Date 2012
Author Deyzen, A . F J . van, Keuning, J.A. and R.H.M. Huijsmans AcJciress Delft University of Technology
Ship Hydromechanics and Structures Laboratory Mekelweg 2, 2628 CD Delft
Delft University of Teclinology
TUDelft
Smart control of fast ships.
Part 1: A setup for automated proactive control of the thrust used to increase the operability of a small planning monohull sailing in head s e a s
by
A.F.J, van Deyzen, J.A. Keuning and R.H.M. Huijsmans
Report No. 1 8 6 2 - 1 - P 2012 Publislied in International Sliipbuflding P r o g r e s s ,
Marine Teclinology Quarterly, Volume 5 9 , Numbers 1,2, 2 0 1 2 , I S S N 0 0 2 0 8 6 8 X
Intemational
Shipbuilding
Progress
M a r i n e Technology Q u a r t e r l yIntemational Shipbuilding Progress 59 (2012) 1-19 DOT 10.3233/ISP-2012-0077
lOS Press
1
Smart control of fast ships = Part 1: A setup for
automated proactive control of the thrust used
to increase the operability of a small planing
monohull sailing in head seas
A . F J . v a n D e y z e n *, J . A . K e u n i n g a n d R . H . M . H u i j s m a n s
Ship Hydromechanics and Structures, Delft University of Technology, Delft, The Netherlands
Received 16 February 2012
Revised 20 August 2012,29 November 2012 Accepted 18 December 2012
WhUe sailing in head or bow quartering seas operators on board of small planing boats try to avoid unacceptably large vertical peak accelerations, the main limiting factor for operability, by temporary speed reductions. The operators observe the incoming wave, roughly estimate whether or not the next impact might be too severe, and i f so, they choose a certain amount of thrust reduction. Results of fuU scale trials suggested that an increase of operability may be realised using this so-called thrust control.
In this paper the concept of smart control for smaU planing monohulls sailing in head seas has been introduced. The idea of smart control is that a solution for the increase of the operabüity of small planing monohulls may be found by using automated proactive control of the thrust. The main purpose of smart control is to keep the vertical accelerations below a predefined threshold value while striving at the highest possible average forward speed during a trip. Three elements are essential for such a control system: a shipboard wave measurement system, a computational model that predicts the response and a stable control system for the thrust.
Keywords: Ship motion control, proactive control, thrust control, prediction of response, planing monohuUs, nonlinear seakeeping behaviour, f u l l scale trials
Nomenclature
Latin letters
-^Zbridge Vertical accelerations at the bridge [m/s^] ^ ^ b o w Vertical accelerations at the bow [m/s-^]
CG Centre o f gravity [ - ]
* Corresponding author: A.F.J, van Deyzen, PhD student. Ship Hydromechanics and Structures, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netheriands. Tel.: +31 15 2786603; E-maü: A.FJ.vanDeyzen@TUDelft.nl.
2 A.F.J, van Deyzen et al. / Smart control of fast ships - Part 1
Loa Length over all [ m ]
"t^ Forward speed [m/s]
1. Introduction
The demand to sail at high forward speeds i n both cahn water and i n seaway remains high. Fast transportation o f personel, passengers or goods may give ship-owners an economical advantage. For various patrol, search and rescue or mihtary operations attaining high forward speeds is essential.
Ship-owners and operators still tend to favour the planing monohull, i n particular i n various military, rescue and patrol applications. The planing monohull is w e l l -established and can be considered a proven concept (one o f the first fast monohulls appeared i n the late 1800 s). I t is a relatively uncomplicated design compared to al-ternative design concepts f o r fast ships, such as multihulls, submerged h u l l supported vessels, f o i l supported vessels and air-cushion supported vessels. Monohulls are easy to b u i l d and the operational costs are low. Planing monohulls come i n different sizes, varying between approximately 10 m up to 50 m i n length. Their manoeuvrabihty characteristics, important f o r military, rescue and patrol operations, are good, and improve w i t h decreasing length.
Sailing i n heavy weather conditions at a high f o r w a r d speed is very demanding f o r both crew and ship. W h e n sailing i n head or bow quartering seas large motions and especially large (vertical) accelerations can occur that may cause discomfort, fatigue or even serious injuries. The forces acting on the h u l l can become so large that struc-tural failure can occur. I n beam seas unpleasant r o l l motions may occur. I n f o l l o w i n g and stem quartering seas a tendency to broach may exist. This can have severe con-sequences f o r both crew and ship. I n many cases the speed has to be reduced and/or heading has to be altered. Fast monohulls can attain high f o r w a r d speeds i n calm water, but seaway imposes a l i m i t to the m a x i m u m attainable f o r w a r d speed.
A w e l l understood factor f o r voluntary speed reduction o n a planing monohull is the occurrence o f large vertical peak accelerations while sailing at high speeds i n head and bow quartering seas (see f o r example [4,5,7]). R o l l i n g i n beam seas on i t -self is not dangerous f o r the ship and crew. Broaching is statistically a phenomenon of very rare occurrence and the mechanism causing i t is complex and poorly under-stood.
