Lab test 4 Seakeeping test with a model of an oil tanker

Transcription

1 Lab test 4 Seakeeping test with a model of an oil tanker The response amplitude operators (RAO) in head seas of a 1:100 scale model of a 257 m long oil tanker shall be determined by model testing in the Marine Cybernetics Laboratory (MCLab). During the tests, the model will be towed, suspended in a system of wires and springs, to avoid the complications of propulsion and steering of the model. Different groups test at different speeds, but all groups shall test the same regular waves. See list of waves and speeds below. The following parameters will be measured during the test: Wave elevation in front of wave maker Wave elevation at FP on the model Vertical accelerations at FP, CoG, and AP 6 DoF motions by use of optical position measurement system Towing force force is measured fore and aft, and the resistance is found from the difference Carriage speed. You shall determine the following Response Amplitude Operators: Heave Pitch Vertical acceleration at AP Vertical acceleration at CoG Vertical acceleration at FP Added resistance (not a normal RAO see Faltinsen Sea Loads page 149) In addition, you shall do a decay test to determine the natural period in surge, and discuss whether the measured period is OK with respect to the speeds and waves that you tested. For the tests in the irregular wave, do the following: Splice the runs to one time series. Check and if necessary correct the time series so that big jumps in the time series at the point of splicing is avoided. Compute wave spectrum and the response spectra for the same responses as you have computed RAOs for (excluding added resistance). Derive simple statistics (like st.dev., max, min, mean etc.) It is recommended to revisit the presentation made by Valentin Chabaud on data processing, and to apply his script clean_data.m. The script is available from the Lab tests-section of the TMR7 web page. A ShipX input file for the ship (model) has been made available on the web page. It can be used for computation of natural frequencies and RAO s, and/or you can use the ones attached to this document. Note that the acceleration RAO s from ShipX are divided by acceleration of gravity g. The results and discussions shall be compiled in a report. The report should contain the following: A brief description of test set-up, including model. A list of performed tests Presentation of results. This should include the requested RAO and other results (see above), and some sample time series. Comparison of RAOs from experiments and ShipX Veres computations.

2 Discussion of the results and their accuracy. Note that a rigorous uncertainty analysis is not expected here. Also, a thorough discussion of error sources is not required. Please be brief and to the point. See additional suggestions at the end of this document. Deadline for submission of the report is Friday 18 of November All tests to be run at 15 knots (0.77 m/s) Waves: All groups shall run 2 runs in the irregular wave Regular waves Wave Full scale Model scale # Wave per.[s] Length [m] Height [m] Wave per. [s] Length [m] Height [m] Amplitude [m] Irregular wave Jonswap spectrum, with gamma=3.3 Schedule Group # Test time 4 Tuesday 25/10 08:30-11:30 3 Tuesday 25/10 12:30-15:30 2 Wednesday 26/10 08:30-11:30 1 Wednesday 26/10 12:30-15:30 6 Thursday 27/10 08:30-11:30 5 Thursday 27/10 12:30-15:30 Avail. Friday 28/10 08:30-11:30 For the attached report pages from ShipX, please note that M2402 is equal to M2298. The ShipX-file was made for M2298.

3 Suggestions lab test 4 The aim of the report of labtest4 is mainly to show post-processed data. All the points asked in the text are required, but no detailed discussion is expected. However post-processing (cleaning, filtering, computing PSDs ) has to be done carefully. If you get unexpected results, it is likely due to the limiting facility, i.e. short time series, non-linear (steep) waves, tank-wall effect However, giving this as a reason to unexpected results should always come with a short justification. With unexpected we mean very different from numerical simulations. The comparison should show the same trends and orders of magnitude, no more. One has to remember that model tests are usually taken as the truth to validate numerical models upon. Here they contain uncertainties, and so do numerical simulations. Therefore a quantitative comparison is meaningless. It is good to have an overlapping period to merge the signals together. Multiply the end of the first time series by a ramp down, the start of the second time series by a ramp up, and add the 2 obtained signals to get the total signal in the overlapping region. Let X1 and X2 be the first and second time series to splice. Noverlap=2/dt; %2 seconds overlap. Play around with this to get a smooth result. Ramp=0:Noverlap; %Ramp function X1(end-Noverlap:end)= X1(end-Noverlap:end).*(1- Ramp/Noverlap)+X2(1:Noverlap).*Ramp/Noverlap; X=[X1 X2(Noverlap+1:end)]; %Spliced signal Be aware that the wave probe on the carriage feels the encounter frequency, not the wave frequency. Instead of using the theoretical periods and wave numbers for plotting the RAO, it is better to derive the from the measured encounter period. Indeed shallow water waves have shorter periods and smaller wave lengths than deep water waves. General information regarding the writing of a technical report: - Make use of your findings as much as possible to discuss your results. - Give details on what the reader (e.g. the customer of commercial tests) should be informed of and cannot guess. It may be assumptions, corrections if things did not go as planned, post-processing On the other hand the report should be kept as brief as possible. - Use scientific writing. Get the difference between an abstract and an introduction. - Do the necessary to be sure about your statements. May should appear as seldom as possible. - Standard deviation should be used for quantifying oscillation amplitudes. Neither the average of the max and min peaks, nor even that of all peaks, is an accurate averaging method. In frequency domain a standard deviation may be computed by integrating the power spectral density over the frequency range of interest. It is equivalent to filtering and using std(filtered time series). Using peaks of frequency response is wrong

