Wind and wave frequency distributions for sites around the British Isles

HSE

Health & Safety Executive

Wind and wave frequency distributions for sites around the British Isles

Prepared by Fugro GEOS for the Health and Safety Executive

OFFSHORE TECHNOLOGY REPORT

2001/030

HSE

Health & Safety Executive

Wind and wave frequency distributions for sites around the British Isles

Fugro GEOS Southampton Oceanography Centre Empress Dock Southampton S014 32H United Kingdom

HSE BOOKS

© Crown copyright 2001 Applications for reproduction should be made in writing to: Copyright Unit, Her Majesty’s Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ First published 2001 ISBN 0 7176 2088 3 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.

This report is made available by the Health and Safety Executive as part of a series of reports of work which has been supported by funds provided by the Executive. Neither the Executive, nor the contractors concerned assume any liability for the reports nor do they necessarily reflect the views or policy of the Executive.

TABLE OF CONTENTS

ACKNOWLEDGEMENT.............................................................................................. 1 SUMMARY ................................................................................................................. 2 1

INTRODUCTION ............................................................................................. 4 1.1 USE OF THIS REPORT....................................................................... 4 1.2 DATA SOURCE ................................................................................... 4 1.3 FACTORS INFLUENCING THE WIND AND WAVE CLIMATE AROUND THE BRITISH ISLES............................................................ 7

2

GLOSSARY OF TERMS ................................................................................. 10 2.1 WIND PARAMETERS.......................................................................... 10 2.2 WAVE PARAMETERS......................................................................... 10

3

FREQUENCY DISTRIBUTIONS AND ROSES ................................................ 13 3.1 PERCENTAGE EXCEEDENCE DISTRIBUTIONS............................... 13 3.2 WIND AND WAVE FREQUENCY DISTRIBUTIONS AND ROSES BY DIRECTION ...................................................................... 15 3.3 WIND SPEED AND WAVE HEIGHT FREQUENCY DISTRIBUTIONS.................................................................................. 19 3.4 WAVE HEIGHT AND WAVE PERIOD FREQUENCY DISTRIBUTIONS.................................................................................. 21

4

BEAUFORT SCALE ........................................................................................ 23

5

REFERENCES ................................................................................................ 24

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TABLES AND FIGURES

Table 1 – NEXT Model Grid Points ...............................................................................6 Table 2 – NEXT Validation; Data Sources and Availability............................................7 Table 3 – NEXT Validation; Summary of Biases in NEXT Hs........................................7 Table 4 – Wind and Wave Characteristics around the British Isles ...............................8 Table 5 – Average Monthly and Annual Percentage Exceedence Distributions of Wind Speed .........................................................................................................14 Table 6 – Average Monthly and Annual Percentage Exceedence Distributions of Wave Height .........................................................................................................14 Table 7 – Beaufort Scale ............................................................................................23

Figure 1 – Chart Showing Location of NEXT Grid Points and Sub-regions....................3 Figure 2 – Marginal Distributions of Spectral Peak Period Around the British Isles........9 Figure 3 – Average Annual Joint Frequency Distribution of Wind Speed & Direction...17 Figure 4 – Average Annual Wind Speed & Direction Rose ..........................................17 Figure 5 – Average Annual Joint Frequency Distribution of Wave Height & Direction..18 Figure 6 – Average Annual Wave Height & Direction Rose .........................................18 Figure 7 – Average Annual Joint Frequency Distribution of Wave Height & Wind Speed ....................................................................................................................20 Figure 8 – Average Annual Joint Frequency Distribution of Wave Height & Peak Period ....................................................................................................................22

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ACKNOWLEDGEMENT The Health & Safety Executive would like to acknowledge the support of the following NEXT User Group participants:











BP/Amoco Exploration. Conoco Inc. Exxon Production and Research Co. Elf Petroland bv. Marathon Oil Company. Norsk Hydro. Rijkswaterstaat. Statoil. Shell UK Exploration and Production. Texaco.

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SUMMARY This report presents wind and wave frequency distributions and roses for forty sites around the British Isles. These have been produced using hindcast wind and wave time series data from the NEXT model, for the combined periods January 1977 to December 1979 and January 1989 to December 1994. The database has been subdivided into eight areas (Figure 1):









Hebrides Shelf West Shetland Shelf Northern North Sea Central North Sea Southern North Sea English Channel Celtic Sea Irish Sea

Seven grid points. Four grid points. Eight grid points. Eight grid points. Six grid points. Two grid points. Three grid points. Two grid points.

