NTNU Norges teknisknaturvitenskapelige

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1 Postadresse: NTNU Norges teknisknaturvitenskapelige universitet RAPPORT Fakultet for informasjonsteknologi, matematikk og elektroteknikk Institutt for teknisk kybernetikk NTNU Institutt for teknisk kybernetikk O.S. Bragstads plass D N 79 TRONDHEIM Rapportnummer -3-T Gradering OPEN ISBN Sentralbord: Instituttet: Telefax: Rapportens tittel Observer and Controller Design for an Offshore Crane Moonpool System Dato Revised May Antall sider og bilag 3 Ansv. sign. Saksbearbeider/forfatter Tor Arne Johansen and Thor Inge Fossen Prosjektnummer Oppdragsgiver Norsk Hydro Oppdr. givers ref. Svein Ivar Sagatun Ekstrakt A control system for active wave synchronization and heave compensation in heavy-lift offshore crane operations is described. reduces the hydrodynamic forces via minimization of the relative speed between payload and moonpool waves using a wave amplitude measurement. Experimental results using a scale model of a semi-submerged rig with a moonpool in the NTNU MC lab shows that wave synchronization leads to significant improvements in performance. Depending on the sea state and payload, experiments indicate that typical reduction in the wire tension standard deviation is between and 5 %. Experiments with regular waves and irregular waves (JONSWAP spectrum) are described and analyzed in detail. Acknowledgements We are grateful to Marintek and NTNU for giving us access to the MCLab in Trondheim where the experiments were conducted. We are also grateful to Norsk Hydro AS for supporting this project. Stikkord Offshore Moonpool Cranes Marine Operations Wave feedforward

2 Contents Background 3 Dynamics 3 3 Control system 3. Control objectives Frequencies Control strategy Active heave compensation Observer and Filter Design 8. Heave position and velocity estimator Wave amplitude velocity estimator Combined Tension and Motor Speed Observer Motor Speed Observer (No Tension Measurements)....5 Combined Payload Position and Velocity Observer Payload position measurement Motor encoder measurement Payload position reconstruction in the frequency domain Experimental setup and instrumentation Experimental results. Regular waves at T =:5 s and A =cm.... Regular waves at T =: s and A =cm JONSWAP spectrum at T =: s and A =3cm Simulations 3 8 Discussions 9 Full scale implementability Suggestions for future work Conclusions

3 Background Higher operability of installations offshore of underwater equipment will become increasingly more important in the years to come. Offshore oil and gas fields will be developed with all processing equipment on the seabed and in the production well itself. The lower cost in using subsea equipment compared to using a floating or fixed production platform is penalized with lower availability for maintenance, repair and replacement of equipment. Production stops due to component failure are costly, hence a high operability on subsea intervention is required to operate subsea fields. High operability implies that subsea intervention must be carried out also during winter time, which in the North Sea and other exposed areas implies underwater intervention in harsh weather conditions. The objective of the marine operation is to safely install a product on the seabed. One of the critical phase of the installation is the water entry phase on which this report focuses. We study active control of heave compensated cranes or module handling systems (MHS) during the water entry phase of a subsea installation, where it is of interest to minimize loads due to the hydrodynamic forces caused by interactions between payload and waves. The principal idea is to utilize a wave amplitude measurement in order to compensate directly for the water motion due to waves inside the moonpool. Dynamics We will only consider the vertical motion of a payload moving through the water entry zone. The payload is handled from a floating vessel. The vessel is kept in a mean fixed position and heading relative to the incoming wave only moving due to the first order wave forces. Effects from the vessel s roll and pitch motion are neglected. In this section we describe the dynamics of a laboratory scale model moonpool crane-rig (scale :3). Figure shows a setup consisting of an electric motor and a payload connected by a wire that runs over a pulley suspended in a spring. We notice that this setup contains no passive heave compensation system or damping device. Figure shows the coordinate system definitions: z = rig position in heave (m) z = payload position (m) z m = motor position (m) = moonpool wave amplitude, relative to mean sea level (m) = moonpool wave amplitude, relative to vessel-fixed position (m) m = payload mass (kg) F t = wire tension (N) The coordinate systems of z and z m are positive downwards and fixed in the crane rig, while the coordinate systems of and z are positive upwards and Earth-fixed. All have the origin at the still water mean sea level. In [], the following transfer functions from motor speed _z m to payload position z and wire tension F t are derived z _z m (s) = F t _z m (s) = m!3!!3! s» s +! s +! s s +! 3» s +! s +! s s + 3! 3 s +! 3 where the resonance frequencies satisfies the relationship! 3 <!. The value! 3 = 8:8 rad/s is the experimentally determined wire resonance frequency with a nominal payload mass of.57 kg when the payload is moving in air, [3]. When the payload is partly or fully submerged, the hydrodynamic force must be taken into account. This leads to increased damping and contributions from added mass and its time derivatives. Depending on the size, shape, mass, position and velocity of the payload, significant () () 3

