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CTD lowering mechanics
Deep-SeaResearch.Vol. 3 I. No. 2, pp. 181to 194. 1984. Printedin Great Britain.
0198-0149/84 $3.00 + 0.00 ~ 1984PergamonPressLtd.
INSTRUMENTS
AND
METHODS
CTD lowering mechanics H. O. BERTEAUX*and R. G. WALDEN* (Received 12 January 1983; in revisedform 30 August 1983; accepted I November 1983) Abstract--The difficulties encountered in the deployment of CTD instruments, including the loss of an appreciable number of them, has prompted a comprehensive study of CTD lowering mechanics. Conditions causing large cyclic cable tensions, shock loads, or cable slack were to be investigated, modeled, and hopefully, measured in situ. The dynamic behaviour of the instrument package needed to Ix assessed. Methods to improve the path of the instruments as they travel through the water column and recommendations for safer and more efficient lowering procedures had to be formulated. This paper describes the study and presents salient findings and recommendations. We point out the limits of present CTD lowering systems and advocate that v~'vo-controlled motion-compensating winches be used. NOTATION Ap Cc Cp d V/dt Dc K mC mp mep mv $ t T(s, 0 V IVI wL We X!
X2 I .x2 I X2 P
horizontal cross section of CTD package longitudinal drag coefficient of lowering cable normal drag coefficient of CTD package time derivative of V diameter of lowering cable cable spring constant mass of lowering cable mass of CTD package added mass of CTD package virtual mass of CTD package length of cable paid out time cable tension, a function ofs and t instantaneous speed of hoisting cable and CTD package absolute value of V immersed weight per unit of length of lowering cable immersed weight of CTD package displacement of cable upper end displacement of CTD package speed of CTD package absolute value of X 2 acceleration of CTD package seawater density. INTRODUCTION
A COMMON technique for measuring parameters such as seawater conductivity and temperature as a function of depth (CTD) is to lower sensing instrument packages (Fig. 1) * Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. 181
182
H.O. BERTEAUXAND R. G. WALDEN
E l M LOWERING CABL E
DOWN ROLL
UP ROLL
Fig. 1. ShiploweringCTD instnnnentpackage. with electromechanical cables. Because of the lengths deployed the tension due to cable weight represents a large fraction of the cable strength. Cyclic stresses induced by ship motion, when superimposed on the high tension levels, result in rapid deterioration of armor wires and conductors. Ship motion can cause momentary cable slackness, which may permit kinks to form and snap loads to occur. The problems often result at best in loss of electrical signal due to short or open circuits and at worst in cable rupture and instrumentation loss. If not properly designed a C T D (Fig. 2) may well kite, tumble, and spin. Such motions obviously impair orderly lowering, reduce the package free-fall speed, and most probably introduce degradation of the data. To understand the problems better a comprehensive study of CTD instrument lowering mechanics was initiated in 1979. The objectives were to identify the causes of cable damage, to predict their occurrence and quantify their effects, to observe and measure the dynamic behaviour of tethered and free-falling CTD instruments, and to draw recommendations for a more reliable and efficient use of instruments and ship time. MECHANISMS
OF LOWERING
CABLE DAMAGE
Reduction of original strength and loss of electrical conductivity are the two principal modes of electrome~:hanical cable damage. Typical cables are made of a number of twisted and electrically insulated copper conductors protected by two layers of galvanized steel wires wound in opposite directions (Fig. 3). The steel armor provides the cable strength, and
32
II
(0.Srn) TENS~ON CELL
PROTECT/VE FRAME
----ROSETTE OF 24 WATER SAMPLING BOTTLES 6"-'I " (I.85m)
~
~ Fig. 2.
=
WHOI CTD instrument-~ckage.
~,~!~i ~
IL4 ''
~(.4m) S TABIL /ZING FIN
J (5around)
,.:;o;- ii !i -*----PROTECTI VE FRAME I-
J
CTD INSTRUMENT ACOUSTIC PINGER Fig. 6.
Half-scale model of CTD instrument package.
21.0,, 0.62 m PARTIAL ROSETTE OF 24 WATER tAMPLING BOTTLES
5"2"
"TD INSTRUMENT
Im
EAD BALLAST (not shown) PLACED AROUND CTD INSTRUMENT
WEIGHT IN AIR : 40Blbs. ( i 8 5 k g ) WEIGHT IN WATER : 280 Ibs. (I~Tk9 ~) Fig. 7. Compact CTD package.
i 87
CTD lowering mechanics OUTER ARMOR
~.
BEBO,.G BRA,B_'NSULAT,ON~
INNER ARMOR"
,/
CONDUCTOR
BINDER TAPE
Fig. 3.
Typical CTD lowering cable construction.
damage to it reduces the cable strength. Loss of electrical signal may result from broken conductors (open circuit) or punctured insulation (short circuit). Cable deterioration can happen progressively or at once. Uniform wear and tear and constant corrosion slowly degrade the cable. Repeated cyclic loading may force the armor wires to fail in a shorter time. Sudden damage to the armor, the conductors, and their insulation will occur if the cable jumps a sheave, if it is crushed at a cross-over point on the drum, if kinks are allowed to form, and if the cable is subjected to shock loads. Conditions leading to such failures may sometimes be eliminated by correcting lowering procedures or equipment. For example, crushing of the cable at cross-over points - - a wellknown cause of conductor damage (BERTE^UX et aL, 1979) - - can be avoided if the cable is correctly wound on the drum. However, when lowering oceanographic instruments to great depths from a rolling ship, the mechanisms of cable damage are less obvious and more difficult to isolate and correct.
High cyclic tensile loads As the winch turns and the ship heaves and rolls in a rough sea, cable and equipment travel through the water at varying speeds. The tension at the head sheave is then the sum of the static load due to cable and instrument weight and of the dynamic load due to drag and inertia forces on the cable and attached equipment. With long lengths of cable deployed, drag and inertial forces can combine to add substantially to an already high static load. The high cyclic stresses can force the cable to fail prematurely by accelerated fatigue.
