REPORT
SSL 91
December 1962
SHIP STRUCTURES LABORATORY
TECHNOLOGICAL UNIVERSITY - DELFT
MEASUREMENTS ÒN DERRICKS
IÑ FULL ACTION
by.
December 1962
MEASUREMENTS ON DERRICKS
IN FULL ACTION
by
IR F. XP. SOEJADI
REPORT NSSL 91
SHIP STRUCTURES LABORATORY
CONTENTS Introduction page i Measurements i - Programme i - Recordings i - Registration 2 Single Derrick 2 - Measuring arrangements 2 - Deflections 3 - Axial Force 4 - Bending Moment 5 - Distribution of Stresses 5 - Dynamic Loading 7 Union Purchase 8
INTRODUCTION
Measuring aboard a ship, when not by desire of the owners themselves., may sometimes be objectionable in that the measuring may hinder the proper exploitation of the ship.
The Dockers Training School (Havenvakschool) in Rotterdam however has also the disposal of a complete cargo-handling equipment mounted on a replica of a part of a ship (see photo, fig. 1) for instructing cargo-handling to boys and men, dockers-to-be. This offered a unique possibility
to make measurements on derricks in full action, without being in the
way.
The managing-board of the school kindly grandted permission and cooperation. As it does not concern equipment on a real floating ship the factor heeling of the ship by handling cargo is not present; the influence of heeling however is small as only light loads are moved.
An investigation on real cargo-handling equipment would be incomplete if no attention was paid to dynamic influences (braking, accelerating etc.), so recordings were made concerning this aspect of the matter too.
Rough handling is another feature to -reckon with.
Except recordings about working with a single derrick measurements were also made while burtoning.
i MEASUREMENTS.
The derricks beside the mast ( fig,. 2) were the objects of MEASUREMENTS ON DERRICKS IN FULL ACTION
investigation. Programme
measurements with a single derrick measurements with Union Purchase Recordings
I
stresses in the derrick(-s) - by means of electriq resistance strain gauges
the real loading force in the cargo falls - strain gauge dynamometer
acceleration of the load - accelerometer.
each at 3 topping positions
S.W.L. 3T-SIN6LE PURCHASE ST- DOUBLE PURCHASE
1. ETC STRAIN GAUGE NUMBER
ALL GAUGES ARE PLACED LONGIIUDINALLY
FlG 3 DIMENSIONS OF DERRICK A ND LOCATION
OF STRAIN GAUGES
Registration.
by. a light ray oscilligraph (Siemens Oscillof il) feeded by a 16 channels strain indicator.
Fig. 3.. shows the dimensions of the derricks in question and the location of the strain gauges.
Because the upper half of a derrick boom is the most interesting part (on account of the top bending moment) nearly all gauges were fitted in this region.
More measuring stations would have been needed to get a more complete picture of the distribution of stresses along the length of the boom. However the object of these measurements was not to obtain complete diagrams; the main point was to verify the concurrence between measured values and values
according to thé present-day design method.
For measuring the condition of working with a single derrick 14 channels were available to record the strains in the derrick, for the Union Purchase method 13 channels.
The other 2 (respectively 3) channels were meant to record
- the accelerations - by means of an accelerometer, placed on the load; - the instantaneous pull in the cargo falls - by means of a dynamometer
between the rope and the load (with the U.P. method one dynamometer for each rope).
Unfortunately channel no 14 (connected to strain .gauge no 14) did not function as it should.
The programme comprised the following items: 1. slow hoisting and lowering
quick hoisting and lowering swinging the load
rough handling (seizing the load with a jerk; hitting the bottom with force, etc.).
2. SINGLE DERRICK
Measuring arrangements:
angle relative to the horizontal 36°, 44° and 53°
load 4 TF.
Deflect ions.
According to the results the deflection of the derrick was upwards
(opposite to the effect of the own weight) in a plane almost vertical through the longitudinal axis of the derrick.
Whether any initial deflection was present is unknown; no device for measuring this deflection was available at that moment.
The strain gauges were bonded to the surface with the derrick in hori-zontal position; the influence of the own weight on the deflection is
greatest then. The steeper a derrick is topped the smaller will be the lateral effect, the greater the axial effect of own weight of derrick,, blocks etc.
FIG. 4: Top OF DERRICK.
(FOR MORE THAN 3TONS: 2P4RTS PURCHASE)
According to a regulation of the derrick-designer a load of 4 tons has to be transported on 2 parts (fig. 4).
Working on 2 parts involves a translation of the line of action of the load-force. For the arrangement in question this meant an upward bending end-moment for all 3 topping-positions.
Because the zero-reading for every measurement was taken after adjusting. the desired topping angle the influences of the axial as well as of the lateral components of weight are expelled from the measured values.
All registered values therefore are exclusive of the influences of own weight.
Axial Force.
The Euler value for buckling, with regard to the middle tube of 14.00 m. length, amounts to 61.3 tons.
The application of a safety factor of 5 results in P = 12.2 tons. admissable
A diagram of forces. as applied in current practice, gives.
