rr392 wave mapping in uk waters
TRANSCRIPT
HSEHealth & Safety
Executive
Wave mapping in UK waters
Prepared by PhysE Ltd for theHealth and Safety Executive 2005
RESEARCH REPORT 392
HSEHealth & Safety
Executive
Wave mapping in UK waters
Martin O Williams BSc MPhil CMarSci FIMarEST Metocean Adviser PhysE Ltd
The Old Customs House The Quay Yarmouth
Isle of Wight PO41 0PG
This work updates the contour map of 50-year extreme significant wave height that is provided in OT 2001/010 (previously Section 11 of Guidance). The updated map, now presented for the 100 year return period, presents extreme significant wave height in UK waters derived from 373 data sets from the NEXTRA hindcast, calibrated against measured wave data and verified against established criteria.
This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.
HSE BOOKS
© Crown copyright 2005
First published 2005
All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.
Applications for reproduction should be made in writing to: Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to [email protected]
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ACKNOWLEDGEMENTS
The author and editor gratefully acknowledge the valuable contribution made to this study by the following industry representatives:
Dr Colin K Grant
Advisor - Metocean
BP Exploration
Mr Ian M Leggett
Discipline Head - Metocean Engineering
Shell EP Europe
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TABLE OF CONTENTS
EXECUTIVE SUMMARY..............................................................................................VII
1. INTRODUCTION ................................................................................................... 1
2. DATA SOURCES .................................................................................................. 2
2.1 THE NESS, NEXT AND NEXTRA WAVE HINDCASTS .................................................. 2
2.2 VERIFICATION DATA...................................................................................................... 5
2.3 ESTABLISHED CRITERIA............................................................................................... 7
3. THE MAPPING PROCESS ................................................................................... 9
3.1 DERIVATION OF EXTREME VALUES............................................................................ 9
3.2 GRIDDING AND MAPPING ............................................................................................. 9
4. ASSESSMENT OF INITIAL RESULTS............................................................... 11
4.1 COMPARISON WITH MEASUREMENTS ..................................................................... 11
4.2 CONTOURS OF NEXTRA WAVE HEIGHTS ................................................................ 12
5. CALIBRATION OF RESULTS ............................................................................ 14
5.1 ADJUSTMENT OF NEXTRA WAVE HEIGHTS............................................................. 14
5.2 EXTREME WAVE HEIGHT MAPPING .......................................................................... 15
5.3 CHECK AGAINST ESTABLISHED CRITERIA .............................................................. 16
6. THE FINAL MAP OF 100 YEAR HS ................................................................... 18
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FIGURES
Figure 1: 100 year extreme significant wave height (m) ......................................................... viii
Figure 2: The NEXTRA archive grid .................................................................................. 4
Figure 3: NEXTRA grid points selected for analysis................................................................ 5
Figure 4: Locations of verification data sets ......................................................................... 6
Figure 5: Locations of established criteria ........................................................................... 8
Figure 6: 100 Year Hs (m) – measured data vs. closest NEXTRA...............................................11
Figure 7: Unadjusted 100 year NEXTRA Hs contours (m) ........................................................13
Figure 8: Calibration curve used to adjust NEXTRA 100 year Hs................................................15
Figure 9: 100 Year Hs – NEXTRA results after calibration........................................................16
Figure 10: 100 Year Hs – calibrated NEXTRA contours with established criteria..............................17
Figure 11: 100 Year Hs – final contour map.........................................................................18
TABLES
Table 1: NEXTRA metadata ........................................................................................... 3
Table 2: Verification data ............................................................................................... 7
Table 3: Hs100 (m) – measured data vs. closest NEXTRA........................................................11
Table 4: Calibration data for NEXTRA Hs100 .......................................................................14
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EXECUTIVE SUMMARY
The UK Department of Energy provided guidance on design, construction and certification of offshore structures between 1974 and June 1998. This guidance was first published to support the Offshore Installations (Construction and Survey) Regulations 1974 (SI 1974/289) by providing a consistent basis for the certification of offshore installations by government-appointed certifying authorities. The guidance was regularly updated to keep up with evolving technical knowledge and the fourth and final edition was published in 1990. The document, in its final format, was entitled:
Department of Energy: Offshore installations: Guidance on design, construction andcertification (4th Edition, 1990). HMSO Consolidated Edition 1993 (plus Amendment No. 3, 1995).
