• Nie Znaleziono Wyników

The positioning of offshore constructions, research and training by simulation

N/A
N/A
Protected

Academic year: 2021

Share "The positioning of offshore constructions, research and training by simulation"

Copied!
6
0
0

Pełen tekst

(1)

-ARCH1EF

EUR 21

THE POSITIONING OF OFFSHORE CONSTRUCTIONS,

RESEARCH AND TRAINING BY SIMULATION

© Copyright 1978, European Offshore Petroleum Conference and Exhibition

Lab.

y.

Schepsbouwkund3

Technische Hogesc!

DefI

8ibJotbeek VEin

de

Afdeling Sc.

n Szar;je

Te'iv h

'cC'-o, Det

DCCUENTAE

f:

532c-/2j

fDATUMI

_j_

161

PUL. NO.

OF THE N.S.M.3.

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

(2)

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

0.2

knots to O knots in the target area

had to be realized. As already mentioned, the target

area was circular and had a diameter of

15

m. 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

15

minutes.

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

15

consecutive 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

215

degrees at

0.5

or 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

1

or 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

(3)

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.

(4)

L

"SMIT ROTTERDAM"

22,000 HP

"NOORDZEE" "RODE ZEE"

12,500 HP

1

12,500 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 FACTOR

1 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

(5)

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 POSITION

(6)

K1 (R,

)

a R2. a2 I

i2, en

K2 (t1)

.-

max. (K1 (R. ) 1)

t.t1 -14

IN WHICH: K1 COST-FUNCTION i K2 COST-FUNCTION 2

R

RADIUS OF THE TARGET-AREA (15M)

DEVIATION FROM REQU1RED FINAL HEADING

a1 a2

COEFFICIENTS CHOSEN SUCH THAT R AND

EQUALLY 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.

Cytaty

Powiązane dokumenty

Y es que, como consecuencia de su capacidad para comunicar un cúmulo de informaciones en el tráfico económico, la inclusión de una indicación geográfica en la presenta- ción de

Er is behelve enkele octrooien geen literatuur over dit proces. De evenwichtdconstanten van de reakties zijn niet bekend. uit de elementen zijn onbekend. Deze zijn

The paper aims at the investigation of partial cavity flows around a scaled-down model of guide vanes (GV) of a high-pressure turbine at different attack

Platon jest przekonany, że człowiek wierzący nie czyni dobrowolnie zła, a zdarzyć się to może jedynie ateistom, O istnieniu bogów i ich dobroci oraz opiece

Max Weber kommt aufgrund seiner religionssoziologischen Untersuchun- gen zu dem Ergebnis, dass jedes religiöse Gemeinschaftshandeln auf die eine oder andere Weise

przybyło kilka tysięcy nider- landzkich rzemieślników i rolników, zwanych „olędrami”, którzy zagospo- darowywali ziemie na Pomorzu (Żuławy Wiślane), Kujawach,

W części podziemnej kościoła odsłonięto w jego m urze północnym niszę ostrołuko- wą ze śladam i otw oru wejściowego oraz uchwycono posadzkę ceglaną niższej

While the loop filter can achieve even lower swing for N = 6, the maximum tolerable input frequency of the zoom ADC decreases [see (9)] and the accuracy requirements of the SAR