bluuebunny/crossref_metadata_2025 huggingface dataset:
Wellhead Fatigue Damage Based on Indirect Measurements
Authors: Grytøyr Guttorm , Coral Fredy , Lindstad Halvor Borgen , Russo Massimilliano
Published: October 2015

Enabling safe and reliable operations of subsea wellheads has a high priority in the global oil and gas industry. The objective of the current paper is to provide a novel method for bending moment estimates at the wellhead based on indirect moment measurements; this moment, together with fatigue properties are then used for fatigue damage estimation. Indirect bending moments are based on inclinations and accelerations measured by motion reference units (MRU) attached to blowout preventer (BOP), lower marine riser package (LMRP) and lower riser joint (LRJ) immediately above the lower flexible joint (LFJ). Also, required is the tension time history in the same period at the LRJ. The proposed methodology here can be implemented and integrated into a portal for data acquisition and visualisation.

In order to validate the proposed method for indirect bending moment estimation, strain gages have been attached to a BOP and marine riser during drilling operations offshore Norway. Strain gage readings are transformed to bending moment which is used as reference (the so-called direct moment). The proposed method is used to calculate the moment indirectly, the so-called indirect moment. The resulting indirect moments agree very well with the direct moments.
Calculation of Shell Element Failure Based on the State of Stress Inside of a Neck
Authors: Dragt R. C. , Kraus J. , Walters C. L.
Published: October 2015

Simulation of failure in thin-walled structures is critical for the correct determination of crash performance of ships and offshore structures. Typically, shell elements are used, but these elements are not able to adequately capture local failure, especially inside of a neck. This paper addresses these gaps by adapting the Bridgman (1952) model of a neck inside of a plate by making it three-dimensional and offering an estimate of the relationship between state parameters of a shell element and the geometry inside of a neck. Finally, recommendations are also made about how to interface this information with the Modified Mohr-Coulomb failure locus to create a practical algorithm for assessing failure in shell elements.
Structural Integrity of Buckled Steel Pipes
Authors: Pournara Aglaia E. , Papatheocharis Theocharis , Karamanos Spyros A. , Perdikaris Philip C.
Published: October 2015

Local distortions on steel pipeline wall in the form of buckles may constitute a threat for the structural integrity of the steel pipeline. In the present paper, experimental research supported by numerical simulation is reported to investigate the structural integrity of buckled steel pipes. A series of six (6) full-scale experiments has been carried out on 6-inch X52 pipes, followed by finite element simulations. The buckled steel pipes are subjected to cyclic loading (bending or pressure) in order to estimate their residual strength and remaining fatigue life. The finite element analysis simulates the experimental procedure for each type of deformation and loading case, in order to estimate the local strain distributions at the buckled region. Based on the numerical results, fatigue life is predicted and compared with the experimental results using an appropriate defined damage factor. The results of the present study are aimed at evaluating existing guidelines and methodologies towards appropriate assessment of local wall distortions in steel pipelines.
Added Wave Resistance Computations Using Desingularized Source and Panel Methods
Authors: Zhang Xinshu , Li Wei , You Yunxiang
Published: October 2015

A three-dimensional time-domain approach has been developed to compute large-amplitude motion response and the second-order added wave resistance for ships traveling in waves. The proposed method is an extension of a well established linear approach developed in a previous paper [1]. The numerical model is developed based on boundary integral equation, which is solved at each time step by distributing desingularized sources above the calm water surface and employing constant-strength panels on body surface. The nonlinear Froude-Krylov and wave diffraction forces are computed. Equations of motion are solved with including the effects of Euler angles. A broad range of different hull forms, including two Wigley hulls, a Series 60 hull, and a S-175 hull, are employed to validate the present computational model. By comparing the obtained numerical results to experiments, it is demonstrated that the present model using double-body basis flow can well predict added wave resistance.
First Step Towards the Co-Design of Planing Craft and Active Control Systems
Authors: Castro-Feliciano Esteban L. , Sun Jing , Troesch Armin W.
Published: October 2015

This paper takes a novel approach to the design of planing craft with active control systems (ACS) by co-designing the longitudinal center of gravity (lcg) and ACS, and compares its performance with a vessel where the lcg and ACS are designed sequentially (traditional approach). The vessels investigated are prismatic in shape. The ACS are modeled as forces on the vessel. The ACS controller is a Linear Quadratic Regulator designed using a reduced order model of the vessel. In the design, only the calm water drag is optimized. The simulated co-designed vessel had 10% lower calm water and mean seaway drag than the sequentially designed vessel. However, the co-designed vessel’s seakeeping was poorer — vertical acceleration doses 25% higher. Results indicate that the traditional sequential design approach does not fully exploit the synergy between a planing craft and its ACS; as a first step, the stability constraints should be relaxed in the design exploration.
ECAs, FE and Bi-Axial Loading: A Critique of DNV-OS-F101, Appendix A
Authors: Cosham Andrew , Macdonald Kenneth A.
Published: October 2015

Controlled lateral buckling in offshore pipelines typically gives rise to the combination of internal over-pressure and high longitudinal strains (possibly exceeding 0.4 percent).