The document was generally referred to as ‘Guidance’ or ‘The Guidance Notes’; sometimes as ‘The Fourth Edition’. Section 11 of that document was entitled ‘Environmental Considerations’ and included maps of indicative values of environmental parameters with a 50-year return period; this return period being selected in accordance with the requirements of SI 1974/289. Section 11 of the ‘Guidance’ remained unchanged since initial publication in 1974.
HSE withdrew the Fourth Edition from publication in June 1998 at the end of the certification regime. However, withdrawal was not a reflection on the soundness of the technical information it contained. Section 11 that addressed ‘Environmental Considerations’ was republished as Offshore Technology Report 2001/010. On re-publication additional text was added to OT 2001/010 in the form of a warning as follows:
“It should be noted that the technical content of the ‘Guidance’ has not been updated as part of the re-formatting of the OTO publication… The user…must therefore assess the appropriateness and currency of the technical information for any specific application.”
It remained a concern that, through OT 2001/010, Section 11 of the 4th Edition remained in everyday use, principally by individuals and organisations that do not have the facilities or data that would enable them to assess the appropriateness and currency of the technical information therein. Indeed, substantial advances have been made since Section 11 was prepared, principally with respect to the hindcasting of wave data. The use of grid-mapped hindcast data, in combination with appropriate validation against measurements, provides a new and substantially more robust method for the derivation of contour maps of metocean parameters.
This work therefore updates the contour map of 50-year extreme significant wave height that is provided in OT 2001/010 (previously Section 11 of Guidance). The updated map, now presented for the 100 year return period, presents extreme significant wave height in UK waters derived from 373 data sets from the NEXTRA hindcast.
The values of 100 year extreme significant wave height obtained directly from NEXTRA were verified against extreme values derived from measured wave data. It was thus found necessary to apply an adjustment to the NEXTRA results. In broad terms, in the North Sea, NEXTRA was found to produce approximately the same value as the verification data when 100 year significant wave height was of the order of 14 metres. Below 14m NEXTRA tended to produce higher values, and above 14m NEXTRA tended to produce lower values.
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Failure to correct this feature of the hindcast would have resulted in unacceptably low extreme values in the Northern North Sea. A final check was performed by comparing the resulting contour map with established design criteria.
The calibration process extended only to approximately Hs 100 years of 18 metres. The validity of extrapolating the calibration curve beyond the range for which data are available is questionable, and therefore the region of the map in which Hs 100 years is shown to be over 18 metres has been excluded. Site specific studies should be performed in connection with any developments planned for this region.
The resulting contour map is presented below as Figure 1.
Figure 1: 100 year extreme significant wave height (m) Important note: the information displayed on this map is intended as a guide and should not be treated as a substitute for site specific study.
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1. INTRODUCTION
The procedure employed to derived the contour map of 100 year extreme significant wave height (Hs100) may be summarised as follows:
a) Source Hs data were selected as follows:
NEXTRA Hindcast – to provide wide area spatial coverage.
Measured data sets – to provide verification of the hindcast values.
Satellite-based observations.
