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recovery crane
Ocean Engng, Vol. 10, No. 4, pp. 295--300, 1983.
0029-8018/83 $3.00+ .00 © 1983 Pergamon Press Ltd.
Printed in Great Britain.
TECHNICAL A MOTION-COMPENSATED
NOTE
LAUNCH/RECOVERY
CRANE
E.H. KIDERA Ocean Engineering, U.S. Naval Academy, Annapolis, MD 21402, U.S.A. Abstract--Oceanographic instrumentation towed from a surface platform is normally subjected to unwanted vertical motion imparted by the rolling, heaving, and pitching of the platform. This motion can cause severe shock-loading conditions in the tow cable and induces unplanned depth variations that reduce the instruments' resolution. This article discusses a unique motion-compensation system for use with towed instrumentation that eliminates shock loading and minimizes depth variation. The system is operational in sea states as high as 5 and effectively decouples 96% of the vessel's motion.
BACKGROUND
ONE OF the primary oceanographic instrumentation systems used by APL's Submarine Technology Division is a faired cable system instrumented over much of its length with thermistors, fluorometers, and conductivity cells (hence, "TFC chain"); the chain is towed from a surface vessel. The use of a depressor combined with the low drag of the chain results in a nearly vertical catenary. Components of surface platform motion (pitch, roll, and heave) are transferred to the chain through the tow sheave. The resultant vertical motion of the chain has limited its utility for wave physics measurements and has restricted its safe operation to low sea states to avoid excessively high shock loading. A passive boom-motion compensator was conceived, designed, and built to handle the TFC chain and reduce the vertical sensor motion. Figure 1 shows the articulated crane that was modified to provide motion compensation in addition to normal launch/recovery functions. A passive compensator acts as a soft spring with a natural period significantly longer than the natural period of platform motion. Theoretically, this action allows it to maintain constant cable tension, independent of platform motion, thus minimizing depth variations. System performance is limited by several factors so that the actual cable tension is not constant but varies slightly about some nominal value determined by the pressure in the system. The following are key design factors: (1) Boom sensitivity threshold - - the force required to initiate movement of the boom. "Stiction" (static friction) in the pivot bearings and piston seals determine the sensitivity. (2) Natural period of the boom - - determined by the length of the boom and the volume of the support gas. A soft spring that corresponds to a large volume is desired. (3) System damping - - primarily a result of losses in the hydraulic oil flow. It should be kept to a minimum for best performance. (4) Boom stroke - - the dynamic range of the boom. Sufficient stroke is needed to allow operation in the most adverse sea states that will be encountered. 295
296
E.H. KIDERA
(5) Depressor effective mass (DEM) - - the mass of the depressor itself plus the entrained water mass. For optimum performance, a depressor with a high DEM and low weight in seawater is desirable. A winged depressor has a high DEM because of the water above the wing that must be accelerated with the depressor. Since the boom sensitivity threshold is fixed, the larger the DEM the smaller the acceleration will be and, thus, the smaller the amplitude of depth variation. Passive compensation is simple in theory but relatively tricky in practice. In designing a motion compensation system, all of the above factors must be carefully weighed and matched to the platform characteristics, chain catenary, and ocean conditions that are involved. As in most such matters, compromises dictated by limitations of time and funds must also be made.
System description The launch/recovery m o t i o n - c o m p e n s a t e d crane incorporates several key components: (1) (2) (3) (4)
The IMTCO model 1725 crane, Motion-compensation modifications and equipment added to the crane, Bottle rack for a large gas volume and adjustment, and Power pack for launch/recovery and general crane use.
The basic crane is an articulated, hydraulic truck crane. Features such as dual main-boom cylinders, a high capacity, a long reach, a rotation system and low cost make it the ideal choice for modification to shipboard use as a launch/recovery motion compensator. The crane was modified by the factory to APL's specifications. A counterweight was added to balance the boom, and the main boom cylinders were ported for connection t o the motion-compensation components. At APL, further changes were made to the crane to allow its use at sea. It was oceanized, a 4-ft-diameter towing sheave and a depressor saddle were mounted, and a fairlead was added to restrain the thermistor chain during operation. However, the primary modifications were to allow its operation as a motion compensator. Accumulators were added via the special porting to the extension side of the main-boom cylinders. The accumulators act as an interfac~e between the hydraulic oil in the cylinder and the nitrogen that provides the spring. Bladder accumulators were chosen because of their high sensitivity to pressure change. In the event of failure of the motion compensation system, all components can be isolated and the launch/recovery operations carried out normally. A bottle rack connected to the accumulators supplied a large adjustable gas volume. This configuration allowed the spring constant to be varied at will. The bottle rack includes a high-pressure source and a low-pressure sink for adjustment of system pressure to match the tow loads at any given time. Adjustment is through solenoid valves controlled either manually from the operator's console or automatically from boom-position limit switches. The automatic feature allows the system to operate unattended for hours because changes in loading and boom position are adjusted by the limit switch to maintain the boom in the desired position.