Engineering critical assessments (ECAs) are commonly conducted during design to determine tolerable sizes for girth weld flaws. ECAs are primarily conducted in accordance with BS 7910, often supplemented by guidance given in DNV-OS-F101 and DNV-FP-F108. DNV-OS-F101 requires that finite element (FE) analysis is conducted when, in the presence of internal over-pressure, the nominal longitudinal strain exceeds 0.4 percent. It recommends a crack driving force assessment, rather than one based on the failure assessment diagram. FE analysis is complicated, time consuming and costly. ECAs are, necessarily, conducted towards the end of the design process, at which point the design loads have been defined, the welding procedures qualified and the material properties quantified. In this context, ECAs and FE are not an ideal combination for the pipeline operator, the designer or the installation contractor.

A pipeline subject to internal over-pressure is in a state of bi-axial loading. The combination of internal over-pressure and longitudinal strain appears to become more complicated as the longitudinal strain increases, because of the effect of bi-axial loading on the stress-strain response.

An analysis of a relatively simple case, a fully-circumferential, external crack in a cylinder subject to internal over-pressure and longitudinal strain, is presented in order to illustrate the issues with the assessment. Finite element analysis, with and without internal over-pressure, are used to determine the plastic limit load, the crack driving force, and the Option 3 failure assessment curve. The results of the assessment are then compared with an assessment using the Option 2 curve. It is shown that an assessment based Option 2, which does not require FE analysis, can potentially give comparable results to the more detailed assessments, when more accurate stress intensity factor and reference stress (plastic limit load) solutions are used.

Finally, the results of the illustrative analysis are used to present an outline of suggested revisions to the guidance in DNV-OS-F101, to reduce the need for FE analysis.
Testing of Full Scale Composite Journal Bearings for Offshore Underwater Applications
Authors: Dragt R. C. , van den Heuvel W. , Gelinck E. R. M. , Slot H. M.
Published: October 2015

In the offshore industry, fibre reinforced sliding journal bearings are increasingly used due to their low-maintenance and self-lubricating characteristics. To be able to use the bearings in the rough offshore environment, under heavy loading, full scale tests are essential to assess the friction and wear rate development during use. For this purpose, two full scale wear tests were performed on composite sliding journal bearings of 300 mm in diameter. These tests were performed at the laboratory of TNO Structural Dynamics in Delft, The Netherlands, over a period of eight months.

For this purpose, a test rig was designed in which a shaft runs through the journal bearing. The latter is loaded in the range 50 to 700 kN and the shaft is given triangle-like cyclic displacements. To be able to test under realistic conditions, the bearing is constantly submerged in flowing seawater. Forces, displacements and temperatures were constantly monitored to manage the system and determine the parameters.

Besides a description of the test rig, a rationale of choices made, observations during testing and detailed cycle-by-cycle friction characteristics are presented, next to generalized trends for the wear and Coefficients of Friction (CoFs) under the various loading conditions.
Effects of Third-Order Nonlinearities on the Fourier Representation of Deterministic Wave Trains
Authors: Alford Laura K. , Maki Kevin J.
Published: October 2015

The accurate estimation of extreme responses for offshore platforms and supply vessels is critical to the design process. Long time Monte Carlo-type simulations can theoretically capture extreme responses, but this type of simulation is not feasible in the early design process due to the extreme computational expense. An alternative is to use shorter simulations that are tailored to create statistically equivalent extreme responses. The latter requires judicious specification of expected extreme sea conditions.

Extreme responses can be estimated using a variety of processes, including the Design Loads Generator (DLG). Assuming a vessel response is Gaussian, stationary, and ergodic, a given response can be approximated as a sum of Fourier amplitudes with random phase angles. The DLG is a process that can calculate innumerable sets of random phase angles that result in a given extreme response or design event. Using linear-systems theory, the corresponding incident-wave phase angles are calculated from the response phase angles and the incident-wave-train profile is determined.