Established design criteria currently in use in the North Sea.
b) Each data set was processed to derive the extreme value of Hs, based on the 3-parameter Weibull distribution fitted to the uppermost 95% of the available data.
c) The results provided by the NEXTRA hindcast were verified against the available measurements. The results indicated a tendency for NEXTRA to give:
higher values, relative to measurements, when extreme Hs is less than 14m.
lower values, relative to measurements, when Hs is greater than 14m.
d) A calibration of the Hs100 at each available measurement location (NEXTRA Hs vs. Measured Hs) was performed. The resulting algorithm was applied to the NEXTRA data to produce a final data set. This approach is in full accordance with best practice recommended in ISO 19901-1.
e) Using commercially available contouring software, the adjusted 100 year extreme values were processed to produce the final contour map of Hs100.
f) Final checks were performed by comparing values indicated on the final contour map with established criteria.
g) The measured data available for calibration extended only to approximately 18m extreme Hs. For this reason the validity of the map was capped at 18m, and the region west of the continental shelf in which indications are that 100 year extreme Hs exceeds 18m has been shaded and labelled “>18m”. Site specific studies are recommended for any developments that might be proposed in this region.
The following sections expand on each stage of the process.
Note: the extreme wave height values given in this report are the results of extrapolating a single statistical distribution, to maximise consistency, and should not be considered a basis for design. Established design wave criteria for the presented locations are based on the careful analysis of many extrapolations and may vary from the extreme values quoted here.
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2. DATA SOURCES
2.1 THE NESS, NEXT AND NEXTRA WAVE HINDCASTS
2.1.1 NESS The North European Storm Study (NESS) was initiated with the intention of producing a high quality hindcast database of winds, waves, currents and water levels for the North European continental shelf. The wave model consisted of a coarse (150 km) grid for the North Atlantic and a fine (30 km) grid for the North European shelf. Wind fields were specified by the UK Meteorological Office (Met Office) and the Norwegian Meteorological Institute. Wave modelling for NESS was performed by GKSS Forschungszentrum using a version of their HYPAS spectral wave model.
The hydrodynamic model, used to determine tide and surge parameters was ‘System 21’ developed by the Danish Hydraulic Institute. This model operated using the 150 km coarse grid, within which was nested a 10 km grid covering the Southern North Sea where the bathymetry is more variable.
The NESS model was run for the period October 1964 to March 1989, although output was not continuous:
October 1964 to March 1989 – Winter data only hindcast: (October to March), except… October 1976 to March 1980 – The hindcast included the summers of 1977, 78 and 79 A number of significant summer storms considered worthy of inclusion were also hindcast.
2.1.2 NEXT Following criticism that the NESS hindcast was providing only a poor representation of wind and wave conditions, the model was re-run using a third generation wave model of the WAM type (WAMDI Group, 1998). Additional wind fields were generated by Oceanweather Inc using pressure fields supplied by the USA National Oceanic and Atmospheric Administration (NOAA) to cover the period 4/1989 to 3/1995, although the earlier wind fields (1964 to 1989) remained unchanged. The hydrodynamic model was also extended to 1995.
The NEXT model therefore spanned the following periods:
October 1964 to March 1989 – Winter data only hindcast: (October to March), except… A number of significant summer storms in the period 1964 to 1989 were also hindcast. October 1976 to March 1980 – The hindcast included the summers of 1977, 78 and 79. April 1989 to March 1995 – Continuous hindcast, including both summer and winter data.
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2.1.3 NEXTRA Unfortunately the questionable performance of NESS was not resolved by NEXT and doubts persisted with respect to both wind and wave criteria, which did not compare favourably with measurements from the North Sea. The problems were attributed to inconsistencies in the wind field which comprised a mix of wind fields prepared by:
The Met Office The Norwegian Meteorological Institute Oceanweather Inc.
The model was therefore re-run again, this time using a homogenous wind field prepared entirely by Oceanweather Inc. The period of the hindcast was further extended to 1998, although on this occasion the hydrodynamic model was not extended. As summarised in Table 1, the period covered by the NEXTRA hindcast is:
October 1964 to March 1989 – Winter data only hindcast: (October to March), except… A number of significant summer storms in the period 1964 to 1989 were also hindcast. October 1976 to March 1980 – The hindcast included the summers of 1977, 78 and 79. April 1989 to March 1998 – Continuous hindcast, including both summer and winter data. Hydrodynamic model ends in March 1995.
