bluuebunny/crossref_metadata_2025 huggingface dataset:
Snap Loads on Mooring Lines of a Floating Offshore Wind Turbine Structure
Authors: Hsu Wei-ting , Thiagarajan Krish P. , Hall Matthew , MacNicoll Michael , Akers Richard
Published: September 2014

There are a number of design challenges facing mooring systems of floating offshore wind turbine (FOWT) platforms in an offshore environment. Some unique aspects of the FOWT industry should be considered when examining applicability of established offshore mooring practices. Important among these are: economy and cost effectiveness; light weight minimal platforms; and water depths ranging from 50–300 m. A lighter displacement platform in shallow water, supported by lines with light to moderate pre-tension can result in a higher probability of slack line events and hence snap loads during re-engagement. Such loads can result in shock on the line material and considerably reduce the fatigue life. Such events have the potential to occur in various sea states, and not necessarily limited to extreme conditions. These conditions will be dependent on structure resonant motions, which are influenced by wind loads and moments, wave conditions and mooring line properties. Model tests of typical concepts for FOWT reported in literature have shown occasional slack line episodes.

This paper is a review of literature on snap load occurrence in marine applications, including lifting and lowering operations, ROV and diving bell operations. This paper presents a case study of a FOWT. Special focus is on mooring systems which are affected by impact load conditions. Criteria are reviewed and consequences are documented.
Crack Initiation and Growth in Bimetallic Girth Welds
Authors: Carlucci Antonio , Bonora Nicola , Ruggiero Andrew , Iannitti Gianluca , Gentile Domenico
Published: September 2014

Bimetallic girth welds are characteristics of clad pipe technology. When dealing with propagation issues, fracture mechanics concepts usually are no longer applicable as a result of the extensive and non-homogeneous plastic deformation along bi-material interface that occur at the crack tip even below design allowables. In this study, ductile crack initiation and propagation in bi-material girth welds was investigated using a Continuum Damage Mechanics (CDM) model proposed by Bonora [1]. For the base, weld and clad metal, ductile damage model parameters have been determined by means of inverse calibration technique using fracture data obtained on smooth and round notched tensile bar specimens. Firstly, the damage model was validated predicting ductile crack growth occurring in single end notch (SEN(T)) geometry sample comparing the applied load vs crack mouth opening displacement with experimental measurements. Successively, the model was used to investigate ductile crack initiation and propagation for under clad circumferential weld crack under remote tension.
Wave-in-Deck Forces on Fixed Horizontal Decks of Offshore Platforms
Authors: Abdussamie Nagi , Thomas Giles , Amin Walid , Ojeda Roberto
Published: September 2014

The problem of wave-in-deck loading on offshore structures involves complex physical mechanisms which require close study. In this paper, the wave-in-deck forces generated on the bottom plate of a rigidly mounted, box-shaped structure subjected to unidirectional regular waves are quantified by means of two approaches. The first is an analytical momentum approach recommended by classification societies and the second is a computational fluid dynamics (CFD) approach based on the volume of fluid (VOF) method implemented in the commercial code FLUENT. The change in force due to very small variations in wave steepness and air gap is investigated and discussed.

Several numerical trials are conducted to optimise the computational domain and model discretisation suggestions are made. The numerical results are compared with physical model tests recently carried out at the Australian Maritime College (AMC). The results of the successive wave impacts are analysed using a discrete wavelet tool to ensure that the temporal information of slamming events is not lost in signal analysis and filtering. By comparing the theoretical and experimental results it was found that in many cases the momentum method underestimates the magnitude of the horizontal and upward directed wave-in-deck forces. Although the three-dimensional CFD cases tested are noticeably time-consuming, these simulations were found to be in good agreement with the experimental measurements.
Numerical Study on Coastal Wave and Near-Shore Current Interaction
Authors: Tang Jun , Shen Yongming , Lv Yigang
Published: September 2014

Coastal waves and near-shore currents have been investigated by many researchers. This paper developed a two-dimensional numerical model of near-shore waves and currents to study breaking wave induced current. In the model, near-shore water wave was simulated by a parabolic mild slope equation incorporating current effect and wave energy dissipation due to breaking, and current was simulated by a nonlinear shallow water equation incorporating wave exerted radiation stress. Wave radiation stress was calculated based on complex wave amplitude in the parabolic mild slope equation, and this result in an effective method for calculating wave radiation stress using an intrinsic wave propagation angle that differs from the ones of using explicit wave propagation angle. Wave and current interactions were considered by cycling the wave and current equation to a steady state. The model was used to study waves and wave-induced longshore currents at the Obaköy coastal water which is located at the Mediterranean coast of Turkey. The numerical results for water wave induced longshore current were validated by measured data to demonstrate the efficiency of the numerical model, and water waves and longshore currents were analyzed based on the numerical results.
An Investigation on Circumferentially Cracked Bar Geometry for Critical CTOD Determination
Authors: Carlucci Antonio , Bonora Nicola , Ruggiero Andrew , Iannitti Gianluca , Persechino Italo
Published: September 2014

