bluuebunny/crossref_metadata_2025 huggingface dataset:
Time Domain Simulation of the 3D Bending Hysteresis Behaviour of an Unbonded Flexible Riser
Authors: Tan Zhimin , Quiggin Peter , Sheldrake Terry
Published: May 2009

This paper presents a ‘state of art’ in the development of the time domain dynamic simulation of 3D bending hysteresis behaviour of a flexible riser under offshore environment loading. The main technical challenge is to understand and model the riser tensile armour behaviour under continuous changes in both the magnitude and direction of bending, and its subsequent impact on the riser’s bending hysteresis characteristics. Because of this technical obstacle, the current industry practice is to model the riser as a linear structure, with certain conservatism enforced, and then to extract the global dynamic loads to a detailed local model for stress and life assessment. This paper introduces two 3D flexible riser bending hysteresis models, developed by Wellstream and Orcina respectively, and their calibrations against the bending hysteresis loops measured in full scale tests. Both models are implemented using the analysis program OrcaFlex; the Wellstream model is a detailed model that calculates both the total bending moment and the stresses in the tensile armour; the Orcina model is a simpler model that only calculates the total bending moment. A study is presented to illustrate the difference in the riser dynamic responses with and without consideration of the bending hysteresis behaviour, and to assess the difference in the dynamic responses between the Wellstream and Orcina 3D bending hysteresis models. This development permits more realistic riser structural properties to be modeled in the dynamic simulation, and reports detailed time history stress or strain results of the strength components of the riser. This expands the current practice of riser fatigue analysis of only using the regular wave approach, to using an irregular wave approach employing the rainflow counting method.
Development of High Strength 100 ksi Seamless Weldable for Deepwater Riser Application
Authors: Izquierdo Alfonso , Quintanilla Hector , Di Schino Andrea , Anelli Ettore
Published: May 2009

The deepwater and ultradeepwater production and exploration technical demands are increasing on a daily basis. The benefit of having pipes with high strength and reduced wall thickness is very well recognized by all the oil community. In order to develop a high strength seamless 100 ksi weldable riser, a Joint Industrial Development Program (JIP) was launched in 2003. This paper describes the results of the first phase of this JIP, devoted to the development and production of seamless 100 ksi pipes. A detailed metallurgical analysis, including the role of chemical composition and quenching and tempering process conditions on the microstructure and precipitation was performed. The influence of the type of microstructure on strength, toughness and fracture mechanics behavior was studied in detail for both laboratory and industrial materials. The 100 ksi grade was accomplished for seamless pipes with wall thickness of 16 mm and 25 mm. The second phase, devoted to evaluate field weldability of this new product, is in progress.
Experimental Investigation of Roll and Heave Excitation and Damping in Beam Wave Fields
Authors: Grant David J. , Etebari Ali , Atsavapranee Paisan
Published: May 2009

This paper will discuss a systematic series of model tests performed at NSWCCD on a conventional combatant hull form at varying Froude numbers, beam wave steepness, and beam wave frequencies near the natural roll frequency of the model. The model was free in roll and heave while constrained in the remaining four degrees of freedom. The model was fully appended, yielding lateral force on the bilge keels, lift, drag and torque on the rudders and all six force and moment components on the propellers. Stereo particle-image velocimetry (SPIV) measurement was used to study the three-dimensional unsteady flow patterns around the bilge keels at the model LCG. Model motion results were analyzed with respect to the excitation waves to yield normalized heave and roll amplitudes and phase angles. Force and moment results were analyzed to yield individual appendage viscous drag and added mass coefficients which were in turn used to extend and refine previous theoretical models to include the effects of beam seas. SPIV results were compiled and analyzed to provide a verification and validation dataset for CFD computations.
Experimental Investigation of Viscous Roll Damping on the DTMB Model 5617 Hull Form
Authors: Atsavapranee Paisan , Carneal Jason B. , Grant David , Percival A. Scott
Published: May 2009

A systematic series of model tests have been performed at NSWCCD to explore the mechanisms of roll damping around a conventional combatant hull form (DTMB model #5617) and an advanced tumble-home hull form (DTMB model #5613-1). Both free roll decay and forced oscillation experiments were carried out in calm water and in waves, over a range of forward speeds. These experimental investigations were performed within the overall context of continuing efforts to advance the capability to assess seakeeping, maneuvering, and dynamic stability characteristics of a surface combatant. Data gathered in these experiments are currently being utilized to develop empirical and analytical roll damping models and to validate the accuracy of simulation programs in the calculation of various components of hydrodynamic forces. This paper will specifically discuss a single-degree-of-freedom free roll decay experiment, with measurements of the appendage lateral force and the associated flow field generated during ship roll motion on the DTMB #5617 model. Using particle-image velocimetry (PIV) measurements, two-dimensional unsteady flow patterns around the bilge keels were performed to study the mechanisms of viscous roll damping due to bilge keels. In addition, lateral forces and moments on the bilge keels, rudders, and propellers have been measured to provide a direct assessment of component roll damping. Analysis for appendage forces and correlation with the measured flow field yield several new important insights into the physical mechanisms of bilge keel roll damping. Flow field observation reveals complex phenomena of viscous flow separations and vortex formation around the bilge keel during different phases of the roll motion cycle. The lateral force on the bilge keels was modeled as the sum of an added mass component and a viscous drag component. The viscous drag coefficients are found to depend strongly on ship forward speed and roll amplitude, but the added mass coefficients are relatively constant for the range of forward speed and roll amplitude investigated.
Nonlinear Finite Element Analysis of Pressurized Spherical Shell for Deep Water Structure
Authors: Li Liangbi , Wang Renhua , Yu Minghua , Wang Zili
Published: May 2009