Results o f f u l l scale measurements showed that the crew reacts to the extremes rather than to the significant or average values [5,7]. They do not necessarily lower the f o r w a r d speed because o f large heave and pitch motions, but rather after one or a few severe impacts on the incoming waves. The occurrence o f a large vertical peak acceleration is experienced as dangerous. I n order to avoid this, the speed is reduced. The challenge f o r designers o f fast monohulls is to explore different possibilities to increase the operability. The operability is defined as the percentage o f time a ship can operate at its design speed given the scatter diagram o f the area o f operation. The occurrence o f large vertical accelerations while sailing i n head seas prevents
A.F.J, van Deyzen et al. / Smaii control of fast ships - Part 1 3
the ship to sail at its design speed. This Hmits the operability. Fast monohulls are designed to sail at a high forward speed, but the comfort on board must not reach an unacceptable level. The crew still must be able to perform their tasks, not only w h i l e sailing, but once they have reached their destination they still need to be fit enough as well. Therefore, an increase of operability has been realised i f i n a certain sea state a higher average f o r w a r d speed can be attained, w h i l e the level o f accelerations remain acceptable.
A solution f o r increasing the operability o f fast monohulls sailing i n head seas was found b y increasing the ship's length. The other dimensions, the design speed and the functionality are kept the same. This change made i t possible to optimise the h u l l shape f o r w a r d w i t h emphasis o f reduced accelerations sailing i n head waves. The length to beam ratio o f the ship is increased, as w e l l as the length to displacement ratio, the longitudinal radius o f gyration i n pitch is reduced and the flare at the bow is decreased. This concept, called the Enlarged Ship Concept (ESC), was later further evolved into the A x e B o w Concept ( A B C ) w i t h more radical bow sections and a significantly improved operability [ 5 ] .
Increasing the ship's length is not necessarily a possible or a desired solution f o r all kinds o f planing monohuUs. The new challenge is to improve the operability o f small, fast ships (L < 25 m ) , like interceptors, patrol vessels. Search A n d Rescue vessels (SAR) and r i g i d inflatable boats (RIB's). The disadvantage o f small, planing boats is that the m o t i o n and acceleration levels become more significant, quickly imposing limits to the operability.
Exploring different possibihties to increase the operabihty o f small, planing mono-hulls is becoming an important issue. The application of the A B C has been explored f o r smaller planing monohulls, e.g. f o r a new design o f a SAR [6]. Results o f model tests proved that this h u l l geometry is effective f o r this ship. The positive effect o f the A B C on the operability, however, is expected to decrease as the ship's length decreases. The length to beam ratio and the length to displacement ratio decrease. The ship often submerges w i t h the middle sections first, where the sectional deadrise angles are small. The radius o f gyration i n pitch direction increases. The principle o f the ESC has been violated.
Active control is another method f o r improving the operability. This is i n partic-ular useful f o r fast ships due to the h i g h efficiency o f the various control devices. A possibility to increase the operability o f a planing monohull sailing i n head seas is to use active controlled stem flaps or interceptors [9,11]. Another possibility is active control o f the f o r w a r d speed, the so-called thrust control. This can be very effective f o r smaller high speed ships w i t h a h i g h power to weight ratio (specific power).
The rationale behind this method is that aheady i n practice operators on board o f small, fast ships avoid severe impacts b y applying thrust control manually. They apply thrust control proactively. Proactive control is required, because intervention is required before the ships encounters a wave that leads to an unacceptably large vertical peak acceleration. Large vertical peak accelerations have a low frequency of occun'ence. Temporary speed reductions are not required each wave encounter.
4 A.F.J, van Deyzen et al./Smart control of fast ships - Part 1
The operators observe the incoming wave. I f they anticipate that the next impact might be too severe, they reduce the speed by decreasing the thrust. The vertical peak acceleration, w h i c h is experienced by the ship and the crew, is therefore much smaller. A f t e r impact they increase the f o r w a r d speed again.
Results o f f u l l scale measurements showed that i f helmsmen are free to influence the thrust, a higher average forward speed is maintained [ 8 ] . This impUes that an increase o f operability has been reahsed. One o f the disadvantages o f this way o f applying thrast control is that the operators need to rely on their intuition and expe-rience. They can neither predict the magnitude o f the slam, nor can they determine the optimal thrast reduction. Large vertical accelerations may still occur when ap-p l y i n g thrast control manually. The reasons f o r this can be due to the oap-perator's misjudgement, loss o f concentration or fatigue. Their judgement depends on v i s i b i l -ity: Excessive amount o f spray makes i t d i f f i c u l t to use thrast control; at night i t is d i f f i c u l t to cairy out thrast control.
This proactive control has been termed smart control. A s a first step the influence of smart control on the operability using only one control variable is explored. This study is narrowed down to smart control o f the f o r w a r d speed i n head seas. This is also defined as automated proactive thrast control. The response i n 3 degrees o f freedom is considered (surge, heave and pitch motion). The main purpose o f smart control is to keep the vertical accelerations below a certain threshold value while striving at the highest possible average f o r w a r d speed. Response predictions are re-quired f o r smart control. The proactive control o f the thrast is based on predicted vertical peak accelerations.
Automated proactive thrast control is expected to be more effective than thrast control apphed manuaUy. The experience o f the operator, misjudgements, fatigue, concentration loss and poor visibility do not play a role anymore. I f the response is predicted accurately, the required amount o f thrast reduction, resulting i n a d i m m -ished vertical peak acceleration, may be determined more accurately than an operator does. The extent o f increase o f operability using automated proactive thrast control is therefore expected to be significantly greater than when a operator performs thrast control manually.
