Validation of a time domain panel code for predicting the seakeeping behaviour of a rigid hull inflatable boat

F(1ST

2017

NA\TES-FRANCE

VALIDATION OF A TIME DOMAIN PANEL CODE FOR PREDICTING

T H E SEAKEEPING BEHAVIOUR OF A RIGID H U L L I N F L A T A B L E

BOAT

Callan Bird - Defence Science and Teclinology Group (DST), Melbourne, Australia Frans van Walree - Marine Research Institute Netherlands ( M A R I N ) , Wageningen, Netherlands

Daniel Sgarioto - Defence Science and Technology Group (DST), Melbourne, Australia

Lew Thomas'^ - United States Coast Guard (USCG), Baltimore, United States Terry Turner - Defence Science and Technology Group (DST), Melbourne, Australia

This paper provides an overview o f on-going development and validation o f a numerical simulation method for predicting the seakeeping behaviour o f a Rigid H u l l Inflatable Boat (RHIB) operating in waves. A series o f model tests were performed in the Seakeeping and Manoeuvring Basin at M A R I N using a 1:6.67 scale model o f a 10.6 m RHIB. The model was equipped with instrumentation for measuring six degree-of freedom motions and accelerations, impact pressures on the console and water levels within the cockpit. The paper presents a number of validation cases for motions and accelerations in a variety o f irregular seas (up to Sea State 4, head through to following seas) at forward speeds o f up to 12 Imots (Froude number o f 0.65). Results are shown for two versions o f the simulation tool, a semi-linear and a semi-nonsemi-linear version. Directions for further work are also provided.

1. I N T R O D U C T I O N

Rigid-Hull Inflatable Boats (RHIBs) generally possess lighter structures, higher payload capacity and greater static stability characteristics relative to other vessels o f the same size and performance. These qualities have led to the adoption of the RHIB around the world as the small boat workhorse of choice for navies, coast guard and lifeboat organisations. In Royal Australian Navy (RAN) service, RHIBs provide major fleet units with a versatile and rapidly deployable small boat capability called upon for various tasks ranging from man overboard recovery to high speed interception and boarding operations. I n military service, RHIBs often need to be launched, recovered and operated in harsh seaway environments, which can result in considerable risk to both vessel and crew. A better understanding of the seakeeping motions of RHIBs would allow for this risk to be quantified and mitigated against. Numerical tools capable o f predicting the seakeeping motions of RHIBs would enable a readily available and cost effective method to investigate the safety of RHIB hull forms in a seaway. Such a tool could then be expanded to other areas of interest such as dynamic stability, slamming loads and vessel-vessel interactions during launch and recovery. For such a tool to be useful, it must first be validated with physical experiments to ensure that simulated results are representative o f real world behaviours.

Presented here is a validation study o f the time domain panel method PanShip for the numerical prediction o f RHIB motions in large irregular seas. This paper first presents a brief description o f the simulation method and an overview o f the experiments undertaken by the United States Coast Guard (USCG) at the Maritime Research Institute o f the Netherlands (MARIN's) Seakeeping and Manoeuvring Basin (SMB), before presenting a comparison o f the numerical and experimental results for the seakeeping o f a RHIB in a variety o f sea conditions.

2. S I M U L A T I O N M E T H O D

PanShip is a time domain panel method developed by M A R I N for investigating the hydrodynamic loads and seakeeping response o f high speed and non-conventional hull forms. PanShip is available in two versions, a semi-linear version (PanShip) and a semi-nonlinear version (PanShipNL). Both versions employ a linearized boundary condition at the free surface and transient Green's functions to calculate forward speed radiation and diffraction forces. A three-dimensional panel method is used to calculate hydrostatic and Froude-Krylov forces on the instantaneous submerged body, with a Kutta condition enforcing atmospheric pressure for ventilated transoms. PanShip uses several empirical and semi-empirical models to account for viscous drag (the cross f l o w drag method) and roll damping (Fast Displacement Ship or Ikeda method), propulsion (propellers or waterjets), steering (Proportional-Integral-Derivative (PID) control-based autopilots) and lifting surfaces.