•m o ~ o q ~ lu~md!nba ap!s~l~op ltu.unp uo!~!sod ~ o l u! u~oqs ~ueJ~ p~msu~lmo~-uo!lol,~
"! "gx-I
Technical Note
299
DISCUSSION The motion-compensated launch/recovery system had its maiden voyage in April 1981 aboard R/V Cape off the Florida coast. It was tested extensively and performed admirably. The system is capable of a 25-ft stroke and has a 25 sec natural period. In sea state 3, the system limited the motion of the depressor to 4 cm rms for periods of 10 sec or less. After the April test, several significant improvements were made to the system. They resulted in a reduced boom-sensitivity threshold and in a substantially reduced system-damping value. The unit was taken to sea in August 1981 in support of an instrumentation checkout. A sea state 5 was encountered, during which the boom performed flawlessly. Motion was reduced from 4 to 3 em rms in band. This performance corresponds to up to a 96% reduction in vessel-induced vertical motion at the characteristic ship pitch period. Figure 2, a power spectrum of depressor depth, shows the significant reduction in motion between compensated and uncompensated tow. Note the absence of motion peaks on the compensated spectrum. The run shown is for a 6-knot tow in sea state 4. CONCLUSIONS The motion compensator has proved itself to be a highly useful tool in the study of ocean structure and a valuable piece of equipment for launch/recovery and towing. It offers several advantages over alternate methods:
102
I
[
II
101_
I
I
I Ill
I
t
It
I
~ompensator off
100 E 10-1 %
Cornpen
10-2 10-3 10-4 10-5 10-6
10 degreesof freedom
10-7 J 10-3 FiG. 2.
, ~ll
10--2
,
i J~l
t I
I I,i
10-1 100 Frequency(Hz)
I
I
I
10
Depth spectral density for compensator on and for rigid tow (compensator off).
300
E.H. KIDERA
(1) Reduction in chain motion over a wide frequency range of platform motion; (2) Combined launch/recovery and motion-compensation capabilities in one efficient unit; (3) Minimum cable fatigue through reduction in bending rates and shock loading, even in rough seas; (4) High sensitivity combined with low damping, providing chain-motion reduction in low and high sea states; (5) The capability of continuous compensation operation, with only intermittent attention; and (6) Fail-safe features that permit continued launch and recovery operations in case of malfunction. Further improvements of the system are possible; they will be carried out if and as time and funding permit. Ultimately, it may be feasible to limit chain motion to only a few centimeters overall.
Acknowledgement--This work
was supported by the Department of the Navy.
0029-8018/83 $3.00+ .00 © 1983 Pergamon Press Ltd.
Printed in Great Britain.
TECHNICAL A MOTION-COMPENSATED
NOTE
LAUNCH/RECOVERY
CRANE
E.H. KIDERA Ocean Engineering, U.S. Naval Academy, Annapolis, MD 21402, U.S.A. Abstract--Oceanographic instrumentation towed from a surface platform is normally subjected to unwanted vertical motion imparted by the rolling, heaving, and pitching of the platform. This motion can cause severe shock-loading conditions in the tow cable and induces unplanned depth variations that reduce the instruments' resolution. This article discusses a unique motion-compensation system for use with towed instrumentation that eliminates shock loading and minimizes depth variation. The system is operational in sea states as high as 5 and effectively decouples 96% of the vessel's motion.
BACKGROUND
ONE OF the primary oceanographic instrumentation systems used by APL's Submarine Technology Division is a faired cable system instrumented over much of its length with thermistors, fluorometers, and conductivity cells (hence, "TFC chain"); the chain is towed from a surface vessel. The use of a depressor combined with the low drag of the chain results in a nearly vertical catenary. Components of surface platform motion (pitch, roll, and heave) are transferred to the chain through the tow sheave. The resultant vertical motion of the chain has limited its utility for wave physics measurements and has restricted its safe operation to low sea states to avoid excessively high shock loading. A passive boom-motion compensator was conceived, designed, and built to handle the TFC chain and reduce the vertical sensor motion. Figure 1 shows the articulated crane that was modified to provide motion compensation in addition to normal launch/recovery functions. A passive compensator acts as a soft spring with a natural period significantly longer than the natural period of platform motion. Theoretically, this action allows it to maintain constant cable tension, independent of platform motion, thus minimizing depth variations. System performance is limited by several factors so that the actual cable tension is not constant but varies slightly about some nominal value determined by the pressure in the system. The following are key design factors: (1) Boom sensitivity threshold - - the force required to initiate movement of the boom. "Stiction" (static friction) in the pivot bearings and piston seals determine the sensitivity. (2) Natural period of the boom - - determined by the length of the boom and the volume of the support gas. A soft spring that corresponds to a large volume is desired. (3) System damping - - primarily a result of losses in the hydraulic oil flow. It should be kept to a minimum for best performance. (4) Boom stroke - - the dynamic range of the boom. Sufficient stroke is needed to allow operation in the most adverse sea states that will be encountered. 295
296
E.H. KIDERA
(5) Depressor effective mass (DEM) - - the mass of the depressor itself plus the entrained water mass. For optimum performance, a depressor with a high DEM and low weight in seawater is desirable. A winged depressor has a high DEM because of the water above the wing that must be accelerated with the depressor. Since the boom sensitivity threshold is fixed, the larger the DEM the smaller the acceleration will be and, thus, the smaller the amplitude of depth variation. Passive compensation is simple in theory but relatively tricky in practice. In designing a motion compensation system, all of the above factors must be carefully weighed and matched to the platform characteristics, chain catenary, and ocean conditions that are involved. As in most such matters, compromises dictated by limitations of time and funds must also be made.
