Experimental studies of springing and whipping of container vessels

Transcription

1 Experimental studies of springing and whipping of container vessels Ole Andreas Hermundstad CeSOS Highlights and AMOS Visions Conference 27-29th May 2013 in Trondheim

2 Outline Background and motivation Description of containership models and instrumentation Test program and sample results Conclusions

3 Pitch [deg], Rel. wave [m], VBM/50 [knm] Background and motivation Wave impact in the bow may give large high-frequency hull girder vibrations (whipping). This occurs in rough seas and adds to the wave-frequency loads Hence, the loads under such impacts will generally be the most severe wave loads experienced by the ship. Whipping will also contribute to fatigue Pitch Rel. wave elev. VBM amidships Press. lower panel Press. upper panel Pressure [kpa] time [s]

4 Background and motivation (II) If the frequency of the wave loads coincide with one of the natural frequencies of the hull girder, resonant vibrations (springing) will occur. This will contribute to fatigue Wave elevation Bending moment Power spectrum Ordinary wave loads Resonance (springing) Frekvens (Hz) 4

5 Background and motivation (III) Ultra Large Containerships (ULCS) Larger ships Larger natural periods in bending and torsion Larger ships generally go faster More high-frequency wave loads Larger bow flare More nonlinear wave loads Little experience with large (>9000 TEU) containerships in N. Atlantic / N. Pacific. Classification rule formulae uncertain. Need for model tests with realistic designs JIP : DNV, CeSOS,, BV and Hyundai (HHI) Objective: Perform model tests with two large containerships in realistic conditions to provide data for further analysis: How important is springing and whipping wrt. fatigue and extreme loading? Which sea-states contribute the most? How well do available numerical methods predict the loads and responses?

6 Two large containerships 8600 TEU TEU Lpp [m] B [m] Design draught [m] Displacement [ton] node bending freq. [Hz] Torsional freq. [Hz] Damping ratio [%]

7 Design of models Criteria: Realistic natural frequencies in the 2-node vertical mode and in torsion Measure 6 DOF forces and moments at 3 locations Low damping Speeds up to 27 knots Significant wave heights up to 11.5 m To be tested in head and oblique seas To be towed

8 Ship model concepts Fully elastic Backbone Hinged

9 Fully elastic models The hull itself is made flexible Realistic deformation pattern Expensive to produce Difficult to adjust flexibility Forces and moment measurements require extensive strain gauge instrumentation and calibration

10 Elastic backbone models Flexibility modelled with an elastic backbone Hull is segmented Responses measured at the backbone (strain gauges) Difficult to adjust stiffness after production 10

11 Segmented models Flexibility is only in the hinges Relatively easy to manufacture Hinges can be made with adjustable stiffness -> Model can be calibrated to give the correct vibration frequencies in the lowest modes Forces and moments measured close to the hinges New challenge: Include torsional flexibility top view MS A B C D E longitudinal at centre line propeller motor A B C D E horizontal at 267 mm above base line 11

12 Segmented containership models 4 segments. 3 flexible connections Forces and moments are measured close to the flexible connections.

13 Requirements to the flexible connections Minimum damping Flexible in vertical bending and torsion Adjustable stiffness Low complexity, robust

14 Design of frame and connection details Verified using finite element analysis

15 Flexible connection

16 Instrumentation 6 DOF motion measurements (optical system) Accelerometers (x,y,z-directions) Wave probes (conductive and acoustic types) Slamming panels Pressure gauges Force/moment transducers close to the flexible connections Measure towing speed and towing forces

17 Slamming panels in the flare Sampled at 4800 Hz

18 Accelerometers Located fore, aft and amidships

19 Wave probes Some fixed to the model Some fixed to the carriage

20 Optical motion measurement system Pressure gauge Slamming panel Wave probes

21 Pressure gauge for green water events

22 Test set-up Towed model Towing connection at the aft segment

23 Test program Decay tests To measure eigenperiods and damping Forward speed tests in calm water To measure steady forces and moments Forward speed tests in regular waves For comparison with numerical predictions Forward speed tests in irregular waves Focus on realistic conditions In head seas: 16 sea-states (4 Tp and 4 Hs values) + 2 extreme In oblique seas (13000TEU only): 9 longcrested + 9 shortcrested Speed is adjusted to obtain a realistic speed in each sea-state Test duration: At least 30 min in each sea-state. Up to 3 hours for selected extreme conditions.

24 13000TEU model in head seas

25 13000TEU model in oblique seas

26 Spectrum of VSF (N 2 S) VSF (N) VSF (N) VSF (N) 4 x 10 7 Project 8600TEU Run 4161 Comparison of low pass f iltered VSF at Cut3 ( < 0.35Hz ) TEU Time(s) x 10 7 Comparison of high pass f iltered VSF at Cut3 ( > 0.35Hz ) 4 2 VSF V=24 knots Hs=5.5m, Tp=10.5s Time(s) x 10 7 Comparison of total VSF at Cut Time(s) x Comparison of Calculated and Measured Power Density Spectrum of Total VSF 15 Calculation 10 Measurement Frequency (Hz) 26

27 References with analysis of measured data Focus on fatigue and extreme hull girder loads Storhaug et al. Consequence of whipping and springing on fatigue for a 8600TEU container vessel in different trades based on model tests. PRADS Storhaug et al. Consequence of whipping and springing on fatigue and extreme loading for a 13000TEU container vessel based on model tests. PRADS Storhaug et al. Effect of whipping on fatigue and extreme loading of a 13000TEU container vessel in bow quartering seas based on model tests. OMAE 2011.

28 References where measured data is used for validation of numerical tools Wu et al. Comparative Study of Springing and Whipping Effects in Ultra Large Container Ships, ITTC Workshop on Seakeeping Suji Zhu, Investigation of Wave-Induced Nonlinear Load Effects in Open Ships considering Hull Girder Vibrations in Bending and Torsion, Ph.D. thesis CeSOS, 2012.

29 Conclusions Most of the fatigue damage comes from high-frequency hull girder vibrations, caused by whipping and springing Changing the ship's course is from head seas to bow quartering seas is not effective to reduce fatigue Wave energy spreading does not reduce fatigue significantly Extreme VBM was higher in oblique seas than in head seas, and well above the IACS rule values. Whipping was found to give a significant contribution to the extreme vertical, torsional and horizontal bending moments. Current numerical tools do not completely capture all mechanisms that produce the whipping and springing loads.

30 References Storhaug G., Choi, B-K, Moan, T. and Hermundstad, O.A Consequence of whipping and springing on fatigue for a 8600TEU container vessel in different trades based on model tests. Proc. PRADS Storhaug G., Malenica, S., Choi, B-K, Zhu, S. and Hermundstad, O.A Consequence of whipping and springing on fatigue and extreme loading for a 13000TEU container vessel based on model tests. Proc. PRADS Storhaug G., Derbanne, Q., Choi, B-K, Moan, T. and Hermundstad, O.A Effect of whipping on fatigue and extreme loading of a 13000TEU container vessel in bow quartering seas based on model tests. Proc. OMAE Wu, M-K, Hermundstad, O.A. and Zhu, S. Comparative Study of Springing and Whipping Effects in Ultra Large Container Ships, Proc. ITTC Workshop on Seakeeping Zhu, S., Hermundstad, O.A. Iijima, K. and Moan T. 2010, Wave-induced Load Effects of a Backbone Model under Oblique Seas in a Towing Tank. Proc. PRADS Suji Zhu, Investigation of Wave-Induced Nonlinear Load Effects in Open Ships considering Hull Girder Vibrations in Bending and Torsion, Ph.D. thesis CeSOS, 2012.