bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of Remote Sensing Technology in Tianjin Binhai New Area Coastal Zone Monitoring
Authors: Wang Juan , Bu Zhiguo , Li Zhongqiang
Published: January 2011

The coastal zone is the belt influenced by land and ocean interactions, as well as human factors. So its evolution depends not only on natural factors but also on human socio-economic activities. It has very good instructive meaning to provide timely accurate coastal zone changeing information for exploiting and protecting the coast. Using 5 periods’ remote sensing images covering 20 years from 1987 to 2008 of Tianjin city, this paper extracted the coastline and the wetlands from different years utilizing different methods and techniques of data image processing and visual interpretation based on the characteristic of each RS image. The paper analyzed the law of the coastline and the wetlands changes in both spatial and temporal aspects, and then discussed the major influential factor to the changes by analyzing natural and artificial factors. The results indicated that the total coastline and the natural coastline increased, while the artificial seashore and wetlands decreased in large scale in the 20 years, due to the development of the coastal industry. Thanks to the protection and reinstatement for wetlands, the area of wetlands increased in the past two years.
Effects of Free Spans on On-Bottom Stability of Offshore Pipelines: A Global Approach
Authors: Aamlid Olav , Vedeld Knut , Sollund Ha˚var
Published: January 2011

On-bottom stability design of pipelines normally assumes that the pipeline rests on a flat seabed without any free spans. This paper presents on-bottom stability calculations accounting for the presence of free spans. It should be noted however that the calculations presented in this paper are only meant to be indicative, and not for design purposes. The absolute static stability criterion in DNV-RP-F109 is given on a form that directly compares the hydrodynamic loads on the pipeline to the resistance provided by the pipe soil interaction. Hence, by following this line one can estimate the effect that the presence of free spans has on both the load and the resistance. It is known that hydrodynamic loads, lift and drag, are higher close to the seabed than away from it. This will obviously be beneficial from an on-bottom stability point of view. The effect that the free spans have on the pipe soil resistance is not so obvious. Often, one assumes that this resistance consists of two components, pure Coulomb friction and so-called passive resistance due to penetration. The Coulomb friction part is herein assumed to be independent on the span fraction as the total weight remains the same, whereas the soil models in DNV-RP-F109 predicts an overall reduction in passive resistance due to the presence of free spans. For the on-bottom stability calculations it turns out the approach predicts a beneficial effect for pipelines on a sandy seabed. However, under most conditions on clay, where the beneficial passive resistance is more important, the prediction is that the presence of free spans is not beneficial.
On the Vortex-Induced Vibration Response of a Model Riser and Location of Sensors for Fatigue Damage Prediction
Authors: Shi C. , Manuel L. , Tognarelli M. A. , Botros T.
Published: January 2011

This study is concerned with vortex-induced vibration (VIV) of deepwater marine risers. Riser response measurements from model tests on a densely instrumented long, flexible riser in uniform and sheared currents offer an almost ideal set-up for our work. Our objectives are two-fold: (i) we use the measured data to describe complexities inherent in riser motions accompanying VIV; and (ii) we discuss how such data sets (and even less spatially dense monitoring) can be used effectively in predicting fatigue damage rates which is of critical interest for deepwater risers. First, we use mathematical tools including Hilbert and wavelet transforms to estimate instantaneous amplitudes and phases of cross-flow (CF) and in-line (IL) displacements for the model riser as well as scalograms to understand time-frequency characteristics of the response; this work confirms that the motion of a long flexible cylinder is far more complex than that of a rigid cylinder, and that non-stationary characteristics, higher harmonics, and traveling waves are evident in the riser response. Second, a well-established empirical procedure, which we refer to as Weighted Waveform Analysis (WWA), is employed to estimate the fatigue damage rate at various locations along the length of the riser from strain measurements at only eight sensors. By iterating over numerous different combinations of these eight strain sensors as inputs (from among all the twenty-four available locations on the riser), optimal locations for the eight sensors on the riser are identified by cross-validation, whereby predicted strains and fatigue damage rates at locations of instrumented sensors are compared with strains and fatigue damage rates based on actual recorded measurements there. We find that, if properly placed, as few as eight sensors can provide reasonably accurate estimates of the fatigue damage rate over the entire riser length. Finally, we demonstrate how more accurate fatigue damage prediction can result when non-stationary response characteristics are considered and a modified WWA method (that more effectively accounts for traveling waves than the WWA method alone does) is employed.
Alternative Empirical Procedures for Fatigue Damage Rate Estimation of Instrumented Risers Undergoing Vortex-Induced Vibration
Authors: Shi C. , Manuel L. , Tognarelli M. A.
Published: January 2011