In the semi-linear version, certain assumptions are made that allow the Green's function to be computed at the start o f the simulation. The submerged geometry is considered to be constant with respect to the still waterline. This results in the linear treatment o f radiation and diffraction forces. The semi-nonlinear version derives nonlinear radiation and diffraction forces by applying linear transient Green's functions in a non-linear manner by way o f a vertical hull transformation. This method requires the re-discretisation o f the geometry and Green's function calculations to be performed at each time step, demanding additional

3. M O D E L T E S T S

The United States Coast Guard (USCG) has recently established a project to develop a standard process to define operability limits for small high speed boats supporting naval missions. This undertaking has included a model testing program to examine the behaviour o f two different USCG RHIBs in extreme seas, in addition to obtaining a database o f vessel motions to assist with the validation o f various analytical and numerical tools (see van Walree and Thomas (2017) for more details).

These tests represent one o f the first model testing programs that specifically investigates the behaviour o f RHIBs in large waves. Seakeeping basins are typically designed to test ship models with scale factors o f between approximately 1 ;20 and 1:40, with the necessary testing equipment optimised for this model scale range. A RHIB model scaled to such a size would be far too small to carry the required instrumentation. Moreover, generating waves o f high enough frequency for a RHIB model would be problematic for larger model sizes. These shoitcomings were overcome during the reported program by sizing the models based on the largest waves able to be generated within the SMB facility. This resulted in a chosen model scale o f 1:6.67. The validation study presented here uses the larger o f the two tested RHIBs, the 10.6m Long Range Interceptor I I (LRI-II). Full scale and model scale particulars o f the L R I - I I are given in Table 1.

Table 1 - M a i n Particulars of the L R l - 1 1

Load Condition T l - Peri brmance T2 - Full Load Unit

Particular Full Scale Model Scale Full Scale Model Scale

Displacement 9.097 29.9 10.094 33.19 t/kg

Length W L 9.259 1.388 9.218 1.382 m

Beam W L 3.072 0.461 3.082 0.462 m

Draft AP 0.653 0.098 0.438 0.115 m

Draft FP 0.499 0.074 0.764 0.065 m

A l l tests were conducted using a free-sailing model powered by twin water jets shown in Figure 1. A t forward speed, the model was self-propelled with a constant motor R P M based on the calm water RPM-speed relationship with some allowance for added resistance in waves. Due to the free running nature o f t h e model, all equipment had to be carried on board including the position measurement system, autopilot computer, power supply, measurement instrumentation and data storage. The measurement instrumentation suite fitted to the model consisted of:

• a 6 DoF Certus optical position monitoring system

• Resistance type wave level probes on top o f collar (4 locations) and in cockpit (3 locations)

• I M U unit at the CoG for measurement o f accelerations and rotational velocities • Revolution (RPM) counters for waterjet motors

• Miniature PC for autopilot and data storage • W i F i data transmission system

Figure 1 - The L R I - I I R H I B model (left) and a photo of the model undergoing testing (right)

The testing regime consisted of calm water roll decay tests, zero speed irregular seas tests and irregular seas tests in transit. The tests were performed for two loading conditions, the performance and f u l l load condition, hregular seas tests in transit were conducted using two nominal forward speeds, 6 and 12 knots, at various headings (head through to following seas). The duration o f the tests was such that at least 180 waves were encountered for each condition. The wave conditions investigated during testing were:

• Moderate irregular waves generated using a Pierson-Moskovitz spectrum with 1.7m significant wave height and 6.9 s peak wave period (Sea State 4)

• Steep (breaking) irregular waves generated using JONSWAP spectra with 2.5 or 3.0 m significant wave height and 5.2 s peak wave period (Sea State 5)

The present investigation is limited to SS4 conditions. The more extreme SS5 conditions w i l l be dealt with in a future paper.

4. V A L I D A T I O N

For the validation o f PanShip, the motions predicted by the simulations are compared with the model test results. For the results shown i n this paper this is performed statistically, comparing the standard deviations or 1/10 significant amplitudes o f simulated and model test results. Validation studies were undertaken for both the semi-linear and semi-nonlinear versions o f PanShip. A l l simulations were conducted with the T l - Performance loading condition.

4.1. Semi-linear PanShip

A total o f 16 model tests were simulated using the semi-linear version o f PanShip. This included 6 and 12 Imot nominal speeds (Froude numbers o f 0.33 and 0.65) for a variety o f headings in Sea State 4 wave conditions as described above. Headings investigated were following seas (vessel travelling in the same directions as waves, 0°), beam seas (90°) and head seas (180°) as well as stern quartering (45°) and bow quartering (135°) headings. Waterjet RPMs were set for each run so that the mean simulation forward speed matched the realised experimental mean forward speed.