This paper presents the setup f o r smart control. Results o f f u l l scale trials, ad-dressing the current application o f thrast control and its infuence on the operability, are discussed i n Section 2 o f this paper. A preliminary criterion f o r the vertical ac-celerations at both the bow and bridge, used as threshold values below w h i c h smart control should keep the vertical accelerations, has been chosen. Section 3 proposes a setup f o r smart control and i t explains w h y i t differs significantly f r o m the more conventional ship motion control systems. Section 4 presents conclusions and rec-ommendations f o r further work.
Part 2 presents a theoretical elaboration o f automated proactive thrast control f o r small, planing monohulls. The resuhs o f simulations carried out w i t h a concepfiial model o f automated proactive thrast control are used to show that an increase o f the operability may be expected i f automated proactive thrust control is applied on board o f a planing monohull sailing i n head seas.
A.F.J, van Deyzen et al. /Smart control of fast ships - Part 1 5
2. F u l l scale trials with thrust control
2.1. Setup full scale trials
Considering thrust control, the f o l l o w i n g variables can be distinguished. The ship and the sea state are the independent variables. The control variable is the f o r w a r d speed and the dependent variable is the response o f the ship. The dependent variable of interest is the vertical acceleration. For the purpose o f this chapter i t is therefore sufficient to explore the current application o f thrust control on a typical small, plan-ing monohull, o f w h i c h i t is known that the crew generally applies thrust control.
The Royal Netherlands Sea Rescue Institution ( K N R M ) provided the D e l f t U n i -versity o f Technology w i t h the opportunity to carry out f u l l scale trials on two SAR's (including crew). The ships used were the 'Jeanine Parqui' f r o m rescue station H o e k van Holland and the 'Koos van Messel' f r o m rescue station IJmuiden. The 'Jeanine Parqui' and the 'Koos van Messel' belong to the A r i e Visser class (28 t, 18.8 m ) (see Fig. 1). They have t w o engines, each having a power o f 736 kW, yielding a specific power of 52.6 k W / t . The trials were carried out on the North Sea and the relative wave direction was head seas. Table 1 presents an overview o f the trials, sorted ac-cording by sea state. A s depicted i n Table 1 each trial day one helmsman operated the ship.
Both ships were instramented equally. D i e f o r w a r d speed was measured using a GPS. Unfortunately, the sample frequency o f the GPS was 1 Hz, w h i c h is very coarse. Accelerometers were postioned at 40% and 65% o f the total length, measured f r o m the aft. The first position corresponds w i t h the lengthwise position o f the bridge, the second is assumed to be the bow. The sample frequency o f the accelerometers was 500 Hz. The throttle and the pitch angle were also measured (sample frequency
6 A.FJ. van Deyzen et al. /Smart control of fast ships - Part 1
Table 1
Full scale trials with SAR of Arie Visser class
Trial Trial Ship Operator Sea state Hs Tp Duration
number date (m) (s) (min)
1 3 May 2011 Jeanine Parqui 1 calm 1.00 3.90 13 2 3 May 2011 Jeanine Pai'qui 1 calm 1.00 3.90 14 3 3 May 2011 Jeanine Parqui 1 calm 1.00 3.90 14 4 9 March 2011 Jeanine Pai'qui 2 moderate 2.00 5.30 13 5 22 Feb 2012 Koos van Messel 3 moderate 1.95 4.60 13 6 26 AprU 2012 Koos van Messel 3 moderate 1.80 5.80 20 7 7 Sept 2011 Jeanine Parqui 1 rough 2.40 5.20 14 8 15 Dec 2011 Koos van Messel 4 rough 2.65 5.70 20
equal to 50 H z ) . A l l signals were filtered w i t h a frequency o f 10 Hz, except the forward speed, which remained unfiltered.
The results o f all trials (except trial 1) are used to examin more closely the current application o f thrust control applied manually b y helmsmen on board o f these plan-i n g boats. A number o f tplan-ime-traces o f the throttle openplan-ing, the surge acceleratplan-ion, the f o r w a r d speed and the vertical acceleration at the bow are depicted. This is to show the relation between a reduced throttle opening and the corresponding deceleration, speed reduction and decreased vertical peak acceleration i n more detail.
The results o f trials 1, 2 and 3 are used to show the influence o f thrust control on the operabihty. During trial 1 the operator had to choose a desired throttle opening before the start o f the ran. The effect o f thrast control is illustrated b y comparing the distributions o f the vertical accelerations, taking into account the average f o r w a r d speed during the trials.
I t has been assumed that during the trials, where the operator was allowed to use thrast control, he strived to attain the highest forward speed. The operator sailed at a speed where i n his opinion the level o f accelerations was still acceptable. He tried to avoid severe impacts using thrast control, but nevertheless some unacceptably large vertical peak accelerations stih occurred during the trial. Hence, vertical peak accelerations w i t h a low frequency o f occurrence indicate what the relevant operator considered the maximum acceptable vertical peak acceleration.
2.2. Illustration ofthe current application, of thrust control
I t was observed that the operators adjust their heading and thrast when they an-ticipate an unacceptably large vertical peak acceleration. Each wave encounter they judge i f they have to reduce the f o i w a r d speed temporarily. They act on the next
oc-curring vertical peak acceleration. This implies that they have little time to effectuate control. They reduce the thrast before the crest o f the incoming wave.