One of the main problems in developing the deep water structure is the stability of the pressurized spherical shell. If the pressurized spherical shell belongs to moderate thick shell, the transverse shear deformation must be taken into account. Based on the nonlinear finite element method, the stability of the pressurized spherical shell under deep water was studied in this paper. The influence of out of roundness due to the machining, the interaction between material nonlinear and geometry nonlinear were considered. They are caused by material yield, strain intensifying and big plastic deformation of the whole shell structure buckling. Results from calculation showed that the critical buckling pressure decreases with the increasing of the radius under the same thickness but increases with the thickness under the same radius. The relationship charts of the critical buckling pressure, the thickness, the radius of the pressurized spherical shell were derived. The reasonable geometric parameters were then selected.
VIV Response Prediction for Long Risers With Variable Damping
Authors: Jaiswal Vivek , Vandiver J. Kim
Published: May 2009

This paper describes the Vortex Induced Vibration (VIV) response of long cylinders equipped with strakes and present damping measurements from Gulf Stream Experiments conducted in October 2006. The experiments used a 500.4 foot (152.4 m) long composite pipe with an outside diameter of 1.43 inches (3.63 cm). The strakes were a triple helix design with a pitch to diameter ratio of 17.5. The pipe was equipped with varying amounts of strake coverage and was towed in the Gulf Stream which had current profiles which varied from nearly uniform to highly sheared. The measured response showed the characteristic of traveling waves. Modeling such response using mode superposition and Green’s Function is presented.
Rationale of Riser System Selection for an FPSO Application
Authors: Sun Liping , Yao Xiongliang , Hu Zhenghong
Published: May 2009

Floating Production, Storage and Offloading (FPSO) system has been used widely in the offshore oil and gas industries worldwide. In recent years, FPSO concept has been deployed in deep and ultra-deepwater field development including West of Africa, Offshore Brazil, and Offshore China. Meanwhile, more and more deepwater risers have been deployed from concept to offshore installation. Compared to other type deepwater floaters (e.g. Spar, TLP, Semisubmersible), the severe motion characteristics of an FPSO makes the riser concept selection and design much more challenges. Other constraints imposed by field reservoir properties (e.g. high temperature\high pressure), disconnection requirements for early production FPSO system, and significant increase in water depth even exclude some riser concepts. This paper presents detailed evaluation of potential deepwater riser concepts applicable for an FPSO. Among the deepwater concepts, steel catenary risers (SCR), flexible risers, hybrid risers have been successfully deployed to FPSO application. In addition, there are also other viable riser concepts for FPSO application including lazy wave SCR, Tension Leg Riser (TLR), Hybrid ‘S’ Riser System (HySR), Hybrid Catenary Riser (HCR), and more to come. The evaluation of deepwater riser concepts for FPSO application is conducted in terms of technical feasibility, design constraints, track records, fabrication, offshore construction, and cost with focus on the existing and field proven riser concepts. Technical challenges and technology gaps are highlighted. The technical feasibility of some of the riser concepts for certain FPSO application has been demonstrated through analysis and calculation. Novel ideas have been generated for particular requirements. Particular attention is paid to the discussion with the application to offshore China. This paper shows the steel riser and hybrid riser are the most viable ones for deep and ultra-deepwater FPSO. Flexible riser and hybrid riser are the preferred solution for FPSO in China.
Combined Effects of Waves and Currents on Offshore Pipeline Bundles
Authors: Kamarudin M. H. , Thiagarajan K. P. , Czajko A.
Published: May 2009

