WAfrica Metocean Data Rev20

5/25/2018 WAfrica Metocean Data Rev20

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Chevron Energy Technology Company (ETC)San Ramon, CA

RESTRICTED TO COMPANY USE

Metocean and Hydrodynamic Criteria forShallow Fixed Structures and Pipelines Off W. Africa

Version 20

C. K. Cooper, M. J. Santala


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W. Africa Metocean Design Basis Rev 20.0

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Table of Contents

1 Introduction ...........................................................................................51.1 Scope and Limitations ........................................................................... 51.2 Conventions, Symbols & Abbreviations ................................................. 81.3 General West Africa Climate ................................................................. 91.4 Revision History ..................................................................................... 10

2 Nigeria Metocean Criteria .....................................................................112.1 Extremes ............................................................................................... 112.2 Wave Fatigue Criteria ............................................................................ 162.3 Operating Criteria .................................................................................. 18

3 Cabinda Metocean Criteria ...................................................................233.1 Extremes ............................................................................................... 233.2 Wave Fatigue Criteria ............................................................................ 293.3 Operating Criteria .................................................................................. 31

4 Region-Wide Ancillary Parameters ......................................................404.1 Marine Growth ....................................................................................... 404.2 Rainfall ................................................................................................... 40

5 Metocean Conditions for Other Locations ..........................................416 Wind Gust Factors and Spectra ...........................................................427 Wave Spectra .........................................................................................43

7.1 Wave Spectral Shape and Spreading ................................................... 437.2 The Gaussian Spectrum ........................................................................ 447.3 The JONSWAP Spectrum ..................................................................... 447.4 The Ochi-Hubble Spectrum ................................................................... 457.5 Directional Spreading ............................................................................ 45

8 Hydrodynamics .....................................................................................468.1 Extreme Force Calculations .................................................................. 468.2 Fatigue Calculations .............................................................................. 47

9 References .............................................................................................52


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List of Figures

Figure1.11MapofStudyArea.............................................................................................................5Figure1.12Nigeriaareaofapplicability(inblue)..................................................................................7Figure1.13Cabindaareaofapplicability(inblue).................................................................................7Figure2.11:WaveheightfactorandcrestheightversuswaterdepthforoffshoreNigeria...................14Figure2.31:SeasonalvariationinmeanwindspeedforoffshoreNigeria............................................19Figure2.32:MonthlymeanHsincludingwindandswellcomponents,Nigeria.....................................20Figure2.33:Seasonalvariationofmeanmonthlyairtemperatureandrainfall,offshoreNigeria.........22Figure3.31:HsfactorandcrestfactorsversuswaterdepthforoffshoreCabinda................................36Figure3.32:Seasonalvariationin50%,90%,95%and99%nonexceedencewindspeed,offshore

Cabinda......................................................................................................................................36Figure3.33:Seasonalvariationin50%,90%,95%and99%nonexceedencesignificantwaveheight

offshoreCabinda(nominalwaterdepthof155ft).......................................................................37Figure3.34:Seasonalmedian,0.1%and99.9%exceedenceairtemperatureandmedianseasurface

temperatureoffshoreCabinda....................................................................................................38Figure3.35:SeasonalvariationinmeanmonthlyrainfallnearthecoastofCabinda(PointeNoire,Rep.

Congo).......................................................................................................................................38Figure 3.3-6: Seawater temperature and salinity profil e criteria off shore Cabinda.

.......................................

39

Figure4.21:Rainfallintensityversusaveragingtimefor10 and100yrstormsoffshoreWestAfrica...40Figure8.21:Hydrodynamicforcecoefficientsfor1yrstorminwaterdepthsof250400ft,offshore

Cabinda......................................................................................................................................50Figure8.22:Hydrodynamicforcecoefficientsfor100yrstorminwaterdepthsof250400ft,offshore

Cabinda......................................................................................................................................50Figure8.23:Hydrodynamicforcecoefficientsfor100 and1yrstormsinwaterdepthsof50ft(MSL),

offshoreNigeria. .......................................................................................................................51Figure8.24:Hydrodynamicforcecoefficientsfor100 and1yrstormsinwaterdepthsof16ft(MLW),

offshoreNigeria.........................................................................................................................51


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List of Tables

Table 1.2-1 Symbols..........................................................................................................................8Table2.11:NyrSwellandAssociated ParametersforSitein50ft,Nigeria.........................................12Table2.12:NyrWindSeaandAssociated ParametersforaSitein50ft,Nigeria................................13Table2.13:SecondaryExtremeLoadingCaseWhichAssumesCurrentMaximumandanAssociated

Wavefor

Offshore

Nigeria

..........................................................................................................

14

Table2.14:FactorsforWave,Wind,andCurrentbyDirection,Nigeria................................................15Table2.15:NigeriaAirandWaterTemperatureExtremes,Nigeria......................................................15Table2.21:PercentoccurrenceofSignificantWaveHeightandPeakPeriodforShallowWaterNigeria

..................................................................................................................................................17Table2.31:JointFrequencyofOccurrenceofWindSpeedandDirection,OffshoreNigeria..................19Table2.32:SignificantWaveHeightPersistenceforNovMayDeepwaterNigeria,365days................20Table2.33:SignificantWaveHeightPersistenceforJunOctDeepwaterNigeria, 292Days.................21Table2.34:PercentFrequencyofOccurrenceofNearSurfaceCurrentSpeedandDirection,Offshore

Nigeria.......................................................................................................................................21Table2.35:PercentFrequencyofOccurrenceofNearBottomCurrentSpeedandDirection,Offshore

Nigeria.......................................................................................................................................22Table

3.1

1:

Metocean

Criteria

for

aSite

in

250

ft

water

depth,

Cabinda

..............................................

25Table3.12:WaveHeight,WavePeriodandDirectionFactors,Cabinda...............................................26

Table3.13:ExtremeBottomCurrentCase,Cabinda............................................................................26Table3.14:ExtremeWindCase,Cabinda............................................................................................27Table3.15:ExtremeSurfaceCurrentCase,Cabinda............................................................................28Table3.16:DirectionalScalingFactorsfortheSurfaceCurrentExtremes,Cabinda..............................28Table3.21:PercentTimeofOccurrenceandParametersforOchiHubbleSpectrumfor250ft,Offshore

Cabinda1.....................................................................................................................................30

Table3.31:JointFrequencyofOccurrenceofWindSpeedandDirectionforOffshoreCabinda(33ft,

10minaverage).(20nmoffshore)................................................................................................32Table3.32:AnnualWaveHeightPersistenceforOffshoreCabinda.....................................................32Table3.33:JanuaryMarchWavePersistenceforOffshoreCabinda....................................................33Table3.34:AprilJuneWavePersistenceforOffshoreCabinda............................................................33Table

3.3

5:

July

September

Wave

Persistence

for

Offshore

Cabinda

...................................................

34

Table3.36:OctoberDecemberWavePersistenceforOffshoreCabinda..............................................34Table3.37:PercentFrequencyofOccurrenceofNearSurfaceCurrentSpeed2ftbelowthesurfaceand

Direction(toward)for250ftwaterdepthOffshoreCabinda.......................................................35Table3.38:PercentFrequencyofOccurrenceofNearBottomCurrentSpeedandDirection(toward)for

250ftOffshoreCabinda..............................................................................................................35Table3.39Min/Mean/MaxAirTemperatureOffshoreCabinda..........................................................37Table3.310PercentOccurrenceofAirTemperatureVersusRelativeHumidityOffshoreCabinda........37Table3.311:SeawaterTemperatureandSalinityProfileCriteriaOffshoreCabinda.............................39Table8.21:DefaultValuesofForceCoefficients..................................................................................47Table8.22:CurrentBlockageFactors

1.................................................................................................48

Table8.23:ValuesfortheWaveExcursionLength,A,forVariousSitesandReturnIntervals...............48Table

8.2

4:

Fatigue

Analysis

Wave

Periods

and

Heights

for

Transfer

Function

.....................................

49Table8.25:FatigueAnalysisInertiaCoefficientsforVariousWaterDepths.........................................49


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1 INTRODUCTION

1.1 SCOPE AND LIMITATIONS

Scope. This report summarizes metocean and hydrodynamic criteria for design

of jackets and pipelines for the shallow waters of offshore equatorial West Africa.The enclosed information should also prove useful for calculating downtimestatistics for facilities installation, marine terminals, etc. Figure 1.1-1 shows theoverall area of interest. Criteria are provided for waves, winds, tides, currents,temperature, drag and inertial coefficients, current blockage factors, marinegrowth, and shielding factors. Data are included for fatigue, operations, andextremes.

Figure 1.1-1 Map of Study Area.

Criteria applicability. The primary intention of this document is to providedesign criteria for steel piled jackets and pipelines in West African offshore areasof high interest to Chevron. In the specific areas addressed high quality datawere available to develop the criteria. The nominal locations presented in thedocument are a 250 ft water depth site offshore Cabinda, and a 50 ft depth siteoffshore Nigeria. For both regions the depth range of applicability is extended towater depths of 10 ft through 400 ft with the provision of depth adjustmentfactors.

The light blue region in Figure 1.1-2 shows the specific region offshore Nigeriawhere the criteria in this document may be considered adequate for the final


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design of a fixed platform or pipeline. This is the Nigerian shelf in water depthsof 10ft to 400ft in the range of longitudes between 4o40E and 5o50E.

The light blue region in Figure 1.1-3 shows the specific region offshore Cabindawhere the criteria in this document may be considered adequate for the final

design of a fixed platform or pipeline. Note that in this figure a dark blue regionlabeled SMAD has been identified. In this sub-region a more detailed sitespecific report has been developed [2].

In general, while the criteria are adequate for final design in the blue regions ofFigure 1.1-2 and Figure 1.1-3, there may be advantages for projects to developsite specific metocean criteria. The two principal advantages of more sitespecific criteria are: (1) that criteria can be presented in tables of values insteadof in terms of the depth and directional factoring approach presented in thisreport (which should be easier for designers to interpret) and (2) more recentdata obtained by ETC may be used to update the criteria for a specific region.

As an example, the surface current criteria for Cabinda are nominally based onthe Sanha site. Currents at the Sanha site are, most likely, more severe than inthe region of the GS-Fox platform (Figure 1.1-3). If surface currents were adesign driver, platforms in the region of GS-Fox would probably benefit from asite specific analysis. Therefore, in some cases, projects will be able to furtheroptimize their designs if they have a site specific criteria document developed.

