Tank and wind tunnel tests for a drill-ship with dynamic position control

(1)

- -

JULI 19Th

ARCHIEF

See note inside cover

e

Lh

t.

Scheepbouwkue

Technische

Hogeschap

Report Ship 194

DeIff

November 1975

National

Physical

Laboratory

Ship Division

TANK AND WIND TUNNEL TESTS FOR A

DRILL-SHIP WITH DYNAMIC POSI TI ON

CONTROL

(Reprint of Paper presented at the Seventh Annual

Offshore Technology Conference, Houston, Texas,

May 1975)

by DAWise*and JWEnglish

G.E.C. Electrical Projects Limited.

(2)

SMTR-.7 543

Extracts from this report may be reproduced providing the source is acimowledged

(3)

6200 North Central Expressway

Dallas, Texas 75206

THIS PRESENTATION IS SUBJECT TO CORRECTION

Tank and Wind Tunnel Tests for

_{a Dri Il-ship with}

Dynamic Position Control

By

D. A. Wise, GEC Electrical Projects Limited and J. W. English, National Physical Laboratory

PAPER

NUMBER OTO 2345

Offshore Technclogy

Petroleum Erci neers, Petroleurr Engineers)

cal Engineers, Amen

Institute of Electri

ation Geophysicists,

Conference on behalf of the American Institute of Mining, Metallurgical, and Inc. (Society of Mining Engineers, The Metallurgical _{Society and Societj of} American Association of Petroleum Geologists, American _{Institute of} Chemi-can Society of Civil Engineers, AmeriChemi-can Society of Mechanical _Engineers, cal and Electronics Engineers, Marine Technology _{Society, Society of}

Explor-and Society of Naval Architects Explor-and Marine _Engineers.

This paper was prepared for presentation at the Seventh Annual Offshore Technology Conference to be held in Houston, Tex., May 5-8, 1975. Permission to copy is restricted to an abstract of not more thar 300 words. Illustrations may not be copied. Such use cf an abstract shculd contain conspicuous acknowledornent of where and by whom the paper is

presented.

1. Introduction

"Wimpey Sealab" is a multi-purpose ocean engineering motor vessel operated by Wimpey Laboratories Limited and is equipped with a computer-based dynamic positioning system by GEC Electrical Projects Limited. Originally

a cargo ship, the "Wimpey Sealab" was

converted to a drill-ship, arid completed her commissioning trials in a dynamic positioning mode of operation in November

_1974.

A view of "Winipey Sealab" at sea is shown in

fig. 1; and the main elements of the dynamic positioning system are shown in fig.

_2.

The design of the dynamic positioning system depends on the mathematical models (i.e. the

set of equations) that define the ship and its environment. A completely theoretical

approach to the formulation of these equations is not feasible, and they are determined by means of tests on scaled down physical models of the ship. Phis paper describes results obtained from the tank and wind tunnel tests, carried out by the

National Physical Laboratory, on models of

"Wimpey Sealab".

References and illustrations at end of paper

Dynamic positioning for "Wimpey Sealab" is required in water depths between ₃₀ arid 300

metres. The ship position must be held within a circle of 7 metres radius (or 3% of water depth, whichever is the greater) in a

steady wind of up to

12.87

rn/see, with waves

of significant height

_3.54

metres and significant length

_91.44

metres, and with a

steady sea current of up to

_1.54

rn/sec.

Under the above conditions, but with the wind gusting up to

_20.59

rn/see, the ship position

must be held within a circle of 11 metres

radius. Ship heading is allowed to vary. Propulsive devices for dynamic positioning may take several forms, including the use of the main engine in conjunction with tunnel

thrusters, steerable thrusters, and cycloidal types of thrust unit. Wimpey Laboratories Limited decided to use steerable thrusters, and the hull of the "Wimpey Sealab" was modified to accommodate four retractable, rotatable and controllable pitch thrusters. These thrusters are each rated at

₇₄₆

kW;

two are located side by side at the stern of the ship, and two are located, slightly offset from one another, at the bow.