For each grid point, the following frequency distributions have been generated: • • • • • • • •

Average monthly and average annual percentage exceedence distributions of Mean Wind Speed (Example in Section 3.1:Table 5). Average monthly and average annual percentage exceedence distributions of Significant Wave Height (Example in Section 3.1: Table 6). Average annual joint frequency distribution of Mean Wind Speed and Direction (Example in Section 3.2: Figure 3). Average annual Mean Wind Speed and Direction rose (Example in Section 3.2: Figure 4). Average annual joint frequency distribution of Significant Wave Height and Dominant Wave Direction (Example in Section 3.2: Figure 5). Average annual Significant Wave Height and Dominant Wave Direction rose (Example in Section 3.2: Figure 6). Average annual joint frequency distribution of Significant Wave Height and Mean Wind Speed (Example in Section 3.3: Figure 7). Average annual joint frequency distribution of Significant Wave Height and Spectral Peak Period (Example in Section 3.4: Figure 8).

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Figure 1 – Chart Showing Location of NEXT Grid Points and Sub-regions

14212

14318

Northern North Sea

West Shetland Shelf 14533

14376

14703

60ºN Hebrides Shelf

14824

15000

14651

14541 14594

15354

15298

14773

15121

14832 15482

14715

15424

14897 14956

15194

15609

15138

15021

Central North Sea

15318 15143 15321

55ºN

15327

15447 15920

Southern North Sea

15571

Irish Sea

15631

15512

16230 15697 16462

16694

Celtic Sea 50ºN

16357

English Channel

16700 16991

10ºW

0º

5ºW

3

5ºE

1. INTRODUCTION 1.1

USE OF THIS REPORT

This report is designed as a decision making tool, primarily for those persons concerned with offshore operations. These persons may or may not have had formal training in meteorology or oceanography, yet may require a readily accessible independent view of the hindcast weather data for a variety of locations on the UK continental shelf. The report aims to satisfy an identified need for a document that could be readily used by operational and managerial personnel in the design, planning, implementation and verification of:



Logistical Operations - involving Supply Vessels, Helicopters, Cranes etc. Evacuation, Escape and Rescue arrangements - involving Standby Vessels, Fast Rescue Craft, Daughter Craft etc.

Notes concerning the use of the Tables and Plots with examples and basic calculations are given in Section 3. 1.2 1.2.1

DATA SOURCE The ‘NEXT’ Model

The North European Storm Study (NESS) was initiated in order to produce a high quality hindcast database of winds, waves, currents and water levels for the North European continental shelf (Peters et al., 1993). The wave model made use of a coarse (150 kilometres) grid for the North Atlantic and a fine (30 kilometres) grid for the North European shelf. The hydrodynamic model was operated using the coarse grid, nested within which was a hyperfine grid with a resolution of 10 kilometres. Wind fields were specified by the UK Meteorological Office (UKMO) and by the Norwegian Meteorological Institute (NMI). GKSS Forschungszentrum performed the wave modelling using a version of the spectral wave model, HYPAS. The hydrodynamic model, System 21, which was used to determine the tide and surge parameters, was developed by the Danish Hydraulic Institute. In addition, the wave model was run using a hyperfine (10 kilometres) grid for the Southern North Sea where the sea bed topography is highly variable. Wave parameters were archived for significant storms that occurred during the period October 1964 to March 1989. Advances in wave modelling improved the predictive capabilities considerably. Shortcomings in the first and second-generation wave models, developed during the 1960s and 1970s, were identified soon after and in some cases during development. These shortcomings are discussed in detail in the SWAMP (1985) wave-model intercomparison study. The SWAMP study concluded that none of the previously existing wave models were applicable for all wind fields and that none were reliable during

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extreme weather for which forecasts are most often needed. Despite the basic forcing mechanisms (the source functions) of wave generation being understood since the 1950s, first and second-generation models were always based on ad hoc assumptions to force the wave spectrum to some pre-conceived shape. The third-generation models compute the wave spectrum from first principles alone by solving the components of the source function (WAMDI Group, 1988). The source function for deep water may be thought of as a superposition of the wind input, non-linear transfer due to wave-wave resonance and a dissipation term caused by white capping and turbulence. Shallow water adjustments are also made to take into account loss of energy due to bottom friction and percolation. The response of the model for arbitrary wind fields is then determined solely by the structure of these source functions. The NESS extension (NEXT) was commissioned in order to make use of these advances in wave model technology and to incorporate the effects of severe storm events that have occurred since March 1989. As a result a hindcast database was generated using the NESS grids, a third generation wave model of the WAM type (WAMDI Group, 1988) and wind fields for the period 1964 to 1995. Wind fields for the period 1964 to 1989 were produced by the UKMO using a combination of UKMO and NMI pressure fields. For the six years post-1989, the wind fields were generated by Oceanweather Inc. (OWI) using pressure fields supplied by the National Oceanic and Atmospheric Administration (NOAA). The wave model makes use of the coarse and fine grids and the hydrodynamic model uses the fine and hyperfine grids. The wave modelling was performed by Oceanweather Inc.; the hydrodynamic modelling by the Danish Hydraulic Institute. The study produced wind, wave, current and water level parameters every hour for the following time periods: • • •

for the winters (October-March) from 10/1964 to 3/1995. for the summers (April-September) for 1977 - 1979 and 1989 – 1994. for significant storms that occurred during the months April - September.

