Flow Induced Vibration A Review of Current Assessment Methods

Transcription

1 Flow Induced Vibration A Review of Current Assessment Methods David Fielding, Matt Straw (Norton Straw Consultants) Alex Graham, Phil Shorter (CD-adapco)

2 Introduction Presenting a joint study into flow-induced vibration (FIV) by Norton Straw Consultants CD-adapco Summary of work presented more fully at Institution of Mechanical Engineering Offshore Engineering 2015 Designing for Flow Induced Vibration with Simulation - Webinar on CD-adapco website (linked from Norton Straw news page) Flow Induced Vibration (FIV) and assessment Energy Institute Guidelines approach Use of CFD & FEA (STAR-CCM+ & wave 6 )

3 Background to FIV Flow Induced Vibration - flow internal to structures that, due to a number of mechanisms, may cause vibration of the structure If oscillation frequencies are close to the natural frequencies of the structure, resonance can occur leading to excessive and potentially damaging vibration e.g. fatigue or fretting We will present a case study that : Introduces current energy industry practice Demonstrates what simulation has to offer us

4 Case Study CLAMPED SUPPORT DEADLEG INTERNAL DIAMETER = 43 mm INLET CLAMPED SUPPORT DEADLEG (NON- FLOWING BRANCH) THERMOWELL 1 (INTRUSIVE ELEMENT) INTERNAL DIAMETER = 87 mm FLOW DIRECTION THERMOWELL 2 (INTRUSIVE ELEMENT) CLAMPED SUPPORT OUTLET

5 Typical Approach EI Guidelines Energy Institute guidelines allow the potential for FIV to be assessed * A case study is presented initially using the EI guidelines approach The same case is then evaluated further using CFD and FEA methods * Guidelines for the avoidance of vibration induced fatigue failure in process pipework, Energy Institute, 2 nd edition

6 Case Study Parameter Case 1 Case 2 Case 3 Gas flow rate m³/h Liquid flow rate m³/h Effective density kg/m³

7 Case Study Results EI Guidelines Mechanism Case 1 Case 2 Case 3 Flow induced turbulence - -! Flow induced pulsation!!! Slug flow Low liquid flow rate not estimated to occur (flow maps) VIV from intrusive elements -!! Small bore connections!!!! Further action required: redesign, further detailed analysis and/or vibration monitoring - Further action: only visual inspection for good, as analysed construction

8 Case Study Results EI Guidelines Corrective actions suggested by the EI guidelines vary for the likelihood of failure (LOF) predicted: Redesigning or re-supporting the line or Carrying out detailed analysis and/or Carrying out vibration monitoring Detailed analysis is not defined Change in design is often the most simple action (and then re-evaluating using the EI guidelines approach) Design changes may be not be possible Re-design may be unnecessary if the EI approach is providing an over-conservative answer So what detailed analysis could we do?

9 What can CFD and FEA offer us? CFD: Local flow modeling of specific phenomena e.g. Flow-induced turbulence Impact of intrusive elements Pulsation Cavitation Multiphase flow e.g. presence of slugging System-wide modelling CFD to assess flow mechanisms and input to Finite Element Analysis (FEA) to assess Natural frequencies of system How the flow-induced forces may excite the system Coupled CFD & FEA analysis could offer the complete solution

10 Local Model - Intrusive Elements The EI guidelines use the following equation to predict a vortex shedding frequency: St = fl/v This equation is based on a long cylinder in free stream flow. Is this what we have? Not in most cases Not in our case study VORTEX SHEDDING AROUND CYLINDER IN CROSS-FLOW

11 Intrusive Elements SKETCH OF THERMOWELL GEOMETRY Using CFD need to be consider: Time step size Grid resolution Turbulence modelling FLOW AROUND INTRUSIVE ELEMENT IS DIFFERENT TO LONG CYLINDER IN FREE STREAM

12 Whole System Analysis CFD simulation can be used in conjunction with a finite element structural model Here STAR-CCM+ (CFD) was used with wave 6 (FEA) CFD to assess the frequencies of flow phenomena FEA enables the assessment of whether flow frequencies coincide with natural frequencies of the system A transient flow solution from STAR-CCM+ imported for random vibration analysis in wave 6 a frequency domain aero-vibro-acoustic solver Summary of this presented here

13 Whole System Analysis Flow modeling: Unsteady (time accurate) Volume of Fluid method for multi-phase flow (VOF) Large Eddy Simulation (LES) turbulence model Ideal gas law applied to gas phase, liquid phase assumed to be incompressible Objectives: Predict internal flow regime Record transient wall pressure data, apply this to FE structural model in wave 6