A fundamental question that must be answered is how can a nonlinear seaway that produces an extreme event be described such that a wave maker or numerical wave boundary condition be controlled to recreate the nonlinear and extreme seaway. A common practice employs linear wave theory to shift the design event in space and time. The shifted phase angles can then be used to drive a wavemaker with the intent of producing the desired wave train at a given point. However, nonlinearities in wave propagation result in an inaccurately generated design wave train. This paper focuses on the role of wave nonlinearity in the evolution of the wave system and how this can be accounted for in the control of a physical or virtual wave maker.
Measured Wellhead Loads During Drilling Operations: Paper 1 — Data Processing and Preliminary Results
Authors: Russo Massimiliano , Wolak Urszula , Myhre Erling , Grytøyr Guttorm
Published: October 2015

The growing size of BOPs, longer drilling campaigns on wells, and operations in harsher environments has resulted in increased challenges in properly documenting wellhead fatigue during planned or executed drilling operations. The industry has started directing its efforts toward the calibration of analytical tools which are typically adopted for predicting wellhead fatigue. The ultimate goal for achieving this ambitious scope is to identify a benchmark set of analytical results that will predict field measurements. Early on Statoil identified a major obstacle: the absence of a good and comprehensive dataset of field measurements to serve as point of reference. Statoil and Aker Solutions cooperated on a pilot project with the intent of collecting a dataset of full scale measurements during drilling operations to be used to validate and calibrate the theoretical wellhead fatigue calculation methodologies. The main objective of the instrumentation campaign was to measure sectional forces as close as possible to typical wellhead hotspots by the use of three sets of strain gauges installed on the outside surface of the conductor and on the outside of the surface casing. With the objective of collecting an exhaustive dataset of measurements, accelerometers and inclinometers were installed on the BOP, the riser adapter, the riser below the upper flex joint and on the rig. An additional set of six strain gauges was installed on the riser to record riser tension variations. Environmental conditions were logged on board the rig and by the hindcast data provider. Operational events were carefully logged. This paper presents the following:

• Data processing used for quality assurance and calibration of the measured data and the associated data challenges

• Highlights of the instrumentation system capabilities to capture salient events of a typical drilling campaign and of ad-hoc performed rig operations to calibrate and validate the measured data

• Effect of a controlled rig cross motion test, performed to evaluate quasi static loads on the well and calibrate strain gauge sensor orientations

• A riser pull test, performed to validate strain gauge functioning

• Several landing and disconnecting of the LMRP

• Manipulation of the preload between the high pressure housing and the low pressure housing to investigate the effect of the preloading on the load sharing between the casings

Since King and Soloman [2], the industry is still lacking quality field data to be used in order to validate the various analytical models used in the analyses of subsea conductor and wellheads. The results will confirm the quality of the measured data and will represent a first data point of comprehensive measured field data. This data will be used for future required work in calibrating the different building blocks pertaining to the analytical tools dedicated to well head fatigue predictions [3].
Data Mining Pt. Reyes Buoy for Rare Wave Groups
Authors: Seyffert Harleigh C. , Troesch Armin W.
Published: October 2015

This paper addresses the existence of rare wave groups, as defined by Kim and Troesch [1], by examining time series data from the Pt. Reyes buoy. The buoy is operated by the Coastal Data Information Program (CDIP), University of California San Diego. The definition of rare wave groups [1] used in this paper differs from the more commonly used wave group definition based on threshold crossings. With the time series data from the Pt. Reyes buoy, these rare wave groups are shown to be a naturally occurring phenomenon. The nature of the data is examined, as well as the analysis methods and findings. By sifting through 17 years of wave elevation data from the Pt. Reyes buoy, this preliminary work addresses not only the question to what extent rare wave groups exist in nature, but also, what their probability of occurrence is.
Validation of Volterra Series Approach for Modelling Parametric Rolling of Ships
Authors: Somayajula Abhilash S. , Falzarano Jeffrey M.
Published: October 2015

Parametric motion is the phenomenon where a structure is excited into large amplitude motion even when there is no direct excitation. A well-known example of this type of motion is the parametric roll of ships in head or following seas. Parametric roll of container ships in head seas is relatively a new problem which has gained much importance after the catastrophic incidence of APL China in 1998. Although a lot of analytical techniques are available on the assessment of parametric roll in regular excitation, not many investigations have explored its occurrence in irregular seas. A consensus on the stability criteria to assess the danger due to this phenomenon in actual ocean has not yet been reached making it an active area of investigation.

A precursor to the development of stability criteria is a simple model to capture the phenomenon of parametric rolling. However, it is important that the model is not over simplified and ignores important dynamics of the process. Therefore it is necessary to perform validation studies between the simplified model and the complete nonlinear model capturing all the physics of the phenomenon.