4 Figure : Experimental :3 scale model of vessel with moonpool.

5 Figure : Definition of the coordinate systems. 5

6 reduction in the resonance frequencies (! ;! 3 ) and increase in the relative damping factors ( ; 3 ) are experienced. A typical value of! 3 with the payload in submerged condition is 3 rad/s to rad/s, [3]. Typical values for the damping factors are 3 ß :5 and slightly higher, when moving in air [3]. Identified models of these factors under submerged conditions are also found in [3]. Theoretical relationships between! ;! 3 ; and 3 can be found in []. A first order model of the transfer function from the reference motor speed _z d to the motor speed _z m is found in []: _z m (s) = e :s (3) _z d +:s The time-delay is mainly due to digital communication and control within the motor drive and control units. 3 Control system 3. Control objectives The performance measures of interest are the following: ffl Wire tension; The minimum value must never be less than zero to avoid high snatch loads, and the maximum value and variability should be minimized. ffl Payload acceleration; Peak values should be minimized as this gives an indication of impulsive forces on the payload. ffl Payload impulse; The impulsive hydrodynamic forces on the payload should be minimized. 3. Frequencies The following are the key frequencies in the system: ffl! 3 =8:8 rad/s is the experimentally determined wire resonance frequency with a nominal payload mass of.57 kg when the payload is moving in air, [3]. When the payload is submerged in water, the added mass may lead to a significant reduction of! and! 3. A typical value of! 3 with the payload in submerged condition is 3 rad/s to rad/s, [3]. ffl The frequency-dependent ratio between wave amplitudes inside the moonpool and in the basin is shown in Figure 3. The data are experimental and based on a frequency-sweep using regular waves at cm. We notice the characteristic resonance near the period T moonpool =:3 s, or! moonpool = :83 rad/s. In [] the moonpool resonance frequency is found to be 5.3 rad/s, pbased on WAMIT analysis. This is more accurate than the simple theoretical formula! moonpool = (g=`m) =5:8 rad/s, where `m is the moonpool depth `m =:9 m. ffl Typical vessel heave frequencies are in the range :»! heave» 9: rad/s. In [], the natural frequency of the heave motion of the vessel is found to be.55 rad/s, using WAMIT analysis. In [], this frequency is found to be. rad/s. In the experiment where the moonpool resonance frequency was determined, the vessel heave acceleration was also measured. The natural frequency of the heave motion of the vessel was found to be approximately! heave =:8 rad/s. It should be remarked that significant differences between the theoretical value and the WAMIT analysis are expected, since the WAMIT analysis is based on the main dimension specifications of the rig, while the experimental ballasting was significantly different in order to stabilize the rig. The sampling frequency is chosen at Hz = 5 rad/s. Although the main frequency components compensated for are around rad/s, such a high sampling frequency gives considerable flexibility for digital filtering and data analysis.

7 .5 moonpool amplification ( ) wave period (s) Figure 3: Ratio between wave amplitudes in moonpool and basin as a function of basin wave period. 3.3 Control strategy The wire elasticity introduce resonances that give fundamental limitations to the achievable feedback control bandwidth. Analysis and simulations indicate that highest performance and robustness is achieved using a pure feed-forward strategy. This report describes a control system with the following main components ffl Payload speed tracking control. The objective of this low level controller is to make the payload track a time-varying speed reference. This reference signal is composed of three components: Constant speed signal from the crane operator, heave compensation and wave synchronization. ffl. A component is added to the payload speed reference that compensates for the heave motion of the rig. Ideally, the payload then moves with specified speed relative to an Earth-fixed coordinate frame. This is based on a heave velocity estimate generated from an accelerometer in the vessel. ffl. A component is added to the payload speed reference in order to synchronize its motion through water entry in order to minimize the relative velocity of the water and payload, and thereby minimizing the hydrodynamic forces. This signal is generated by feedforward from a wave amplitude sensor, and overrides the heave compensation during the water entry phase. 3. Active heave compensation The objective of a heave compensator can be formulated as follows: Make the payload track a given trajectory in an Earth-fixed vertical reference system. This means that the payload position will not be influenced by the heave motion of the vessel. This is easily implemented using feed-forward where an estimate _^z of the vessel s vertical speed (in an Earth-fixed reference system) is added to the motor speed reference signal _z d = _z Λ m + _^z () 7

