Regular and random wave loading on a small diameter cylinder

gab.

v. Scheepsbouwunde

Technische Hogeschool

VREGULAR

PRINCIPAL INVESTIGATOR Dr.A.G.L.Borthwick, University of Salford

CO-TNVESTIG.TOR Dr.G.N.Bullock, Plymouth Polytechnic

INSTITUTION/CONTACT ADDRESS: Department of Civil Engineering University of Salford

SA LFORD

M5 4WT

Tel: 061-736--5843 (x7110)

INTRODUCTION

Wave loading on small diameter cylinders is of interest to offshore engineers. Through the fluid loading committee, several research projects at the University of Salford have attracted grants from the Science and

Engineering Research Council. The project described herein will continue from previous investigations started by Dr (3.N. Bu1lock'2 and Dr A.G.L.

Borthwick. In the stage which formed part of the 1981/83 fluid loading programme regular and random wave loading on a fixed small diameter cylinder placed at various orientations was examined. This was followed by two series of regular wave tests (funded within the 1983/85 and 1985/87 fluid loading prograes) on the same cylinder in a vertical position, hut with a spring mounting rather than a fixed base.

Regular wave loading on a flexibly mounted vertical cylinder results in complex fluid-structure interactions which require careful analysis by

offshore engineers. For example, the dynamic responses of slender members

in fixed jacket platforms, marine risers and compliant structures are

susceptible to resonant vibrations. Stresses associated with the dynamic

responses alter the fatigue life of the structure under cperating

conditions and may affect the strength under extreme waves.

A] though advances in research have led to a much improved understanding of wave loading, catastrophic failures such as those of Texas Tower No 4 in

1961 and the Alexander Kielland in 1980 have highlighted the need for

further research, especially regarding vortex-induced fluctuating

tr.wsverse forces. In Salford, experiments were conducted usirg an

instrumented cylinder mounted in a 39.6m long wave flume. Forces and response displacements of the cylinder were measured and analysed to

provide a substantial data base°'4'5'6 consisting of force coefficients,

etc. As part of this study, analytical4 and numerical6 models of the wave-vortex-induced responses of the cylinder were developed. These models

showed that the dynamic behaviour of the cylinder depended on various

dimensionless groups, particularly the ratio of wave frequency to natural

frequency fw/fNw and the force coefficients OD, and CL0 (J) The

experimental data and numerical predictions indicated that maximum response occurred when fw/fN was an integer submultiple, in agreement with Sawaragj

et al.7' and Hayashi9. At resonance, the in-line and transverse forces

and responses increased due to synchronised vortex shedding as the

fluid-structure motions formed closed path Lissajous figures for the range

of Keiilegan-Carpenter numbers considered. This was in accordance with data

presented by McConnell and Park'° and McConnell and Jiao1 who oscillated a

cylinder sinusoidally in otherwise still water.

The most significant finding was that the force coefficients from the flexibly mounted cylinder tests were always larger than those obtained

previously at Salford1243 with the same

cylinder hut with a

frequency ratio fw/fNw was the most important parameter influencing the

force coefficients. At higher Keulegan-Carpenter ni.bers the force

coefficients from the flexibly mounted cylinder became closer to those from the fixed cylinder and showed dependence on the orbital shape parameter.

From the foregoing it is obvious that wave flows past bluff bodies result in very complex flow conditions even in controlled laboratcry exoerirnents. Only a few studies such as those by Bearman et al.14, Starisby et

al.12,

Bullock13 and Borthwick et al.,3 ' '

have measured the particie kinematics in determining force coefficients, etc. Thus, although progress is being made in understanding the fluid loading and response mechanics, many questions remain unanswered. For example, how should the effects of

three-dimensionality in waves be quantified? To what extent does

vortex-structure interaction influence in-line loading? Eow should Morison's equation be applied in random wave conditions? Do interference

effects from random components disrupt the vortex-wave--structure

interactions at resonahce with consequent reductions in amDlitudes of

forces and responses?

The series of tests proposed herein is designed to investigate several of the above questions; in particular, the effect of widening the sectr of wave components about a given mean resonant frequency on the fluid loading

and responses of a flexibly mounted cylinder. Such information would be invaluable to offshore engineers who are restricted to data, from regular

wave experiments, which may result in over-predictions of loading and

response when extrapolated to the psuedo-random conditions prevaiing in

the North Sea.