This paper provides the validation studies of a 1-DOF (degree of freedom) simplified model for roll motion against a standard 6-DOF time domain simulation approach. The 1-DOF model is based on the Volterra series representation of the hydrostatic stiffness in waves while accounting for the heave and pitch motions of the model. It also includes a nonlinear damping model capturing the radiation and the viscous damping. The 6-DOF model solves for the nonlinear equations of motion based on Euler angles and also includes the nonlinear Froude Krylov excitations and nonlinear hydrostatic forces on the vessel. Details of the modeling in the two approaches are described and comparisons are performed to assess the validity of 1-DOF simplified model.
Estimation of Risk of Progressive Drifts in a Wind Farm Caused by Collision of Drift Ship
Authors: Hirokawa Eiji , Suzuki Hideyuki , Hirabayashi Shinichiro , Muratake Minon
Published: October 2015

In off-shore wind turbine, it is difficult to determine the risk of accident caused by the mooring destruction through experiment. In this paper, the authors discuss the risk, with the case of a drifting ship wanders into the wind farm. In the design of a floating offshore wind turbine (FOWT), drift of a FOWT is considered as a serious failure mode and the mooring system must be designed to avoid the failure. The failure of mooring line is not initiated just by extreme environmental load but can be initiated by collision with a drifting ship, which enters the wind farm. This phenomenon is difficult to investigate by a tank experiment. So far, little knowledge exists about the phenomenon. In this research, a simulator to reproduce the collision process of a FOWT and a drift ship and a progressive drift of FOWTs in a wind farm was developed. Using this simulator and statistics of drift incidents of a ship, a procedure to evaluate risk of progressive drifts in a wind farm was established. In that case, second accident that a wind turbine which has started drifting caused by the drifting ship collides with one another wind turbine is expected. As a result, the risk mainly depends on the risk of drifting caused by a large displaced ship. In addition, the risk partly depends on the arrangement of wind farm.
FEM Analysis of the Ultimate Strength of Aluminum Stiffened Panels With Fixed and Floating Transverse Frames
Authors: Li Chenfeng , Ren Huilong , Zhu Zhiyao , Guedes Soares Carlos
Published: October 2015

The aim of this study was to analyze the ultimate strength of stiffened aluminum panels by the non-linear finite element method. A new type of stiffened aluminum alloy panel has been designed which has fixed longitudinal and alternating floating transverse frames. Based on material tensile tests, the material properties of the aluminum alloy were obtained. Then, the simulation method of welding residual stresses and the effect of heat-affected zone are investigated. The FEA software ABAQUS V6.11 is used to estimate the ultimate strength of these stiffened panels under axial compression. The results show that: 1) The mechanical imperfections have significant effect on ultimate strength of stiffened panels; 2) residual stresses may have positive effect on ultimate strength; 3) the new stiffened panels also have good performance on ultimate bearing capacities.
A Revised Methodology for the Calculation of Wellhead Fatigue due to VIV
Authors: Williams Dara , O’Hara Niall , Dunne Kenneth
Published: October 2015

In regions of high current velocity such as East Africa, South America and the GoM, VIV (Vortex Induced Vibrations) of drilling riser systems is a very real risk. Much work has been carried out by the offshore industry in order to predict the VIV and resultant fatigue in top tensioned production and drilling risers and to develop standardized methodologies for performing these calculations. In addition a number of commercially available VIV analysis tools have been developed over the years. These VIV analysis tools and industry methodologies however, are focused on the global response of the riser string alone and have not been developed to account for the intricacies of wellhead loading & stress distributions. VIV response of the top tensioned riser (TTR) system (whether production TTR or drilling riser) results in load transfer into the wellhead via VIV induced motions. As commercially available VIV analysis tools generally only consider a riser model consisting of a slender beam, the more complex geometry and load path associated with the wellhead and conductor are not accurately modeled. Thus VIV induced damage of the wellhead cannot be predicted with confidence using these VIV analysis tools alone.

Based on a review and critique of both existing and proposed methods it has become apparent that existing methods for the calculation of VIV induced fatigue of the wellhead and conductor system are overly conservative and do not capture the level of detail required to predict the level of fatigue damage with confidence. Thus a revised methodology for calculation of wellhead and conductor fatigue due to VIV is required to build up a more representative picture of the associated fatigue damage accumulation. Proposed VIV fatigue calculation methodologies, including the use of VIV analysis tools alone and application of the results of the VIV analysis tools in combination with non-linear global finite element models, are described and the challenges associated with each are discussed herein.
Sensitivity Study of Hydrodynamic Derivative Variations on the Maneuverability Prediction of a Container Ship
Authors: Shenoi R. Rajita , Krishnankutty P. , Selvam R. Panneer
Published: October 2015