Vortex-induced vibration (VIV) is a topic of great importance in fatigue damage assessment and life prediction for marine risers. In order to gain insight into riser motions and estimated fatigue damage due to VIV, data loggers such as strain sensors and/or accelerometers are sometimes installed on risers to monitor their motion in different current velocity conditions. Accurate reconstruction of the riser response and empirical estimation of fatigue damage rates over the entire riser length using measurements from a limited number of sensors is important for efficient utilization of the costly measurements recorded. In this study, different empirical methods are employed to analyze the VIV response of a long flexible cylinder subjected to uniform and sheared current profiles. The methods include weighted waveform analysis (WWA), proper orthogonal decomposition (POD), modal phase reconstruction (MPR), a modified WWA procedure, and a hybrid method which combines MPR and the modified WWA method. Fatigue damage rates estimated using these different empirical methods are compared and cross-validated against measurements. Formulations for each method are briefly presented and discussed with examples. Results show that all the empirical methods, despite different underlying assumptions in each of them, can be employed to estimate fatigue damage rates quite well from limited strain measurements.
The Focusing of Uni-Directional Gaussian Wave-Groups in Finite Depth: An Approximate NLSE Based Approach
Authors: Adcock Thomas A. A. , Yan Shiqiang
Published: January 2011

The non-linear changes to a NewWave type wave-group are helpful in developing our understanding of the non-linear interactions which can lead to the formation of freak waves. In addition, Gaussian wave-groups are used in model tests where it is useful to have a simple model for their non-linear dynamics. This paper derives a simple analytical model to describe the nonlinear changes to a wave-group as it focuses. This paper is an extension to finite depth of the theory developed for deep water in Adcock & Taylor (2009) (Proc. Roy. Soc. A 465(2110)). The model is derived using the conserved quantities of the cubic nonlinear Schrodinger equation (NLSE). In deep water there are substantial changes to the group shape and spectrum as the wave-group focuses, and the characteristics of these changes are governed by the Benjamin-Feir Index. However, in finite depth the characteristics of the non-linear interactions change, reducing the non-linear changes to the group shape. The analytical model is validated against simulations using the NLSE and against full potential flow solutions using a QALE-FEM numerical scheme. We also compare its predictions against experiments in a physical wavetank. We find that the NLSE, and thus analytical theories derived from it, capture the dominant physics in the evolution of narrowbanded wave-groups.
Structural Failure Assessment of the Engine Room Region for a Post-Panamax Class Containership: Lessons Learned From the MSC Napoli Accident
Authors: Koh Teuk-Jin , Paik Jeom Kee , Kim Bong Ju , Woo Je-Hyouk , Sohn Sang Yong , Kim Man Soo
Published: January 2011

This paper presents a structural failure assessment of the engine room region for a post-Panamax containership, outside 0.4L amidships, subject to an unexpected excessive hull girder bending moment and shearing force, with reference to the hull failure accident of the post-Panamax containership MSC Napoli. The ultimate strength of both the hull girder and stiffened panels in an as-built post-Panamax containership is estimated using ANSYS nonlinear finite element (FE) analysis and ALPS/HULL and ALPS/ULSAP programs to clarify the safety of the structures concerned. Finally, this paper proposes an assessment procedure for preventing an unexpected catastrophe such as that experienced by the MSC Napoli. Such accidents, although rare, result in significant economic losses and marine pollution.
Prediction of Roll Damping in the Frequency Domain Using the Discrete Vortex Method
Authors: Hajiarab Mohammad , Graham J. Michael R. , Downie Martin
Published: January 2011

This paper describes a theoretical approach to predict roll damping for a three-dimensional barge shaped vessel in the frequency domain by matching a simple discrete vortex method (DVM), describing local separated flow, to an inviscid 3-D seakeeping code. The results are compared with model test experiments to demonstrate validity of the method. A good agreement between the model test RAO and the damped RAO is achieved.
Reliability-Based Factors of Safety for VIV Fatigue Using Field Measurements
Authors: Tognarelli Michael , Fontaine Emmanuel , Beynet Pierre , Santosa Mikhail , Marcollo Hayden
Published: January 2011

The development of a Vortex Induced Vibration (VIV) fatigue Factor of Safety (FoS) consistent with state-of-the-art industry design practice is cast within the coherent framework of reliability analysis. The proposed methodology consists of the following steps: (i) define the failure criteria or limit-state function, (ii) set-up a deterministic analysis model, (iii) characterize the uncertainties involved in the problem, (iv) propagate the uncertainties through the deterministic model and assess the probability of failure due to VIV fatigue and (v) calculate the FoS required to achieve a given failure probability. The proposed methodology is demonstrated by determining the FoS associated with using state-of-the-art VIV prediction models to attain varying reliability levels (probabilities of failure) in a hypothetical design scenario. Prediction uncertainty is based herein on measured flow and response data for several full-scale drilling risers working in the field. Results indicate that depending on the reliability level required of a particular design, different FoS than those that currently appear in guidance may be appropriate. Results also indicate the sensitivity of the FoS to the riser and prevailing current type; analysis program and input parameters; and accumulation of conservatism in aggregate vs. single-event damage predictions.
Efficient Fatigue Analysis of Helix Elements in Umbilicals and Flexible Risers
Authors: So̸dahl Nils , Skeie Geir , Steinkjer Oddrun , Kalleklev Arve Johan
Published: January 2011