OBJECTIVES

This work will he directed towards examining the influence of random wave

components on fluid structure interactions at resonance. Particular emphasis will be placed on identifying the width of the spectrum at which

resonant effects are substantially reduced due to interference from broad band components for a frequency ratio fwp/fN where fwp is the dominant

frequency in the wave spectrum;

PROGI?AM?IE

(1) Wave generator control software deve]opiiient 01/09/88 - 31/04/89

As part of previous projects, computer programs have been

developed which control the wave generator so that regular waves

or random waves corresponding to a typical sea spectra may be

created. Further programming will he undertaken to produce the interference spectrum required for stages (ii) and (iv) of the

programme. This stage will occur partly in advance of the grant

st3rt date, the remainder in p:irallel with stage (ii). The Scientific Officer who is to he responsible for this softwnrc wi 1.1 be supplied by the University of Salford.

Measw-eent of particle kinematics 01/12/88 - 31/08/89 This stage is planned to coincide with the appointment of a

Research Assistant funded by SERC. Horizontal and vertical wave particle velocities will be measured using the replacement DANTEC

laser anemometer. This will reduce the estimated measurement time by more than a factor of twc siime the present laser is _extremely difficult to set up and operate. _{Particle accelerations will he} obtained by appropriate numerical differentiation of the particle

velocities.

Structural characteristics _{01/09/89 - 31/10/89}

Before conducting the main test programme, _{the effective mass,}

stiffness, damping ratio and natural frequency of the test

cylinder will be estimated via modal analysis, force-displacement and free vibration tests (similar to those described by Bortliwick

et al.3 for a spring mounted cylinder). _{In thi.s case, the}

cylinder base mounting will consist of a thin cantilever to ensure greater uniformity in directional responses.

Spectral bandwith tests _{01/11/89 - 01/07/90}

A series of tests would be undertaken in which the flexibly mounted cylinder is subjected to pseudo-random waves corresponding

to spectra of various widths centred at a frequency ratio of

. At one extreme, nearly regular waves are tc he

generated with constant period but variable wave height. At the

other, _{a Pierson-Moskowitz type spectrum will be used for wave} generation. _{Response displacements and sectional forces will be}

measured from the instrumented cylinder in conjunction with wave elevation data. This data will then he processed, using the

appropriate particle kinematics obtained in stage (iii) and

structural characteristics from (iv), in order to identify the

effect. of spectral bandwidth on wave-vortex-induced resonance.

Report _{01/07/90 - 31/11/90}

RESO!JRCES REQUESTED

(1) Staff

1 No Research Assistant lB for 24 months commencing 1/12/88 at Grade 5.

(ii) Travel and Subsistence

A total of £1600 is requested to cover _{10 meetings between staff at}

Plymouth/Salford as part of the collaboration and 8 meetings in London with other investigators or the steering committee.

/

(1

Swrwary of .4n ticipa ted Expenditure

1988 1989 1990 Total

Cl- _{1160 13986}

13462 28608

Ecu pment 21727 ₂₁₇₂₇

Travel and Subsistence 600 1000 1600

Total 1160 36313 14462 ₅₁₉₃₅

CASE FOR EQUIPMENT RESOURCES

Most oi the resources requi red for the above prograimne are already

avai1b1e. The wave flume is well established and runs under the control of a PD? minicomputer. The constituent parts of the test cylinder, force elements and displacement transducers are ready for installation. _{There is} a need, however, for three new force elements to he manufactured in order to replace the old elements as they wear out. In the previous project

involving the small diameter cylinder two elements perished.

For more than a decade, measurement of wave-induced particle velocities has

been undertaken at Salford using a DISA LDAO]. _{unidirectional laser}

anemometer. This model has been obsolete for several years and is

extremely time consuming to set up, calibrate and operate. _{In order to} bring the system up-to-date, al least to the level of equipment currently utilised by other institutions engaged _{in wave-related research, it is}

necessary to purchase a DANTEC LDAO4 model laser. The University of

Salford Research Committee has agreed to pay for this, conditional _{on the}

success of this grant application. It is also proposed that the

unidirectiunal laser be extended to a two-directional system which would

halve the number of tests required to _{obtuin horizontal and vertical}

(iii)

Eguipnient

Description Category Cost VAT Total

1 No IBM PS/2/80

microcomputer

Equipment 4706.70 691.31 5398.01

1 No Perspex Panel Equipment 456.62 68.49 525.11

3 No Force elements Equipment 1410.00 212.00 1522.00

Additional Parts for the DANTEC Laser

Equipment 12332.00 1849.80 14181.80

Anemometer

velocity components. _{Funding for this additional equipment is requested as}

part. of this aDplication to SEC. _{In addftion, an extra perspex pane] is} requested so that the light beams would pass through both walls of the wave

flume thus removing the preseni: requirement for _{mirrors to he placed within}

the flume. This would al.so _{eliminate a potential safety hazard from}

reflected light if the mirror is disturbed.