I n Fig. 2 time-traces o f a part o f trial 3 are displayed. Operator 1 steered the boat. The throttle opening, the surge acceleration, the forward speed and the vertical ac-celeration at the bow are depicted. For a consistent convention w i t h Part 2 the z-axis
A.FJ. van Deyzen et al. /Smait control of fast ships - Part 1 7 100 190 195 200 205 t [ s ] 190 195 200 205 t [ s ] 30 195 200 205 t [ s ] -30' ' ' ' 190 195 200 205
Fig. 2. Typical time-traces, obtained during trial 3 (operator 1). (Colors are visible in the online version ofthe article; http://dx.doi.org/10.3233/ISP-2012-0077.)
is pointed downwards, yielding negative values f o r a vertical peak acceleration u p -wards. The surge acceleration is defined parallel to the undisturbed water line and has been approximated using the body fixed accelerations measured at the bridge. The operator most l i k e l y reacted on the peak occurring at 196 s. The reduced throttle opening yielded a significant speed reduction. I t may be assumed that this speed re-duction resulted i n a diminished vertical peak acceleration, compared to what w o u l d have happened i f the operator did not applied thrust control.
Figure 3 shows a part o f the same trial, where the first vertical peak acceleration is much larger than the f o l l o w i n g two. When comparing the throttle w i t h the vertical
8 A.F.J, van Deyzen et al. /Smart control of fast ships - Part 1 100 75 50 25 0 ^ J^^^^ , ,— i
I
280 285 290 295 t [ s ] t [ s ] 30 1 5 ' ^ ^ ' 280 285 290 295 t [ s ] II
280 285 290 295 t [ s ]Fig. 3. Typical time-traces, obtained during trial 3 (operator 1). (Colors are visible in the online version p i
of the article; http://dx.doi.org/10.3233/ISP-2012-0077.) of
acceleration the operator was i n fact too late w i t h reducing the thrust. This sequence o f a large vertical acceleration f o l l o w e d by a throttle reduction was observed more
T P
often during trials 2 and 3. This implies that the operator d i d not always apply proac-tive control, but i n a way carried out reacproac-tive control. He was alarmed by the first
lai'ge vertical peak acceleration and j u d g e d b y looking at the i n c o m i n g wave that the ^ next one or two might also be unacceptable. Hence, he stül decided to reduce speed. ^ ' Operator 2, a less experienced operator than the other three, carried out thrust ^ control i n a similar way as operator 1. The main difference was that he chose a more ^• conservative f o r w a r d speed, probably i n order to reduce the probability o f having ^
A.F.J, van Deyzen et al. /Smart control of fast ships - Part 1 9
Fig. 4. Typical time-traces, obtained during trial 6 (operator 3). (Colors are visible in the online version ofthe article; http://dx.doi.org/10.3233/ISP-2012-0077.)
unacceptably large vertical peak accelerations. Still, he had to reduce the thrust a few times during the trial.
Operator 3 applied thmst control quite often. He often reduced the t h r o t ü e opening a litde b i t (10-30%) and quite briefly (for 1-2 s). He also restored the t h r o t ü e opening before impact had taken place (see F i g . 4). This was necessary i n order to climb the wave and to maintain sufficient steering capacity. He was almost never late, meaning that he could almost always accurately forsee a severe impact coming (practically no misjudgements or loss o f concentration).
10 A.FJ. van Deyzen et al. /Smart control of fast ships - Part 1
Operator 4 had a significantly different way o f carrying out thmst control. H e constantly changed the throttle opening. A clear distinction f o r w h i c h individual peak he had reduced the thmst could not be made.
Generally speaking, each operator chose a desired speed that he would like to maintain during the trial beforehand. I f he anticipated an unacceptably large verti-cal peak acceleration he temporary reduced the speed. Choosing a higher desired forward speed implied that i t is more likely that unacceptably large vertical peak accelerations w o u l d occur. Hence, more thrust reductions were necessary.
The speed reduction that can be realised before impact is an important parameter concerning automated proactive thmst control. I t is only possible to realise sufficient diminishing o f large vertical peak accelerations i f large speed reductions before i m -pact can be reahsed. I t depends on the amount of thmst reduction, the time the thrast reduction is sustained and on the surge force acting on the ship. Ships having a large specific power are more l i k e l y to r e a ü s e a large speed reduction. A large specific power implies a large deceleration.
Based on trials 2, 3 , 4 and 7 the speed reduction, when the thrast was reduced, was i n the range o f 2-10 kts. The average speed reduction was approximately 5 kts. The individual speed reductions measured during trial 6 or 8 were not as clearly distin-guishable. The deceleration after a thrast reduction varied between 0.3 and 1.5 m/s^. F u l l t h r o t ü e reductions were not observed. Ttie m a x i m u m observed desired throt-tle reduction during these trials was around 50%. The reduction o f the thrast force, however, might be more than 50%. The relation between throttle opening, engine, waterjet and thrast force is not straightforward. The time interval the operators sus-tained a reduced thrast varied between 1-5 s.
The speed was sampled at 1 H z . A n accurate reading o f the speed variation due to wave action could not be done. A n analysis o f the surge acceleration o f all trials showed that the speed osciUation due to wave action was i n the order o f magnitude of 2-3 kts. Occasionally, an oscillation o f 4 kts was observed. I t may be concluded that i n general the speed reduction after a thrast reduction is larger than the speed variation due to wave action.