(4)

The S.l. system of units is used throughout this paper. Conversion factors to imperial units are given in Appendix 1.

2. Test Objectives and Ship Details

The objectives of the tests can be considered in three main parts as

follows:-Part 1: the estimation of the steady forces and moments acting on the ship due to the environment of current, wind and waves (see sections 3, 4 and 5 of the paper respectively); and the estimation of wave induced ship motions (see section 5).

Part 2: the estimation of forces and moments applied to the ship by the action of the four thrusters; and, in

particular, the interaction effects that occur between the thruster jeta and the hull, in still water and in the presence of current flow (see

section 6).

Part

3:

the formulation of the equations of motion of the ship, which define its motions due to the forces and

moments applied by the thrusters and by the environment (see section 7). Section 8 of the paper describes the

simulation of the dynamic positioning system, and shows how this depends on the results of

tests covered by parts 1, 2 arid 3 above.

Details of "Wimpey Sealab" and models used for the tank and wind tunnel tests are as

follows:

-Ship designation

Single-screw Diesel Vessel

Displacement (including moonpool water) 5670 tonne

Block coefficient 0.655 LCB 2.46% Lpp aft amidships. Machinery

Main Engine 2014 kW (2700 bhp) diesel

Four 746 kW thrust units capable of vectoring thrust through 360°.

Model scales and materials

Model hull 5209 1/20.09 pclyurethene Wind tunnel model 1/96 wood

Current Forces and Moments

The steady-state forces and moments due to a current were measured on a 20.09th scale model, of 4.7m Lpp, by towing the model through the tank. The model was attached to the carriage with flexures the electrical output of which was integrated and logged. For this purpose the model was fitted with the following appendages,

- main four-bladed propeller with blades fixed at 450 to the vertical

- rudder fixed at zero helm - bilge keels

- open drill well

- thruster recesses faired off

- turbulence stimulating studs at bow and stern, and bottom trip wires extending from the termination of the bilge keels to the bow and stern.

The results have been scaled to ship values without applying any viscous scale

corrections and these are given in figs. 3, 4 and 5 for the X and Y forces and the N moment

in 1, 2 and 3 knot currents. These results apply to deep water i.e. a water depth 6 or

₇

times the ship draft, or 33 to 38 m. For shallower water the Y force in particular can increase considerably.

The X force at zero and low yaw angles is the only force that is influenced significantly

by viscous scale effect and since this force is relatively small, even at 3 knots, the

effect is not serious, A feature of the moment characteristics in fig. ₅ is the lack of rotational symmetry about 4, = 9(P, the

negative moment at 4'

-

1300 being much

greater than the positive moment at 4, = 40°.

This is due, presumably, to the presence of the fixed rudder and screw at the stern.

Steady Wind Forces and Moments

The tests to measure wind forces and moments were conducted in a wind tunnel in Maritime Science Division, NPL. For this purpose a model of the above water form was made to a scale of 1/96 and the tests were conducted at

a wind speed of 25 rn/s. The model was yawed through the complete 360° and the

longitudinal and athwartships forces measured together with the yawing moment referred to

the ship C.G.

In calculating the ship wind forces arid

moments from the test results, the wind velocity was specified at a datum height of 10m above the still water level. An allowance was made for the velocity distribution with height above the still Ship particulars

Length between perpendiculars (Lpp) 94. 49m

Beam (B) 15.24m

Mean Draft (T) 5.49m

Trim by stern between perpendiculars O.61m

(5)

water level according to the following law,

In this expression Uh is the wind velocIty at a height h above the water surface and U denotes the velocity at the datum height h. For a wind speed of 25 knots at the 10 m datum height the forces arid moment on the ship as derived from the model coefficients are given in fig. 6 for the range O to

900.

5.

Wave Drift Forces and Ship Motions Depending upon the sea state a ship in the dynamically positioned mode can be subjected

to very large forces caused by the passage of

waves. There is no possibility of counter-ing these relatively high frequency effects with thrusters. Indeed the system must be desigued so that the thrusters do not respond to these fluctuations to avoid excessive wear in the mechanisms. In

addition to the high-frequency forces the so-called drift forces and moments due to waves must be considered.