P = 9.6 tons (the coefficient of rope tension in the cargo runner hoisting
for hoisting taken to be 0.561, conformable to the British Standard 408 1949). For this diagram half the weight of the derrick and the weight of blocks, shackles etc. (± 515 kg) were added up to the load, as is usual. For lowering, the coefficient being .0,445, p = 9.1 ton.
lowering
+
exclusive of effect of weight ++
weights accounted for in the usual manner; thrust is independent of topping angle then.
In the positions I and II the derrick was plumbing the hatch, i.e. the load was hoisted and lowered out of and into the hatch.
Position III was not appropiate for working a load into and out of the hatch; in this position the load was hoisted and lowered from and to the
quay. To maintain the outboard position the derrick had to be. held by the outboard-guy; the guy thus exerted an extra axial force on the derrick.
In vertical position the weight of the upper part of the derrick and the weight of blocks, shackles etc. would cause a compressive force on the inidlength section amounting to 515 kg.
-4-Measured values Result
of force diagram
Topping position I II III
36° to horizontal 44°
53° hoisting
lowering.o
In position I this force would be reduced to 515 sin 36 = 302 kg., in posi-tion II to 515 sin 440 = 358 kg. (if the derrick be prismatic). Even 1f these values are taken account of no agreement is found between the measured
values and th value according to a diagram of forces. The value mentioned last is on the so-called safe side.
One of the factors possibly contributing to the deviation is the matter of the coefficients of friction of the sheaves, for hoisting and for
lowering. According to theoretical considerations the rope tension in the runner must Increase when hoisting and decrease when lowering; consequently the axial force in the derrick also must Increase or decrease. The recordings
(see fig. 5, 6, 7) however do not show tangible differences whether hanging, lowering or hoisting.
Bending moment.
As mentioned above a top- bndingmoment was working, the mean value of which was 0.8 t.m..
A combination of an axial force = 8.5 tons and a thp-moment of 0.8 t.m. on a cylindrical tube with a section equal to the section of the middle tube of the derrick and of the same length as the derrick would cause an internal bending moment at L of 42700 kg.cm..
In the derrick In question, as measured Mx = = 62000 kg.cm..
The difference is evident and must be caused by stepping the derrick.
Distribution of Stresses.
Because of the small number of measuring-stations per step It is not realistic to present the distribution of stresses as diagrams. Therefore
it Is given as Table I (page 6).
(I)
TABLE.1
MEAN VALUE OF M.
0.8 TM.rABLE. 2: LOAD HANGING IN OUTBOARD DERRICK
TABLE.3.: LOAD HANGING IN BOTH DERRICKS
DENOTES ANGLE RELATIVE TO HORIZONTAL
- DENOTES TENSION
DENOTES DER! VED VAL UES (NOT MEASURED)
6
_8KG/cÑ2
61 134 105-?
oC=36°
201
u326
449
414(629)
*
P85T
=
--8
'
-61
-151
-143
-277
«44°
8?T
281
.
326
496
449
(783)
*
P--17
.-71
-162
-147
-277
«530 30'
P=97
307
s'378
508
496
77 is 13-29
-i?
6?Q(.53°30'
P57
701 's-
158 147 17 s, 10-10
-
OUTBOARD DERRICK=36
P-2T
52
«80
65
-INBOARD DERRICK: NOT MEASURED
55 'S
40
42
38
4
OUTBOARD DERRICK97
'S(75)*
149 122(22
44°
P..4.eT. 126 s' INBOARD DERRICK69
=46°
P6T.
'ra..
P
i'í'
r
I_'u
____
U I4ØPI$A
I44 '4 Fig. 5.i : strain gauges on derrick
2 : accelerometer
3 load-dynamometer
4 : load on bottom
5 : quick application of load
6 hoisting
12
7 effect of sudden exertion of the load-force
8 effect of unraveling of turns of the cargo-runner
9 lowering
10 : effect of braking
11 : slackening
notes:
- topping angle 36° and 44°: lt was mentioned that the derrick deflected upwards while the own weight acts downwards:
For the middle tube the own weight seemed to have effected the defiections in such a maimer that the stresses remained the same.
This influence was smaller for the steps.
- topping angle 53: as mentioned above the values of this topping position
were effected by the force in the outboard-guy.
Dynamic Loading.
One of the uncertaintiès to be taken account of by applying a factor òf safety is the influence of the so-called dynamic loading, especially by;
- sudden braking
- striking the bottom with force - accelerated hoisting
- swinging the load.
During one of the manoeuvres it happened that turns of the cargo rope were unraveled with force; this resulted in a bang, followed by a violent swish of the derrick.
From the recordings (see fig. 5) it became evident that this happening had a much greater effect than all other dynamic loadings. It will be clear from fig. 5 that it is impossible to determine the amplitudes of the oscillations because of the tangling of the lines.
The effect of the above mentioned dynamic loading always proved to be a
vibration superposed_onthe_aireàdy_present_static_or_semistatiC_(when
manoeuvring calmly) loading (see fig. 5 and 6).
The peak-loadings can be high., but they are of very short duration and damping is quick.
