train machine learning models using Random Forest™ regression and Gradient Boosting. Both techniques showed promising results for modeling ROP.
Friction Factor Equations Accuracy for Single and Two-Phase Flows
Authors: Assunção Germano Scarabeli Custódio , Marcelin Dykenlove , Von Hohendorff Filho João Carlos , Schiozer Denis José , De Castro Marcelo Souza
Published: December 2020

Estimate pressure drop throughout petroleum production and transport system has an important role to properly sizing the various parameters involved in those complex facilities. One of the most challenging variables used to calculate the pressure drop is the friction factor, also known as Darcy–Weisbach’s friction factor. In this context, Colebrook’ s equation is recognized by many engineers and scientists as the most accurate equation to estimate it. However, due to its computational cost, since it is an implicit equation, several explicit equations have been developed over the decades to accurately estimate friction factor in a straightforward way. This paper aims to investigate accuracy of 46 of those explicit equations and Colebrook implicit equation against 2397 experimental points from single-phase and two-phase flows, with Reynolds number between 3000 and 735000 and relative roughness between 0 and 1.40 × 10−3. Applying three different statistical metrics, we concluded that the best explicit equation, proposed by Achour et al. (2002), presented better accuracy to estimate friction factor than Colebrook’s equation. On the other hand, we also showed that equations developed by Wood (1966), Rao and Kumar (2007) and Brkić (2016) must be used in specifics conditions which were developed, otherwise can produce highly inaccurate results. The remaining equations presented good accuracy and can be applied, however, presented similar or lower accuracy than Colebrook’s equation.
Numerical Simulations of Turbulent Flow Through an Orifice Plate in a Pipe
Authors: Yin Guang , Nitter Bjørnar , Ong Muk Chen
Published: December 2020

Orifice flow meters are widely used in industries to measure the flow rate in pipelines. The flow rate inside the pipe can be calculated using the relationship between the flow velocity and the pressure drop across the orifice plate. In the present study, numerical simulations have been carried out using three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations combined with the k-ω SST turbulence model to thoroughly investigate the turbulent flow through a circular square-edged orifice with various orifice plate thicknesses and orifice diameters inside a pipe at different Reynolds numbers ranging from 2500 to 40000. The orifice thickness to pipe diameter ratio (t) varies between 0.125 and 2 and the orifice diameter to pipe diameter (β) varies between 0.25 and 0.75. The resulting centerline profiles of the streamwise velocity and pressure of the present study are compared with the previous published numerical results and experimental data as the validation study. The effects of Reynolds numbers and orifice geometries on the pressure, the flow velocity and vorticity distribution in the orifice are discussed in detail.
Annular Frictional Pressure Losses for Drilling Fluids
Authors: Saasen Arild , Ytrehus Jan David , Lund Bjørnar
Published: December 2020

The most common viscosity models used in the drilling industry are the Bingham, the Power-Law and the Herschel-Bulkley models. In addition, it is common to refer to the low-shear yield-point. The scope of the present paper is to discuss numerical methods applicable for calculating annular frictional pressure losses.

The topic of annular frictional pressure loss modelling has been treated in textbooks. None of these couple their models with the selection of viscosity data from measurements at the relevant shear rates.

It is earlier shown how rotation of the inner string in an annulus can complicate the flow due to establishment of Taylor vortices. There are currently no analytical methods to handle such flow. The effect of the vortices depends strongly on the fluid’s composition in addition to the flow conditions. The practical way to handle these situations are by “fingerprinting” during circulation.

In the paper examples will be presented showing how the Herschel-Bulkley fluid can be transferred to simple models for axial flow in an annulus where the inner cylinder does not rotate. It is common to use the narrow slot approximation. This method was used by Founargiotakis et al. In this paper both the modified Herschel-Bulkley model with dimensionless shear rates and the traditional model where the consistency depends on the shear rate will be presented. The dimensionless shear rate model can easily be translated back to the traditional form and vice-versa. Mathematical models will be presented. Hence a framework is given that is easier to use for digitalization and automation and in correlations including pressure, temperature and composition.
Peridynamic Analysis of Fragmentation of Ice Plate Under Explosive Loading With Thermal Effects
Authors: Zhang Yuan , Wang Chao , Guo Chunyu , Tao Longbin
Published: December 2020