8 Here _z Λ m is the motor speed commanded by the operator or a higher level control system. The vessel vertical speed _z can be estimated using a state estimator which essentially integrates an accelerometer signal and removes bias using a high-pass filter, see Section Wave amplitude measurements can be used in a feed-forward compensator to ensure that the payload motion is synchronized with the wave motion during the water entry phase. An objective is to minimize variations in the dynamic forces on the payload, jf zd (z r )j, where f zd (z r ) = ρo (z r ) :: z r Z :: z r (z r ) :: z z r (z r ) : z r ρc : fi DA pz zr fi : fi zrfi (5) and the relative position is z r = z ( z )=z. The first term of (5) is the Froude-Kriloff pressure force. The second term represents the contribution of the added mass, while the third term contains the slamming loads. The fourth term is the viscous drag on the payload. Rather than minimizing this expression explicitly, we observe that a close to optimal solution is achieved by minimizing variations in _z r. This is achieved by the feed-forward compensator _z d = _z Λ m _^ exp( kz) + _^z ( exp( kz)) () where k is the wave number and _^ is an estimate of the derivative (speed) of the wave amplitude inside the moonpool (in a vessel-fixed coordinate frame), see section.. Since this control should only be applied during the water entry phase, it is blended with the active heave compensator as follows: _z d = _z Λ m ff(z) _^ exp( kz) + _^z ( exp( kz)) (7) The position-dependent factor ff(z) goes smoothly from zero to one when the payload is submerged: 8 < ; z< : ff(z) = (z +:); :» z» (8) : ; z> It is remarked that the factor exp( kz) accounting for the dependence of the water vertical velocity on water depth is based on assumptions that are not realistic in the moonpool, and might be replaced by a more accurate expression. In particular, one might except that the water vertical velocity is approximately constant from the wave surface to the bottom of the moonpool. Observer and Filter Design. Heave position and velocity estimator Since the wave motion has zero mean, the following filter removes the accelerometer bias and reconstructs the heave position and velocity ^z (s) = H hp(s) s z (s); _^z (s) = H hp(s) z (s) s The high-pass filter H hp (s) is a nd order filter with critical damping = :5 and cutoff frequency! c =:5 rad/s (full scale) or! c =:37 rad/s (model scale): s H hp (s) =! c +! (9) cs + s In addition there are analog and digital lowpass filters to remove high-frequency noise from the accelerometer measurement. 8

9 . Wave amplitude velocity estimator The wave amplitude relative to a vessel-fixed position inside the moonpool is measured. The wave amplitude velocity _ is estimated by filtering and numerical differentiation: _^ (s) = sh lp (s)h notch (s) (s) () The low pass filter H lp (s) is composed of a nd order critically damped filter at rad/s and a nd order critically damped filter at rad/s. In order to avoid exciting the wire resonance, we have introduced a notch filter H notch (s) = s + :! n +! n () s + :5! n +! n typically tuned at! n ß! 3. In the experiments we used! n = 37 rad=s. Notice that this tuning would depend on the payload mass, hydrodynamic properties such as added mass and drag, in addition to wire elasticity. Ideally, the notch frequency should be adapted online in order to match the observed resonance. The factors : and :5 in () are chosen empirically to give good performance, i.e. to give sufficient damping without too much phase loss..3 Combined Tension and Motor Speed Observer Consider the transfer functions for tension and closed-loop motor speed:» F t!3 s +! s +! (s) = m s _z m! s + 3! 3 s +! 3 _z m _z m ref (s) = (3) Ts+ Here =T represents the bandwidth of the closed-loop motor control system. Combining these two expressions, yields:» F m!3 s s +! s +! _z m ref (s) = m ()! Ts+ s + 3! 3 s +! 3 A state-space model with tension measurement F t and encoder measurement z m is: This implies:» Ft z m 3 _x _x _x 3 _x 7 5 =! 3 3! 3! 3 T z T } A () _x= Ax + Bu + w (5) y= Cx + Du + v () x x x 3 x ! 3 T T z } B _z m ref + 3 = m! 3!! m 3 3! m!!3! 3 T! 5 z } C " #» m! 3 + T! _z ref v m + v z } D w w w 3 w (7) x x x 3 x (8) 9

10 A linear fiixed-gain observer based on pole-placement or the algebraic Riccati equation (ARE) XA + AX T XC T RCX + Q = (9) becomes _^x= A^x + Bu + K(y ^y) () ^y= C^x + Du () where is the steady-state Kalman filter gain. K = XC T R () Tension measurement If a force ring is used to measure the tension F t ; the measurement equation simply becomes: y = F t (3) Payload acceleration measurement If the tension y computed from the payload acceleration z as = F t cannot be measured directly, y can be. Motor Speed Observer (No Tension Measurements) If only encoder measurements z m are available the model becomes:»»»» _x x = _x + x T T Λ» x z m = x y = m z + F w () _z ref m (5) In this case a linear observer for motor speed ^x using encoder x = _z m and speed reference _z m ref measurements can be constructed as:» _^x _^x =» T» ^x» ^x ^y =[; ] ^x where K and K are two positive design gains. ^x +» T _z ref m.5 Combined Payload Position and Velocity Observer Define The observer model then becomes: +» K K () (y ^y) (7) (8) x = z (payload position) (9) x = _z (payload speed) (3) u = z (measured payload acceleration) (3) _x = x (3) _x = x 3 + u + b + w (33) y = x + v (3)

