ansv. sign. - institutt for teknisk kybernetikk, ntnu · 2008. 4. 16. · norsk hydro ekstrakt ......

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NTNU Norges teknisk- naturvitenskapelige universitet

RAPPORT Rapportnummer 2001-13-T

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ISBNFakultet for informasjonsteknologi, matematikk og elektroteknikk Institutt for teknisk kybernetikk

NTNU Institutt for teknisk kybernetikkO.S. Bragstads plass 2D N 7491 TRONDHEIM

Sentralbord: 73 59 40 00 Instituttet: 73 59 43 76 Telefax: 73 59 43 99

Rapportens tittel Observer and Controller Design for an Offshore Crane Moonpool System

Dato Revised 11 May 2002

Antall sider og bilag 43 Ansv. sign.

Prosjektnummer

Saksbearbeider/forfatter Tor Arne Johansen and Thor Inge Fossen

Oppdr. givers ref. Svein Ivar Sagatun

Oppdragsgiver Norsk Hydro

Ekstrakt A control system for active wave synchronization and heave compensation in heavy-lift offshore crane operations is described. Wave synchronization 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 0 and 50 %. 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.

Offshore Moonpool Cranes Marine Operations Wave feedforward

Contents

1 Background 3

2 Dynamics 3

3 Control system 63.1 Control objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2 Frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3 Control strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.4 Active heave compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.5 Wave synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4 Observer and Filter Design 84.1 Heave position and velocity estimator . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.2 Wave amplitude velocity estimator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.3 Combined Tension and Motor Speed Observer . . . . . . . . . . . . . . . . . . . . . . . 94.4 Motor Speed Observer (No Tension Measurements) . . . . . . . . . . . . . . . . . . . . 104.5 Combined Payload Position and Velocity Observer . . . . . . . . . . . . . . . . . . . . 10

4.5.1 Payload position measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.5.2 Motor encoder measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.6 Payload position reconstruction in the frequency domain . . . . . . . . . . . . . . . . . 11

5 Experimental setup and instrumentation 11

6 Experimental results 126.1 Regular waves at � � �� s and � � � cm . . . . . . . . . . . . . . . . . . . . . . . . 146.2 Regular waves at � � �� s and � � � cm . . . . . . . . . . . . . . . . . . . . . . . . . 156.3 JONSWAP spectrum at � � �� s and � � � cm . . . . . . . . . . . . . . . . . . . . . 15

7 Simulations 34

8 Discussions 40

9 Full scale implementability 41

10 Suggestions for future work 42

11 Conclusions 42

2

1 Background

Higher operability of installations offshore of underwater equipment will become increasingly more im-portant in the years to come. Offshore oil and gas fields will be developed with all processing equipmenton the seabed and in the production well itself. The lower cost in using subsea equipment compared tousing a floating or fixed production platform is penalized with lower availability for maintenance, repairand replacement of equipment. Production stops due to component failure are costly, hence a high op-erability on subsea intervention is required to operate subsea fields. High operability implies that subseaintervention must be carried out also during winter time, which in the North Sea and other exposed areasimplies underwater intervention in harsh weather conditions. The objective of the marine operation is tosafely install a product on the seabed. One of the critical phase of the installation is the water entry phaseon which this report focuses. We study active control of heave compensated cranes or module handlingsystems (MHS) during the water entry phase of a subsea installation, where it is of interest to minimizeloads due to the hydrodynamic forces caused by interactions between payload and waves. The principalidea is to utilize a wave amplitude measurement in order to compensate directly for the water motion dueto waves inside the moonpool.

2 Dynamics

We will only consider the vertical motion of a payload moving through the water entry zone. The payloadis handled from a floating vessel. The vessel is kept in a mean fixed position and heading relative to theincoming wave only moving due to the first order wave forces. Effects from the vessel’s roll and pitchmotion are neglected.

In this section we describe the dynamics of a laboratory scale model moonpool crane-rig (scale 1:30).Figure 1 shows a setup consisting of an electric motor and a payload connected by a wire that runs overa pulley suspended in a spring. We notice that this setup contains no passive heave compensation systemor damping device. Figure 2 shows the coordinate system definitions:

�� rig position in heave (�)� � payload position (�)�� motor position (�)� � moonpool wave amplitude, relative to mean sea level (�)�� moonpool wave amplitude, relative to vessel-fixed position (�)� � payload mass (��)�� = wire tension ( )

The coordinate systems of � and �� are positive downwards and fixed in the crane rig, while thecoordinate systems of � and �� are positive upwards and Earth-fixed. All have the origin at the still watermean sea level. In [2], the following transfer functions from motor speed �� to payload position � andwire tension �� are derived

�

��

��

��

��

��

�

� ��

�(1)

��

��

��

��

��

��

�

� ��

�(2)

where the resonance frequencies satisfies the relationship �� . The value �� rad/s is theexperimentally determined wire resonance frequency with a nominal payload mass of 0.572 kg when thepayload is moving in air, [3]. When the payload is partly or fully submerged, the hydrodynamic forcemust be taken into account. This leads to increased damping and contributions from added mass and itstime derivatives. Depending on the size, shape, mass, position and velocity of the payload, significant

3

Figure 1: Experimental 1:30 scale model of vessel with moonpool.

4

Figure 2: Definition of the coordinate systems.

5

reduction in the resonance frequencies (�� ) and increase in the relative damping factors (�� ) areexperienced. A typical value of �� with the payload in submerged condition is 31 rad/s to 46 rad/s,[3]. Typical values for the damping factors are �� and �� slightly higher, when moving in air[3]. Identified models of these factors under submerged conditions are also found in [3]. Theoreticalrelationships between �� and �� can be found in [2].

A first order model of the transfer function from the reference motor speed �� to the motor speed ��is found in [2]:

��

� ��

� ��(3)

The time-delay is mainly due to digital communication and control within the motor drive and controlunits.

3 Control system

3.1 Control objectives

The performance measures of interest are the following:

� Wire tension; The minimum value must never be less than zero to avoid high snatch loads, andthe maximum value and variability should be minimized.

� Payload acceleration; Peak values should be minimized as this gives an indication of impulsiveforces on the payload.

� Payload impulse; The impulsive hydrodynamic forces on the payload should be minimized.

3.2 Frequencies

The following are the key frequencies in the system:

� �� rad/s is the experimentally determined wire resonance frequency with a nominal pay-load mass of 0.572 kg when the payload is moving in air, [3]. When the payload is submerged inwater, the added mass may lead to a significant reduction of �� and ��. A typical value of �� withthe payload in submerged condition is 31 rad/s to 46 rad/s, [3].

