Hutton Field development Water depth measurement and tide prediction model Contract 13860 TS 55 Second progress report, June 1983

INTERNAL DOCUMENT

Hutton Field Development

Water depth measurement and tide predictior model. Contract 13860-TS-55

Second Progress Report, June 1983. G.A. Alcock

[This document should not be cited in a published bibliography, and is supplied for the use of the recipient only].

INSTITUTE OF OCEAN OQRAPHIC SCIENCES

\

INSTITUTE OF OCEANOGRAPHIC SCIENCES

Wormley, Godalming, Surrey GU8 SUB

(042-879-4141)

(Director: Dr. A. 8. Laughton)

Bidston Observatory, Birkenhead,

Merseyside L43 7RA (051-653-8633)

(Assistant Director: Dr. D. E. Cartwright)

Crossway, Taunton,

Somerset TA1 2DW (0823-86211)

/ . INTERNAL DOCUMENT I85

INSTITUTE €F OCEANOGRAPHIC SCIENCES

NATURAL ENVIRONMENT RESEARCH COUNCIL

Hutton Field Development

Water depth measurement and tide prediction

model. Contract I386O-TS-55

Second Progress Report, June 1983" G.A. Alcock

1. INTRODUCTION

2. DEPLOYMENT AND RECOVERY

3. DATA PROCESSING

4. DATA ANALYSIS AND RESULTS

5. CONCLUSIONS

1 . Introduction

Conoco is developing the Mutton Field located in the northern North Sea and

utilising a single Tension Leg Platform (TLP) for drilling, production and

personnel accommodation. Water level variations due to tide and storm surge have significant effects on the mean tension in the tension legs, therefore it is desirable for the operating personnel to have available reliable predictions of the tidal levels and variations due to storms.

The Institute of Oceanographic Sciences (lOS) is contracted to collect,

analyse and interpret water level data, and to develop a tide prediction model and use it to calculate predicted tide levels for the Mutton Field for a twenty year period. Data acquisition, using two pressure gauges, is planned to take place over one year, with periodic recovery and redeployment of one gauge to examine the data quality. This report concerns the processing of data collected over the period l4th January to 15th May I983, and the analysis of data from 31st October I982 to

15th May 1983.

2. Deployment and Recovery

Aanderaa WLR-5 pressure gauge number 445 was originally deployed on 31st

October I983 and recovered on l4th January 1983s to yield six week's of good quality data (see Ref. 1). It was then redeployed from the "Dundee Kingsnorth"

(DKN) at 2229 GMT, l4th January, after a new battery and tape had been fitted. D. Piatt of IOS travelled to the "Odin" on 11th - 12th May but operational

difficulties meant that the recovery was not attempted until l6th May. The gauge was on board at 0105 GMT l6th May I983, checked, and found to be in good working order. A new battery and tape were fitted and the gauge redeployed at 0200 GMT.

3. Data Processing

The magnetic tape Arom tl^ pressure gauge was copied onto a 9 track magnetic

tape and the channel counts listed using the CAMAC work station at Bidston. There were no gaps in the record and only one translation error which was corrected interpolation.

Pressure frequencies were calculated from the channel counts and the bottom

pressure calculated from the pressure/frequency calibration and stored on disk.

The 2 hourly values of bottom pressure are plotted in Figure 1 and show a very

2

-An interpolation programme was used to produce an output of hourly values,

on the hour (GMT), of the bottom pressure record. This programme smoothed the

data using a low-pass filter, FLPO7, of half length 12 and cut-off frequency

(half-. - 1

power point) of 0.375 c h ; this reduces the amplitude response at the Mg tidal

frequency by 1% but had negligible effect at other tidal frequencies. The resulting

series was then interpolated, using a cubic spline, to obtain the hourly values,

on the hour (GMT), applying a time correction as the recorder clock had gained

2 seconds over the 121 day period. (Exact times of so&ns at the beginning and end

of the record were noted prior to deployment and after recovery).

The resulting bottom pressure record obtained was for the period 0500 GMT

15th January to I7OO GMT 15th May I983. As the previously processed record had

ended at 1100 GMT l4th January (see Ref. 1), there was a gap of 17 hours in the

record and this was interpolated graphically using predicted values as a guide.

