B= ring ge, in d.
Qk-
L7
cboriumv.
VRlTELT
Lkth
Mekeweg Z
GD DeIfti
Ote-TheTh -.oie .leia2e
An experimental Study of the Dynamic Holding Capability
Characteristics of Anchors
Hiroshi Kikutani*, Kiyoshi Oikawa and Shihci Nornura**
* Tokyo University of Mercantile Marine2-1-6 Etchu/ima, Koto-ku, Tokyo. Japan
**yuge Mercantile Mariize College Yuge, Och i-gull, E/urne, Japan
Abstract
When an anchor is pulled shipward to try to obtain its good bottom hold, or when a ship
is lying at anchor in a strong wind, the forces acting on the anchor are dynwnical ones
caused by ship motions. We used model experiments to investigate the seabed behavior of anchors under such circumstances. The results showed that
when an anchor is pulled shipward its bottom penetration dept/i and stabilizing tenden-cy attained are relative to its retrieval speed, and therefore, if we can measure the inddent anchor cable strain we should be able to determine whether or not the anchor has a good bite; and
when an anchor is subIect to the dynamical forces generated bi' its being brought in. it drags quickly, bui does nor rotate or come out of the bottom, and retains its original bottom;: hold when ¡he dynamic forces cease, and ils dragging distance is less 1/ian several times its length. ¡n our study we found rizal anchor !zoldimzg resistance could net simply be analyzed and that further experimental and theoretical study is necessary.
1. Introduction
There have been many papers published in Japan, the United States, and the United Kingdom on
studies of anchor holding capabilitiesdone for real and model anchprs, at sea and in test tanks. Because. of the conclûsions drawn in these papers (e.g., from the results of experiments with the JIS type anchor. it was said that its holding-power factor was between 3.5 and 5.0, and that the posture of its bottom bite was not enough to obtain a good hold), all shiphandlers have worried about the effectiveness of their anchorages when lying at anchor under heavy weather. But we realized that the experiments reported in these papers were performed by pulling the anchor along the bottom at a constant speed (whereas the actual forces acting on a ship anchor are not constant, but vary in intensity in response to the inertia forces generated by a ship's swinging motions in wind and waves), and knew that even in heavy weather incidents where anchor dragging caused ship damage or wrecks, some ships rode out the weather without mishap, so we decided to investigate, and try to clarify the anchor dragging mechanics.
In the summer of 1962 we carried out an experiment aboard the training ship Shioji Maru (148.99 G.T.; Lpp, 30.00m; bower anchor weight, 360kg) and obtained some results. But it is difficult to per-form experiments aboard ship (there are many limiting factor: data measuring methods, test manning, expense, scheduling) we decided, to perform test tank experiments with an anchor model, using eui'p-ment that was capable of duplicating, on a reduced scale, the motions of a moored ship that produce the inertia forces that affect anchor behavior.
2. Description and arrangement of test equipment components
2.1 Drag test tank
This is an ordinary anchor holding-power drag test tank. Its dimensions are:
Lx B x Dx d = lO.Sm x l.5m x l.2m x 0.7m
where d = bottom depth.
The tank is built of steel plates, with one
longitudinal side partially equipped with glass
observation windows. Along the top edges of the
tank are rails used for arranging the bottom
sur-face. The bottom material is ordinary river sand: and its specific gravity and angle of repose in the water are 1 .54 and 32.5°. respectively.
The motive power for the constant speed anchor drag rope is a ring-corn, i H.P. motor,
located apart from the tank. The motor has
a variable pulling speed of from 2.0 to 5.0 cm/s.To measure the strain on the wire rope that
pulls the anchor, a proving-ring and a load-cellare
connected in series and suspended from a frame-work erected over the terminal end of the tank. An electric meter for direct readings of the drag force measured by the proving-ring, and an oscillograph for are located on a table near the tank.
The anchor speedmeter is located at the end of the tank opposite the measuring instrument sup-port. The anchor speed is obtained by piano wire (load strain, 200 to 300 gr.) wound around the
speed-meter reel and fastened to the anchor. When the anchor is pulled, the pianowire pays out, and the
speedmeter transmits a signal every 1 cm. to an oscillograph. (Since the anchor model is about 10kg, the piano wire strain load is not significant, and need not be considered in test measurements.)
2.2 Weight vehicle and run-way details
As shown in Fig. 2.1, the weight vehicle run-way is erected near and parallel to the test tank. To
obtain the dynamic force generated by the vehicle for the anchor, its pulling rope is engaged by the
vehicle as it runs down its track. The vehicle track length is 14m; and the first 4m has a 1:10 gradient to
give the vehicle its initial velocity. (This velocity, 0 to 230 cm/s. may be adjusted by changing the distance the vehicle travels on the 1:10 gradient track section.) The final 10msection of the track is horizontal, but may be adjusted foruptoa 1:100
gradient. The height of the structure at the track
surface, is Ll8m, and its overall width is 0.8m.