� The frequency-dependent ratio between wave amplitudes inside the moonpool and in the basin isshown in Figure 3. The data are experimental and based on a frequency-sweep using regular wavesat 2 cm. We notice the characteristic resonance near the period �� s, or �� rad/s. In [4] the moonpool resonance frequency is found to be 5.43 rad/s, based on WAMITanalysis. This is more accurate than the simple theoretical formula ��

��

rad/s, where �� is the moonpool depth �� m.

� Typical vessel heave frequencies are in the range �� rad/s. In [4], the naturalfrequency of the heave motion of the vessel is found to be 4.55 rad/s, using WAMIT analysis.In [1], this frequency is found to be 4.22 rad/s. In the experiment where the moonpool resonancefrequency was determined, the vessel heave acceleration was also measured. The natural frequencyof the heave motion of the vessel was found to be approximately �� rad/s.

It should be remarked that significant differences between the theoretical value and the WAMIT analysisare expected, since the WAMIT analysis is based on the main dimension specifications of the rig, whilethe experimental ballasting was significantly different in order to stabilize the rig.

The sampling frequency is chosen at 200 Hz = 1256 rad/s. Although the main frequency componentscompensated for are around 10 rad/s, such a high sampling frequency gives considerable flexibility fordigital filtering and data analysis.

6

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50

0.5

1

1.5

2

2.5

wave period (s)

moonpool amplification (−)

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 feedbackcontrol bandwidth. Analysis and simulations indicate that highest performance and robustness is achievedusing a pure feed-forward strategy. This report describes a control system with the following main com-ponents

� Payload speed tracking control. The objective of this low level controller is to make the payloadtrack 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.

� Heave compensation. A component is added to the payload speed reference that compensatesfor the heave motion of the rig. Ideally, the payload then moves with specified speed relative toan Earth-fixed coordinate frame. This is based on a heave velocity estimate generated from anaccelerometer in the vessel.

� Wave synchronization. A component is added to the payload speed reference in order to syn-chronize its motion through water entry in order to minimize the relative velocity of the waterand payload, and thereby minimizing the hydrodynamic forces. This signal is generated by feed-forward from a wave amplitude sensor, and overrides the heave compensation during the waterentry phase.

3.4 Active heave compensation

The objective of a heave compensator can be formulated as follows: Make the payload track a giventrajectory in an Earth-fixed vertical reference system. This means that the payload position will not beinfluenced by the heave motion of the vessel. This is easily implemented using feed-forward where anestimate �� of the vessel’s vertical speed (in an Earth-fixed reference system) is added to the motor speedreference signal

�� (4)

7

Here �� is the motor speed commanded by the operator or a higher level control system. The vesselvertical speed �� can be estimated using a state estimator which essentially integrates an accelerometersignal and removes bias using a high-pass filter, see Section 4.1.

3.5 Wave synchronization

Wave amplitude measurements can be used in a feed-forward compensator to ensure that the payloadmotion is synchronized with the wave motion during the water entry phase. An objective is to minimizevariations in the dynamic forces on the payload, �� , where

� ��

��

��

��

��

��

��

� ��

��

�� (5)

and the relative position is �� . The first term of (5) is the Froude-Kriloff pressureforce. The second term represents the contribution of the added mass, while the third term contains theslamming 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 isachieved by minimizing variations in ��. This is achieved by the feed-forward compensator

�� (6)

where � is the wave number and �� is an estimate of the derivative (speed) of the wave amplitude insidethe moonpool (in a vessel-fixed coordinate frame), see section 4.2. Since this control should only beapplied during the water entry phase, it is blended with the active heave compensator as follows:

�� (7)

The position-dependent factor �� goes smoothly from zero to one when the payload is submerged:

��

��

��

(8)

It is remarked that the factor �� accounting for the dependence of the water vertical velocity onwater depth is based on assumptions that are not realistic in the moonpool, and might be replaced by amore accurate expression. In particular, one might except that the water vertical velocity is approximatelyconstant from the wave surface to the bottom of the moonpool.

4 Observer and Filter Design

4.1 Heave position and velocity estimator

Since the wave motion has zero mean, the following filter removes the accelerometer bias and recon-structs the heave position and velocity

��

��

��

��

The high-pass filter �� is a 2nd order filter with critical damping � � �� and cutoff frequency�� rad/s (full scale) or �� rad/s (model scale):

��

��

(9)

In addition there are analog and digital lowpass filters to remove high-frequency noise from the ac-celerometer measurement.

8

4.2 Wave amplitude velocity estimator

The wave amplitude �� relative to a vessel-fixed position inside the moonpool is measured. The waveamplitude velocity �� is estimated by filtering and numerical differentiation:

�� (10)

The low pass filter �� is composed of a 2nd order critically damped filter at 60 rad/s and a 2nd ordercritically damped filter at 200 rad/s. In order to avoid exciting the wire resonance, we have introduced anotch filter

��

�

� � � ��

(11)

typically tuned at �� . In the experiments we used �� . Notice that this tuningwould depend on the payload mass, hydrodynamic properties such as added mass and drag, in additionto wire elasticity. Ideally, the notch frequency should be adapted online in order to match the observedresonance. The factors �� and �� in (11) are chosen empirically to give good performance, i.e. to givesufficient damping without too much phase loss.

4.3 Combined Tension and Motor Speed Observer

Consider the transfer functions for tension and closed-loop motor speed:

��

��

��

��

��

��

�

� ��

�(12)

��

��

� ��

� �(13)

Here �� represents the bandwidth of the closed-loop motor control system. Combining these twoexpressions, yields:

��

��

� � ��

��

��

��

� �

��

�

� ��

�(14)

A state-space model with tension measurement �� and encoder measurement �� is:

�� (15)

�� (16)

This implies:�

��

� � �

�

� � � �

��

��

�

� � � ��

�

� �

� ��

�

��

� �

�

��

�

��

�

� �

� ��

��

�

��

��

��

��

� � (17)

��

��

��

� �

��

��

��

��

��

��

��

�

��

��

��

� ��

�

��

� �

��

�

��

��

��

�

��

��

�(18)

9

A linear fiixed-gain observer based on pole-placement or the algebraic Riccati equation (ARE)

�� (19)

becomes

�� (20)

�� (21)

where� � �� (22)

is the steady-state Kalman filter gain.

