rr621 wave mapping in uk waters: supporting document [4.32mb]

Executive Health and Safety

Wave mapping in UK waters Supporting document

Prepared by PhysE Limited for the Health and Safety Executive 2008

RR621 Research Report

Executive Health and Safety

Wave mapping in UK waters Supporting document

Martin O Williams Metocean Advisor PhysE Limited 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 the Guidance Notes). 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 2008

First published 2008

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording or otherwise) without the prior written permission of the copyright owner.

Applications for reproduction should be made in writing to: Licensing Division, Her Majestys Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to [email protected]

ii

ACKNOWLEDGEMENTS

The author and editor gratefully acknowledge the valuable contribution made to this study by the following industry representatives:

Dr Colin K Grant Metocean Specialist /Deepwater Facilities

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

1.1 BACKGROUND..................................................................................................................... 1

2. DATA SOURCES ................................................................................................ 2

2.1 THE NESS, NEXT AND NEXTRA WAVE HINDCASTS .................................................... 2

2.2 VERIFICATION DATA .......................................................................................................... 5

2.3 ESTABLISHED CRITERIA ................................................................................................... 8

3. THE MAPPING PROCESS................................................................................ 10

3.1 DERIVATION OF EXTREME VALUES ............................................................................. 10

3.2 GRIDDING AND MAPPING ............................................................................................... 10

4. ASSESSMENT OF NEXTRA PERFORMANCE ................................................ 14

4.1 COMPARISON WITH MEASUREMENTS ........................................................................ 14

4.2 CONTOURS OF NEXTRA WAVE HEIGHTS.................................................................... 15

4.3 SELECTION OF OPTIMUM GRIDDING METHOD.......................................................... 17

5. CALIBRATION OF NEXTRA HS100 ................................................................... 18

5.1 CALIBRATION METHOD ................................................................................................... 18

5.2 CALIBRATION OPTIONS................................................................................................... 18

5.3 RESULTS OF THE CALIBRATION EXERCISE ............................................................... 20

6. THE FINAL MAP OF 100 YEAR HS.................................................................. 28

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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

: Locations of verification data sets .................................................................. 6Figure 4: Locations of established criteria ..................................................................... 8Figure 5

Figure 6: Contours of Hs100 from the software gridding options ................................... 12

Figure 7: Hs100 (m) measured verification data vs. closest unadjusted NEXTRA...... 14

Figure 8: Contours of unadjusted NEXTRA Hs100 (m) from Kriging gridding................ 16

Figure 9: Contours of unadjusted NEXTRA Hs100 (m) from Local Polynomial gridding 16

Figure 10: Tested calibrations for adjusting NEXTRA Hs100 ........................................ 20

Figure 11: Comparison of the potential NEXTRA Hs100 calibrations ............................ 20

Figure 12: Grid values of NEXTRA Hs100 (m) adjusted with the four calibrations......... 22

Figure 13: Contours of adjusted NEXTRA Hs100 (m) ................................................... 23

Figure 14: Adjusted NEXTRA Hs100 compared to established 100yr design criteria .... 24

Figure 15: Relationship between NEXTRA Hs50 and Hs100 (m) ................................... 24

Figure 16: Comparison of NEXTRA Hs50 contours (m) and those from OT 2001/010.. 25

Figure 17: Comparison of OT 2001/010 and NEXTRA Hs50 grid values (m)................ 26

Figure 18: Hs100 (m) the final contour map ............................................................... 28

TABLES

Table 1: NEXTRA metadata.......................................................................................... 3

Table 2: Verification data .............................................................................................. 7

Table 3: Established criteria.......................................................................................... 9

Table 4: Data gridding options .................................................................................... 11

Table 5: Hs100 (m) measured data vs. closest unadjusted NEXTRA......................... 14

Table 6: Verification and unadjusted NEXTRA Hs100 (m) ............................................ 19

Table 7: Adjustment of wave heights by application of the four calibrations ................ 21

Table 8: Tabulated comparison from Figure 17........................................................... 26

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EXECUTIVE SUMMARY

A contour map of 100 year extreme significant wave heights in UK waters has been produced by processing, interpolating and contouring 373 data sets from the NEXTRA model hindcast.

The work updates the contour map of 50 year extreme significant wave height that was provided in:

Department of Energy: Offshore installations: Guidance on design, construction and

certification (4th Edition, 1990).