Effect o f Speed

A n estimate o f ttie experimental uncertainty for motions is 10% mainly due to statistical uncertainty and wave reflections, which are relatively large for small models. Yet these factors do not f u l l y account for the large differences observed between simulated and experimental results.

A t 12 knots the simulations provide a close match to the experiments for heave motions in beam seas (within 0.01 m, 2%) and a reasonable approximation for stern quartering seas (0.05 m, 10%). However, heave in both bow quartering seas (0.09 m, +19%) and head seas (0.15 m, +33%) is predicted. Similarly, following seas results were also considerably over-predicted relative to other headings, with the numerical value (0.49 m) far greater than that observed in the experiments (0.13 m).

Notionally, the heave to wave height ratio in terms o f standard deviation is relatively constant for the waves encountered by the model during the testing program, i.e. approximately equal to 1. This does not hold for following seas. The reason for this might be due to surf-riding during which the heave and pitch are relatively constant. Surf-riding was experienced by the model during certain 12 knot runs, but this phenomenon was not observed during corresponding 6 Imot conditions.

For all headings the simulations provide better prediction o f heave motions at 6 Imots compared to 12 loiots, with the exception o f stern quartering seas where the opposite occurs (i.e. better prediction at 12 loiots). This may imply that the nonlinear effects are dominant at the higher 12 knot speed and therefore are not adequately captured with the semi-linear version of PanShip. 1.0 0.8 • Exp - 6 kn + PS - 6 kn • Exp -1 2 Kn X PS - 1 2 kn "E Q O . 6 U l (U S O . 4 ) I < J V X X

t

Figure 2 - Heave motion standard deviation results for 6 and 12 knots in Sea State 4

Figure 3 presents the pitch standard deviation results. A t 6 Imot forward speed, there is a reasonable match between simulation results and the experiments for head, bow quarter and beam seas (6%, 1.4% and 10% respectively). The differences observed between the predicted and the experimentally obtained resuhs increased in stern quartering (24%) and following seas (32%).

A t 12 loiots the simulation results over-predicted the model test results for all headings considered, with the largest differences occurring at following seas (53%) and beam seas (237%).

6.0 5.0 - • Exp - 6 kn + P S - 6 k n • Exp - 1 2 Kn X P S - 1 2 k n X "^4.0 2 , ë 2.0 a. > < • < > _{• f} X • 1.0 O 0.0 0.0 f _{1 1 1} 0 45 Heading (deg) 135 180

Figure 3 - Pitcli motion standard deviation results for 6 and 12 linots in Sea State 4

The roU motion resuhs can be seen in Figure 4. Tlie roll motion is under-predicted at 6 knots by up to 25% for beam, stern quartering and following seas.

A t 12 knots, there is an over-prediction of approximately 27% at beam seas and 42% for bow quartering seas, but an under-prediction o f 40% in stern quartering seas. As mentioned previously, the reason for these large discrepancies may be due to increased nonlinearity that is not effectively accounted for in the semi-linear version.

The numerical simulations do not predict any roll motions in head and following seas where the experiments all experienced roll standard deviations between 0.5 and 0.6 degrees. This can be explained by the ability to specify exact symmetry in simulations while in experiments it is impossible to achieve a perfect symmetry and small deviations away from 0° or 180° degrees w i l l result in some roll motion.

10.0 8.0 "55 B 6 . 0 Q 1/1 = 4.0 o q : 2.0 - X • • + _X IJ t • Exp - 6 kn + P S - 6 kn • Exp -1 2 Kn X P S - 1 2 k n 0.0 i

1

Figure 4 - Roll motion standard deviation results for 6 and 12 Imots in Sea State 4

4.2. Semi-linear vs. semi -nonlinear PanShipNL

Figure 5 provides a comparison between the experimental data and predictions obtained using both the semi-linear and semi-nonlinear versions. In general, the semi-nonlinear method provides more representative estimates o f RHIB motions compared to the semi-linear approach, especially at the higher foi^ward speed (12 knots). However, at low speed (i.e. 6