The study of maneuverability of a ship involves the determination of the hydrodynamic derivatives in the equations of motion. The standard maneuvers are simulated by integrating the equations of motion and the maneuvering parameters are checked for compliance with appropriate standards set by IMO. The numerically or experimentally predicted hydrodynamic derivatives may differ from actual values of the built and operated ship. Hence, it is worth to understand the sensitivity of these variations on the actual maneuvering performance of the ship. This paper deals with a study on the sensitivity of the hydrodynamic derivatives in the equations of motion of a container ship (S175). The sensitivity analysis of all the hydrodynamic derivatives is performed by deviating each derivative in the range of −50% to +50% from the experimentally derived values, in steps of 10%. The standard maneuvering tests like turning tests at rudder angle, δ = 35° and 20°/20° zig-zag maneuvers are performed for each case and their effects on the standard maneuvering parameters are estimated. The hydrodynamic derivatives that are important and which have to be estimated with high level of accuracy in maneuvering studies for a container ship are identified through this study.
Diffraction and Instability of Short-Crested Limited-Length One-Dimensional Coherent Wave Trains
Authors: Babanin Alexander V. , Waseda Takuji
Published: October 2015

Alternative representations of the wave field (as opposed to superposition of Fourier components) are possible. In this paper, behaviour of short-crested limited-length one-dimensional coherent wave trains is investigated. Experiments were conducted in the three-dimensional wave tank of the University of Tokyo. Description of the directional wave tank and its capacity to generate short-crested coherent wave trains, including those concurrent, superposed and directionally-superposed is provided. If the crest is shorter than the lateral extent of the wave tank, diffraction tends to redistribute the wave energy into clear surfaces, and thus energy of the wave trains is reduced and the modulational instability bandwidth changes correspondingly. Rates of such nonlinear lateral spread are estimated, and they are proportional to mean wave steepness. To avoid the diffraction, in further tests concurrent trains were mechanically generated, each of which occupied half of the lateral width of the wave tank and had the same energy as another half. The trains had the same frequency, and in order to keep them separate phase shift of 180 degrees was used. Sideband growth was significantly impaired by comparison with the long-crested evolution of the train with the same steepness.
Fatigue Crack Propagation Rate Test of Q235 Steel in Ship Hull
Authors: Wang Yitao , Feng Guoqing , Ren Huilong , Li Chenfeng , Lin Yi
Published: October 2015

A series of crack propagation rate tests of compact tension specimen for Q235 steel are conducted. Compliance method and pixel method are introduced to measure the crack length, and the comparison of the two methods is proposed. Incremental polynomial method and least square method are applied to fitting and regressing the test data. By statistical analysis, the statistical characteristics of crack propagation parameters C and m are obtained and the fatigue propagation rate described by Δ CMOD and ΔK is proposed. On the basis of probability fracture mechanics theory, the functions of crack propagation rate under different probability standards are presented.
Dynamic Response Analysis of Spar-Type Floating Wind Turbines and Mooring Lines With Uncoupled vs Coupled Models
Authors: Xu Xue , Srinil Narakorn
Published: October 2015

Offshore floating wind turbines (OFWT) are supported by the flexible mooring systems subjected to nonlinear hydrodynamic wave and current forces. Depending on the floater type and environmental condition, the mooring responses can have a significant impact on the overall dynamic performance of OFWT. To evaluate the dynamic responses of OFWT, both uncoupled (quasi-static) and coupled (dynamic) mooring models have been proposed in the literature and in practice based on the use of the well-known FAST software and the FAST-Orcaflex package, respectively. This paper will investigate and compare the dynamics of the OFWT and the mooring lines using uncoupled vs coupled models, based on the OC3-Hywind Spar platform supporting the 5MW wind turbines developed by the National Renewable Energy Laboratory. Preliminary numerical studies in several load cases reveal substantial differences in the OFWT and mooring dynamics obtained by the two approaches, e.g. under regular and irregular waves. The levels of differences are reported, and the comparisons with available experimental results are also made to validate the model analyses and outcomes. The importance of mooring line dynamics and their contributions to the overall 6-DOF responses of OFWT are highlighted which should be recognised in the analysis and optimization design.
Investigation of Parametric Resonance in Roll for Container Carrier Ships
Authors: Ghamari Isar , Faltinsen Odd M. , Greco Marilena
Published: October 2015

Parametric roll resonance is of concern for container and fishing vessels, especially in head-sea waves. Here this phenomenon is investigated with a numerical method based on potential-flow theory with viscous corrections for the roll damping. The seakeeping problem is handled by considering a strip theory and assuming a 5-DOF system. Nonlinearities are accounted for in the Froude-Krylov and hydrostatic loads. The solver has been validated against experiments on a C11 class container carrier ship in terms of parametric resonance occurrence and features for different ship forward speeds and headings, wavelengths, wave amplitudes and wave headings. The overall agreement is good but there are some discrepancies. For instance, the simulations show capsizing in some cases while it does not happen in the experiments. The results from present method can be used to generate 2D and 3D polar diagrams identifying the zones with parametric roll occurrence, and are very handy for masters aboard ships. This type of information is valuable at design stage and can be used aboard vessels for a safer voyage.
Optimising Numerical Wave Tank Tests for the Parametric Identification of Wave Energy Device Models
Authors: Ringwood John V. , Davidson Josh , Giorgi Simone
Published: October 2015