Slack conditions Cable relaxation can be the prelude to eatastrophy. A slack cable can easily jump out of a sheave, kink, or be subjected to severe snap loads\As they are payed out, most free-ended electromechanical cables have a tendency to unlay. So long as the cable is taut, no kinks can form. However, if the cable becomes slack it will relieve its torsional energy by forming one or more twisted loops at the point of slack. When tension is reapplied the loops are pulled fight, the armor wires and the conductors are then severely bent, permanently damaging the cable at the kink. If a body heavier than water is allowed to free-fall to the sea floor, it will first accelerate, gain speed, and eventually reach terminal velocity. If a combination of pay-out rate (line speed) and sheave motion produces a cable speed higher than the terminal velocity, the cable will override the equipment and hang below it. If the cable speed is higher than the terminal velocity of the cable and instrument combined, the cable will become slack at the shipboard end.
188
H.O. BERTEAUX and R. G. WALDEN
Snap loads Let us assume that the ship forces the head sheave to pull the cable and attached instrument towards the surface. A few instants later the ship reverses its motion and the head sheave starts to fall. The pull of the cable of the instrument diminishes and soon stops. The instrument then travels on its own, still going upwards but slowing down until gravity stops it. It then reverses its direction and starts to fall. In the meantime the cable follows the ship down-roll giving time for the package to acquire downward momentum and perhaps reach terminal velocity. Now comes the next up-roll. The cable rushes back to the surface. When the distance between the upper end of the cable and the position of the CTD equals the relaxed length of cable payed out, the lower end of the cable catches the free-falling instrument. The force required to stop the instrument and rapidly change its direction cf motion constitutes a snap load. If the wave frequency permits the procedure to repeat itself, the cable will be subjected to a series of snap loads and will probably break. PREDICTING
CABLE LOADS AND CTD PACKAGE
DYNAMIC
BEHAVIOR
Early in the study, mathematical models were used to predict and quantify the occurrence of high tensile loads, zero loads, and snap loads as well as to investigate the flight stability of tethered and free-falling CTD instruments.
Peak tensile loads If the cable pulls the instrument in a vertical path the tension T(s, t) at the head sheave can be computed using Morisson's equation
T(s,t)= We+wLs +½p(Cc~zDcs+CeAp) VI Vl+(mc+me+m'e)dV/dt.
(1)
The expression can be used to calculate maximum expected cable tension other than snap loads. Calculations of tension levels expected while hauling a CTD from the R.V. Atlantis H are presented in Fig. 4.
Snap loads As proposed by GOELLERand LAURA(1970), a single degree of freedom spring-mass system with non-linear damping (Fig. 5) can be used to investigate cable relaxation followed by snap loads. In the model the cable is treated as an elastic spring that changes its length as the winch pays the cable out or reels it in. The upper end of the cable [Xl(t )] is forced to follow a prescribed displacement such as a sinusoidal or a known time series. The ensuing displacement of the CTD package [X2(t)] is found by numerical integration of the instrument equation of motion, i.e., K ( X , - x~) - ~pC pA pJC21 X2 I = mvX'~.
(2)
The instantaneous cable tension at the instrument is found from T(s, t) = Wp + K(X, - X2).
(3)
When the tension T(s, t) goes to zero, the motion of the instrument is governed by the free flight equation -Wp-
½PCpA ,,.f ,I X2 I= m, )(2.
(4)
CTD
SEA
STATE
lowering
SEA
O
8000
6000
STATE
3
Roted B+'eokmg SW~h
- f . j /~
........
. _ __R°,_~_~ ~i_,,=_St,~,_h . . . . . .
~u
189
mechanics
/ -
Ykeld S~eaglk ..................
zoo0 tOOO
l
t 2
O0
i
i 4
z
t 6
z
i 8
i
LENGTH OF CdlBLE P,~/D OUT
ElM CABLE CHARACTERISTICS Immersed ~l~i(~lt = .145 tb/ft (22kg/m) ~meter : 303in (77cm) Bveoki~ strelwth : 7400 Ibs (3360 kq)
i
i
i
2
i
4
l 6
~
i 8
( m e t e r s x I 0 :I )
INSTRUMENT CHARACTERISTICS
Tyae CTD Package Immersed weight : 350 tbs (16Okg)
HEAVE AMPLITUDE= 3 f t (9m) ROLL AMPLITUDE : 15degrees WAVE PERIOD : 8 seconds VESSEL
Fig. 4.
Peak
tension
R/V ATLANTIS
at head sheave
as a function
of cable paid out.
When the tension becomes positive, equation (2) prevails. Speed and displacements used when switching from one equation to the other are those computed at the time immediately preceding the switehover. The model was used to predict the behavior of typical CTD packages when excited by regular sinusoidal waves and to reproduce reasonably actual measurements of cable tension and package motion (Fig. 8).
Xt
, ~r'.
t
CABLE UPPER END OISPLACEMENT
CABLE
J
',I,"
T , CABLE TENSION
MY ~ ~
UNEAR (;ABLE
SPRIt~ W1TH
NONLINEAR OAMPING
CONS'rAwr K
~KAGE
J , w D, IMMERSED WEIGHT - OF PACKAGE
K , E._.~A L CABLE
OISII:~MENT
X2
0" ORAG ON PACKAGE
MY
• ~ pCp Ap ~2 I)~t
SPRING-MASS MODEL OF INSTRUMENT PACKAGE AND
PACKAGE FREE BODY DIAGRAM
LOWERING CABLE
Fig. 5.
Spring-mass
model.