HMSO Consolidated Edition 1993 (plus Amendment No. 3, 1995).

Following withdrawal in 1998, this document was republished as Offshore Technology Report OT2001/010, with the 50 year map reproduced therein.

The values of 100 year extreme significant wave height (Hs100) obtained directly from the NEXTRA data were found to be at variance with current industry criteria and with extreme values based on measured verification data sets. 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.

A precautionary approach was therefore adopted; a calibration was derived through the assessment of the NEXTRA Hs100 values against equivalent values from measured data. The resulting algorithm was applied to adjust the NEXTRA results to a level consistent with those indicated by the measured data, and it is from these adjusted values from that the contour map has been derived.

The validity of extrapolating the calibration curve beyond the range for which verification data are available is questionable, and therefore the region of the map in which Hs100 is shown to be over 18 metres has been blanked off.

The resulting contour map is presented below as Figure 1.

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Figure 1: 100 year extreme significant wave height (m) Important note: the information displayed on this map is intended to provide guidance and should not be treated as a substitute for site specific study.

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1. INTRODUCTION

1.1 BACKGROUND 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 and

certification (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 usermust 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. 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.

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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 spectral wave model. HYPAS.

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 the 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

Resolution

Duration

Data availability

Relevant Products

45.4N-72.4N, 21.2W-36.6E

30 km

1st March 1964 to 30th September 1998

(i) Winters only 1964 to 1977 (ii) Continuous 1977-79, 1985-1998 (iii) 40+ selected summer storms

Significant wave height archived at 3 hour intervals;

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.

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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 (e.g. the Southern North Sea) 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 and Buoy Measurements The measured verification data were obtained from either oil platform-mounted sensors or wave buoys including, in the Atlantic west of the UK, Met Office K-buoys. Except at Magnus the extreme values from the platform 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. 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.

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The same procedure was carried out on frequency distributions compiled from the K-buoy measurements, which were collected intermittently between 1984 and 2004. The locations of the measurements are illustrated in Figure 4 and the verification data are summarised in Table 2.

Figure 4: Locations of verification data sets

2.2.2 Satellite Observations Observations of wave height made by satellite altimeter were downloaded in frequency distribution format from an internet database1, for the areas shown in Figure 4. Each area covers 200 x 200 km2, except area 7 at 100 x 100 km2, chosen to ensure sufficient observations were included in the extreme value analysis. An extreme value of Hs in each area was derived by extrapolation of a 3-parameter Weibull distribution fitted to the upper 95% of the observations. The satellite areas are shown in Figure 4 and the extreme values given in Table 2.

1 www.waveclimate.com

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Table 2: Verification data

Group Name Individual I.D. Lat Long Period 50 yr Hs 100 yr Hs (m) (m)

K2 (62081) 51.0N 13.3W 91 to04 18.0 18.7 Atlantic K- K4 (62185) 54.5N 12.4W 92 to 04 18.4 19.2

Buoys K5 (64045) 59.1N 11.4W 84 to 94 17.8 18.5

Magnus 61.6N 1.3E 85 to 04 16.8 17.5

North Cormorant 61.2N 1.2E 83 to 98 16.2 16.8

Buchan 57.9N 0.04E 81 to 95 12.8 13.2

Forties 57.8N 0.9E 74 to 95 12.7 13.2

Auk 56.4N 2.1E 76 to 98 12.4 12.9

West Sole 53.7N 1.15E 72 to 90 8.4 8.7

K13 53.2N 3.2E 80 to 98 8.7 9.0

Leman 53.1N 2.2E 72 to 97 7.1 7.4

Sat Area 1 57.8N 1.0E 85 to 02 11.7 11.4

Sat Area 2 59.5N 3.0E 85 to 02 12.7 14.4

Sat Area 3 55.0N 0.0E 85 to 02 10.3 10.6

Sat Area 4 52.5N 3.0E 85 to 02 8.2 8.6

Sat Area 5 50.0N 1.0W 85 to 02 7.7 9.7

Sat Area 6 49.0N 6.3W 85 to 02 13.9 15.4

Sat Area 7 51.0N 7.0W 85 to 02 11.8 12.3

Sat Area 8 50.0N 12.0W 85 to 02 17.1 17.7

Sat Area 9 53.5N 13.0W 85 to 02 17.6 18.3

Sat Area 10 57.0N 12.0W 85 to 02 18.6 19.3

Sat Area 11 61.0N 11.0W 85 to 02 19.0 19.7

Sat Area 12 62.0N 1.0W 85 to 02 14.9 15.4

Sate

llite

Obs

erva

tions

Pl

atfo

rm M

easu

rem

ents

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2.3 ESTABLISHED CRITERIA

Established design criteria were cross referenced against the final map in order to define the degree of consistency between the two. The locations of these criteria are illustrated in Figure 5, the corresponding data being presented Table 3.