Fatigue analysis of helix elements such as tensile armors and steel tubes are critical design issues for dynamic umbilicals and flexible pipes. The basis for assessment of fatigue damage of such elements is the long-term stress cycle distribution at critical locations on the helix elements caused by long-term environmental loading on the system. Establishment of the long-term stress cycle distribution will hence require global dynamic time domain analysis followed by cross-sectional analysis in a large number of irregular sea states. An overall computational consistent and efficient fatigue analysis scheme is outlined in this paper. A detailed description is given of cross-sectional analysis techniques required for fatigue stress calculation in helix elements exposed to bending, tensile and pressure loading. Special emphasis is placed on assessment of friction stresses caused by the stick/slip behaviour of helix elements in bending. Such effects are known to be of special importance for fatigue life assessments of deepwater applications of flexible pipes and umbilicals due to increased contact pressure between the individual elements in the cross-section. The described cross-sectional analysis techniques are based on an extensive literature survey and hence considered to represent industry consensus. The performance of the described calculation scheme is illustrated by case studies.
Fatigue of Copper Conductors for Dynamic Subsea Power Cables
Authors: Karlsen Stian
Published: January 2011

Dynamic subsea power cables are used for distribution of electric power to subsea units for oil and gas production and for power distribution from offshore wind mills. As the cables are suspended from the sea level to sea floor, a dynamic analysis is normally required to ensure that the cables are able to withstand the dynamic tension and bending caused by waves and vessel movements. The fatigue property of a conductor is determined by the mechanical properties of the individual wires and stress concentration caused by wire interaction. Previous publications on steel wires and ropes have brought light on the effect of R-ratio and wire interaction such as fretting and inter wire slippage. For copper conductors the complexity of fatigue is multiplied by the fact that the individual wires have a nonlinear stress-strain behavior well below the defined yield stress limit and poor creep properties below maximum cable operation temperatures. Moreover, the interaction between the wires is determined not only by the contact between copper surfaces but also by the water blocking compound in between the wires normally required for subsea cables. A test method, simulating the fatigue mechanism in a dynamic power cable, including the effect from friction, fretting, creep properties of copper and high tension at deep waters is presented together with test results for a representative conductor.
An Investigation on Nonlinear Effects of Gaps in Dolphin Structures Analysis
Authors: Moharrami Mohammad Reza , Zargar Ehssan , Mirzaei Mahdi
Published: January 2011

Dolphins are essential structures used in gas and oil terminals for berthing and mooring purposes. In essence, design of these structures is mostly based on linear analysis, so that the effect of nonlinearity on the actual responses is not considered. In this paper, the effect of nonlinearity caused by the gap modeling between parts of the structure is investigated and the modeling is carried out using accurate and efficient finite element software. As a case study, a mooring dolphin located in the Persian Gulf is simulated in order to demonstrate the nonlinear analysis effects. In contrary to the most common mooring dolphins, the dolphin is designed to have flexible structural behavior. As a result, it is comprised of three short piles connected by a triangle-shape box girder from the top. The box girder is composed of two triangle plates as flanges, which are connected by means of vertical stiffeners acting as web. Besides, three torsion bars are used to change the direct shear to a torsion shear to get a more efficient force transmission. In the piles-box girder connection, where piles pass through the holes prepared in the flanges, there is a considerable difference between the diameter of piles and holes as a tolerance (gap) for construction and codes provisions (BS 6349). The nonlinear effects are mainly caused by the gap in this connection besides the vertical supports of the box girder and the torsion bars. This paper contains the results of a nonlinear static analysis in which the gap between different parts is modeled using nonlinear spring elements to reach a convergence in solving the equations. Results are then compared with the linear static analysis of a similar dolphin, which gaps are not modeled in it and nonlinear effects are presented.
OPTI-EX Deck Design: A Novel Approach to Deck Mating
Authors: Yang Zhiyong , DaSilva Otto , Nagel Ed
Published: January 2011