The large peaks i n Figs 3 and 4 show the typical short duration o f the vertical peak accelerations, approximately 0.1-0.2 s. The peaks i n F i g . 2 are less profound because o f the decreased f o r w a r d speed. They seem somewhat longer i n duration, but the rise tune o f these vertical accelerations was also approximately 0.1 s. Concerning automated proactive thrast control, an accurate approximation o f the magnitude o f the vertical peak accelerations, having a short duration, is o f great importance. The amount o f thrast reduction is determined by the predicted magnimde o f the vertical peak acceleration.
2.3. The influence of thrust control on the operability
The distributions o f the vertical peak accelerations at both the bridge and bow, measured during trials 1, 2 and 3, ai-e given i n F i g . 5. These figures, the so-called
A.F.J, van Deyzen et al. /Smart control of fast ships - Part 1 11
Fig. 5. Rayleigli plots of tlie vertical accelerations obtained during trials 1, 2 and 3. (Colors are visible in the online version ofthe article; http://dx.doi.org/10.3233/ISP-2012-0077.)
Rayleigh plots, indicate the probability o f exceedance o f the plotted signal. The probability is given on the horizontal axis, which is deformed i n such way that the probability o f exceedance o f Rayleigh distributed maxima and minima appear as a straight line i n the Rayleigh plot (for more information on Rayleigh plots as a t o o l f o r nonlinear analysis see [ 1 ] , pp. 4 3 - 4 6 ) . The minima o f the vertical acceleration are depicted (upward vertical acceleration, 2:-axis pointing downwards). The average forward speed during trial 1 (constant thmst) was 23 kts. D u r i n g trial 2 i t was equal to 27 kts, but during trial 3 the GPS failed halfway. The measured f o r w a r d speed f o r that part was also equal to 27 kts, although this does not say m u c h about the average speed over the complete trial.
On small, planing boats, such as these SAR's, the occurrence of vertical peak ac-celerations at the wheelhouse proved to be the l i m i t i n g criterion f o r speed reduction, not the vertical accelerations at the bow [ 7 ] . The difference i n the level o f accelera-tions between these three trials is larger at the bow than at the bridge. The vertical acceleration at the bow gets an additional contribution f r o m the pitch acceleration. The differences are therefore more profound.
I t seems remarkable that the highest level o f accelerations, both at the bridge and bow, was f o u n d when the operator was allowed to use thrast control. This has to do w i t h the so-called 'feeling o f being i n control'. When he knew that he was not al-lowed to adjust the throttle i n case o f a possible high, steep incoming wave, he rather chose his desired f o r w a r d speed more conservatively. During the second trial w i t h thrast control, trial 3, the operator remembered trial 2, so he pushed the boundaries a
12 A.F.J, van Deyzen et al. / Smart control of fast ships - Part 1 30 I 25 constant thrust <> thrust control 50 20 10 5 2 1 .5 .2.1 Probability of Exceedance [%] 30 25 20 % 10 - I 1 1 1 — I 1 — r 0 100 50 20 10 5 2 1 .5 .2.1 . Probability of Exceedance [%] Fig. 6. Rayleigh plots of the vertical accelerations obtained during fuU scale trials with SAR 'Kapiteins Hazewinkel'. (Colors are visible in the online version of the article; http://dx.doi.org/10.3233/ISP-2012-0077.)
bit further. M o r e thrust reductions were observed and a higher level o f accelerations was f o u n d .
The results obtained during trial 1, 2 and 3 are very similar to the results Nieuwen-huis obtained during her experiments [ 8 ] . The boat she used was a Dutch S A R o f the Johannes Frederik class, called 'Kapiteins Hazewinkel'. The Johannes Frederik class is the predecessor o f the A r i e Visser class. I t is smaller boat (14.6 t, 14.4 m ) and has a specific power o f 68.6 k W / t . The trials were carried out on the N o r t h Sea near the Dutch village Hoek van Holland. The sea state during the trials had a significant wave height o f 2.0 m and a peak period o f 4.6 s. The operator was operator 1, the same one as on the 'Jeanine Parqui' during trials 1, 2, 3 and 7.
Figure 6 shows the distributions o f the vertical peak accelerations at both the bridge and bow measured during two trials. D u r i n g one trial the operator was free to use thmst control, during the other he had to choose a desired throttle opening before the start o f the m n . The average forward speed during the trial w i t h a constant thmst was equal to 18 kts. D u r i n g the trial w i t h thmst control i t was equal to 22 kts. The highest level o f accelerations at the bridge was also f o u n d when the operator was allowed to use thmst control. The difference w i t h the other trial is much larger than what was observed on the 'Jeanine Parqui'. The operator could have chosen a very conservative constant speed, or he could have been very motivated to show h o w fast he could sad when he was allowed to use thrast control.