Figure 5: Locations of established criteria

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Table 3: Established criteria Location Lat N Long -W/E Supplier 50yr Hs (m) 100yr Hs (m) Northern North Sea Magnus 61.6 1.3 BP 15.7 16.4 Thistle 61.4 1.6 BP 15.3 15.9 N. Corm 61.2 1.2 BP 15.4 16.0 Rhum 60.1 1.8 BP 13.6 14.2 NW Hutton 61.1 1.3 BP 15.0 15.6 Brent A 61.0 1.7 Shell 15.0 15.6 Bruce 59.8 1.7 BP 13.6 14.2 Central North Sea Harding Miller Cyrus Andrew Goldeneye Buchan N Everest Fulmar Kittiwake Mungo Monan Marnock Lomond Anasuria Gannet Shearwater Ula Machar Curlew Auk Southern North Sea Tyne Ketch Cleeton West Sole Carrack Inde K13 Sean Leman Moray Firth Beatrice

59.3 58.7 58.2 58.1 58.0 57.9 57.8 57.5 57.5 57.4 57.3 57.3 57.3 57.2 57.2 57.1 57.2 57.1 56.7 56.4

54.5 54.1 54.1 53.8 53.6 53.4 53.2 53.2 53.1

58.1

1.5 1.4 1.4 1.4 -0.4 0.0 1.8 2.1 0.5 2.0 1.9 1.7 2.2 0.9 1.0 1.9 2.9 2.1 1.3 2.1

2.5 2.5 1.3 1.2 2.8 2.6 3.2 2.9 2.2

-3.1

BP BP BP BP Shell BP BP Shell Shell BP BP BP BP Shell Shell Shell BP BP Shell Shell

Shell Shell Shell Shell Shell Shell Shell Shell Shell

Encana

13.4 13.6 13.1 13.0 12.8 12.6 13.0 12.8 12.8 13.0 13.1 12.9 13.4 12.8 12.8 12.8 13.4 13.1 12.8 12.8

8.8 9.6 9.8 8.6 8.7 8.0 8.2 8.1 7.5

9.0

14.0 14.2 13.6 13.5 13.3 13.2 13.5 13.3 13.3 13.5 13.6 13.4 14.0 13.3 13.3 13.3 14.0 13.6 13.3 13.3

9.3 10.1 10.3 9.0 9.2 8.4 8.6 8.5 7.9

9.3 West of UK Clair Foinaven Morecambe N. Morecambe S.

60.7 60.3 54.0 53.9

-2.6 -4.3 -3.7 -3.6

BP BP HSE HSE

15.8 17.3 N/A N/A

16.6 18.0 8.7 7.4

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3. THE MAPPING PROCESS

3.1 DERIVATION OF EXTREME VALUES

3.1.1 NEXTRA

Values of 100 year significant wave height (Hs100) 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 analysis of the selected 373 NEXTRA grid points.

3.1.2 Verification Data Extreme Hs values were extracted from relevant entries in the appropriate criteria reference documents. Where possible the Hs100 values 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, extracted from design reports or as supplied by HSE.

3.2 GRIDDING AND MAPPING

3.2.1 Gridding Golden Software Inc, 2002. Surfer, Version 8, was selected as the preferred application for gridding and mapping.

Gridding is the process of creating a regularly spaced, rectangular grid of values from irregularly spaced input data. The grid is the base from which the contour map is created. 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 software offers twelve gridding algorithms, as summarised in Table 4.

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Table 4: Data gridding options

No. Gridding Method Comment

1 Inverse Distance to a Power Weighted average interpolator, either exact or smoothing. Produced noisy contours.

2 Kriging Well-proven v. flexible method, represents irregularly spaced data well. Expresses trends in data producing accurate grid. Can be either exact or smoothing interpolator. Default Surfer method as it generally provides best representation of data.