While linear and nonlinear system identification is a well established field in the control system sciences, it is rarely used in wave energy applications. System identification allows the dynamics of the system to be quantified from measurements of the system inputs and outputs, without significant recourse to first principles modelling. One significant obstacle in using system identification for wave energy devices is the difficulty in accurately quantifying the exact incident wave excitation, in both open ocean and wave tank scenarios. However, the use of numerical wave tanks (NWTs) allow all system variables to be accurately quantified and present some novel system tests not normally available for experimental devices. Considered from a system identification perspective, this paper examines the range of tests available in a NWT from which linear and nonlinear dynamic models can be derived. Recommendations are given as to the optimal configuration of such system identification tests.
Springing and Ringing Response of Offshore Triceratops
Authors: Chandrasekaran Srinivasan , Nassery Jamshed
Published: October 2015

This paper presents a numerical study of ringing and springing response of a relatively new Offshore Triceratops. The platform is subjected to irregular waves using JONSWAP spectrum by varying the significant wave height, average wave period, vertical asymmetric factor and the over shoot parameter. Strongly asymmetric waves, which are capable of causing impact loads on triceratops, are used to simulate ringing in offshore triceratops. Heave response shows bursts but there are no rapid built ups. Gradual decay, which is seen in the present case, resembles a beat phenomenon.
Carcass Tearing in Flexible Pipes
Authors: Hansen Rasmus , Lyckegaard Anders , Cappeln Christian , Mcgeorge Dag , Sødahl Nils , Bendiksen Erik
Published: October 2015

This paper presents a novel model of carcass tearing in flexible pipes. The model is based on a simple parameterization of the pipe design in terms of isotropic layers, together forming the composite pipe structure. The model allows evaluations of interfacial shear stresses between the inner pipe layers, as well as axial normal stress and strain levels in response to gravitational and thermal loading.

Interfacial slip of a given interface, i.e. axial sliding of adjacent layers relative to each other, is accounted for by introducing a maximal possible value of the interfacial shear stress for a given interface, amounting to a static friction capacity. The model shows how a cut-off of the shear stress at the shear stress capacity implies an interfacial slip, which is followed by a significant increase in axial strain of the carcass layer.

Detailed quantitative results of the model are presented for a particular 11.5 inch K-carcass riser design. In order to improve engineering practice, an analytical expression of the governing shear stress is derived in terms of the gravitational and thermal loads. This analytical expression is easily applied for particular design evaluations.

The model directs attention to critical design parameters related to the carcass tearing failure mode and thereby supports continued safety in the design and operation of flexible pipes from a carcass tearing perspective.
Probabilistic Analysis and Risk Assessment of Deep Water Composite Production Riser Against Fatigue Limit State
Authors: Singh Manander , Ahmad Suhail
Published: October 2015

Deep water composite risers are subjected to randomly fluctuating loads, induced by wind and waves in the presence of fluctuating axial tension which may be critical in deep sea conditions. Therefore, risers experience the extreme bending and randomly fluctuating stresses throughout their service life. Cumulative fatigue damage is a critical assessment of riser life in the presence of large dynamic stresses. Probabilistic analysis and risk assessment of composite risers for cumulative fatigue is a vital design requirement for its satisfactory service and survival for stipulated period. Without addressing the reliability assessment, composite risers may not be recommended for deep water oil and gas exploration and production. Hence, the reliability assessment is a critical issue that is to be addressed for the safety of the deep water composite riser. It is studied for the entire system for all possible sea states occurring in the exploration region. Unlike conventional risers, the wall structure of a composite riser is more complicated. Therefore, multiple failure mechanisms are used jointly to assess the safety of the composite riser. Fatigue reliability is a challenging task due to complex nature of dynamic response and associated uncertainties caused by the material and external loads. The present study is focused on reliability assessment using stochastic finite element analysis. Response time histories for random sea plus current have been obtained. Requisite numbers of sea states are considered for the simulation of a wide range of off-shore environment and estimation of accumulated damage. By using the S-N data, damage fractions are calculated then summed linearly using Miner-Palmgren rule. The total damage has been obtained by summing the accumulated damages over all the sea states under consideration. Non-linear limit state function is derived based upon the above given approach to calculate the fatigue life. Important uncertainties associated with random variables are considered while deriving the limit state function. Numerical methods, such as Monte Carlo simulation and Advanced First Order Reliability Method, are used for the calculation of the reliability. The sensitivities of various random variables on overall probability of failure have been studied and design points have been located on failure surface. Probabilities of failure for important parameters are investigated.
Pushover Load Factors for Fixed Steel Platforms Sensitive to Topside Load and Wave-in-Deck Load
Authors: Fraser Ramsay , Manzocchi Mark , Gibson Richard
Published: October 2015

The objective of this paper is to present recent work in the development of a method for the strength assessment of offshore jacket structures subject to storm loading that is consistent across all global offshore regions.