It is common practice to accompany offshore pipelines by smaller diameter service lines or umbilicals to create a bundle. This gives rise to the so-called piggyback configuration. The flow behavior around the bundle is not well-known, leading to concerns on the stability of the configuration. This paper investigates the influence of the piggyback on the hydrodynamic loadings on the bundle in wave plus current condition using Computational Fluid Dynamics (CFD). Key parameters of the configuration that were investigated were the orientation of the smaller pipe with respect to the main pipeline and the flow conditions (different Keulegan-Carpenter numbers). The gap between the seabed and the main pipe was set to zero for all cases investigated. It was found that the hydrodynamic characteristics of the main pipe were significantly influenced by the presence of the piggyback. The numerical results also showed that the orientation of the piggyback plays an essential role in determining the drag, lift and inertia coefficients for the bundle. This phenomenon is explained by examining the vortex flow patterns around the cylinders. It is shown that the established industry practice of assuming the hydrodynamic characteristics of the bundle to be the same as an equivalent diameter cylinder may underestimate the forces on the bundle, and lead to a non-conservative design.
Weathervaning Instabilities of a FPSO in Regular Waves and Consequence on Response Amplitude Operators
Authors: Munipalli J. , Pistani F. , Thiagarajan K. P. , Winsor F. , Colbourne B.
Published: May 2009

A series of model tests in regular seas were performed on a 1:60 scaled model of a generic FPSO. The model was moored by means of four instrumented mooring lines attached to an internal turret. The wavelength to model length ratio was varied from 0.5 to 2.0, while the ratio of the wavelength and the wave amplitude was kept constant as per ITTC seakeeping recommendations. The goal of the tests was to evaluate the response of the hull to swell conditions that may happen in open sea environments and to assess the weathervaning characteristics of the internal turret arrangement. Three set of tests were performed. 1. Model free to rotate about the turret; test duration 15 minutes full scale and initial position set at head seas. In this case yaw instability was observed where the model showed a tendency to rotate towards starboard. 2. Model free to rotate about the turret; test duration 2 hours full scale and initial position was varied. The effect of initial condition on the instability and any slowly varying oscillations were observed. 3. Model slightly restrained by a constant line force applied at the stern. In this case, the model maintained its heading towards waves. The results of these runs were used to evaluate the Response Amplitude Operators for different headings of the model. During the tests the six accelerations and six displacements have been measured by means of an inertial and optical system respectively, together with the wave elevation in the basin. From the results, motion response amplitude operators were obtained at different heading conditions. The experimental results for the model displacements were compared with simulations from the well-known diffraction program WAMIT/WADAM. The paper will discuss possible reasons for the yaw instabilities, including the effect of moorings and test duration. The influence of the heading changes on the RAO of the hull with and without constraints will be discussed. Finally, RAO comparison with SESAM will demonstrate similarities and differences with RAOs at different sea headings.
Extreme Value Prediction on Decreasing Depth by Means of a Nonlinear Wave Transformation Model
Authors: Belibassakis K. A. , Stefanakos Ch. N. , Georgiou Y. G.
Published: May 2009

In the present work a weakly nonlinear wave model originally developed by Rebaudengo Lando` et al (1996) is applied to the transformation of wave spectra from offshore to nearshore, and subsequently, it has been systematically applied to the derivation of long-term time series of spectral wave parameters on decreasing depth from corresponding offshore wave data. The derived long-term series of nearshore parameters have been used as input to a new method, recently developed by Stefanakos & Athanassoulis (2006), for calculating return periods of various level values from nonstationary time series data. The latter method is based on a new definition of the return period, that uses the MEan Number of Upcrossings of the level x* (MENU method), and it has been shown to lead to predictions that are more realistic than traditional methods. To examine the effects of bottom topography on the nearshore extreme value predictions, Roseau (1976) bottom profiles have been used for which analytical expressions are available concerning the reflection and transmission coefficients. A parametric (JONSWAP) model is used to synthesize offshore spectra from integrated parameters, which are then linearly transformed based on the previous transmission coefficient to derive first-order nearshore wave spectra. Second-order random sea states have been simulated by following the approach of Hudspeth & Chen (1979) (see also Langley 1987, Lando et al 1996), exploiting the quadratic transfer functions on decreasing depth to calculate the second-order nearshore spectra. Finally, wave parameters are extracted from the nearshore spectra by calculating the first few moments.
A Coupled-Mode Technique for the Prediction of Wave-Induced Set-Up and Mean Flow in Variable Bathymetry Domains
Authors: Belibassakis K. A. , Gerostathis Th. P. , Athanassoulis G. A.
Published: May 2009

In the present work, a complete, phase-resolving wave model is coupled with an iterative solver of the mean-flow equations in intermediate and shallow water depth, permitting an accurate calculation of wave set-up and wave-induced current in intermediate and shallow water environment with possibly steep bathymetric variations. The wave model is based on the consistent coupled-mode system of equations, developed by Athanassoulis & Belibassakis (1999) for the propagation of water waves in variable bathymetry regions. This model improves the predictions of the mild-slope equation, permitting the treatment of wave propagation in regions with steep bottom slope and/or large curvature. In addition, it supports the consistent calculation of wave velocity up to and including the bottom boundary. The above wave model has been further extended to include the effects of bottom friction and wave breaking, which are important factors for the calculation of radiation stresses on decreasing depth. The latter have been used as forcing terms to the mean flow equations in order to predict wave-induced set up and mean flow in open and closed domains. Numerical results obtained by the present model are presented and compared with predictions obtained by the mild-slope approximation (Massel & Gourlay 2000), and experimental data (Gourlay 1996).
Three-Dimensional Numerical Simulations of Flows Past Smooth and Rough/Bare and Helically Straked Circular Cylinders Allowed to Undergo Two Degree-of-Freedom Motions
Authors: Pontaza Juan P. , Menon Raghu G. , Chen H. C.
Published: May 2009