Along with the development in arctic region, the icebreaking technologies are gradually becoming the focus. As one of the most powerful and effective way to breaking ice, especially in the ability to solve ice jams, the study of the behaviour of the sea and river ice under dynamic loads is an urgent subject of scientific research and it attracts extensive attention. In addition, the temperature change in the process of ice failure cannot be neglected since that temperature plays an important role in the mechanical properties of the ice. In this study, a fully coupled thermoelastic ordinary state-based Peridynamic model is employed to investigate fragmentation of ice cover subjected to an underwater explosion. Both the deformation effect on the thermal effects and the thermal effects on deformation are taken into consideration. The pressure shocks generated by the underwater explosion are applied to the bottom surface of the ice cover for non-uniform load distributions. Crack propagation paths are investigated, the damage is predicted and compared with existing experimental results. The corresponding temperature distributions are also examined. Furthermore, the ice failure mode in both the top surface and the bottom surface of the ice sheet is investigated.
Local Buckling of Dented Subsea Pipelines Under the Combined Loadings
Authors: Chen Yanfei , Hou Fuheng , Dong Shaohua , He Guoyan , Zhang Qi , Liu Hao , Li LuLu
Published: December 2020

Dents has long been regarded as a potential hazard for integrity of subsea pipelines in the oil and gas industry. The dents in subsea pipeline are usually caused by the collision and extrusion with other objects resulting in permanent plastic deformation in pipe wall. Meanwhile, seabed deformation or motion due to subsidence, mudslides, and seismic activities may load the pipeline to an ultimate state, where the pipeline is under combined external pressure and bending moment. It is of great importance to carry out the research on the buckling failure and bending capacity of dented subsea pipelines under these combined loads.

Nonlinear finite element method is adopted to investigate the local buckling failure of the dented subsea pipeline under combined loads. The numerical results show that the buckling behavior and bending capacity of the pipeline can be largely reduced due to the existence of external pressure and axial force. Based on numerical results, a formula for predicting the bending capacity of dented subsea pipelines is proposed. Finally, the accuracy of propose solution is validated.
An Uncertainty Evaluation of Different Fidelity Methods to Predict Ship Motions and Structural Loading in Waves
Authors: Husser Nicholas , Brizzolara Stefano
Published: December 2020

In this study, four approaches are investigated to predict the motions and structural loads on a containership in waves. The Flockstra (1974) containership model is used as the benchmark for this study as extensive experimental data is available to compare to the predictions. The hydrodynamic loads and motions are predicted using strip theory, a zero speed Green’s functions panel method with forward speed correction, a fully unsteady 3D panel method and unsteady RANSE simulations for limited cases. Simulations are performed at Fn = 0.245 in head, stern quartering, and bow quartering seas for wave length to ship length ration λ/L of 0.35–1.40. The accuracy of each method, relative to experimental results, in predicting the amplitudes of heave, pitch, and roll are investigated. Vertical and horizontal bending moments, shear forces, and the torsional moment on the hull at midships and 0.25LBP forward and aft of midships are also calculated and compared with the measured values. Through comparison with experimental data, the relative uncertainty of all four methodologies in predicting both motions and structural loads are assessed and discussed. Overall, all linearized potential flow methods show a large discrepancy with the experimental loads, motivating the need for further studies on non-linear effects for this particular ship type. This paper has been prepared in the framework of the ISSC-ITTC special joint committee on uncertainty quantification in wave load estimation.
Hydrodynamic Analysis of Floating Docks With Alternative Geometries for Floating Wind Turbine Installation
Authors: Gran Viktor A. , Jiang Zhiyu , Pan Zhiyuan
Published: December 2020