This paper describes deck design and mating operation of the semisubmersible Floating Production Unit (FPU) OPTI-EX. It also addresses engineering challenges, and how these were met by a novel design and proper analysis. OPTI-EX is the first production semi-submersible mated by a combination of guide plates and shoe bracket at 8 contact points. The challenges of mating include dimensional control of the mating, hull deflections control by guide plates, dimensional tolerance criteria, and welding connections. Because of the flexibility of OPTI-EX pontoon in the water, the horizontal deflection of four columns is significant, which makes the adjustment of column position important. The use of half “Y” shape guide plates installed on column top helps the deck bring the column back into desired location under deck self weight. The deck self weight is shed at 8 contact points. The dimensional tolerance criteria are critical, since fatigue is sensitive in the deck column connection area. By selecting the placement of guide plates, the tolerance can be achieved. The mating operation went very well in terms of time and final tolerance. Welding work after the landing of truss deck on column is easy to access and of simple design. All the work is done in open space rather than confined in a small space. A sensitivity study was done to check the contact forces during the mating process. The results show that there will be significant changes in contact forces assuming different initial gaps present between deck and column top guides during the mating. However, all these forces can be accommodated by a proper local reinforcement design. While mating the deck and column, a built-in stress due to the loads in mating is normally involved. The support conditions change between mating stage and in-place condition. This means modeling the in-place condition only will lead to an unrealistic result. In this paper, it is shown that a prestress approach has to be utilized to predict the final in-place strength condition with the consideration of built-in stress in deck at mating stage.
Mosaicing of Acoustic Video Images for Underwater Structure Inspection
Authors: Tong Jianfeng , Han Jun , Shen Wei , Xu Panlin
Published: January 2011

With the development of international communication and the advent of terrorism, safety and security of the underwater structures such as shipping dock, bridge pier and the bottom of ship hull become a key issue. Generally, in order to inspect the underwater structures, divers are sent down to make tactile examination, or with a camera to take photos or video tapes for further study and examination. However, in dark, turbid water, the quality of the optical images is practically not good enough to give much useful information of the underwater structures. And the working efficiency of this kind of inspection is relatively low. To overcome these problems, it is imperative that more efficient and effective inspection methods are developed. Recently, an acoustic lens camera called DIDSON (Dual-frequency Identification Sonar), which is invented by the Applied Physics Laboratory, University of Washington, can provide almost-video-quality images to identify objects even in turbid water. It has been proved to be very useful in underwater objects searching, especially in dark, turbid water. Although the quality of every single frame provided by DIDSON is quite good, its field of view is narrow, only 29 degrees in horizontal and 14 degrees in vertical, making it hard to comprehend the overall condition of the inspection area and guarantee the safety and security of underwater structures. In this paper, we present a new mosaicing method for the time-sequential images provided by DIDSON, which enables us to inspect the whole underwater structure in a computer screen. An experiment was carried out to inspect the underwater part of a shipping dork. Our experimental result has shown that the proposed method can successfully mosaic the images provided by DIDSON, and thereby expand the visual field of DIDON indirectly which enables it to be effectively used in underwater structures inspection.
Dynamic Response of Blast-Loaded Stiffened Plates by Rigid-Plastic Analysis
Authors: Yang Ping , Peng Ying
Published: January 2011

The dynamic response of one-way stiffened plates with clamped edges subjected to uniformly distributed blast-induced shock loading is theoretically investigated using a singly symmetric beam model. The beam model is based on the rigid-perfectly plastic assumption. The bending moment-axial force capacity interaction relation or yield curve for singly symmetric cross-section is derived and explicitly presented. The deflection condition that a plastic string response must satisfy is determined by the linearized interaction curve and associated plastic flow rule. Moreover, the possible motion mechanisms of the beam are discussed under different load intensity. Finally the dynamic response of a one-way stiffened plate is calculated theoretically and numerically. Good agreements are obtained between the presented theoretical results and those from numerical calculations of the FEM software ANSYS and ABAQUS/Explicit. It is concluded that the basic assumptions and approximations for simplifying calculations are reasonable and the beam model in theoretical analysis is adoptable. The example also shows that an arbitrary blast load can be replaced equivalently by a rectangular type pulse.
Establishing Platform Foundation Capacity Using Topside Vibration Measurements
Authors: Black Jared L.
Published: January 2011

This paper describes how a structural vibration monitoring procedure was used in combination with structural analysis modeling to determine the foundation capacity of an existing offshore platform. The project was initiated due to concern that the foundation capacity was marginal for a needed expansion of the topsides process system. The field work consisted of conducting a sequence of vibration measurement for a range of increasing deck loads. Since the platform’s resonant frequencies depend on the structural stiffness and mass, each deck loading (mass) produced a slightly different resonance condition. The set of resonant frequencies obtained for the different deck loads allows one to deduce the stiffness of the structure-foundation system. The stiffness of the steel structure (jacket and deck) was established with structural analysis. The remaining unknown is the axial and lateral stiffness of the pile foundation. Using the torsional resonant frequencies, the lateral stiffness of the pile array and individual piles was established. Then using the translational resonance, the axial pile stiffness was found. Based on these analyses, the platform’s foundation capacity was estimated to be significantly greater than that predicted by the standard pile capacity design recipe. The structure was deemed adequate for the planned topside modification.
Development of Simple Design-Oriented Procedure for Predicting the Collision Damage of FPSO Caisson Protection Structures
Authors: Cho Sang-Rai , Jeong Jang-Hyun , Frieze Paul A.
Published: January 2011