These criteria may also serve as a basis for the preliminary designs offshoreWest Africa outside the specific application regions. It is recommended that theETC Metocean group be contacted prior to such uses so that the userunderstands the approximations which are being made when adopting these

criteria outside the regions for which they were specifically developed.

Limitations on use. The criteria in this document do not apply in water depthsgreater than 400 ft. These criteria are not appropriate for the design of floatingstructures.

Report organization. Most readers will be interested in either Nigeria orCabindaso the metocean criteria for these regions are covered in the next twosections. Criteria applicable to both regions are provided in Section 4.

Outside Nigeria and Cabinda, the coverage is much more sparse and issummarized in Section 5. This guidance is only adequate for preliminary design.

The wind gust and wave spectrum Sections (6 & 7) are referenced whereappropriate from the regional criteria.

Section 8 provides hydrodynamic criteria which applies uniformly to all regions.


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Figure 1.1-2 Nigeria area of app licabil ity (in blue).

Figure notes: 1. The region of applicability for final design is on the Nigerian shelf in water depths of 10ft to 400ftin the range of longitudes from 4o40E to 5o50E.

Figure 1.1-3 Cabinda area of appl icabil ity (in b lue).

Figure notes: 1. The region of applicability for final design is on the Cabinda shelf in water depths of 10ft to 400ftin the range of latitudes from 5o48S to 5o15S.2. Note that in the dark blue region labeled SMAD a site specific criteria document has beendeveloped. So, though this document applies in the SMAD area and beyond, more detailedinformation is provided in the SMAD Metocean Design Basis.


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1.2 CONVENTIONS,SYMBOLS &ABBREVIATIONS

Direction convention. Criteria for all parameters specified in this document arein terms of direction toward. Wave directions are the direction towards whichwaves propagate, wind directions are the direction toward which winds blow and

current directions are the direction toward which currents flow. All directions arespecified in the nautical convention of being measured in degrees clockwiserelative to true north.

Water depths. In this document mean low water is used as the referenceelevation.

Symbols and abbreviations.Commonly used symbols and abbreviations arefound in the following tables.

Table 1.2-1 Symbols

Hmax maximum wave height

Hs significant wave height (subscripts denote sea, swell , etc.)

N,NE,E, directions north, northeast, east, etc.

N-year, N-yr Return period in years

THmax period of maximum wave

Tp peak spectral period (subscripts denote sea, swell, etc.)

ws wind speed (reference elevation and averaging interval specified with speed)

z distance below the sea surface

zSL distance above sea surface

max maximum crest elevation relative to the water level

JONSWAP spectrum peak enhancement factor (Section 7.3)H direction toward which waves are travelling

w direction toward which wind is blowing

Ochi-Hubble spectrum peak enhancement factor (Section 7.4)


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1.3 GENERAL WEST AFRICA CLIMATE

Winds. In general the climate of the region is mild. The trade winds dominate

the day-to-day conditions. Extreme winds are due to fairly brief but intensesquall (thunderstorm) events. Squalls are much more intense in the north thanthe south.

Waves. Normal wave conditions are a mix of swells and seas characterized bymulti-modal spectra. Extreme waves originate from swells that are generated farto the south of the study area by South Atlantic storms. Swell intensity peaksduring the months of March through September. The 100-yr deepwatermaximum wave height ranges from about 27 ft in the south to 20 ft in the north.

Currents and tides. Currents in the region are more complicated than waves

and winds, and not well understood. Limited measurements suggest mean near-surface currents in shallow water of about 0.2 kt. Currents will increasesubstantially within about a mile of large rivers where they can reach 3-4 kt butthey probably extend only about 10 ft beneath the surface. Tidal currents aregenerally less than 0.1 kt.

Tidal elevations in the study region have a spring range of about 4 ft. Stormsurges are insignificant because of the lack of strong large-scale local winds.

Air and water temperature. The air temperature is generally between 72-93F.Near-surface water temperatures are in the low 80F range.


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1.4 REVISION HISTORY

Rev Date Change

3 April 19 2002: Increases extreme winds for Nigeria to reflect Bonga measurements. Also changed from 1-hr to 1-min since this is more appropriate for most structures.

4 Nov 11, 2002 Increase max wave height in Table 2.1-1 & Table 2.1-2 based on Shell ratio of 2.156.

5 Feb 23, 2003 Decreased max wave height in Table 2.1-1 & Table 2.1-2 based on work by Nerzic.

6 Oct 24, 2003 Added Agbami wave and wind information to Nigeria section. Replaced original Angola wave scattertable with Bereks. Other modifications are too numerous to list.

7 Feb 9, 2004 Changed typo, Table 2.1-1 to Table 2.1-4. Added Wave factors to Table 2.1-4 based on version 5modified slightly by spot checks with Agbami DB.

8 Feb 12, 2004 Added footnote 3 to Table 2.1-4.

9 May 6 2004 Augmented footnote 1 in Table 2.1-1 & Table 2.1-2. Added wave persistence for Nigeria.

10 June 14 2004 Added winds to Table 3.1-3 & clarified footnotes.

11 June 28 2004 Added River current case for Cabinda.

12 April 25, 2005 Replaced Table 2.2-1 with a table derived from BOP measurements. Table 2.2-1 in Rev 11 wasconsidered by structural engineers to be too complex.

13 Sept 16, 2005 Corrected crest heights in Figure 3.3-1 & Table 2.1-1. Similar changes to Figure 2.1-1 & Table 3.1-1.14 Apr 12, 2006 Changed cover page to Chevron

15 April 20, 2006 Changed rainfall for Cabinda. Old graph was too low.

16 June 16, 2006 Removed references to Appendix A (Appendix A was removed in Rev 13)

17 Nov 1, 2006 Increased near-surface currents in Table 3.1-1 & Table 3.1-3 to include mean river current. Createdseparate extreme wind case (squalls, Table 3.1-4). Replaced mid-column current jet case (Table3.1-3) with maximum bottom case.

18 Jan 18, 2007 Modified Figure 2.1-1 and Figure 3.3-1 to account for detailed wave refraction studies done by Berek.Increased Hmax (and crest height) in Table 2.1-1 and Table 3.1-1 to include effect of wind wave.

19 Feb 11, 2009 Corrected reference to Figure 2.1-1 in Table 3.1-5. Added footnote to Table 3.3-7.

20 Sep 14, 2009 1. Provided directional scaling for Cabinda near-surface currents (Table 3.1-6).2. Revise Cabinda near-surface current percent occurrence table (Table 3.3-7).3. Revise operational wind table for Cabinda (Table 3.3-1).4. Add spectral shape parameters for Cabinda extreme wave heights.5. Revised air and seawater temperatures (and seawater salinity). Table 3.3-9 to Table 3.3-11.6. Revised Nigeria sea dominant THmax in Table 2.1-2.7. Clarified additional footnotes.8. Changed all directions to direction to.9. Provided more specific guidance on regions where criteria are appropriate for final design

(Figure 1.1-2, Figure 1.1-3).10.Revised figure and table numbers to be by sub-section.11.Add List of Tables, List of Figures and List of Symbols. Use uniform convention for symbols.


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2 NIGERIA METOCEAN CRITERIA

2.1 EXTREMES

Waves. Table 2.1-1 to Table 2.1-3 summarize extreme criteria for drag-dominated structures in 50 ft of water, offshore Nigeria. For other depths, the

extreme waves must be multiplied by the wave height factor in Figure 2.1-1. Thedominant wave direction is toward the northeast. For other directions, Table2.1-4 provides modification factors. When using Figure 2.1-1 make sure thewave height never exceeds the breaking wave height of 0.78 * (h + 4) where h isthe local mean low water (MLW) depth.

Waves off Nigeria usually have multi-peaked spectra. The swell wave peakscan be characterized with a Gaussian shape (as specified in Section 7.2) and thewind sea peak can be characterized with a JONSWAP shape (as specified inSection 7.3).

The designer should evaluate the extreme wave cases in both Table 2.1-1 andTable 2.1-2 and design to the case that causes the highest load. Both cases areequally probable.

Winds. Two types of winds are given in Table 2.1-1. The first is the wind to beused in conjunction with the design wave (associated wind). Because extremewaves are generated by storms hundreds to thousands of miles from the site, theassociated wind is based on climatological (average) conditions. The secondwind value shown in Table 2.1-1 is due to gusts during squalls (thunderstorms).These storms do not generate large waves so this value should not be used withother values in the table. It is provided for the design of appurtenances such as

flare booms that are dominated by wind.

Wind gusts may be converted to alternate averaging intervals and elevationsusing the NORSOK gust factors (see Section 6).

Tides, Currents, and Surges. The tide range in Table 2.1-1 is based on spring(maximum) tide conditions which occur several days each month. Levels formean low water, mean low spring, and lowest astronomical tide are 1.7, 2.3, and3.4 feet below mean water level, respectively. No storm surge is included in thetable - local winds are too weak or have too small a fetch to generate ameasurable surge. Currents are uncorrelated to extreme waves, so mean values

are used. ETC should be contacted for more refined estimates of current at siteswithin a mile of a major river, or in water deeper than 400 ft.

Pipelines. The extreme load on pipelines in shallow water will probably begoverned by the maximum wave conditions; either Table 2.1-1 or Table 2.1-2.However, another loading case, Table 2.1-3, should also be checked. This caseassumes the current load is dominant and provides the associated wave. Thetable which generates the larger load should be used.


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Other. The temperatures in Table 2.1-5 show the expected minimums andmaximums. These are approximately the 1 in 100 yr event, although theuncertainty is high because of limited data. The mean relative humidity is 85-90%. The maximum rainfall expected is given in Figure 4.2-1.