Installation of spar floating wind turbine offshore is a challenging task. Usually, the spar platform is upended first, and mating of the tower assembly with the spar platform is assisted by a crane vessel. Due to motions of the spar platforms and of the crane vessel, such an operation often takes place in shielded areas with relatively small wave heights and wind speed. The floating dock concept has been recently proposed to expand the weather window for installing spar floating wind turbines. The idea is to use a cylindrical dock to shield the spar platform from wave excitations. However, because of the trapped internal fluid, the cylindrical geometry is subjected to piston mode and sloshing mode excitations, and these modes may fall in the wave periods and cause unfavourable response characteristics. This paper investigates the influence of floating dock geometries on the piston and sloshing modes. We assumed a homogeneous mass distribution of the floating dock and changed the geometry of the cylindrical dock by expanding or reducing the neck area. Then, hydrodynamic analysis of the alternative geometries was carried out using a potential flow code. By comparing the system’s piston modes and sloshing modes, we identified the trend of variation and found that an expanded neck area can lead to increased sloshing period and piston mode. This indicates a potential improvement of the dock responses in operating sea states. The results of this analysis can be used in the shape design optimisation in a future work.
Time Domain VIV Analysis Tool VIVANA-TD: Validations and Improvements
Authors: Wu Jie , Jin Jingzhe , Yin Decao , Lie Halvor , Passano Elizabeth , Sævik Svein , Tognarelli Michael A. , Grytøyr Guttorm , Andersen Torgrim , Karunakaran Daniel , Igland Ragnar
Published: December 2020

An empirical time-domain (TD) vortex-induced vibration (VIV) prediction model has been implemented in a software called VIVANA-TD based on its earlier development by Thorsen at NTNU. It models the synchronization of VIV loads and structural responses with a set of empirical parameters generalized from model tests. Combining this time domain hydrodynamic load model with a non-linear finite element structural model makes it possible to account for structural non-linearities and time-varying flow.

A joint industry project (JIP), i.e., Lazy Wave Riser JIP has been organized to improve the design basis for SLWRs. This JIP is executed by SINTEF Ocean with support from NTNU. The industry participants are Equinor, BP, Subsea7, Kongsberg Maritime and Aker Solutions. The overall objective of this JIP is to systematically validate VIVANA-TD, in order to establish it as an industrial tool for VIV prediction. It is also aimed to improve the empirical basis and methods for calculation of VIV of deep-water steel lazy wave risers (SLWRs).

In the present paper, the validation study is presented for selected model tests in constant flow conditions with uniform and sheared profiles. The test model includes bare pipe, pipe with partial strake coverage and riser model with staggered buoyancy elements. The empirical parameters have been generalized based on extensive model test data. Limitations and improvement of the model have been also been explored. The results show that the present TD model can represent reasonably the VIV loads and that the prediction has good agreement with measurements in general.
Effects of Scale and Configuration of Air Chamber of OWC Type WECs on Air Chamber Characteristics
Authors: Ikoma Tomoki , Hirai Shota , Aida Yasuhiro , Masuda Koichi , Eto Hiroaki
Published: December 2020

This paper describes scale effects and influence of configurations of oscillating water column type wave energy converters from model tests and theoretical calculations. Many researches regarding wave energy converters (WECs) have been conducted. The behavior of an oscillating water column of an OWC type WEC is complicated because of including wave-air-turbine interaction, and thus several issues remain. One of the issues is that influence of difference in scale between small scale experimental models and full scale models is unclear. It is important to understand its characteristics accurately to improve design technologies for such as complicated systems. In this study, we carried out forced oscillation tests using multiple scales and shapes of OWC models in still water, and measured the pressure inside the air chamber and the internal mean water level with a multi-line wave probe. The experimental models used have a box like air chamber or manifold type air chamber, and which scales were 1/1, 1/2 and 1/4.The difference of the two air chambers is an orifice or a duct to be inlet-outlet of air. As a result, the difference in scale and configuration of the air chamber affected the characteristics of the air chamber. In addition, as a result of numerical calculation using the linear potential theory and comparison with experimental results, the experimental results could be reproduced by numerical calculation. Besides, we could discuss the effects and the influences of the air chamber basically.
Large-Scale Tests With Hydraulic and Pneumatic Overpressure for Monopile Decommissioning of Offshore Wind Turbines
Authors: Hinzmann Nils , Gattermann Jörg , Lehn Patrick
Published: December 2020

The complete decommissioning of an offshore wind farm can be considered as a highly complex and hazardous approach. An unknown number of variables and unforeseen circumstances are involved in the decommissioning process.