The complete bottom pressure record obtained was for the period I6OO GMT 31st

October I982 to I7OO GMT 15th May I983.

Each hourly value of the bottom pressure obtained was the total pressure

measured by the recorder, i.e. the sum of the pressures due to the water column and

air column above the sensor. The latter was subtracted using hourly values of

atmospheric pressure, extracted from barometer chart records, supplied by Conoco's

Norops Division. Records for the period 0800 GMT 11th November I982 to 2300 GMT

6th February were from the DKN, those from 0000 GMT 7th February to I7OO GMT 15th

May were from the Murchison platform.

No calibration information was available for the DKN records and so data was

extracted assuming that the pressure and time scales of the barometer chart

recorder were correct. A correction was made to reduce the atmospheric pressures

to mean sea level, assuming a barometric height of 20m. The Murchison barometer

was calibrated by Marex on l8th May I983 and was evidently under-recording the

atmospheric pressure by ^mb - iWiis vMis allowed for whan processing the data. The

calibration also indicated a timing error of 4l minutes, the actual "pen lift-off"

time being given as 1329 (no time zone given) whilst the chart time was l4lO.

Periodic time checks over the recording period indicated a progressive error with

the chart recorder clock gaining time, and this was allowed for extracting

data from the charts. Time annotations on the charts did not have a reference to

the time zone being used, for example the annotation "local time" on 3rd April

-3-Time, British Summer -3-Time, Central European Time etc. In the absence of further information, EST was assumed. A correction was made to reduce the atmospheric pressures to mean sea level, assuming a barometric height of 58.3m. Gaps in the record of 6 hours on 20th February, 35 hours during 1st to 3rd April and 4l hours during 5th to 6th May were interpolated using data from the Daily Weather Report issued by the U.K. Meteorological Office. These gaps were due to power failures on the Murchison platform.

The computed water pressures were converted to elevations using the hydro--3

static equation. A sea water density value of 1027.5 Kg m was used, as

determined by lOS following measurements of temperature and salinity during the recovery/redeployment in May. The resulting sea level elevation record obtained was from 1200 GMT l4th January to 1700 GMT 15th May; when added to the previously processed data, the complete record available was from 0800 GMT 8th November I9B2

to 1700 GMT 15th May.

4. Data Analysis and Results

A tidal analysis of a I83 day period of the hourly sea level data was carried out using the IOS TIRA programme which utilises the harmonic method of analysis and which performs a least-squares fit to the data. The method models the tidal

level, ^^4^ , as a finite number, N, of harmonic constituents with an Amplitude H and angular speed CT ,

A/

r I) . r i

A/ ,

^ Cos6?;,6 4-1/^ J.

At I

is the mean level referred to the sensor level, V is the initial phase at an arbitrary time origin t = 0 and G is the constituent's phase lag with respect to the equilibrium tide. f and u are slow modulating functions mostly with the period l8.6y of the lunar node. The amplitude (H in cm) and phase lag (G in°) relative to Greenwich epoch of 54 major and 2 related constituents were computed,

the time zone being G.M.T. and these are given in Table 1. The constituents T T )

and T are not separable with six months of data, and so they were related to the major constituents and S respectively using values derived from the harmonic

analysis of nine years of data from the nearby permanent coastal station at

Lerwick.

Table 1 also gives the mean value (Z^ in cm) of the sea level elevation

above the sensor level for the six months' period. This value should only be

site i) because of the difficulties of obtaining absolute measurements of water

levels with a bottom pressure gauge, and ii) because of the annual, seasonal and

monthly variations of msl:

i) The difficulties are the accurate determination of a) the sensor

level with respect to the seabed or known datum, b ) the

atmospheric pressure, so that bottom pressure can be converted to

water column pressure, and c) water density, so that the pressure

can be converted into a water level.

ii) Annual variations in msl are mainly dependent on a) time variations

of wind stress and air pressure and b) time variations in

oceanographic forces due to changes in temperature, salinity or

currents. Seasonal changes of British msl are mainly due to

density changes of the adjacent North Atlantic; monthly changes can

be related to the seasonal changes and to changes in local air

pressure and to the influence of winds over the continental shelf.

Ref. 2 contains a detailed analysis and explanation of variations

in monthly British msl data.