Table 2.1-1: N-yr Swell and Assoc iated Parameters for Site in 50 ft, Nigeria

Parameter/Return Period 1-yr 10-yr 100-yr

MAXIMUM INDIVIDUAL WAVE1

Hmax(ft)2 17.4 20.5 23.1

THmax(sec) 15.0 16.6 17.9

Crest Elevation (ft)2,3 - - 17.3

SWELL WAVESHs,swell(ft)

2 7.5 9.2 10.5

H,swell(toward) -5 to 30 -5 to 30 -5 to 30Tp,swell(sec) 15.0 16.6 17.9(Gaussian spectral width) 0.007 0.056 0.0046

ASSOCIATED WIND WAVE

Hs, wind wave(ft) 3.6 3.9 4.3H, wind wave(toward) 0 to 65 0 to 65 0 to 65Tp,wind wave(sec) 6.3 6.3 6.3

(JONSWAP) 1.8 1.9 1.9TIDE (ft) 4.0 4.0 4.0

WIND

ws (10-min, 33', kt) 14 14 14

w(toward) -15 to 75 -15 to 75 -15 to 75extreme ws(1-min, 33, kt)

4 49 60 70w(toward) Any Any Any

CURRENT (Inline with Wave)

Surface Speed5(kt) 0.5 0.5 0.5

3 ft off bottom5(kt) 0.3 0.3 0.3

1These individual wave parameters are intended for analyses (such as static analyses) where the largest N-year individual wave is required. For spectral analyses use the bi-modal seastate which results fromcombining the Swell Waves and Associated Seas.

2These values are for a water depth of 50 feet. Wave height in other water depths is found by multiplying

the wave height in this table by the appropriate factor in Figure 2.1-1.3

Crest elevation is the height above mean water at the time of maximum wave. A 4' tide MUST be added to setdeck elevations, i.e. 17.3 + 4 + air gap.4This is the extreme wind value to be used for designing platform quarters, cranes, flare towers, etc. For

other elevations and time periods use the elevation factors specified in Section 6. DO NOT USE INCONJUNTION WITH N-YR WAVE.

5Current is in the direction of the wave.


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Table 2.1-2: N-yr Wind Sea and Associated Parameters for a Site in 50 ft, Nigeria



Hmax(ft)2 19.7 20.7 21.8

THmax(sec) 7.1 7.3 7.4WIND WAVESHs, wind wave(ft) 7.2 8.2 9.2H, wind wave(toward) 10 to 40 10 to 40 10 to 40Tp,wind wave(sec) 7.1 7.3 7.4

(JONSWAP) 1.8 1.9 1.9ASSOCIATED SWELL

Hs,swell(ft)2 5.2 5.2 5.2


WINDws (10-min, 33', kt) 27 29 33w(toward) 0 to 90 0 to 90 0 to 90

CURRENT (Inline with Wave)



1These individual wave parameters are intended for analyses (such as static analyses) where the largest N-year individual wave is required. For spectral analyses use the bi-modal seastate which results fromcombining the Wind Waves and Associated Swell.

2These values are for a water depth of 50 feet. Wave height in other water depths is found by multiplying

the wave height in this table by the appropriate factor in Figure 2.1-1.3Current is in the direction of the wave.


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Table 2.1-3: Secondary Extreme Loading Case Which Assumes Current Maximum and an AssociatedWave for Offshore Nigeria


ASSOCIATED WIND WAVES

Hs, wind wave(ft) 3.3 3.3 3.3H, wind wave(toward) 10 to 40 10 to 40 10 to 40Tp,wind wave(sec) 6.2 6.2 6.2(JONSWAP) 1.6 1.6 1.6

ASSOCIATED SWELLHs,swell(ft)

23.6 3.6 3.6


ASSOCIATED WINDws (10-min, 33', kt) 14 14 14

CURRENT (along isobath)



1Current is a maximum parallel to local isobaths. For other directions use Table2.1-4. ETC should be consulted for locations within 30 miles of major rivers or inwater deeper than 400 ft.

Figure 2.1-1: Wave height f actor and crest height versus water depth for offshore Nigeria.

Figure notes: 1. To be used in conjunction with Table 2.1-1 to Table 2.1-3 to calculate extreme waves in waterdepths other than 50 ft.

2. When calculating deck elevations an air gap and a 4 tide should be added to the crest height.

0.5

0.6

0.7

0.8

0.9

1.01.1

1.2

1.3

10.0 100.0 1000.0

F

ac

t

o

r

s

MLW Depth (ft)

Hs

Crest


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Table 2.1-4: Factors for Wave, Wind, and Current by Direction, Nigeria

(otoward)1Wave Ht.Factor

WavePeriod3

WindFactor

CurrentFactor

0 1.0 Table

2

1.0 0.645 1.0 Table2 1.0 0.690 0.7 7,14 0.8 1.0135 0.4 6,13 0.8 1.0180 0.4 6,13 0.4 0.6225 0.4 6,13 0.3 0.6270 0.5 6,13 1.1 0.8315 0.5 6,13 0.6 1.0

1For example a heading of 90 means toward the east; a heading of 180means toward the south.

2See Table 2.1-1 and Table 2.1-2.

3Use the shorter period for the wind sea case and the longer period for theswell case.

Table 2.1-5: Nigeria Ai r and Water Temperature Ext remes, Nigeria

Temperature

Parameter Min Max

Air (F) 64 92Sea (F, near-surface ) 80 85Sea (F, bottom, 50 ft) 80 85Sea (F, bottom, 150 ft) 70 85


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2.2 WAVE FATIGUE CRITERIA

Wave Spectrum and Hs-Tp Statistics. In general, the multimodal spectralmodel should lead to longer calculated fatigue lives than a unimodal spectralmodel. However, there may be some cases involving dynamic structures where

the multimodal spectra create more onerous loads. In any event it is stronglyrecommended that the multimodal spectra be utilized as it is more realistic thanunimodal spectra for West Africa. The recommended spectrum for use is theOchi-Hubble form (see Section 7.4).

Table 2.2-1 is based on roughly two years of measurements taken from the BOPplatform off the Escravos River in roughly 60 ft. of water. These data are quiteconsistent with data collected by Shell at the deepwater Bonga site.


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Table 2.2-1: Percent occurrence of Significant Wave Height and Peak Period for Shallow WaterNigeria

H(toward)

Hs1(ft) Tp1 (s) Hs2 (ft) Tp2 (sec) -450 0o 45o 90o Counts %Total

2.46 11.5 2.46 5.5 0.61 2.14 2.14 1.22 299 6.11

2.46 12.5 2.46 5.5 0.57 1.99 1.99 1.14 278 5.68

0.82 11.5 0.82 4.5 0.44 1.52 1.52 0.87 213 4.35

0.82 11.5 0.82 5.5 0.41 1.43 1.43 0.82 200 4.09

2.46 11.5 2.46 6.5 0.36 1.27 1.27 0.72 177 3.62

2.46 12.5 0.82 5.5 0.35 1.22 1.22 0.70 171 3.50

0.82 12.5 0.82 5.5 0.35 1.21 1.21 0.69 169 3.45

0.82 13.5 0.82 5.5 0.33 1.14 1.14 0.65 160 3.27

0.82 12.5 0.82 4.5 0.33 1.14 1.14 0.65 159 3.25

2.46 11.5 0.82 5.5 0.32 1.11 1.11 0.63 155 3.17

2.46 13.5 2.46 5.5 0.30 1.04 1.04 0.60 146 2.98

2.46 12.5 2.46 4.5 0.29 1.03 1.03 0.59 144 2.94

2.46 13.5 0.82 6.5 0.28 0.97 0.97 0.56 136 2.78

2.46 12.5 2.46 6.5 0.27 0.94 0.94 0.54 131 2.68

2.46 12.5 0.82 6.5 0.26 0.92 0.92 0.53 129 2.64

2.46 13.5 0.82 5.5 0.24 0.84 0.84 0.48 118 2.41

0.82 13.5 0.82 4.5 0.24 0.83 0.83 0.47 116 2.37

2.46 10.5 2.46 5.5 0.23 0.81 0.81 0.46 113 2.31

2.46 11.5 0.82 6.5 0.23 0.80 0.80 0.46 112 2.29

2.46 11.5 2.46 4.5 0.21 0.74 0.74 0.43 104 2.13

2.46 11.5 0.82 4.5 0.21 0.73 0.73 0.42 102 2.09

0.82 11.5 0.82 6.5 0.20 0.72 0.72 0.41 100 2.04

4.1 13.5 2.46 5.5 0.20 0.71 0.71 0.40 99 2.02

0.82 8.5 2.46 6.5 0.20 0.69 0.69 0.39 96 1.96

2.46 10.5 2.46 6.5 0.19 0.68 0.68 0.39 95 1.94

2.46 13.5 2.46 4.5 0.19 0.67 0.67 0.38 93 1.90

2.46 13.5 2.46 6.5 0.19 0.67 0.67 0.38 93 1.902.46 12.5 0.82 4.5 0.18 0.64 0.64 0.37 90 1.84

2.46 14.5 2.46 5.5 0.18 0.62 0.62 0.36 87 1.78

4.1 12.5 2.46 5.5 0.16 0.57 0.57 0.32 79 1.61

0.82 8.5 4.1 6.5 0.15 0.52 0.52 0.29 72 1.47

4.1 15.5 2.46 5.5 0.13 0.47 0.47 0.27 65 1.33

4.1 14.5 2.46 5.5 0.13 0.45 0.45 0.26 63 1.29

4.1 13.5 2.46 4.5 0.12 0.42 0.42 0.24 59 1.21

2.46 11.5 4.1 5.5 0.12 0.41 0.41 0.24 58 1.19

4.1 15.5 2.46 6.5 0.11 0.39 0.39 0.22 54 1.10

4.1 13.5 4.1 5.5 0.11 0.38 0.38 0.22 53 1.08

2.46 11.5 4.1 6.5 0.11 0.38 0.38 0.22 53 1.08

4.1 11.5 2.46 5.5 0.11 0.37 0.37 0.21 52 1.06

4.1 14.5 2.46 6.5 0.11 0.37 0.37 0.21 52 1.064.1 13.5 2.46 6.5 0.10 0.36 0.36 0.20 50 1.02

2.46 13.5 4.1 5.5 0.10 0.35 0.35 0.20 49 1.00

4.1 13.5 4.1 6.5 0.10 0.34 0.34 0.20 48 0.98

4892 100.00

Notes: 1. Use 1=6.0, 2=0.75 for the peak factors in the Ochi-Hubble spectrum.


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2.3 OPERATING CRITERIA

Winds.Table 2.3-1 shows the joint frequency of occurrence of wind speed anddirection. Winds are very constant reflecting the dominance of the trade winds.The mean speed is about 5 kt, and the direction is towards the northeast. Figure

2-3 shows that the monthly-mean wind speed varies by about a factor of twoduring the course of the year, reaching a maximum in June-August.