4 Note that M2402 is equal to M2298. The ShipX-file was made for M2298.

5 ENCL. 1) HYDROSTATICS HULL MODEL NO.: M2298 Model Scale: Loading condition: Not given - Exp. met test Draught AP/FP: / [m] Symbol Unit SHIP MODEL Length overall L OA [m] Length betw. perp. L PP [m] Breadth moulded B [m] Depth to 1 st deck D [m] Draught at L PP /2 T [m] Draught at FP T FP [m] Draught at AP T AP [m] Trim (pos. aft) t [m] Rake of keel [m] Rise of floor [m] Bilge radius [m] Water density ρ s [kg/m 3 ] Shell plating thickness [mm] Shell plating in % of displ. [%] Length on waterline L WL [m] Breadth waterline B WL [m] Volume displacement [m 3 ] Displacement [t] Prismatic coefficient* C P [-] Block coefficient* C B [-] Midship section coefficient C M [-] Longitudinal C.B. from L PP /2 LCB [m] Longitudinal C.B. from L PP /2* LCB [% L PP ] Longitudinal C.B. from AP LCB [m] Vertical C.B. VCB [m] Wetted surface S [m 2 ] Wetted surf. of transom stern A T [m 2 ] Waterplane area A W [m 2 ] Waterplane area coefficient C W (L WL ) [-] Longitudinal C.F. from L PP /2 LCF [m] Longitudinal C.F. from AP LCF [m] Immersion DP 1 [t/cm] Trim moment MT 1 [t m/cm] Transv. metacenter above keel KM T [m] Longit. metacenter above keel KM L [m] Remarks: *Refers to L PP Hydrostatic corrections not included Appendages: None. No rudder. Turbulence stimulator: 1 mm cotton tread ShipX (RepGen version ) 20-Oct :24:24 - Licensed to: Steen (NTNU)

6 ENCL. 2) DATA-CHECK PROPERTIES Run name: Ship name: Petrobras Tanker Loading condition description: Exp. met test ShipX exported data Main dimensions (from input): Length between perpendiculars (m) Breadth (m) Draught, midship (m) Sinkage (m) Trim, + = aft (deg) Coefficients for data check etc.: Type Specified Calculated Displacement (tonnes) * Vertical center of bouyancy, KB 6.452* Vertical center of gravity, VCG * Longitudinal center of bouyancy, LCB * Longitudinal center of gravity, LCG * Block coefficient, Cb Water plane area coefficient, Cw Prismatic coefficient, Cp Mid section area coefficient, Cm * Longitudinal metacentric height, GMl * Transverse metacentric height, GMt 3.761* Roll radius of gyration, r * Pitch radius of gyration, r * Yaw radius of gyration, r * Roll-yaw radius of gyration, r * * - Applied in the hydrodynamic calculations ShipX :24:33 - Licensed to: Steen (NTNU)

7 ENCL. 3) HULL GRID Run name: Ship name: Petrobras Tanker ShipX :24:33 - Licensed to: Steen (NTNU)

8 ENCL. 4) RESPONSE AMPLITUDE OPERATORS DISPLACEMENTS RAO HEAVE η(3)/a WAVE PERIOD [sec] Exp. met. ; 10.00kn 0.0 Exp. met. ; 12.50kn 0.0 Exp. met. ; 15.00kn 0.0 Project: RAOs for Exp met ShipX :29:48 - Licensed to: Steen (NTNU)

9 ENCL. 5) RESPONSE AMPLITUDE OPERATORS DISPLACEMENTS RAO PITCH η(5)/ka WAVE PERIOD [sec] Exp. met. ; 10.00kn 0.0 Exp. met. ; 12.50kn 0.0 Exp. met. ; 15.00kn 0.0 Project: RAOs for Exp met ShipX :29:56 - Licensed to: Steen (NTNU)

10 ENCL. 6) RESPONSE AMPLITUDE OPERATORS ACCELERATIONS Position: AP RAO HEAVE ACC. η(3)/a [g-force/m] WAVE PERIOD [sec] Exp. met. ; 10.00kn 0.0 Exp. met. ; 12.50kn 0.0 Exp. met. ; 15.00kn 0.0 Project: RAOs for Exp met ShipX :32:11 - Licensed to: Steen (NTNU)

11 ENCL. 7) RESPONSE AMPLITUDE OPERATORS ACCELERATIONS RAO HEAVE ACC. η(3)/a [g-force/m] WAVE PERIOD [sec] Exp. met. ; 10.00kn 0.0 Exp. met. ; 12.50kn 0.0 Exp. met. ; 15.00kn 0.0 Project: RAOs for Exp met ShipX :32:23 - Licensed to: Steen (NTNU)

12 ENCL. 8) RESPONSE AMPLITUDE OPERATORS ACCELERATIONS Position: FP RAO HEAVE ACC. η(3)/a [g-force/m] WAVE PERIOD [sec] Exp. met. ; 10.00kn 0.0 Exp. met. ; 12.50kn 0.0 Exp. met. ; 15.00kn 0.0 Project: RAOs for Exp met ShipX :32:51 - Licensed to: Steen (NTNU)