-ARCH1EF
EUR 21
THE POSITIONING OF OFFSHORE CONSTRUCTIONS,
RESEARCH AND TRAINING BY SIMULATION
© Copyright 1978, European Offshore Petroleum Conference and Exhibition
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EUROPEAII
OFFSHORE
PETROLEUITI
CONFERENCE
& EXHIBITION
This paper was presented at the European Offshore Petroleum Conference and Exhibition in I.ondon 24-27, October. 1978. The material is subject to correction by the author. Permiusion to copy is restricted to an abstract of not more than 300 words.
ABSTRACT
In 1976, two research program concerning the positioning of offshore gravity platforms were carried
out on the ship manoeuvring simulator of the N.S.M.B.
for Smit International Marine Services B.V., which company is specialized in the towage and positioning
of offshore constructions like the Ekofisk i and Mobil Beryl 'A' units.
The results and the consequent recommendations have meanwhile been applied by Smit International in the organization and execution of several positioning
operations.
The "tug order display", designed by the Netherlands Ship Model Basin to give the towmaster a view of all standing tug orders has also been used for instance in towing the ANDOC-built Dunlin-platform out of
Euro-poort.
INTRODUCTION
Putting a gravity structure down somewhere on the North Sea bottom is a job in which a variety of
specialisms of different nature in involved. Already in the design and construction stage, knowledge, ideas and convictions of many experts have to converge into
a total agreement on the planned effort.
It is obvious that differences in opinion on
decisions to be taken during such an operation are very likely to result in grounding or loss of the platform. To avoid this possibility and to ensure a
smooth handling of such a tow, a very strict
organi-zation is of crucial importance, especially for the last and most critical phase of the transport: the positioning. In a number if cases this positìoning involved bringing the platform to a predetermined location and keeping it there during the sinking of the structure in its definite position. The position as well as the heading of the structure had to be correct within very narrow accuracy limits. The accuracy of positioning, which in first instance was a target area with a diameter of 15 m within which
Illustrations at end of paper.
the centre of gravity of the structure was to be kept long enough to allow the platform to sink by ballasting
its tanks.
The accuracy of its heading was 5 degrees on either side of the intended heading. Rightly, the question was asked whether such an accuracy would be achievable
in the manner planned for the operation. The way in which the positioning was to be carried out involved the manipulation of six tugs which were grouped around
the platform in a star-shaped pattern.
The total tug power was intended to be 70,000 HP.
(Fig. i). A townaster was charged with the actual command of the positioning. This towmaster usually is a man with a long and extensive towing experience. It
is his task to direct the tugs, with the available
data and the aid of a staff of navigation specialists,
in such a way that the ultimate position is reached
and kept during the sinking.
Fig. 2 gives a block-diagram of the working scheme
during positioning. The positioning of large objects
like these with a weight in excess of 300,000 tons
with an accuracy of less than 15 ro, during which a
great number of strong tugs have to be directed, is an extremely difficult job.
Therefore, both research programs reviewed in this
article, served first to answer the question: "Is it
anyhow possible to carry out such an operation with
sufficient accuracy while taking into account
distur-bances like tidal stream etc." and secondly: "Which
accuracy can be reached?". A favourable circumstance of tho "real-tine" simulation of these operations is the fact that the townaster who has to do the ob can
get a specific training for this complicated task by
active participation in the research program.
THE RESEARCH PROGRAM
In order to answer the above-mentioned questions a real-time "nan-in-the-loop" simulation was set up. This simulation contained three elements. The first element was a simulation of the behaviour of the plat-form, which could be realized to an adequate extent
by three non-linear equations of motion; two for the track and one for the heading of the platform. The
by Franz G. J. Witt and Klaas Meurs, Netherlands Ship Model Basin
resistance, the stability and the inertia were
repre-sented by a specific set of input co-efficients
(Fig. 3). The co-efficients were determined by tests
carried out with a scale model of the platform in a
basin as well as by theoretical conclusions.