The monthly msl values at Lerwick for the period October I982 to May I983 are

plotted in Figure 2, together with the monthly mean values of the Hutton data

obtained by separate analyses of 29 day's data from each calendar month. Note

that the msl curves are offset for clarity and do not imply that Hutton msl is

"below" Lerwick msl. Msl variations at Hutton closely follow those at Lerwick

and we therefore have some confidence in assuming that long term variations

in msl will be similar at the two locations.

At Lerwick over the period 1957-1980, annual msl values had a standard

deviation of 30 mm about the 24 year mean of 993nim and monthly msl values

had a s.d. of 185mm. The February I983 value of 8l3mm at Lerwick was the second

lowest monthly value over the 24 year period (minimum = 771mm during April 1974);

the January 1983 value of 1187mm has been exceeded on six occasions (maximum =

1227mm during October 1967); the January - February difference of 374mm is the

largest change between consecutive monthly values (next largest = 259mm,

January - February I962).

-5-pne in terms of msl variations and illustrates the problems of using short

period data to estimate long period means. The mean of Lerwick msl November

-April was 1029mm, 36mm higher than the 24 year mean of 993mm. Application of

the same difference to the Hutton 6 month value yields an estimate of l44644mm

as the long term mean. Note that mean sea level values at Hutton are given

relative to the level of sensor 44$, which was designed to be 864.0mm above

the bottom deck level of the sensor table.

The values of Z , and S have been used to compute the tidal parameters

of Mean High Water Springs, Mean High Water Neaps, Mean Low Water Neaps and Mean

Low Water Springs (MHW5, MHWN, MLWN and MLWS respectively), and these are given

in Table 2. Values of Highest and Lowest Astronomical Tide (HAT and LAT) are

also given in Table 2 and have been estimated by inspection of predicted High and

Low Waters during I983 - I985 arid 2002 - 2004; the years when HAT and LAT are

most likely to occur (J.M.Vassie, personal communication). The predictions were

computed using equation (l). The values of HAT and LAT given in Table 2 are

only approximate as the true values depend on seasonal variations and other

constituents not derivable from six months of data.

5. Conclusions

Good quality bottom pressure data has been obtained from the Hutton Field

site for the period 31st October 1982 to May I983, and has been processed to

yield a hourly sea level record from 0800 GMT 8th November I982 to I7OO GMT 15th

May 1983.

Tidal statistics have been computed from an analysis of 6 months' data but

the results should be treated with caution because of the short span of data

available. In particular, mean sea level variations at Hutton closely followed

those at Lerwick and indicate large inter-month variability. Use of the 6 months'

msl value should therefore be treated with caution and the msl variations at

Lerwick over a 24 year period have been used to produce a best estimate of the

long term msl at Hutton of l44.644m.. Errors due to instrumental accuracy and

calibration are estimated to be l^mm, those due to determination of atmospheric

pressure and sea water density to be 4&mn.

It is recommended that personnel involved in operating the Murchison

barometer be instructed to annotate future charts with "pen lift off and on"

6

-6. References

1. Hutton Field Development, Water depth measurement and tide prediction model. First progress report, March I983. G.A. Alcock. I.O.S. Internal Document No. I80.

2. An analysis of British monthly mean sea level. K.R. Thompson. Geophys, J.R.astr.Soc., 63 pp. 57-73, I98O.

Acknowledgements

MAY 1 9 8 3 P R E S S U R E ( M B )

A P R I L 1 9 8 3 P R E S S U R E ( M B )

MARCH 1 9 8 3 PRESSURE ( M B I

F E B R U A R Y 1 9 8 3 P R E S S U R E ( M B )

J A N U A R Y 1 9 8 3 P R E S S U R E ( M B )

MUTTON F I E L D J A N ' 8 3 / MAY ' 8 3 A A N D E R A A WLR 4 4 5 / 2

I , k

-I—^ 4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

k-I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—4—I—I—I—I—I—I—I—I—I—^—I—I—I—I—I—I—I—I—,—,—,—,—,11,

