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A Numerical Simulation for Predicting Sea WavesCharacteristics and Downtime for Marine and Offshore

Structures Installation Operations

Kabir Sadeghi1Girne American University, Department of Electrical and Electronic Engineering,

TRNC

AbstractIn this paper, a numerical simulation of sea wave characteristics and operationdowntimes of offshore structures is presented. The simulation was based on availablewind data and seawater temperature recorded by an oceanography buoy installed inthe Caspian Sea. Wave characteristics were simulated for deepwater parts of theCaspian Sea by applying the Bretschneider spectrum and equations using following

recorded data: wind velocity, wind duration, fetch length, and water/air temperaturedifferences. Since recorded wave data were only available for a one-year period, theywere solely used for validation of the simulation results with recorded data but fornot the simulation itself. Some practically established thresholds for wave velocity,wave period, and wind velocity were considered as constrains, limiting the operationof offshore installations. The numerical simulation model revealed that it is possibleto operate offshore installations for 250 days per year in the southern parts of theCaspian Sea. A worst-case scenario showed that the maximum waiting time forrestarting the offshore installations is 17 days. Considering the swell parameter, itwas concluded that the annual downtime period of offshore installation operations insouthern parts of the Caspian Sea is about one third of a year and the maximumwaiting time for this operation is about two third of a month.

Keywords: Simulation, Bretschneider, Wave Height/Period, Caspian Sea, Marine

IntroductionIn order to design, calculate, construct, and install coastal, port, marine, andoffshore structures, environmental data are needed, in particular wave and winddata. To determine wave data, simulation models, measuring devices, and remotesensing by satellites are used. Obtaining valuable and long-term environmentaldata by measuring devices or satellites is time consuming and very expensive. Inaddition, these types of data are not available for all regions.

1 ksadeghi@gau.edu.tr

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Because of the above-mentioned drawbacks of the direct measuring methods,simulation models are widely used in establishing environmental data. They areeasy to use and are applicable for every geographical area, provided validequations and spectrums are chosen, calibrated, and well adapted to the specificsituation. To predict the wave characteristics in lakes, gulfs, seas, and oceans, the

equations and spectrums of Sverdrup-Munk-Bretschneider (S.M.B), Bretschneider,Pierson-Moskowitz, and JONSWAP (Joint North Sea Wave Project) are mainlyused. These models can be adapted to every point in any water depth in the seawaters (Sadeghi 2001).

To evaluate the validity of the Bretschneider spectrum, a case study was carried outby the author for the Caspian Sea. The Bretschneider spectrum was selected for thisstudy because its equations and spectrums consider the very important parameter ofwater/air temperature differences.

The Caspian Sea was chosen as study area, sincesufficientwave characteristicsdata are not available for some parts of the Caspian Sea allowing the evaluation ofsensitive operations such as installations of offshore petroleum platforms and thelike. Simulation of wave characteristics was carried out based on available winddata recorded by Khazar Oceanography Buoy (KEPCO 2001). This buoy is locatedin the south-eastern part of Caspian Sea, 30 km from Neka Harbour at a waterdepth of 35 m and operated by KEPCO (Iranian Company for Exploration of Oil inCaspian Sea).

Wave characteristics were simulated for deep water parts of the Caspian Sea basedon recorded wind data using the Bretschneider spectrum (Bhattacharyya 1972,Cold Bay Study [CBS] 1999) and various modeling equations (U.S. Army CoastalEngineering Research Center 1980, Sadeghi 1989, 2001). Wind duration, windvelocity, fetch length, and water/air temperature differences were considered in thesimulation. Some constrains for wave velocity, wave period and wind velocitywere used as limitation criteria for offshore installation operations. It is importantto mention that only one-year wave data were available for the study area. Thus,these data have only local validity and are not valid for other points of the CaspianSea, particularly for deep-water areas. Therefore, the recorded wave data were onlyused for comparison purpose and were not applied in the simulation.

Information and data usedGeneral information on environmental conditions (wave, wind, current, etc.)obtained from literature were used for general consideration and overallengineering judgments (U.S. Army Coastal Engineering Research Center 1980,Sadeghi 1989, 2001). The data used in the wave characteristic simulations weretaken from KEPCO (2001).

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Formulas used in this studyThe Bretschneider equations and spectrum were applied to predict forecasting thewave characteristics (U.S. Army Coastal Engineering Research Center 1980, Sadeghi1989, 2001, CBS 1999). The Bretschneider equations take the effects of windblowing duration, wind velocity, air-sea temperature difference, and fetch length

into account.

Summary of assumptions and analysis approachThe analysis is based on one-year environmental data recorded in the Caspian Sea,and Bretschneider equations and Rayliegh distributions for prediction of wavecharacteristics (Bhattacharyya 1972, U.S. Army Coastal Engineering Research Center1980, Sadeghi 1989, 2001, 2004, DNV Classification Notes, 2000).