Over the last decade and a half many papers have been published for calculating drift forces and moments based on potential theory,

refs. 3,

4,

5, 6, 7, 8, 9 and 10. However few papers have given calculated results and in those cases where comparisons between experiment and theory have been made they are not always satisfactory - although in view of the difficulty of the problem the progress is very encouraging. Accordingly in this work

it was decided to attempt to measure these

quantities. For this purpose the

_4.7

m model was dynamically balanced and then

subjected to the passage of regular wave trains in the towing tank. The model was lightly restrained by three and sometimes four nearly horizontal strings passing over pulleys to which were attached scale-pans and

weights. After some experience results were obtained but it was clear that these were not entirely free from subjective impressions and this was reflected in the relatively poor repeatability and large scatter in the

results. This difficulty in measuring the small 'steady' forces in the presence of large fluctuating forces is well known and has been found by other experimenters. The measurements in regular waves were expressed in coefficient form so that,

CyH

(cxw)

lateral (or longitudinal) drift force

Pg Lpp

_A2

Yaw drift moment and C

pgLpp

A2

and plotted against ''/Lpp or wave frequency.

Drift forces and moments in irregular seas To calculate the drift forces and moments in irregular seas it was necessary to invoke the superposition principle. If the enerr

density of the irregular wave spectrum at frequency f is denoted by S(f), where the wave amplitude corresponding to the enerr contained in a frequency bandwidth of s(f)

is Ç -

,,/2 S(f) df, then the mean drift

forces are given by

rF

X p g Lpp J s(f) C df

uO

PgLpp

_¡

S(f)Cydf

JO

and the drift moment by,

N= pgL2pp

where F is the upper frequency limit.

In the absence of knowledge of the specific operating areas of the ship the ITTC

irregular wave spectrum was used in

calculating the drift forces. This is given

by

s(f)

499.8

x io-3

e/4

f5

1.995 x io-3

where B

2

where is the significant wave height in

metres. The results of these calculations are shown in fig.

_7.

At

₉₀₀

the

regular drift moment measurements were

unrealistically high in addition to the large scatter present and, therefore, this point is not included in fig.

_7.

Ships motions

Measurements of the model's motion in the horizontal plane were recorded on video tape and subsequently analysed. These showed

that at = 900 the longitudinal motion was negligible, as also was the lateral motion at

= O. Also in all the tests the oscillatory yaw appeared to be negligible. Examples of the measured sway and surge in regular waves are given in figs. 8 and

_9.

The other motions in regular and irregular seas were all calculated.

6.

Thruster/Hull Interaction

When a ship is held stationary in a current or moved in still water under the action of thrusters and main propeller(s) the resultant force and turning moment acting on the hull are not simply the force and moment due to

s

105

JO

(6)

the thrusters acting as though they are in

isolation. Interactions arise which can cause the hull force and moment to be greater or less than the thruster forces and moment. In an attempt to obtain more information on this subject an extensive series of model tests were performed with thrusters operating beneath the hull. These thrusters were capable of being turned in azimuth about their vertical axes and the model was attached to the tank carriage with flexures

so that the resultant force and moment on the hull could be measured with or without a current being simulated. It was not possible to measure the thrust and power input to the thrusters during these tests and instead the previously measured static thrust versus rotational speed characteristics obtained on the isolated thrusters was used to set the specified thrust. At the low advance ratios that the units operate at, this procedure was considered satisfactory. At some azimuth angles however the slipstream from one unit will enter another unit and

then the performance of the downstream unit will be affected. When this occurred on the model the thrust from the downstream thruster was less than it should have been by an unspecified amount. On the ship this will not arise because controllable pitch

propellers are installed and full power can

be maintained.

Interactions with no current

Figs. 10 and 11 give the results of tests using the forward thrusters on the stationary hull in still water. The full-power static

thrust for the two forward units is nominally 200 kN, i.e. loo kN per unit. In these

tests both units were ganged in azimuth so that their directions relative to the ship

X axis was defined by a single angle 8 The thrust axes of the units coincided at

8= .-lO° and 170° and from flow observation tests with a model thruster, in the presence of a flat surface, it was seen that the slip-stream subtended an arc of about ±20° at the separation distance between the forward

thrusters. This gives sorne idea of maximum

angular range over which thruster/thruster interaction can occur.