We simulate the flow past smooth and rough rigid circular cylinders that are either bare or outfitted with helical stakes. We consider operating conditions that correspond to high Reynolds numbers of 105 and 106, and allow for two degree-of-freedom motions when the structure is allowed to respond to vortex-induced cross flow and in-line forces. The computations are performed using a parallelized Navier-Stokes in-house solver using overset grids. For smooth surface simulations at a Reynolds number of 105, we use a Smagorinsky Large Eddy Simulation (LES) turbulence model and for the Reynolds number cases of 106 we make use of the unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with a two-layer k-epsilon turbulence model. The rough surface modifications of the two-layer k-epsilon turbulence model due to Durbin et al. (ASME J. Fluids Eng., 2001) are implemented to account for surface roughness effects. In all our computations we aim to resolve the boundary layer directly by using adequate grid spacing in the near-wall region. The predicted global flow parameters under different surface conditions are in good agreement with experimental data and significant VIV suppression is observed when using helically straked cylinders.
Experimental Research on Flow Induced Oscillations in Moonpool Encountered Through Waves
Authors: Kang Zhuang , Yao Xiongliang , Salman Sadiq
Published: May 2009

Model experiments in this paper explains the response of the moonpool encountered through waves at different wave heights, time periods and attack angles, which reflect close to real situation at sea for ships equipped with moonpool. EMD method is used to have a meaningful data collected through hydrophone, pressure sensors, acceleration sensor and wave height meter. Results show that fluid natural oscillating frequency remains constant at 0.7 Hz in calm water with forward model speed condition (Fr = 0.26∼1.3), while by increasing the time period, wave height or both; it fluctuates between 0.6 Hz∼0.9 Hz. The fluid inside the moonpool faces internal sloshing, resulting in transverse breaking waves that are added to the vertical motions. Sloshing motion in the moonpool couples with the piston mode due to which results of calm water with forward speed, and combined waves and flow condition are quite different. Square shaped moonpool at different attack angles and circular shaped moonpool are also discussed for fluctuating pressure level (dB) at increasing time period and wave height. Moreover, Oscillation pressure results at only-wave condition and wave and flow condition together are represented by using frequency ratio between ‘oscillation pressure frequency’ and ‘sloshing natural frequency’ with varying ‘KC’ number. KC varies from 4.16 to 5.67 at 1-sec time period having frequency ratio f/fo of 1. Once time period increases to 2-sec, KC becomes around 22.69 at same frequency ratio. Dynamic magnification due to waves and flow combined situation may cause slamming on diving bells or ROV that are launched, green water over the edge of the moonpool which can be dangerous for the crew, or can increase drastically the resistance of the vessel in transit conditions therefore, this experimental research gives very important factors and results for designing moonpool; we believe that this paper has a significant contribution in the permanent literature.
Continuum FE Modeling of Lateral Buckling: Study of Soil Effects
Authors: Konuk Ibrahim , Yu Shenkai
Published: May 2009

2D and 3D Arbitrary Lagrangian Eulerian (ALE) Finite Element (FE) models are developed to study soil-pipe interaction and lateral buckling problems. A new in-situ testing method to characterize seabed soils and to obtain soil constitutive parameters is proposed. The influence of the soil properties on the soil-pipe interaction and lateral buckling processes are presented. Implications and observations for the design of high temperature and high pressure pipelines are discussed.
Cross Modal Strain Energy Method for Damage Localization and Severity Estimation
Authors: Wang Shuqing , Li Huajun , Hu Sau-Lon James
Published: May 2009

A cross modal strain energy (CMSE) damage diagnosis method, which is capable of localizing the damages and estimating their severities, is presented. The numerical study uses measurements synthesized from a finite element model of a laboratory-scaled offshore platform. Several damage scenarios are investigated, including: single/double damaged element(s), with/without measurement noise. Numerical results suggest that good performance on both damage location and severity assessment can be achieved by implementing the CMSE method.
Extreme Bending Moments on Long Catenary Risers Due to Heave Excitation
Authors: Chatjigeorgiou Ioannis K. , Passano Elizabeth , Larsen Carl M.
Published: May 2009

