bluuebunny/crossref_metadata_2025_split huggingface dataset:
Parametric Study Related to the Collision Probability Between Two Risers
Authors: Fu Ping , Leira Bernt J. , Myrhaug Dag 
Published: October 2016

Efficient and accurate methods for assessment of riser collision become increasingly important as offshore oil and gas exploration moves into deeper water. Collisions may occur as a result of the environmental loading (current and waves), flow interaction and surface floater motions, or combinations of these effects. This paper provides a parametric study on the influence of these factors, especially the influence of the fluid interference on the minimum distance statistics. The conditional collision probability for two flexible risers in a tandem arrangement is also estimated for sea-states with a 100 year return period.
The drag coefficient of the downstream riser is reduced due to flow-field interaction according to the relative position of the risers. The reduced drag force results in a decrease of the relative distance between two risers when current is applied. This leads to an increase of collision probability. In this paper, the drag coefficient is obtained by Blevins wake model and used for both static and dynamic analysis. The statistics of the minimum distance between two risers are calculated according to the relative position of Finite Element nodes which are obtained from dynamic analyses. Based on a set of such dynamic analyses, the probability distribution function of the minimum distance is estimated. The probability of riser collision can then readily be computed.
Towards an Improved Understanding of Green Water Exceedance at the Bow of an FPSO
Authors: van ‘t Veer Riaan , Boorsma Anne 
Published: October 2016

When a permanently moored FPSO in deep draft condition finds itself in harsh weather conditions it most likely will experience freeboard exceedance at the bow and consequently green water on deck. Knowledge about how and how much water will come onto the deck, is relevant for both a turret moored and spread-moored FPSO since both are expected to experience the most severe design conditions in head to bow quartering waves.
In this paper we focus on green water phenomena observed on a Suezmax FPSO in severe head seas in the model test basin. Using an on-board video in combination with deck-mounted wave probes, green water events are recorded in detail. This provides unique insights in how the water floods onto the deck. Very different flooding phenomena are observed between different events and they can strongly deviate from a dam-breaking kind of event.
Through a detailed description of three typical flooding events, the present paper improves the understanding of how green water flows onto the deck. It highlights the benefit of detailed model tests and emphasizes that new and more detailed modelling is required since the dam-breaking theory has limited value for the present problem.
Fatigue Modelling of Semi-Elliptical Surface Cracks in Welded Pipe Geometries
Authors: Hoh Hsin Jen , Pang John H. L. , Tsang Kin Shun 
Published: October 2016

Offshore pipelines and risers transfer oil and gas across long distances, from seabed to production facility to the surface. The long pipelines are formed by welding together pipe segments. The welded joints formed are a source of stress concentration and defects from which fatigue cracks can grow. Hence, it is imperative that the effect of the weld geometry on the stress concentration be understood so that appropriate measures can be taken to assess the potential remaining service life of the welded structure.
The effects can be understood by the linear elastic fracture mechanics approach, where the stress intensity factors quantify the stress concentration. While the classical equations of Newman and Raju have been long available for semi-elliptical surface cracks in plates, no similarly elegant stress intensity factor solutions are available for pipes. There have been solutions in tabular form which can be cumbersome in practice. Moreover, solutions of welded pipe geometries have not been developed. The objectives of the current work are to develop closed-form solutions for stress intensity factors for external semi-elliptical surface cracks in plates. The welded pipe geometry will also be studied to develop solutions for the weld toe magnification factors of welded pipe geometries. The stress intensity factors can be used to determine the propagation rate of cracks in pipe or welded pipe geometries.
The stress intensity factors are obtained by the J-integral output of the three-dimensional finite element method. First, a plate with a circular crack is modelled. The initial step transforms the model to a plate with a semi-elliptical crack with the appropriate crack aspect ratio and width. A second transformation follows to transform the geometry to pipe form. The main parameters studied are the relative crack depth to thickness, crack aspect ratio, radius and thickness. The developed stress intensity factor solutions can be reduced to the classical equations. The new solutions show good agreement compared to previous work. A similar approach is developed to study the welded pipe geometry to develop weld toe magnification factor solutions. The weld toe magnification factor solutions for certain geometries are presented as a function of the relative crack depth.
The stress intensity factor solutions are then applied to predict the crack growth rates of cracks in pipe geometries. The prediction was conducted by a program written to assess the fatigue life of single and multiple cracks in pipes and welded pipes. The fatigue life assessment of welded pipes using the weld toe magnification factor solutions shows how significantly the weld geometry affects fatigue life.
Drag Reduction and VIV Suppression Behaviour of LGS Technology Integral to Drilling Riser Buoyancy Units
Authors: Marcollo H. , Potts A. E. , Johnstone D. R. , Pezet P. , Kurts P. 
Published: October 2016