3 Minimum Curvature Widely used in earth sciences, generates smooth, elastic surface through each data point with minimum bending. Can introduce high magnitude artefacts in areas of no data. Here, produced noisy contours.

4 Modified Shepherds method Similar to inverse power. Produced spurious contours in some areas.

5 Natural Neighbour Complex polygon interpolator. Good for irregularly spaced data, but does not generate data in areas of no observations. Here, produced good representation of data but slightly noisy contours.

6 Nearest Neighbour Assigns value of nearest point to each grid node. Here, produced unacceptable results.

7 Polynomial Regression Defines large scale trends and patterns in data. Here, produced unacceptable results.

8 Radial Basis Functions Series of exact interpolation functions. Flexible and produces similar results to Kriging.

9 Triangulation with Linear Interpolation

Generates triangular faces between data points. Here, produced angular contours.

10 Moving Average Assigns values to grid nodes by averaging data within defined search ellipse. Here produced unacceptable results.

11 Data Metrics Used to provide information about gridded data on a node by node basis. Here produced unacceptable results.

12 Local Polynomial Assigns values to grid nodes by using weighted least squares fit to data within defined search ellipse. Here, generated smoothest, most natural contours.

Contours of Hs100 from each of the methods in Table 4 are reproduced in Figure 4.1, using the same numbering scheme.

11

12

1. Inverse Distance 2. Kriging 3. Min. Curvature

4. Modified Shepherds 5. Natural Neighbour 6. Nearest Neighbour

7. Polynomial Regression 8. Radial Basis Functions 9. Triangulation

10. Moving Average 11. Data Metrics 12. Local Polynomial

Figure 6: Contours of Hs100 from the software gridding options

Methods 1, 6, 7, 10 and 11 were rejected at the first pass and from the others method 2 (Kriging) and method 12 (local polynomial) were selected by experimentation as candidate methods for producing contours of Hs100:

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1. Kriging interpolation. Well-proven, flexible method, representing irregularly spaced data well. The algorithm expresses trends in the underlying data producing an accurate grid. Block Kriging estimates the average value of blocks centred on each grid node, the size and shape of which are the same as a grid cell. Since it is not estimating a value at a single point block Kriging is not an exact interpolator, but it was employed here as it does generate a smoother grid.

2. Local Polynomial interpolation. Assigns values to grid nodes by using a weighted least

squares fit to data within a defined search ellipse. For each grid node, the neighbouring data are identified by the user-specified sector search, the specification being the width of each sector the software searches for data. Using only the identified data, a local polynomial is fitted and the grid node value is set to this value. The polynomial can be order 1, 2 or 3.

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 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. Contour lines are drawn as a series of straight-line segments between adjacent grid nodes; a finer grid will therefore create smoother contours. A resolution of x = 50, y = 33 was found by experimentation to provide an appropriate level of detail for Kriging interpolation and x = 100, y = 66 for Local Polynomial interpolation. In both cases additional contour smoothing was necessary. 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.

3.2.4 Filtering and Smoothing The gridded data were filtered and smoothed prior to final presentation.

A low pass linear convolution filter was 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 NEXTRA PERFORMANCE

4.1 COMPARISON WITH MEASUREMENTS An assessment of NEXTRA model performance was carried out by comparing Hs100 from each verification data set with that obtained from the closest NEXTRA grid point. The wave height comparisons are plotted in Figure 6 and the corresponding values are given in Table 4.

NEXTRA/MEASURED VERIFICATION

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

16.0

17.0

18.0

19.0

20.0

Mag

nu

s

N.C

orm

oran

t

Bu

ch

an

Fo

rti

es

Au

k

West

So

le

K1

3

Lem

an

K2 (

62081)

K4 (

62185)

K5 (

64045)

10

0 y

ear H

s (

m)

60

70

80

90

100

110

120

130

%

Measured

NEXTRA

NEXTRA as % of measured

Figure 7: Hs100 (m) measured verification data vs. closest unadjusted NEXTRA

Table 5: Hs100 (m) measured data vs. closest unadjusted NEXTRA

Area Location NEXTRA GP Verification Hs100 (m)

Unadjusted NEXTRA Hs100 (m)

Distance from verification (km)

NEXTRA as % of verification

Magnus 14158 17.5 16.8 12.3 90 N. Cormorant 14267 16.8 15.4 11.2 91

Buchan 14894 13.2 12.8 13.1 102 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

UK

Wat

ers

Leman 15635 7.4 8.7 17.0 118 K2 buoy 16977 18.7 17.5 1.9 94

K4 16213 19.2 18.2 3.8 95

Atla

ntic

O

cean

K5 15294 18.5 18.3 13.3 99

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Comparison of the NEXTRA extreme values against the measured verification data reveals 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 15%.