Protection structures for FPSO caissons are required to be strong enough not to contact caisson pipes even when the protection structure is damaged by impact by attendant vessels. In the present structural design process, non-linear commercial packages are employed for the collision analyses. However, non-linear collision analyses using commercial packages are still time-consuming and expensive to operate especially at the initial design stage. In this study, validation of the adopted commercial package was firstly performed using collision test results on unstiffened tubulars. Then, a rigorous parametric study was conducted on simple protection structures by changing the collision velocity and the scantling of the protection structure. A simple analytical expression was derived assuming that the kinetic energy of the striking vessel is dissipated by plastic elongation of tubulars and rotation of plastic hinges. Using the parametric study results, an equation for the modification factor was obtained by which the effects of local denting and dynamic behavior can be considered. The developed procedure was also substantiated using numerically predicted extent of damages of an actual protection structure.
Time-Domain Nonlinear Wave Making Simulation of the Catamaran Based on Velocity Potential Boundary Element Method
Authors: Feng Dakui , Wang Xianzhou , Zhang Zhiguo , Guan Yanming
Published: January 2011

The catamaran is composed of two monohulls, the flow fields between the inner and outer side of each monohull are different, the bodies must be considered as lifting bodies. So it is very important to know the lifting effect on hydrodynamic characteristics of catamaran hull at the preliminary design stage of its hull form. The pressure Kutta condition is imposed on the trailing-surface of the lifting body by determining the dipole distribution, which generates required circulation on the lifting part. The method is based on Green’s second theorem. Rankine Sources and dipoles are placed on boundary surfaces. Time-stepping scheme is adopted to simulate the wave generated by the catamaran with a uniform speed in deep water. The values of the potential and position of the free surface are updated by integrating the nonlinear Lagrangian free surface boundary conditions for every time. A moving computational window is used in the computations by truncating the fluid domain (the free surface) into a computational domain. The grid regeneration scheme is developed to determine the approximate position of the free surface for the next time step. An implicit implement of far field condition is enforced automatically at the truncation boundary of the computational window, Radiation condition is satisfied automatically. The influences on the wave making resistance of the distance between the twin hulls of the Wigley catamaran on the hydrodynamic characteristics are discussed. The numerical results are presented compared with the existing simulation result. The method can be used to simulate the flow fields around the foil near free surface.
Ultimate Strength of Steel and Glass Fibre Reinforced Composite Plates With Square Opening Under Axial and Out of Plane Loads
Authors: Sundaravadivelu R. , Alagusundaramoorthy P. , Suneel Kumar M. , Rahima Shabeen S.
Published: January 2011

The weight of glass fiber reinforced polymer composite (GFRP) plate is about one fourth of the steel plate and can be used in ship and offshore structures, so that the payload can be increased. However comparative studies on the behaviour of steel and GFRP composite plates with square opening have not been studied in detail. The experimental studies on steel and GFRP plates with and without openings are carried out for the combined loading of axial compression and out-of-plane loads. The in-plane and out-of-plane deflections are measured. The reduction in the axial load carrying capacity of the plates due to out-of-plane load is quantified. The effect of column slenderness ratio and plate slenderness ratio on the collapse load of simply supported stiffened plates is presented. Two sets of interaction equations are developed, one for the steel plate and another for the GFRP composite plate.
Approximation of Second-Order Low-Frequency Wave Loading in Multi-Directional Waves
Authors: Rezende Flavia C. , Chen Xiao-bo
Published: January 2011

Further to the studies by Chen & Rezende (OMAE2009) on the quadratic transfer function (QTF) of low-frequency wave loading in which the QTF is developed by the series expansion associated with the difference-frequency up to the order-Δω2, new formulations have been developed in order to take into account the effect of interactions between waves of different headings. It provides a novel method to evaluate the low-frequency second-order wave loads in a more accurate than usual order-Δω approximation (often called Newman approximation) and more efficient way comparing to the computation of complete QTF in multi-directional waves. New developments including numerical results of different components of QTF are presented here. Furthermore, the time-series reconstruction of excitation loads by quadruple sums in the motion simulation of mooring systems is analyzed and a new efficient and accurate scheme using only a triple sum is demonstrated.
Robustness of an SCR in Gulf of Mexico Hurricane Conditions
Authors: Sharma Partha , Mo̸rk Kim , Hansen Vigleik , Raposo Celso , Vishnubhotla Srinivas
Published: January 2011