Codes and standards for oil&gas industry, as OS-DNV-F101, recommend the use of single edge cracked plate in tension (SEN(T)) for the experimental determination of the material critical CTOD. For clad pipe welds, this specimen geometry is difficult to be obtained for the weld material or clad corrosion resistance alloy due to the specimen shape and minimum dimensions. Alternatively, circumferential cracked bar geometry, CCB(T) could be used. This geometry configuration can be machined when limited material quantity is available and used for both quasi-static and dynamic fracture characterization. In this paper, an extensive elastic-plastic finite element investigation has been carried out on both SEN(T) and CCB(T) geometries in order to select equivalent configurations in the J-Q space. Ductile crack initiation and growth has been simulated using continuum damage mechanics model. Numerical simulation results indicate that CCB(T) with a crack depth ratio r/a = 0.2 realizes constraint loss similar to that of SEN(T) with a crack depth ratio a/W = 0.5. Similar crack resistance curves have been obtained for these two configurations confirming the equivalence of the selected sample geometries.
Experimental Assessment of the Behaviour of a Pipe Vibration Damper Underwater
Authors: Bordalo Sergio N. , Morooka Celso K. , Tochetto Luan G. , Pavanello Renato , Song Gangbing , Bartos John C.
Published: September 2014

Submarine petroleum pipelines, risers and jumpers suffer static and dynamic loads due to sea currents and waves, due to the displacements of the floating production units and due to the internal flow, among other causes. Mitigating the oscillations caused by such excitations is critical to the reliability and fatigue of those underwater bodies. The Pounding Tuned Mass Damper (PTMD) is one device that may be employed to absorb and dissipate vibrations. These devices have long been used for mechanical systems operating in the atmosphere, but are new for underwater applications. This paper presents a study of the behaviour of a PTMD working underwater.

A small scale laboratory apparatus was built to assess the effect of the absorber on the oscillation of a pipe submerged in a water tank. The PTMD was attached to a test pipe section mounted on an elastic suspension harness. The PTMD model is a lumped mass-spring attachment similar to a tuned mass dumper (TMD) suppressor, but with the addition of a pounding layer, which limits the motion of the PTMD mass, dissipating the energy of the oscillating pipe through the impact of the PTMD mass against that layer. Free and forced oscillation experiments were executed in air and in water, with and without the oscillation absorber, to determine the effectiveness of the PTMD. The tests were run on a range of excitation frequencies and the amplification factors were obtained for each case.

The data show a remarkable influence of the surrounding media on the dynamics of the pipe-absorber system, therefore the interaction with the water must be taken into consideration in the design of the system. Although the results are only a preliminary step on the development of a device applicable to an actual petroleum submarine pipeline, it was observed that the PTMD does indeed suppress the vibrations, but it must be properly configured to achieve an optimum performance. The data gathered from this work will also be useful in the improvement of a numerical model of the pipe-PTMD system for use in a computer simulator.
A New Approach to Model Hyperbaric Rock Cutting Processes
Authors: Helmons Rudy L. J. , Miedema Sape A. , van Rhee Cees
Published: September 2014

In deep sea mining processes, rock is being cut in a hyperbaric pressure environment. The effect of such a high pressure environment is of major influence on the cutting process. Due to deformation of the rock matrix, local fluid pressure differences will occur. This can result in a higher apparent strength of the rock, but also in a higher (loading) rate dependency.

Our new modeling approach aims at combining both rock mechanics and fluid dynamics to model the high deformation (rate) behavior experienced in seabed excavations. This new approach is based on the Discrete Element Method to simulate the rock mechanics, combined with the use of Smoothed Particle Methods to model the influence of the fluid (pressure) on and in the rock. In this paper, emphasis is put on estimating the local (volumetric) deformation rate with the new approach. Eventually, the new technique will be validated with experiments and data available in literature. The new approach will give more insight in the physical processes that occur during cutting.
Performance Evaluation of High Quality HFW Linepipe by a Series of Full-Scale Pipe Tests
Authors: Igi Satoshi , Sadasue Teruki , Oi Kenji , Yabumoto Satoru , Toyoda Shunsuke
Published: September 2014

Newly-developed high quality high frequency electric resistance welded (HFW) linepipes have recently been used in pipelines in reel-lay applications and low temperature service environments because of their excellent low temperature weld toughness and cost effectiveness. In order to clarify the safety performance of these HFW linepipes, a series of full-scale tests including a hydrostatic burst test at low temperature, fatigue test and tension test under high internal pressure was conducted, together with small-scale tests such as impact energy and standard fracture toughness tests, which are generally used in mill production and pre-qualification tests.