Following assumptions and analysis steps were applied:

a. Since there is lack of wind data for all points in south Caspian Sea, thewind data recorded in the location of the above-mentioned buoy wereused for all points of the south Caspian Sea considering different fetch

lengths. Comparing the wind data for different parts of the southCaspian Sea presented in literature (Kosarev & Yablonskaya 1994)showed that this assumption could be considered as a valid.

b. A summary of used formulas based on the Bretschneider spectrum ispresented.

c. Data on seawater temperature, wind velocity, direction of wind, windduration, and air temperature were taken from KEPCO EngineeringDepartment measured by the KEPCO buoy (KEPCO 2001) and wereused for the numerical simulation of wave characteristics of the southCaspian Sea. It is to be underlined that the available one-year datacovers only a certain period (1988/1989).

d. In the available data (KEPCO 2001), the period of each set of recordis three hours. It is important to note that this 3-hour period is notnecessarily the wind blowing duration. Therefore following criteriawere used for evaluating the wind duration:

If the directions of wind for two consecutive sets of recorded datawere different, the wind duration is considered as 1.53 hours(because of difference between measuring period and real windduration by considering minimum probable duration of 4.5 hours).

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If the differences among wind directions between every twoconsecutive periods of recorded data were less than 7 degrees, theconsecutive accumulated wind measuring duration wereconsidered as wind blowing duration (t). In this case the averagewind velocity was also used.

e. With regard to wind velocity and blowing duration, the requiredrelated minimum fetch was calculated from the Bretschneiderequation which should be less than maximum effective fetch existingin the south Caspian Sea (i.e. 450 km).

f. Based on the Bretschneider spectrum, the ratio of maximum waveheight over significant wave height is normally bigger than two. Inthis study the Rayliegh ratio for Hmax/Hs was used for the benefit ofsimplicity (i.e. Hmax/Hs = 1.85) (DNV Classification Notes 2000,Sadeghi 2001).

g. Significant wave height, significant wave period, and peak periodwere calculated based on the Bretschneider equation considering theair-sea water temperature difference.

h. Constraints for the limitation of installation operation were defined asfollows:

Maximum wind velocity equal to 20 Knots Maximum wave height equal to 2 meters Maximum wave period equal to 8 seconds Installation operation duration equal to 5 days (This duration is

considered for mooring and installation operation of the Iran-Alborz semi-submersible drilling platform in water depth of 970m in the Caspian Sea).

Wave characteristics simulation results

Comparison of simulated and recorded wave characteristicsSimulated wave height and period for the south Caspian Sea were compared withrecorded wave height and period for a distinct point in this sea (30 km from NekaHarbour, at 35 m water depth). It is important to note that the results of simulatedvalues were different from that of existing recorded values at that certain point. Thereason is that the reference point in this study was located at the south-east corner

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0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

1 41 81 121 161 201 241 281 321 361 401 441 481

Time (Number of Registration 3 hours period)

Maximum

WaveHeight(m)

Hmax (Simulated)

Non Operational Days = 17

Non Operational Days = 2

Non Operational Days = 7

Limit Line as per Criteria

of Caspian Sea and thus, the fetch will be different from other points at southernbasin of Caspian Sea.

Figures 1, 2, 3, and 4 present the maximum wave height (Hmax in meters) versustime (sets of 3 hours registered data). In the horizontal axis of these figures, 40means 40 3 hours that equals to five days and 720 means 720 3 hoursequaling three months.

Figures 5, 6, 7 and 8 show the recorded values of Hmax. As it can be seen from thesefigures the simulated values are generally well adopted with the recorded valuesbut a little bigger than them due to longer fetches.

Figure 1. Simulated maximum wave height for months 11, 12, year 1988 and month1, year 1989.

As shown in figures 7 and 8 for an about two-month periods, the wave heightswere not recorded, but for the time period, the wave heights were simulated and arepresented in figures 3 and 4.

Figures 9, 10, 11 and 12 show the simulated peak period (Tm) versus time (sets of 3hours registered data).

On the above-mentioned figures, the limitation criteria are shown by full blocklines and also the non-operational days are presented. It is to be mentioned that therecorded and simulated wave periods were moderately different. This is mainly dueto the swell effect that has not been considered in Bretschneider formula and as aresult of the numerical simulation.

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0.0

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7.0

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1 41 81 121 161 201 241 281 321 361 401 441 481 521 561 601 641 681

Time (Number of Registration 3 hour s period)

MaximumWaveHeight(m

)

Hmax (Simulated)

N. Op. D. = 1N. Op. D. = 1

N. Op. D. = 1

Non Operational Days = 7

N. Op. D. = 1 N. Op. D. = 1

N. Op. D. = 1

Limit Line as per Criteria

0.0

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1 41 81 121 161 201 241 281 321 361 401 441 481 521 561 601 641 681 721

Time (Number of Registration 3 hours period)

Maximum

WaveHeight(m)

Hmax (Simulated)

N. Op. D. = 1 N. Op. D. = 3

Non Operational Days = 12N. Op. D. = 9

N. Op. D. = 1

Limit Line (Criteria)

Figure 2. Simulated maximum wave height for months 2, 3 and 4, year 1989.

Figure 3. Simulated maximum wave height for months 5, 6 and 7, year 1989.