The NEXTRA hindcast is restricted to use by members of the NESS User Group (NUG – of which HSE is a member) and contractors working on their behalf. The data are supplied by Oceanweather Inc. on 4 DVDs and a CD-ROM.
At the time of writing, work is proposed with a view to further improving the model. Oceanweather Inc. has recently acknowledged that the ‘wind’ parameter output in NEXTRA is an intermediate value in the calculation of the wave height, and should not be taken as directly representative of the wind speed. For this reason a contour map of extreme wind speed has not been attempted as part of the current work. It is anticipated that any additional studies, if approved, will resolve the wind issues and further improve the quality of the hindcast.
Table 1: NEXTRA metadata
Domain 45.4°N-72.4°N, 21.2°W-36.6°E
Resolution 30 km
Duration 1st March 1964 to 30th September 1998
Data (i) Winters only 1964 to 1977
availability (ii) Continuous 1977-79, 1985-1998 (iii) 40+ selected summer storms
Relevant
Products Significant wave height at 3 hour intervals;
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The NEXTRA grid spans the Northwest European Continental Shelf and the Northeast Atlantic. There are more than 3000 grid points in total and the model domain is illustrated in Figure 2.
Figure 2: The NEXTRA archive grid
It was, however, neither practicable nor necessary to process all 3000 points. Therefore the 373 representative grid points were manually selected to provide appropriate detail over the region of interest. The selected grid points are illustrated in Figure 3. Note the increased density of selected points in shallow regions where conditions were anticipated to change rapidly with location.
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Figure 3: NEXTRA grid points selected for analysis
2.2 VERIFICATION DATA
2.2.1 Platform Measurements The measured verification data were obtained from either oil platform-mounted sensors or wave buoys. Except at Magnus the extreme values from the measurements were extracted from relevant design or data reports. Where possible, to ensure consistency with the NEXTRA analyses undertaken for this project, only the extreme results from fitting a 3-parameter Weibull distribution to the upper 95% of the data were used, although in a very small number of cases these were not presented, in which case values derived from a fit to the uppermost 10% of the available data were selected. At Magnus, measured 3-hourly wave heights between April 1985 and July 2004 were extrapolated by fitting a 3-parameter Weibull distribution to upper 95% of the data. The locations of the measurements are illustrated in Figure 4 and the verification data are summarised in Table 2.
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Figure 4: Locations of verification data sets
2.2.2 Satellite Observations Observations of wave height made by satellite altimeter were downloaded from an online database1, in the areas shown in Figure 4. Each area covers 40,000 km2, chosen to ensure sufficient observations were included in the extreme value analysis. An extreme value of Hs for each area was derived by extrapolation of a 3-parameter Weibull distribution fitted to the upper 95% of the observations.
1 www.waveclimate.com
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Table 2: Verification data
Location Lat Long Duration 50yr Hs (m)
100yr Hs (m)
Magnus 61.6°N 1.3°E 85 to 04 16.8 17.5 North Cormorant 61.2°N 1.2°E 83 to 98 16.2 16.8 Buchan 57.9°N 0.0°E 81 to 95 12.8 13.2 Forties 57.8°N 0.9°E 74 to 95 12.7 13.2 Auk 56.4°N 2.1°E 76 to 98 12.4 12.9 West Sole 53.7°N 1.2°E 72 to 90 8.4 8.7 K13 53.2°N 3.2°E 80 to 98 8.7 9.0 Leman 53.1°N 2.2°E 72 to 97 7.1 7.4 Satellite Area 1 57.8°N 1.0°W 85 to 02 11.7 12.2 Satellite Area 2 59.5°N 3.0°E 85 to 02 12.7 13.2 Satellite Area 3 55.0°N 0.0°E 85 to 02 10.3 10.8 Satellite Area 4 52.5°N 3.0°N 85 to 02 8.2 8.5
Estimates of extreme Hs given in the preceding table were kindly made available by: BP Exploration - Magnus, Buchan, Forties, West Sole Shell EPE - North Cormorant, Auk, K13, Leman
Note: the values given in this table are the results of a single statistical extrapolation and hence must not be considered a basis for design. Established design wave criteria at each of the locations may vary from the quoted values.