Tension measurement If a force ring is used to measure the tension �� the measurement equationsimply becomes:

� � �� (23)

Payload acceleration measurement If the tension � � �� cannot be measured directly, � can becomputed from the payload acceleration �� as

� � �� (24)

4.4 Motor Speed Observer (No Tension Measurements)

If only encoder measurements �� are available the model becomes:��

��

��

�

� ��

�

��

�

�� (25)

��

� � ��

�(26)

In this case a linear observer for motor speed �� using encoder �� and speed reference ��

measurements can be constructed as:��

��

��

�

� ��

�

��

�

��

�!�

!�

�� (27)

� � ��

��

�(28)

where !� and !� are two positive design gains.

4.5 Combined Payload Position and Velocity Observer

Define

�� (payload position) (29)

�� (payload speed) (30)

" � �� (measured payload acceleration) (31)

The observer model then becomes:

�� (32)

�� " # � (33)

� �� (34)

10

where � and � are zero-mean Gaussian white noise processes. The bias # is found apriori as:

# � ��"�� m/s� (35)

where � �� Hence, the observer becomes:��

��

��

� ��

�

��

��" #

�!�

!�

�� (36)

� � ��

��

�(37)

where pole-placement of a double integrator suggest that the observer gains are chosen as:

!� � �� (38)

!� � ��

� (39)

4.5.1 Payload position measurement

If a camera or another position sensor used to measure the payload position �� the measurement equationsimply becomes:

� � (40)

4.5.2 Motor encoder measurement

If the payload position � � cannot be measured directly, can be computed from the motor position�� simply by setting � ��. The limitation of this is that the motor position signal only contains thelow-frequency part of the payload position. However, this is not a major problem since the accelerationsensor contains information mainly on the high-frequency part of the payload position. Another problemis 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 �� and �� . The bandwidth ofthe observer should be placed below the resonance frequency since the encoder signal does not containuseful information at this frequency.

4.6 Payload position reconstruction in the frequency domain

The motor encoder measurement can be used to reconstruct the low-frequency part of the payload posi-tion signal and the payload acceleration measurement can be used to reconstruct the high-frequency partof the payload position signal. The following estimate combines these signals

��

��

We choose �� and �� equal to a 2nd order Butterworth high-pass filter with cut-off fre-quency at �� .

5 Experimental setup and instrumentation

The winch is an AC servomotor with an internal speed control loop. There are vertical accelerometersin both the payload and vessel, and a wire tension sensor. In the moonpool there are wave metersmeasuring the wave amplitude in a vessel-fixed coordinate frame, i.e. �� . The motor position�� is measured using an encoder.

The total scale model mass is 157 �� with a water plane area of 0.63 ��.Two payloads where tested: a sphere and a pump inside a frame, see Figure 4

11

Figure 4: Payloads used during experiments.

6 Experimental results

Experimental results for the scenaria summarized in Table 1 are described in this section. The notation”Ws” is an acronym for ”Wave synchronization” while ”Hc” is an acronym for ”Heave compensation”.Notice that the reported values of the wave period � and amplitude � are values specified to the wavegenerator, and will therefore deviate somewhat from the values measured (especially the amplitude).Three sea states are considered:

� Regular waves with period � � �� s and amplitude � � � cm. This wave frequency is closeto the moonpool resonance frequency, see Figure 3, and the wave amplitude inside the moonpoolis about 2 times the basin wave amplitude. The wave height (full scale) is around �� m.

� Regular waves with period � � �� s and amplitude � � � cm. At this frequency there is noresonance. The wave height (full scale) is around �� m.

� JONSWAP spectrum with � � �� s and � � � cm. The significant wave height (full scale) isaround �� m.

We present both raw and filtered data. The filtered data contains mainly components in the frequencyband between 0.5 and 1.5 Hz, where the significant wave motion is located1. This filtering allows theeffects of the wave synchronization to be separated from other effects, since this is the frequency bandwhere the wave synchronization is effective. It is therefore natural to verify the performance of thewave synchronization on the filtered data, neglecting low-frequency components due to buoyancy andhigh-frequency components due to measurement noise and wire resonances. Still, it is also of interest toevaluate 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 containpassive heave compensation or damping. Also, the excitation caused by signal noise and winch motordrive is not directly scalable to full scale implementation and must be considered in the context of thetechnology used for implementation. Thus, we will base our conclusions mainly on the filtered data.

1The filtering is carried out using 4th order Butterworth highpass and lowpass filters using the MATLAB filtfiltalgorithm to ensure no phase shift.

12

Wave spectrum Payload Controller TestsRegular, � � �� s, � � � cm Sphere Ws A,BRegular, � � �� s, � � � cm Sphere Hc A,BRegular, � � �� s, � � � cm Sphere Ws+Hc A,BRegular, � � �� s, � � � cm Sphere - A,BRegular, � � �� s, � � � cm Sphere Ws A,BRegular, � � �� s, � � � cm Sphere Hc A,BRegular, � � �� s, � � � cm Sphere Ws+Hc A,BRegular, � � �� s, � � � cm Sphere - A,BJonswap, � � �� s, � � � cm Sphere Ws A,B,C,D,EJonswap, � � �� s, � � � cm Sphere Hc A,B,C,D,EJonswap, � � �� s, � � � cm Sphere Ws+Hc A,B,C,D,EJonswap, � � �� s, � � � cm Sphere - A,B,C,D,ERegular, � � �� s, � � � cm Pump Ws A,BRegular, � � �� s, � � � cm Pump Hc A,BRegular, � � �� s, � � � cm Pump Ws+Hc A,BRegular, � � �� s, � � � cm Pump - A,BRegular, � � �� s, � � � cm Pump Ws A,BRegular, � � �� s, � � � cm Pump Hc A,BRegular, � � �� s, � � � cm Pump Ws+Hc A,BRegular, � � �� s, � � � cm Pump - A,BJonswap, � � �� s, � � � cm Pump Ws+Hc A,B,C,D,EJonswap, � � �� s, � � � cm Pump Ws A,B,C,D,EJonswap, � � �� s, � � � cm Pump Hc A,B,C,D,EJonswap, � � �� s, � � � cm Pump - A,B,C,D,E

Table 1: Overview of experiments.

13

Time series from selected tests will be shown. In addition we show the following statistics for eachtest:

� �� - significant wave height (model scale) or wave height (if the waves are regular).

� $�� - standard deviation for tension signal �� where signal components under 0.5 Hz are

removed.

� �� - peak minimum value of tension.