190
H . O . BEaaX^ux AND R. G. WALDEN
C T D flight stability
To fall in a plumb, orderly way, the CTD package must be statically and dynamically stable. The package is statically stable if it has a natural tendency to return to an upright steady-state vertical flight. It is dynamically stable if it returns to such a condition with oscillations of decaying amplitude. The static stability of CTD packages was modeled as hereafter reviewed. The dynamic stability was not modeled but was observed as described in the next section. Static stability of the package was determined by computing the resultant moment with respect to the center of gravity of the package of all the forces acting on the package, namely its weight, its buoyancy, the tangential and normal drag forces on the package components, and the tension in the lowering line. Using the above approach the righting (stable) and capsizing (unstable) moments of different CTD configurations were calculated for prescribed initial tilt angles and various speeds of fall (COOK, 1981). The technique pointed out that asymmetrical packages have inherent static instability and can flutter or tumble when falling freely. It also demonstrated the beneficial effects of drag reduction, weight increase, and overall package symmetry on stability and terminal velocity. The above numerical models permitted us to verify and quantify the occurrence of large peak loads and of possible cable relaxation followed by impact loads, they confirmed trends and limitations that were later used to draw conclusions and formulate recommendations, and they helped us design the scale model tests and the sea tests, which are next presented. O B S E R V A T I O N S A N D M E A S U R E M E N T S OF C T D P A C K A G E F L I G H T C H A R A C T E R I S T I C S
T a n k tests
-:
A series of tests using a half-scale model of the CTD were run at the Naval Surface Weapons Command tank at White Oak, Maryland. The vertical tank is 15 m in diameter and 30 m deep. The model consisted of a steel frame and removable cylinders representing the different components of a CTD package (Fig. 6). The cylinders were placed in the frame at a location similar to the actual package. The density of each cylinder was adjusted to maintain the immersed weight:drag ratios equal for both model and instrument package. Criteria of flow similitude followed in the model design are further explained by COOK (198 I). Lowerings were made with an adjustable speed winch. Maximum payout speed exceeded the terminal velocity of all configurations tested thus enabling free-fall observations to be made. A total of 42 lowerings were made. The simplest configuration - - frame, CTD, rosette was used as a reference. Pingers and nephelometers were modeled and added to the base line, and stabilizing fins were used in some lowerings. At low lowering speeds and high cable tensions, all configurations behaved well. Undesirable flight patterns could be observed only at high lowering speeds or terminal velocity. At high speeds the reference, with or without a pinger, oscillated around its center of gravity with an amplitude of one CTD instrument diameter and a frequency of I Hz (wobbling). Strapping a pinger on the frame caused the instrument to tilt. A nephelometer mounted across the bottom frame induced spin. Adding three equally spaced vertical fins suppressed wobbling and made the axisymmetrical configurations fall in a plumb line. However the fins forced nonsymmetrical packages to glide off the vertical in a stable flight pattern. Violent tumbling could not be observed even when the packages were dropped with an initial angle of 30 ° from the vertical. The tests are described in detail in BERTEAUXet al. (1983). -
-
CTD lowering mechanics
!9 !
Fable 1. CTD termhlal relocities Package weight Lowering no. I 2 3
Package Standard CTD package (no tension cell) Same as No. I except 223 Ib of lead added Compact CTD package (Fig. 7)
In air (kg)
In water (kg)
Terminal velocity (m s t)
191
116
2.3
295
210
3. I
185
127
3.8
Measurements o f terminal velocity An instrument was designed and constructed to measure the terminal velocity of different C T D packages. Typical results are in Table 1. The results show the remarkable gain in terminal velocity obtained by increasing the weight and more efficient packaging. All packages reached terminal velocity in 1 to 1.5 s. Measurements at sea A standard W H O I C T D instrument was modified to measure the tension immediately above the package and also the package inclination. Simultaneous records of cable tension and of instrument depth and tilt could thus be obtained. Two lowerings of the modified instrument were made from the R.V. Oceanus on 14 December, 1981 and 16 January, 1982. The first lowering was made in fair weather and calm sea. A sea state 3 with I0 to 15-kn winds and a 2-m swell prevailed at the time of the second lowering. Cable speed o f f a l l Pressure records can be used to hindcast the speed of cable fall. By differentiating the pressure record one obtains a record of instrument rate of travel in the vertical direction. On a down-roll the cable can fall faster than the instrument and therefore down-speed measuremerits cannot always be used to infer cable speed. On the other hand when the instrument is pulled upwards the cable and instrument travel together. If at that moment the winch is paying the cable out at a given rate, the sheave must go up at a speed equal to the sum of the payout rate and the rate of instrument climb. In general a hard up-roll is followed by an equally hard down-roll. Thus one can expect the sheave to fall down as fast as it came up. The cable down speed would then be the down speed of the sheave augmented by the payout rate, or the terminal velocity of the cable, whichever is the smaller. Applying the above reasoning to the pressure data (Fig. 8), the upward speed of the instrument at time t = 8 s is found to be 1.6 m s -~. With the winch paying out at a rate of 0.73 m s -t, the sheave must climb at 2.33 m s -t. On the next down-roll the cable will fall at 2.3 + 0.73 = 3 . 0 3 m s -t. The pressure record shows the instrument fell at t = 10s at 2.8 m s -~. If the cable falls faster it goes slack, as shown by the tension record. Tension measurements Tension measurements, particularly those of the second lowering, showed large deviations from the weight of the immersed instrument (138 kg). Low levels of tension (including zero) were often encountered followed by values as high as three times the instrument weight.
H . O . BEnam^ux AND R. G. WALDEN
192
Typtco/ ShOp Lood I
TIENSIOIM RECORD
300
2oo 1OO
O 2
4
8
6
I0
12
14
10
12
14
T/ME secs ) 2
4
6
[
8
50
",
54
5s
_ .-___ ~
de¢,uM soe
62
RECORE}
-
",.. -
""
,
} Poyou~ R o l e : 7 3 m / s e e
R/V OCEANUS CRUISE ~ I 1 2
Fig. 8.