2.3 ESTABLISHED CRITERIA
Established design criteria were cross referenced against the final map in order to identify the degree of consistency between the map produced by this study and values in present use. The locations of these criteria are illustrated in Figure 5. For reasons of confidentiality, the location names and geographical co-ordinates of the established criteria have been omitted.
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Figure 5: Locations of established criteria
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3. THE MAPPING PROCESS
3.1 DERIVATION OF EXTREME VALUES
3.1.1 NEXTRA Extreme values of Hs were derived by extrapolation of the 3-parameter Weibull distribution fitted to the upper 95% of the cumulative frequency distribution. The analyses were performed using OceanStats2 software (PhysE Limited) that uniquely provides a batch processing facility, thus facilitating the rapid analysis of the selected 373 NEXTRA grid points.
3.1.2 Verification Data In most cases extreme Hs values were extracted from relevant entries in the appropriate criteria reference documents. Where possible the Hs100 derived from the 3-parameter Weibull distribution fitted to the upper 95% of available data was extracted, but in some cases this was not presented and therefore values derived from fits to the upper 10% were used.
One exception was the Magnus Field; because of the importance of the Northern North Sea, and the fact that new data had recently become available, the Magnus data were re-processed for this study. Here, 3-hourly measured wave heights between April 1985 and July 2004 were extrapolated by fitting the 3-parameter Weibull distribution to the upper 95% of the available data.
3.1.3 Established Criteria Final values were cross referenced against the established 100 year design values of Hs, as extracted from relevant design reports or supplied by HSE.
3.2 GRIDDING AND MAPPING
3.2.1 Software Gridding is the process of creating a regularly spaced, rectangular grid of values from irregularly spaced input data. Care must be taken in the choice of gridding algorithms as the chosen method can have a significant impact on the final contour map; different methods can produce very different, and sometimes inappropriate, results.
The grid is the base from which the contour map is created. Contour lines are drawn as a series of straight-line segments between adjacent grid nodes. A finer grid will therefore create smoother contours.
Additional smoothing of the contours can be introduced through the use of filters and spline smoothing tools.
Golden Software Inc, 2002. Surfer, Version 8, was selected as the preferred application for gridding and mapping.
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3.2.2 The Base Map The coastline and bathymetry were downloaded from the US National Geophysical Data Centre web facility2.
The coastline was extracted from the World Vector Shoreline data set at a scale of 1:250000. The mapping software was configured to recognise the coastline as a boundary to the contouring process, thus blanking regions that represent land.
The bathymetry was downloaded from the ETOPO2 bathymetric database gridded at two minute (latitude/longitude) resolution.
Both required conversion to Surfer format. The bathymetry was interpolated on to a 50 x 33 grid using the Kriging method.
3.2.3 Kriging Interpolation Based on the results of tests on several alternative gridding methods, Kriging was selected as the preferred option as it most closely represented the variability in the NEXTRA data. Kriging is a recognised contouring algorithm, widely used in geostatistics. In this study, on the advice of Golden Software, the default linear variogram was used.
3.2.4 Grid Resolution and Blanking Grid resolution refers to the number of columns and rows of data in the interpolated grid and hence the number or intersections or ‘nodes’. Greater grid density produces visually smoother contour maps, but greatly increasing the number of interpolated nodes can also introduce artefacts and reduce computing efficiency. A resolution of x = 50, y = 33 was found by experimentation to provide an appropriate level of detail although additional contour smoothing was necessary at this resolution.