� �� - peak maximum value of tension.

� $� �� - standard deviation for payload acceleration signal �� where signal components under0.5 Hz are removed.

� �� - peak minimum value of payload acceleration.

� �� - peak maximum value of payload acceleration.

The statistics presented are corrected for variations in average speed and length of experiments, such thatthe standard deviations presented are averages over a constant number of wave periods during the waterentry phase of each operation.

Experimental data.The experimental data can be found in MAT-files. The files are named such asreg 1.25 2.0 paysphere a.matwhere reg indicates regular waves, 1.25 is the wave period, 2.0 is the wave amplitude, paysphere

indicate the payload and the last letter a indicates test A. The results can be plotted in MATLAB usingthe plot exp script.

6.1 Regular waves at � � �� s and � � � cm

The results of tests A and B are summarized in Table 2. Figures 5 - 8 shows some relevant signals fortest B with the spherical payload. Figures 9 - 12 shows some relevant signals for test A with the pumppayload. The following observations and remarks can be made:

� For the spherical payload, the wave synchronization reduces the tension standard deviation by25.2 % in test A and 21.4 % in test B, compared to no control, when considering filtered data. Wavesynchronization in combination with heave compensation reduces the tension standard deviationby 22.0 % in test A and 21.8 % in test B, compared to no control, when considering filtered data.

� For the spherical payload, the peak minimum tension is increased from 1.21 N to 1.58 N usingwave synchronization in test A and from 1.36 N to 1.58 N in test B, compared to no control, whenconsidering filtered data. This is beneficial as it reduces the possibility of wire snatch. The peakmaximum tension is somewhat increased with the wave synchronization.

� The use of heave compensation does not give any significant reduction of tension variability inthis sea state. However, it gives significant reduction of the standard deviation of the payloadacceleration.

� The payload acceleration is significantly increased using wave synchronization.

� When considering the unfiltered data, similar qualitative conclusions can be made in most cases.

14

� For the pump-in-frame payload, the wave synchronization reduces the tension standard deviationby 7.2 % in test A and 12.8 % in test B, compared to no control, when considering filtered data.Wave synchronization in combination with heave compensation reduces the tension standard de-viation by 8.9 % in test A and 15.3 % in test B, compared to no control, when considering filtereddata.

6.2 Regular waves at � � �� s and � � � cm

The results of tests A and B are summarized in Table 3. Figures 13 - 16 shows the relevant signals fortest B with the spherical payload. Figures 17 - 20 shows the relevant signals for test A with the pumppayload. The following observations and remarks can be made:

� For the spherical payload, the wave synchronization reduces the tension standard deviation by 31.0% in test A and 33.1 % in test B, compared to no control, when considering filtered data. Wavesynchronization in combination with heave compensation reduces the tension standard deviationby 48.1 % in test A and 54.2 % in test B, compared to no control, when considering filtered data.

� For the spherical payload, the minimum tension is reduced from 1.36 N to 1.23 N using wavesynchronization in test A and from 1.59 N to 1.67 N in test B, compared to no control, whenconsidering filtered data.

� For the spherical payload, the use of heave compensation alone gives significant reduction of ten-sion variability in this very rough sea state, namely 50.3 % in test A and 25.9 % in test B. Thisindicates that significant reduction in tension variance can be achieved by either heave compensa-tion or wave synchronization, but largest reduction is achieved by combining them.

� The payload acceleration is significantly reduced using any combination of heave compensationand wave synchronization.

� When considering the unfiltered data with the spherical payload, similar qualitative conclusionscan be made in most cases. With the pump-in-frame payload the unfiltered data shows increasedvariance with wave synchronization and heave compensation.

� For the pump payload the wave synchronization reduces the tension standard deviation by 36.9% in test A and 34.5 % in test B. Wave synchronization in combination with heave compensationreduces the tension standard deviation by 61.4 % in test A and 53.2 % in test B, when consideringfiltered data. Hence, the pump and spherical payloads give similar results in this sea state.

6.3 JONSWAP spectrum at � � �� s and � � � cm

The averaged results of tests A to E are summarized in Table 4. Figures 21 - 24 shows the relevantsignals for test B with the spherical payload, which is the test with poorest performance of the wavesynchronization. The following observations and remarks can be made:

� For the spherical payload, the wave synchronization reduces the tension standard deviation by27.6 % on average, compared to no control, when considering filtered data. Wave synchronizationin combination with heave compensation increases the tension standard deviation by 1.2 % onaverage, compared to no control, when considering filtered data.

� For the spherical payload, the minimum tension is reduced from 1.61 N to 1.68 N using wave syn-chronization on average, compared to no control, when considering filtered data and the sphericalpayload.

15

Mode �� $�� $� ��

Sphere Test A Filtered (0.5 - 1.5 Hz)Ws 0.037702 0.267813 1.456132 5.795719 0.311217 -0.602558 0.614468Hc 0.036499 0.401340 1.138118 5.582255 0.038327 -0.067921 0.071675

Ws+Hc 0.033925 0.279348 1.583275 6.034342 0.306320 -0.617523 0.617327none 0.038299 0.357952 1.213946 5.722934 0.089129 -0.127943 0.130912

Sphere Test A UnfilteredWs 0.037702 0.432547 0.696337 6.684045 0.600266 -2.162592 1.903956Hc 0.036499 0.507332 0.822350 6.100927 0.406043 -1.064087 0.827034

Ws+Hc 0.033925 0.447314 0.827344 6.704614 0.611461 -1.832424 1.710669none 0.038299 0.528371 0.641684 5.848055 0.522571 -1.263501 1.120482

Sphere Test B Filtered (0.5 - 1.5 Hz)Ws 0.037784 0.268407 1.592595 6.202517 0.280106 -0.593456 0.601818Hc 0.035806 0.372235 1.360973 5.841397 0.039713 -0.074419 0.069177

Ws+Hc 0.034092 0.267272 1.580037 5.915207 0.270912 -0.595244 0.600746none 0.037597 0.341721 1.366221 5.822916 0.085884 -0.126354 0.130272

Sphere Test B UnfilteredWs 0.037784 0.433747 0.880140 6.821880 0.561937 -1.969048 1.801773Hc 0.035806 0.464373 0.978126 6.170447 0.356264 -0.985576 0.778424

Ws+Hc 0.034092 0.433610 0.680507 6.712235 0.568871 -2.037143 1.383322none 0.037597 0.492832 0.873798 5.989654 0.482955 -0.977351 1.038329