Records of tension and pressure measurements.
A typical section of the record was selected for detailed analysis. Figure 8, based on 32 data points per second, shows the simultaneous measurements of cable tension and instrument depth as a function of time, starting from an arbitrary origin. The record illustrates cable relaxation followed by impact loading. At time t = 4 s the instrument started to fall. The cable overrode the instrument 1.5 s later and the tension went to zero (t = 5.5 s). The cable remained slack for I s. At t = 6.5 s the instrument started decelerating and was brought to zero speed 0.25 s later. The tension jumped from zero to 318 kg in this time. The snap load was followed by typical shock waves in the cable. As the instrument traveled upwards the pull of the cable became less. Eventually the instrument stopped climbing (t = 9 s), started to fall again acquiring terminal velocity at t = 10 s, at which time the cable again overrode the instrument. A second relaxation took place, followed by a second snap load (t = 11 s). Peak tensions reached in the series (318 kg or so) may seem small compared to the 3360 kg of cable strength. On the other hand more severe weather undoubtedly would have produced much larger peaks. Of more significance perhaps is that relaxation - - which can permit kinks to form - - happened repeatedly. Repeated snap loads would then be applied to a damaged cable. Tilt measurements
The records indicated that the package was slightly tilted (5 to 6 ° ) and turned slowly while being lowered and raised. The slow spin was probably caused by cable unlaying. No violent tumbling was observed. CONCLUSIONS
AND RECOMMENDATIONS
Causes of lowering cable damage were identified and measured in situ. They include high stress levels at the head sheave, cable relaxation, and snap loads. The flight pattern of typical
CTD loweringmechanics
193
CTD packages has been observed with the help of scale models. Mathematical models were used to quantify tension levels, calculate terminal velocities, and investigate instrument package stability. The following recommendations to reduce or suppress the cause of cable damage and improve the package flight pattern are made:
Modus operandi As a first measure, limits should be set and observed as to maximum depth of casts and allowable winch speeds. To help plan safe casts, predictions of tension at the head sheave should be readily available. If, for example, one had graphs of peak tension vs cable length for different hauling speeds and sea states of the type shown in Fig. 4, the allowable cable length could be found from the intersection of the pertinent tension curve with the safe load (or the elastic limit) of the cable. During CTD lowering one should try to avoid a slack cable. Winch payout rates that would cause the cable to become slack as a function of known (preferably measured) package terminal velocity and sea state should be calculated and presented in a tabular or nomogram form, and payout rates set accordingly. Examples of such calculations and nomograms are shown in BERTEAUXet al. (1983).
Improving the CTD package Short of major changes - - such as enclosing the different instruments and water bottles in a hydrodynamically profiled and properly ballasted casing - - the following should help improve the free-fall stability and the terminal velocity of present CTD packages. When attaching instruments on the protective frame an effort should be made to distribute evenly weight and drag surfaces around the vertical frame axis. Axisymmetry will greatly enhance flight stability. Packages should not be too top-heavy, and the center of buoyancy should be well above the centre of gravity. Adding ballast to the lower part of the frame would lower the center of gravity and increase stability. The added weight, combined with a possible reduction of the surface area normal to the water flow, would also increase the terminal velocity (BERTEAOX et al., 1983).
Placemenl of head sheave The judicial placement of the head sheave can help reduce the combined effects of pitch and roll. An attempt should be made to place the head sheave as close as practical to the intersection of the ship eenterline with the axis of the ship's pitch.
Motion compensators The above recommendations are meant to help reduce the failure rate of CTD lowerings. Limits on deployment depth to avoid high stresses or payout rates to prevent slack conditions should be considered temporary measures. The goal is to make deeper and faster casts in more severe weather. Automatic motion compensators could greatly reduce or suppress the undesirable effects of ship motion. Shock loads can be substantially reduced with the help of multiple sheave hydraulic or pneumatic accumulators. An active boom to suspend the head sheave can also be used. Another approach is to modulate the speed of the winch in response to measured sheave displacement rates. Accumulators and active booms are complex systems, they require ample deck space, and are stroke-limited. For open-ocean applications, a winch with a servo-control option appears to be a practical solution. By enabling the package to travel at a
194
H.O. BERTEAUXANDR. G. WALDEN
preset c o n s t a n t speed such a winch would eliminate the high tension peaks due to violent sheave motion. It would also greatly reduce the potential for cable relaxation.
Seamanship and good handling practices A last r e m a r k should be m a d e to emphasize the i m p o r t a n c e o f good cable handling practices. A wealth o f information on the subject is in the Handbook on oceanographic winch, wire, and cable technology (DRIsCOLL, 1982).
Acknowledgements--The assistance of P. CLAY,R. McDEvrrT, and M. Cook is gratefully acknowledged. The study was funded by the U.S. Naval Office of Naval Research, Code 421, under Contract No. N0014-79-C-0071. Contribution No. 5311 from the Woods Hole Oceanographic Institution, Woods Hole, Massachusetts. REFERENCES
BERTEAUXH. O., R. G. WALDEN,D. A. MOLLERand Y. C. AGRAWAL(1979) A study of CTD cables and lowering systems. WHOI Technical Report 79-81, 60 pp. BER'n~AUX H.O., R.G. WALDEN, P.R. CLAY and R.E. McDEvn'r 0983) Expcrimental evaluation of CTD hydrodynamic behavior and recommendations for improved lowering techniques. WHOI Technical Report 83-21, 50 pp. COOK M. (1981) Hydrodynamics of CTD instrument packages. WHOI Technical Report 81-76, 60 pp. DRISCOLLA. H. (1982) Handbookof oceanographic winch, wireand cable technology,University of Rhode Island, Kingston, Rhode Island, pp. 7- I-7-23. GOELLJERJ. E. and P. A. LAtno, (1970) A theoretical and experimental investigation of impact loads in stranded steel cables during longitudinal excitation. Catholic University of America, Themis Program, Report 70-2, 50 pp.