Waves. Swells will be weakest from November to May as shown by Figure2.3-2. Conversely, it is roughest from June to October. Table 2.3-2 and Table2.3-3 give the wave persistence for the mild and stormy seasons. These arebased on combined swell and wind sea in deepwater so they can beconservatively applied to shallow water. As an example of how to use the tables,assume you have a lift with threshold of 1.5 m in the June-October time frame.Table 2.3-3 tells you that there were 15 occurrences in the 292 days of data inwhich Hs exceeded 1.5 m. The average event lasted 13.52 days but the longest

lasted 42.37 days.

Current. Table 2.3-4 (Table 2.3-5) summarizes the joint frequency of occurrenceof near-surface (near-bottom) currents in a water depth of 60 ft. A comparison ofthe two tables suggests that near-bottom currents are about half the near-surfacecurrents in 60 ft of water. Currents at other levels can be calculated using linearinterpolation. At the bottom the currents are directed nearly uniformly. The tableapplies to other water depths with the exception of sites within a mile of a largeriver, or deeper than 400 ft. In these two cases, ETC should be contacted.

Other. Figure 2.3-3 shows the seasonal variation in the mean monthly air

temperature and rainfall based on 3 years of offshore measurements.


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Table 2.3-1: Joint Frequency of Occurrence of Wind Speed and Direction, Offshore Nigeria.

ws (1-hr, 33', kt)

w(toward) 2 6 10 14 18 22 26 Total

0 0.99 2.2 0.76 0.19 0.01 0 0 4.1522.5 1.96 8.04 6.88 1.88 0.06 0 0 18.82

45 2.54 13.67 11.05 1.18 0.02 0 0 28.46

67.5 2.66 10.97 7.34 0.63 0.02 0 0 21.62

90 2.21 4.68 2.39 0.2 0.01 0 0 9.49

112.5 1.64 2.25 0.75 0.04 0 0 0 4.68

135 1.17 1.3 0.34 0.07 0.02 0 0 2.9157.5 1.1 1.43 0.31 0.06 0.02 0 0 2.92

180 0.89 0.57 0.08 0 0 0 0 1.54

202.5 0.64 0.08 0 0 0 0 0 0.72

225 0.56 0.03 0 0 0 0 0 0.59

247.5 0.36 0.02 0.01 0 0 0 0 0.39

270 0.33 0.19 0.07 0.05 0.02 0.01 0.01 0.68

292.5 0.39 0.29 0.16 0.09 0.08 0.01 0.01 1.03

315 0.36 0.28 0.05 0.01 0 0 0 0.7

337.5 0.73 0.5 0.07 0.01 0 0 0 1.31

Total 18.53 46.5 30.26 4.41 0.26 0.02 0.02 100

Figure 2.3-1: Seasonal variation in mean wind speed for offshore Nigeria.

M o n t h

W i nd Spd (kt )

0

2

4

6

8

10

Jan M ar M ay July Sep N ov


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Figure 2.3-2: Monthly mean Hs including wind and swell components, Nigeria.

Table 2.3-2: Sign ificant Wave Height Persistence for Nov-May Deepwater Nigeria, 365 days.

Hs(m)

0 0.5 1 1.5 2

# of Occur 3 3 18 32 9Avg Days 121.62 121.62 17.01 2.35 0.69

Max Days 202.25 202.25 96.75 14.25 3.63

Min Days 48.13 48.13 0.13 0.13 0.12

Std Dev 77.31 77.31 28.4 3.27 1.2

% Occur 100 100 83.9 20.62 1.71

%>= 0.3 d 99.9 99.9 94.44 78.13 33.33

%>= 0.5 d 99.9 99.9 94.44 62.5 22.22

%>= 0.8 d 99.9 99.9 94.44 56.25 22.22

%>= 1.0 d 99.9 99.9 94.44 50 22.22

%>= 1.5 d 99.9 99.9 88.89 37.5 22.22

%>= 2.0 d 99.9 99.9 83.33 34.38 11.11

%>= 2.5 d 99.9 99.9 77.78 28.13 11.11

%>= 3.0 d 99.9 99.9 77.78 28.13 11.11

%>= 3.5 d 99.9 99.9 72.22 21.88 11.11

%>= 4.0 d 99.9 99.9 72.22 21.88 0

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

1 2 3 4 5 6 7 8 9 10 11 12

Month

Hm0(m)

MonthlyMean

MinimumMonthlyMeanMaximumMonthlyMeanStandardDeviation


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Table 2.3-3: Signif icant Wave Height Persi stence for Jun-Oct Deepwater Nigeria, 292 Days.

Hs(m)

0 0.5 1 1.5 2 2.5

# of Occur 4 4 5 15 27 6Avg Days 73.12 73.12 57.97 13.52 1.47 0.4

Max Days 81.25 81.25 81.25 42.37 8 0.88

Min Days 63.75 63.75 34.38 0.25 0.12 0.13

Std Dev 7.19 7.19 21.18 13.36 1.65 0.3

% Occur 100 100 99.1 69.32 13.59 0.81

%>= 0.3 d 99.9 99.9 99.9 99.9 81.48 66.67

%>= 0.5 d 99.9 99.9 99.9 93.33 74.07 33.33

%>= 0.8 d 99.9 99.9 99.9 93.33 66.67 16.67

%>= 1.0 d 99.9 99.9 99.9 93.33 51.85 0

%>= 1.5 d 99.9 99.9 99.9 86.67 29.63 0

%>= 2.0 d 99.9 99.9 99.9 86.67 29.63 0%>= 2.5 d 99.9 99.9 99.9 80 22.22 0

%>= 3.0 d 99.9 99.9 99.9 80 11.11 0

%>= 3.5 d 99.9 99.9 99.9 66.67 3.7 0

%>= 4.0 d 99.9 99.9 99.9 66.67 3.7 0

Table 2.3-4: Percent Frequency of Occurrence of Near-Surface Current Speed and Direction,Offshore Nigeria.

directionCurrent Spd (kt)

(otoward) 0-0.2 0.2-0.4 0.4-0.6 0.6-0.8 0.8-1.0 1.0-1.2 1.2-1.4 1.4-1.6 Total

0 to 45 1.6 6.8 3.9 1.1 0.3 0.1 0 0 13.8

45 to 90 1.9 4.4 1.7 0.2 0.1 0 0 0 8.3

90 to 135 3.3 6.2 4.7 1.8 0.9 0.5 0.1 0 17.5

135 to 180 2.7 5.7 4.9 2.3 1.3 0.8 0.4 0.1 18.2

180 to 225 0.9 3.7 2 0.5 0.3 0.1 0 0 7.5

225 to 270 1.1 2.7 1 0.1 0 0 0 0 4.9

270 to 315 2.5 4.7 2.1 0.7 0.2 0 0 0 10.2

315 to 360 2.5 7.4 5.5 3.1 0.7 0.3 0.1 0 19.6

Total 16.5 41.6 25.8 9.8 3.8 1.8 0.6 0.1 100


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Table 2.3-5: Percent Frequency of Occurrence of Near-Bottom Current Speed and Direction,Offshore Nigeria.

direction Speed (kt)

(otoward) 0-0.2 0.2-0.4 0.4-0.6 0.6-0.8 0.8-1.0 Total

0 to 45 4.2 3.2 0.7 0.2 0 8.345 to 90 4.6 3.3 0.7 0.1 0 8.7

90 to 135 9.6 4.5 1 0.1 0.1 15.3

135 to 180 8.5 3.9 1.8 0.2 0 14.4

180 to 225 5.2 4.1 1.4 0.2 0 10.9

225 to 270 8.4 4.8 1.5 0.2 0 14.9

270 to 315 7.9 5.1 1.7 0.3 0.1 15.1

315 to 360 6.5 4.7 1 0.2 0 12.4

Total 54.9 33.6 9.8 1.5 0.2 100

Month

Air Temp. (F)

70

71

72

73

74

75

76

77

78

79

80

81

Jan Mar May July Sept Nov

0

2

4

6

8

10

12

14

16

18

Rainfall (in)

Air Temp

Rainfall

Figure 2.3-3: Seasonal variation of mean monthly air temperature and rainfall, offshore Nigeria.


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23

3 CABINDA METOCEAN CRITERIA

3.1 EXTREMES

Wave Height. Table 3.1-1 and Table 3.1-3 to Table 3.1-5 summarize extremeload cases for drag-dominated structures and pipelines in 250 ft of water,

offshore Cabinda. For other depths, the extreme waves in Table 3.1-1 must bemultiplied by the wave height factor in Figure 3.3-1. The dominant wave directionis toward the northeast. For other directions, Table 3.1-2 provides modificationfactors. This table assumes the local isobaths are aligned North-South as in theapplicability region shown in Figure 1.1-3. For cases where this is not true, thenthe table should be adjusted by shifting the axis.

Seastates offshore Cabinda may exhibit multi-peaked spectra. When multi-peakspectra are specified for Cabinda the Ochi-Hubble spectral form is recommended(Section 7.4). In some cases, especially where waves are a value associatedwith a peak wind or current, the seastates may be specified with a single peak.

In these single peak cases for Cabinda the JONSWAP spectral shape isspecified (Section 7.3).

Wave Period. The peak wave period shown in Table 3.1-1 covers a range ofvalues. The designer should use the period which causes the largest forces for aparticular design. The range reflects uncertainty in the estimate of peak period.This uncertainty results because the normal procedure for deriving peak periodfrom a regression with wave height does not work well for swell where the twoparameters are poorly correlated.

Winds. Because extreme waves are generated by storms thousands of miles

from the site, the associated winds Table 3.1-1 are based on climatological(average) conditions.

A maximum wind case is given in Table 3.1-4. The winds given in Table 3.1-4are due to gusts during thunderstorms. These storms do not generate largewaves so their associated waves are quite modest. These wind gust extremesare provided for the design of appurtenances such as flare booms that aredominated by wind.

Currents. The near-surface (tens of feet below waterline) currents are driven bythe local trade winds and a thin lens of freshwater from the Congo River.

Currents peak in intensity from November through January during the peakmonths of the Congo River outflow. The thin upper layer can reach 4 knots nearthe river mouth. In the applicability region shown in Figure 1.1-3 these near-surface currents flow towards northwesterly headings. Extreme surface currentsare provided in Table 3.1-5. The directional variation in intensity of extremenear-surface currents is provided in Table 3.1-6.


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Currents deeper in the water column are typically mild (less than 0.3 knots) andare not coherent throughout the water column. Occasionally currents in differentsub-surface layers may intensify to greater than 0.5 knots. Because of themodest intensity of mid-depth currents and their lack of correlation to the strongersurface currents, mid-depth current criteria are not provided in this basis intended

for design of fixed structures.