Pump Test A Filtered (0.5 - 1.5 Hz)Ws 0.039584 0.167990 3.959354 6.040978 0.284068 -0.589020 0.624885Hc 0.038119 0.180205 3.718327 5.793527 0.048950 -0.075886 0.078593

Ws+Hc 0.033350 0.164843 3.916865 5.993941 0.272455 -0.603828 0.668273none 0.039988 0.180951 3.839583 6.026410 0.104844 -0.157119 0.149506

Pump Test A UnfilteredWs 0.039584 0.336488 3.416551 7.039119 0.569014 -2.287737 2.299409Hc 0.038119 0.299483 3.289559 6.244836 0.389238 -1.083781 0.833700

Ws+Hc 0.033350 0.327978 3.269907 6.501920 0.547977 -2.050921 2.273515none 0.039988 0.247486 3.597863 6.241724 0.276047 -0.737595 0.568726

Pump Test B Filtered (0.5 - 1.5 Hz)Ws 0.039982 0.158423 3.819313 5.812970 0.261576 -0.567374 0.597118Hc 0.038632 0.162784 3.573695 5.765090 0.033465 -0.052580 0.064725

Ws+Hc 0.035470 0.153891 3.840655 5.841607 0.253084 -0.595864 0.629548none 0.040687 0.181612 3.560337 5.572643 0.106293 -0.154130 0.153616

Pump Test B UnfilteredWs 0.039982 0.314047 3.127796 6.422724 0.519078 -1.695554 1.982500Hc 0.038632 0.293130 3.156398 6.223716 0.401000 -1.172815 0.920834

Ws+Hc 0.035470 0.311899 3.246430 6.242517 0.520933 -2.172067 1.457749none 0.040687 0.243299 3.310918 5.913031 0.270390 -0.720871 0.667572

Table 2: Summary of results with regular waves at � � �� s and � � � cm.

16

260 265 270 275 280−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)

Wave synchronization

185 190 195 200−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)

Heave compensation

95 100 105 110 115−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)

Wave sychronization and heave compensation

345 350 355 360 365−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)

No control

260 265 270 275 280

−0.2

0

0.2

0.4

0.6

0.8

time (s)

mot

or p

ositio

n (m

)


185 190 195 200

−0.2

0

0.2

0.4

0.6

0.8

time (s)

mot

or p

ositio

n (m

)

Heave compensation

95 100 105 110 115

−0.2

0

0.2

0.4

0.6

0.8

time (s)

mot

or p

ositio

n (m

)


345 350 355 360 365

−0.2

0

0.2

0.4

0.6

0.8

time (s)

mot

or p

ositio

n (m

)

No control

Figure 5: Regular waves at � � �� s and � � � cm, Sphere, Test B

17

260 265 270 275 2800

2

4

6

8

time (s)

tens

ion (N

) and

LF

(cya

n)


185 190 195 2000

2

4

6

8

time (s)

tens

ion (N

) and

LF

(cya

n)

Heave compensation

95 100 105 110 1150

2

4

6

8

time (s)

tens

ion (N

) and

LF

(cya

n)


345 350 355 360 3650

2

4

6

8

time (s)

tens

ion (N

) and

LF

(cya

n)

No control

260 265 270 275 280−2

−1

0

1

2

time (s)

acc p

ayloa

d (re

d) a

nd L

F (c

yan)


185 190 195 200−2

−1

0

1

2

time (s)

acc p

ayloa

d (re

d) a

nd L

F (c

yan)

Heave compensation

95 100 105 110 115−2

−1

0

1

2

time (s)

acc p

ayloa

d (re

d) a

nd L

F (c

yan)


345 350 355 360 365−2

−1

0

1

2

time (s)

acc p

ayloa

d (re

d) a

nd L

F (c

yan)

No control


18

260 265 270 275 280−0.5

0

0.5

time (s)

acc v

esse

l


185 190 195 200−0.5

0

0.5

time (s)

acc v

esse

l

Heave compensation

95 100 105 110 115−0.5

0

0.5

time (s)

acc v

esse

l


345 350 355 360 365−0.5

0

0.5

time (s)

acc v

esse

lNo control

0 0.5 1 1.5 2 2.5 30

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Wave frequency (Hz)

Powe

r spe

ctrum

Moonpool (green and blue) and basin (red)


19

260 265 270 275 280−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) m

oonp

ool


185 190 195 200−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) m

oonp

ool

Heave compensation

95 100 105 110 115−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) m

oonp

ool


345 350 355 360 365−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) m

oonp

ool

No control

260 265 270 275 280−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) b

asin

(red

)


185 190 195 200−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) b

asin

(red

)

Heave compensation

95 100 105 110 115−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) b

asin

(red

)


345 350 355 360 365−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) b

asin

(red

)

No control

Figure 8: Regular waves at � � �� s and � � � cm, Pump, Test B

20

250 255 260 265 270 275−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)


170 175 180 185 190−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)

Heave compensation

90 95 100 105 110−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)


335 340 345 350 355−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)

No control

250 255 260 265 270 275

−0.2

0

0.2

0.4

0.6

0.8

time (s)

mot

or p

ositio

n (m

)


170 175 180 185 190

−0.2

0

0.2

0.4

0.6

0.8

time (s)

mot

or p

ositio

n (m

)

Heave compensation

90 95 100 105 110

−0.2

0

0.2

0.4

0.6

0.8

time (s)

mot

or p

ositio

n (m

)


335 340 345 350 355

−0.2

0

0.2

0.4

0.6

0.8

time (s)

mot

or p

ositio

n (m

)

No control

Figure 9: Regular waves at � � �� s and � � � cm, Pump, Test A

21

250 255 260 265 270 2750

2

4

6

8

time (s)

tens

ion (N

) and

LF

(cya

n)


170 175 180 185 1900

2

4

6

8

time (s)

tens

ion (N

) and

LF

(cya

n)

Heave compensation

90 95 100 105 1100

2

4

6

8

time (s)

tens

ion (N

) and

LF

(cya

n)


335 340 345 350 3550

2

4

6

8

time (s)

tens

ion (N

) and

LF

(cya

n)

No control

250 255 260 265 270 275−2

−1

0

1

2

time (s)

acc p

ayloa

d (re

d) a

nd L

F (c

yan)


170 175 180 185 190−2

−1

0

1

2

time (s)

acc p

ayloa

d (re

d) a

nd L

F (c

yan)

Heave compensation

90 95 100 105 110−2

−1

0

1

2

time (s)

acc p

ayloa

d (re

d) a

nd L

F (c

yan)