To avoid producing erroneous onshore values the coastline around the UK, Ireland, Europe and all major islands was digitised and assigned a blanking value that the mapping software recognised as a boundary to the contouring process.
3.2.5 Filtering and Smoothing The gridded data were filtered and smoothed prior to final presentation.
A low pass linear convolution filter was further applied to reduce small scale variability in the interpolated data.
In addition to filtering the grid, further smoothing was carried out by re-constituting the grid with 5-point cubic spline interpolation. The original grid node values were preserved in this process.
2 http://www.ngdc.noaa.gov/mgg/mggd.html
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4. ASSESSMENT OF INITIAL RESULTS
4.1 COMPARISON WITH MEASUREMENTS An assessment of the performance of NEXTRA was carried out by comparing the Hs100 from each measured data set with that obtained from the closest NEXTRA grid point.
The wave height comparisons are plotted in Figure 6. The corresponding values are given in Table 4.
NEXTRA/MEASURED VERIFICATION 20.0 130
19.0
18.0 120
17.0
10
0 y
ea
r H
s (
m) 16.0
15.0
14.0
13.0
12.0
11.0
10.0 Measured
NEXTRA
NEXTRA as % of measured
6.0 60
%
110
100
90
80
9.0
8.0 70
7.0
Ma
gn
us
N.C
orm
ora
nt
Bu
ch
an
Fo
rtie
s
Au
k
We
st
So
le
K1
3
Le
ma
n
Figure 6: Hs100 (m) – measured data vs. closest NEXTRA
Table 3: Hs100 (m) – measured data vs. closest NEXTRA
Location NEXTRA GP
100 year Verification Hs (m)
100 year Raw NEXTRA Hs (m)
Distance from Verification (km)
NEXTRA as % of Verification
Magnus 14158 17.5 15.8 12.3 90%
N. Cormorant 14267 16.8 15.4 11.2 92%
Buchan 14894 13.2 12.8 13.1 97%
Forties 14836 13.2 14.0 4.3 106%
Auk 15021 12.9 14.1 9.3 109%
West Sole 15570 8.7 9.0 7.7 102%
K13 15514 9.0 9.8 7.9 109%
Leman 15635 7.4 8.7 17.0 118% Note: the values given in this table are the results of a single statistical extrapolation and hence must not be considered a basis for design. Established design wave criteria at each of the locations may vary from the quoted values.
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The verification data reveal a clear trend:
In the northern North Sea NEXTRA extreme Hs values are less than the corresponding verification data in the northern North Sea by 8% to 10%.
In the central North Sea NEXTRA extreme Hs values are in general greater than the verification data by 6% to 10% (Buchan is an outlier).
In the southern North Sea NEXTRA extreme Hs values are greater than the verification data by up to 3 to 18%.
Further inspection of the data reveals that, in general:
If Hs100 > 14m then NEXTRA < Verification Data
If Hs100 ≈ 14m then NEXTRA ≈ Verification Data
If Hs100 < 14m then NEXTRA > Verification Data
The degree of spatial variability in the comparisons indicates that:
NEXTRA model performance is not consistent across the mapping domain.
Any adjustment of NEXTRA will not be linear across the range of wave heights within the database.
4.2 CONTOURS OF NEXTRA WAVE HEIGHTS Contours of Hs100 derived from NEXTRA data prior to any form of further adjustment are given in Figure 7. The contours were derived from Kriging gridding of the 373 NEXTRA grid points using a grid size of x = 50, y = 33; resulting in 1650 interpolated grid points.
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Figure 7: Unadjusted NEXTRA Hs100 contours (m)
Cross checking the contours against the verification data shows:
The NEXTRA wave heights are low in the exposed Atlantic, northern North Sea and Faroe Shetland Channel.
The contours are in reasonable agreement with the verification data in the central North Sea.
The NEXTRA wave heights are high in the southern North Sea.
It was therefore concluded that calibration was required prior to production of the final map.