11 where w and v are zero-mean Gaussian white noise processes. The bias b is found apriori as: b = mean(u( : N )) ß 9:8 m/s (35) where N fl : Hence, the observer becomes:»»» _^x ^x = _^x ^x ^y =[; ]» ^x ^x +» (u + b) +» K K (y ^y) (3) (37) where pole-placement of a double integrator suggest that the observer gains are chosen as:.5. Payload position measurement K = f! f (38) K =! f (39) If a camera or another position sensor used to measure the payload position z; the measurement equation simply becomes: y = z ().5. Motor encoder measurement If the payload position y = z cannot be measured directly, y can be computed from the motor position z m simply by setting y = z m. The limitation of this is that the motor position signal only contains the low-frequency part of the payload position. However, this is not a major problem since the acceleration sensor contains information mainly on the high-frequency part of the payload position. Another problem is that there will be an offset that depends on the payload mass due to the steady state wire tension. A reasonable tuning of the observer is f = and! f =! 3 =5 ß rad=s. The bandwidth of the observer should be placed below the resonance frequency since the encoder signal does not contain useful information at this frequency.. Payload position reconstruction in the frequency domain The motor encoder measurement can be used to reconstruct the low-frequency part of the payload position signal and the payload acceleration measurement can be used to reconstruct the high-frequency part of the payload position signal. The following estimate combines these signals ^z(s) =H lp (s)z m (s) +H hp (s) s z(s) We choose H lp (s) = and H hp (s) equal to a nd order Butterworth high-pass filter with cut-off frequency at! 3 =5 ß rad=s. 5 Experimental setup and instrumentation The winch is an AC servomotor with an internal speed control loop. There are vertical accelerometers in both the payload and vessel, and a wire tension sensor. In the moonpool there are wave meters measuring the wave amplitude in a vessel-fixed coordinate frame, i.e. = z. The motor position z m is measured using an encoder. The total scale model mass is 57 kg with a water plane area of.3 m. Two payloads where tested: a sphere and a pump inside a frame, see Figure

12 Experimental results Figure : Payloads used during experiments. Experimental results for the scenaria summarized in Table are described in this section. The notation Ws is an acronym for while Hc is an acronym for. Notice that the reported values of the wave period T and amplitude A are values specified to the wave generator, and will therefore deviate somewhat from the values measured (especially the amplitude). Three sea states are considered: ffl Regular waves with period T = :5 s and amplitude A = cm. This wave frequency is close to the moonpool resonance frequency, see Figure 3, and the wave amplitude inside the moonpool is about times the basin wave amplitude. The wave height (full scale) is around H s =: m. ffl Regular waves with period T =: s and amplitude A =cm. At this frequency there is no resonance. The wave height (full scale) is around H s =: m. ffl JONSWAP spectrum with T = : s and A = 3 cm. The significant wave height (full scale) is around H s =:7 m. We present both raw and filtered data. The filtered data contains mainly components in the frequency band between.5 and.5 Hz, where the significant wave motion is located. This filtering allows the effects of the wave synchronization to be separated from other effects, since this is the frequency band where the wave synchronization is effective. It is therefore natural to verify the performance of the wave synchronization on the filtered data, neglecting low-frequency components due to buoyancy and high-frequency components due to measurement noise and wire resonances. Still, it is also of interest to evaluate the performance of the wave synchronization with respect to excitation of the wire resonance. This evaluation is, however, not straightforward to carry out since the laboratory model does not contain passive heave compensation or damping. Also, the excitation caused by signal noise and winch motor drive is not directly scalable to full scale implementation and must be considered in the context of the technology used for implementation. Thus, we will base our conclusions mainly on the filtered data. The filtering is carried out using th order Butterworth highpass and lowpass filters using the MATLAB filtfilt algorithm to ensure no phase shift.

13 Wave spectrum Payload Controller Tests Regular, T = :5 s, A = cm Sphere Ws A,B Regular, T = :5 s, A = cm Sphere Hc A,B Regular, T = :5 s, A = cm Sphere Ws+Hc A,B Regular, T = :5 s, A = cm Sphere - A,B Regular, T = : s, A = cm Sphere Ws A,B Regular, T = : s, A = cm Sphere Hc A,B Regular, T = : s, A = cm Sphere Ws+Hc A,B Regular, T = : s, A = cm Sphere - A,B Jonswap, T =: s, A =3cm Sphere Ws A,B,C,D,E Jonswap, T =: s, A =3cm Sphere Hc A,B,C,D,E Jonswap, T =: s, A =3cm Sphere Ws+Hc A,B,C,D,E Jonswap, T =: s, A =3cm Sphere - A,B,C,D,E Regular, T = :5 s, A = cm Pump Ws A,B Regular, T = :5 s, A = cm Pump Hc A,B Regular, T = :5 s, A = cm Pump Ws+Hc A,B Regular, T = :5 s, A = cm Pump - A,B Regular, T = : s, A = cm Pump Ws A,B Regular, T = : s, A = cm Pump Hc A,B Regular, T = : s, A = cm Pump Ws+Hc A,B Regular, T = : s, A = cm Pump - A,B Jonswap, T =: s, A =3cm Pump Ws+Hc A,B,C,D,E Jonswap, T =: s, A =3cm Pump Ws A,B,C,D,E Jonswap, T =: s, A =3cm Pump Hc A,B,C,D,E Jonswap, T =: s, A =3cm Pump - A,B,C,D,E Table : Overview of experiments. 3