335 340 345 350 355−2

−1

0

1

2

time (s)

acc p

ayloa

d (re

d) a

nd L

F (c

yan)

No control


22

250 255 260 265 270 275−0.5

0

0.5

time (s)

acc v

esse

l


170 175 180 185 190−0.5

0

0.5

time (s)

acc v

esse

l

Heave compensation

90 95 100 105 110−0.5

0

0.5

time (s)

acc v

esse

l


335 340 345 350 355−0.5

0

0.5

time (s)

acc v

esse

lNo control

0 0.5 1 1.5 2 2.5 30

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Wave frequency (Hz)

Powe

r spe

ctrum



23

250 255 260 265 270 275−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) m

oonp

ool


170 175 180 185 190−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) m

oonp

ool

Heave compensation

90 95 100 105 110−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) m

oonp

ool


335 340 345 350 355−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) m

oonp

ool

No control

250 255 260 265 270 275−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) b

asin

(red)


170 175 180 185 190−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) b

asin

(red)

Heave compensation

90 95 100 105 110−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) b

asin

(red)


335 340 345 350 355−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) b

asin

(red)

No control


24

Mode �� $�� $� ��

Sphere Test A Filtered (0.5 - 1.5 Hz)Ws 0.140929 0.119137 1.233426 5.787420 0.137208 -0.323603 0.314699Hc 0.129563 0.085772 1.566849 5.849324 0.085160 -0.183688 0.177011

Ws+Hc 0.145822 0.089639 1.441192 5.672045 0.071665 -0.172134 0.196842none 0.133366 0.172708 1.363261 5.783954 0.197983 -0.357805 0.403569

Sphere Test A UnfilteredWs 0.140929 0.270422 0.897056 6.123736 0.377981 -1.452998 1.232566Hc 0.129563 0.310364 0.825410 6.321483 0.434072 -1.166016 1.282921

Ws+Hc 0.145822 0.268914 0.885345 6.116791 0.369244 -1.260728 1.184828none 0.133366 0.359688 0.839921 6.020499 0.475496 -1.228008 1.104505

Sphere Test B Filtered (0.5 - 1.5 Hz)Ws 0.146149 0.126486 1.676149 6.391967 0.146075 -0.330311 0.351247Hc 0.125508 0.140045 1.812155 5.955645 0.088839 -0.196299 0.210371

Ws+Hc 0.132829 0.086440 1.634072 5.786055 0.066158 -0.158606 0.150261none 0.147696 0.188946 1.591259 5.968182 0.209952 -0.422405 0.444642

Sphere Test B UnfilteredWs 0.146149 0.322511 1.220800 6.985844 0.434190 -1.462208 1.062934Hc 0.125508 0.341766 1.077042 6.340165 0.452463 -1.205510 1.179610

Ws+Hc 0.132829 0.280850 1.041029 6.271269 0.394364 -1.072351 1.046822none 0.147696 0.407262 0.968331 6.211750 0.529311 -1.537896 1.348271

Pump Test A Filtered (0.5 - 1.5 Hz)Ws 0.125626 0.083144 3.643843 5.728446 0.133995 -0.357737 0.366876Hc 0.147884 0.063287 3.698299 5.592405 0.086909 -0.191490 0.177141

Ws+Hc 0.155504 0.050923 3.765016 5.584578 0.061904 -0.141546 0.135270none 0.138238 0.131814 3.681908 5.791855 0.196529 -0.333658 0.357109

Pump Test A UnfilteredWs 0.125626 0.233171 3.134766 5.947250 0.381145 -1.316349 1.178281Hc 0.147884 0.233697 3.258585 6.097025 0.380192 -1.126635 0.932620

Ws+Hc 0.155504 0.233955 3.210500 6.068031 0.383082 -1.066368 1.109607none 0.138238 0.202107 3.409899 6.188390 0.315419 -0.821098 0.874770

Pump Test B Filtered (0.5 - 1.5 Hz)Ws 0.131482 0.093484 3.854790 5.875933 0.131105 -0.306519 0.300447Hc 0.170757 0.074580 3.914421 5.846177 0.091627 -0.203026 0.200025

Ws+Hc 0.159757 0.066779 3.892946 5.792726 0.061494 -0.132914 0.131479none 0.145883 0.142775 3.754977 5.916678 0.200955 -0.361894 0.374791

Pump Test B UnfilteredWs 0.131482 0.240533 3.315567 6.244126 0.383532 -1.219221 1.336926Hc 0.170757 0.239702 3.471249 6.235620 0.373924 -1.394553 1.069778

Ws+Hc 0.159757 0.255866 3.354529 6.422333 0.413939 -1.317648 1.333159none 0.145883 0.211895 3.518764 6.449334 0.314398 -0.828859 0.807912

Table 3: Summary of results with regular waves at � � �� s and � � � cm.

25

260 265 270 275 280−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)


185 190 195 200 205−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)

Heave compensation

110 115 120 125 130−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)


335 340 345 350−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)

No control

260 265 270 275 280

−0.2

0

0.2

0.4

0.6

0.8

time (s)

mot

or p

ositio

n (m

)


185 190 195 200 205

−0.2

0

0.2

0.4

0.6

0.8

time (s)

mot

or p

ositio

n (m

)

Heave compensation

110 115 120 125 130

−0.2

0

0.2

0.4

0.6

0.8

time (s)

mot

or p

ositio

n (m

)


335 340 345 350

−0.2

0

0.2

0.4

0.6

0.8

time (s)

mot

or p

ositio

n (m

)

No control


26

260 265 270 275 2800

2

4

6

8

time (s)

tens

ion (N

) and

LF

(cya

n)


185 190 195 200 2050

2

4

6

8

time (s)

tens

ion (N

) and

LF

(cya

n)

Heave compensation

110 115 120 125 1300

2

4

6

8

time (s)

tens

ion (N

) and

LF

(cya

n)


335 340 345 3500

2

4

6

8

time (s)

tens

ion (N

) and

LF

(cya

n)

No control

260 265 270 275 280−2

−1

0

1

2

time (s)

acc p

ayloa

d (re

d) a

nd L

F (c

yan)


185 190 195 200 205−2

−1

0

1

2

time (s)

acc p

ayloa

d (re

d) a

nd L

F (c

yan)

Heave compensation

110 115 120 125 130−2

−1

0

1

2

time (s)

acc p

ayloa

d (re

d) a

nd L

F (c

yan)