A.FJ. van Deyzen et al. /Smart control of fast ships - Part 1 13
Based on these f u l l scale trials, an unambiguous claim that thrust control increases the operability o f a small planing monohull sailing i n head seas cannot be given. Figures 2 - 4 illustrate that the operator reacted to anticipated vertical peak accel-erations w h i c h he considered unacceptable. These figures also suggest that thrust control does decrease the magnitude o f the vertical pealc acceleration, i f the f o r w a r d speed has decreased sufficiently. Overall, a significant higher average forward speed has been attained using thrust control, but at the same time the level o f accelerations was also increased. Large veitical peak accelerations still occurred during the trials w i t h thrust control. They could be ascribed to misjudgements or loss o f concentra-tion. The level o f accelerations (despite a f e w large peaks) measured during the trials w i t h thrust control, however, was still acceptable f o r the crew. During trials 2 and 3 the crew sometimes mentioned that a slam was too severe, but they generaUy d i d not complain about the accelerations they experienced. I f the operator had to choose a constant throttle opening before the trial he chose a more conservative desired f o r -ward speed, yielding a relatively low level o f accelerations and less extreme impacts. I f we therefore assume that the level o f accelerations during these trials were c o m -parable and since we have found that the average f o r w a r d speed was significantly higher during the trials w i t h thrast control, these f u l l scale trials strongly suggests that thrast control yields an increase o f operability. The extent o f increase o f oper->- ability, expressed as speed increase, was approximately 20%.
2.4. Preliminary criterion for the vertical accelerations
.s
The motivation and mindset o f the crew and the (anticipated) duration o f the j o u r -i- ney have a significant influence on what the crew generally accepts. Tiredness also e has an influence. The development of an accurate criterion, however, is not w i t h i n s the scope o f the smdy smart control. For smart control, a reliable threshold value, s that w i l l be used to show that automated proactive thrast control is able to keep the r vertical accelerations below that value, is sufficient. The question is: What can be
ii considered a reliable value?
e Figure 7 presents the distributions o f the vertical peak accelerations at the bridge and bow measured during trial 4, 6, 7 and 8. D u r i n g trial 4 the accelerometer at the e bridge failed. Figure 5 showed the distributions during trial 1, 2 and 3. The distri-3 butions show a large variation. The operators shared a different opinion about what e is considered the m a x i m u m acceptable vertical peak acceleration. For the crew on ;t board o f the S A R o f the A r i e Visser class the m a x i m u m vertical peak acceleration e lies somewhere between 6 to 14 m/s^ at the bridge and 15-32 m/s^ at the bow (see s also F i g . 5). D u r i n g the f u l l scale trials w i t h the 'Kapiteins Hazewinkel' operator 1 1 indicated a m a x i m u m vertical acceleration o f 13 m/s^ at the bridge and 25 m/s^ at
the bow [7,8].
t Based on these limited number o f trials, the preliminary criterion used f o r smart control is chosen to be 10 m/s^ at the bridge and 20 m/s^ at the bow.
14 A.F.J, van Deyzen et al. / Smart control of fast ships - Part 1
A
trial 4 0 trail 6 25 - X trial? trials 20 ^ 50 20 10 5 2 1 .5 .2.1 Probability of Exceedance [%] 30 25 20 1I
15 10/ y
éi'' ' • ' •
0 100 50 20 10 5 2 1 .5 .2.1 Probability of Exceedance [%] Fig. 7. Rayleigh plots ofthe vertical accelerations obtained during trials 4, 6, 7 and 8. (Colors are visible in the online version ofthe article; http://dx.doi.org/10.3233/ISP-2012-0077.)3. Setup smart control
3.1. Proposed control system
The M l scale trials carried out on board o f the S A R o f the A r i e Visser class teach us that thmst control is actuated before impact (proactive control o f the thmst). Con-trol is based on the anticipated vertical peak accelerations. There is l i t ü e time to effectuate control. Operators scan the wave over approximately one wave length. I f they anticipate that the next impact w i l l result i n an unacceptably large acceleration, they temporary reduce the speed. I f a sequence o f large vertical peak accelerations is anticipated, the thmst remains reduced.
I n relation to the setup o f smart control i t can be concluded that an operator: • chooses a desired forward speed that he w o u l d like to maintain during the trip
on forehand,
• observes the incoming wave and judges the wave and especially the next trough, • predicts the response using the observation o f the wave and his experience and • determines and applies an amount o f the thmst reduction that he thinks w i l l result i n a sufficient reduction of the next occurring vertical peak acceleration. I f based on this knowledge automated proactive thmst control (smart control) is set up, the f o l l o w i n g three components are essential:
A.FJ. van Deyzen et al. / Smart control of fast ships - Part 1 15
1. a shipboard wave measurement system that provides a sufficiently accurate description o f the incoming wave(s) over the next f e w seconds,
2. a computational model that predicts the response o f the ship based on the mea-sured incoming wave faster than real-time and
3. a stable control system that determines the thmst force continuously.
I n this part of the project, i t is assumed that the waves used f o r predicting the response can be measured b y state-of-the-art techniques (laser, radar, lidar) i n the near future. The aim o f the smart control o f fast ships -is to show that smart control is a feasible way o f increasing the operabihty of small, fast ships sailing i n head seas and i f so, to what extent the operability inay be increased i f automated proactive thmst control is applied on board o f a planing monohull. A thorough research into possibilities to measure the incoming wave on board o f violently m o v i n g ship having a high f o r w a r d speed is not o f interest at this stage. A f e w recent publications concerning shipboard wave measurements using a radar or lidar can be f o u n d i n [2,3,10]. Real-time wave measurements f r o m a object m o v i n g i n waves and its transformation i n time to the ship's location are still very much state-of-the-art. Concemrng smart control, the question is i f i t is possible to carry out proactive control o f one control variable (the forward speed), where the control is based on predicted vertical accelerations. The main focus lies on the response predictions and the control system f o r the thrust. The waves are assumed to be k n o w n .