The second element consisted of the forces and
the moments, exerted on the platform by the six tugs
determined by magnitude, direction and time as
ordered by the towmaster. The resulting motion was
calculated by the computer every second. The orders
were given to the tugniasters by simulated
V.H.F.-communication. The roles o.f the six tugnasters were
played by two operators who repeated the towniaster's
orders in a professional way from a room in which they
could operate the tugs in a simple way (Fig. 14).
The direction of pull of the tow-line in degrees true
could be set by a small wheel. The pulling force
could be set by moving a slide knob. The time delays
between the orders and their execution were
incor-porated in the computer program of the simulated
operation. The orders to the tugs concerning the
direction if pull were given by the towmaster as a
three-digit number, denoting the desired true
direction of the towing hawser. The desired pulling
force was given as a number between 1 and 7. Each
step in the power setting from 1
to 6 corresponded
with 1000 HP for the tug. Power setting 7 meant full
power for each of the tugs. Consequently for the tug
"Witte Zee" the step from 6 to 7 only meant an
in-crease in pulling force of 100 HP, because the
maximum power for this tug had then been reached.
For the TTSmit Rotterdam", however, the step from
power setting 6 to
7,meant a tripling of its total
power as the maximum power of this tug was
22,000 HP.The third element concerned the simulated command
cabin for the towicaster, which also contained all
instruments relevant to the operation such as a
gyro-compass, a doppler log, position information displays,
communication equipment, charts etc.
In addition to these instruments the towmaster was
given a "tug-order-display" as a memory aid for course
and power orders which he gave to tugs during
posi-tioning. (Other investigations of similar situations,
concerning the steering of slow processes with a
complicated "control system", have indicated
diffi-culties experienced by the operator in keeping in
memory orders given and actions taken. This resulted
in repetitions of given orders, forgetting the given
orders and concentration of attention on only a few
aspects of the task). The "tug-order-display" was
designed as a panel, on which the towinaster himself,
simultaneously with giving the order to one of the
six tugs, could make the order visible. Each tug
was shown in this panel on a circular disk, which
could be turned in the direction of pull. A knob which
could be moved in a slot in the disk could be set on
the power setting which was ordered. (Fig.
5).In
this way it could be seen in a wink in which
direction and with which power each tug was pulling.
The task of the towinaster was to carry out a
manoeuvre which would bring the centre of gravity of
the platform to the target area starting from a
certain position. During the manoeuvre a speed
decrease of
0.2knots to O knots in the target area
had to be realized. As already mentioned, the target
area was circular and had a diameter of
15m. It was
drawn in the plotting chart as a circle. Once brought
within this circle the centre of gravity had to be
kept there during 15 minutes. In the first program a
heading between 355 and 005 degrees had to be
main-tained. In the second program the headings had to be
kept between 335 and 3145 degrees during the
15minutes.
When during the 15 minutes the centre of gravity moved
outside the target area or the heading limits were
exceeded the time was reset at zero. When toth
condI-tions had been satisfied for
15consecutive minutes
the manoeuvre was considered successful. If after 60
minutes from the start the task had not been fulfilled
the manoeuvre was considered unsuccessful.
During the operation there was a tidal stream, going
South at 0.5 knots in the first program and in a
direction
215degrees at
0.5or 0.8 knots in the
second program (Fig. 6).
In the first program the manoeuvre was started
from a position exactly downstream of the target area
and with the strongest tug in forward centre position
pulling exactly into the stream. A consequence of this
starting condition was that the manoeuvre always began
from a perfect equilibrium in the forces. Observations
and practical experience led to a different set-up
in the second program. Unlike the first program, this
time the manoeuvre was started from a position not
exactly downstream with a direction of pull not quite
in the direction of the target area.
In order to be able to establish whether all conditions
were satisfied and a run could be ended, two cost
functions were calculated every second (Fig. T). The
calculated values of both cost functions gave a
momentary indication of the accuracy with which the
positioning took place.