The Charpy transition curve of the developed HFW pipe occurred at a temperature much lower than −45°C. Based on the low-temperature hydrostatic burst test with a notched weld seam at −45°C, the weld of the HFW linepipe presented the same level of leak-before-break (LBB) behavior, as observed in UOE pipes. A full-pipe fatigue test of HFW pipes with repeated internal pressurizing was conducted. The fatigue strength of the developed HFW linepipe shows better performance than butt weld joints because of the smoothness at its weld portion, which is achieved by mechanical grinding of the weld reinforcement. Full-scale pipe tensile tests of girth welded joints were performed with an artificial surface notch at the heat affected zone in the girth weld. The influence of internal pressure was clearly observed in these tests.

Based on the above-mentioned full-scale tests, the safety performance of high quality HFW linepipe is discussed in comparison with the mechanical properties in the small-scale tests such as the Charpy and standard fracture toughness tests, especially when the notch was placed in the welded seam.
Design of Mooring Lines of Floating Offshore Wind Turbine in Jeju Offshore Area
Authors: Kim Hyungjun , Choung Joonmo , Jeon Gi-Young
Published: September 2014

This paper presents a mooring design procedure of a floating offshore wind turbine. The offshore environment data of Jeju south sea collected from Korea Hydrographic and Oceanographic Administration (KHOA) are used as environmental conditions for hydrodynamic analysis. A semi-submersible floating wind turbine system is considered based on Offshore Code Comparison Collaborative Continuation (OC4) DeepCWind platform and the National Renewable Energy Laboratory (NREL) 5MW class wind turbine. Catenary mooring with studless chain is chosen as the mooring system. Important design decisions such as how large the nominal sizes are, how long the mooring lines are, how far the anchor points are located, are demonstrated in detail. Considering ultimate limit state and fatigue limit state based on 100-year return period and 50 year design life, respectively, long-term predictions of breaking strength and fatigue are performed.
Modeling Stalk-On Operation and its Application
Authors: Hong Jin Wuk , Kim Jong Wook , Kim Kyu Tae , Lee Woo Seob
Published: September 2014

Tie-in between a pipeline and a riser is an important operation in the offshore oil and gas development. Normally, divers need to carry out the tie-in operation on the seabed by welding or using flanges in order to connect the pre-laid pipeline end to the jacket riser. However, working on the seabed can have leakage or cost problems. As one of solutions, the surface-tie-in method so called stalk-on method is used in the shallow water. In this method, the riser is welded to the pipeline which is lifted over the sea surface, and the welded pipeline-riser is lowered to the target position.

The main purpose of this paper is to demonstrate the safety evaluation procedure of the stalk-on operation and to share the experience learnt from actual field application. For the sake of the safety evaluation, the stalk-on operation is divided into 3 phases which are the phases of lifting a pipeline by davit cranes, holding a lifted-pipeline and lowering the weld-connected pipeline and the riser. Each phase is numerically simulated in time-domain analysis. The several lesson learnt from actual application are summarized including the consideration of discrepancy between time domain simulation result and the measured one during the installation.
Physical and Numerical Modelling of Mooring Forces and Displacements of a Wave Activated Body Energy Converter
Authors: Angelelli Elisa , Zanuttigh Barbara , Martinelli Luca , Ferri Francesco
Published: September 2014

The paper analyses the forces on the mooring system and the device motions of a floating Wave Activated Body Energy Converter under ordinary and extreme wave conditions. The investigation has been carried out with physical and numerical approaches. The physical tests were performed in 1:60 scale in the shallow-water wave tank of Aalborg University, whereas the numerical simulations were performed in 1:1 scale with the AQWA code developed by ANSYS. The spread mooring system tested in the laboratory allowed for an efficient device keeping while minimising the space. The loads on the moorings increase with increasing the significant wave height and show a modest trend with the peak wave length.