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0.0

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1 41 81 121 161 201 241 281 321 361 401 441 481 521 561 601 641 681 721 761

Time (Number of Registration 3 h ours period)

Maximum

WaveHeight(m

)

Hmax (Simulated)

N. Op. D. = 1

N. Op. D. = 1

Non Operational Days = 12

N. Op. D. = 1 N. Op. D. = 1

N. Op. D. = 2

N. Op. D. = 7

Limit Line as per Criteria

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

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1 41 81 121 161 201 241 281 321 361 401 441 481

Time (Number of Registration 3 h ours period)

MaximumWaveHeight(m)

Hmax (Recorded)

Non Operational Days = 13

N. Op. D. = 2 N. Op. D. = 8

Limit Line as per Criteria

Figure 4. Simulated maximum wave height for months 8, 9, 10 and 11, year 1989.

Figure 5. Recorded maximum wave height for months 11, 12, year 1988 and month 1,year 1989

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0.0

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1 41 81 121 161 201 241 281 321 361 401 441 481 521 561 601 641 681

Time (Number of Registration 3 h ours period)

Maximum

WaveHeight(m

)

Hmax (Recorded)

N. Op. D. = 1

N. Op. D. = 4N. Op. D. = 1

Non Operational Days = 6

N. Op. D. = 1 N. Op. D. = 1

N. Op. D. = 1

Limit Line as per Criteria

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

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1 41 81 121 161 201 241 281 321 361 401 441 481 521 561 601 641 681 721

Time (Number of Registration 3 hours period)

Maximu

m

WaveHeight(m)

Hmax (Recorded)

N. Op. D. = 1 Non Operational Days = 3

Not Recorded

Limit Line as p er Criteria

Figure 6. Recorded maximum wave height for months 2, 3 and 4, year 1989.

Figure 7. Recorded maximum wave height for months 5, 6 and 7, year 1989.

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1 41 81 121 161 201 241 281 321 361 401 441 481 521 561 601 641 681 721 761

Time (Number of Registration 3 h ours period)

Maximum

WaveHeight(m)

Hmax (Recorded)

N. Op. D. = 1

Non Operational Days = 11

N. Op. D. = 5

N. Op. D. = 4 N. Op. D. = 1

N. Op. D. = 3

Limit Li ne (Criteria)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

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Time (Number of Registration 3 hours period )

PeakPeriod(s)

Tm (Simulated)

Limit Line as per Criteria

Figure 8. Recorded maximum wave height for months 8, 9, 10 and 11, year 1989.

Figure 9. Simulated peak period for months 11, 12, year 1988 and month 1, year 1989.

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Tm (Simulated)

Limit Line as per Criteria

0

1

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1 41 81 121 161 201 241 281 321 361 401 441 481 521 561 601 641 681 721

Time (Number of Registration 3 ho urs period)

P

eakPeriod(s)

Tm (Simulated)

Limit Line as per Criteria

Figure 10. Simulated peak period for months 2, 3 and 4, year 1989.

Figure 11. Simulated peak period for months 5, 6 and 7, year 1989.

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1 41 81 121 161 201 241 281 321 361 401 441 481 521 561 601 641 681 721 761

Time (Number of Registration 3 hours period)

PeakPeriod(s)

Tm (Simulated)

Limit Line as per Criteria

Figure 12. Simulated peak period for months 8, 9, 10 and 11, year 1989.

ConclusionDue to the limitation of the existing recorded data (only one year available data fora certain point near the shoreline), this simulation can be considered only as aguide for evaluation of situations and can be used only for preliminary estimations.

Considering the operational constraints (Hmax =2 m, Tm =8 s and gust velocity =20 Knots) used for numerical simulation, an installation operation of 250 days peryear is possible. The maximum waiting time for restarting the installation operationis estimated to last 17 days.

As only the seas are simulated and due to lack of data and information for swell, itcan be in general concluded that the installation operation is possible for 2/3 of theyear and the maximum waiting time for this operation is about 2/3 of months.

References

Bhattacharyya R, 1972. Dynamics of marine vehicles.

Cold Bay Study [CBS], 1999. Section IV (Climatology).

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DNV Classification Notes, 2000. Environmental conditions and environmentalloads, No. 30.5, March 2000.

KEPCO Engineering Department, 2001. "Work Report on data obtained fromKhazar oceanography buoy and related CD electronic file", winter of 2001.

Kosarev AN, Yablonskaya EA, 1994. The Caspian Sea, translated from Russia byWinstin AK, published by SPB Academic Publishing, The Hague.

Sadeghi K, 1989. Design and analysis of marine structures. Published by KhajehNasirroddin Toosi University of Technology, Tehran, Iran, 1989, 456 pp.

Sadeghi K, 2001. Coasts, ports and offshore structures engineering. Published byPower and Water University of Technology, Tehran, Iran, 501 pp [ISBN: 964-93442-0-9]

Sadeghi K, 2004. An analytical approach to predict downtime in Caspian Sea forinstallation operations. 6th International Conference on Ports, Coasts and MarineStructures (ICOPMAS 2004), Tehran, Iran, Dec. 2004.

U.S. Army Coastal Engineering Research Center, 1980. Shore protection manual,Vol. 1 & 2, 4th edition.