The weight vehicle dimensions are:
LxBxD= l.Smxl.OmxO3m; the distancebe.
tven its fore and after wheel axes is 0.78m; and Lts weight 143kg.To adjust its weight, there are one hundred 10kg pieces of pig iron, and to engage the anchor
ull rope and to measure the drag force,a hook md a load-cell are mounted together on its bottom.
A ring support frame for the end link of the
DRAGGING TEST TANK SPEEDMETER OF ANCHOR TO DRAGGING
WINCH PUWNG ROTE
FOR MODEL ANCHOR
Fig. 2.1 General view of the experimental tank and its arrangements
the photo.copying of the load-cell measurements
USSECH. TIME MARE
PIANO WIRE FOR SFEEDMETER
RUN-WAY OF W1. VEHICLE
MARK FOR TRANSFER
DISTANCE OF W, VEHICLE. I
MARK FOR TRANSFER
DIETASTE OF w, VEHICLE. I STRAIN OF PJLUNG ROPE
TRANSFER DISTANCE OF MODEL ANCHOR
Fig. 2.2 One of hand-copy of oscilograph recorded on the experiment of wt. of wt. vehicle l003kg,a= 210cm wt. of model anchor 10.9kg. WINCH FOR W1. VEHICLE pulling roi same as th to an oscil 3. Genera 3.1 Stud The behavior ai Som to bite mt We believe speed. In oi pulled at vehicle. TI wheel axis the 1:10 vehicle rut and is indi 5 times fc altogether) load, the y anchor spe
in Fig. 3.
value ilust values can i As si seemed to bottom, bL and it cha. observed di the a was I fluke tende it was not observed, tI end of the t When fluke held was furthe tance lengtl When the a judge the at As in is the load was the bacWhen
a exceeds I
and T is s
pulling rope is located 4m 'from the end of the vehicle track.) A speedmeter. arranged and operated the same as the anchor speedmeter, measures the vehicle speed, and transmitts signals every 1 cm and 10 cm to an oscillograph. (A sample hard-copy oscilograph record is shown in Fig. 2.2.)
3. General aspects and results of experiments 3.1 Study of the anchor fluke bottom bit process
The purpose of this study was the determination of the effect of ship stern way inertia on anchor behavior and bottom bite during the retrieval process.
Some papers on anchor holding power state that in most observed instances the anchor fluke failed to bite into the bottom, even in soft sand, and that the failure rate for mud bottoms was almost 100%. We believe that the probable causes of these failures were the anchor shape, bottom position and drag
speed.
In our investigation, the anchor model was
pulled at various speeds by the test tank weight vehicle. The distance between the vehicle's after wheel axis and the lowest point of the surface of
the 1:10 gradient track section of the weight
Io:vehicle run-way was used as a speed parameter, and is indicated by a. Tests were conducted 3 to
5 times for each of li different a's (43 times
altogether). A diagram of the pulling rope strain load, the weight vehicle speed variation, and theanchor speed data recorded for one test is shown
in Fig. 3.1. (Although the distribution of each value illustrated in Fig. 3.1 varied, their mean
values can be obtained from Fig. 3.2.)
As shown in Fig. 3.1, the anchor at
firstseemed to jump and its fluke to bite into the
bottom, but this was because its head was lifted 'T,. T, STRAIN OF PVLUNC ROPE
and it changed position rapidly In no instance SPEED OF W VEHICLE
observed did the fluke bite into the bottom when
the a was 110cm. As the a increased, however, the T.
fluke tended to dig in; but even for a a of 140cm
it was not deeply buried, and in some cases we
observed, the fluke was free of the bottom at the end of the test run.
When the a exceeded 150 cm, the anchor
fluke held the bottom satisfactorily. As the a
IOwas further increased, the anchor transfer
dis-tance lengthened, and the fluke was buried deeper. When the a exceeded 190 cm, it was impossible to judge the anchor depth.
As indicated in Fig. 3.1, the pulling rope strain had two remarkable peak values. The first, T1,
is the load peak for the strain imposed in countering the anchor's initial static resistance; the second, 12, was the load peak of the strain imposed by friction as the anchor was dragged along the bottom.
When the values of L and T2 increase, as shown in Fig. 3.2, T is always larger than T2 until the a exceeds 150 cm, when it becomes smaller. From this we concluded that when the a exceeds 150 cm,
and T is smaller than T2, the anchor is functioning satisfactorily. Therefore, if we can measure the
anchor chain strain, it should be possible to determine whether or not the anchor has a good bite.