335 340 345 350−2

−1

0

1

2

time (s)

acc p

ayloa

d (re

d) a

nd L

F (c

yan)

No control


27

260 265 270 275 280−0.5

0

0.5

time (s)

acc v

esse

l


185 190 195 200 205−0.5

0

0.5

time (s)

acc v

esse

l

Heave compensation

110 115 120 125 130−0.5

0

0.5

time (s)

acc v

esse

l


335 340 345 350−0.5

0

0.5

time (s)

acc v

esse

lNo control

0 0.5 1 1.5 2 2.5 30

0.5

1

1.5

2

Wave frequency (Hz)

Powe

r spe

ctrum



28

260 265 270 275 280−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) m

oonp

ool


185 190 195 200 205−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) m

oonp

ool

Heave compensation

110 115 120 125 130−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) m

oonp

ool


335 340 345 350−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) m

oonp

ool

No control

260 265 270 275 280−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) b

asin

(red)


185 190 195 200 205−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) b

asin

(red)

Heave compensation

110 115 120 125 130−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) b

asin

(red)


335 340 345 350−0.08

−0.06

−0.04

−0.02

0

0.02

0.04

0.06

time (s)

wave

(m) b

asin

(red)

No control

Figure 16: Regular waves at � � �� s and � � � cm, Pump, Test B

29

225 230 235 240 245−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)


150 155 160 165 170−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)

Heave compensation

75 80 85 90 95−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)


300 305 310 315 320−0.2

−0.1

0

0.1

0.2

time (s)

mot

or sp

eed

(m/s)

No control

225 230 235 240 245

−0.2

0

0.2

0.4

0.6

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32

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33

� The use of heave compensation does not give any significant reduction of tension variability.

� The payload acceleration is significantly reduced using heave compensation but wave synchro-nization gives some increase in acceleration.

� For the pump-in-frame payload, the wave synchronization reduces the tension standard deviationby 4.2 % on average. Wave synchronization in combination with heave compensation increasesthe tension standard deviation by 10.0 % on average, when considering filtered data.

� When considering the unfiltered data with the spherical payload, similar qualitative conclusionscan be made in most cases. With the pump-in-frame payload the variance is somewhat increased.

� It should be remarked that averaging not very meaningful because there are only five tests.

Mode �� $�� $� ��

Sphere Avg. Test A-E Filtered (0.5 - 1.5 Hz)Ws 0.056344 0.077631 1.678904 5.852417 0.103220 -0.223124 0.223629Hc 0.052885 0.120011 1.586839 5.779087 0.047672 -0.107292 0.111874

Ws+Hc 0.058565 0.108441 1.693125 6.066377 0.119405 -0.316455 0.316524none 0.063855 0.107173 1.608691 6.055017 0.082840 -0.187113 0.183162

Sphere Avg. Test A-E UnfilteredWs 0.056344 0.314209 0.995342 6.298145 0.434457 -1.523909 1.116269Hc 0.052885 0.302401 1.030220 6.238224 0.391227 -1.326201 1.012649

Ws+Hc 0.058565 0.348831 0.947103 6.593276 0.471319 -1.719319 1.393762none 0.063855 0.381432 0.940792 6.431565 0.506801 -1.258611 1.133136

Pump Avg. Test A-E Filtered (0.5 - 1.5 Hz)Ws 0.051406 0.058548 3.919543 5.787056 0.106257 -0.274711 0.260420Hc 0.050830 0.062965 3.887495 5.794023 0.043820 -0.112190 0.108023

Ws+Hc 0.061632 0.054974 3.902177 5.664070 0.110833 -0.295802 0.297791none 0.061182 0.061126 3.864759 5.811901 0.087255 -0.203082 0.194729

Pump Avg. Test A-E UnfilteredWs 0.051406 0.241929 3.291288 6.137570 0.398284 -1.512659 1.356862Hc 0.050830 0.223636 3.394861 6.363680 0.350418 -1.126899 0.944196

Ws+Hc 0.061632 0.240381 3.319888 6.076801 0.398873 -1.456790 1.101630none 0.061182 0.160626 3.548589 6.315000 0.247505 -0.679781 0.598222

Table 4: Summary of results with JONSWAP spectrum at � � �� s and � � � cm, averaged over 5tests.

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 [2].

Figures 25, 26, and 27 show the payload acceleration ��, wire tension �� and hydrodynamic forces� during a lifting operation with irregular waves, typical moonpool wave amplitude 0.04 � and waveperiod 1.0 (1.2 � and 6.3 in full scale), and a typical vessel heave amplitude 0.025 � and period 1.6

34

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Figure 21: JONSWAP spectrum at � � �� s and � � � cm, Sphere, Test B

35

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Figure 24: JONSWAP spectrum at � � �� s and � � � cm, Pump, Test B

38

(0.75 � and 8.7 in full scale). The figures also show the vertical payload, vessel and wave positionsduring the water entry phase. The required motor speed (not shown) is within realistic limits. Thedifferent strategies are simulated with the same external excitations (waves) and similar improvementsin performance are typically achieved for other realizations of the random waves. It can be seen thatpayload acceleration is smallest with heave compensation, while the wave synchronization reduces thehydrodynamic forces and wire tension during water entry. The remaining hydrodynamic forces and wiretension variations are to a large degree high-frequency (at the wire resonance). They might be furtherdamped by a standard passive heave compensation system as the motor bandwidth does not allow activecontrol at such high frequencies.

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Figure 25: Simulations without active control, i.e. constant speed.

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Figure 26: Simulations with heave compensation.

39

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Figure 27: Simulations with wave synchronization.

8 Discussions

Wave synchronization, either alone or in combination with heave compensation, significantly reducestension 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 inorder to synchronize with the wave motion, while under some sea states the payload acceleration is alsoreduced. The experiments with regular waves showed high repeatability, with small variations betweenexperiments under the same sea state.

There are significant differences in performance when comparing the spherical and pump-in-framepayload. The wave synchronization shows consistently greater improvements with the spherical payloadthan with the pump-in-the frame. Reasons for this may be that the sphere is more sensitive to the wavesbecause it is a closed structure, while the pump is in an open frame. The pump-in-frame has significantlyless 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 interms of reduced tension variability and peaks at least under some sea states and with some payloads.