0198-0149/84 $3.00 + 0.00 ~ 1984PergamonPressLtd.
INSTRUMENTS
AND
METHODS
CTD lowering mechanics H. O. BERTEAUX*and R. G. WALDEN* (Received 12 January 1983; in revisedform 30 August 1983; accepted I November 1983) Abstract--The difficulties encountered in the deployment of CTD instruments, including the loss of an appreciable number of them, has prompted a comprehensive study of CTD lowering mechanics. Conditions causing large cyclic cable tensions, shock loads, or cable slack were to be investigated, modeled, and hopefully, measured in situ. The dynamic behaviour of the instrument package needed to Ix assessed. Methods to improve the path of the instruments as they travel through the water column and recommendations for safer and more efficient lowering procedures had to be formulated. This paper describes the study and presents salient findings and recommendations. We point out the limits of present CTD lowering systems and advocate that v~'vo-controlled motion-compensating winches be used. NOTATION Ap Cc Cp d V/dt Dc K mC mp mep mv $ t T(s, 0 V IVI wL We X!
X2 I .x2 I X2 P
horizontal cross section of CTD package longitudinal drag coefficient of lowering cable normal drag coefficient of CTD package time derivative of V diameter of lowering cable cable spring constant mass of lowering cable mass of CTD package added mass of CTD package virtual mass of CTD package length of cable paid out time cable tension, a function ofs and t instantaneous speed of hoisting cable and CTD package absolute value of V immersed weight per unit of length of lowering cable immersed weight of CTD package displacement of cable upper end displacement of CTD package speed of CTD package absolute value of X 2 acceleration of CTD package seawater density. INTRODUCTION
A COMMON technique for measuring parameters such as seawater conductivity and temperature as a function of depth (CTD) is to lower sensing instrument packages (Fig. 1) * Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. 181
182
H.O. BERTEAUXAND R. G. WALDEN
E l M LOWERING CABL E
DOWN ROLL
UP ROLL
Fig. 1. ShiploweringCTD instnnnentpackage. with electromechanical cables. Because of the lengths deployed the tension due to cable weight represents a large fraction of the cable strength. Cyclic stresses induced by ship motion, when superimposed on the high tension levels, result in rapid deterioration of armor wires and conductors. Ship motion can cause momentary cable slackness, which may permit kinks to form and snap loads to occur. The problems often result at best in loss of electrical signal due to short or open circuits and at worst in cable rupture and instrumentation loss. If not properly designed a C T D (Fig. 2) may well kite, tumble, and spin. Such motions obviously impair orderly lowering, reduce the package free-fall speed, and most probably introduce degradation of the data. To understand the problems better a comprehensive study of CTD instrument lowering mechanics was initiated in 1979. The objectives were to identify the causes of cable damage, to predict their occurrence and quantify their effects, to observe and measure the dynamic behaviour of tethered and free-falling CTD instruments, and to draw recommendations for a more reliable and efficient use of instruments and ship time. MECHANISMS
OF LOWERING
CABLE DAMAGE
Reduction of original strength and loss of electrical conductivity are the two principal modes of electrome~:hanical cable damage. Typical cables are made of a number of twisted and electrically insulated copper conductors protected by two layers of galvanized steel wires wound in opposite directions (Fig. 3). The steel armor provides the cable strength, and
32
II
(0.Srn) TENS~ON CELL
PROTECT/VE FRAME
----ROSETTE OF 24 WATER SAMPLING BOTTLES 6"-'I " (I.85m)
~
~ Fig. 2.
=
WHOI CTD instrument-~ckage.
~,~!~i ~
IL4 ''
~(.4m) S TABIL /ZING FIN
J (5around)
,.:;o;- ii !i -*----PROTECTI VE FRAME I-
J
CTD INSTRUMENT ACOUSTIC PINGER Fig. 6.
Half-scale model of CTD instrument package.
21.0,, 0.62 m PARTIAL ROSETTE OF 24 WATER tAMPLING BOTTLES
5"2"
"TD INSTRUMENT
Im
EAD BALLAST (not shown) PLACED AROUND CTD INSTRUMENT
WEIGHT IN AIR : 40Blbs. ( i 8 5 k g ) WEIGHT IN WATER : 280 Ibs. (I~Tk9 ~) Fig. 7. Compact CTD package.
i 87
CTD lowering mechanics OUTER ARMOR
~.
BEBO,.G BRA,B_'NSULAT,ON~
INNER ARMOR"
,/
CONDUCTOR
BINDER TAPE
Fig. 3.
Typical CTD lowering cable construction.
damage to it reduces the cable strength. Loss of electrical signal may result from broken conductors (open circuit) or punctured insulation (short circuit). Cable deterioration can happen progressively or at once. Uniform wear and tear and constant corrosion slowly degrade the cable. Repeated cyclic loading may force the armor wires to fail in a shorter time. Sudden damage to the armor, the conductors, and their insulation will occur if the cable jumps a sheave, if it is crushed at a cross-over point on the drum, if kinks are allowed to form, and if the cable is subjected to shock loads. Conditions leading to such failures may sometimes be eliminated by correcting lowering procedures or equipment. For example, crushing of the cable at cross-over points - - a wellknown cause of conductor damage (BERTE^UX et aL, 1979) - - can be avoided if the cable is correctly wound on the drum. However, when lowering oceanographic instruments to great depths from a rolling ship, the mechanisms of cable damage are less obvious and more difficult to isolate and correct.
High cyclic tensile loads As the winch turns and the ship heaves and rolls in a rough sea, cable and equipment travel through the water at varying speeds. The tension at the head sheave is then the sum of the static load due to cable and instrument weight and of the dynamic load due to drag and inertia forces on the cable and attached equipment. With long lengths of cable deployed, drag and inertial forces can combine to add substantially to an already high static load. The high cyclic stresses can force the cable to fail prematurely by accelerated fatigue.