Bottom current criteria have been provided in Table 3.1-3.

Tides and Surges. The tide range in Table 3.1-1 is based on spring (maximum)tide conditions which occur several days each month. Levels for mean low water,mean low spring, and lowest astronomical tide are 1.7, 2.3, and 3.4 feet belowmean water level, respectively. No storm surge is included in the table - localwinds are too weak or have too small a fetch to generate a measurable surge.ETC should be contacted for more refined estimates of current at sites near amajor river.

Pipelines. The extreme load on pipelines in shallow water will probably begoverned by Table 3.1-1. This table assumes the wave is dominant, andprovides an "associated current" that accounts for joint statistics. However, asecond loading case, Table 3.1-3, should be checked for pipelines. This caseassumes the current load is dominant and provides the associated wave. Thetable which generates the larger load should be used. Figure 3.3-1 can be usedto adjust the waves in Table 3.1-1 for different water depths.

Other. Extremes of air and seawater temperature are listed in Section 3.3 wherethe normal occurrence statistics are also discussed. Figure 4.2-1 can be used tocalculate rainfall rates.


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Table 3.1-1: Metocean Criteria for a Site in 250 ft water depth, Cabinda

Parameter/Return Period 1-yr 5-yr 25-yr 100-yr


Hmax(ft)

2

18.6 20.5 24.3 26.3THmax(sec) 14-16 14-16 15-17 15-17

Crest Elevation (ft)2,3 - - - 13.9

SWELL WAVESHs,swell(ft)

2 8.0 9.0 11.0 12.0H,swell(toward)

4 20 to 70 20 to 70 20 to 70 20 to 70Tp,swell(sec) 14-16 14-16 15-17 15-17swell(Ochi-Hubble) 3-6 3-6 3-6 3-6

ASSOCIATED WIND WAVES

Hs, wind wave(ft) 4 4 4 4H, wind wave(toward) 0 to 70 0 to 70 0 to 70 0 to 70

Tp,wind wave(sec) 6 6 6 6sea(Ochi-Hubble) 1-3 1-3 1-3 1-3

TIDE(ft) 4 4 4 4

WINDws (1-min, 33', kt) 11 11 11 11

CURRENTSpd @ 0 ft (kt) 1.8 1.8 1.8 1.8Spd @ 13 ft 1.5 1.5 1.5 1.5Direction @ 0-25 ft (toward)5 260 to 360 260 to 360 260 to 360 260 to 360Spd @ 26 ft 0.3 0.3 0.3 0.3Direction below 25 ft (toward) Any Any Any Any3 ft off bottom (kt) 0.2 0.2 0.2 0.2

1These individual wave parameters are intended for analyses (such as static analyses) where the largest N-yearindividual wave is required. For spectral analyses use the bi-modal seastate which results from combining theWind Waves and Associated Swell.

2These values are for a water depth of 250 feet. Wave height in other water depths is found by multiplying the waveheight in this table by the appropriate factor in Figure 3.3-1. The peak spectral period may be associated with Hmax(THmax=Tp).

3Crest elevation is the height above mean water at the time of maximum wave. A 4' tide MUST be added to it to set

deck elevations, i.e. 14 + 4 + air gap.4To calculate wave heights from other directions multiply the wave height in this table by the appropriate factor in Table

3.1-2. Do not use Table 3.1-2 to modify associated currents or wind.5Current in the upper 0-25 ft can be reduced to 0.3 kt aligned with the deeper current when considering directions

outside the 0-260 range.


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Table 3.1-2: Wave Height, Wave Period and Direct ion Facto rs, Cabinda

(otoward)1HeightFactor

WavePeriod (s)

CurrentFactor2

0 0.8 12 0.745 1.0 15-17 0.8

90 0.7 12 0.8135 0.5 8 1.0180 0.3 4 0.8225 0.3 4 0.6270 0.4 6 0.8315 0.5 8 0.8

1For example a heading of 90 means to the east; a heading of 180 means to the south.

2Not to be used with surface currents in Table 3.1-5. Surface currents are directionally scaled

with the factors in Table 3.1-6.

Table 3.1-3: Extreme Bottom Current Case, Cabinda


WAVES1

Hs(ft)2 4 4 4 4

Tp(sec) 12 12 12 12H(toward) -45 to 45 -45 to 45 -45 to 45 -45 to 45

(JONSWAP) 2-3 2-3 2-3 2-3WIND

ws (1-min, 33', kt) 11 11 11 11w(toward) -45 to 45 -45 to 45 -45 to 45 -45 to 45

TIDE(ft) 0 0 0 0CURRENTNear-surface (kt) 1.8 1.8 1.9 2.1Spd @ 13 ft 1.5 1.6 1.9 2.1Spd @ 25 ft 1.3 1.6 1.9 2.1Dir. 0-25 ft (toward) 260 to 360 260 to 360 260 to 360 260 to 360Dir. below 25 (toward)3

135 135 135 135

Spd @ mid-depth 1.0 1.3 1.6 1.83 ft off bottom (kt) 0.7 1.0 1.3 1.5

1Waves in this table are specifically associated with the given directional sector andshould not be scaled according to Table 3.1-2.

2If Hmax values are needed assume Hmax=2Hsand assume THmax=Tp.

3For current in the other directions use Table 3.1-2 but do not decrease the current in theupper 25 ft to less than 1 kt. Do not use Table 3.1-2 to modify associated wind or wave.


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Table 3.1-4: Extreme Wind Case, Cabinda


ASSOCIATED WAVES1,2

Hs(ft) 4 4 4 4H( toward)3

10 to 45 10 to 45 10 to 45 10 to 45

TP (sec) 5-12 5-12 5-12 5-12

(JONSWAP) 2-3 2-3 2-3 2-3WINDws (1-min, 33', kt) 37 43 47 53w(toward)

Any Any Any any

TIDE(ft above MLW) 0-4 0-4 0-4 0-4

N-YR CURRENT (kt)4Spd @ 0 ft (kt) 1.8 1.8 1.8 1.8Spd @ 13 ft 1.5 1.5 1.5 1.5Spd @ 25 ft 0.3 0.3 0.3 0.3Direction @ 0-25 ft ( toward) 260 to 360 260 to 360 260 to 360 260 to 3603 ft off bottom 0.3 0.3 0.3 0.3Direction below 25 ft ( toward) Any Any Any Any

1Use a JONSWAP spectra with the parameters indicated. Use a cos

2 spreading law for

components with of < 4 and a cos4for larger .2If Hmax values are needed assume Hmax=2Hsand assume THmax=Tp.

3In all other directions Hs= 2 feet, Tp=4 to 8s.

4Currents between levels can be linearly interpolated.


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Table 3.1-5: Ext reme Surface Current Case, Cabinda


ASSOCIATED WAVES1

Hs(ft) 4 4 4 4H(toward )3 10 to 45 10 to 45 10 to 45 10 to 45

TP (sec) 5-12 5-12 5-12 5-12

(JONSWAP) 2-3 2-3 2-3 2-3ASSOCIATED WINDws (10-min, 33', kt) 11 11 11 11w(toward)

4 315 to 45 315 to 45 315 to 45 315 to 45

TIDE(ft above MLW) 0-4 0-4 0-4 0-4

N-YR CURRENT (kt)2Spd (kt) @ 0 ft 3.4 3.7 3.8 4.2Spd (kt) @ 5 ft 2.8 3.1 3.2 3.5Spd (kt) @ 10 ft 2.2 2.5 2.6 2.8Spd (kt) @ 25 ft 0.6 0.6 0.6 0.6Direction in layer 0 ft to 25 ft See Table 3.1-6Spd (kt) @ 60 ft and lower 0.3 0.3 0.3 0.3Direction @ depth 60 ft (toward) Any Any Any Any

1Use a JONSWAP spectra with the parameters indicated. Use a cos

2 spreading law for

components with of < 4 and a cos4for larger .2Current extremes vary directionally according to the scaling factors in Table 3.1-6. Currentsbetween levels can be linearly interpolated.

3In sectors outside 10

oto 45

o; Hs=2ft and , Tp=4 to 8s.

4Wind speed (1 min, 33ft) in all other directions = 7 kt

Table 3.1-6: Directional Scaling Factors for the Surface Current Extremes, Cabinda

Direction (o toward) 0 45 90 135 180 225 270 315

scaling factor 1.0 0.7 0.8 0.8 0.8 0.8 1.0 1.0


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3.2 WAVE FATIGUE CRITERIA

Wave Spectrum and Hs-Tp Statistics. Table 3.2-1 lists waves spectralparameters and percent time of occurrence. It is recommended that the Ochi-Hubble spectrum be used (Section 7.4). For Cabinda we have specified double-

peaked spectra. The dominant wave direction for each component, , is alsogiven in the table. This can be combined with a cos2 spreading law for theshorter period components (< 13 s) and a cos4for the longer-period components(Section 7.5). In general the bimodal spectrum should lead to longer calculatedfatigue lives than a unimodal spectrum.

Water Depth. Strictly speaking Table 3.2-1 only applies to a water depth of 250ft but should be similar in deeper water. For shallower water, the wave energywill tend to decrease as the depth decreases. Using these heights in shallowerwater should be conservative but keep in mind that wave direction changes in asystematic way with water depth. For applications where wave direction is

considered in water depths much shallower than 250fta site specific analysis canbe carried out to refine the results.


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Table 3.2-1: Percent Time of Occurrence and Parameters for Ochi-Hubble Spectrum for 250 ft,Offshore Cabinda1.

