VORTEX INDUCED VIBRATIONS

Transcription

1 VORTEX INDUCED VIBRATIONS EXPERIMENTAL METHODS LECTURE 26 SEPTEMEBER 2017 Chittiappa Muthanna Research Manager, Measurement Systems and Instrumentation Offshore Hydrodynamics, MARINTEK

2 Testing of part problems Dynamics of risers are most important subject! Vortex Induced Vibrations (VIV) Structural testing

3 Current Vortex Induced Vibrations Vortex shedding The cylinder starts to oscillate f st = St U/D, St SPAR with D=30 m, U=1.5 m/s, f St 0.01 Hz (T St =100 s) Riser with D=0.30 m, U=1.5 m/s, f St 1 Hz

4 VIV problem areas Statens veivesen

5 More specifically Increased diameter and drag Risers: Reduced fatigue life Increased axial tension Increased extreme loads Increased drag SPAR: Increased global motions Increased drag (Off-set) Increased mooring line tensions (ULS &FLS) T d 2d Increased axial tension

6 Riser eigenmodes n: f 1 f 2 f 3 f 4 f 5 f 6 f To each mode, n, there corresponds an eigenfrequency, f n. The riser will oscillate when the Strouhal frequency is close to an eigenfrequency: f n f s = St U/D Hence, the speed of the current will determine which mode (n) will respond.

7 Complex hydroelastic interactions for long risers in sheared flow Riser Current profile, U Strouhal Frequency f s = St U/d Natural frequencies: f 1 f 2 f 3 f 4 f 5 f 6 Competing modes Varying current profile: Many possible frequencies of oscillation exist. Competition between modes. Difficult to predict frequency.

8 CF and IL fatigue vs. tow speed for bare riser in uniform flow

9 Two Typical VIV test set-ups 2D tests with rigid cylinder with various geometrical shapes that are either elastic mounted, free to move or with forced motion and towed in still water 3D test with long elastic cylinder with varying geometries and boundary conditions, free to vibrate. Various flow condition and current profiles may be arranged

10 Typical Test Requirements Well defined flow with small (neglectable) turbulence Correct (e.g. Froude) scaled models Accurate measurements of motion / deformation / reaction forces For free vibration test: low and documented material damping No unintentionally interference between VIV response frequencies ( Strouhal frequencies) and eigen frequencies of test rig

11 Strouhal Number vs. Reynolds Number FS MS (& CFD)

12 Rigid cylinder section (2D) tests (L/D = 10-25) Determine hydrodynamic coefficients Study in-line and/or cross-flow oscillations Study effect of VIV suppression devices

13 Cross-Flow VIV Behaviour

14 Free oscillation of rigid cylinder; amplitude vs. Re Godvardsen and Williamson Susan Swithenbank (post doc. at CeSOS)

15 Prototype Rigid Riser Section Test, Elastic Mounted 15

16 Bare Cylinder Tests Sub-critical and Critical Reynolds-Numbers Ur=U/(fn D) Rn=U D/ν 16

17 VIV Behaviour of Drilling Riser with Protection Fins 17

18 Principle sketch of one test apparatus for 2-D free oscillation tests

19 Principle sketch of a pendulum test apparatus for 2-D free oscillation tests

20 Test set-up for forced motion test in Marine Cybernetics Laboratory (MCLab)

21 Pendulum test of in-line oscillations with horizontal pipe

22 Photo of test cylinder in test apparatus Force gauge Triple-start strakes P/D=13.2 H/D=0.25

23 Cross-flow displacements (rms value/ diameter). Bare riser and straked risers 1.0 Bare pipe P/D=5, H/D=0.14 P/D=17.5, H/D=0.25 x RMS / D rms x/d [-] Reduced velocity, Vr [-] V r =U/f n D

24 Systematic study of triple-start straked risers mm 160 3D velocity vector plot based on the PIV measurements Arrows present velocity in the paper plane Colours the velocity normal to paper plane Vector map: 3D vectors, vectors (11904)Burst#; rec#: 1; 41 (6), Date: , Time: 02:43:59: Analog inputs: ; ; ; mm 2

25 Soft marine growth (slimy marine grass) Soft marine as a model Soft marine growth on a real riser

26 Hard marine growth (Shell, wart barnacle, etc.) Hard marine growth on a real riser Hard marine growth as modeled

27 Models of various cross sectional geometries

28 Using transient for extracting lift coefficient

29 Cross-flow response w/ strakes Amplitude Time

30 In-line VIV - important for free spanning pipelines

31 Lift Coefficient from Forced Motion 2D Test (Gopalkrishnan) A/ D fˆ f D / U osc

32 Flexible beam (3D) tests 3-D Pipe & Umbilical Tests (L/D =50-150) To study multimode oscillations of free spanning pipelines in uniform current 3-D Riser Tests (L/D > 300) To study multimode oscillations in uniform and sheared current

33 Instability of Faired Riser, 3 D Test Fairing Riser

34 Pure IL VIV: Free spanning pipelines Current Measured orbits From free spann model tests Small amplitudes, but still more fatigue damage from pure IL than CF in many cases

35 Coupling between IL and CF oscillations; free spanning pipelines

36 NDP High Mode VIV Test in Ocean Basin 3D tests with 38m long riser model, Do=27mm

37 NDP High Mode VIV Test in Ocean Basin Test Set-up for Uniform and Sheared Flow gondol riser gondol riser Riser model Clump weight

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39 Summary of Part II: Vortex Induced Vibrations Phenomenon & problem areas Objective of testing VIV behavior Suppression devices Determine empirical coefficients used as input to prediction codes Establish experimental data used for validation of semi-empirical codes and CFD codes Typical test set-ups 2D tests 3D tests

40 Teknologi for et bedre samfunn