14 Time series from selected tests will be shown. In addition we show the following statistics for each test: ffl H s (m) - significant wave height (model scale) or wave height (if the waves are regular). ffl ff Ft (N ) - standard deviation for tension signal F t where signal components under.5 Hz are removed. ffl min(f t )(N ) - peak minimum value of tension. ffl max(f t )(N ) - peak maximum value of tension. ffl ff z (m=s ) - standard deviation for payload acceleration signal F t where signal components under.5 Hz are removed. ffl min( z) (m=s ) - peak minimum value of payload acceleration. ffl max( z) (m=s ) - peak maximum value of payload acceleration. The statistics presented are corrected for variations in average speed and length of experiments, such that the standard deviations presented are averages over a constant number of wave periods during the water entry phase of each operation. Experimental data. The experimental data can be found in MAT-files. The files are named such as reg.5. paysphere a.mat where reg indicates regular waves,.5 is the wave period,. is the wave amplitude, paysphere indicate the payload and the last letter a indicates test A. The results can be plotted in MATLAB using the plot exp script.. Regular waves at T =:5 s and A =cm The results of tests A and B are summarized in Table. Figures 5-8 shows some relevant signals for test B with the spherical payload. Figures 9 - shows some relevant signals for test A with the pump payload. The following observations and remarks can be made: ffl For the spherical payload, the wave synchronization reduces the tension standard deviation by 5. % in test A and. % in test B, compared to no control, when considering filtered data. Wave synchronization in combination with heave compensation reduces the tension standard deviation by. % in test A and.8 % in test B, compared to no control, when considering filtered data. ffl For the spherical payload, the peak minimum tension is increased from. N to.58 N using wave synchronization in test A and from.3 N to.58 N in test B, compared to no control, when considering filtered data. This is beneficial as it reduces the possibility of wire snatch. The peak maximum tension is somewhat increased with the wave synchronization. ffl The use of heave compensation does not give any significant reduction of tension variability in this sea state. However, it gives significant reduction of the standard deviation of the payload acceleration. ffl The payload acceleration is significantly increased using wave synchronization. ffl When considering the unfiltered data, similar qualitative conclusions can be made in most cases.

15 ffl For the pump-in-frame payload, the wave synchronization reduces the tension standard deviation by 7. % in test A and.8 % in test B, compared to no control, when considering filtered data. in combination with heave compensation reduces the tension standard deviation by 8.9 % in test A and 5.3 % in test B, compared to no control, when considering filtered data.. Regular waves at T =: s and A =cm The results of tests A and B are summarized in Table 3. Figures 3 - shows the relevant signals for test B with the spherical payload. Figures 7 - shows the relevant signals for test A with the pump payload. The following observations and remarks can be made: ffl For the spherical payload, the wave synchronization reduces the tension standard deviation by 3. % in test A and 33. % in test B, compared to no control, when considering filtered data. Wave synchronization in combination with heave compensation reduces the tension standard deviation by 8. % in test A and 5. % in test B, compared to no control, when considering filtered data. ffl For the spherical payload, the minimum tension is reduced from.3 N to.3 N using wave synchronization in test A and from.59 N to.7 N in test B, compared to no control, when considering filtered data. ffl For the spherical payload, the use of heave compensation alone gives significant reduction of tension variability in this very rough sea state, namely 5.3 % in test A and 5.9 % in test B. This indicates that significant reduction in tension variance can be achieved by either heave compensation or wave synchronization, but largest reduction is achieved by combining them. ffl The payload acceleration is significantly reduced using any combination of heave compensation and wave synchronization. ffl When considering the unfiltered data with the spherical payload, similar qualitative conclusions can be made in most cases. With the pump-in-frame payload the unfiltered data shows increased variance with wave synchronization and heave compensation. ffl For the pump payload the wave synchronization reduces the tension standard deviation by 3.9 % in test A and 3.5 % in test B. in combination with heave compensation reduces the tension standard deviation by. % in test A and 53. % in test B, when considering filtered data. Hence, the pump and spherical payloads give similar results in this sea state..3 JONSWAP spectrum at T =: s and A =3cm The averaged results of tests A to E are summarized in Table. Figures - shows the relevant signals for test B with the spherical payload, which is the test with poorest performance of the wave synchronization. The following observations and remarks can be made: ffl For the spherical payload, the wave synchronization reduces the tension standard deviation by 7. % on average, compared to no control, when considering filtered data. in combination with heave compensation increases the tension standard deviation by. % on average, compared to no control, when considering filtered data. ffl For the spherical payload, the minimum tension is reduced from. N to.8 N using wave synchronization on average, compared to no control, when considering filtered data and the spherical payload. 5

16 Mode H s ff Ft min(f t ) max(f t ) ff z min( z) max( z) Sphere Test A Filtered ( Hz) Ws Hc Ws+Hc none Sphere Test A Unfiltered Ws Hc Ws+Hc none Sphere Test B Filtered ( Hz) Ws Hc Ws+Hc none Sphere Test B Unfiltered Ws Hc Ws+Hc none Pump Test A Filtered ( Hz) Ws Hc Ws+Hc none Pump Test A Unfiltered Ws Hc Ws+Hc none Pump Test B Filtered ( Hz) Ws Hc Ws+Hc none Pump Test B Unfiltered Ws Hc Ws+Hc none Table : Summary of results with regular waves at T = :5 s and A = cm.