The force vectors in fig. 10 show that

significant interactions occur in the rangeo

±60°. For example when 8 = 300 and the

thrusters still produce a significant ahead thrust, the resultant force on the hull is only about 80% of the thruster forces. Also it occurs at an angle of about 15° instead of

300 i.e., - -15°. With the thrusters operating in this direction the slipstreams sweep aft along the hull probably adhering to it on account of the Coanda effect - the effect whereby a jet clings to a curved

surface. The rearward flow of the slip-steams will cause a skin friction on the hull

and also jet impingement Lerce on the forward facing surface of the drill well. These

will both detract from the thruster forces. Another force - a pressure force on the hull

as a result of the Coanda effect - acting generally in the direction starboard to port will cause the force vector to swing in the anticlockwise direction.

When the forward thrusters are operating in the general athwartships direction i.e.

8 = 900

± 30°

or 270°

_{± 30°,}

the

interactions are less and about 90% of the full thrust is achieved. Again this force reduction is probably caused by the Coanda

effect.

Interactions with current

With a current flowing the matrix of possible operating conditions increases considerably. The results in figs. 12, 13 and 14 provide an example of the interactions that can occur and refer to the vessel at 300 to the current when the bow thrusters are trained in azimuth

through 0-360°. The interactions arise as a

consequence of the thruster slipstreams

combining with the current flow and modifying the pressure distribution over the hull. It

is essential, therefore, that such

experiments are conducted on a complete model hull fitted with thrusters.

The main features of the force interaction in fig. 12 is the large reduction in force that occurs when ô increases from about 1000 to about 1500, and the increase in force at about 8= 2200. These interactions are well removed from the 8 regions where

thruster/thruster interaction predominates and may be accounted for as follows.

As 8 increases from 60° the thruster slip-streams become increasingly directed into the current until at 5 - 150° they oppose it completely. The jets flowing against the current diffuse more rapidly than they would

in still water and tend to arrest and deflect the oncoming current some distance upstream

of the hull. As a consequence of this so-called 'shielding effect' the ship does not experience the full current force in addition to the thrust from the thrusters. It is

possible that a similar effect is responsible for the favourable interaction on the turning moment in the 8 range around

40-8C°

when the diffused slipstreaxns will tend to shield the stern of the ship from the current. It may be noted that with the thrusters operating in

this range they are propelling the ship in the general current direction and perhaps this is an unlikely mode of operation. At and around ô = 2200 the thruster

slip-streams discharge on the leeward side cf the hull and then the interactions are associated with a strong swirling flow that has been

observed to pass along the leeward side

(7)

again caused by the slipstrearns combining with the current flow. However in this case

the explanation for the interaction is less obvious and must await the results of more detailed tests.

A more detailed general description of thruster/hull interactions that occur with dynamically positioned ships is given in

ref. 11.

7.

Equations of Motion

The dynamic positioning system controls the surge, sway and yaw motions of the ship. Treating the ship as a rigid body having freedom in surge sway and yaw, but restricted in heave pitch and roll, the equations of motion, for a co-ordinate system fixed in the body with origin at the C. of G.

are:-m(-rv) = XA+XE

7.1 m(+ru)

=

7.2 Ir

= N + NH

7.3

where (a) _{XA A and NA represent forces and} moments on the ship due to

thrusters and environment.

(b) XH YE and NH the hydrodynamic forces and moments which depend on relative motion between ship and

water.

For a ship moving in still water, XH _{H and} NH may be expressed as functions of the instantaneous velocities and accelerations

u,

y,

r, t, and

To determine the equations of motion, expressions for XE H and NH are required, applicable to a ship making small movements about a fixed position relative to earth. Approaches to this problem include tests on a

model using plarr motion mechanisms, tests on a free running model having known forces and moments applied to it, and simple theory substantiated by model tests. The latter approach was used for "Wimpey Sealab", as described below.