Wind and wave parameters were obtained from the fine grid (30 kilometres) wave model archive held by Oceanweather Inc. For each model grid point there are a total of 179,308 records. The grid points are given in Table 1. The benefits accorded by a hindcast model, compared with measurements made using environmental monitoring systems, are as follows: •

Data are available for the entire eastern North Atlantic (grid resolution 150 kilometres) and the North-west European shelf (grid resolution 30 kilometres) within the area bounded by the model grid, whereas metocean measurements tend to originate from sites that are clustered around areas of industrial activity.

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•

Continuous data are available for thirty-one winters and nine summers, whereas data from environmental monitoring systems often contain gaps in the record, particularly during storm events.

•

The fine grid resolution increases the likelihood of obtaining data from a source close to the location of interest.

•

The data are not affected by uncertainties in instrumentation or in the sampling scheme used to measure the wind or wave climate.

However, one should bear in mind that the hindcast model data will reflect any uncertainties in the wind and pressure fields used to drive the model, the model bathymetry and the components of the numerical wave model itself. Furthermore, the model will not include geographic features with dimensions less than the grid spacing. Table 1 – NEXT Model Grid Points

1.2.2

NEXT Grid Point

Latitude

Longitude

NEXT Grid Point

Latitude

Longitude

14212 14318 14533 14376 14703 14824 15000 14651 14541 14594 15354 15121 15298 14773 14832 14715 15482 15424 14897 14956

61.507°N 61.373°N 60.604°N 60.513°N 60.191°N 59.579°N 59.399°N 59.389°N 59.362°N 59.300°N 58.854°N 58.788°N 58.546°N 58.473°N 58.404°N 58.378°N 57.692°N 57.579°N 57.398°N 57.336°N

0.942°E 0.670°W 2.400°W 0.863°E 4.226°W 4.315°W 6.991°W 0.713°W 1.225°E 0.475°E 12.218°W 7.021°W 9.604°W 0.008°W 0.733°W 1.142°E 9.799°W 8.209°W 1.064°E 0.362°E

15194 15609 15138 15021 15318 15143 15321 15327 15447 15920 15571 15631 15512 15697 16230 16462 16694 16357 16700 16991

56.748°N 56.609°N 56.361°N 56.319°N 55.850°N 55.565°N 55.391°N 54.438°N 54.332°N 53.741°N 53.587°N 53.529°N 53.480°N 52.528°N 52.503°N 51.455°N 50.537°N 50.413°N 49.773°N 49.504°N

1.583°W 7.996°W 0.311°E 2.064°E 0.911°W 2.250°E 0.263°E 2.513°E 1.229°E 3.805°W 1.422°E 0.797°E 2.405°E 2.304°E 5.524°W 5.986°W 6.783°W 1.650°W 4.657°W 8.078°W

Validation of the NEXT Model against Measured Data

In May 2000 Fugro GEOS first published the results of a study commissioned by the Health & Safety Executive which compared measured estimates of Hs with those from the NEXT numerical model for three areas of the North Sea. Spatial information on the validity of the model was also provided together with information on the validity of the NEXT data for the combined period 1977-1979 and 1989-1995. In addition, the report presents the results of an investigation into changes in the NEXT data that have occurred over time due to changes in the specification of the wind fields. The data sources used for the validation study are shown in Table 2.

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Table 2 – NEXT Validation; Data Sources and Availability Area

Data Source Measured

Period of Simultaneous Data

Southern

K13 & NEXT 15514

28 May 1977-31 Dec 1979 & 1 Apr 1989-31 Mar 1995

Central

Auk & NEXT 15021

1 Jan 1977-31 July 1979 & 1 Apr 1989-31 Mar 1995

Northern

Brent & NEXT 14268 North Cormorant & NEXT 14267

1 Jan 1977-31 Dec 1979 1 Dec 1990-1 Oct 1994

This study has shown significant differences between the measured wave data and the data from the NEXT model for all three areas considered. Table 3 summarises the biases revealed by the omni-directional analysis performed using the combined period 1977-79 and 1989-95. Table 3 – NEXT Validation; Summary of Biases in NEXT Hs Area

Height Band

Combined Period 1977-79 & 1989-95

Southern

Storm Peaks Higher (3-5m) Intermediate (1-2.9m) Lower (