In the Atlantic area west of the UK, NEXTRA extreme Hs values are less than the verification data by up to 6%.

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 as follows: Figure 8: Kriging gridding of the 373 NEXTRA grid points using a grid size of x = 50, y = 33; resulting in 1650 interpolated grid points. Figure 9: Local Polynomial gridding of the 373 NEXTRA grid points. The grid resolution was x = 100, y = 66 giving a total of 6600 interpolated grid nodes.

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Figure 8: Contours of unadjusted NEXTRA Hs100 (m) from Kriging gridding

Figure 9: Contours of unadjusted NEXTRA Hs100 (m) from Local Polynomial gridding

17

Cross checking the contours on the maps against the measured verification data shows: Values of Hs100 derived from NEXTRA data 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.

Values of Hs100 derived from NEXTRA data are high in the southern North Sea.

It was therefore concluded that calibration of the extreme NEXTRA wave heights was required prior to production of the final map.

4.3 SELECTION OF OPTIMUM GRIDDING METHOD As evidenced by Figures 8 and 9, interpolation by the Kriging and Local Polynomial gridding methods gives significantly different results. The Kriging grid (Figure 8) has retained the small scale variability in the unadjusted NEXTRA extreme values to a greater level than the Local Polynomial grid (Figure 9). In contrast, the contours from the Local Polynomial grid represent the smoother spatial transition of wave conditions that may be expected to occur naturally. Experimentation with Local Polynomial gridding of adjusted NEXTRA extreme values produced maps of smoothly varying Hs contours, which after some adjustment matched the verification data to an acceptable degree. However, the process had a number of weaknesses: Gridding the adjusted model data by Local Polynomial interpolation produced contours

that in some areas were inconsistent with spot values of adjusted NEXTRA Hs100.

Optimum contour positioning was only possible by applying two geographically dependent calibrations. Combining the grids was complex and resulted in a contrived wave map.

An additional, somewhat arbitrary adjustment of the calibrated wave heights was required to incorporate a degree of conservatism in the contours.

On the basis of this assessment, the wave height map based on the Local Polynomial gridding method was rejected in favour of the Kriging technique. The advantages of this approach are: A simple, more robust calibration was possible.

The contours more closely reflected spatial trends in underlying NEXTRA data.

In relation to the underlying grid of NEXTRA model data, the positioning of the contours was thus optimal.

Development of the optimised wave map using the Kriging technique is described in the following sections.

18

5. CALIBRATION OF NEXTRA Hs100 5.1 CALIBRATION METHOD Calibration of the unadjusted NEXTRA extreme wave heights was carried out in the following sequence:

1. Extract the Hs100 from the closest NEXTRA grid point to each verification data location or, for the satellite data, the average of unadjusted NEXTRA extreme values from grid points lying in each satellite area.

2. Plot these NEXTRA extreme values against the corresponding verification data.

3. Fit a curve to the plotted data, such that the equation of the curve describes the adjustment to be applied to the NEXTRA extreme values.

4. Apply the equation of the fitted curve to the 373 unadjusted NEXTRA extreme values.

5. Interpolate the adjusted NEXTRA extremes and contour the interpolated grid.

6. Cross-check the contours and underlying grid against the verification data and established design criteria.

5.2 CALIBRATION OPTIONS Previous attempts at mapping with Local Polynomial interpolation showed that a number of potential calibration options were possible. Since the satellite extreme values were based on data from a relatively wide area, they were expected to be at variance with nearby location specific measured verification data; the calibration exercise therefore included verification data sets with and without satellite extreme values. The following potential calibrations were tested: 1. All verification data - North Sea measurements, Atlantic K-Buoy and satellite areas 1-12

2. North Sea measurements and Atlantic K-Buoy measurements

3. North Sea measurements and satellite areas 1-4

4. North Sea measurements only

Table 6 summarises the verification data and the unadjusted NEXTRA values used in the calibration.