The system used to non-dimensionalise the equations of motion (applicable to a ship with zero speed reference conditions) was as

follows:

-u' = u/JLp

y' = v/jT

r' = r/,pp/g

= /g = /g ' = Lpp/g

= kzz/Lpp t' = t/j7

X' = X/m. g Y' = Y/m. g N' N/rn. g. Lpp

The hydrodynamic forces and moment dependent on the lateral components of ship velocity

(u and

y)

were derived from the Current Force and Moment Tests (see section

_3).

The

relationships obtained for "Wimpey Sealab"

were:

-0.092

y'2

-=

-2.58 v'U'

N' =-0.764 u'v'

where U'

=Ju'2

+

The hydrodynamic forces and moment dependent on the angular velocity r can be estimated from the underwater profile of the ship. Considering the ship turning about a vertical axis through the centre of gravity it is assumed

that:-total hydrodynarnio moment

JJcy* PV2Xdk

profi e area

where dA is an elemental area of profile, distance L from the vertical axis through the centre of gravity, V is the velocity of the area dA through the water, and Cy* is the local transverse force coefficient.

If it is assumed that the overall side force

at

90° is

uniformly distributed in the direction, then from the results of the Current Force and Moment Tests (see section

3),

*

= 1.0. Equation

_7.7

can now be used to calculate the hydrodynamic moment due to angular velocity r. The hydrodynamic force in the Y direction due to r can be similarly calculated and for "Wimpey Sealab" the

following values of force and moment due to angular velocity r were

obtained:-Y' =

0.068

r'Jr'I

7.8

N' =

-0.162

r'jr'j

_7.9

Estimates of the added masses in surge and sway and the added inertia in yaw were based on experimental data measured by Motora, see ref. 1, and presented in a convenient form in

ref.

2.

For "Wimpey Sealab" the added masses and inertia terms were found to

be:-7.7

The equations of motion for "Vlimpey Sealab"

were derived by using the results given in equations

7.4

to

7.6

and

7.8

to

7.12

in equations

7.1

to

7.3;

and are given

by:-(i +

0.044) i' -

r'v'

7.13

= XA' +

0.092 y'2 - 0.138

u'U'

(i + 0.84) ',' +

r'u'

7.14 - 2.58 v'U' - 1.8

+ 0.068

r' Ir'!

(k55'2

+ 0.0431)'

7.15 = NA' - 0.764 u'v' + 0.258

v'U' -

0.162

_r'lr'l

X' Y' N' = = =

0.044

0.84

'

0.0431

'

7.10

7.11

7.12 0.138 v'U'

7.4

7.5 -

1.84--4-0.258

v'U'

7.6 2345

107

(8)

108 TAK AI'D WIND TUEL TESTS FOR A DRILL-SHIP WITH DYWC POSITION CONTROL

2345

8.

Simulation of the Dmrnic

Positioning System

A hybrid simulation of the dynamic

positioning system for 'Winxpey Sealab" was used in the design of the control system and to assess its performance. The layout of the simulation is shown in fig. 15, and can conveniently be considered in four parts as

follows:

-Part 1: the mathematical models that describe the ship behaviour under the influence of its thrusters and of the environment of current, wind

and waves.

Part 2: the functions of the digital computer used for dynamic position

control.

Part

3:

the response of the thrusters to output demands from the control

computer.

Part 4: the characteristics of t devices

used to measure ship position and heading and the direction and strength of the wind.

The mathematical models of the ship are built up from the data obtained from the tank and wind tunnel tests described in this paper.

The equations of motion refer to motion in still water, in the horizontal plane (see section

7)

and it is assumed that, for the motions associated with the dynamic

positioning system, the added masses and other coefficients do not vary with frequency. This assumption cannot be made for the

oscillatory type of ship motion induced by

the waves; estimation of these motions is based on the use of the sea spectrum combined with the response characteristics of the ship to waves (see section

5).