These experimental measurements were compared with the numerical results, suggesting that AQWA model is able to accurately reproduce the standard deviation of the forces on the mooring lines. From the simulations it is highlighted that the device should work far from resonance condition, because when the typical wave period is near to the natural period (in surge) the forces acting on the mooring lines increase, resulting in a high level of energy loss.
Numerical Investigation of the Flow Features Around Heave Plates Oscillating Close to a Free Surface or Seabed
Authors: Garrido-Mendoza Carlos A. , Thiagarajan K. P. , Souto-Iglesias Antonio , Bouscasse Benjamin , Colagrossi Andrea
Published: September 2014

Performance of heave plates used in offshore structures is strongly influenced by their added mass and damping, which are affected by proximity to a boundary. A previous paper by the authors presented numerical simulations of the flow around a circular solid disk oscillating at varying elevations from seabed [1]. The force calculated was used to evaluate the added mass and damping coefficients for the disk. The simulations suggest that as the structure moves closer to the seabed the added mass and damping coefficients (Ca and Cb) increases continuously. In order to understand the physics behind the added mass and damping trends, when a heave plate is moving near a seabed or closer to the free surface, the flow characteristics around the heave plate are examined numerically in this paper. Flow around oscillating disks is dominated by generation and development of phase-dependent vortical structures, characterized by the KC number and the distance from the seabed or free surface to the heave plate. Numerical calculations presented in this paper have comprised the qualitative analysis of the vortex shedding and the investigation of the links between such vortex shedding and, on one hand the damping coefficient, and on the other hand, pairing mechanisms such as the shedding angle.
Derivation of Wave Height and Peak Period Statistics Accounting for Physical Limitations
Authors: Melis Cécile , Bonnaffoux Guillaume
Published: September 2014

When assessing the joint-probability of significant wave height and peak period, (Hs,Tp) measured over years at a given site, it is customary to fit a log-normal distribution to assess Tp dependence on Hs. The parameters of this distribution are then used either to compute N-year return period design curves in order to compute extreme response by means of short-term analysis, or response distributions, by means of response-based analysis.

The main drawback of the Log-Normal distribution to represent the variability of Tp wrt. Hs is that its lower bound is zero, while physics tell us that wave steepness cannot be infinite, hence the lower bound, Tplim(Hs) should be greater than zero. If the distribution is kept unbounded, the resulting statistical fitting tends to predict occurrences of sea-states with (Hs,Tp) pairs having unphysical or unlikely steepness. This is particularly true in the range of 10–15s, where some ship-shaped units mooring systems responses are at their maximum.

Attempts have been made in the past to introduce a lower bound to the log-normal distribution, for instance by Drago et al, [1], by shifting it by a predefined value of limit steepness. By doing so, some points of the original dataset had to be discarded as they were falling below the lower bound. An evolution of their methodology is proposed in this paper, which uses the points of the dataset in a relevant region which will be defined hereafter, and then uses this limit to shift the Log-Normal distribution. The obtained environmental contours are then compared against observed data to check which one fits most accurately the original set of measured (Hs,Tp) pairs.
An Investigation on the Discrepancies Between RANSE and BEM Approaches for the Prediction of Marine Propeller Unsteady Performances in Strongly Non-Homogeneous Wakes
Authors: Gaggero Stefano , Villa Diego , Viviani Michele
Published: September 2014

In the present work an analysis of the reliability of different numerical approaches, namely BEM and RANSE solver, for the prediction of unsteady performances of marine propellers is presented. To this aim, the well-known Seiun-Maru Highly Skewed Propeller operating in different wakes is considered. The results of this analysis show that, in correspondence to particularly challenging conditions, i.e. when very pronounced ship wakes are present, the two approaches may provide considerably different results in terms of propeller mechanical characteristics and pressure distributions on the blades. This problem, which is implicitly eliminated when the thrust identity approach is applied (as almost always performed, as an example, for propeller analyses aimed to the evaluation of cavitation extension and / or induced pressures), may become critical in case the codes are used for numerical self-propulsion tests, where a high accuracy in the prediction of the unsteady propeller performances in correspondence to a prescribed value of the advance coefficient, instead of the thrust coefficient, is mandatory. The analyses carried out allow to underline which are the potentially more problematic cases, in terms of wakes characteristics, and to suggest some possible reasons of the encountered discrepancies, which will need further analyses to enhance the prediction capability of numerical codes.
Optimization of Mooring System for Multi-Arrayed Tidal Turbines in a Strong Current Area
Authors: Rho Yu-ho , Jo Chul-hee , Kim Do-youb
Published: September 2014

Recently, focus has been placed on ocean energy resources as environmental concerns regarding the exploitation of hydrocarbons are increasing. The enormous energy potential in tidal current power fields has been exploited by installing floating tidal current power turbines. Due to other floating structures operating within a limited area, stationkeeping is needed in order to keep the motions of the floating structures within permissible limits. In this study, methods for selecting and optimizing the mooring system for floating tidal current power systems in shallow water are investigated. The mooring system provides restoring forces and moments on a floating structure, pulling the structure back toward its equilibrium position. Typically, the mooring lines are segmented in order to optimize the force and displacement characteristics known as the mooring line characteristics. The mooring system concepts investigated in this study include the distributed mass, clump weight, and buoyancy element mooring systems.
Dynamics of Unstable Stokes Waves: A Numerical and Experimental Study
Authors: Chabchoub Amin , Perić Robinson , Hoffmann Norbert P.
Published: September 2014