The following limitations to performance has been identified:

1. The feedforward approach leads to a phase error, due to the dynamics of the motor and signalfiltering. In our experimental setup this error is significant, but not compensated for. For a waveperiod of 1.0 s, the phase loss in the motor is 15 degrees and the phase loss in the filtering of thewave amplitude measurement is about 25 deg. The total phase loss is around 40 deg, which issignificant. It is therefore clear that the use of some model-based predictor of the wave amplitudemay lead to significant improvement of performance.

2. The wire resonance is being excited, also without the use of wave synchronization or heave com-pensation. The use of heave compensation or heave compensation reduces the excitations of theresonance in most cases with the spherical payload, while the excitations of the resonance seemsto be typically increased with the pump-in-frame payload. There are several possible sources forthese excitations:

(a) Even with constant speed reference the AC motor speed measurement exhibit oscillations

40

with significant frequency components in area where the resonance frequency is. This situa-tion may be improved by the use of a gear that will increase operating range of the motor andshift the frequencies. Notice that the AC motor control system is built into the motor driveand cannot be modified.

(b) The water flow inside the moonpool may contain frequency components that give hydrody-namic forces that excite the resonance.

(c) With heave compensation and wave feed-forward is it clear that measurement noise mayexcite the resonance frequency. Since there is very little damping in the suspension and theresonance frequency is less than a decade above from typical wave frequencies, any filteringwill lead to phase loss as described above. A further complication is that the resonancefrequency depend on payload mass and position-dependent hydrodynamic properties suchas added mass. Improvements may be expected by adaptive notch filtering, in combinationwith wave prediction. Obviously, the technical solutions for wave measurements, signaltransmission and electronics may be modified to reduce the overall noise level. Noise isamplified due to numerical differentiation when computing the wave surface speed from thewave amplitude measurements. This may be avoided via the use of position control of theAC motor rather than velocity control.

It may be that the degree of damping in the scale model is too small compared to typical full scalecranes, 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 tobe added to an existing active heave compensation system. The key requirement is the availability of ameasurement or estimate of the water vertical velocity within the moonpool. In the scale experimentswe utilized a standard laboratory-type wave amplitude measurement system from DHI (wave gauge202/201), which relies on measurement of the water-level dependent conductivity between two parallelelectrodes. In order to determine the water vertical velocity the measured amplitude was differentiatedafter low-pass and notch filtering. In full scale implementation the same measurement principle mayprove useful, but alternatives such as water pressure and optical/radar systems should be evaluated. Ifthe water vertical velocity can be measured directly, it would be an advantage since the noise-sensitivedifferentiation of the amplitude measurement can be avoided. Very rough estimates indicate that moon-pool wave amplitude measurements at 0.05 m accuracy or water vertical velocity measurements at 0.05m/s accuracy is sufficient.

A measurement or estimate of the payload position relative to the mean sea level is required. Sincethis measurement is only used to gradually activate/deactivate the wave synchronization as the payloadmoves through the water entry phase, high accuracy is not required (0.50 m accuracy is expected to besufficient).

A model scale sampling frequency on 200 Hz was utilized, corresponding to a full scale samplingfrequency of 36 Hz. Such a high sampling frequency was chosen mainly to allow accurate analysisof data (for presentation and performance evaluation) and digital signal processing (rather than analogfiltering). In order to implement the core functionality of the wave synchronization, a significantly lowersampling frequency is expected to be sufficient. In any case, the computational requirements are verysmall, essentially limited to a few filters.

The motor time constant � � �� s corresponds to approximately � � �� s in full scale, and itstime-delay � � �� corresponds to � � �� s in full scale. It is assumed the motor is equipped witha speed controller.

Tuning of the controller involves the following aspects:

41

� Bandpass filters must be tuned to reduce sensor signal high-frequency noise and low-frequencydrift. The cutoff frequencies depend on the wave spectrum and may be adapted online for optimalperformance.

� The notch filter is tuned to avoid excitation of the wire resonance. Since these will depend on thepayload position, mass and hydrodynamic parameters, in addition to the type of wire, it shouldbe considered to implement an adaptive filter that estimates the resonance frequency in real timeduring operation. In order to get information for the adaptive filter, a tension or accelerationmeasurement on payload or wire is useful.

� An estimate of how the water vertical velocity reduces with the water depth must be made, basedon the moonpool geometry.

10 Suggestions for future work

Below are some ideas for future work:

1. Redesign of rig. In particular the deplacement (main dimensions) of the rig should be increased tosimplify ballasting and to improve metacentric stability.

2. The springs used for station-keeping should be mounted in the horizontal plane in order to avoidexcitation of the heave mode. In addition the optimal stiffness of the springs should be computedaccording the P-gain in DP control system.

3. Improved design and tuning of filters, including predictors to reduce phase error due to filteringand motor dynamics as well as to avoid exciting resonant wire elasticity. This may give significantperformance improvements.

4. Motor position control rather than speed control: avoid numerical differentiation of wave ampli-tude measurements. This improves phase margins and performance of the wave synchronizationalgorithm.

5. Compensation of roll and pitch for acceleration measurements (VRU-functionality).

6. Redesign of suspension (four wires instead of one) to improve horizontal stability and to get real-istic 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 ve-locity.

9. Test the method also for water exit.

10. Evaluate alternative control strategies such as combination of feedback/feedforward and optimiza-tion of payload trajectories.

11 Conclusions

Wave synchronization has been suggested to improve the performance of heave compensated offshorecranes during the water entry phase in moonpool marine operations. The method requires measurementof the wave amplitude in the moonpool, and is implemented as a feedforward compensator. Experimentalresults using a scale model shows that typical reduction of the wire tension standard deviation is between0 to 50 % depending on the sea state and payload.

42

References

[1] M. P. Fard and F. G. Nielsen. Simulation of coupled vessel-load dynamics. Technical Report NH-00036960, Norsk Hydro, Exploration and Production, Bergen, Norway, 2001.

[2] T. I. Fossen and T. A. Johansen. Modelling and identification of offshore crane-rig system. TechnicalReport 2001-12-T, Department of Engineering Cybernetics, NTNU, Trondheim, Norway, 2001.

[3] T. A. Johansen and T. I. Fossen. Hydrolaunch free decay tests. Technical Report 2001-18-T, Depart-ment of Engineering Cybernetics, NTNU, Trondheim, Norway, 2001.

[4] F. G. Nielsen. Coupled hydrodynamics for the hydrolaunch vessel with load. Technical report, NorskHydro, Exploration and Production, Bergen, Norway, 2001.

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ansv. sign. - institutt for teknisk kybernetikk, ntnu · 2008. 4. 16. · norsk hydro ekstrakt ......

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