Drilling risers are regularly deployed in deep water (over 1500 m) with large sections covered in buoyancy modules. The smooth cylindrical shape of these modules can result in significant vortex-induced vibration (VIV) response, causing an overall amplification of drag experienced by the riser.
Operations can be suspended due to the total drag adversely affecting top and bottom angles. Although suppression technologies exist to reduce VIV (such as helical strakes or fairings), and therefore reduce VIV-induced amplification of drag, only fairings are able to be installed onto buoyancy modules for practical reasons, and fairings themselves have significant penalties related to installation, removal, and reliability.
An innovative solution has been developed to address this gap; LGS (Longitudinally Grooved Suppression)1. Two model testing campaigns were undertaken; small scale (sub-critical Reynolds Number flow), and large scale (post-critical Reynolds Number flow) to test and confirm the performance benefits of LGS.
The testing campaigns found substantial benefits measured in hydrodynamic performance that will be realized when LGS modules are deployed by operators for deepwater drilling operations.
Fatigue Algorithm for Modeling Many Weld Toe Cracks Propagating at Welded Joints
Authors: Tsang Kin Shun , Pang John H. L. , Hoh Hsin Jen 
Published: October 2016

Fatigue crack growth at welded joints often propagates from as many as tens to hundreds of small weld toe cracks along the weld toe line in offshore welded structures. This paper will present a fatigue algorithm for modeling many small weld toe cracks propagating from a welded joint. Cracks usually initiate at the weld toe region of the structures and propagate as surface cracks at the stress concentration regions of the weld-toe line. The presence of such weld defects or crack-like flaws can have a severe detrimental effect on their fatigue life and fracture resistance. Currently, there is a lack of studies that considers the effects of multiple cracks and their distribution density in welded joints. This work focuses on the fatigue analysis and modeling of multiple weld toe cracks, specifically in T-butt joints.
Fatigue crack growth prediction is usually determined by the stress intensity factor range and crack propagation rate through Paris law. To predict the Stress Intensity Factor (SIF) of a weld toe crack, the magnification (Mk) factor was used. The Mk factor is influenced by the size of the welded attachment, as well as the size and depth of the weld toe crack. Simplified solutions for practical prediction of Mk factors were determined from 3D extended finite element method (XFEM) by modelling a semi-elliptical weld toe crack in a T-butt weld for cracks of different dimensions. The accuracy of the Mk factor solutions was verified by comparison to HSE fatigue data on 16 mm thick tubular joints.
The Mk factor solutions were used to predict the growth of fatigue cracks using a model based on Paris Law and SIF solutions by Newman and Raju with plastic zone size corrections. Fatigue life was predicted for plates with and without attachments. It could be seen that the predicted life of a weld toe crack was severely reduced with the addition of a welded attachment. The model was extended to the multiple surface cracks commonly observed at the weld toe, where each crack is treated as independent, following established code procedures. The multiple cracks will coalesce as they propagate, until a single dominant crack emerges and fracture occurs. In this paper, the relationship between the fatigue life and the number and density distribution of the initial cracks was investigated. Fatigue life was predicted for plates with attachments with 1, 2, 10 and 100 cracks initially. The results show that as the number of cracks increases, the predicted fatigue life decreases.
A Rubber Bag for Liquid Cargo to Improve Ship Collision Safety
Authors: Kubiczek Jan M. , Liang Boyuan , Molter Lars , Ehlers Sören 
Published: October 2016

Collisions and grounding accidents of ships, but also the failure of the hull-integrity, can lead to oil leakage. Examples are the Rena in 2011, the Hebei Spirit in 2007 and the much known accident of the Prestige in 2002.
Consequently research regarding the enhancement of the structural design to increase the safety-level of ships in case of accidents is important. In this paper the use of a rubber bag as a second barrier is presented as an alternative concept to prevent oil leakage in case of accidents.
The influence of the rubber bag is investigated using the example of a ship collision. A simplified tanker side structure as well as a box shaped rubber bag are analyzed with the finite element method. The material model for the rubber bag is calibrated with tensile tests to obtain the required material parameters.
The reaction forces and the associated penetration depth are analyzed. The comparison is done between the structure with and without the rubber bag. For the latter, the general behavior is compared with large-scale experimental results. Furthermore an additional increase of the survivability of the ship due to the rubber bag without changing the common structural design is discussed.
Correlation Allowances in Model Tests Results: A Delicate Balance Between Performance, Accuracy and Commercial Interests?
Authors: Hagesteijn Gerco , Hooijmans Patrick , van der Meij Karola 
Published: October 2016