Being an appropriate prototype to describe oceanic rogue waves, the Peregrine breather solution of the nonlinear Schrödinger equation is investigated numerically and experimentally to analyze the dynamics of modulationally unstable Stokes waves. The evolution of the water surface elevation is studied numerically by solving the Navier-Stokes equations using a finite-volume approach and a volume of fluid method. The comparison of the numerical results with wave tank experiments show a very good agreement. The results confirm the ability of the chosen method to model the modulation instability of Stokes waves, in particular, breather dynamics in water waves with high accuracy even up to the onset of breaking. We also investigate the sub-surface flow fields, which may be of significance for the short-term prediction of extreme wave focusing in narrow-banded sea state conditions and therefore, for ocean engineering applications.
Bursting Capacity and Debonding of Ultra Deep Composite Production Riser: A Safety Assessment
Authors: Singh Manander , Ahmad Suhail
Published: September 2014

Due to depleting sources of oil and gas reserves in shallow water depths, exploration and production activities have moved into ultra-deep offshore oil fields. Risers are an essential part of any offshore drilling facility. A riser tensioner located on the drilling platform has to provide an adequate vertical tension to maintain the stability of the riser. It is essential for a successful operation. Composite risers in deep sea conditions require much lower top vertical forces due to their high strength to weight ratio. Carbon/epoxy composite has been considered in the present study to carry out the burst analysis and to assess the safety of the composite riser under internal and external pressures and other environmental loads due to random sea currents. In order to ensure the permissible pressure and no fluid leakage, composite risers are provided with an internal steel liner. Initiation and propagation of debonding between the liner and composite has been studied and probability of failure is obtained. In burst analysis, maximum internal pressure is applied to a riser section and the stresses in all (hoop and longitudinal layers) the composite layers are checked against the failure. High pressures are incremented in small steps until fiber rupture occurs due to bursting. Maximum normal stress theory is employed for checking the failure. The same theory provides the limit-state to assess the safe pressure considering uncertainties associated with random input parameters involved. A finite element analysis has been carried out in ABAQUS/AQUA for random sea motion and fluctuating axial tension considering salient non-linearities. A small riser section modelled as a hybrid beam element (for global analysis) has been considered to study the bursting and debonding behavior. It is further discretized into thin shell elements (S4R). Steel liner and composite pipes are modeled separately and assembled together to ensure the overlapping various layers and sharing nodes. The composite body sustains the major stresses in the inner layers that diminish on moving outwards radially. An implicit time domain analysis has been carried out to obtain the response. The debonding through circumference and length are studied. The stresses obtained are compared with their ultimate strength.
Viscoelasticity and Drilling Fluid Performance
Authors: Lee John , Tehrani Ahmadi , Young Steve , Nguyen Christine
Published: September 2014

Viscoelastic properties of drilling fluids are not often measured due to a lack of understanding of their impact on fluid performance as well as a lack of field equipment suitable for such measurements. A study has been conducted recently to evaluate the viscoelastic properties of xanthan gum and invert drilling fluids and their impact on barite suspension quality and rheology. Both a Brookfield YR-1 rheometer and a Bohlin Gemini 150 rheometer were used to generate data for comparison. The impact of viscoelasticity on steady-state rheology, thixotropy and shear thinning was evaluated using a multi-speed rheometer. A soon-to-be-adopted API recommended procedure was used to measure the barite sag tendency under dynamic conditions.

Aqueous solutions of xanthan gum showed that viscoelasticity, shear thinning and thixotropy increased with increasing polymer concentration. When the solutions were weighed up with barite, they became more viscoelastic, slightly more thixotropic, but less shear thinning. Proper suspension of barite was observed at a xanthan gum concentration of 2 lb/bbl without any other additives.

Compared to xanthan-barite suspensions, invert drilling fluids of similar density exhibited a greater viscoelasticity but less thixotropy and shear thinning. Treatment of invert drilling fluids with viscoelastic polymers resulted in a further enhancement of viscoelasticity and thixotropy, but a slight deterioration in shear thinning. Barite suspension quality showed a certain degree of correlation with viscoelasticity as well as steady-state rheology; however, these properties were temperature dependent for invert drilling fluids.

