When a current impacts on a circular pipe, fluctuating forces are created due to vortex-shedding in the wake. In offshore industry the interest for vortex induced vibration (VIV) is focused on the fatigue that pipe can experience. The fatigue due to VIV can be the dominant fatigue in some pipelines, subsea structures and risers.
The assessment of VIV fatigue for pipelines and subsea structures involves two steps. The step one is to determine the structures response under the combined current and/or wave conditions, using analytical, semi-analytical or numerical approach. The second step is to identify the hot spots in the structure and process their loading states to calculate the corresponding damage or life, such as using S-N curve, which is a common practice in the industry. Since simulating the structures response under fluid loads is the key challenge in the VIV fatigue assessment, this paper focuses on the first step with an emphasis on the FSI approach.
The current study developed a finite element model in the frame work of the ABAQUS that can be used in the cross flow VIV analysis of the pipelines and jumpers. The fully three dimensional computational fluid dynamics (CFD) solutions are combined with structural models of pipeline and jumper to predict vortex induced motion. The use of three dimensional CFD solutions is aimed to eliminate the guess work for VIV analysis. The proposed method uses finite element methods that are tolerant of sparse meshes and high element aspect ratios. This allows economical solutions of large fluid domains while retaining the important features of the large fluid vortex structures. The method can also be extended to sheared currents whose velocity varies with depth. The proposed method is applied to pipeline and jumper and benchmarked against published results. It also confirms the validity of the simplification of a jumper to a straight pipe during jumper VIV analysis based on DNV RP-F105, which is a common practice in offshore industry. The developed model might be used to reduce the conservatism in the fatigue assessments of pipeline guided by codes, such as DNV RP-F105.
Wittkower, Bob (J P Kenny) | Poblete, Ben (Cameron) | Botto, Adriana (Wood Group Integrity Management) | Garcia, Jose (Wood Group Integrity Management) | Singh, Binder (Wood Group Integrity Management) | Jukes, Paul (MCS Kenny Houston)
The 2010 Deepwater Horizon explosion and loss and the Piper Alpha Disaster of 1988 resulted in sweeping changes across the industry, both in their respective countries and around the world. Changes were in reaction to these major events from the event itself and the governmental actions that compelled changes to the industry. The events were too horrific for the public to contemplate or to absorb and governments projected these public reactions into regulations and new oversight structures to allow the industry to move forward: protect people, environment and property: and restore economies, public trust and work. The events define themselves uniquely by their own circumstances but each contain similar reactions to tragedy, grief, anger, reaction, response, questions and answers, followed by resolve, regulation and restoration. Latent in these histories is an unsolved and pragmatic question: Could each of these disasters have been avoided where management shapes a permanent safety culture to not lose the rationale or articulation of risks? An answer is provided to us by governmental mandated reorganization and regulations that bind a cohesive framework that endeavors to fit an industry into expanded and ever encompassing tasks. The better answer is for industry to organize management actions now that create predictable future successful outcomes which I paraphrase with the catch phrase “Create Futures Today”. That is, each company creates its own management plan which is incentivized, organized, staffed and resourced to deliver an expected safety culture in the future, at expected timeframes, through management champions and a comprehensive and integrated integrity management (IM) program. This program operates within the regulatory framework which has its external factor in shaping safety culture. INTRODUCTION Trevor Kletz’s many influential and proactive safety publications provide reference points in time by reason of their effect on safety culture and organizational safety process.
Chica, Leonardo (University of Houston, Department of Mechanical Engineering Technology) | Pascali, Raresh (University of Houston, Department of Mechanical Engineering Technology) | Jukes, Paul (MCS Kenny) | Ozturk, Burak (MCS Kenny) | Gamino, Marcus (University of Houston, Department of Mechanical Engineering Technology) | Smith, Kevin (University of Houston, Department of Mechanical Engineering Technology)
Wind turbines and renewable energy devices are important components for thefuture of energy sector. This is due to fast depleting sources of oil and gasall around the globe. Offshore wind turbines offer numerous advantages, due totheir remote location from land, thereby reducing noise, higher power output(due to large wind speeds) and utilization of existing offshore platforms fortheir installation and operation.
This paper presents a general analytical solution for calculation of dynamicbending stresses induced in a wind turbine tower as a result of rotatingimbalance in the rotor blades. The formulation is based on elastic beam theory,with inclusion of rotating imbalance forces. The formulation neglects the bladegyroscopic effects and models only the lateral tower vibrations.