If a justifiable factor for compensation of dynamic influences is wanted an investigation is necessary into the behaviour of steel which Is exposed to short, fast decreasing peak-loadings, superposed on a static loading.
1K/NG THE BOTTOM
The oscillation of the load was often so violent that the velocity of the light ray of the Oscillof il for recording the accelerations, was too large for the photographic paper.
A rough idea of the relationship between the measured quantities is given in the following:
a variation of acceleration with a peak amplitude of 0.07 g. caused a load variation of 0.22 tons; the stresses in the derrick hardly alter.
3. UNION PURCHASE.
Fundamentally the problem is the same whether loading is done with a single derrick or with coupled derricks in Union Purchase.
But it is a well known fact that when a certain load is transported on the U.P. method larger forces will act than when the load is moved by means of a single derrick.
For the derricks in question the maximum load for V.P. Is l tons and working with a single part purchase is allowed then.
To determine, the instantaneous pull in each of the load-ropes a
dynamometer was Interconnected between the common hook and each fall. As in the case of working with a single derrick an accelerometer was placed on the load.
The derrick which was used for single work acted as the outboard derrick, the other as inboard derrick. On the latter derrick only 4 gauges were fixed in way of the section at L.
Table 2 gives the-di-s.tr-i-bu-t-ion-ofst-resses-a-t-a-moment that the load was
still hanging onthe outboard derrick only; the gauges on the inboard derrick then showed zero strain.
Table 3 contains the distribution of stresses for two different arrangements, the load hanging on both derricks.
The unification of the two cargo-falls introduces a 3-dimens±onal loading,, where-as the case of a single derrick can be considered as
2-dimensional, except in certain instances (for example when a guy comes into action).
The graphical determination of the thrust In the derrick(s) in V.P. requires an exercise with force diagrams in three projections, but the application of this method by laymen may be confusing.
-8-The authors Bennet -(1)- and Soldà -(2)- each devised a method to determine calculatively the forces in derrick, topping lift, and guys. The starting-point for all methods is of course essentially the same: the, position of load, derrick, topping lift and guys must be determined in some way or other.
'The method of Soldà makes use of cartesian coordinates, while Bennet has recourse to angles e.g. the angle between cargo-falls and derrick.
The estimation of angles Is much more difficult than estimating distances as for the method-Soldà; preference is therefore given to the latter.
An attempt is made to verify the results of the application of the method Soldà with regard to the measurements and to the results according to a force
diagram (see table 4).
Table 4: a comparison of thrust values.
Arrangement' I: outboard derrick 36 to horizontal inboárd derrick 36° to horizontal angle between load ropes 570
Arrangement II: outboard derrick 44° to the horizontal inboard derrick 46° to the horizontal angle between load ropes 890.
As may be observed the values are too divergent within each case, as well as the two cases compared, to draw any conclusion.
4. SU11MARY añd CONCLUSIONS.
The recordings show that the influences of dynamic factors are visible only when a swish of the derrick (vibration) is brought about by the dynamic loading In question.
1. The British Dock Regulations 1934 prescribe that proof loading of a derrick by moveable weights shall include swinging of the loads as far as possible
In both directions.
9
arrangement I arrangement II calculated (Soldà) 2.6 tons 4.1 tons force diagram 3.2 tons 4.5 tons
OAD ON BOTTO
FIG. O
LOAD A GING FREE
-L OAD HA GING FREE
s OF HOISTING TING
I__
J -IUI_
H
H__
i
_
!1TT
I____i
__I=__I__
.g;==
I---STAAIN GAUGE ON DERRICK
I CELEROETR ' DYNAMOM E I OTHER LI ES
---
--
I IThe influence of swinging of the load however proved to be negligible
(fig. 7).
Usually a difference Is made between a "static" load and a "running" load, concerning the effect of either of these loads on the magnitude of the axial thrust on the derrick.
Again the difference proved to be too small to cause a change In the recordings (fig. 8), nor Is there a real difference between the Influence of hoisting and of lowering. (fig. 6).
Accelerations/deceleratIons and braking are not directly endangering; the danger lies In the vibrations which may be caused by these phenomena. In general the effect of dynamic loading Is a damped vibration superposed on an already present static loading.
As vibration has been frequently observed In ordinary derricks the statements above may be seen as an emphásis on the necessity of a further investigation of this aspect, In order to arrive at a scientifically justified factor of safety.
A further investigatIon is also necessary if certainty is desired concerning the force-distribution in the several components of a Union Purchase arrangement.
The observations about the small influence of the above factors lead to the following alternatives:
6.1. scientific; another measurement, with the apparatus In a more sensitive adjustment;
6.2. practical: another measurement, just to verify the present one; less value shall then be attached to factors which really prove to be of small Influence.
References.
R. Bennet: "Krf te beim Laden mit gekuppelten Ladebäumen" Schiff stechnik, Band 2, September 1955.
G. Soldà: "Formula's for the rapid calculation of Stresses in Burtoning Derricks",
European Shipbuilding, vol. 5, 1956 no. 2.