Hydraulic analyses indicated that viscoelastic additives can impact fluid viscosity thus affecting pressure loss, equivalent circulating density and hole cleaning. Viscoelasticity enhancement may be used to improve barite suspension quality under certain conditions, but its impact on hydraulics must be carefully considered.
Comparison of Fully Nonlinear and Weakly Nonlinear Potential Flow Solvers for the Study of Wave Energy Converters Undergoing Large Amplitude Motions
Authors: Letournel Lucas , Ferrant Pierre , Babarit Aurélien , Ducrozet Guillaume , Harris Jeffrey C. , Benoit Michel , Dombre Emmanuel
Published: September 2014

We present a comparison between two distinct numerical codes dedicated to the study of wave energy converters. Both are developed by the authors, using a boundary element method with linear triangular elements. One model applies fully nonlinear boundary conditions in a numerical wavetank environnment (and thus referred later as NWT), whereas the second relies on a weak-scatterer approach in open-domain and can be considered a weakly nonlinear potential code (referred later as WSC). For the purposes of comparison, we limit our study to the forces on a heaving submerged sphere. Additional results for more realistic problem geometries will be presented at the conference.
Wave Energy Assessment and Wind Correlation for the North Region of Scotland, Hindcast Resource and Calibration, Investigating for Improvements of Physical Model for Adaptation to Temporal Correlation
Authors: Lavidas George , Venugopal Vengatesan , Friedrich Daniel , Argawal Atul
Published: September 2014

Wave energy sites around Scotland, are considered one of the most energetic waters, as they are exposed to the Atlantic Ocean. The amount of energy reaching the shoreline provides an opportunity for wave energy deployments.

Currently, considerations on wave devices expect them to be installed at nearshore locations. That means that the potential wave resource has to be investigated, since deep to shallow water interactions alter the shape of propagated waves. Resource assessment for these regions is essential in order to estimate the available and extractable energy resource. Although several numerical models exist for wave modelling, not all are suitable for nearshore applications.

For the present work, the nearshore wave model SWAN has been used to simulate waves for the Hebridean region. The set-up, calibration and validation of the model are discussed. The resulting wave conditions are compared with buoy measurements. Results indicate that the modelling technique performed well.
Shakedown Analysis of Ocean Engineering Structures Subjected to Repeated Dynamic Loads
Authors: Jun Wang , Yao Xiongliang , A'man Zhang
Published: September 2014

Ocean engineering structures are frequently subjected to repeated dynamic loads caused by slamming of wave, impact of ice, dropped objects, collisions of store ship and grounding. Shakedown analysis is an extension of plastic limit analysis. Meanwhile, the dynamic strength analysis and shakedown analysis of offshore platform structure have an important place in ensuring the safety and reliability of ocean engineering structures under repeated dynamic loads. Therefore the shakedown analysis theory was introduced to the ultimate strength analysis of brace strut of semisubmersible drilling platform considering cyclic wave load. Based on the kinematic shakedown theorem, a theoretical method of shakedown analysis for typical ocean engineering structures under repeated dynamic loads was presented and compared with existing results to verify the reasonableness. According to the method of finite elastic-plastic theory, the strength of brace strut was analyzed through the overall model of semisubmersible drilling platform. Then based on the boundary conditions getting from the overall three-dimensional model, locally refined model of brace strut was obtained. By applying the theoretical method to shakedown analysis of brace strut under repeated dynamic loads, influence of shell thickness, stiffener thickness and stiffener spacing on shakedown limit were studied. The results show that the theoretical calculation method is consistent with the existing results. The limit load increases with the increase of shell thickness and stiffener thickness, while decreases with the increase of stiffener spacing.
Application of Focusing Wave Groups in Model Testing Practice
Authors: Schmittner Christian , Brouwer Joris , Hennig Janou
Published: September 2014

For hydrodynamic model testing different types of model waves are applied, where the most common ones are regular (monochromatic) and irregular (multichromatic) waves. In addition to these wave types the application of focusing wave groups, which are also often denoted as wave packages or transient wave packets, can give insight into aspects that cannot be assessed by the conventional model waves.

This paper describes the different applications of focusing wave groups for hydrodynamic model testing. The paper starts with the historical background, followed by a theoretical description and the generation procedure.

The main part of the paper is dedicated to the practical application of focusing wave groups in the basin. Items that will be described are a) the derivation of transfer functions for floating structures and for anti-roll tanks b) the determination of hydraulic and electrical transfer function of wave makers c) the verification of position and calibration of wave probes in the basin d) the generation of extreme wave events e) the assessment of reflection coefficient of beaches f) the investigation of non-linear aspects of transfer functions.