No.%

OccurrenceHs1(ft)

Tp1(sec)

Hs2(ft)

Tp2(sec)

1 21

(o

towards)2

(o

towards)

291 15.3 3 12 2.5 7.7 7 2.1 27 21231 12.1 2 11 2.3 7.2 7.3 1.6 25 23208 10.9 2 12 2.3 7.7 7.3 1.8 29 19154 8.1 3 11 2.5 7.2 7.1 1.8 22 23117 6.1 4 12 2.7 8.1 7.4 2.9 27 22103 5.4 2 10 2.4 6.3 5.4 1.4 21 2794 4.9 3 13 2.5 8.4 7.5 2.3 30 1890 4.7 4 13 2.8 8.2 7.2 2.2 32 2172 3.8 2 13 2.6 7.9 7.1 2.2 33 1965 3.4 3 14 2.9 8.7 7.5 2.4 33 1763 3.3 2 14 2.6 8.9 7.7 2.3 35 18

52 2.7 5 13 3 8.7 8.1 2.4 29 2247 2.5 4 14 3.2 9.4 8.3 2.7 31 2137 1.9 5 14 3.2 9.4 7.3 3.1 32 1936 1.9 4 11 2.9 7.7 7.5 2 21 2235 1.8 3 10 2.5 5.9 4 1.4 21 3132 1.7 2 15 3 9.6 8.4 2.4 32 2229 1.5 3 15 2.9 9.4 8.7 2.9 34 1728 1.5 1 12 2 8.6 8.7 3.3 32 1527 1.4 5 12 2.7 8.6 7.5 5 27 2126 1.4 1 11 2.5 7 8.1 1.9 28 1824 1.3 5 15 3.3 9.8 8.1 2.5 31 2023 1.2 6 14 2.9 8.7 7 1.7 32 18

20 1.1 2 17 3.4 10.2 9 2.8 31 231Use an Ochi-Hubble spectrum (Section 7.4) with the parameters indicated.


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3.3 OPERATING CRITERIA

Winds. Table 3.3-1 shows the joint frequency of occurrence of wind speed anddirection. Winds are very constant reflecting the dominance of the trades. The

mean speed is about 7 kt toward the northerly quadrant. Figure 3.3-2 shows thevariation in wind speed intensity over the year.

Waves. Swells will be weakest from October to April as suggested in Figure3.3-3. Conversely, it is roughest from May-September.

Table 3.3-2 summarizes the wave persistence during the year. The table showsthe number of events with significant wave height exceeding ranges from 3-10 ft.For example, the table shows there were 6 events where the significant waveheight exceeded 9 feet. These events averaged 4.5 hrs in duration, with thelongest event lasting 6 hrs and the shortest 3 hrs. Table 3.3-3 to Table 3.3-6

provide similar information for the four seasons.

Current. Table 3.3-7 (Table 3.3-8) summarizes the joint frequency of occurrenceof near-surface (near-bottom) currents in a water depth of 250 ft. Near-surfaceand near bottom currents are poorly correlated.

Air temperature, humidity and rainfall. The air temperature and humiditycriteria in this section are based on 5 years of measurements from Soyo. Airtemperature extremes were also checked against measurements made at Kuitoand Sanha. Seasonal variations in air temperature are plotted in Figure 3.3-4and monthly mean rainfall is plotted in Figure 3.3-5.

Seawater temperature and salinity. Seawater temperature and salinity on theCabinda shelf has been updated using the measurements from the World Ocean

Atlas [2].


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Table 3.3-1: Joint Frequency of Occurrence of Wind Speed and Direction for Offshore Cabinda (33 ft,10min average). (20nm offshore).

Speed (kt)

Direction 0-3 3-6 6-9 9-12 12-15 15-18 18-21 >21 Total

0-45 0.4 2.8 8.3 10.4 7.5 3.4 1.1 0.3 34.2

45-90 0.1 0.3 0.6 0.8 0.5 0.2 0.0 0.0 2.590-135 0.1 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.2

135-180 0.1 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.2180-225 0.0 0.0 0.1 0.1 0.0 0.0 0.0 0.0 0.2

225-270 0.1 0.4 1.0 0.9 0.6 0.1 0.0 0.0 3.1

270-315 0.5 4.5 8.9 7.0 2.4 0.5 0.1 0.0 23.9

315-360 0.7 7.1 13.2 10.0 3.7 0.9 0.1 0.0 35.7

Total 2.0 15.3 32.1 29.2 14.7 5.1 1.3 0.3 100.0

Table 3.3-2: Annual Wave Height Persist ence for Offshore Cabinda.

Hs(ft)

3 4 5 6 7 8 9 10

No. of Occ. 123 146 106 41 13 7 6 1

Avg (hr) 57 25 13 11 14 12 5 3

Max (hr) 744 198 159 129 108 24 6 3

Std Dev (hr) 121 41 21 21 28 9 2 0

% > 6 hr 63 59 53 39 31 57 50 0

% > 9 hr 51 39 35 24 31 57 0 0

% > 12 hr 45 32 26 20 23 57 0 0% > 18 hr 40 27 17 12 15 43 0 0

% > 24 hr 34 26 12 10 15 29 0 0

% > 36 hr 28 23 9 7 8 0 0 0

% > 48 hr 26 20 7 7 8 0 0 0

% > 60 hr 24 16 5 2 8 0 0 0

% > 72 hr 21 14 3 2 8 0 0 0

% > 84 hr 20 11 2 2 8 0 0 0

% > 96 hr 17 8 1 2 8 0 0 0


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Table 3.3-3: January -March Wave Persis tence for Offshore Cabinda

Hs(ft)

3 4 5 6

No. of Occ. 61 34 4 0

Avg (hr) 22 8 9 0Max (hr) 231 69 18 0

Std Dev (hr) 61 14 2 0

% > 6 hr 66 62 75 0

% > 9 hr 54 24 50 0

% > 12 hr 46 18 25 0

% > 18 hr 38 6 25 0

% > 24 hr 28 6 0 0

% > 36 hr 18 3 0 0

% > 48 hr 16 3 0 0

% > 60 hr 11 3 0 0

% > 72 hr 7 0 0 0

% > 84 hr 5 0 0 0

% > 96 hr 2 0 0 0

Table 3.3-4: April-June Wave Persistence for Offshore Cabinda

Hs(ft)

3 4 5 6 7 8

No. of Occ. 23 29 37 17 4 1

Avg (hr) 81 40 12 7 10 3

Max (hr) 723 198 81 48 30 3

Std Dev (hr) 158 83 18 11 12 0

% > 6 hr 61 62 46 35 25 0

% > 9 hr 48 48 30 17 25 0

% > 12 hr 43 48 24 12 25 0

% > 18 hr 39 45 19 6 25 0

% > 24 hr 39 38 14 6 25 0

% > 36 hr 35 35 11 6 0 0

% > 48 hr 26 28 8 6 0 0% > 60 hr 26 28 5 0 0 0

% > 72 hr 26 24 3 0 0 0

% > 84 hr 26 24 0 0 0 0

% > 96 hr 26 21 0 0 0 0


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34

Table 3.3-5: Ju ly-September Wave Persis tence for Offsho re Cabinda

Hs(ft)

3 4 5 6 7 8 9 10

No. of Occ. 24 36 35 15 8 6 6 1

Avg (hr) 84 35 16 18 18 14 5 3Max (hr) 627 195 159 129 108 24 6 3

Std Dev (hr) 146 49 60 32 34 9 2 0

% > 6 hr 46 56 57 40 38 67 50 0

% > 9 hr 38 44 37 33 38 67 0 0

% > 12 hr 33 44 26 33 25 67 0 0

% > 18 hr 33 42 14 27 13 50 0 0

% > 24 hr 33 42 14 20 13 33 0 0

% > 36 hr 33 36 14 13 13 0 0 0

% > 48 hr 33 33 9 13 13 0 0 0

% > 60 hr 33 19 9 7 13 0 0 0% > 72 hr 33 17 6 7 13 0 0 0

% > 84 hr 33 14 6 7 13 0 0 0

% > 96 hr 33 8 3 7 13 0 0 0

Table 3.3-6: October-December Wave Persistence for Offshore Cabinda

Hs(ft)

3 4 5 6 7

No. of Occ. 15 47 30 9 1Avg (hr) 118 21 10 5 3

Max (hr) 744 159 51 12 3

Std Dev (hr) 182 35 11 3 0

% > 6 hr 87 57 53 44 0

% > 9 hr 67 40 37 22 0

% > 12 hr 67 23 30 11 0

% > 18 hr 60 21 17 0 0

% > 24 hr 53 21 10 0 0

% > 36 hr 53 19 3 0 0

% > 48 hr 53 17 3 0 0% > 60 hr 53 17 0 0 0

% > 72 hr 53 15 0 0 0

% > 84 hr 47 9 0 0 0

% > 96 hr 40 4 0 0 0


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35

Table 3.3-7: Percent Frequency of Occurrence of Near-Surface Current Speed 2 ft below the surfaceand Direction (toward) for 250 ft water depth Offshore Cabinda.

Direction Speed (kt)

(otoward) 0-0.5 0.5-1.0 1.0-1.5 1.5-2.0 2.0-2.5 2.5-3.0 >3.0 Total

0-45 0.60 0.79 0.32 0.02 0.00 0.00 0.00 1.73

45-90 0.42 0.36 0.09 0.01 0.00 0.00 0.00 0.88

90-135 0.56 0.50 0.48 0.29 0.11 0.02 0.00 1.96

135-180 0.89 1.55 0.18 0.01 0.00 0.00 0.00 2.63

180-225 0.81 0.77 0.01 0.00 0.00 0.00 0.00 1.59

225-270 0.80 2.33 1.53 0.37 0.01 0.00 0.00 5.04

270-315 1.20 8.55 22.24 21.52 7.97 0.85 0.03 62.36

315-360 1.05 4.49 8.02 7.18 2.74 0.30 0.03 23.81

Total 6.33 19.34 32.87 29.40 10.83 1.17 0.06 100.00

WARNING! These data are based on measurements at 2ftbelow the seas surface. Current severity and directionalityvary fairly rapidly with depth. If currents at an alternate depth are needed contact ETC.

Table 3.3-8: Percent Frequency of Occurrence of Near-Bottom Current Speed and Direction (toward)for 250 ft Offshore Cabinda.

Direction Speed (kt)

(otoward) 0-0.1 0.1-0.2 0.2-0.3 0.3-0.4 0.4-0.5 0.5-0.6 0.6-0.7 0.7-0.8 Total

0-45 12.8 3.6 3.2 1.1 0.2 0.0 0.0 0.0 20.9

45-90 2.4 2.0 1.7 1.0 0.4 0.0 0.0 0.0 7.5

90-135 2.2 2.0 1.6 0.9 0.2 0.0 0.0 0.0 6.9

135-180 2.8 3.4 3.0 2.0 0.8 0.4 0.2 0.0 12.6180-225 2.2 4.6 4.5 2.7 1.3 0.9 0.4 0.0 16.6

225-270 2.5 3.4 2.6 1.7 0.6 0.2 0.1 0.1 11.2

270-315 3.8 2.4 1.6 1.0 0.5 0.1 0.1 0.1 9.6

315-360 4.3 3.5 4.1 1.9 0.6 0.3 0.0 0.0 14.7

Total 33.0 24.9 22.3 12.3 4.6 1.9 0.8 0.2 100


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36

Figure 3.3-1: Hs factor and crest factors versus water depth for offshore Cabinda.Figure notes: 1. Multiply Hsfrom Table 3.1-1 by these factors to account for effect of water depth.