This paper deals with the dynamic behaviour of catenary shaped risers under imposed motions applied at the top. Particular attention is paid to the heave component of motion which is of substantial importance for practical applications as it results to the amplification of large bending moments in the touch down region. In fact, the bending moment obtains its maximum value at the vicinity of the touch down point and very close to the location of its maximum static counterpart. This singular behaviour is discussed using the results from the solution of the eigenvalue problem. To this end the eigenfrequencies and the corresponding mode shapes are calculated using the WKB approximation making no assumption regarding the variation of the static components. In addition, the feature of the correlation between the axial component of the velocity of the excitation and the extreme bending moments at the lower part (Passano and Larsen, 2006) is further investigated through comparative numerical calculations of the problem using both frequency and time domain techniques.
H-Shaped Pontoon Deepwater Floating Production Semisubmersible
Authors: Mansour Alaa M. , Huang Edward W.
Published: May 2009

This paper presents a new deepwater floating production semisubmersible concept, namely H-Pontoon Semi. The H-pontoon semi consists of an H-shaped pontoon structure which has a pair of parallel longer main pontoons and a pair of shorter secondary central pontoons disposed inwardly from the outer ends of the main pontoons and supported by the two main pontoons. The H-pontoon includes four columns placed at or inboard of the edge of main pontoons. The H-shaped pontoon structure improves the hydrodynamic performance of the platform as compared to conventional ring pontoon structures. The asymmetric pontoon spacing in the horizontal plan provides a reduced horizontal and angular motion in one direction over the other. Additionally, the inward position of the secondary central pontoons facilitates the support of SCR and allows a reduced vertical motion at the SCR fairlead locations. This reduced vertical and horizontal motion at the SCR fairlead help the SCR fatigue and strength design and consequently reduce the SCR associated cost. Numerical results for the global performance of the H-Pontoon platform are presented to illustrate the advantages of the newly developed H-Pontoon Semi concept over the conventional semisubmersible.
Parametric Optimization of a Semi-Submersible Platform With Heave Plates
Authors: Aubault Alexia , Cermelli Christian , Roddier Dominique
Published: May 2009

The hydrodynamic responses of a semi-submersible platform are driven by its mass properties and geometric parameters, e.g. column size, spacing, draft and pontoon size. The mooring system also influences the platform responses. Heave plates added to the base of each column have been proposed to enhanced stability of semi-submersible platforms, particularly in the lower payload range. Optimization of a platform typically involves a compromise among a large number of factors including the structural weight, vertical, horizontal motion and rotations in operating and extreme sea-states, airgap, mooring size, etc. Optimization methods are reviewed. The complexity of the problem leads to the choice of a genetic algorithm presented herein. To allow systematic platform optimization assuming primary project parameters are given, i.e. payload, waterdepth, environmental conditions. A simplified hydrodynamic model is developed to capture the parametric sensitivity of the platform responses to primary design parameters.
A Monte Carlo Approach to Prediction of Extreme Response Statistics of Drag Dominated Offshore Structures
Authors: Naess A. , Gaidai O.
Published: May 2009

The focus of the present paper is the extreme response statistics of drag dominated offshore structures subjected to harsh weather conditions. More specifically, severe sea states both with and without strong current are considered. The nature of the hydrodynamic forces acting on the structure becomes highly nonlinear. Additionally to the drag forces, the so called inundation effect due to the wave elevation, corrected to include second order waves, is also taken into account. In the present paper the Monte Carlo method along with a special extrapolation technique is applied. The proposed method opens up the possibility to predict simply and efficiently long-term extreme response statistics, which is an important issue for the offshore structures design.
Resonant Water Motion Between a Ship and a Terminal in Shallow Water
Authors: Kristiansen Trygve , Faltinsen Odd M.
Published: May 2009

This work focus on the hydrodynamical problem of an LNG carrier near a GBS-type offshore terminal subject to incoming waves in medium deep to shallow water conditions. The work is restricted to 2D and the ship is restrained from moving. The resonant behaviour of the fluid in the gap between the ship and the terminal is investigated. The problem is investigated by means of a numerical model and model tests. Potential theory is assumed and a linear as well as a nonlinear time-domain numerical wavetank based on a boundary element method with a Mixed Eularian-Lagrangian approach is implemented for this purpose. Model tests (near 2D) of a mid-ship section near a vertical wall is carried out in a 26.5m long and 0.595m wide wave flume in model scale 1:70. In full scale the ship beam is 45m and the ship draft 12m. The ship model is constructed in such a way as to avoid flow separation, i.e. no sharp corners. Several parameters are varied: Water depth, wave period and wave steepness. Wave elevation is measured at twelve locations.
Experimental Investigation of the Dynamic Behaviour of a Water Intake Riser
Authors: Kuiper Guido , Metrikine Andrei , Efthymiou Mike
Published: May 2009