19

Table 6: Verification and unadjusted NEXTRA Hs100 (m)

Area Location NEXTRA GP Verification Hs100 (m)

Unadjusted NEXTRA Hs100 (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 91 Buchan 14894 13.2 12.8 13.1 102 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 Sat area 1 Average 12.2 11.4 N/A 93 Sat area 2 Average 13.2 14.4 N/A 109 Sat area 3 Average 10.8 10.6 N/A 98

Nor

th S

ea

Sat area 4 Average 8.5 8.6 N/A 101 Sat area 5 Average 8.0 9.7 N/A 121 Sat area 6 Average 14.4 15.4 N/A 107 Sat area 7 Average 12.3 14.7 N/A 119

Oth

er U

K

Wat

ers

Sat area 12 Average 15.4 16.1 N/A 104 K2 buoy 16977 18.7 17.5 1.9 94 K4 buoy 16213 19.2 18.2 3.8 95 K5 buoy 15294 18.5 18.3 13.3 99 Sat area 8 Average 17.1 17.7 N/A 100 Sat area 9 Average 18.3 17.9 N/A 98 Sat area 10 Average 19.3 18.3 N/A 95 A

tlant

ic O

cean

Sat area 11 Average 19.7 17.9 N/A 91

In order to optimise the positioning of the adjusted Hs100 contours in relation to the verification data and established criteria, the calibration requirements were: 1. To amend with a single calibration the unadjusted NEXTRA Hs100 values in areas where

offshore activities are prevalent.

2. To increase the unadjusted Hs100 values above 15m, to represent conditions in the Faroe-Shetland Channel and Northern North Sea.

3. To maintain unadjusted Hs100 values of around 14m, to represent conditions in the Central North Sea.

4. To decrease slightly Hs100 values below 13m, to represent conditions in the Southern North Sea.

An exponential curve was found by experimentation to be the most robust solution to these requirements. Other types of fit e.g., linear, logarithmic and power law were less successful in meeting all the specified requirements and hence were rejected. A quadratic curve fitted to the data in calibrations 1, 3 and 4 was of similar quality as the exponential fit, although the placement of the associated contours was sufficiently poor to reject this. It may have been possible to force a higher order polynomial to fit the data, but this would have resulted in an excessively complicated calibration and hence a contrived contour map. The curves from the four tested calibrations are given in Figure 10.

20

Calibration 1: North Sea measured, K-Buoy & satellite

verification data vs unadjusted NEXTRA

y = 3.8458e0.0887x

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21Unadjusted NEXTRA Hs100 (m)

Verif

icati

on

Hs

100 (

m)

Calibration 2: North Sea measured & K-Buoy verification data vs

unadjusted NEXTRA

y = 3.6063e0.0942x

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21Unadjusted NEXTRA Hs100 (m)

Verif

icati

on

Hs

100 (

m)

Calibration 3: North Sea measured & satellite areas 1-4

verification data vs unadjusted NEXTRA

y = 3.5709e0.0975x

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

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21

6 7 8 9 10 11 12 13 14 15 16 17 18 19Unadjusted NEXTRA Hs100 (m)

Verif

icati

on

Hs

100 (

m)

Calibration 4: North Sea measured verification data vs

unadjusted NEXTRA

y = 3.1153e0.1073x

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

6 7 8 9 10 11 12 13 14 15 16 17 18 19Unadjusted NEXTRA Hs100 (m)

Verif

icati

on

Hs

100 (

m)

Figure 10: Tested calibrations for adjusting NEXTRA Hs100 Note differing axis scales

5.3 RESULTS OF THE CALIBRATION EXERCISE

5.3.1 Calibration Selection To assess their individual merits, the four potential calibrations were applied to a range of wave heights between 5m and 18m, and the calibrated values plotted against their unadjusted counterparts (Figure 11). The adjusted wave heights are given in Table 7.