Recent hurricanes in Gulf of Mexico, most notably Ivan (2004), Katrina & Rita (2005), Ike (2008), were more severe than the local 100 year extremes in the Gulf of Mexico (GoM). As a result API has issued an interim metocean bulletin, API Bulletin 2INT-MET [1]. Concurrently, API also issued API Bulletin 2INT-EX [2] for assessment of existing offshore structures for hurricane conditions. API Bulletin 2INT-EX recommends a robustness check to evaluate floating structure critical components including production and export risers. The robustness check for risers as a minimum should consider the capacity and ductility of the key riser components. This paper investigates the robustness of a steel catenary riser (SCR) suspended from a deepwater tension leg platform (TLP) unit in Central GoM. The robustness assessment is performed for the 1000 year Central GoM hurricane conditions provided in API 2INT-MET. Time domain coupled analysis using the program DeepC is performed to determine the TLP motions and the associated loading on the SCR. SCR strength capacity checks are performed as per the methods outlined in new ISO 13628-12 [3].
Improved Efficiencies in Truss Spar Designs Without Dry Transportation Constraints
Authors: Murray John , Choi Guibog
Published: January 2011

Truss Spars that are designed to accommodate dry transportation encounter certain constraints because the overall length and hard tank diameter of the Spar cannot exceed the limits of the transport vessel. When these constraints are removed, hulls can be longer. Increased length allows the fixed ballast and hard tank steel weights to be reduced. Although the truss steel weight is increased for deeper drafts, there is an overall savings in steel when the hard tank and soft tank are taken into account. The heave response of a Spar can be adversely affected by tensioners used to support TTRs. Deeper drafts can improve heave response and an additional heave plate can be introduced if necessary.
Global Performance of the Perdido Spar in Waves, Wind and Current: Numerical Predictions and Comparison With Experiments
Authors: Liapis Stergios , Bhat Shankar , Caracostis Constantine , Webb Carl , Lohr Curtis
Published: January 2011

In recent years, spars have become a preferred industry solution for certain offshore deepwater developments. Perdido is the first spar platform to be operated by Shell. The Perdido spar has been recently installed in the ultradeepwater Gulf of Mexico Alaminos Canyon and is scheduled for first oil in 2010. This Direct Vertical Access (DVA) spar will operate at a water depth of 7,825′ and will be the deepest spar production and drilling facility in the world. Numerical predictions of the spar global motions in waves, wind and current are presented in this paper. Motivation for this study comes from two facts: 1. Each spar platform design is unique in terms of its size, number and geometry of heave plates, riser system and mooring system. 2. Metocean design criteria have been increased in view of the recent hurricanes. Model tests of the Perdido spar were conducted at MARIN at a scale of 1:59.94. In these experiments, several Gulf of Mexico (GOM) wave, current and wind environments were considered. The six-degree-of-freedom motions, deck accelerations, air gap, as well as the loads on the heave plates, mooring lines and risers were measured. In this paper, global motion predictions of the Perdido spar are given using Shell’s in-house COSMOS/ WAMIT suite of programs. Extensive comparisons between the numerical predictions and the experimental results were undertaken. In all cases, the comparisons are very good. In order to include heave viscous loads and damping, special line members were included at the bottom of the hard tank, the bottom of the soft tank and each heave plate, in addition to standard line members used to describe the truss. These special members contribute heave viscous loads with drag coefficients selected from the Perdido experiments. Several heave plate configurations were considered to systematically study the impact of heave plates on the spar global motions. The influence of the heave plate geometry on the heave added mass and on the global motions was derived using WAMIT. The strakes’ actual geometry was also included in the WAMIT diffraction analysis. Most of the moonpool area at the bottom of the Perdido hard tank is closed. As a result, the pumping mode was not excited during the experiments. However, numerical simulations with WAMIT showed a sharp peak at the “pumping mode” resonant frequency. This peak was suppressed by introducing a second floating body that capped the moonpool at the water surface. Based on these learnings, recommendations for global motion modeling are presented in this paper.
Research on the Line Style of Shaft Housing Pairs of Progressing Cavity Pump
Authors: Song Yujie , Wen Houzhen
Published: January 2011

The progressing cavity pump (PCP) has a good usage foreground on the seabed oilfield. The most important invalidation reason of PCP is the rubber shaft housing’s abrasion. This article introduced the hypocycloid single-screw hydraulic machinery’s primary line style and isometric profiles equations, analyzed the contact status between the primary line styles of rotor and stator, and explained why the isometric curves have been used as the cross section of PCP. Defined the concept of turning point, deduced the turning point and midpoint’s contact angle formula of the hypocycloid singlescrew hydraulic machinery, then compared the contact angles of three models as i = 1/2, 2/3, 3/4. Concluded that if shorten the contact time of rotor with stator’s turning point, the abrasion of stator’s turning point will be relieved, the life of the hypocycloid single-screw hydraulic machinery will be longer.
The Effect of Reeling on the Fatigue of Welded Risers
Authors: Buitrago Jaime , Rinehart Adam J. , Wang Howard , Miller John E.
Published: January 2011