Slack conditions Cable relaxation can be the prelude to eatastrophy. A slack cable can easily jump out of a sheave, kink, or be subjected to severe snap loads\As they are payed out, most free-ended electromechanical cables have a tendency to unlay. So long as the cable is taut, no kinks can form. However, if the cable becomes slack it will relieve its torsional energy by forming one or more twisted loops at the point of slack. When tension is reapplied the loops are pulled fight, the armor wires and the conductors are then severely bent, permanently damaging the cable at the kink. If a body heavier than water is allowed to free-fall to the sea floor, it will first accelerate, gain speed, and eventually reach terminal velocity. If a combination of pay-out rate (line speed) and sheave motion produces a cable speed higher than the terminal velocity, the cable will override the equipment and hang below it. If the cable speed is higher than the terminal velocity of the cable and instrument combined, the cable will become slack at the shipboard end.
188
H.O. BERTEAUX and R. G. WALDEN
Snap loads Let us assume that the ship forces the head sheave to pull the cable and attached instrument towards the surface. A few instants later the ship reverses its motion and the head sheave starts to fall. The pull of the cable of the instrument diminishes and soon stops. The instrument then travels on its own, still going upwards but slowing down until gravity stops it. It then reverses its direction and starts to fall. In the meantime the cable follows the ship down-roll giving time for the package to acquire downward momentum and perhaps reach terminal velocity. Now comes the next up-roll. The cable rushes back to the surface. When the distance between the upper end of the cable and the position of the CTD equals the relaxed length of cable payed out, the lower end of the cable catches the free-falling instrument. The force required to stop the instrument and rapidly change its direction cf motion constitutes a snap load. If the wave frequency permits the procedure to repeat itself, the cable will be subjected to a series of snap loads and will probably break. PREDICTING
CABLE LOADS AND CTD PACKAGE
DYNAMIC
BEHAVIOR
Early in the study, mathematical models were used to predict and quantify the occurrence of high tensile loads, zero loads, and snap loads as well as to investigate the flight stability of tethered and free-falling CTD instruments.
Peak tensile loads If the cable pulls the instrument in a vertical path the tension T(s, t) at the head sheave can be computed using Morisson's equation
T(s,t)= We+wLs +½p(Cc~zDcs+CeAp) VI Vl+(mc+me+m'e)dV/dt.
(1)
The expression can be used to calculate maximum expected cable tension other than snap loads. Calculations of tension levels expected while hauling a CTD from the R.V. Atlantis H are presented in Fig. 4.
Snap loads As proposed by GOELLERand LAURA(1970), a single degree of freedom spring-mass system with non-linear damping (Fig. 5) can be used to investigate cable relaxation followed by snap loads. In the model the cable is treated as an elastic spring that changes its length as the winch pays the cable out or reels it in. The upper end of the cable [Xl(t )] is forced to follow a prescribed displacement such as a sinusoidal or a known time series. The ensuing displacement of the CTD package [X2(t)] is found by numerical integration of the instrument equation of motion, i.e., K ( X , - x~) - ~pC pA pJC21 X2 I = mvX'~.
(2)
The instantaneous cable tension at the instrument is found from T(s, t) = Wp + K(X, - X2).
(3)
When the tension T(s, t) goes to zero, the motion of the instrument is governed by the free flight equation -Wp-
½PCpA ,,.f ,I X2 I= m, )(2.
(4)
CTD
SEA
STATE
lowering
SEA
O
8000
6000
STATE
3
Roted B+'eokmg SW~h
- f . j /~
........
. _ __R°,_~_~ ~i_,,=_St,~,_h . . . . . .
~u
189
mechanics
/ -
Ykeld S~eaglk ..................
zoo0 tOOO
l
t 2
O0
i
i 4
z
t 6
z
i 8
i
LENGTH OF CdlBLE P,~/D OUT
ElM CABLE CHARACTERISTICS Immersed ~l~i(~lt = .145 tb/ft (22kg/m) ~meter : 303in (77cm) Bveoki~ strelwth : 7400 Ibs (3360 kq)
i
i
i
2
i
4
l 6
~
i 8
( m e t e r s x I 0 :I )
INSTRUMENT CHARACTERISTICS
Tyae CTD Package Immersed weight : 350 tbs (16Okg)
HEAVE AMPLITUDE= 3 f t (9m) ROLL AMPLITUDE : 15degrees WAVE PERIOD : 8 seconds VESSEL
Fig. 4.
Peak
tension
R/V ATLANTIS
at head sheave
as a function
of cable paid out.
When the tension becomes positive, equation (2) prevails. Speed and displacements used when switching from one equation to the other are those computed at the time immediately preceding the switehover. The model was used to predict the behavior of typical CTD packages when excited by regular sinusoidal waves and to reproduce reasonably actual measurements of cable tension and package motion (Fig. 8).
Xt
, ~r'.
t
CABLE UPPER END OISPLACEMENT
CABLE
J
',I,"
T , CABLE TENSION
MY ~ ~
UNEAR (;ABLE
SPRIt~ W1TH
NONLINEAR OAMPING
CONS'rAwr K
~KAGE
J , w D, IMMERSED WEIGHT - OF PACKAGE
K , E._.~A L CABLE
OISII:~MENT
X2
0" ORAG ON PACKAGE
MY
• ~ pCp Ap ~2 I)~t
SPRING-MASS MODEL OF INSTRUMENT PACKAGE AND
PACKAGE FREE BODY DIAGRAM
LOWERING CABLE
Fig. 5.
Spring-mass
model.