COMPARISON OF TESTED NEXTRA Hs100 CALIBRATIONS

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

7 8 9 10 11 12 13 14 15 16 17 18 19

Unadjusted wave height (m)

Ad

juste

d w

ave h

eig

ht

(m

)

Cal 1: North Sea measured, Atlantic K-Buoy & all satellite areas

Cal 2: North Sea measured & Atlantic K-Buoy

Cal 3: North Sea measured & satellite areas 1-4

Cal 4: North Sea measured only

Figure 11: Comparison of the potential NEXTRA Hs100 calibrations

21

Table 7: Adjustment of wave heights by application of the four calibrations Adjusted Wave Height (m)

Calibration 1 Calibration 2 Calibration 3 Calibration 4 Unadjusted Wave

Height (m) North Sea measured K-Buoys Satellite areas 1-12

North Sea measured K Buoys

North Sea measured Satellite areas 1-4 North Sea measured

8 7.8 7.7 7.8 7.4 10 9.3 9.3 9.5 9.1 12 11.1 11.2 11.5 11.3 14 13.3 13.5 14.0 14.0 16 15.9 16.3 17.0 17.3 18 19.0 19.7 20.7 21.5

Examination of Figure 11 and the data in Table 7 shows that: Calibrations 1, 2 and 3 gave comparable results up to a wave height of 13m. Wave heights

in the lower classes were adjusted downwards more by calibration 4.

For wave heights above 12m, calibrations 1 and 2 provided less upwards adjustment than calibrations 3 and 4.

Calibrations 3 and 4 maintained the unadjusted 14m wave height requirement.

Inclusion of the satellite data in the calibration had the effect of narrowing the range of adjusted wave heights.

To assist the calibration selection, the 373 NEXTRA Hs100 values were adjusted with each of the calibrations, gridded using the Kriging technique and compared to the verification data (Figure 12). For the platform measurements the closest gridded Hs100 to each was extracted from the grids; for comparison with the satellite extreme values the Hs100 of all grid nodes in a satellite area were averaged to give a single representative extreme wave height for that area. In Figure 12, data lying above the 1:1 reference line indicates that the calibration and gridding processes have resulted in adjusted extreme values higher than the verification data, and hence would incorporate an element of conservatism in the final mapped wave heights.

22

GRIDDED, ADJUSTED NEXTRA Hs100 / VERIFICATION

CALIBRATION 1

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6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Verification Hs100 (m)

Grid

Hs

100 (

m)


CALIBRATION 2

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6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22


Grid

Hs

100 (

m)


CALIBRATION 3

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Grid

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100 (

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CALIBRATION 4

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6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22


Grid

Hs

100 (

m)

Figure 12: Grid values of NEXTRA Hs100 (m) adjusted with the four calibrations and plotted against corresponding verification extreme values

From Figure 12: Calibrations 1 and 2 were rejected on the grounds that there was insufficient margin for

conservatism in the mapped wave heights in the Central North Sea, Northern North Sea and Atlantic regions.

Calibration 4 was rejected on the grounds that the mapped wave heights in the Faroe-Shetland Channel and Atlantic regions were excessively high in relation to the verification data.

Calibration 3 (North Sea measurements and satellite areas 1-4) was thus the preferred option, and the NEXTRA extreme values were accordingly adjusted by:

Adjusted Hs = 3.5709e0.0975[NEXTRA Hs]

Contours of adjusted Hs100 are shown in Figure 13. The individual (adjusted) extreme values upon which the map is based are also shown.

23

Figure 13: Contours of adjusted NEXTRA Hs100 (m)

5.3.2 Comparison of Adjusted Hs100 with Established Criteria As evidenced in Figure 14, the adjusted NEXTRA Hs100 data agree well with established design values throughout the region, with the majority of NEXTRA data lying close to or slightly above the 1:1 reference.

NEXTRA Hs100 adjusted with calibration 3

Kriging gridding Grid resn x = 50, y =33 Grid low pass filtered and spline

smoothed

24

GRIDDED, ADJUSTED NEXTRA Hs 100 / ESTABLISHED CRITERIA -

CALIBRATION 3

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Established 100 year criteria (m)

Gri

d H

s1

00 (

m)

`

Figure 14: Adjusted NEXTRA Hs100 (m) compared to

established 100 year design 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.

5.3.3 Comparison with OT 2001/010 50 Year Wave Map Contours of 50 year wave heights from the map published in OT 2001/0103 (see Section 1 for details) were digitised, such that they could be compared to equivalent contours from NEXTRA. Examination of the ratios of Hs50 to Hs100 from the 373 unadjusted NEXTRA grid points showed that the relationship was linear (Figure 15):

NEXTRA Hs 100 /NEXTRA Hs 50

y = 0.96x - 0.04

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

NEXTRA Hs 100 (m)

NE

XT

RA

Hs

50 (

m)

Figure 15: Relationship between NEXTRA Hs50 and Hs100 (m)

3 BOMEL Limited, 2002. OT 2001/010 Environmental Considerations. Prepared for the Health and Safety Executive by HSE Books. Published by HMSO. ISBN 0 7176 2379 3

25

The ratio between 50 year and 100 year verification wave heights was 0.96 at all locations except three, where it varied by 0.01. It was thus determined that 50 year wave heights may be derived from the 100 year map by:

NEXTRA Hs50 = 0.96*NEXTRA Hs100 The 373 adjusted NEXTRA Hs100 values were scaled according to this relationship, gridded and contoured. Figure 16 shows the Hs50 contours together with those from the OT 2001/010 map.