System description The launch/recovery m o t i o n - c o m p e n s a t e d crane incorporates several key components: (1) (2) (3) (4)
The IMTCO model 1725 crane, Motion-compensation modifications and equipment added to the crane, Bottle rack for a large gas volume and adjustment, and Power pack for launch/recovery and general crane use.
The basic crane is an articulated, hydraulic truck crane. Features such as dual main-boom cylinders, a high capacity, a long reach, a rotation system and low cost make it the ideal choice for modification to shipboard use as a launch/recovery motion compensator. The crane was modified by the factory to APL's specifications. A counterweight was added to balance the boom, and the main boom cylinders were ported for connection t o the motion-compensation components. At APL, further changes were made to the crane to allow its use at sea. It was oceanized, a 4-ft-diameter towing sheave and a depressor saddle were mounted, and a fairlead was added to restrain the thermistor chain during operation. However, the primary modifications were to allow its operation as a motion compensator. Accumulators were added via the special porting to the extension side of the main-boom cylinders. The accumulators act as an interfac~e between the hydraulic oil in the cylinder and the nitrogen that provides the spring. Bladder accumulators were chosen because of their high sensitivity to pressure change. In the event of failure of the motion compensation system, all components can be isolated and the launch/recovery operations carried out normally. A bottle rack connected to the accumulators supplied a large adjustable gas volume. This configuration allowed the spring constant to be varied at will. The bottle rack includes a high-pressure source and a low-pressure sink for adjustment of system pressure to match the tow loads at any given time. Adjustment is through solenoid valves controlled either manually from the operator's console or automatically from boom-position limit switches. The automatic feature allows the system to operate unattended for hours because changes in loading and boom position are adjusted by the limit switch to maintain the boom in the desired position.
•m o ~ o q ~ lu~md!nba ap!s~l~op ltu.unp uo!~!sod ~ o l u! u~oqs ~ueJ~ p~msu~lmo~-uo!lol,~
"! "gx-I
Technical Note
299
DISCUSSION The motion-compensated launch/recovery system had its maiden voyage in April 1981 aboard R/V Cape off the Florida coast. It was tested extensively and performed admirably. The system is capable of a 25-ft stroke and has a 25 sec natural period. In sea state 3, the system limited the motion of the depressor to 4 cm rms for periods of 10 sec or less. After the April test, several significant improvements were made to the system. They resulted in a reduced boom-sensitivity threshold and in a substantially reduced system-damping value. The unit was taken to sea in August 1981 in support of an instrumentation checkout. A sea state 5 was encountered, during which the boom performed flawlessly. Motion was reduced from 4 to 3 em rms in band. This performance corresponds to up to a 96% reduction in vessel-induced vertical motion at the characteristic ship pitch period. Figure 2, a power spectrum of depressor depth, shows the significant reduction in motion between compensated and uncompensated tow. Note the absence of motion peaks on the compensated spectrum. The run shown is for a 6-knot tow in sea state 4. CONCLUSIONS The motion compensator has proved itself to be a highly useful tool in the study of ocean structure and a valuable piece of equipment for launch/recovery and towing. It offers several advantages over alternate methods:
102
I
[
II
101_
I
I
I Ill
I
t
It
I
~ompensator off
100 E 10-1 %
Cornpen
10-2 10-3 10-4 10-5 10-6
10 degreesof freedom
10-7 J 10-3 FiG. 2.
, ~ll
10--2
,
i J~l
t I
I I,i
10-1 100 Frequency(Hz)
I
I
I
10
Depth spectral density for compensator on and for rigid tow (compensator off).
300
E.H. KIDERA
(1) Reduction in chain motion over a wide frequency range of platform motion; (2) Combined launch/recovery and motion-compensation capabilities in one efficient unit; (3) Minimum cable fatigue through reduction in bending rates and shock loading, even in rough seas; (4) High sensitivity combined with low damping, providing chain-motion reduction in low and high sea states; (5) The capability of continuous compensation operation, with only intermittent attention; and (6) Fail-safe features that permit continued launch and recovery operations in case of malfunction. Further improvements of the system are possible; they will be carried out if and as time and funding permit. Ultimately, it may be feasible to limit chain motion to only a few centimeters overall.
Acknowledgement--This work
was supported by the Department of the Navy.