The run was ended when the second function cecanie
smaller than
1or when the total elapsed time of the
manoeuvre exceeded 60 minutes. The manoeuvre was
considered unsuccessful in the second.
Besides, during each run predictions were made of
the values of both cost functions. Every minute of
titee to go, a calculation of the cost-functions was
made, based on the expected development of the
manoeuvre in case no more orders would be given. After
completion of the manoeuvre, the results of these
calculations were compared. The lowest values for each
minute were then recorded. The predicted vulues
therefore gave an indication of the effectiveness with
which the operation was carried out.
Both research programs comprised a total of about
100 positioning manoeuvres carried out by twelve
experienced tugmasters of Smut International Marine
Services B.V., who functioned as towmaster in an equal
number of runs each.
Besides the cost functions, during each run the track
of the centre of gravity of the platform, the heading,
the speed, the rate of turn and the tug orders were
recorded, as well as the forces and moments exerted on
the platform by the six tugs as a function cf tine.
These recordings were nade on magnetic tape for further
analysis in a CDC-6600 computer.
Besides the additional advantage of training for the
townasters, the main reason why each person made a
considerable number of runs is the stochastic
charac-ter of systems in which a human operator is involved.
"Chance" is unavoidable in these systems, because no
two persons behave equally. Even one person cannot
make two identical manoeuvres. The best way to make
analyses is to figure with the average and the spread
around the average of a number of runs that were made
under identical circumstances.
These stochastics are also the reason why the results
are always expressed in average values and why
predic-tions and generalizapredic-tions are always given as "the
chance of an occurrence".
THE RESULTS
In Fig. 8 the results of the research programs are given in their simplest form. The number of
successful versus unsuccessful manoeuvres can be read.
It has to be remarked that in these simulations "unsuccessful" only means that the earlier mentioned
criteria of precision and duration could not be
satisfied. A statistical analysis of the results of the first program shows that two factors have an
important influence on the manoeuvre:
The differences between the subjects. These
differences show up in the track, the heading,
the speed decrease and the rate of turn of
the platform.
The learning effect. The effect of training shows up especially in the track, the speed
decrease and the values of the cost functions.
In Fig. 9 the average curve of such a function is shown for the first three and the last three runs that the eight subjects made during the first research program. Based on the results of both programs, a few examples of which have just been reviewed, a number of recommendations can be made for the positioning of platforms in general.
A basic rule for the execution of the described kind of positioning manoeuvres is to find a condition of
equilibriimi of the disturbing forces, like for
in-stance the tidal stream and the tug forces on the platform. From this condition of equilibrium it is possible, by careful manoeuvring, to reach the final position and maintain it for some time. For this
careful manoeuvring a number of indications can be
given as well. These are:
- Wait to give a new order until the former
order has had a noticeable effect.
- Be aware of the fact that the speed over the ground of the platform can only be braked by
counter-action of the tugs. (This is due to
the great inertia and the relatively small
drag of the platform at very low speeds).
- Use as few tugs as possible and direct them to
their original position after action.
- Preferably use little steps for change of power
or course.
Besides the acquisition of concrete results and recommendations, one may conclude from this research
program that a real-time simulation of the towing,
relocating and precise positioning of offshore constructions can be useful and can contribute to
cost-saving in the following activities:
- The planning of multiple tug operations.
- Advising number of tugs and ways to use them.
- Predicting the feasibility and accuracy of
positioning manoeuvres.
- Evaluation of electronic position indicating
system and other aids used in positioning.
- Training of persons involved in such operations in which the dynamics of the constructions and manipulation of the tugs can be simulated in
a realistic way.
- To facilitate discussions between the persons in charge which may lead to an optimization
of such manoeuvres.
L
"SMIT ROTTERDAM"
22,000 HP
"NOORDZEE" "RODE ZEE"
12,500 HP
112,500 HP
TIDAL STREAM GRAV TY PLATFORM FORWARD DIRECTION"SMIT WAGENINGEN"
"ZWARTE ZEE"
10500 HP
10,500 HP
"WITTE ZEE"
8,000 HP
Fig, i - Arrangements of tugs around the platform for positioning.