This paper considers the dynamic stability of free-hanging water intake risers. Suspended from a barge, these risers convey a great volume of cooling water, which is needed for offshore liquefaction process of natural gas. There is a contradiction between theoretical predictions and experiments for cantilever pipes pumping up water. Reported small-scale experiments did not show any instability, while theory predicts instability beyond a critical fluid velocity. To investigate whether the previous experimental setups did not allow to observe the instability or the pipe aspirating water is unconditionally stable, a new test setup was built which could attain a higher internal fluid velocity than the predicted critical velocities. A cantilever pipe of about 5 m length was partly submerged in water. The experiments clearly showed that the cantilever pipe aspirating water becomes unstable by self-excited oscillatory motion (flutter) beyond a critical velocity of water convection through the pipe. Below this velocity the pipe is stable, whereas above it, the pipe shows a complex motion that consists of two alternating types of motion. The first type is a nearly periodic orbital motion with the amplitude of a few pipe diameters and the second one is a quasi-chaotic motion with very small amplitude. Translating these results to offshore water intake risers, shows that for realistic internal flow velocities the riser might become unstable.
Detection of Corrosion in Offshore Risers Using Guided Ultrasonic Waves
Authors: Edwards Graham R. , Gan Tat-Hean
Published: May 2009

Guided wave ultrasonics is now an established technology for detecting corrosion in pipes. The technique is principally applied to pipe that is insulated, buried or otherwise inaccessible to conventional NDT. TWI has experience in applying the technique to offshore risers, where there are specific problems. Access beneath the cut-off valves of the platform is restricted and the clamps used to hold the riser and the coatings used to protect it in the splash zone can affect test performance. This paper will describe these problems in detail and discuss ways of overcoming them. Case studies will be used to illustrate the current state-of-the-art and TWI’s research and development programme in long range ultrasonics for inspecting a wide range of components from pipes to railway lines will highlight future developments.
Delivery Characteristics of Newly Designed Twin Screw Pumps for Offshore Applications
Authors: Aleksieva Galabina , Scharf Alexander , Mewes Dieter , Rohlfing Gerhard , Reichwage Mark
Published: May 2009

Twin screw pumps with declining pitches designed for multiphase operation are experimentally investigated. The pumps are operated in single, serial or parallel mode within a closed loop pipe system of half technical scaled size. The continuously transported mixtures consist of air, water and oil. In the theoretical part of the investigation a model is deduced for the calculation of the delivering behavior of the multiphase screw pumps. The measured and the predicted flow rates as well as the pumping efficiencies are given as a function of the differential pressure, which is built up by the pumps.
Electrical Power Generation by Tidal Flow Acceleration
Authors: Roddier Dominique , Cermelli Christian , Aubault Alexia
Published: May 2009

Hydropower is a significant contributor to the renewable power generation sector, but the energy in tidal currents is not commonly used to generate electricity. This is due to the relatively slow speed of tidal currents which does not allow for the economic development of underwater turbines in tidal regions. This paper investigates whether it is possible to increase locally the current speed in regions where the tidal current is normally not strong enough to generate significant power. The device proposed to increase current speed is composed of an arrangement of vertical walls made of poles supporting a thin membrane with suitable profile, referred to as Tidal Current Accelerating Structure or TCAS. Current turbines are to be placed in areas of accelerated flow to convert the current energy into electricity. In this paper, results of model tests that were performed to quantify the ability to increase current speed are discussed. It was found that the increase in flow velocity was not as significant as expected, probably due to interactions between the turbines and the current accelerating devices. Recall potential theory’s flow speed around a disk yields a velocity factor increase of 2 at 90 degrees from the stagnation point.
Strength Evaluation of LNG Containment System Considering Fluid-Structure Interaction Under Sloshing Impact Pressure
Authors: Wang Bo , Kim Jang Whan
Published: May 2009

As LNG carriers become larger and new operating conditions are being designed, it is essential to develop a new procedure for the strength evaluation of a membrane-type LNG containment system under sloshing loads. The conventional comparative method based on existing service experiences and previous damage cases is currently used in most cases, but this method is only valid for designing new LNG carriers with similar size and type of existing ones. In this study, an analytical solution of acoustic-solid interaction has been derived and a simple 2D coupled acoustic-solid model has been simulated to investigate hydro-elastic effects for the verification purpose. After validation of FE modeling, a coupled model considering the fluid-structure interaction between LNG and containment system has been developed for structural analysis of LNG Mark III containment system. For LNG Mark III containment system, nonlinear dynamic FE analysis under sloshing impact pressure has been conducted using the fluid-structure coupling model. In FE simulations, the hydro-elastic effect in structural response has been studied through considering LNG as an acoustic medium, foam as a visco-elastic material, plywood as an orthotropic material, and mastic as an isotropic material. Parametric study has also been done to investigate the effects of material properties and loading patterns on hydro-elastic response in the coupled fluid-structure model. Based on FE results and experimental data, the strength of LNG Mark III containment system has been evaluated in terms of acceptance criteria. Finally, the new procedure has been developed for the strength evaluation of membrane-type LNG containment systems.
Fatigue Characterization of Long Dynamic Risers in Deep Waters
Authors: Iranpour Mohammad , Taheri Farid , Vandiver J. Kim
Published: May 2009