17 .. motor speed (m/s).. motor speed (m/s) motor speed (m/s).. motor speed (m/s) motor position (m).. motor position (m) motor position (m).. motor position (m) Figure 5: Regular waves at T = :5 s and A = cm, Sphere, Test B 7

18 8 8 tension (N) and LF (cyan) tension (N) and LF (cyan) tension (N) and LF (cyan) tension (N) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) Figure : Regular waves at T = :5 s and A = cm, Sphere, Test B 8

19 .5.5 acc vessel acc vessel acc vessel acc vessel Moonpool (green and blue) and basin (red)..5 Power spectrum Wave frequency (Hz) Figure 7: Regular waves at T = :5 s and A = cm, Sphere, Test B 9

20 .. wave (m) moonpool.... wave (m) moonpool wave (m) moonpool.... wave (m) moonpool wave (m) basin (red).... wave (m) basin (red) wave (m) basin (red).... wave (m) basin (red) Figure 8: Regular waves at T =:5 s and A =cm, Pump, Test B

21 .. motor speed (m/s).. motor speed (m/s) motor speed (m/s).. motor speed (m/s) motor position (m).. motor position (m) motor position (m).. motor position (m) Figure 9: Regular waves at T = :5 s and A = cm, Pump, Test A

22 8 8 tension (N) and LF (cyan) tension (N) and LF (cyan) tension (N) and LF (cyan) tension (N) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) Figure : Regular waves at T = :5 s and A = cm, Pump, Test A

23 .5.5 acc vessel acc vessel acc vessel acc vessel Moonpool (green and blue) and basin (red) Power spectrum Wave frequency (Hz) Figure : Regular waves at T = :5 s and A = cm, Pump, Test A 3

24 .. wave (m) moonpool.... wave (m) moonpool wave (m) moonpool wave (m) moonpool wave (m) basin (red).... wave (m) basin (red) wave (m) basin (red) wave (m) basin (red) Figure : Regular waves at T = :5 s and A = cm, Pump, Test A

25 Mode H s ff Ft min(f t ) max(f t ) ff z min( z) max( z) Sphere Test A Filtered ( Hz) Ws Hc Ws+Hc none Sphere Test A Unfiltered Ws Hc Ws+Hc none Sphere Test B Filtered ( Hz) Ws Hc Ws+Hc none Sphere Test B Unfiltered Ws Hc Ws+Hc none Pump Test A Filtered ( Hz) Ws Hc Ws+Hc none Pump Test A Unfiltered Ws Hc Ws+Hc none Pump Test B Filtered ( Hz) Ws Hc Ws+Hc none Pump Test B Unfiltered Ws Hc Ws+Hc none Table 3: Summary of results with regular waves at T = : s and A = cm. 5

26 .. motor speed (m/s).. motor speed (m/s) motor speed (m/s).. motor speed (m/s) motor position (m).. motor position (m) motor position (m).. motor position (m) Figure 3: Regular waves at T = : s and A = cm, Sphere, Test B

27 8 8 tension (N) and LF (cyan) tension (N) and LF (cyan) tension (N) and LF (cyan) tension (N) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) Figure : Regular waves at T = : s and A = cm, Sphere, Test B 7

28 .5.5 acc vessel acc vessel acc vessel acc vessel Moonpool (green and blue) and basin (red).5 Power spectrum Wave frequency (Hz) Figure 5: Regular waves at T = : s and A = cm, Sphere, Test B 8

29 wave (m) moonpool wave (m) moonpool wave (m) moonpool.... wave (m) moonpool wave (m) basin (red) wave (m) basin (red) wave (m) basin (red).... wave (m) basin (red) Figure : Regular waves at T = : s and A = cm, Pump, Test B 9

30 .. motor speed (m/s).. motor speed (m/s) motor speed (m/s).. motor speed (m/s) motor position (m).. motor position (m) motor position (m).. motor position (m) Figure 7: Regular waves at T =: s and A =cm, Pump, Test A 3

31 8 8 tension (N) and LF (cyan) tension (N) and LF (cyan) tension (N) and LF (cyan) tension (N) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) Figure 8: Regular waves at T =: s and A =cm, Pump, Test A 3

32 .5.5 acc vessel acc vessel acc vessel acc vessel Moonpool (green and blue) and basin (red) Power spectrum Wave frequency (Hz) Figure 9: Regular waves at T =: s and A =cm, Pump, Test A 3

33 .. wave (m) moonpool.... wave (m) moonpool wave (m) moonpool.... wave (m) moonpool wave (m) basin (red).... wave (m) basin (red) wave (m) basin (red).... wave (m) basin (red) Figure : Regular waves at T =: s and A =cm, Pump, Test A 33