190
H . O . BEaaX^ux AND R. G. WALDEN
C T D flight stability
To fall in a plumb, orderly way, the CTD package must be statically and dynamically stable. The package is statically stable if it has a natural tendency to return to an upright steady-state vertical flight. It is dynamically stable if it returns to such a condition with oscillations of decaying amplitude. The static stability of CTD packages was modeled as hereafter reviewed. The dynamic stability was not modeled but was observed as described in the next section. Static stability of the package was determined by computing the resultant moment with respect to the center of gravity of the package of all the forces acting on the package, namely its weight, its buoyancy, the tangential and normal drag forces on the package components, and the tension in the lowering line. Using the above approach the righting (stable) and capsizing (unstable) moments of different CTD configurations were calculated for prescribed initial tilt angles and various speeds of fall (COOK, 1981). The technique pointed out that asymmetrical packages have inherent static instability and can flutter or tumble when falling freely. It also demonstrated the beneficial effects of drag reduction, weight increase, and overall package symmetry on stability and terminal velocity. The above numerical models permitted us to verify and quantify the occurrence of large peak loads and of possible cable relaxation followed by impact loads, they confirmed trends and limitations that were later used to draw conclusions and formulate recommendations, and they helped us design the scale model tests and the sea tests, which are next presented. O B S E R V A T I O N S A N D M E A S U R E M E N T S OF C T D P A C K A G E F L I G H T C H A R A C T E R I S T I C S
T a n k tests
-:
A series of tests using a half-scale model of the CTD were run at the Naval Surface Weapons Command tank at White Oak, Maryland. The vertical tank is 15 m in diameter and 30 m deep. The model consisted of a steel frame and removable cylinders representing the different components of a CTD package (Fig. 6). The cylinders were placed in the frame at a location similar to the actual package. The density of each cylinder was adjusted to maintain the immersed weight:drag ratios equal for both model and instrument package. Criteria of flow similitude followed in the model design are further explained by COOK (198 I). Lowerings were made with an adjustable speed winch. Maximum payout speed exceeded the terminal velocity of all configurations tested thus enabling free-fall observations to be made. A total of 42 lowerings were made. The simplest configuration - - frame, CTD, rosette was used as a reference. Pingers and nephelometers were modeled and added to the base line, and stabilizing fins were used in some lowerings. At low lowering speeds and high cable tensions, all configurations behaved well. Undesirable flight patterns could be observed only at high lowering speeds or terminal velocity. At high speeds the reference, with or without a pinger, oscillated around its center of gravity with an amplitude of one CTD instrument diameter and a frequency of I Hz (wobbling). Strapping a pinger on the frame caused the instrument to tilt. A nephelometer mounted across the bottom frame induced spin. Adding three equally spaced vertical fins suppressed wobbling and made the axisymmetrical configurations fall in a plumb line. However the fins forced nonsymmetrical packages to glide off the vertical in a stable flight pattern. Violent tumbling could not be observed even when the packages were dropped with an initial angle of 30 ° from the vertical. The tests are described in detail in BERTEAUXet al. (1983). -
-
CTD lowering mechanics
!9 !
Fable 1. CTD termhlal relocities Package weight Lowering no. I 2 3
Package Standard CTD package (no tension cell) Same as No. I except 223 Ib of lead added Compact CTD package (Fig. 7)
In air (kg)
In water (kg)
Terminal velocity (m s t)
191
116
2.3
295
210
3. I
185
127
3.8
Measurements o f terminal velocity An instrument was designed and constructed to measure the terminal velocity of different C T D packages. Typical results are in Table 1. The results show the remarkable gain in terminal velocity obtained by increasing the weight and more efficient packaging. All packages reached terminal velocity in 1 to 1.5 s. Measurements at sea A standard W H O I C T D instrument was modified to measure the tension immediately above the package and also the package inclination. Simultaneous records of cable tension and of instrument depth and tilt could thus be obtained. Two lowerings of the modified instrument were made from the R.V. Oceanus on 14 December, 1981 and 16 January, 1982. The first lowering was made in fair weather and calm sea. A sea state 3 with I0 to 15-kn winds and a 2-m swell prevailed at the time of the second lowering. Cable speed o f f a l l Pressure records can be used to hindcast the speed of cable fall. By differentiating the pressure record one obtains a record of instrument rate of travel in the vertical direction. On a down-roll the cable can fall faster than the instrument and therefore down-speed measuremerits cannot always be used to infer cable speed. On the other hand when the instrument is pulled upwards the cable and instrument travel together. If at that moment the winch is paying the cable out at a given rate, the sheave must go up at a speed equal to the sum of the payout rate and the rate of instrument climb. In general a hard up-roll is followed by an equally hard down-roll. Thus one can expect the sheave to fall down as fast as it came up. The cable down speed would then be the down speed of the sheave augmented by the payout rate, or the terminal velocity of the cable, whichever is the smaller. Applying the above reasoning to the pressure data (Fig. 8), the upward speed of the instrument at time t = 8 s is found to be 1.6 m s -~. With the winch paying out at a rate of 0.73 m s -t, the sheave must climb at 2.33 m s -t. On the next down-roll the cable will fall at 2.3 + 0.73 = 3 . 0 3 m s -t. The pressure record shows the instrument fell at t = 10s at 2.8 m s -~. If the cable falls faster it goes slack, as shown by the tension record. Tension measurements Tension measurements, particularly those of the second lowering, showed large deviations from the weight of the immersed instrument (138 kg). Low levels of tension (including zero) were often encountered followed by values as high as three times the instrument weight.
H . O . BEnam^ux AND R. G. WALDEN
192
Typtco/ ShOp Lood I
TIENSIOIM RECORD
300
2oo 1OO
O 2
4
8
6
I0
12
14
10
12
14
T/ME secs ) 2
4
6
[
8
50
",
54
5s
_ .-___ ~
de¢,uM soe
62
RECORE}
-
",.. -
""
,
} Poyou~ R o l e : 7 3 m / s e e
R/V OCEANUS CRUISE ~ I 1 2
Fig. 8.
Records of tension and pressure measurements.