The response should be predicted f o r a certain time interval, called the prediction window. The response should be predicted a number o f times each wave encounter I f i t has been predicted a number o f times, the probability that the next occurring vertical peak acceleration has been predicted i n time, leaving sufficient time l e f t to decelerate, increases. Figure 8 depicts the principle o f the control system used f o r smart control. The dashed frame represents the control system. The elements outside the frame are elements i n the real w o r l d . Real-time simulations are performed at a certain frequency, preferably at 1, 2 or 4 Hz. A t first, on time ti, the response f o r the duration o f the prediction window w i l l be predicted using an unchanged thmst. I f no unacceptable vertical peak acceleration is anticipated, nothing has to be done;
incoming wave
instantaneous forward speed, fieave and pitch
motion s i m u l a t i o n m o d e l change control settings no predicted motion response predicted vertical peak acceleration less than criterion?
apply control settings
yes
16 A.F.J, van Deyzen et al. / Smart control of fast ships - Part 1
the thrust remains unchanged. I f an unacceptably large vertical peak acceleration is predicted speed reduction w i l l be necessary. Dependent on the time available the response w i l l be predicted using a reduced thrust. For a number o f control settings the response must be predicted. I n this way, a relation between the control setting, here the thrust force, and the predicted magnitude o f the vertical acceleration can be found. The m a x i m u m possible thrast force, f o r which the predicted vertical acceler-ation is smaller than the preset criterion, is chosen. A f t e r impact the thrast may be restored to its original value, i f no new unacceptable vertical peak acceleration has been predicted.
A significant increase of operability may only be realised i f the occurrence and magnitude o f vertical peak accelerations can be predicted accurately. The computa-tional model should be able to predict the response o f a planing monohull sailing i n head seas i n 3 degrees of freedom (surge, heave and pitch motion). Especially the magnitude of the vertical peak acceleration should be predicted accurately. The re-sponse o f a planing monohull saihng i n head seas is nonlinear to the amplimde o f the incoming wave [4]. The occurrence o f an unacceptably large vertical peak accel-eration is a result o f the complex interplay between the ship, the incoming wave, the motions o f the ship before impact and the f o r w a r d speed at impact. The computa-tional model needs to incorporate the influence hereof on the response. The instanta-neous f o r w a r d speed, heave and pitch m o t i o n at ti, the starting point f o r a response prediction, are therefore relevant f o r an accurate response prediction. The predic-tions should be carried out much faster than real-time due to high relative velocity between the ship and the incoming wave.
3.2. Practical issues concerning smart control
Carrying out real-thne simulations on board o f a planing ship saihng i n waves and moreover using the outcome f o r control o f the vertical motions is a new concept. I f the predicted vertical accelerations (output computational model) prove to be i n -accurate, the chosen amount o f thrast reduction might be inaccurate. The cause o f inaccurate predicted vertical peak accelerations may be f o u n d i n two factors:
1. the computational model m i g h t not provide satisfying results, or
2. the input to the model is incorrect: measurement incoming wave and instanta-neous f o r w a r d speed, heave and pitch motion at the beginning o f a response prediction.
I f the magnitude o f the vertical peak accelerations is not predicted accurately, the extent o f increase o f operability reduces. The worst case, i f the prediction o f the vertical peak accelerations proves to be inaccurate, is that no thrast control has been applied during a trip. This yields no reduction o f the level o f accelerations. Or, the f o r w a r d speed has been reduced f o r peaks that were acceptable to begin w i t h . This results i n a very l o w average f o r w a r d speed. I t is not expected that a speed reduction increases the next occurring vertical peak acceleration. I t may therefore be expected
A.F.J, van Deyzen et al. /Smart control of fast ships - Part 1 17
that automated proactive ttrrust control does not decrease the operability o f a small planing monohuU saihng i n head seas (that using thrast control during a trip a higher level o f accelerations is observed).
The extent o f increase o f operability is expected to be larger i f a large speed re-duction before impact can be realised. The possible m i n i m u m value o f the control variable is an important quantity w i t h respect to the extent o f increase o f operabihty. I f the prediction window is large, there is more time available to decelerate. For ships having a large specific power, the m a x i m u m possible deceleration is large. Hence, the speed decrease that can be realised before impact is large.
The response predictions consume time. I f this calculation time becomes signifi-cantly large, the remaining time interval to reduce speed becomes short. The time l e f t to decelerate reduces and so the m a x i m u m speed reduction before impact. Moreover, i f the response predictions are carried out sequentially, the next prediction can only start once the previous one is finished. A large calculation time reduces the number o f predictions that can be performed each wave encounter
The time step used f o r the response predictions may also have a large influence on the predicted magnitude o f the vertical peak accelerations. For an accurate de-scription o f the peaks a small time step is desired. O n the other hand, the response predictions should be carried out faster than real-time, w h i c h imposes liirüts to the m i n i m u m time step that may be used. A trade-off exists between little calculation time (large time step) and the accuracy o f the predicted vertical peak accelerations (small time step).
The influence o f the m a x i m u m speed reduction before impact, the degree o f ac-curacy o f the predicted vertical peak accelerations, including the influence o f the calculation time step used, on the extent o f increase o f operability are important is-sues f o r further research. The influence o f incorrect input to the computational model are not further elaborated.