Finally, characteristics of the analysis of focusing waves are introduced and compared to conventional methods based on regular and irregular waves.
Optimized Propulsor Retrofit for Slow Steaming for a Post Pan-Max Container Vessel
Authors: Will Johannes , Greitsch Lars
Published: September 2014

Ships are typically designed for a life cycle of about 20–25 years, with a specific transportation task, that suits the instant or estimated requirements of the owner or operator. Anyhow the business model might change due to a multitude of reasons and force the owner to buy new ships or modify the existing. Such an event took place in the year 2008 with the beginning of the financial crisis and the resulting recession, which lead to a drastic decrease of the global volume of cargo. This paper describes the re-design of the propulsion system of an existing post pan-max container vessel to suit the new economical boundary conditions.
Experimental Study of Cuttings Transport Efficiency of Water Based Drilling Fluids
Authors: Ytrehus Jan David , Taghipour Ali , Lund Bjørnar , Werner Benjamin , Opedal Nils , Saasen Arild , Ibragimova Zalpato
Published: September 2014

One important requirement for a drilling fluid is the ability to transport the cuttings out of the borehole. Improved hole cleaning is a key to solve several challenges in the drilling industry and will allow both longer wells and improved quality of well construction. It has been observed, however, that drilling fluids with similar properties according to the API standard can have significantly different behavior with respect to hole cleaning performance. The reasons for this are not fully understood.

This paper presents results from laboratory tests where water based drilling fluids with similar rheological properties according to API measurements have been tested for their hole cleaning capabilities in a full scale flow loop. Thorough investigation of the viscoelastic properties of the fluids were performed with, among other instruments, a Paar-Physica rheometer.

Improved understanding of the fluid properties relevant to hole cleaning performance will help develop better models of wellbore hydraulics used in planning of well operations. Eventually this may lead to higher ROP with water based drilling fluids as obtained with oil based drilling fluids. This may ease cuttings handling in many operations and thereby significantly reduce the drilling cost using (normally) more environmentally friendly fluids.

The experiments have been conducted as part of an industry-sponsored research project where understanding the hole cleaning performance of various oil and water based drilling fluids is the aim. The experiments have been performed under realistic conditions. The flow loop includes a 12 meter long test section with 2″ OD freely rotating drillstring inside a 4″ ID wellbore made of concrete. Sand particles were injected while circulating the drilling fluid through the test section in horizontal position.
A New Riser Fatigue Monitoring Methodology Based on Measured Accelerations
Authors: Ge Michael Long , Kannala Jomon , Li Songcheng , Maheshwari Himanshu , Campbell Mike
Published: September 2014

In support of its commitment to safe and reliable operations, BP has been continuously developing a program to assess and maintain structural integrity for offshore drilling risers and conductors. This paper presents recent efforts by BP, in conjunction with 2H Offshore, to develop a new fatigue monitoring methodology for drilling riser systems due to both wave and vortex-induced-vibration (VIV) damage.

BP has been monitoring structural response, including the fatigue damage, of riser systems in the Gulf of Mexico over the past ten years. To date, the focus has predominantly been on determining the fatigue damage due to VIV, since VIV and its effects on structural response are considered a not well-understood phenomenon. In addition to VIV fatigue, direct wave loading and vessel motions also contribute to the total fatigue damage, and sometimes wave fatigue may have a larger contribution than VIV fatigue damage. Therefore, it is necessary to determine fatigue due to both wave and VIV effects to confirm the long-term fatigue integrity of the drilling risers.

To take full advantage of the accumulated monitoring data, a new fatigue monitoring methodology was developed using an analytical solution to account for the damage due to both wave and VIV effects. With this method, the measured acceleration data are converted into curvature, and then fatigue damage along the length of riser and conductor are calculated. This new methodology has been validated with both finite element analysis (FEA) and field data, and sensitivities to various parameters have been considered.
Enhanced Collapse Resistance of Compressed Steel Pipes
Authors: Krishnan Venkat R. , Baker David A.
Published: September 2014

Pipe collapse is a primary design consideration for deep water locations and offshore areas with sharp seabed curvatures or spans, where bending reduces collapse resistance due to ovalization. Previous numerical and experimental work has shown that collapse resistance of steel pipes can be enhanced significantly by using compression instead of expansion during the final stage of the pipe forming process.

ExxonMobil has recently undertaken a rigorous numerical modeling and experimental testing program to investigate the collapse resistance of compressed (JCOC) steel pipes under combined loading of external pressure and bending, and this paper presents the main results from the program. The first part of the paper presents results of sensitivity studies from three dimensional (3D) finite element analyses (FEA) of the pipe forming process, and the second part focuses on the collapse modeling under combined loading as well as a comparison of the numerical results with the experiments. The results indicate that the collapse envelope for steel pipes under combined external pressure and bending can be enhanced by up to 35% by adopting pipe compression rather than expansion as the final step of the forming process.
Seafloor Pressure Signatures of a High-Speed Boat in Shallow Water With SPH
Authors: Tafuni Angelantonio , Sahin Iskender
Published: September 2014