The basic function of the control computer is to input error signals of ship position and heading, and operate on them to output thrust magnitude and direction commands to the thrusters, so that ship position and heading are maintained at their fixed reference values, against the environmental

disturbances.

The power of the thrusters is generally insufficient to counteract the oscillatory

forces and moments due to the waves. ttempts to counteract these forces cause unnecessary wear and tear on the thrusters, a wastage of power, and a reduction in the

capacity of the thrusters to counteract wind, current and wave drift forces. To minimise these undesirable effects, the components of oscillatory ship motions which appear in the error measurements of ship position and heading are removed by wave filters. Other

features of the control system

include:-Heading priority

The ship must head into heavy weather, so that sufficient power is available from the thrusters for dynamic position control. If

this heading is lost, forces and moments can build up to values which exceed thruster

capacity. To counteract this effect, priority is given to the heading control at the expense of the position control, when thrusters become overloaded.

Wind feed forward

Instead of waiting for a build-up of error to allow the control system to compensate for wind gusts, the computer continuously measures the magnitude and direction of the wind, and outputs appropriate thruster commands to counteract the gusts. This

technique improves the performance of the dynamic position control; however its

effectiveness is dependent on the results of the tank and wind tunnel tests in providing an accurate wind model and a reasonable estimate of how demanded thrust differs from actual thrust due to the interaction effects between the thrusters and hull.

Optimal heading

On "VTimpey Sealab" the computer selects the ship heading so that thruster power is

rninimised.

9.

Discussion and Conclusions

Model experiments have been conducted to obtain information for predicting the full-scale environmental forces and moments that will be experienced by "Wimpey Sealab". It

is assumed that simple vectorial addition of the current, wind, and wave components is all that is necessary to obtain the net

externally applied force and moment due to the environment, i.e. interactions between the three components have not been considered. Possibly the interactions between wave and current, and wave and wind, will be the most

significant, but these effects are expected to be of second order importance.

The interactions between the thrusters and the hull have been found to be surprisingly large in some cases. Some explanations for these have been given but further work on this topic is necessary. The Coanda effect is seen to play a role when the thru.sters are operated with the ship in stationary water. In a current the interactions between the thruster slipstreams and the current can lead to significant changes in the force and

moment acting on the hull. The so-called "shielding effect" is a particular example of this type of interaction.

(9)

The equations of motion used in the

simulation work apply to a vessel moving in calm conditions, although wave drift forces and moment are included in the externally applied forces and moment X', Y' and N'. The external forces and moment should also include the thruster/hull interaction.

10. Nomenclature

A Area

Longitudinal current force coefficient X

P Lpp TV2

Cy Traverse current force coefficient y

P Lpp TV2

CN Yaw moment coefficient due to current N

i.

PLTV

2 2

Cj

Longitudinal wave drift force coefficient

xW

p gLpp _A2

CyW Transverse wave drift force coefficient

P gLpp A2

C Wave drift yaw moment coefficient

P gL2pp _A2 f Frequency

F Upper frequency limit

g Gravitational acceleration

h

Height

above still water surface

h Datum height above still water surface Significant wave height

IZZ Moment of inertia of ship in yaw about

axis through C.G.

kz Radius of inertia of ship in yaw about axis through C.G.

Lpp Length between perpendiculars m Ship mass

N Yaw moment

N; Added inertia in yaw

r Angular speed of ship rotation in yaw

s(f) Enerr wave density at frequency f

t Time

T Ship draft

u Ahead_velocity

u

fu2+v2

U Wind velocity at datum height h y Transverse velocity

V Current speed

Longitudinal distance from CG.

X Longitudinal force X0 Surge amplitude

X Longitudinal added mass coefficient Y Transverse force

Sway amplitude

Y Transverse added mass coefficient

t

Angular phase

8 Thruster azimuth angle from ahead (clockwise in plan view)

Angle between ship's head and oncoming current

Angle between ship's head and waves or wind

X Wave length

Mass density sea water Wave amplitude

Suffices

A Externally applied forces and moments (current, wind, waves and thrusters)

Hydrodynamically applied forces and moments

W Refers to wave drift forces and moment w Refers to wind forces and moment NB The dot above a symbol indicates

differentiation with respect to time t

Dashed symbols are non-dimensiorialised, i e.