Continuous current action on risers develops vortices. These vortices cause risers to respond in a dynamic manner, leading to relatively large oscillations in the flexural and circumferential stresses. Vortex-induced vibration (VIV) and the resulting changes in the level of stresses in a riser could therefore cause detrimental fatigue induced problems, ranging from leakage to catastrophic failure of the riser. As a result, the VIV induced fatigue has always been a critical parameter in the design of risers, particularly when considering risers in deep waters. An experimental investigation into the fatigue crack propagation was conducted using a series of field data obtained on the high mode vibration of a relatively long flexible model riser. The field test data clearly illustrated the variable amplitude nature of the loading imposed on the riser by the current profile. There are currently several methods available for consideration of the crack growth rate due to overload and/or underloads; however, none has been developed based on data obtained for an actual riser. Indeed, fatigue experimental data on risers subject to VIV are relatively scarce in the literature. In this study the influence of the different harmonics resulting from VIV on the overall fatigue damage of the material is investigated. It is shown that the available basic approaches for assessing the fatigue crack propagation of components under variable amplitude loadings may significantly underestimate the VIV-induced fatigue damage of risers.
Reliability of Switched Model-Based Controller for Vessel Dynamic Positioning With Switching Under Estimated Motion Frequency
Authors: Nguyen Dong T. , Quek Ser Tong
Published: May 2009

In switched-control methodology which integrates several controllers handling different environmental conditions for dynamic positioning (DP) systems in marine vessels, the supervisor uses the estimated wave peak frequency (WPF) to classify the existing environmental condition so as to switch to the appropriate controller. The WPF is assumed to be same as the peak frequency of the vessel’s response spectrum at wave frequency estimated by Fast Fourier Transform (FFT) of the measured position and heading. Using a short window width leads to error in the WPF estimates whereas a longer width causes significant time lag in control activation. This paper addresses the effect of uncertainty in the estimation of WPF by FFT on the reliability of the switched-controller based DP vessel. The probability of the supervisor switching to a wrong controller due to WPF estimation error is first evaluated for four different sea states. The failure probabilities of the system with and without WPF uncertainty are then evaluated and compared. Numerical simulation based on a supply vessel shows that the failure probability of the switched-controller DP system due to the uncertainty in the estimated WPF by FFT did not increase much compared to the failure probability of a perfect switched-controller DP system.
Simulation of Fully Nonlinear Waves Interaction With Submerged Breakwater in a Numerical Wave Tank
Authors: El Safty Hoda M. , Mansour Alaa M. , Abul-Azm A. G.
Published: May 2009

The fully nonlinear wave interaction with submerged breakwaters that possess various configurations has been investigated using a fully nonlinear numerical wave tank model. In the numerical wave tank model, the fully nonlinear dynamic and kinematic free-surface boundary conditions have been applied and the boundary integral equation (BIE) solution to the Laplacian problem has been obtained using the Mixed Eulerian-Lagrangian (MEL) approach. Numerical results are presented for wave transmission for various breakwater and wave parameters. The model results have been verified against the available experimental data. The nonlinear solution has been compared with the results of other solutions based on the linear wave theory. Breakwater efficiency based on the nonlinear solution has been evaluated and compared for four different types of breakwater configurations, namely, vertical breakwater, sloped breakwater, breakwater with berm, and a pair of breakwaters.
Wave Load and Structural Analysis for a Jack-Up Platform in Freak Waves
Authors: el Moctar Ould , Schellin Thomas E. , Peric Milovan
Published: May 2009

The paper analyzed effects of freak waves on a mobile jack-up drilling platform stationed in exposed waters of the North Sea. Under freak wave conditions, highly nonlinear effects, such as wave run-up on platform legs and impact-related wave loads on the hull, had to be considered. Traditional methods based on the Morison formula needed to be critically examined to accurately predict these loads. Our analysis was based on the use of advanced CFD techniques. The code used here solves the Reynolds-averaged Navier-Stokes equations and relies on the interface-capturing technique of the volume-of-fluid type. It computed the two-phase flow of water and air to describe the physics associated with complex free-surface shapes with breaking waves and air trapping, hydrodynamic phenomena that had to be considered to yield reliable predictions. Lastly, the FEM was used to apply the wave-induced loads onto a comprehensive finite element structural model of the platform, yielding deformations and stresses.
Atlantis Oil Export SCR: Treatment of Girth Welds for Enhanced Fatigue Performance
Authors: Brooks Jim , Masson Craig , Reeves David
Published: May 2009