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5. CALIBRATION OF RESULTS
5.1 ADJUSTMENT OF NEXTRA WAVE HEIGHTS The adjustment, or calibration, process was based on comparison of the extreme value from each measured verification data set and its nearest NEXTRA grid point. For the satellite data comparisons, the NEXTRA values from grid points lying within each area were averaged. A scatter plot of the two sets of wave heights was fitted by a curve, the equation of which was then applied to the NEXTRA 100 year extreme values. Table 5 summarises the calibration data.
Table 4: Calibration data for NEXTRA Hs100
100 year 100 year Location NEXTRA GP Verification Hs NEXTRA Hs
(m) (m)
Magnus 14158 17.5 15.8
N. Cormorant 14267 16.8 15.4
Buchan 14894 13.2 12.8
Forties 14836 13.2 14.0
Auk 15021 12.9 14.1
West Sole 15570 8.7 9.0
K13 15514 9.0 9.8
Leman 15635 7.4 8.7
Satellite Area 1 Average 12.2 11.4
Satellite Area 2 Average 13.2 14.4
Satellite Area 3 Average 10.8 10.6
Satellite Area 4 Average 8.5 8.6
Note: the values given in this table are the results of a single statistical extrapolation and hence must not be considered a basis for design. Established design wave criteria at each of the locations may vary from the quoted values.
The best fit was obtained using an exponential distribution, as illustrated in Figure 8.
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100 Year Hs - Verification Data vs. NEXTRA V
eri
fica
tio
n H
s (m
)
19
18
17
16
15
14
13
12
11
10
9
8
7
6
y = 3.5709e0.0975x
6 7 8 9 10 11 12 13 14 15 16 17 18 NEXTRA Hs (m)
Figure 8: Calibration curve used to adjust NEXTRA Hs100
5.2 EXTREME WAVE HEIGHT MAPPING
The NEXTRA extreme Hs values were modified according to the calibration curve:
]Modified Hs = 3.5709e0.0975[NEXTRA Hs
The resulting modified values of Hs were gridded using the Kriging algorithm at a resolution of 50 x 33. Filtering and spline smoothing were applied to the data in accordance with Section 3.2.5 and the resulting smoothed data were then contoured, as shown in Figure 9.
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Figure 9: Hs100 – NEXTRA results after calibration
5.3 CHECK AGAINST ESTABLISHED CRITERIA In order to facilitate cross-referencing, established criteria were compared against the closet NEXTRA grid point, as plotted in Figure 10.
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GRIDDED, ADJUSTED NEXTRA Hs100 / ESTABLISHED CRITERIA CALIBRATION 3
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19
Gri
d H
s100 (
m)
18
17
16
15
14
13
12
11
10
9
8
7
6 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Established 100 year criteria (m)
`
Figure 10: Adjusted NEXTRA Hs100 (m) compared to established 100 year design criteria
Inspection of Figure 10 reveals generally good agreement with established criteria. There is some scatter in the lower wave height classes, which represent the Southern North Sea and Liverpool Bay areas, but this is to be expected given the complications in the modelling process afforded by the shallow and variable bathymetry. Notwithstanding the two obvious outliers the grid values are typically within 10% of the established criteria.
There remains, however, concern that it has not been possible to adequately verify the NEXTRA Hs100 values in the deep water off the continental shelf west of the UK. The positions of the contours in this region, as shown in Figure 10, are based on an extrapolation of the calibration curve beyond the limit of the available verification data.
For this reason it is considered imprudent to rely on contours corresponding to higher values of Hs. It is therefore recommended that all regions for which the 100 Year Hs values are greater than 18m should be excluded and simply labelled “>18m”. When this is implemented the final map, shown as Figure 11 in Section 6, is produced.
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6. THE FINAL MAP OF 100 YEAR Hs
Figure 11: 100 Year Hs – final contour map
Important note: the information displayed on this map is intended as a guide and should not be treated as a substitute for site specific study.
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Published by the Health and Safety Executive11/05
RR 392