On the one hand the decommissioning of the top structure, such as blades, turbine and mast, can be handled relatively risk free by reversing the installation steps. More focus needs to be given to the recycling method and logistic. The foundation decommissioning on the other hand is a much more challenging procedure. Originally designed for high axial and lateral loads, the foundations are generally oversized concerning the loading capacity. With a diameter up to nine meter, an embedment of about 40 meter and a set up effect over 25 years, the necessary force to pull the pile out of the seabed can be assumed, if at all determinable, to be enormous.

Different methods and techniques for a complete removal of offshore pile foundation are currently investigated within the project DeCoMP. Vibratory extraction and jet drilling aim for a reduction of the pile skin friction by creating a layer of less density between the pile shaft and pending soil.

In a different approach the seabed is used as an abutment and a pressing force is applied by creating an overpressure inside the pile. The results of pilot test, presented in this paper, show the capability of overpressure pile decommissioning. On this basis scaled test with an extensive measurement concept will be carried out in 2020.
A Numerical Study on the Evolution of Random Seas With the Occurrence of Rogue Waves
Authors: Zhou Zhuowei , Zhang Ningchuan , Huang Guoxing
Published: December 2020

Numerous numerical and experimental investigations show that rogue waves present a much larger probability of occurrence than expected from the linear random wave model, i.e., Gaussian distributed waves. The deviation from normal statistical events excites a continuous concern about rogue-wave research. In this study, rogue waves under random wave seas are addressed within the framework of the horizontal 1-D fully nonlinear Euler equations. The JONSWAP wave spectra with a different set of random phases are selected as the initial state of the recurrences of incoming wave trains. Different values of spectrum parameters (i.e., enhancement factor γ and significant wave height Hs) for JONSWAP spectra are chosen in order to reproduce different random sea states with different BFI values. The results of the numerical method using in this study are compared with classical experimental studies of rogue waves and show good agreements. Nonlinear wave interactions and the evolution of simulated waves are investigated in order to study the emergence of rogue waves. Statistics analysis is applied to the simulating results to find the deviations with normal distributions. Numerical results reveal that the initial unstable waves need some space to evolve, i.e., around 20 wavelengths, and will keep in an energetic state for the formation of rogue waves.
Feasibility of Using Hindcast Data for Fatigue Assessment of Permanently Moored Offshore Units in West-Africa
Authors: Hageman Remco , Aalberts Pieter , L’Hostis Didier , Ledoux Alain
Published: December 2020

To register the wave loads acting on offshore floating structures, wave buoys have been the generally accepted as the most accurate method. However, over the last couple of years, hindcast data has become increasingly accurate. The availability of this data allows assessment of the response of offshore floating structure without the need of local wave buoys.

The accuracy when using hindcast data to assess fatigue accumulation on FPSO hulls is assessed in this paper. For this analysis, calculations have been executed for multiple spread-moored production units in the West-Africa region. The purposely deployed wave buoys provide a full wave spectrum including multiple wave components and directional variation of the wave energy. This data is used to derive statistical data description of the sea state.

In this paper, we consider the use of WaveWatch III, ERA-5 and Copernicus hindcast models. These are all models which provide sea state descriptions on a global scale. The statistical data from the hindcast models is compared against the reference data provided by the wave buoys. Overall, the three models provide decent results in this mild environment. However, it is found that Copernicus provides more accurate estimates for the larger wave heights.

Fatigue analyses have been executed. The full spectral data provided by the wave buoys is used to conduct a spectral fatigue assessment. The hindcast models generally provide more limited details of the sea state, information such as spectral shapes and directional spreading may be missing. Understanding the influence of these missing characteristics is vital for reliable long term fatigue assessment. Multiple fatigue analyses have been executed to examine the sensitivity to these missing characteristics.