Recent developments in the application of response based metocean analysis together with the establishment of a standardized structural reliability model has resulted in an assessment method that can be applied in a consistent manner to all fixed jacket structures in all global regions. Applying this method in the “assessment” stage of the SIM process, as published by API and ISO, would aid global standardization of both the assessment methods and the performance criteria.

The assessment methods have evolved over the last 3 years and have been subject to application and testing on several projects involving the assessment of existing offshore jacket structures. They have proven to be capable of discriminating between the differing extreme environments and differing evacuation and unmanning procedures that occur in different regions while being compatible with existing approaches to structural integrity. The paper will describe the standardized structural reliability model and provide examples of load factors for pushover and pushdown analysis for fixed steel offshore platforms that have a storm load capacity that is sensitivity to topside load and wave-in-deck load.
On the Wavemaker Theory for Two Fluids
Authors: Abadie Thomas , Dias Frédéric , Cox Ted , Ghidaglia Jean-Michel
Published: October 2015

The wavemaker theory for a single layer fluid is a classical chapter of books on water waves. Indeed, linear waves with a desired amplitude, wave length, and frequency can be generated with the horizontal motion of a vertical plate, which is estimated from the wavemaker theory. However, there is much less work on the wavemaker theory when two fluids are superposed. Here we consider a variant of the classical wavemaker theory, which consists in a semi-infinite container with two layers of fluid of finite depths, bounded above by a rigid roof and on one vertical side by a piston. The lower fluid is incompressible (typically a liquid) and the upper fluid is a lighter gas (or liquid), which is considered to be incompressible as well. When the piston is set in motion, waves are created at the interface between the liquid and the gas. Resulting wave profiles and velocity fields based on linearized theory will be shown and compared to the classical one fluid wavemaker theory and to numerical simulations of wave generation with two layers of fluid. Two approaches will be presented, a stationary one, which is the equivalent to the wavemaker theory encountered in books in the case of a single layer, and the initial value problem that takes into account the transient motion from water and wavemaker at rest. The effects of both the density ratio, as well as the effects of the rigid roof are analyzed.
Analysis and Probabilistic Modeling of the Unstationary Ice Loads Stochastic Process, Based on Experiments With Models of Offshore Structures
Authors: Zvyagin Petr , Sazonov Kirill
Published: October 2015

Experiments with models of platforms and offshore structures with vertical and inclined panels, which were conducted at Krylov Research Center (St. Petersburg), demonstrated that sometimes ice loads time series registered in these experiments cannot be considered as stationary. At the same time until nowadays methods and algorithms of probabilistic modeling were mainly based on the assumption of ice loads time series stationarity. That is because the analysis and modeling for stationary stochastic process is easier than for those unstationary.

In the paper the method for determining the presence of unstationarity in ice loads time series, based on statistical analysis, is described. This method employs sample mean normality. Fuzzy C-means algorithm is used to cluster autocorrelation vectors, which are built for different fragments of time series. In the paper ice loads time series, got in experiments in ice tank with offshore structure columns and basement models, are investigated on their unstationarity. The algorithm of unstationary ice loads time series simulation is offered.
Risk-Based Evaluation of Subsea Pipeline Leak Detection Technologies
Authors: Hillier Alan , Imtiaz Syed , Khan Faisal , Thodi Premkumar
Published: October 2015

Continuous monitoring of subsea pipelines is the best way to detect leaks quickly. A number of systems and technologies exist for this purpose. The present work describes an analytical approach to making decisions related to best technology selection. The proposed approach uses a risk-based methodology for identifying the best technology. The key focus of the present work is to develop a method to quantify uncertainties involved with leak detection technologies on subsea arctic pipelines and use the quantified uncertainty in decision-making. The proposed methodology considers a broad range of factors. It gives an overall risk, as well as, revealing the strengths and weaknesses of the leak detection technologies in different subcategories. The proposed methodology is demonstrated using a case study.
Qualified Assessment for Lifetime Extension of Flexible Risers
Authors: Leira Bernt J. , Løtveit Svein Are , Berge Stig , Fergestad Dag , Langhelle Nina
Published: October 2015

There have been many challenges with respect to long term operation of flexible pipes, in many cases with the need to replace or repair them before the end of the initial design life. The number of identified failure mechanisms and observed failure events is also increasing.