Due to the nature of the PD? control system it is not possible to carry out

innuediate processing of the data without disrupting the collection

pragrrmrte _{At present, data is transferred overnight to a Prime mainframe}

computer where it is processed the following day. _{Currently the only} method for displaying information is via _{an oscilloscope which monitors} voltage from the amplifiers. _{It is proposed therefore to purchase an IBM}

PS2 model 80 microcomputer which would be _{connected by a high speed} parallel link to the PDP minicomputer and _{be capable of inmiediate data}

analysis with graphic display. _{It should he noted that the prices}

quoted on the for _{include a hardware reduction of 409 below}

the list price.

COMMENTS

The research work decrihed herein is both fundamental and _highly

applicable to Industry. _{Previous investigations on related topics have} been well supported by Salford University and the Science and Engineering

Research Council. _{It should also be noted that Salford has been selected}

by the U _{coordinating body to host an EEC funded short course on "Wave and} Ice Forces on Offshore Structures" in April _{1989 as part of the COMETT}

progranme.

Dr. _{A.G.L. Borthwick has more than 9}

years post-graduate experience in engineering and research including four (4) _{years University research in} fluid loading, one and a half (la) years _{in the offshore oil and gas}

industry and four (4) years as a University Lecturer. _{He has obtained}

three SERC grants as part of the 1983/85 and 1985/87 fluid loading prograiTnes.

Dr. G.N. Bullock joined the Department of Civil _{Engineering at Plymouth} Polytecinic as Reader in 1983. _{Formerly: 1971-83 Lecturer, then Senior} Lecturer at Salford University and 1967-71 Engineer, _{Hydraulics Group,} George Wimpey & Co Ltd. _{Wide experience of hydraulics research}

involving

various wave related phenomena and large scale model testing.

_Between

l978-3 obtained three SERC grants for work on wave loading of small diameter cylinders. Currently working on wave loading of full-scale

breakwaters.

REFERENCES

Bullock, G.N.,

and

Bullock, G.N., Stanshy, P.R.,

_and

of Cylinders in Waves", Coastal Engineering, 1978,

pp2415-2432.

Borthwick, A., Bullock, ('.N., _{and Herbert, D.t'1.,}

_{"Wave Loading on a}

Flexibly Mounted Small Diameter

Vertical

Civil

Salford, December 1086.

V

by the Journal of Fluids and Structures, 1988.

Borthwick, A.G.L., Bullock, G.N., and Herbert, D.M., _{"Harmonic Force} Coefficient from a Flexibly Mounted Cylinder in Waves", _tentatively accepted by \SME, 8th Tnternational Conference on Offshore Mechanics and Arctic Engineering, The Hague, The Netherlands, March 1989.

Borthwick, A.G.L., and Herbert, D.M., "Resonant and _Non-Resonant

Behaviour of a Flexibly Mounted Cylinder in Waves", _{Internal Report,} Department of Civil Engineering, University of Salford, May 1988.

Sawaragi, T., Naknmura, T., and Kita, H., _{"Characteristics of Lift} Forces on a Circular Pile in Waves", Coastal Engineering in Japan,

Vol.19, 1976, pp59-71.

Sawaragi, T., Nakamura, T., and Miki, H., _{"Dynamic Behaviour of a}

Circular Pile due to Eddy Shedding in Waves", Coastal Engineering _in

Japan,

Vol.20, 1977,

Hayashi, K., "The Non-Linear Vortex-Excited Vibration of a Vertical Cylinder in Waves", PhD Thesis, University of Liverpool, December 1984.

McConnell, K.G., and Park, Y-S., "The Response and the Lift-Force

Analysis of an Elastically-Mounted Cylinder Oscillating in Stil.1

Water", Proceedings of the Third International Conference on the

Behaviour of Offshore Structures, Vol.2., August 1982,

_pp671-680.

McConnell, K.G., and Jiao, Q., "The In-line Forces Acting on a Elastically Mounted Cylinder Oscillating in Still Water", 1985 SEM Conference on Experimental Mechanics, Las Vegas, USA, 1985.

Stanshy, P.R., Bullock, G.N., and Short, I., _{"Quasi-2-D-Forces on a}

Vertical Cylinder in Waves", Journal of Waterway, _{Port, Coastal and} Ocean Engineering, ASCE, Vol.109, No.1, February

_1983,

Bullock, G.N., "Wave Loading on a Generally Orientated Small Diameter Cylinder", Internal Report No 83/190, Department of Civil Engineering,

University of Salford, September 1983.

Bearman, P.W., Chaplin, 3.R., Graham, J.M.R., Kostense, J.H., Hall,

P.F., and Klopman, G., "The Loading on a Cylinder in Post-Critical Flow Beneath Periodic and Random Waves", Proceedings of 4th International Conference on Behaviour of Offshore Structures, Delft, _{1-5 July 1985,}