2. A 4 tide and air gap should be added to the crest height when calculating deck elevations.

Figure 3.3-2: Seasonal variation in 50%, 90%, 95% and 99% non-exceedence wind speed, off shoreCabinda.

0.5

0.7

0.9

1.1

1.3

1.5

1.7

10.0 100.0 1000.0

F

a

c

t

o

r

s

-

MLW Depth (ft)

Hs

Current

Crest

J F M A M J J A S O N D0

5

10

15

20

25

month

ws,1

0min

(knots)

SMAD 20n.m. Offshore

9995

90

50


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37

Figure 3.3-3: Seasonal variation in 50%, 90%, 95% and 99% non-exceedence sign ifi cant wave height

offshore Cabinda (nominal water depth of 155ft

).

Table 3.3-9 Min/Mean/Max A ir Temperatu re Offshore Cabinda

Statistic Tair (oF)

Minimum 59

Mean 78

Maximum 94

Table 3.3-10 Percent Occurrence of Air Temperature Versus Relative Humidity Offshore Cabinda

% Relative Humidity


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38

Figure 3.3-4: Seasonal medi an, 0.1% and 99.9% exceedence air temperature and median sea-surface

temperature offshore Cabinda.

Figure 3.3-5: Seasonal variation in mean monthly rainfall near the coast of Cabinda (Pointe Noire,Rep. Congo)

J F M A M J J A S O N D60

66

72

78

84

90

96

month

temperature(oF)

Air and Sea Temperature

99.9% Tair

50% Tair

0.1% Tair

50% Tsurface water

0

50

100

150

200

250

1 3 5 7 9 11

rain

(mm)

Month

Rain


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39

Figure 3.3-6: Seawater temperature and salinity profile criteria offshore Cabinda.

Table 3.3-11: Seawater Temperature and Salinity Profi le Criteria Offshore Cabinda

water temperature (oF) salinity (psu)

depth, z (ft) minimum mean maximum minimum median maximum

5 62.6 76.2 87.6 5.2 22.6-32.0 35.7

10 61.9 74.5 86.9 11.7 26.8-33.3 35.8

20 61.3 72.8 86.2 18.3 31.1-34.7 36.0

40 60.6 71.1 85.5 24.4 34.3-35.6 36.1

60 60.2 70.1 85.1 27.4 34.8-35.6 36.2

100 59.7 68.8 84.4 31.0 35.4-35.6 36.3

150 59.0 66.4 79.5 32.2 35.4-35.6 36.4

200 58.4 64.7 76.0 33.1 35.4-35.6 36.4250 58.0 63.3 73.3 33.7 35.4-35.6 36.5

Notes: 1. For salinity the median value is estimated to lie in the range listed.2. Values may be assumed to be constant over the upper 5ftof the water column.

44 50 56 62 68 74 80 86-10

3

-102

-101

T (oF)

depth(ft)

Shelf / Inner Slope

min

mean

max

2 8 14 20 26 32 38-10

3

-102

-101

salinity (psu)

Shelf / Inner Slope

min

median

max


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40

4 REGION-WIDE ANCILLARY PARAMETERS

4.1 MARINE GROWTH

Marine growth profile. Offshore West Africa the marine growth thickness

should be taken as 4 inches from mean high water down to a depth of 150ftbelow the sea surface.

4.2 RAINFALL

Rainfall. Maximum rainfall rate events offshore most of West Africa should bedue to thunderstorm event. As such, we expect similar peak rates throughout theregions and the rainfall rates in Figure 4.2-1 are presently recommended for allregions offshore West Africa.

Figure 4.2-1: Rainfall in tensity versus averaging time for 10- and 100-yr storm s of fshore West Af rica.


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41

5 METOCEAN CONDITIONS FOR OTHER LOCATIONS

Criteria for Nigeria Cabinda have been included in the previous two sections.These two regions lie on the northern and southern boundaries of the generalstudy area denoted in Figure 1.1-1. Establishing criteria for the large region

between Cabinda and Nigeria is more difficult because of the lack of good qualitysite data.

Results from the WAX JIP suggest a modest and nearly linear variation in the100-year Hs. This is not surprising given the lack of variability between Nigeriaand Cabinda as documented in Sections 2 and 3.

For preliminary design, it is suggested that a linear interpolation of the resultsfrom Nigeria and Cabinda be used. For example, suppose the site of interest lieson the equator (Gabon) in 100 ft of water. Using Table 2.1-1 and Figure 2.1-1,give a 100-year H

s in 100 ft of water of 9.7 ft for Nigeria. Similarly, Table 3.1-1

and Figure 3.3-1 give a 100-yr Hs of 12.8 ft for Cabinda. Using linearinterpolation and assuming Gabon is halfway between Nigeria and Cabinda givesa 100-yr Hsof 11.2 ft. A similar approach is recommended for the other criteria.


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42

6 WIND GUST FACTORS AND SPECTRA

Gust and elevation conversion from reference wind. The wind speed atdifferent elevations and averaging intervals (gusts) can be calculated using thefollowing relationships from the NORSOK standard [3]+.

o

SLuSLSLst

tzIzWtzw ln41.01,

where:

10ln1 SLso

zCwzW ,

5.015.010573.0 sowC ,

22.0

10043.0106.0

SLsoSLu

zwzI

where:

wsois the 1-hr average wind speed (m/sec) at 10melevation,

to=3600s,

zSL=new elevation above sea level (m),

t=new averaging interval in sec (tto).

Wind spectrum. The NORSOK wind spectrum recommended is recommendedfor computation of dynamic wind loads:

nn

SLso

ww

f

zw

fS3

5

45.02

~1

1010320

)(

,

where n=0.468 and the non-dimensional frequency f~

is given by:

75.03/2

1010172

~

sowz

ff .

+Equivalent relationships in English units are provided in API [4].


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43

7 WAVE SPECTRA

7.1 WAVE SPECTRAL SHAPE AND SPREADING

Multi-peaked spectra. Offshore West Africa seastates are frequently composedof waves from different storms. As a result, the wave spectra are normally multi-

peaked. The dominant component is usually associated with swells coming fromthe extra-tropical storms in the deep South Atlantic. There may be more thanone swell peak related to multiple distant storms. In addition, there may be ahigher frequency sea component due to local winds. In the case of a singleswell and a single sea component the total spectrum would be:

)()()( fSfSfS seaswelltotal

where f is the wave frequency (f=1/T). It should be recalled that wave spectra

are proportional to wave elevation-squared. Therefore, if component significantwave heights are specified the total seastate significant wave height would be:

,, ,

Throughout this specification three different types of wave spectra are specified:

Gaussian Spectra. Used to characterize the extremely narrow-band

Nigeria swells. JONSWAP Spectra. Used to characterize wind-driven sea offshoreNigeria (and some associated seastates offshore Cabinda).

Ochi-Hubble Spectra. Used offshore Cabinda (and in some tables forNigeria) to characterize multi-modal seastates.

The specific mathematical forms of these spectra are listed in the followingsections.


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44

7.2 THE GAUSSIAN SPECTRUM

Spectrum for Nigerian swell seas: The Gaussian spectrum. The Gaussianswell spectrum is specified in terms of its significant wave height Hs, peak period

Tp(peak frequency fp=1/Tp) and spectral width parameter :

2

2

2exp

2

po ffm

fS ,

where the zerothspectral moment mo is

16

2

s

o

Hm .

7.3 THE

JONSWAPS

PECTRUM

Spectrum for local seas: The JONSWAP spectrum. The JONSWAP wavespectrum was originally formulated in terms of wind speed and non-dimensionalfetch. A form expressed in terms of significant wave height Hs, peak spectral

period Tp (peak frequency fp=1/Tp) and peak enhancement factor is much moreconvenient for engineering purposes [5]:

4542 25.1exp fTfTHfS pps where

19.1185.00336.0230.00624.0

2

2

2

1exp

fTp,

p

p

ff

ff

when09.0

when07.0 .


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45

7.4 THE OCHI-HUBBLE SPECTRUM

The Ochi-Hubble form. Throughout this document the Ochi-Hubble spectrum isspecified for use. Each of the Ochi-Hubble spectral partitions Si (where i=1 maybe swell and i=2 may be sea)+is described by the expression:

]))(

4

14(exp[

)2()(

})14(4{

2)( 4

,

14

2

,

4

,

4

f

f

f

HffS

ipiis

i

ipi

ii

i

For example, for most of the criteria in this document for first component, the

three parameters Hs, fpand correspond to Hs,swell, Tp,swell, and swelland for thesecond component correspond to Hs,sea, Tp,sea, and sea. Note fpis the frequencyof the spectral peak or 1/Tp. The Gamma Function (), is a mathematical seriesgiven in tables or as a functional call in Matlab or the IMSL math library.

7.5 DIRECTIONAL SPREADING

Directional spreading function. The cosine spreading function isrecommended for use with these criteria. This spreading function is applied

relative to the mean wave direction and is non-zero within 90oof the mean wavedirection:

,

90,0

90,cos

o

H

o

HHH

n

H

for

forD

where the constant is defined such that the energy at each frequency ispreserved:

oH

oH

HH dD

90

90

.1

+Or more generally more than two spectral peaks may be specified with more than two i values.


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46

8 HYDRODYNAMICS

These criteria apply to static analysis of jacket structures. ETC should becontacted for specialized or highly dynamic structures or components like free-standing conductors, deepwater jackets, or caissons. The criteria given below

are based on API RP2A (20th

edition).

Marine growth in the region is 4 inches added to the radius of members betweenmean high water and 150 ft below mean water. Member diameters in this zoneshould be increased by 8 inches and rough drag and inertial coefficients shouldbe used. Members outside this range can use smooth force coefficients.