FEEDBACK TO SUVTGAT1ON AIDS USO TOW MASTER
Fig. 2 - Block scheme of the positioning operation.
Fig. 3 - Relation between the mathematical simulation of the platform and the real' simulation of the command cabin.
a--Z X
COEFFICIENTS
r,, 7.2RE7 kg NCU'I W MASS
t,
. SGElO kg IT UPU' MOMENT OF INERTIA X0,, .Y00 . 4 S7ES kg SeC'I TT DAMPING FACTOR1 R5E9 kg SPC OAMPINO FACTOR
y
CAeIN
I
TEM OF AXES. FIXED
THE PLATFORM
COMMAND
EQUATIONS OF MOTIONS
r,,,-or) X .U,,, U, LONGITUDINAL FORCE
T'I(AAF).Y50 VreI TtUg TRANSVERSE FORCE
SORR Nr r R, RreI N,,9 MOMENT AGOUT THE Z -AXE
WRePS reI' U
-V-Va
R are the CArrenT velocItIes
POSITION INFORMATION DISPLAY AND GYRO - COMPASS '
TOW MASTER j
VHF-TUG ORDERS TUG FORCES
I
- J X
Y , R TOTAL VECTOR OF TUG FORCES tog tog SAg
RELATED 10 THE THREE AXES
ENVIRONMENTAL EON OIT IO NS TUS TUG MASTER POWER TUG AG A R HEU DIM TUG TUG 3 MASTER
INPUT. TANG TOW NF COMMUNICAITON TUG GRAVITY OUTPUT. ACTUALPOSITION
MASTER FORCES PLATFORM
TUG ROWE TUG 4 MASTER POWER TUG U MASTER READING TUG TUGS MASTER ST TO
r
Fig. 5 - Tug order memory display.
Fig. 4 - Control panels for a grasp of two tugs.
Fig. 6 - Map of the situation. NORTH TARGET AREA
r
15M 150 M STARTING POSITIONK1 (R,
)a R2. a2 I
i2, enK2 (t1)
.-
max. (K1 (R. ) 1)t.t1 -14
IN WHICH: K1 COST-FUNCTION i K2 COST-FUNCTION 2R
RADIUS OF THE TARGET-AREA (15M)
DEVIATION FROM REQU1RED FINAL HEADING
a1 a2
COEFFICIENTS CHOSEN SUCH THAT R ANDEQUALLY CONTRIBUTE TO THE VALUE OF K1
t1 = TIME
Fig. 7 - Cost functions K1 and K2 that were used to obtain suantitative information about the progress of the manoeuvres.
Fig. B - Results of both experiments in their simplest form.
200 VALUE 0F 151E 100 COST-FUNCTiON 2 200 O o VALUE 0F THE PREDICTION OF THE 100-COST-FUNCTION 2
AVERAGE 0F TRIAL 1.2 AND 3 VERAGE OF TRIAL 4.5 ANO 6
20 40
TIME (MINUTES)
AVERAGE 0F TRIAL 1, 2 AND 3 AVERAGE OF TRIAL 4, 5 AND 6
60
EXPERIMENT 1 EXPERIMENT 2
SUCCES FAILURE SUCCES FAILURE
TOTAL NUMBER OF RUNS 31 33 11 21
NUMBER OF TUG ORDERS
PER MINUTE 1.67 1.83 1.18 1.61
/ ORDERS OF TRIS TO:
TUGS FORE 42.6 52.7 35.7 66.6
AFT 57.4 47,3 64.3 334
DIVIDED IN' POWER ORDERS 5. 57.3 53.6 87.4 909
COURSE ORDERS 5. 42.7 46.4 12.6 9 1
20 40 60
TIME (MINUSES)
Fig. 9 - Average values of the cost functions K2 compared with the average value of the prediction of the cost function.