A typical section of the record was selected for detailed analysis. Figure 8, based on 32 data points per second, shows the simultaneous measurements of cable tension and instrument depth as a function of time, starting from an arbitrary origin. The record illustrates cable relaxation followed by impact loading. At time t = 4 s the instrument started to fall. The cable overrode the instrument 1.5 s later and the tension went to zero (t = 5.5 s). The cable remained slack for I s. At t = 6.5 s the instrument started decelerating and was brought to zero speed 0.25 s later. The tension jumped from zero to 318 kg in this time. The snap load was followed by typical shock waves in the cable. As the instrument traveled upwards the pull of the cable became less. Eventually the instrument stopped climbing (t = 9 s), started to fall again acquiring terminal velocity at t = 10 s, at which time the cable again overrode the instrument. A second relaxation took place, followed by a second snap load (t = 11 s). Peak tensions reached in the series (318 kg or so) may seem small compared to the 3360 kg of cable strength. On the other hand more severe weather undoubtedly would have produced much larger peaks. Of more significance perhaps is that relaxation - - which can permit kinks to form - - happened repeatedly. Repeated snap loads would then be applied to a damaged cable. Tilt measurements
The records indicated that the package was slightly tilted (5 to 6 ° ) and turned slowly while being lowered and raised. The slow spin was probably caused by cable unlaying. No violent tumbling was observed. CONCLUSIONS
AND RECOMMENDATIONS
Causes of lowering cable damage were identified and measured in situ. They include high stress levels at the head sheave, cable relaxation, and snap loads. The flight pattern of typical
CTD loweringmechanics
193
CTD packages has been observed with the help of scale models. Mathematical models were used to quantify tension levels, calculate terminal velocities, and investigate instrument package stability. The following recommendations to reduce or suppress the cause of cable damage and improve the package flight pattern are made:
Modus operandi As a first measure, limits should be set and observed as to maximum depth of casts and allowable winch speeds. To help plan safe casts, predictions of tension at the head sheave should be readily available. If, for example, one had graphs of peak tension vs cable length for different hauling speeds and sea states of the type shown in Fig. 4, the allowable cable length could be found from the intersection of the pertinent tension curve with the safe load (or the elastic limit) of the cable. During CTD lowering one should try to avoid a slack cable. Winch payout rates that would cause the cable to become slack as a function of known (preferably measured) package terminal velocity and sea state should be calculated and presented in a tabular or nomogram form, and payout rates set accordingly. Examples of such calculations and nomograms are shown in BERTEAUXet al. (1983).
Improving the CTD package Short of major changes - - such as enclosing the different instruments and water bottles in a hydrodynamically profiled and properly ballasted casing - - the following should help improve the free-fall stability and the terminal velocity of present CTD packages. When attaching instruments on the protective frame an effort should be made to distribute evenly weight and drag surfaces around the vertical frame axis. Axisymmetry will greatly enhance flight stability. Packages should not be too top-heavy, and the center of buoyancy should be well above the centre of gravity. Adding ballast to the lower part of the frame would lower the center of gravity and increase stability. The added weight, combined with a possible reduction of the surface area normal to the water flow, would also increase the terminal velocity (BERTEAOX et al., 1983).
Placemenl of head sheave The judicial placement of the head sheave can help reduce the combined effects of pitch and roll. An attempt should be made to place the head sheave as close as practical to the intersection of the ship eenterline with the axis of the ship's pitch.
Motion compensators The above recommendations are meant to help reduce the failure rate of CTD lowerings. Limits on deployment depth to avoid high stresses or payout rates to prevent slack conditions should be considered temporary measures. The goal is to make deeper and faster casts in more severe weather. Automatic motion compensators could greatly reduce or suppress the undesirable effects of ship motion. Shock loads can be substantially reduced with the help of multiple sheave hydraulic or pneumatic accumulators. An active boom to suspend the head sheave can also be used. Another approach is to modulate the speed of the winch in response to measured sheave displacement rates. Accumulators and active booms are complex systems, they require ample deck space, and are stroke-limited. For open-ocean applications, a winch with a servo-control option appears to be a practical solution. By enabling the package to travel at a
194
H.O. BERTEAUXANDR. G. WALDEN
preset c o n s t a n t speed such a winch would eliminate the high tension peaks due to violent sheave motion. It would also greatly reduce the potential for cable relaxation.
Seamanship and good handling practices A last r e m a r k should be m a d e to emphasize the i m p o r t a n c e o f good cable handling practices. A wealth o f information on the subject is in the Handbook on oceanographic winch, wire, and cable technology (DRIsCOLL, 1982).
Acknowledgements--The assistance of P. CLAY,R. McDEvrrT, and M. Cook is gratefully acknowledged. The study was funded by the U.S. Naval Office of Naval Research, Code 421, under Contract No. N0014-79-C-0071. Contribution No. 5311 from the Woods Hole Oceanographic Institution, Woods Hole, Massachusetts. REFERENCES
BERTEAUXH. O., R. G. WALDEN,D. A. MOLLERand Y. C. AGRAWAL(1979) A study of CTD cables and lowering systems. WHOI Technical Report 79-81, 60 pp. BER'n~AUX H.O., R.G. WALDEN, P.R. CLAY and R.E. McDEvn'r 0983) Expcrimental evaluation of CTD hydrodynamic behavior and recommendations for improved lowering techniques. WHOI Technical Report 83-21, 50 pp. COOK M. (1981) Hydrodynamics of CTD instrument packages. WHOI Technical Report 81-76, 60 pp. DRISCOLLA. H. (1982) Handbookof oceanographic winch, wireand cable technology,University of Rhode Island, Kingston, Rhode Island, pp. 7- I-7-23. GOELLJERJ. E. and P. A. LAtno, (1970) A theoretical and experimental investigation of impact loads in stranded steel cables during longitudinal excitation. Catholic University of America, Themis Program, Report 70-2, 50 pp.