34 ffl The use of heave compensation does not give any significant reduction of tension variability. ffl The payload acceleration is significantly reduced using heave compensation but wave synchronization gives some increase in acceleration. ffl For the pump-in-frame payload, the wave synchronization reduces the tension standard deviation by. % on average. in combination with heave compensation increases the tension standard deviation by. % on average, when considering filtered data. ffl When considering the unfiltered data with the spherical payload, similar qualitative conclusions can be made in most cases. With the pump-in-frame payload the variance is somewhat increased. ffl It should be remarked that averaging not very meaningful because there are only five tests. Mode H s ff Ft min(f t ) max(f t ) ff z min( z) max( z) Sphere Avg. Test A-E Filtered ( Hz) Ws Hc Ws+Hc none Sphere Avg. Test A-E Unfiltered Ws Hc Ws+Hc none Pump Avg. Test A-E Filtered ( Hz) Ws Hc Ws+Hc none Pump Avg. Test A-E Unfiltered Ws Hc Ws+Hc none Table : Summary of results with JONSWAP spectrum at T = : s and A = 3 cm, averaged over 5 tests. 7 Simulations Some simulations are included, primarily because they include the hydrodynamic forces on the payload, which cannot be measured directly. The simulation model is documented in []. Figures 5,, and 7 show the payload acceleration z, wire tension F t and hydrodynamic forces f z during a lifting operation with irregular waves, typical moonpool wave amplitude. m and wave period. s (. m and.3 s in full scale), and a typical vessel heave amplitude.5 m and period. 3

35 .. motor speed (m/s).. motor speed (m/s) motor speed (m/s).. motor speed (m/s) motor position (m).. motor position (m) motor position (m).. motor position (m) Figure : JONSWAP spectrum at T = : s and A = 3 cm, Sphere, Test B 35

36 8 8 tension (N) and LF (cyan) tension (N) and LF (cyan) tension (N) and LF (cyan) tension (N) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) acc payload (red) and LF (cyan) Figure : JONSWAP spectrum at T = : s and A = 3 cm, Sphere, Test B 3

37 .5.5 acc vessel acc vessel acc vessel acc vessel Moonpool (green and blue) and basin (red)..5. Power spectrum Wave frequency (Hz) Figure 3: JONSWAP spectrum at T = : s and A = 3 cm, Sphere, Test B 37

38 .. wave (m) moonpool.... wave (m) moonpool wave (m) moonpool.... wave (m) moonpool wave (m) basin (red).... wave (m) basin (red) wave (m) basin (red).... wave (m) basin (red) Figure : JONSWAP spectrum at T = : s and A = 3 cm, Pump, Test B 38

39 s (.75 m and 8.7 s in full scale). The figures also show the vertical payload, vessel and wave positions during the water entry phase. The required motor speed (not shown) is within realistic limits. The different strategies are simulated with the same external excitations (waves) and similar improvements in performance are typically achieved for other realizations of the random waves. It can be seen that payload acceleration is smallest with heave compensation, while the wave synchronization reduces the hydrodynamic forces and wire tension during water entry. The remaining hydrodynamic forces and wire tension variations are to a large degree high-frequency (at the wire resonance). They might be further damped by a standard passive heave compensation system as the motor bandwidth does not allow active control at such high frequencies. Position (payload( ), relative( ), heave( ), wave ( )).5 Payload acceleration.3..5 z (m). a p (m/s ) Wire tension Hydrodynamic force f z F t (N) 3 f z (N) Figure 5: Simulations without active control, i.e. constant speed. Position (payload( ), relative( ), heave( ), wave ( )).5 Payload acceleration.3..5 z (m). a p (m/s ) Wire tension Hydrodynamic force f z F t (N) 3 f z (N) Figure : Simulations with heave compensation. 39

40 Position (payload( ), relative( ), heave( ), wave ( )).5 Payload acceleration.3..5 z (m). a p (m/s ) Wire tension Hydrodynamic force f z F t (N) 3 f z (N) Figure 7: Simulations with wave synchronization. 8 Discussions, either alone or in combination with heave compensation, significantly reduces tension variability and peak values in a large majority of the tests. However, under some sea states, in particular with the pump in the frame payload, no or only small improvement was experienced. Payload acceleration is sometimes increased simply because the payload need to move more quickly in order to synchronize with the wave motion, while under some sea states the payload acceleration is also reduced. The experiments with regular waves showed high repeatability, with small variations between experiments under the same sea state. There are significant differences in performance when comparing the spherical and pump-in-frame payload. The wave synchronization shows consistently greater improvements with the spherical payload than with the pump-in-the frame. Reasons for this may be that the sphere is more sensitive to the waves because it is a closed structure, while the pump is in an open frame. The pump-in-frame has significantly less buoyancy and more irregular shape, which is also expected to play a role. Overall, the results are encouraging, showing that wave synchronization has a significant benefit in terms of reduced tension variability and peaks at least under some sea states and with some payloads. The following limitations to performance has been identified:. The feedforward approach leads to a phase error, due to the dynamics of the motor and signal filtering. In our experimental setup this error is significant, but not compensated for. For a wave period of. s, the phase loss in the motor is 5 degrees and the phase loss in the filtering of the wave amplitude measurement is about 5 deg. The total phase loss is around deg, which is significant. It is therefore clear that the use of some model-based predictor of the wave amplitude may lead to significant improvement of performance.. The wire resonance is being excited, also without the use of wave synchronization or heave compensation. The use of heave compensation or heave compensation reduces the excitations of the resonance in most cases with the spherical payload, while the excitations of the resonance seems to be typically increased with the pump-in-frame payload. There are several possible sources for these excitations: (a) Even with constant speed reference the AC motor speed measurement exhibit oscillations