50 kg
loe
SPEED OF ASCEOR. 9
SPEED OF w, VEHICU. t
STRAIN OF POLLING ROPE. T
Fig. 3.1 Pulling experiment (original position of anchor-flat on bottom): values of pulling rope strain; anchor model and
wt. vehicle speeds.
/
y
-A
Fig. 3.2 Mean values of pulling rope strain and anchor model and wt. vehicle speeds for each a
Even though the anchor transfer distance is lengthened for speeds attained from a's exceeding 190 cm, the value of 12 changes little. From this we concluded that the practical useful range of a is from
150 to 190 cm.
In none of our experiments was there any anchor shank axial rotation (this phenomenon is always
observed during constant speed anchor pulling tests). We could not determine the reason it did not
occur: but it may be because the anchor, in our tests, was initially lying flat on the bottom. We must run
more tests to determine the effect on anchor behavior of the initial bottom condition, and the use of
chain instead of wire rope for pulling.
3.2 Study of the effect on anchor behavior of the dynamic forces generated in a holding condition
The Yaw of an anchored ship subjected to high winds is severe, and the impact force developed by this swinging motion seems to be the main cause of anchor dragging. To investigate this problem we performed two experiments; in the first the weight of the weight vehicle was 503 kg, and in the second
1003 kg.
a. 503 kg weight experiment
To obtain the initial anchor holding condition, the anchor, lying flat on the bottom, was set by two runs of the weight vehicle. The a was 150 cm for both run.
To test the anchor pulling power we ran a
series of tests using a's ranging from 150 cm to
350 cm. Three tests were made for each
measure-ment used, and the values of the pulling rope
' vEwc.ostrain, and weight vehicle and anchor speeds were ooOFoo
recorded by oscillograph. ('rl
From these records, even though the
the maximum pulling rope strain was reached. - '° '°
Where there were two anchor speed value peaks,
the first occurred before the maximum pulling 00:
rope strain was recorded and the second after.
After the maximum pulling rope strain value was
attained the anchor speed almost equalled the
'°weight vehicle speed, and then reduced gradually.
The weight vehicle's speed reduction resembled a
:.
cosine curve, but the pulling rope strain measured
...
from 50 to 70kg even when the weight vehicle 30. so
and anchor speeds were almost 0, then it decreased
-
130 V50 350330 00
rapidly. Fig. 3.4
As the initial weight vehicle speed was
in-creased by using larger cr's, the anchor transfer distance lengthened; and as shown in Fig. 3.4,
'3
IO
'O VIO 3/IO
values varied, we calculated the means of the strain imposed on the pulling rope by the vehicle's initial velocity, and the maximum pulling rope strain and anchor transfer distance obtained by the dynamic
force involved.
In the test results shown in Fig. 33, in
the very short time it took the anchor to move the
initial
1 cm, the pulling rope strain increased
from 50 to 100 kg, 4.5 to 9.0 times the anchor Fig. 3.3 An example of anchor behavior subject
weight, before the anchor began to move rapidly.
In the two cases where the peak anchor
speed value occurred only once, it peaked before
TL50SFVR O/SThII 0F 100050 000000*. p
50D OF RODEI. 000000*. 5
ROOD OF 00o. VEOIflLS
0/IO
todynamic force of wt. vehicle
Wy. OF W7. 0/LIBrEE 303 I IO03 STR0I5 OF 501110G ROPE DT di.
SPEED OF W VEHICLE
-TRAS/SFLO DISTAS/CE OF ES/CROO y s,
1/IO
_.1
Reference of value between pulling rope strain and ipitial wt. vehicle
velocity. the pu11ii cm, but t suming ít cm. We c Cofltifluoi. shows th maximum to drag, strain diai ferent fro that when near the r th reason less than any infort
But we t
anchor tr none of t out of the b. Ii
used almo test, with duced to for a's of in Fig. 3. maximum vehicle ve] tance (y) f A ceeded 25 from the the strain know the Ir test did t1 head indiac.T
If are where M i dragging o anchor trat In the combir known factthe pulling rope strain rose until a equalled 230
cm, but then eased suddenly at 250 cm before re- 200
-suming its climb to the point where the a was 350
i'
cm. We cannot determine the reason for this dis- j1
:flj
ìI a270Continuous phenomenon. But from Fig. 3.5, which l7
shows the pulling rope strain variation near the
maximum value recorded after the anchor began ioo
to drag, we can see that the shape trend of the
J IIT'..\
strain diagrams for a s of less than 230 cm is dif.
250
a = ISO
a 210
O SxI0
Fig. 3.5 Pulling rope strain variation; after anchor model began to move.
ferent from the diagram for a's over 250 cm, and that when the a exceeds 250 cm the diagram shape near the maximum value is rounded. This may be
th reason this phenomenon occurs.