Smoothed Particle Hydrodynamics (SPH) based simulations are implemented to evaluate the pressure-induced signatures on the ocean floor due to the passage of a high-speed boat in quiescent shallow water. Along with the standard Weakly-Compressible SPH (WCSPH) equations, the delta-SPH formulation is employed, which modifies the SPH continuity equation by incorporating numerical diffusion. This correction allows for a considerable reduction of the spurious oscillations characterizing pressure fields obtained with WCSPH algorithms. A simple computer model of a planing boat is developed for comparison with similar works in the literature. Simulations are performed using a parallel open-source SPH code on a high-end graphics processing unit (GPU). A convergence study on the size of the optimal computational domain is carried out, with a total number of particles per simulation ranging between 100,000 to 20,000,000. Part of the computational work is directed towards the investigation of the best set of SPH parameters to be employed in this specific study, with particular attention to the choice of a suitable kernel function, particle resolution and viscosity coefficients. Pressure contours and pressure plots at lateral locations at the seafloor are presented, showing good agreement with previous studies. It can be inferred that the SPH methodology is a suitable choice for free-surface problems, offering a good trade-off among the ease of implementation, computational efficiency and accuracy of the results.
Transient Wave Packets: New Application in CFD-Methods
Authors: Clauss Günther F. , Stuppe Sven , Dudek Matthias
Published: September 2014

Detailed knowledge of motion and seakeeping behaviour in an early design stage is indispensable in modern layout of marine offshore structures. Therefore, numerical methods are used to calculate the Response Amplitude Operators (RAO), which are generally based on potential theory or the Reynolds-Averaged-Navier-Stokes-Equation (RANSE).

Calculations with potential-codes are commonly used, well established and time-saving. Main disadvantages are the neglect of viscous effects and the hull structure above the still water level. By using RANSE-methods, these nonlinear effects can be investigated in detail, but at the price of calculation time and extensive grid generation. To achieve sufficient RAOs in frequency domain, time-consuming and intensive calculations would be necessary with these CFD-methods, using seastate applications with regular or irregular waves only. Therefore, these methods are not convenient for standard motion analysis by now.

Transient Wave Packets (TWP) represent an approved method at model tests, revealing the entire RAO for any offshore structure within one single, short test run. Main advantage of this technique is the accurate predictability and short superposition in space and time. Containing all elementary wavelengths of the generated initial wave spectra, the TWP-method could be used in RANSE-methods, implementing all necessary initial conditions to the CFD-solver. To reduce the calculation effort to a minimum in space and time, the superimposed wave train is generated near the investigated offshore structure by using modified, linear wave theory in spatial domain.

To present this method by means of a practical example, the motion and sloshing behaviour of an offshore LNG-carrier (LNGC) are investigated in detail. For validation purpose, all results are compared to model tests, conducted in the seakeeping basin at Technische Universität Berlin (TUB), as well as numerical results of the potential theory solver WAMIT.
Life-Cycle Assessment of Flexible Risers
Authors: Fergestad Dag , Løtveit Svein Are , Leira Bernt J.
Published: September 2014

The number of flexible pipes in operation is increasing due to new field developments and the desire to extend the lifetime of existing fields. There have been many challenges with respect to long term operation of such pipes, in many cases with the need to replace or repair before the end of the initial design life. The number of identified failure mechanisms and observed failure events is also increasing.

The present paper summarizes state of the art technology related to operation of flexible pipes and some recent research and development activities that contribute to resolving these issues. In particular, the following topics are addressed:

• Fatigue testing of flexible pipes subjected to corrosion damage during operation

• Development of improved numerical models and methods for assessment of the residual capacity of flexible pipes

• Integrity assessment of flexible pipes also applying quantified risk and reliability assessment

• Repair methods for damaged pipe sections

In addition, an outline is given of a Handbook (ref. Handbook 2014) which addresses the various topics related to life-cycle design and operation of flexible pipes, see Appendix. Future prioritized research efforts with focus on some of the topical issues given in the list above are finally addressed.
Computation of Wave Loads Under Multidirectional Sea States for Floating Offshore Wind Turbines
Authors: Duarte Tiago , Gueydon Sébastien , Jonkman Jason , Sarmento António
Published: September 2014

This paper focuses on the analysis of a floating wind turbine under multidirectional wave loading. Special attention is given to the different methods used to synthesize the multidirectional sea state. This analysis includes the double-sum and single-sum methods, as well as an equal-energy discretization of the directional spectrum. These three methods are compared in detail, including the ergodicity of the solution obtained. From the analysis, the equal-energy method proved to be the most computationally efficient while still retaining the ergodicity of the solution. This method was chosen to be implemented in the numerical code FAST. Preliminary results on the influence of these wave loads on a floating wind turbine showed significant additional roll and sway motion of the platform.