41 with significant frequency components in area where the resonance frequency is. This situation may be improved by the use of a gear that will increase operating range of the motor and shift the frequencies. Notice that the AC motor control system is built into the motor drive and cannot be modified. (b) The water flow inside the moonpool may contain frequency components that give hydrodynamic forces that excite the resonance. (c) With heave compensation and wave feed-forward is it clear that measurement noise may excite the resonance frequency. Since there is very little damping in the suspension and the resonance frequency is less than a decade above from typical wave frequencies, any filtering will lead to phase loss as described above. A further complication is that the resonance frequency depend on payload mass and position-dependent hydrodynamic properties such as added mass. Improvements may be expected by adaptive notch filtering, in combination with wave prediction. Obviously, the technical solutions for wave measurements, signal transmission and electronics may be modified to reduce the overall noise level. Noise is amplified due to numerical differentiation when computing the wave surface speed from the wave amplitude measurements. This may be avoided via the use of position control of the AC motor rather than velocity control. It may be that the degree of damping in the scale model is too small compared to typical full scale cranes, and the effects of resonances will be much less in full scale. 9 Full scale implementability A full scale implementation of wave synchronization requires essentially a wave feedforward signal to be added to an existing active heave compensation system. The key requirement is the availability of a measurement or estimate of the water vertical velocity within the moonpool. In the scale experiments we utilized a standard laboratory-type wave amplitude measurement system from DHI (wave gauge /), which relies on measurement of the water-level dependent conductivity between two parallel electrodes. In order to determine the water vertical velocity the measured amplitude was differentiated after low-pass and notch filtering. In full scale implementation the same measurement principle may prove useful, but alternatives such as water pressure and optical/radar systems should be evaluated. If the water vertical velocity can be measured directly, it would be an advantage since the noise-sensitive differentiation of the amplitude measurement can be avoided. Very rough estimates indicate that moonpool wave amplitude measurements at.5 m accuracy or water vertical velocity measurements at.5 m/s accuracy is sufficient. A measurement or estimate of the payload position relative to the mean sea level is required. Since this measurement is only used to gradually activate/deactivate the wave synchronization as the payload moves through the water entry phase, high accuracy is not required (.5 m accuracy is expected to be sufficient). A model scale sampling frequency on Hz was utilized, corresponding to a full scale sampling frequency of 3 Hz. Such a high sampling frequency was chosen mainly to allow accurate analysis of data (for presentation and performance evaluation) and digital signal processing (rather than analog filtering). In order to implement the core functionality of the wave synchronization, a significantly lower sampling frequency is expected to be sufficient. In any case, the computational requirements are very small, essentially limited to a few filters. The motor time constant T =: s corresponds to approximately T =: s in full scale, and its time-delay T =: corresponds to T =:5 s in full scale. It is assumed the motor is equipped with a speed controller. Tuning of the controller involves the following aspects:

42 ffl Bandpass filters must be tuned to reduce sensor signal high-frequency noise and low-frequency drift. The cutoff frequencies depend on the wave spectrum and may be adapted online for optimal performance. ffl The notch filter is tuned to avoid excitation of the wire resonance. Since these will depend on the payload position, mass and hydrodynamic parameters, in addition to the type of wire, it should be considered to implement an adaptive filter that estimates the resonance frequency in real time during operation. In order to get information for the adaptive filter, a tension or acceleration measurement on payload or wire is useful. ffl An estimate of how the water vertical velocity reduces with the water depth must be made, based on the moonpool geometry. Suggestions for future work Below are some ideas for future work:. Redesign of rig. In particular the deplacement (main dimensions) of the rig should be increased to simplify ballasting and to improve metacentric stability.. The springs used for station-keeping should be mounted in the horizontal plane in order to avoid excitation of the heave mode. In addition the optimal stiffness of the springs should be computed according the P-gain in DP control system. 3. Improved design and tuning of filters, including predictors to reduce phase error due to filtering and motor dynamics as well as to avoid exciting resonant wire elasticity. This may give significant performance improvements.. Motor position control rather than speed control: avoid numerical differentiation of wave amplitude measurements. This improves phase margins and performance of the wave synchronization algorithm. 5. Compensation of roll and pitch for acceleration measurements (VRU-functionality).. Redesign of suspension (four wires instead of one) to improve horizontal stability and to get realistic amounts of passive damping in the system. 7. Gear to improve motor accuracy. 8. Consider alternative sensor solutions for wave amplitude (such as pressure) or water vertical velocity. 9. Test the method also for water exit.. Evaluate alternative control strategies such as combination of feedback/feedforward and optimization of payload trajectories. Conclusions has been suggested to improve the performance of heave compensated offshore cranes during the water entry phase in moonpool marine operations. The method requires measurement of the wave amplitude in the moonpool, and is implemented as a feedforward compensator. Experimental results using a scale model shows that typical reduction of the wire tension standard deviation is between to 5 % depending on the sea state and payload.