Model tests at ballast and design draught are used to convert the sea trial results from the ballast trial draught to the contractual design draught. Correlation allowances in model test results and their effect on the trial performance prediction are of major importance. Nowadays it is not only typical to verify the contract speed but also the EEDI certification requires a verification of the speed power performance of the vessel. The use of a to favorable CA-value may lead to attractive performance figures, but also leads to higher fuel consumption figures than expected. Furthermore the design point of the propeller is affected, which leads to a too low light running margin and in some cases to erosive cavitation.
During a study, large spreading in the values of the correlation allowances for design draughts have been found for merchant vessels tested at different model test institutes, but at ballast trial draught the spreading is much less. Can it happen that some institutes select favorable correlations allowances on the basis of inaccurate trial data of shipyards? Or should we accept a large spreading in correlation allowances and have these indeed been confirmed by sea trials at design draught? This paper will present a discussion using the experience of a large full scale trial database as well as the accuracy of model and full scale tests.
Aframax in Numerical Wave Tank: From Classic Decay Test to the Ship Moored in Irregular Waves
Authors: Barcarolo Daniel , Thilleul Olivia , le Touzé David , Jacquin Erwan , de Vries Igor , Naciri Mamoun 
Published: October 2016

The prediction of ship motions in extreme seastates is very complex as it involves strong nonlinearities. It deals with high motions of the ship and implies strong mooring system loads. These seastates are usually modeled in tank tests but an alternative in the near future could be CFD computations.
In this article, all required steps to setup and verify the hydrodynamic and numerical model are performed. The setup of the hydrodynamic and numerical model enable us to show that CFD computations of motion RAOS and pitch decay tests provide results in agreement with diffraction-radiation results.
Wave only simulations enable us to verify that irregular waves are accurately modelled in the CFD domain. Since the wavemaker motion used in tank tests to generate irregular waves is not available, a process of linear back propagation is set up from the wave elevation on a wave probe in tank tests. High Order Spectral (HOS) simulations are performed to reproduce the seastate measured in tank tests.
Finally, a test was performed to model the ship motions in irregular extreme waves with ICARE solver coupled to the computed HOS wave field through Spectral Wave Explicit Navier Stokes Equations (SWENSE).
Numerical Analysis of Combined VIV and Slug Flow in Time Domain
Authors: Huse Knudsen Tor , Sævik Svein , Thorsen Mats Jørgen 
Published: October 2016

Vortex induced vibrations (VIV) and slug flow are two important aspects for marine risers conveying a multiphase flow, and should be carefully examined due to the influence on the fatigue life of the structure. This article examines a truncated riser exposed to VIV with an internal two-phase slug flow. The main focus of the article was to examine the effect of internal slug flow on the VIV of a riser. The VIV were simulated in time domain with a linear structural model with constant pretension. Approximately 150 vortex shedding periods were simulated after the response reached steady state. An internal two-fluid flow was introduced, with constant internal velocity, pressure and uniform slug lengths. From the numerical study it was apparent that the slug velocity and slug length had an influence on the response pattern, amplitude and frequency. An analytical model that predicts additional response frequencies due to slug flow was also compared to the numerical studies. The analytical study produced similar additional response frequencies as the numerical study.
The slug length and internal velocity can influence the response of the riser, and should be considered for marine risers conveying multiphase flow.
Friction Behavior Between Epoxy and Flexible Pipes Armor Wires
Authors: Lorio Diego A. , Wedekamper Facundo J. , Bertoni Fabiano , Lopéz Facundo S. , Campello George C. , Strohaecker Telmo 
Published: October 2016

The offshore industry has presented an increasing demand over the last few decades, requiring the production in deep water fields. The end fittings (EF) are critical points within the production system. Therefore, structural and fatigue analyses are essential in the EF design, making it necessary to know the stress distribution experienced by the armor wires along the EF. Numerical and analytical models are often used in order to assess the stress state. However, characteristics like geometries, materials and interactions must be previously known in order to apply these models.
The purpose of this work was to analyze the arithmetic mean surface roughness (Ra) and to determine the friction coefficient (μ) for two types of armor wires when in contact with resin used to fill the EF. The resin used in the interaction with the armor wires was an epoxy filled with metallic particles. For the experimental analysis straight carbon steel armor wires with different cross-sections, typically used in 2.5″ and 8″ flexible pipes were used. Surface profile was obtained for each wire by repeated measurements along two lines over each surface. A total of three repetitions were performed in each measure line. Longitudinal roughness was determined through these profiles. Finally, friction coefficients were obtained experimentally by means of a device that allows to simulate the wire pullout and sliding process. In this device, two epoxy pads were put in contact with the surface of the analyzed wire sample, and rigid bodies in contact with the pads were used to ensure that the normal load applied is transmitted uniformly through the contact surface. The displacement rate, contact pressure between the surface of the wire and the epoxy resin pads, and axial force were recorded.
The roughness in the longitudinal direction of the wires was analyzed through descriptive statistic and compared by Student’s “t” test. The highest values were obtained on wires with larger sections. This behavior is exposed on the results obtained for the friction coefficient as a function of the contact pressure. Friction coefficient for both wires was analyzed and compared using a Mann-Whitney U test. Both friction coefficients have a positive slope, indicating a small increase as the contact pressure raise. The significance value obtained for the means comparisons was p = 0.0001 and confirms that the average friction coefficient of the two wires are really different. Because of that, we conclude that is necessary to treat the EF project for different flexible pipes differentially.
Motion-Based Wave Inference: Monitoring Campaign on a Turret FPSO
Authors: da Silva Bispo Iuri Baldaconi , Queiroz Filho Asdrubal N. , Tannuri Eduardo A. , Simos Alexandre N. 
Published: October 2016