4. Conclusions and recommendations
4.1. Conclusions
I n this paper the concept o f smart control f o r small planing monohulls sailing i n head seas has been brought forward. The idea o f smart control is that a solution f o r the increase o f the operability o f small planing monohulls may be f o u n d b y using automated proactive control o f the thmst. The main purpose o f automated proactive thrast control is to keep the vertical accelerations below a predefined threshold value while striving at the highest possible average f o r w a r d speed during a trip.
Considering thrast control, the f o l l o w i n g variables can be distinguished. The ship and the sea state are the independent variables. The control variable is the f o r w a r d speed and the dependent variable is the response o f the ship. The dependent variable of interest is the vertical acceleration.
18 A.F.J, van Deyzen et al. /Smart control of fast ships - Part 1
The current apphcation o f thrust control used on board o f small planing boats has been investigated by looking into more detail i n results obtained during f u l l scale trails w i t h Dutch Search A n d Rescue vessels. The operators observe the incoming wave, roughly estimate whether or not the next impact m i g h t become too severe, and i f so, they reduce the thrust. Large vertical peak accelerations have a l o w frequency of occurrence. Temporary speed reductions are not required each wave encounter.
A significant higher average forward speed has been attained using thrust control, but at the same time the level of accelerations was also increased. Large vertical peak accelerations still occurred during the trials w i t h thrust control. They could be ascribed to misjudgements or loss o f concentration. I f we assume that the level o f accelerations were comparable, and since we have f o u n d that the average f o r w a r d speed was significantly higher during the trials w i t h thrust control, these f u l l scale trials strongly suggests that thrust control yields an increase o f operability.
A value f o r the m a x i m u m acceptable vertical peak acceleleration, based on a l i m -ited number o f f u l l scale trials w i t h planing monohulls, was estimated on 10 mls^ at the bridge and 20 m/s-^ at the bow. These values are used as a preliminary criterion f o r automated proactive thrust control.
The proposed setup f o r smart control contains:
1. a shipboard wave measurement system (not considered i n this study), 2. a computational model that predicts the response and
3. a stable control system f o r the thrast.
The occurrence o f an unacceptably large vertical peak acceleration is a result o f the complex interplay between the ship, the incoming wave, the motions o f the ship before impact and the f o r w a r d speed at impact (nonlinear seakeeping behaviour). The computational model used f o r the response predictions needs to incorporate the influence hereof on the response o f the ship.
Smart control is unique due to three factors:
1. the control is based on the predicted veitical peak accelerations, 2. control is actuated before impact (proactive control o f the thrast) and 3. there is littie tune f o r effectuating control.
The outcome o f simulations, carried out real-time while sailing, determine i f thrast control is necessary and how much thrast reduction w o u l d suffice. The extent o f increase o f operability may be limited i f the vertical peak accelerations cannot be predicted accurately. The time step used f o r the calculations may have a large i n f l u -ence on the predicted magnitude o f the vertical peak accelerations.
A n important factor that determines to a large extent i f thrast control may y i e l d an increase o f operability is the m i n i m u m attainable speed just before impact. I t depends on the amount o f thrast reduction, the time the thrast reduction is sustained and on the surge force acting on the ship. Ships having a large specific power are more hkely to realise a large speed reduction. A large specific power implies a large deceleration. I f the response calculations consume too m u c h time the time to decelerate reduced. Hence, the m a x i m u m speed reduction before impact reduces.
A.F.J, van Deyzen et al. /Smart control of fast ships - Part 1 19
4.2. Recommendations
The way thrust control is carried out was only investigated f o r one type o f planing boat and f o r f o u r different helmsmen. The estimated value f o r the m a x i m u m accept-able vertical acceleration has been based on a Iknited number o f f u l l scale trials. A larger data set, including the effect o f different operators and crews, different ship types and weather conditions, is desirable.
I n this part o f the project smart control of fast ships the waves are assumed to be known. The development of existing of new shipboard wave measurement tech-niques, l i k e laser, radar or lidar, f o r the purpose of smart control is very m u c h desired.
A conceptual model o f automated proactive thrast control can be setup. The re-sponse of a planing monohull can be m i m i c k e d using an elementary model. A control system that determines the desired thrast force continuously needs to be incorpo-rated. The conceptual model can be used to lay out the relation between variables associated w i t h automated proactive thrast control and to show that an mcrease o f the operabihty may be expected i f automated proactive thrast control w o u l d be ap-p l i e d on board o f ap-planing monohulls sailing i n head seas.
References
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INTERNATIONAL SHIPBUILDING PROGRESS
M a r i n e Technology Q u a r t e r l y
Volume 59, N u m b e r s 1,2, 2012
Altslracled/lDcloNCil in C o n i p e m l e x . C S A l l l i i m i n a . Scopus Contents
A.F.J, van Deyzen, J.A. Keuning and R.H.IVI. Huijsmans Smart control of fast ships - Part 1: A setup for a u t o m a t e d proactive control of the thrust used to increase the
operabil-ity of a small planing m o n o h u l l sailing in head seas 1 A.FJ. van Deyzen, J.A. Keuning and R.H.M. Huijsmans
Smart control of fast ships - Part 2: A conceptual m o d e l of
automated proactive thrust control 21 H. Amini and S. Steen
Theoretical and experimental investigation of propeller
shaft loads in transient conditions 55 A. Akinturk, M.F Islam, B. Veitch and R Liu
Performance of d y n a m i c azimuthing podded propulsor 83 Shen and M.J. Hughes
Effective i n f l o w velocity for rudder calculations 107