A A

H 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1-IFfStw-l

I I—'—I—I—I—I—I—I—:—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—^

-I—I—I—I—t-l5'6o>'i

15^906 -I—I—I—I—I—I—I—I—I—I—I—:—I—I—I—I—I—I—I—I—I—I—I—I—I- _{-I—:—I—;—I—I—:—I—I—I—I—I—I—I—I—t—I—I—I—I—H}

: ' 1 ' ' '0 " '« W u n w r, w WI M :i « M M w :« w ,,

ORY OF THE MONTH

Figure 2 : Mean sea levels at Mutton and Lerwick

Mutton

(mm)

145 0 0 0 i

144 9 0 0

Lerwick

(mm)

1200

1100

144 8 0 0

1000

144 7 0 0 1 9 0 0

144 6 0 0

-K Lerwick msl

(mm)

—"Mutton msl

(mm)

800

144 5 0 0 1 7 0 0

OCT NOV DEC JAN FEB MAR APR MAY

R E L A T E D C O U S T I T U E N T S

NO

1

2

R E L C O N S T P J 1

T 2

R E F C O N S T

K 1

S 2

S P E E D

1 4 . 9 1 7 8 6 4 7 2 9 . 9 5 8 9 3 3 3

0 . 1 4 0 1 0 . 9 9 9 2

1 2 5 . 3 9 7 3 1 7 . 7 6 6

M A J O R C O N S T I T U E N T S

MO N A M E

N S I G :