ö 1/) > ro OJ 1.0 0.8 0.6 0.4 0.2 0.0 • E x p- 6 k n A P S N L - 6 k n XP S - S k n A A 45 ^ .90 135 Heading (deg) 180 1.0 0.8 '0.6 -I 0.2 0.0 X • E x p- 1 2 kn A P S N L - 1 2 k n | X P S - 1 2 kn .X 45 90 135 Heading (deg) 180 6.0 5.0 ^ 3 . 0 ë 2.0 1.0 0.0 O ffiExp - 6 kn A P S N L - 6 k n X PS - 6 kn A A k 45 ..90,^ . 135 Heading (deg) 180 6.0 • 5.0 "^4.0 '^3 0 ë 2.0 -1.0 -0.0 • E x p- 1 2 k n A P S N L - 1 2 kn X P S - 1 2 k n X A X A X 45 L, ^-90,^ , 135 Heading (deg) 180 10.0 8.0 -I :g6.o -J Q i/l = 4.0 K 2.0 0.0 ^-BE x p- 6 k n A P S N L - 6 k n X P S - 6 k n X A X X O 45 90 135 Heading (deg) 180 10.0 8.0 i 6 . 0 a = 4.0 2.0 0.0

Figure 5 - Heave, pitcli and roll motion standard deviation results for 6 knots (left) and 12 knots (right) in Sea State 4

4.3. Peak Accelerations

For the model tests and semi-nonlinear PanShipNL simulations the mean o f t h e 10% highest acceleration values have been determined for a location at the bow o f the model. Figure 6 shows a comparison between experimental and simulation resuhs for both vertical (Vacc) and horizontal (Hacc) accelerations. The agreement between the predictions and experiments is seen to be quite acceptable. The relatively large under-prediction o f accelerations for stern quartering and following seas may be caused by vibrations in the model due to the propulsion system.

Vertical acceleration @ 6 knots

il

I Expsriment

aPanshipNL

Heading [deg]

Horizontal acceleration @ 6 knots

i Experiment HPsnshipNL

Vertical acceleration @ 12 knots

1 »

I Experiment • PanshipNL

Heading Ideg]

Horizontal acceleration @ 12 knots

I Experiment

aPansliipNL

Figure 6 - 10% highest values of vertical acceleration (left) and horizontal acceleration (right) for 6 and 12 knots in Sea State 4

5. C O N C L U S I O N S

In general, the semi-linear version o f PanShip was able to match the experiments for the case with 6 knot forward speed operating in Sea State 4, with the exception o f heave and pitch in following seas. A n increase in speed to 12 knots generally resulted in less accurate motion predictions. Compared to the semi-linear version, the semi-nonlinear version provided more representative predictions o f RHIB motions, especially at higher speeds (12 Imots). This suggests that use o f the semi-nonlinear version is required for conditions where significant nonlinearities exist, such as wave impacts, which are more prevalent at higher speeds. This recommendation is further supported upon consideration of the results depicting the mean o f the 10% highest vertical and horizontal accelerations.

Future Work

Future work concerning the application o f PanShip for the prediction o f RHIB motions in wave includes finalising an uncertainty analysis on the model scale L R l - l I experiments, which w i l l quantify the acceptable difference between numerical and experimental results. A n investigation into viscous roll damping settings will need to be carried out to obtain accurate roll motion results for the semi-linear version at 12 knots. Similarly, the semi-nonlinear version o f PanShip currently under predicts calm water roll decay at 12 knots. This under prediction is expected to have an effect on the predicted roll motions in a seaway. Additional studies w i l l be undertaken to validate both versions o f PanShip in higher sea states, namely Sea State 5. Further validation work is planned to be performed on the 7m Juliet 3 RHIB used by the R A N which recently underwent model testing, providing a further dataset to check the

6. R E F E R E N C E S

de Jong, P. (2011). Seakeeping Behaviour o f High Speed Ships: A n Experimental and Numerical Study, Delf University o f Technology.

van Walree, F. (2002). Development, validation and application of a time domain seakeeping method for high speed craft with a ride control system. Proceedings o f the 24th Symposium on Naval Hydrodynamics.

van Walree, F., Sgarioto, D. and T. Turner (2016). Validation of a Time Domain Panel Code for Prediction o f Impulsive Loads on High Speed Ships. 31st Symposium on Naval Hydrodynamics. Monterey, California.

van Walree, F. and W. L . Thomas (2017). Validation o f Simulation Tools for a RHIB Operating in Heavy Seas. Proceedings o f the 16th International Ship Stability Workshop. Belgrade.

van Walree, F. and T. Turner (2013). Development and Validation o f a Time Domain Panel Code for Prediction of Hydrodynamic Loads on High Speed Craft. International Conference of High Speed Sea Transportation. FAST 2013. Amsterdam.