Figure 16: Comparison of NEXTRA Hs50 contours (m) and those from OT 2001/010

As expected from maps prepared by very different means, there is some disparity between the two sets of contours. Figure 17 plots 50 year values from the map above at a number of locations around the mapping domain, the NEXTRA values being extracted directly from the grid underpinning the contours. The comparison is tabulated in Table 8.

26

NEXTRA Hs 50 / OT 2001/010 Hs 50

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7

8

9

10

11

12

13

14

15

16

17

18

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20

21

50

.0N

,14

.0W

55

.0N

,11

.0W

60

.0N

,10

.0W

61

.0N

, 2.0

E

58

.0N

, 1.0

E

57

.5N

, 4.0

E

56

.5N

, 5.0

E

56

.0N

, 4.0

E

56

.0N

, 7.0

E

55

.5N

, 0.0

E

54

.5N

, 1.0

E

53

.5N

, 4.0

E

51

.5N

, 2.0

E

Hs (

m)

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

125

%

OT 2001/010

NEXTRA

NEXTRA as % of OT

Figure 17: Comparison of OT 2001/010 and NEXTRA Hs50 grid values (m)

Table 8: Tabulated comparison from Figure 17

Area Lat Long 50yr Hs (m)* NEXTRA 50yr Hs (m)

NEXTRA as % of OT

50.0N 14.0W 17.4 18.6 107

55.0N 11.0W 17.9 20.2 113 Atlantic

60.0N 10.0W 18.9 19.9 105

61.0N 2.0E 16 15.0 94

58.0N 1.0E 14 13.2 95 Northern North Sea 57.5N 4.0E 14 14.0 100

56.5N 5.0E 12 13.2 110

56.0N 4.0E 12 12.8 107

56.0N 7.0E 10 12.0 120 Central North Sea

55.5N 0.0E 12 9.7 81

54.5N 1.0E 10 9.9 99

53.5N 4.0E 10 10.0 100 Southern North Sea 51.5N 2.0E 8 6.7 84

* Values extracted from OT 2001/0104 and are the only public domain values

4 BOMEL Limited, 2002. OT 2001/010 Environmental Considerations. Prepared for the Health and Safety Executive by HSE Books. Published by HMSO. ISBN 0 7176 2379 3.

27

The difference in contour positioning between the two maps varies spatially, with the largest differences occurring in the Central North Sea.

Differences in the North Sea arise because:

1. The OT 2001/010 contours predict a much steeper wave height gradient on the western side.

2. The OT 2001/010 contours imply greater energy in the southern and Northern North Sea areas.

6. THE FINAL MAP OF 100 YEAR Hs Adjustment of the NEXTRA extreme values using a calibration based on North Sea data only has been shown to be the most appropriate approach. However, there remains concern that the Hs100 contours off the continental shelf west of the UK are inadequately verified, since the adjustment here is based on an extrapolation of the calibration curve beyond the range of the North Sea data. The large wave heights in this area (shown Figure 13) are a result of adjusting the NEXTRA data such that conditions in the Faroe-Shetland Channel and Northern North Sea oil fields are adequately represented. It is thus considered that the Hs100 contours of more than 18m are unreliable and for the final wave map they have therefore been replaced by a clearly marked area labelled >18m. The final 100 year contour map is presented in Figure 18.

Figure 18: Hs100 (m) the final contour map

Important note: the information displayed on this map is intended to provide guidance and should not be treated as a substitute for site specific study.

Published by the Health and Safety Executive 05/08

Wave mapping in UK watersSupporting document

Health and Safety Executive

RR621

www.hse.gov.uk

This work updates the contour map of 50-year extreme significant wave height that is provided in OT 2001/010 (previously Section 11 of the Guidance Notes). 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.

rr621 wave mapping in uk waters: supporting document [4.32mb]

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