S P E E D

5 5

1 Z O 0 . 1 4 4 6 8 . 0 4 3 2 0 . •

? S S A 0 . 0 8 2 1 3 7 3 1 2 . 3 6 3 9 1 7 5 . 9 5 5

3 MM 0 . 5 4 4 3 7 4 7 4 . 2 2 1 9 8 4 . 1 1 6

4 M S F 1 . 0 1 5 8 9 5 8 1 . 2 1 0 6 1 4 3 . 5 5 5

5 M F 1 . 0 9 8 0 3 3 1 4 . 0 2 0 1 2 0 2 . 1 6 9

6 2 Q 1 1 2 . 8 5 4 2 1 6 2 0 . 2 6 1 9 3 1 1 . 1 5 4

7 S I G 1 1 2 . 9 2 7 1 3 9 8 0 . 4 0 9 9 0 . 5 6 4

8 Q 1 1 3 . 3 9 8 6 6 0 9 1 . 8 8 6 1 3 4 1 . 8 4 8

9 R 0 1 1 3 . 4 7 1 5 1 4 5 0 . 8 2 9 3 0 . 5 2 7

1 ( 1 0 1 1 3 . 9 4 3 0 3 5 6 5 . 4 7 9 1 3 7 . 2 6 8

1 1 M P 1 1 4 . 0 2 5 1 7 2 9 0 . 2 5 1 8 1 5 3 . 7 4 1

1 2 M 1 1 4 . 4 9 2 0 5 2 1 0 . 6 1 1 7 2 2 . 2 6 3

1 3 CH I 1 1 4 . 5 6 ^ 5 4 7 6 0 . 1 0 4 3 2 3 7 . 9 2 2

1 4 P 1 1 4 . 9 5 8 9 3 1 4 2 . 1 9 9 1 1 5 2 . 0 5 7

1 5 K 1 1 5 . 0 4 1 0 6 8 6 5 . 9 1 4 8 1 6 7 . 2 4 7

1 6 P H I 1 1 5 . 1 2 3 2 0 5 9 0 . 2 7 5 5 2 9 8 . 8 5 1

1 7 T H I 1 5 . 5 1 2 5 8 9 7 0 . 1 0 4 7 1 5 7 . 6 3 3

1 0 J 1 1 5 . 5 8 5 4 4 3 3 0 . 4 1 1 9 1 9 1 . 8 8 5

1 9 S O I 1 6 . 0 5 6 0 6 4 4 0 . 0 4 7 2 7 0 . 9 3 0

2 0 0 0 1 1 6 . 1 3 9 1 0 1 7 0 . 2 8 2 0 3 3 6 . 3 3 9

2 1 0 0 2 2 7 . 3 4 1 6 9 6 4 0 . 2 4 1 7 1 9 9 . 1 7 4

2 2 M U S 2 2 7 . 4 2 3 8 3 3 7 0 . 3 2 1 3 2 2 0 . 5 8 1

2 3 2 N 2 2 7 . 8 9 5 3 5 4 8 1 . 3 7 9 0 2 4 5 . 6 1 3

2 4 M U 2 2 7 . 9 6 8 2 0 8 4 1 . 8 6 9 0 2 4 3 . 2 3 7

2 5 N 2 2 8 . 4 3 9 7 2 9 5 1 0 . 2 6 3 7 2 6 7 . 2 7 3

2 6 N U 2 2 8 . 5 1 2 5 8 3 1 2 . 2 1 2 5 2 6 8 . 3 6 2

2 7 O P 2 2 8 . 9 0 1 9 6 6 9 0 . 3 7 1 3 7 5 . 0 3 1

2 8 M 2 2 8 . 9 8 4 1 0 4 2 5 0 . 6 7 7 8 2 8 8 . 3 7 7

2 9 I 1 K S 2 2 9 . 0 6 6 2 4 1 5 0 . 2 6 2 8 2 1 4 . 0 3 3

3 U L A M 2 2 9 . 4 5 5 6 2 5 3 0 . 3 9 3 6 2 7 7 . 5 1 6

3 1 L 2 2 9 . 5 2 8 4 7 8 9 1 . 5 4 4 2 2 9 7 . 1 5 4

3 2 S 2 3 0 . 0 0 0 0 0 0 0 1 8 . 1 2 4 9 3 2 3 . 7 0 6

3 3 K 2 3 0 . 0 8 2 1 3 7 3 5 . 3 6 9 7 3 2 1 . 6 6 0

3 4 M S N 2 3 0 . 5 4 4 3 7 4 7 0 . 1 2 5 9 1 9 5 . 0 1 5

3 5 K J 2 3 0 . 6 2 6 5 1 2 0 0 . 2 0 3 2 1 5 8 . 9 5 0

3 6 2 S M 2 3 1 . 0 1 5 8 9 5 8 0 . 1 2 5 9 1 8 7 . 6 1 5

3 7 M 0 3 4 2 . 9 2 7 1 3 9 8 0 . 2 1 1 6 1 8 4 . 4 5 5

3 8 M 3 4 3 . 4 7 6 1 5 6 3 0 . 4 1 6 9 1 7 5 . 2 5 4

3 9 S 0 3 4 3 . 9 4 3 0 3 5 6 0 . 1 0 0 1 3 1 2 . 5 9 5

4 0 MK 3 4 4 . 0 2 5 1 7 2 9 0 . 1 5 3 7 3 3 5 . 9 1 1

42

M N 4

M4

S N 4 M S 4

MK4

S 4 S K 4

2HN6

M6

M S N 6 2 M S 6 2 M K 6 2 S M 6 M S K 6

5 7 . 5 7 . 5 8 . 5 8 . 5 9 .

6 0 .

60.

8 6 .

8 6 .

8 7 . 8 7 .

8 8 . 8 8 .

8 9 .

4 2 3 0 3 3 7 9 6 8 2 0 8 4 4 3 9 7 2 9 5 9 3 4 1 0 4 2 0 6 6 2 4 1 5 0000000 0 8 2 1 3 7 3 4 0 7 9 3 8 0 9 5 2 3 1 2 7 4 2 3 8 3 3 7 9 6 8 2 0 8 4 0 5 0 3 4 5 7 9 8 4 1 0 4 2 0 6 6 2 4 1 5

0 . 0 1 7 7 0 . 4 8 7 4 0 . 0 8 5 4

0.6261

0 . 2 6 5 5 0 . 1 9 3 7 0 . 0 6 8 9 0 . 2 0 5 2 0 . 4 3 0 3 0 . 1 2 8 4 0 . 4 8 5 9

0.1628

0.1001

261.

2 3 3 . 2 7 . 3 4 2 . 3 5 2 . 1 3 9 .

1 1 0 .

2 2 7 . 2 5 5 . 3 3 8 . 3 0 7 . 3 2 6 . 1 5 . 2 .

TABLE 2. TIDAL STATISTICS AT HUTTON FIELD

MHWS

MHWN

MLWN

MLWS

relative to sensor level

relative

to Z

o

(m)

144.680*

1.01

+0.69

+0.33

-0.33

-0.69

-1.11

Note; 1) All statistics are based on analysis of

183 day period, 09/11/82 to 10/05/83, of

water level data from sensor

445-*2) Best estimate of long term msl at Hutton

is l44.644m relative to sensor level (see