Linear Velocities with respect toj Linear Accelerations with respect to g

Angular Velocity with respect tofL7

Angular Acceleration with respect to Lpp/g Length with respect to Lpp

Time with respect toj/Lpp

Force with respect to mg Moments with respect to mgL

(10)

Acknowledgements

The authors are grateful to Viuipey

Laboratories Limited, GEC Electrical Projects

Limited and the National Physical Laboratory

for permission to publish this paper;

and

also to their collegues at GEC and NPL who

participated in carrying out the model tests

and simulation.

In particular, thanks are

due to rtr.

.D. Gill of the Ship Division of

NPL who developed the "equations of motion",

to Mr. C.F. Cowdrey of the Division of

Íiaritime Science of NPL who conducted the

wind tunnel tests, and to Mr. R.. Sutton of

the GEC Hirst Research Centre, who helped

with the simulation.

References

Motora, S., "On the measurement cf added

mass and added moment of inertia of

ships in steering motion", ist Symposium

on Ships Manoeuvrability, DT

Report

No. 1461, (1960).

English, J.'., "Moving ship sideways",

Shipping world and Shipbuilder, (April

and September 1970), 549-555 and 1236.

Maruo, Hajime, "The drift of a body

floating on waves", J. of Ship Research,

(December, 1960), 1-10.

Hu, Pung Ilien and Eng, King, "Drifting

force and moment on ships in oblique

waves", J. of Ship Research (March,

1966), 18-24.

Newman, J.N., "The drift force and

moment on ships in waves", J. of Ship

Research, (March, 1967), 51-59.

Kaplan, Paul, "Hydrodynamic analyses

applied to a mooring and positioning of

vehicles and systems in a seaway",

Eighth Symposium on Naval Hydrodynamics,

Office of Naval Research, Department of

the Navy, (August, 1970), 1017-1081.

Verhagen, J.H.G., "The drifting force on

a floating body in irregular waves",

Eighth Symposium on Naval Hydrodynamics,

Office of Naval Research, Department of

the Navy, (August, 1970), 955-979.

Chou, F., and Kim, C.FI., "Prediction of

drifting force and moment on an ocean

platform floating in oblique waves",

International Shipbuilding Progress,

(October, 1973), 20, 388-401.

Remery, G.F.M., and van Oortmerssen, G.,

"The mean wave, wind and current forces

on offshore structures and their role in

the design of mooring systems", paper

OTC 1741, presented at the 5th Annual

Offshore Tecbnolor Conference, Houston,

April 29 - May 2, 1973.

Wahab, R., "Yaves induced motions and

drift forces on a floating structure",

Netherlands Ship Research Centre ThO,

Report No. 186S, (March, 1974).

English, J.., "Propeller/hull

interaction", Appendix to the 14th

I.T.T.C. Propeller Committee Report to

be published in the Proceedings of the

14th I.T.T.C., (Ottawa, Canada,

September, 1975).

Appendix i

Conversion factors from S.l. to Imperial

Units.

(Correct to 5 significant figures).

Lenth

1m =

3.2808

ft

Area

1m2 =

10.764 ft2

Volume ira3 =

35.315 ft3

Speed

1 km/h

=

0.53996 International knot

i

=

3.2808 f t/5

Acceleration

1 m/52

=

3.2808 ft/32

Mas s 1

tonne

0.984207 ton (2240 lb)

1 kg

=

2.2046 lb

Specific Volume

1 m3/tonne

=

35.881 ft3/t0

Moment of Inertia

1 kg ra2

=

23.730 lb ft2

Force

1 kN

=

0.10036 ton f

1 N

=

0.22481 lb f

Moment of Force

1 kNm =

0.32927 ton f. ft.