The dynamic stresses are highly important for fatigue evaluation of the steelwind turbine towers. The formulation models the linear elastic response of thetower under the effect of rotor imbalance, and proceeds to compute the dynamicbending stresses as a result of lateral vibrations. The model also includes theeffect of linear viscous damping at the rotor-nacelle at the top of tower, andshows the beneficial effect of damping to suppress vibrations and reducebending stresses. The damping may arise from sources such as aerodynamic dragand internal steel structural dissipation. The motivation of this technicalpaper is to provide a simple analytical desktop tool for an engineer toevaluate the dynamic stresses for a given wind turbine size (tower height,diameter, wall thickness, blades and nacelle mass, rotating imbalance androtational speed), for an initial dynamic stress and fatigue screening, beforeproceeding to perform more comprehensive aero-structural finite element modelsof the wind turbines for simulating the dynamics of the wind turbines.
Wind turbines; dynamics; rotordynamics; bending stresses; dynamic stresses;vibrations; fatigue; critical speed
Currently, the so-called state-of-practice approaches are commonly used inriser VIV analysis. DNV RP F204 has indicated that riser axial stress fatiguedue to VIV is not considered due to the limitation of the state-of-practiceapproach. This paper gives a methodology for considering the top tensionedriser (TTR) axial stress fatigue due to VIV, using nonlinear coupledbeam-column modeling, and proposes a procedure, using Flexcom and Shear7, toconduct TTR axial stress fatigue damage analysis. Case studies are discussed,including TTR VIV in the Gulf of Mexico (GoM) and West of Africa (WoA).
On the other hand, API RP 2RD suggests that TTR with relatively stifftensioning systems may experience tension fluctuations that are significantrelative to the mean tension, leading to significant changes in the lateralstiffness. Further, it was found in field tests in Norway that if VIV frequencycoincides with half the TTR axial mode frequency, extreme axial stressesresult. This paper demonstrates that the phenomenon observed in the field testis due to the Mathieu effects. Using Mathieu theory on TTR axial stressresonance due to VIV is a novel idea and this paper provides a novelmethodology to assess Mathieu Instability (MI), specifically, stabilitydiagrams with damping effects in parameter plans are generated. These diagramsare intended to cover possible combinations of TTR properties, such aspre-tension, mass, damping, axial and bending stiffness etc. Finally, thispaper illustrates applications of new method in total VIV fatigue analysis,including axial fatigue, and MI engineering assessment.
The Ocean Current Monitoring from 500 - 1,000 meters study is part of theTechnology Assessment & Research (TA&R) program of the Bureau of OceanEnergy Management, Regulation and Enforcement (BOEMRE) of the U.S. Departmentof the Interior. The objectives of the study were:
• To assess the characteristics of Gulf of Mexico current, forcing in the 500to 1,000 meter water depth range and occurrence of elevated events; and
• To use those data to assess the importance of such currents on the fatigueand design of risers, moorings and TLP tendons.
The study was conducted as collaboration between MCS Kenny, Fugro GEOS, andPrinceton where the first objective of the study was addressed by Fugro GEOSand Princeton. The first objective is a topic for another paper and it will notbe discussed in detail in this paper. This paper addresses the second objectiveonly and was undertaken by MCS Kenny.
Fugro GEOS study describes how BOEMRE's NTL ADCP data, historical mooringcurrent data and Princeton University's PROFS model data were used to derivethe current profiles to be used in the riser/Tension Leg Platform (TLP) tendonVortex Induced Vibration (VIV) fatigue study described in this paper.
In particular, based on the analysis of current characteristics, ocean currentkinetic energy distribution and observational data availability; fourrepresentative zones (1-4) for Gulf of Mexico were indentified. Zone 1 and zone3 represented the area dominated by Loop Current/Loop Current Eddies andhurricane generated currents. Zone 2 represented an area with relatively strongcurrents found near the continental rise and slope in the northern Gulfespecially where isobaths converge are narrow. Zone 4 represented the new frontarea corresponding to the high kinetic energy at 500 m water depth. For each ofthe zones, both long term and short term full water column current profilecharacterizations (represent current profiles and associated probabilities)were derived and provided for the use in the riser VIV fatigue study, by FugroGEOS.
These current profiles were then used as a part of the study performed by MCSKenny, to address the second objective. In particular, the current profilesprovided by Fugro GEOS were used in determining whether monitoring oceancurrent between 500 m and 1,000 m water depth range should be required and usedin assessing VIV fatigue damage of risers (Drilling, SCR, TTR, and Hybrid) andTLP tendons in the GoM.
As a part of the methodology to achieve this objective was to compare the VIVfatigue damage calculated based on the following current characteristicsbetween 500 m and 1,000 m:
• Existing Current Profile: Current profile typically assumed by designers,with the lack of actual recorded profiles, which is generally an extension ofthe current velocity at 500, dubbed as linear current profile;
• New Current Profile: Current profiles, as monitored and provided by FugroGEOS.
Depending on the relative values of the VIV damage resulting from eithercurrent profile, a conclusion can be drawn as to the importance of monitoringand using current data between 500 m and 1,000 m in assessing VIV damage ofrisers and tendons in the GoM.