-
lili!
ISEClì t
Juli
Q 2zI0 Q 3jIQ O 4xIO
As all these processes were completed in
less than 0.5 seconds, we were unable to obtain
any information about them by visual observation.
But we could see that as the a increased, the
anchor transfer distance lengthened, and that in none of the tests did the anchor rotate or comeout of the bottom.
1003 kg weight experiment
To obtain the initial anchor condition, we =.
used almost the same methods as for the previous
test, with the exception that the a used was re- '°°
\
duced to 125 cm. Tests were run 3 times each aNCHOR BEGIN TO MOVE
for a's of 250, 270, 300 and 350 cm. The diagram
(I
ANCHOR W. 10.9 ka
in Fig. 3.4 shows the mean value variation of the 5r. OF VT. VEHICLE 3003 kg
maximum pulling rope strain (T), the initial weight vehicle velocity (V), and the anchor transfer dis-tance (y) for each a.
As shown in Fig. 3.6, when the a ex-
Fig. 3.6 PullIng rope strain: after anchorceeded 250 cm, the strain diagram trend differed
from the one for the 503 kg test. For larger a's, although the anchor transfer distance lengthened,
the strain value did not increase, and the strain diagram shape resembled a trapezoid curve. We do not know the reason for this.
In these tests, the anchor moved faster than it did with the 503 kg weight vehicle, but in no test did the anchor rotate or come out of the bottom, and there was onlyone case where the anchor head inclined.
Theoretical consideration applied to the experiment
If the weight vehicle frictional resistance and rail height are excluded, the equations of motion
are
M- i = T
m5'TR
where M is the weight vehicle mass, m is the apparent anchor mass, R is the anchor resistance when
dragging on the bottom, T is the pulling rope strain, x is the weight vehicle transfer distance, and y is the anchor transfer distance.
In these equation, the value of M is the only actual quantity now known. We have no data on the combined mass of the anchor and the bottom material it displaces as it is dragged. R is also an un-known factor. T is given by
Us m op till th Intro Conci theory o since on The like prof the theo With by mak these st degrees regardin Con waves n Spea they en operati general Base give a method In t the East Expe In th 2.1
T=k(xy)
where k is the elastic factor of wire rope. The linear value of this rope did not vary so much because the break load of the 6mm wire rope we used in this experiment is 1700 kg, and the maximum test strain
was less than 250 kg.
Because we studied and discussed the dragging anchor mechanics by the use of R, the next euqations are
before the anchor began to drag
R=q.y
after the anchor began to drag R = R0
or
R=qjy
The results, however, of the theoretical ¿alculations were very different from those obtained in the tests, so we tried another formula that was based on the similarities of actual and model ship anchors. The results in this case were doubtful. We are sorry to say that we cannot determine the value of R. We must further analyze the theory of anchor dynamic behavior, and.the effect of the impact load on it.
4. Conclusion
The results of the experimentaf tests vary, and further study is required. But we have drawn the following conclusions:
Ship speed and movement affect the anchor retrieval process. If we could record the chain cable strain diagram, we should be able to determine if the anchor is biting into the bottom effectively. If the anchor is lying flat on the bottom and adequate ship speed will cause it to obtain an adequate bite. Further investigation is needed on the way the anchor striking the bottom and the catenary effect of the chain cable elasticity affect fluke bottom penetration.
When inertia forces act on an anchor embedded in a soft sand bottom, the anchor moves quickly,
but does not rotate or come out. The anchor transfer distance depends on the inertia force acting on
the anchor. This movement softens the impact load placed on an anchor by the ship inertial motion, but we cannot clarify the effect of its holding resistance when the impact load acts on the anchor.
We need further discussion to study the similarity formula for this experiment.
Reference
T. Shraishi and others "Holding power of the anchor in strong wind" (part I and II). Journal of
the Research Institute of J.N.R. No. 58.256 and No. 59-337
K. Honda and others "The holding characteristics of anchor from model test " Journal of the Nautical Society of Japan, Vol. 22
N. Sameshima and others "Experimental studies on anchoring in strong wind" (Part I and II)
Above Journal Vol. 22 and 23
M. Tamaki and others "Experiments on holding power of the anchor and anchor cable of Seikan Ferry Boats" Above Journal Vo. 27
T. Yokota and others "Results of full-scaled trial on a ship bower anchor" Above Journal Vol. 28 S. Oba and others "Research on quality if sea bottom and attitude of dropped anchor in Osaka-wan" Above Journal Vol. 38
K. P. Farrel "Improvements in mooring anchors" TINA, Vol. 92 H. L. Dove and others "Development of anchors" INA, 1960