It has been shown that the multi-modality of the sea state and spectral shape are the most important parameters driving the deviation between the fatigue assessment based on hindcast data and wave buoy data. These differences can accumulate to a factor of 2 on lifetime consumption. The influence of the accuracy of the statistical parameters provided by the hindcast models and wave spreading is considerably less with a typical contribution of around 30% on lifetime consumption.
Experimental Study on the Effects of Mooring System on Air Gap Response of Semi-Submersible Platform
Authors: Li Xu , Xiao Longfei , Wei Handi , Liu Mingyue
Published: December 2020

The air gap response is crucial for the safe design and operation of large-volume floating platforms such as semi-submersible and tension leg platforms. It is a complex task to perform numerical simulation on the air gap response considering the wave free surface elevation and the motions of the floating vessel. Therefore, the prediction of air gap response still relies heavily on model tests. This paper attempts to investigate the effects of the mooring system, especially the effects of the length of mooring lines, on the air gap response of semi-submersible platform based on model tests results. The scaled model of the semi-submersible platform is supported by a symmetric mooring system composed of 8 mooring lines. A set of model tests with different length of mooring lines was performed in the State Key Laboratory of Ocean Engineering basin at Shanghai Jiao Tong University, and the air gap responses of 15 locations were measured using wave probes. The results indicate that the mooring system plays an important role in the air gap response of semi-submersible platform.
Sensitivity Analysis of an ANN-Based System for Detection of Mooring Line Failure
Authors: Sidarta Djoni E. , Tcherniguin Nicolas , Bouchard Philippe , Lim Ho-Joon
Published: December 2020

Monitoring the integrity of mooring lines on floating offshore platforms is one of the key factors in ensuring safe and productive offshore operations. Sensors, such as inclinometers, compressive load cells or strain sensors, can be used to monitor the inclination angles or tensions on mooring lines. An alternative method using only dry monitoring systems, such as DGPS (Differential Global Positioning System), Gyrocompass and/or IMU (Inertial Measurement / Motion Unit), can also be used to monitor the integrity of mooring lines. This method uses the measured motions and positions of a vessel without any information on the environmental conditions to detect mooring line failure. The detection of mooring line failure is based on detecting shifts in low-frequency periods and mean yaw angles as a function of vessel position, mass and added mass. The proposed method utilizes Artificial Neural Network (ANN) to recognize and classify patterns.

The training of an ANN model requires examples/data associated with intact mooring lines and broken mooring line(s). Examples/data of broken mooring line(s) are practically available only from numerical simulations. Therefore, it is important to address these two key topics: (1) Is the real behavior of the floating offshore platform sufficiently aligned with numerical simulations? and (2) The effect of the accuracy of monitoring equipment on the performance of an ANN-based system.

The first topic is reviewed briefly with its possible solution including some sensitivity tests, and this paper focuses on addressing the second topic. A system architecture is discussed in this paper along with the accuracy of the monitoring equipment. As an example, an ANN model has been trained to detect a broken mooring line of a spread-moored FPSO. This ANN model has been tested on its performance in dealing with a range of possible errors associated with the monitoring equipment. Furthermore, the tests have been carried out for a combination of variables that are not included in the ANN training, such as: vessel draft (mass), sea state conditions and directions. This paper presents the results of the tests for various variable sensitivities, which cover vessel positions, mean yaw angles and vessel drafts. These are essentially testing the tolerance of a trained ANN model against error or noise in the data. The results show that a trained ANN model can be error/noise tolerant.
Experimental Study the Performance of a Compact Mooring System for a Dual Chamber Floating Oscillating Water Column Device
Authors: Xu Sheng , Rezanejad K. , Wang Shan , Gadelho J. F. M. , Guedes Soares C.
Published: December 2020

A compact mooring system concept is proposed. This novel mooring is composed of submerged buoy and three segments of nylon ropes, which is suitable for the large wave energy converters and wave energy converter array due to its high flexibility and small mooring radius. The performance of this mooring concept was studied experimentally when it was moored to an oscillating water column. The damping of the oscillating water column was modelled by an orifice on top of the chamber. Both regular and irregular head sea wave tests were conducted. In order to study the influence of wave height on system dynamics, two series of regular wave tests with same periods but different wave heights were conducted. An optical tracking system was installed to capture six degrees of freedom motion responses of oscillating water column. The air pressure in the chamber was measured by the air pressure sensor. Two load cells were installed on the top of mooring lines to measure mooring tension time series. Besides, the wave surface elevations inside the chamber were measured by the wave gauges. According to the experimental results, the six degrees of freedom motion responses of floating wave energy converter and mooring tensions are analyzed. Besides, the energy conversion efficiency is evaluated based on the measured data.