ExxonMobil Upstream Research Company (URC) conducted a pilot testing program to systematically assess the effect of reeling on the fatigue performance of welded risers. Given the criticality of deepwater risers, a robust reeling simulation method was developed. The method properly addresses the fundamental weld sampling issue arising from the randomness of the plane in which risers are reeled. To address that issue the nominal reeling strain is accumulated around the entire circumference of girth weld, thereby capturing the weak fatigue bending plane. The reeling simulation is implemented by using multi-plane bending. The number and orientation of bending planes needed to simulate a total cumulative reeling strain are based on (1) the strain experienced during the process being simulated (2) weld fabrication and inspection records and (3) residual stresses left after reeling calculated via finite element analyses. Thirty production-quality welds were made on 323mm OD × 20.5-mm WT (12.75-in OD × 0.81-in WT) seamless X60 pipe, yielding a total of 14 full-scale specimens. Two different welding procedures and two different cumulative nominal strain levels were used. The effect of reeling on fatigue was assessed by comparing the fatigue lives obtained with companion reeled and non-reeled full-scale specimens. Results indicate that, although there was a distinct deleterious effect of reeling on fatigue life, the performance may still be adequate for most riser applications when high quality welds are used and qualified.
Numerical Investigation on the Influence of Residual Stresses on Ductile Crack Growth Resistance
Authors: Ren X. B. , Zhang Z. L. , Nyhus B.
Published: January 2011

In this paper, a numerical study was performed to assess the effect of residual stresses on ductile crack growth resistance of a typical pipeline steel. A modified boundary layer model was employed for the analysis under plane strain, Mode I loading condition. The residual stress fields were introduced into the finite element model by the eigenstrain method. The complete Gurson model has been applied to simulate the ductile fracture, which is characterized by microvoid nucleation, growth and coalescence. Results show that tensile residual stresses can significantly reduce the crack growth resistance when the crack growth is small compared with the length scale of the tensile residual stress field. With the crack growth, the effect of residual stresses on the crack growth resistance tends to diminish. It has also been found that the effect of residual stresses on crack growth resistance depends on the initial void volume fraction f0 and hardening exponent n.
Computational Simulation of Free Surface Flows Using Stabilized Edge-Based Finite Element Method
Authors: Elias Renato N. , Gonc¸alves Milton A. , Coutinho Alvaro L. G. A. , Esperanc¸a Paulo T. T. , Martins Marcos A. D. , Ferreira Marcos D. A. S.
Published: January 2011

Flows involving waves and free-surfaces occur in several problems in hydrodynamics, such as sloshing in tanks, waves breaking in ship and motions of offshore platforms. The computation of such wave problems is challenging since the water/air interface (or free-surface) commonly present merging, fragmentation and cusps, leading to the use of interface capturing Arbitrary Lagrangian-Eulerian (ALE) approaches. In such methods the interface between the two fluids is captured by the use of a marking function which is transported in a flow field. In this work we simulate these problems with a 3D incompressible SUPG/PSPG parallel edge-based finite element flow solver associated to the Volume-of-Fluid (VOF) method [1]. The hyperbolic equation for the transport of the marking function is also solved by a fully implicit parallel edge-based SUPG finite element formulation. Global mass conservation is enforced adding or removing mass proportionally to the absolute value of the normal velocity at the interface. The performance and accuracy of the proposed solution method is tested in the simulation of pulse wave and the interaction of a fixed square cylinder with a progressive wave.
Numerical Investigation of Vortex-Induced Vibration of a Circular Cylinder Close to a Plane Boundary
Authors: Zhao Ming , Cheng Liang
Published: January 2011

Two-degree of freedom vortex-induced vibration (VIV) of a circular cylinder close to a plane boundary is investigated numerically. Two-dimensional (2D) Reynolds-Averaged Navier-Stokes Equations (RANS) and structural dynamic equation are solved using a finite element method (FEM). If the cylinder is initially very close to the plane boundary, it will be bounced back after it collides with boundary. It is assumed that the bouncing back only alters the cylinder’s velocity component perpendicular to the boundary. After it is bounced back, the cylinder’s velocity are determined by Uc = Uc′, Vc = −bVc′, where Uc and Vc are the cylinder’s velocity parallel to the boundary and that perpendicular to the boundary respectively, Uc′ and Vc′ are the velocities before cylinder is bounced back, b is the bounce back coefficient which is between 0 and 1. Numerical results of the vibration amplitude and frequency of a one-degree-of-freedom vibration (transverse to flow direction) of a circular cylinder close to a plane boundary are compared with the experimental data by Yang et al. [1]. The overall trends of the variation of the VIV amplitude with the reduced velocity were found to be in agreement with the experimental results. The calculated amplitude is smaller than the measured data. The frequency of the vibration increases with the increase of reduced velocity. The calculated vibrating frequency agrees well with the experimental data. It was found in this study that vortex-induced vibration (VIV) occurs even when the gap between the cylinder and the plane boundary is zero. This contradicts a perception that VIV would not occur for a pipeline close to the seabed with a gap ratio smaller than 0.3, this is because it was understood that vortex shedding would have been suppressed if the gap between the cylinder and the plane boundary is less than about 0.3 times of cylinder diameter for a fixed cylinder. Two-degree-of-freedom VIV of a circular cylinder close to a plane boundary is studied. The XY-trajectories, the frequency and the amplitude of the vibration are studied. The effects of the cylinder-to-boundary gap and the bounce back coefficient on VIV and the link between the vortex shedding mode and the VIV are discussed.
Identifications of Spurious Waves in the Wave Tank With Shallow Water
Authors: Zaman M. Hasanat , Peng Heather , Baddour Emile , Spencer Don , McKay Shane
Published: January 2011