In the present paper, some typical issues that arise during the process of integrity assessment and qualification for lifetime extension are outlined. More detailed considerations are illustrated in relation to a particular case study. This involves elaboration of the different steps which are required in order to be able to extend the lifetime of a flexible riser for the case of corrosive annulus environments. The particular case of an outer sheath damage is applied for the purpose of illustration.
A Risk Management Framework for Unmanned Underwater Vehicles Focusing on Human and Organizational Factors
Authors: Thieme Christoph Alexander , Utne Ingrid Bouwer , Schjølberg Ingrid
Published: October 2015

This paper presents a risk management framework (RMF) for unmanned underwater vehicles (UUV). UUV are tethered or untethered vehicles operating in hazardous surroundings. The RMF, as one of the first for UUV, emphasizes the need for considering human and organizational factors impact on risk. A case study on risk assessment has been carried out, covering the risk of loss and mission abort of an autonomous underwater vehicle. It is based on a case study of the Applied Underwater Robotics Laboratory of NTNU, which operates several UUV. The assessment applies human reliability analysis, fault tree analysis, and event tree analysis. The case study identified measures for risk reduction, such as improvement and adaptation of procedures for maintenance, planning of missions, and fault recognition and solving.
Sediment Attractors: Seabed Shear Stress Shadows Around Subsea Pipelines Cause Net Sediment Accretion
Authors: Zhao Fuyu , Griffiths Terry , Shen Wenwen , Draper Scott , An Hongwei , Leggoe Jeremy , Carneiro Daniel
Published: October 2015

This paper presents interpretation of the results of 2D CFD modelling using ANSYS Fluent, which has been undertaken for a parametric range of over 200 cases, including over 60 different seabed geometries, pipe diameters and seabed roughnesses as well as a range of steady current, wave and combined wave / current cases. Through analysis of the results including evaluation of seabed shear stress amplification factors compared to far-field ambient values, integration across the seabed of seabed shear stresses and bedload transport potential, the conditions under which sedimentation can be expected are predicted.

The results have relevance to improving our understanding of sedimentation (backfilling) around subsea pipelines under live bed conditions, since the presence of shear-stress deficits or shadows leads to enhanced accretion of sediment in the region of a pipeline, even where there is localised amplification of shear stress right next to the pipe. The results are expected to enable better approaches design of subsea pipeline stability on erodible seabeds, or on impermeable rocky beds where veneers of mobile sediment are present.
Numerical Study of the Added Resistance of Ship Advancing in Waves Using Far-Field and Near-Field Methods
Authors: Hong D. C. , Kim J. G. , Song K. H. , Lee H. K.
Published: October 2015

When a ship advances in a seaway, it undergoes 6-degree-of-freedom motion. The ship motions and wave loads are very important from operability and survivability points of view. The resistance increase due to waves is also important from the economic point of view. Although the accurate prediction of these seakeeping characteristics should be done using the unsteady CFD computations, the analytical method based on the linear potential flow theory have been widely used to evaluate them at the early design stage since the latter does not require large computing resources. In the present paper, the added resistance of a ship advancing in waves was calculated using both Maruo’s far-field formula and the near-field method. The radiation-diffraction potential over the wetted surface of the ship has been obtained using the three-dimensional frequency-domain forward-speed free-surface Green function (Brard 1948) and the forward-speed Green integral equation (Hong 2000). Numerical solutions are obtained by making use of the 9-node second-order inner collocation boundary element method (Hong et al. 2014b). In the present paper, Maruo’s far-field formula was combined with the exact three-dimensional Kochin function so that the added resistance thereby obtained could show good comparison with experimental results over the entire wave range including the short wave range. The near-field added resistance is the time mean value of the 2nd-order forces acting on the advancing ship in waves. The time-mean hydrodynamic force, obtained by using direct integration of the hydrodynamic pressure due to the sum of the unsteady potential and steady potential approximated by the double-body potential over the wetted surface of the ship, was also presented. Comparison of the present far-field and simplified near-field numerical values and the experimental values reported by Journee (1992) of the added resistance for the Wigley ship models I and II has been made in order to find appropriate numerical values of the far-field added resistance over the entire frequency range of interest.
Volume Averaging of Multiphase Flows With Hydrate Formation in Subsea Pipelines
Authors: Torbati Reza Zeinali , Naterer Greg F. , Odukoya Adedoyin
Published: October 2015

This paper presents new predictive models of multiphase flows in undersea pipelines to describe multiphase flows of gas/water and gas/hydrate particles through a pipeline. The conservation equations of mass, momentum and energy are formulated and simplified based on the flow condition in the pipe of interest in this paper. The equations are solved semi-numerically to determine the phase fractions, gas density, velocity, pressure and temperature. The predictive models are validated against benchmark cases with mixture properties, and the comparison exhibits relatively good agreement.