Power

i kW

=

1.3410 hp

Prefixes

k (kilo)

= i3

M (Mega)

=

io6

TANK AND WIND TUNNEl TESTS FOR A DRILL-SHE 'iITH DYNAÍLC POSITIONING CONTROL

2345

110

(11)

(12)

40 z

X

:

20 u

L

o

D C 4-,

L

20

40 Computer

2 Stern thrusters

(side by side)

Taut wire pdsition

measuring system

7'

hon

Acoustic "

beocon

\

/

'rrtf,r

-I+ir\n

Weight I

,

II r

y measuring

system

POSITION REFERENCE SYSTEM A sonar

and taut wire system to measure the deviation

of ship position from a fixed reference position on

the sea bed; a gyrocompass to measure

the deviation of ship heading from a fixed reference heading.

PROPULSION SYSTEM. Four 746 kW controllable

pitch rotatable and retractable

thrusters to provide the forces and moments on

the ship to maintain its position

and

heading, against disturbances due to wind waves

and current.

CONTROL SYSTEM A GEC 2050T digital computer which receìves inputs of

ship

position and heading deviations and sends

output commands of thrust magnitude

and

direction to the thrusters so that the ship position

and heading are held at their reference settings.

Fig. 2 - Main elements of dynamic positioning

system for 'Wimpey sealab'.

Deep water

2 Bow thrusters

(offset)

Sea bed

3 Knots

2 Knots

I Knot

60

80

100

120

140

160

180 Yaw ongle * /degrees

(13)

700

600 z500

:

₄₀₀

'300

V

200

100

4

2

-10

E

zo

Z2

4-I C

E

o-E

o,

C6

C

L

20

40

60

20

40

60 3 Knots

Deep water

i Knot

80

100

120

140

160

180

Fig. 4 - Side force due to current.

Yaw angle -Jr Idegrees

80

100

120

140

160

180 Deep water

Fig. 5 - Manient due to current.

(14)

90

80

70

60 z

-250

E 200

Z

Z 150

o

z

.

100 >-j 50

><

Wind speed 25 Knots

Waves

Irregular waves_

ITTC spectrum Hi/335m

10

20

30

40

50

60

70 kwI

degrees

Fig. 7 - Wave dr-ift forces and moment.

Z

-0.7

06

05

04

03

02

01

90

Fig. 6 - Wind forces and moment.

10

20

30

40

50

60

70

80

(15)

o-8

0-7

06

05 O-4

4 0-3

>2

O-2

0-1

o-7

0-6

0-5

4

04 _o

>(

0-3

_co

measured

o 3Q0

o00

OE2 0-1

Regular waves

60° measured

o 3Q0

Il

02 Regular waves

OE6

X0

/

0

30°

O-8

,

.,

_/

/

1-O o

Fig. 9 - Wave induced surge ampi itudes.

1-2

1-4

F î. Wave induced sway amp i i tuce. /

L p

l-8

0-B

1-0

1-2

1-4

i-6

1-8

(16)

12-10

E

z8

z

.116

C

E

o

E4

C C

t-

/

Forward units at

full thrust

-zero current

Forward units at

/

full thrust

_-

/

zero current

/

L

180°

41)

-)o

.-

_s

)

ig. IO - Thruster/hull interaction tests.

\

0°

Turning moment

impressed

by

thrusters

Turning moment

experienced by

'

ship

\

D c

o

1

Jet

sprea

20

Fig. Il - Thruster/hull interaction te'E.

S / degrees

(17)

280

160

3Knot

current

j20

_'1fr3Q°

C)

r:.

. .

0

40

80

120

160

200

240

280

320 360 20

.S /degrees

F19. 12 - Thruster/hul I interaction tests. For-ward units at full thrust - 3 knot current.

(O

240

200

40

30

20 -10

-20

-30

¡ I I I I

40

80

120 S Idegrees

I

/%

Force impressed

by thrusters

Jet

spread

o

u)

X

o

4-, U) 'C 160

0 u

C L)

o

ç,

Jet

spread

3 Knot

current

*= 300

/

Resultant

force on

ship minus

current

Jet

force.

spread,1j

u)t

200

240

280

320 t360

20

Fig. 13 - Thruster/hull interaction tests. Forward ur-its at full thrust - 3 knot current.