8.1 EXTREME FORCE CALCULATIONS

The standard drag and inertial force coefficients (Cdand Cm) used by Chevronin other parts of the world may not be applicable to W. Africa. This is

because the W. Africa environment is so mild that the Kuelegan-Carpenternumber often falls below a critical value, particularly for larger members. Theupcoming release of SACS (Version 3.2) will automatically calculate theproper coefficients. For those who are not using SACS, Figure 8.2-1 andFigure 8.2-2 show the force coefficients for offshore Cabinda for the 1- and100-yr return periods, respectively. The figures apply to nearly verticalmembers (< 15 from vertical) in water depths of 250-400 ft. The coefficientsfor return periods between 1- and 100-yr can be interpolated from the figures.The rough (r subscripts) coefficients should be used for members down to150 ft below mean water. The diameter used in the figures to find roughcoefficients should be increased by 8 inches for marine growth. For bracing

members suggested values are Cds=0.65, Cdr=1.05, and Cms=Cmr=2.0.

Figure 8.2-3 and Figure 8.2-4 give the force coefficients for offshore Nigeria in46 and 16 ft of water depth (Mean Low Water, MLW). Table 8.2-1 should beused for coefficients not shown in the figures. These coefficients apply tonear-vertical (< 15 from vertical) members. Each figure shows coefficientsfor the 1- and 100-yr storms. Coefficients for large diameter members inother water depths should not be interpolated from the figures. ETC canprovide these if needed. The diameter used in the figures to find roughcoefficients should be increased by 8 inches for marine growth. For bracingmembers suggested values are Cds=0.65, Cdr=1.05, and Cms=Cmr=2.0.

Streamfunction Wave Theory kinematics are recommended for calculatingextreme loads. Use 20th order for the shallow water (< 50 ft), and 7th orderfor deeper water.

A wave kinematics factor of 1.0 is recommended for W. Africa because oflack of directional spreading expected for extreme wave.


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48

Table 8.2-2: Current Blockage Factors 1

No. of Legs Heading Factor

3 all 0.90

4 end-on 0.804 diagonal 0.85

4 broadside 0.806 end-on 0.756 diagonal 0.85

6 broadside 0.80

8 end-on 0.70

8 diagonal 0.85

8 broadside 0.80

1For free-standing and braced caissons the

current blockage factor should be 1.0.

Table 8.2-3: Values for the Wave Excursion Length, A, for Various Sites and Return Intervals.

Site 1-yr 100-yr

Cabinda (250-400 ft) 9 ft 12 ft

Nigeria (16 ft MLW) 30 28

Nigeria (46 ft MLW) 20 29


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49

Table 8.2-4: Fatigue Analysis Wave Periods and Heights for Transfer Function

Period Height Order of

(sec) (ft) Streamfunction

18.0 3.5 2016.0 3.5 20

14.0 3.5 2012.0 3.5 7

11.0 3.5 7

10.0 3.5 7

9.0 3.5 7

8.0 3.3 7

7.5 3.3 7

7.0 3.2 7

6.5 3.1 7

6.0 3.0 7

5.5 2.9 75.0 2.8 7

4.5 2.7 7

4.0 2.5 7

3.5 2.4 7

3.0 2.2 7

2.5 1.5 7

2.0 1.0 7

Structures with sharp response peaks should bechecked at finer period increments. For this purpose,the table can be linearly interpolated.

Table 8.2-5: Fatigue Analysis Inertia Coefficients for Various Water Depths

Depth (ft) Cm

10 1.7

20 1.9

>30 2.0


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50

Figure 8.2-1: Hydrodynamic force coeffici ents for 1-yr s torm in water depths of 250-400 ft, offshoreCabinda.Figure notes 1. The "r" subscript means "rough"; the "s" means smooth. When selecting rough coefficient, the

smooth diameter should increased by 8".2. Members should be within 15 of vertical.3. Use Table 8.2-1 for coefficients not shown in Figure.

Figure 8.2-2: Hydrodynamic force coefficients for 100-yr storm in water depths of 250-400 ft,offshore Cabinda.

Figure notes 1. The "r" subscript means "rough"; the "s" means smooth. When selecting rough coefficient, thesmooth diameter should increased by 8".2. Members should be within 15 of vertical.3. Use Table 8.2-1 for coefficients not shown in Figure.

Diameter (in)

ForceCoefficien

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30 40 50 60 70 80 90 100

Cds Cdr Cms Cmr

Diameter (in)

ForceCoefficien

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30 40 50 60 70 80 90 100

Cds Cdr Cms Cmr


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51

Figure 8.2-3: Hydrodynamic force coefficients for 100- and 1-yr storms in water depths of 50 ft(MSL), offshore Nigeria. .


Figure 8.2-4: Hydrodynamic force coefficients for 100- and 1-yr storms in water depths of 16 ft(MLW), offshore Nigeria.


Diameter (in)

ForceCoefficien

0.6

0.8

1

1.2

1.4

1.6

1.8

0 20 40 60 80 100

Cdr-1yr

Cds-1yr

Cmr-1yr

Cdr-100yr

Cds-100yr

Diameter (in)

ForceCoefficien

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

0 20 40 60 80 100

Cdr-1yr Cdr-100yr Cds-1yr


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52

9 REFERENCES

1. Cooper, C.K. Metocean and Hydrodynamic Criteria for Shallow Fixed Structures andPipelines Off W. Africa, ETC memo, Version 19, Feb 11, 2009.

2. Santala, M. SMAD Fixed Platform Metocean Criteria, June 2, 2009 ETC report; Version B.3. Norwegian Technical Standards Institute, Actions and Effects, NORSOK N-003, Rev. 1,

February 1999.4. American Petroleum Institute, Recommended Practice for Planning, Designing and

Constructing Fixed Offshore Platforms-Working Stress Design, API RP 2A-WSD, 21st

Edition, October 2005.

5. Goda, Y., Random Seas and Design of Maritime Structures, University of Tokyo Press.1985, Tokyo, Japan.


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WAfrica#20 Metocean & the EGP Pipeline 1/3

Chevron Energy Technology Company6001 Bollinger Canyon RoadSan Ramon, CA 94583FE Department CSME Unit MOSP Team

FROM: Markku Santala (925) 842-4889TO: Desmond Yan 2 Dec, 2009

CC: Cort CooperREF: Recommendations on the use of WAfrica#20 for Final Designof the EGP3 Pipeline

Summary. This EGP3 pipeline project would like to use the Chevron general shallowwater West Africa criteria document [1] for metocean input to its pipeline designs in waterdepths of 200 and less offshore the Escravos River (Figure 1). This memo outlines a fewsite specific modifications that should be applied based on review of [1] and the actualarea of the EGP3 pipeline. In short, three modifications are recommended:

1. A maximum wave height to associate with peak bottom currents has been developed.

2. The directional sector of long-period swells should be varied with water depth.3. Wave heights should not be reduced offshore of the reference water depth of 50.

Areas examined. Since the metocean parameters most relevant to pipeline design arewave heights and currents; this review of [1] focused on these parameters. A separatereport specifies air and seafloor water temperature criteria which should be used for thearea [2].

Maximum wave heights to be associated with peak bottom currents. The peakbottom current criteria in Table 2.1-3 of [1] specify an associated bi-modal seastate. If it is

desired to use an individual maximum wave height with the peak bottom current the wavein Table 1 may be used.

Table 1. Individual Wave to Use with Peak Bot tom Currents in Table 2.1-3 of [1].

Parameter Symbol Value

Maximum Wave Height Hmax 10.3 ft

Associated Wave Period THmax 9.5 s

Note: This wave height is associated with all return period (1-year to 100-year) current extremes in Table 2.1-3 of [1].

It should also be noted that the data which we have indicate that the return period bottom

current extremes (1-year=1.2knots, 100-year=1.8knots) in Table 2.1-3 of [1] should bevalid all the way out to the 200 depth limit considered.

Wind-wave direction, swell direction and wave height modification versus waterdepth. It has been previously pointed out that the wave height modification curve inFigure 2.1-1 of [1] is not consistent with data we have from the recent WANE JIP hindcast[4,5]. Developing the precise wave and crest height modification curve versus depthbased on this data is beyond the scope of this work. Analyses do show that wave heightsshould not be lower in depths greater than 50ft than they are at 50ft. Hence, it isrecommended that wave heights not be reduced from their reference level at 50ft when


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considering sites offshore of the 50ft isobath. Inshore of the 50ft isobath the wavemodification curve may be used as in [1]. Wave directions as a function of water depthhave been estimated directly from the WANE shallow grid point data as they were in [4] fora single depth.

Table 2 lists the wave height modification factor as a function of water depth to be used inplace of the values in Figure 2.1-1 of [1]. This table also lists the principal direction of

swells and wind seas as a function of water depth to be used in place on the values inTables 2.1-1, Table 2.1-2 and Table 2.1-3 of [1]. Due to their longer period the swellwaves turn more strongly towards shore due to refraction as the move into shallow water.

As expected, the directions of short period waves are unaltered with water depth.

Table 2. Wind Sea Direction, Swell Direction and Wave Height Mult iplication factors to beused in p lace of vales in Tables 2.1-1, Table 2.1-3, Table 2.1-3 and Figure 2.1-1 of [1].

MLW depth Hsand Hmax swell direction toward, H,swell wind sea direction toward, H,windsea

(ft) factor low limit (deg) high limit (deg) low limit (deg) high limit (deg)

10 0.61 25 75 25 75

16 0.87 25 75 25 75

25 1.03 20 70 20 70

30 1.05 20 70 20 70

37 1.01 20 70 20 70

50 1.00 15 65 20 70

60 1.00 10 60 20 70

80 1.00 5 55 20 70

100 1.00 0 50 20 70

250 1.00 0 50 20 70


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Figure 1. Local area of EGP3 pipeline. The purp le line is the pipeline. The ligh t gray lines are isobaths. The 30

isobath has been pointed out.

References

1. Cooper, C.K. and M.J. Santala, Metocean and Hydrodynamic Criteria for ShallowFixed Structures and Pipelines Off W. Africa, ETC memo, Version 20, Sept 14, 2009.

2. Santala, M.J., Air and Water Temperature Criteria Offshore the Escravos River (waterdepths of 250ft and shallower). 25 Nov, 2009 ETC report.

3. MH, Design Basis for Pipelines & Power Cable, Doc No. OF@-WIL-GN-DES-PL-00001. Willbros West Africa, Inc. May 5, 2005.

4. Santala, M.J., Commentary on Metocean Criteria adopted for EGP3 Pipeline, 27 Apr,2009 ETC memo.

5. Oceanweather, Inc., West Africa Normals and Extremes Hindcast (WANE2),Submitted to WASP2 JIP Participants July 2008.

Documents

WAfrica Metocean Data Rev20