In recent years, considerable effort has been made in order to validate different methods that aim at estimating the wave spectra from the motions recorded on a ship or on an offshore platform. For more than ten years now, the University of São Paulo has been working on a wave inference method for moored oceanic systems, such as Floating Production Storage and Offloading (FPSO) vessels.
This paper brings the first results from an ongoing field campaign, started in December 2014, for the estimation of wave statistics by means of this system, which is based on a Bayesian inference approach. The performance of the motion-based method is checked against the wave estimations provided by a commercial marine radar system. The radar is installed in a fixed platform close to the FPSO that is being monitored, which is moored in a turret configuration. Comparison between both systems allows one not only to evaluate the performance of the method but also to evaluate the inherent limitations that exist when the estimations are based on a very large vessel, especially one whose heading may be subjected to fast variations.
In this work, a new algorithm for the partition of the wave spectrum is used, differing from the previous works. This technique allows the identification and combination of energy peaks of the directional spectrum into wave systems, providing better modal wave statistics and less noise in the final spectrum.
Previously validation of the wave inference method was made through numerical and small-scale experimental analysis. Also, in previous studies, a 9-month field campaign was also used to validate the Bayesian method using a spread-moored FPSO, but no comparison to other measurement technique was made, instead, numerical forecasts were adopted. In this work this objective is achieved: so far, 10 months of estimates from the Bayesian inference algorithm have already been compared to data supplied by a radar system.
Comparisons with the marine radar readings attest an adequate identification of mean wave directions, with the Bayesian model estimating mean wave directions that are in close agreement to those provided by the radar system most of the time. There are, however, a few cases when the discrepancy is large and the reasons for this are still under investigation.
The comparison between both sources of data provides some insightful results, which are discussed in this paper. First, they confirm and quantify the main biases of the applied method, which are related to the filtering of the energy from high-frequency waves. On the other hand, they attest that the performance of the method for more severe sea states is good overall, showing that the method is able to provide good estimates of wave height and direction and also to capture situations when wave conditions are changing fast, such as the ones that occur with the incoming of cold fronts from South.
A Study of Fluid Flow in Sediments and the Effect of Tidal Pumping
Authors: Whitehead John , Waltrich Paulo , Hughes Richard , Thompson Karsten 
Published: October 2016

Offshore drilling and production operations can result in spills or leaks of hydrocarbons into seabed sediments, which can potentially contaminate these sediments with oil. If this oil trapped later migrates to the water surface it has the potential for negative environmental impacts. For proper contingency planning and to avoid larger consequences in the environment, it is essential to understand mechanisms and rates for hydrocarbon migration from oil containing sediments to the water surface as well as how much will remain trapped in the sediments. It is believed that the amount of oil transported out of the sediment can be affected by tidal pumping, a common form of Subterranean Ground Water Discharge (SGD). However, we could find no study investigating the phenomenon of fluid flow in subsea sediments saturated with oil and the effects of tidal pumping.
This study presents an experimental investigation of tidal pumping to determine if it is a possible mechanism to describe the appearance of an oil on the ocean surface above a sediment bed containing oil. An experimental apparatus was constructed of clear PVC pipe allowing for oil migration to be monitored as it flowed out of a sand pack containing oil, while tidal pressure oscillations were applied in three different manners.
The effect of tidal pumping was simulated via compression of air above the water (which simulated the increasing static head from tidal exchange). Experimental results show that sustained oil release occurred from all tests, and tests with oscillating pressure produced for longer periods of time. Furthermore, the experimental results showed that the oil migration rate was affected by grain size, oil saturation, and oscillation wave type. In the static experiments, a linear relationship between grain size and permeability was observed, as is well-known in fluid flow in porous medium. However, the oil recovery does not show a linear relationship with viscosity, as the oil recovery only changed by 50% for a nearly 400% variation in viscosity. In all oscillating experiments the rate and ultimate recovery was less than the comparable static experiments. This leads to the preliminary conclusion that with an oscillating pressure on top of a sand pack, movement of a non-replenishing source of oil is suppressed by pressure oscillation.