14 Whole system analysis: CFD Domain Flow development region 20m upstream Domain extended 5m downstream in area of interest The main fluid region was meshing using polyhedral elements with a body fitted prism layer. Total element count = 2M cells Flow Direction Average cell size in core mesh ~4mm

15 Whole System Analysis: CFD Flow regime for case study for case 1 All three cases exhibit annular flow regime Some low frequency pumping visible in deadleg

16 Whole System Analysis: CFD The intrusive element interacts with the fluid film and gas streams (vertical thermowell, case 2 shown) An unsteady pumping mechanism results From time domain data & visual inspection it is difficult to discern characteristic spatial and frequency content

17 Frequency Domain Analysis To quantify the risk of structural vibration, CFD results can be processed in the frequency domain. STAR-CCM+ has built in functions to perform spectral analysis on monitored data Fluctuating surface pressures can be compared to modal frequencies of the structure to estimate the response wave6 is a frequency domain aero-vibro-acoustic analysis tool which can read in transient wall pressure data from STAR-CCM+. wave6 can be used to create coupled vibro-acoustic models that combine Finite Element, Boundary Element and Statistical energy analysis methods. Structural/Acoustic FE model is directly excited by the fluctuating surfaces pressures calculated in CFD. Acceleration and Stress RMS levels calculated

18 RMS Pressure on Pipe Wall Case 1 Overall RMS level Flow direction B STAR-CCM+ surface pressure data mapped on wave6 model RMS pressure shows ~30dB dynamic range Highest levels associated with: Separated flow at the bend Near the first thermowell Around the deadleg tee C A A B C

19 Spectral Diagnosis of RMS Pressure on Pipe Wall Case 1 4Hz 32Hz RMS pipe wall pressure shown as a function of frequency Highest levels at lower frequencies with distinct spectral peaks around 4 Hz Second peak around 32 Hz

20 Cause of 4Hz Peak Case 1 Pressure PSD at 4 Hz Pressure fluctuations at 4Hz concentrated on sharp edges on which the annular flow impinges B The liquid holdup trace from CFD run confirms that this is roughly the frequency of fluctuation in liquid loading C B C

21 Cause of 32Hz Peak Case 1 Pressure PSD at 32 Hz Pressure fluctuations at 32Hz comes from the first thermowell pumping The second thermowell does not produce a clear response

22 Structural Modes Free Response Mode 1 17 Hz Mode 2 39 Hz Mode 3 42 Hz Mode 4 60 Hz

23 Random Vibration Response Case 1 32 Hz 37 Hz 17 Hz 0.54 grms 0.49 grms 0.25 grms 0.24 grms Acceleration response is maximum at modal peaks Structure does not respond to general excitation at 4Hz 32Hz excitation in thermowell 1 is just visible in system level response

24 Stress Response Case 1 1 MPa RMS 37 Hz 17 Hz 0.85 MPa RMS 0.77 MPa RMS 0.45 MPa RMS Highest Von-Mises stress responses occur at structural resonances (17Hz, 37Hz and 42Hz) near the support locations Very small contribution from thermowell excitation at 32Hz

25 Case Study Summary EI guidelines identifies flow-induced vibration due to a range of mechanisms. Issues identified can be further investigated by detailed analysis using CFD & FEA. The component-level CFD analysis for intrusive element showed the VIV issue identified in EI guidelines was not present. System-wide analysis using CFD & FEA showed: Characteristic frequency of multiphase flow around 4Hz Unsteady phenomena in thermowell found to occur at 32Hz Natural frequencies of system did not correspond to flow Using STAR-CCM+ coupled with wave 6 little flow-related issues were identified

26 STAR-CCM+ / wave 6 workflow STAR-CCM+ was run on a cluster in the CD-adapco office in Hammersmith, London The 2M cell model was run on up to 64 cores depending on availability. Runtime for 8s of simulation time was two weeks per case on 32 cores, and just over one week on 64 cores. TRN files updated every 1E-4s, giving a frequency resolution of 0.25Hz to 5kHz Longer simulation time is not required for the frequency range of interest, and could be shortened. wave 6 was run on a standard workstation Set up of basic case/workflow: ½ day Basic forced response calculated in one hour, if the workflow exists Further post processing / solving can extend this time, overall << 1 day

27 Conclusions This presentation outlines the approach of using the Energy Institute guidelines to assess flow-induced vibration. Detailed analysis can be used for: Component-level analysis e.g. vortex shedding from intrusive elements System-wide analysis combining both CFD & FEA The system-wide analysis showed: It is possible to use STAR-CCM+ to determine frequency of pressure fluctuations. wave 6 can use the pressure fluctuations to provide magnitudes of response and stress information to be predicted. The STAR-CCM+ & wave 6 approach is relatively low in computational expense. A fully coupled flow and structural response analysis is also now possible example in later presentation.