Accurate generation of the primary waves and the reproduction of the group-induced second-order low and high frequency waves have been considered essential for physical model test in the laboratory. In the laboratory when bi-chromatic primary waves are generated the required bounded waves will be generated naturally at the difference frequency. In addition to that several unwanted free waves are also generated. The free waves, having the same frequency of the bounded wave are reproduced due to mismatch of the boundary conditions at the wave paddle. The other two types of free waves are due to the wave paddle displacement and the local disturbances. We carried out an extensive experimental program to identify the second order spurious waves in shallow water in the Offshore Engineering Basin (OEB) at the Institute for Ocean Technology (IOT) of National Research Council (NRC) Canada. In the experiments water depths are used in the range of 0.3m to 0.8m. The wave periods also have varied from 0.9s to 2.22s. In the experiments mono- and bi-chromatic waves are used. The drive signals of the wave-maker are generated using first-order and second-order wave generation techniques. Total 14 wave probes are used to capture the data in the wave tank. A NRC-IOT code is used to isolate the primary waves, the bounded waves and the unwanted free waves from the measured data at each wave probe. The measured data are analyzed in this paper to illustrate the differences in the waves generated by two different generation techniques.
An Advanced Test Method to Determine Viscous Damping of Floating Structures
Authors: Huang Z. J. , O’Donnell B. J. , Yung T. W. , Slocum S. T.
Published: January 2011

ExxonMobil Upstream Research Company developed an advanced model test method to determine reliable damping values for predicting low frequency motions of an FLNG barge and an LNG carrier. Since viscous damping forces are a very small portion of the total force on the model, how to separate the viscous forces from the total forces is the key technical challenge. To better isolate viscous damping forces, an inertial compensation system consisting of springs was employed in the test. The spring stiffness was designed such that the restoring force cancelled the large inertial loads at the oscillation frequency. Furthermore, double-body models were built and were deeply submerged to minimize surface wave damping. With such an experimental setup, the total force measured was mainly the viscous damping force. Viscous damping was derived from the measured force and motion time histories.
Comparative Reliability Analysis of FAST-Pipe™ and All-Steel Pipe
Authors: Chen Qishi , Salama Mamdouh M.
Published: January 2011

Fiber Augmented Steel Technology Pipe (FAST-Pipe™) is being proposed as an alternative to high strength steel (grade X80 or higher) for high pressure gas transmission pipelines. The FAST-Pipe™ is manufactured by wrapping dry fiberglass on conventional steel pipe. It offers many performance and cost advantages. However, as with any proposed new technology, it is required to achieve a reliability level at least equal to the conventional all-steel technology. This paper summarizes the results of a burst failure probability analysis. It was part of the ongoing work to experimentally and analytically establish the design basis for FAST-Pipe™. The purpose of this study is to estimate the burst failure probabilities of defect-free all-steel and FAST pipes after 30 years of operation, and to identify FAST-Pipe™ design cases which can achieve burst reliability levels comparable to those of an all-steel design. The results of this study suggest that a safety factor of 2.0 on burst pressure is adequate for the FAST-Pipe™.
Study on the Hydrodynamic Characteristics for Heave Plate Structure of Truss Spar
Authors: Shen Wenjun , Tang Yougang , Liu Liqin
Published: January 2011

The hydrodynamic characteristics of heave plates are studied in this paper. Firstly, different motion amplitudes and plate spacing influencing hydrodynamic coefficients are considered. Secondly, heave plates with different thicknesses are calculated, the case of edges with inclined form for heave plate is also taken into account. Numerical simulations are made for the plate forced oscillation, employing the dynamic mesh method and UDF (User defined functions). The values of Cm and Cd for heave plate are calculated. It is found that, in a certain amplitude range, Cm increases with increasing of amplitudes, Cd decreases with increasing of amplitudes. The values of Cm and Cd increase with increasing of plate spacing. Furthermore with the same effective thickness, the hydrodynamic performance of heave plate with inclined form is improved greatly.