Suspended from the Atlantis Production Quarters Semi-submersible in over 7000 ft water depth, the 24-inch diameter Mardi Gras Oil Export Steel Catenary Riser (SCR) presented many challenges for design and installation. In particular the touchdown region of the riser was subject to severe fatigue and extreme loadings. The Engineering Critical Assessment (ECA) at the weld OD generated defect acceptance criteria that would be impractical to apply with the available inspection technology. To mitigate this, the decision was made to remove the weld caps in the most critical portion of the SCR. This paper describes the design drivers that led to this decision and the steps taken to ensure successful implementation.
Fracture Mechanics Approach for the Assessment of a Damaged Pipeline
Authors: Stadie-Frohbo¨s Gundula , Zerbst Uwe , Plonski Thomas
Published: May 2009

Nowadays, the damage assessment of pipelines using fracture mechanics techniques is a state-of-the-art methodology which allows a judgement whether the pipeline is fit-for-purpose over the residual lifetime (perhaps with a reduced pressure load) or has to be repaired or replaced. However, in some circumstances, uncertainty exists about its correct application, as this may be a matter of interpretation. This paper describes the analysis of a damaged pipeline based on a fracture mechanics approach for which the straightforward application of the flaw assessment standard BS 7910 without additional consideration of individual conservatism criteria could lead to misinterpretation. The investigation was carried out for pipelines under internal overpressure. The methodology applied considered large plastic deformation introduced during a repair measure. Since the repair or a subsequent overload resulted in the damage of a pipeline and could also have caused the damage of further components, evidence had to be provided that the structure was still damage-tolerant, i.e. that cracks, the existence of which could not be excluded, would not grow to a critical dimension over the projected lifetime. The investigation comprised the following steps: In a first step, the maximum crack size which could have survived the repair measure had to be specified. In this context, the choice of conservative input data and model parameters was essential in order to avoid any underestimation of the postulated crack size and geometry. Various criteria are discussed briefly here, placing special emphasis on global and local plastic collapse criteria. Secondly, the growth of this initial crack due to the operation stress had to be quantified; this was done by applying the so-called NASGRO approach. Finally, the critical crack dimensions had to be assessed. Fracture assessments were performed using various procedures, such as BS 7910, R6 and FITNET. A parameter study was carried out to demonstrate the potential conservatism of these assessment methods.
On the Effect of Narrow and Long Corrosion Defects on the Collapse Pressure of Pipelines
Authors: Netto T. A.
Published: May 2009

Internal corrosion in pipelines is often caused by water, sediment, or chemical contaminants present in the multi-phase flow. This normally occurs at the bottom of the pipe and at low points in the pipeline where sediment and water can settle out of the product being transported, therefore creating narrow and long defects. The effect of corrosion defects on the collapse pressure of offshore pipelines was studied through combined small-scale experiments and nonlinear numerical analyses based on the finite element method. After calibrated in view of the experimental results, the model was used to determine the collapse pressure as a function of material and geometric parameters of different pipes and defects. An extensive parametric study using 2-D and 3-D numerical models was carried out encompassing different defect geometries and their interaction with pipe ovalization. This paper reports these results which are subsequently used to develop a simple procedure for estimating the collapse pressure of pipes with narrow defects.
Pipeline Integrity Management System: A New Method for Monitoring Pipeline Structural Integrity During Offshore Pipe-Lay
Authors: Cocault-Duverger Vincent , Howard Brett
Published: May 2009

Traditionally, the monitoring of the pipeline structural response to dynamic loads during offshore installation is performed indirectly by comparing the observed sea-states to a matrix of pre-run dynamic analysis cases. Offshore work is planned within a weather window such that the vessel’s station keeping and equipment capacities are not exceeded and pipeline integrity remains within code limits. Assessment of actual seastate offshore is subject to interpretation, possibly introducing undue conservatism with respect to pipe lay operations in some circumstances. This paper describes a proprietary pipeline integrity monitoring method for managing pipe-lay operations. Technip has developed and tested this approach to optimise installation weather windows for the company’s reel-lay vessel, Apache. The method integrates both office-based analysis and offshore real-time motion monitoring. Limiting equations, which represent pipeline stresses and tensions during pipe-lay as a function of the motion of the pipeline top connection, are defined during pre-campaign finite element analysis. Considerable time savings are achieved over conventional approaches by utilising multi-parametric optimisation techniques. Once offshore, the actual motions are measured in real-time using a motion reference unit mounted on the lay ramp. Recorded data can then be compared against pre-defined multi-variate response surface. The system provides a real-time indication of the stress and tension levels in the pipeline. It is believed this method could introduce greater accuracy to pipeline integrity management in some circumstances, which in turn could provide more accurate information for making operational decisions. This novel approach is presented together with a description of current dynamic analysis philosophy and an alternative approach made possible by recent improvements in analytical software and computer processing capabilities.