X-Band Radar as a Tool to Determine Spectral and Single Wave Properties

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Deift University of Technology

Ship Hydromechanics Laboratory

Library

Mekelweg 2, 2628 CD Delfi

The Netherlands

Phone: +31 15 2786873 - Fax: ±31 15 2781836

X-BAND RADAR AS A TOOL TO DETERMINE SPECTRAL AND SINGLE WAVE PROPERTIES

Konstanze Reichert', Katrin Hessner', Jens Dannenberg', ma Tränkmann', Björn Lund'

Abstract: The ve nitoring System WaMoS II was developed for

real time measurements of directional ocean waves spectra to monitor the

sea state from fixed platforms in deep water, in coastal areas or from

moving vessels. The system is based on a standard marine X-Band radar

used for navigation and ship traffic control. WaMoS II digitises the

analogous radar signal and analyses the sea clutter information to obtain

directional wave spectra from the sea surface in real time even under

harsh weather conditions and during night. Spectral sea state parameters

such as significant wave height, peak wave period, and peak wave

direction both for wind sea and swell are derived. Within the EU funded

project MaxWave and the German project SinSee _{new algorithms were}

developed to determine sea surface elevation maps from radar images

which are used to investigate the spatial and temporal evolution of single

waves simultaneously. In this paper a short overview describes the

calculation of surface elevation maps and the detection of individual

waves. Considering two case studies, the results of spatial single wave

detection

and corresponding temporal

single wave properties are

compared and discussed. Individual wave parameters derived from radar

images are compared to individual

_{waves measured by a buoy. An}

application of the method to characterise extreme sea states is discussed.

INTRODUCTION

Sea state forecasts for the offshore industry

_{are based on spectral sea state}

parameters. Up to now, they give no information about the risk

_{to encounter}

extremes, which occur more frequently then previously assumed (Kjeldsen 1984; Haver and Anderson 2000). Therefore there is a need to identify additional wave

parameters that point at an increased risk for extreme waves and/or dangerous_seas.

In recent years X-Band radars were used to image ocean surface waves. Based on

standard marine X-Band radar the wave monitoring system WaMoS Il is proven to be a powerful tool to monitor spectral sea state parameters (Hessner et al. 2001).

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Within the EU funded project Max Wave and the German project SinSee strong

emphasis was put on the development of new analysis software that allows the

determination of single wave events from X-Band radar images. Main topic of the

MaxWave project was the investigation of extreme waves and their influence on

ships and off-shore structures. The ongoing German funded project SinSee is dealing

with the relation between ship movements and particular wave events. In both

projects WaMoS 11 sea surface elevation sequences were analysed to understand the spatial behaviour of single ocean surface waves. New algorithms were developed to

derive sea surface elevation maps which describe the spatial and temporal

development of single waves.

This paper presents two case studies. In the first study single waves are validated by

comparison with data retrieved from wave spectra. It is based on WaMoS II data acquired aboard the oil platform 2/4 k at Ekofisk in the North Sea for the storm

event May 29, 2001. The second case study gives an independent validation between

individual wave parameters derived from radar images and individual

waves

measured by a buoy. This data was acquired aboard the offshore wind test platform FINO in the Southern North Sea during a storm event from January 13-1 5, 2004. For both platforms the identification of extremes is discussed.

WAMOS II INSTALLATION EKOFISK

Since 1994 a WaMoS II is connected to the X-Band radar (Furuno _{FR1510) of} the platform 2/4 K in the Ekofisk oil field (56°33.9 N, 30_{12.4 E). The average water}

depth in this area is about 70 m. Every 2.55 s the radar receives a new image with a

spatial resolution of 8.4 m. Its field of view is a circular area around the antenna,

ranging from 240m to 2160m distance from the antenna. A single

standard WaMoS Il measurement consists of a sequence of 32 radar images.

For the

_{first case study a storm on May 29, 2001 was selected, as}

_{it is}

characterised by fast increasing wave heights and bimodalsea state.

4 5 5 7 R 10 11 C,, E

I

_Fi

350T TN ... 000 6:00 1200 1800

Time Hours of May 29. 2001)

j

4 D I D lU

-

WMoS U ...Buoy

Figure 1: Left: Location of the testing sites in the North Sea. Right: Statistical

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STATISTICAL PARAMETERS

Significant wave height (H,) and peak _{wave period (Tr) are the two most used}

parameters to describe the sea-state (WMO, 1998). These

_{parameters can be} determined directly from the WaMoS II directional wave spectra.

Figure 1 (left side) shows the location of Ekofisk, the right side of the figure

shows a time series of standard sea state parameters during the storm on May 29,

2001 as obtained by WaMoS 11 (marked black). The standard sea state parameters

from a reference buoy are plotted comparative to WaMoS II data (marked blue).

During the storm H., increased from

_{1 .5 m to 4.7 m at the peak of the storm}

(about 10: 10 UTC). In this time T1, increased from

_{about 6 s to 10 s, while the}

direction O, varied slightly between west and northwest. The measurements of the two sensors show a good agreement.

Spectral sea state parameters are not capable to characterise the spatial behaviour of individual ocean surface waves. For this purpose sea surface elevation maps have to be derived.

DETERMINATION OF SINGLE WAVES

To retrieve single waves several WaMoS Il image sequences were inverted into sea surface elevation maps applying a method proposed by Nieto et al. (2004). This

approach assumes shadowing as the main imaging mechanism of ocean waves in

nautical radar images and is based on linear wave theory. By application of a transfer

function and subsequent filtering the surface elevation for each point in the radar

image is calculated from the radar backscatter.

2 E o -1 -2 3--2 -1 °xlkm] 1 2 0 1 H.s = 47m Tp=9.7s Op285 ,.p 145m 1km .4.50 -225 0.00 2.25 4.50 surface elevation [mj 2 x[km] 3

Figure 2: Left: WaMoS II Radar image (Ekofisk,

_{May 29, 2001; 10.10 UTC).}

Right: Sea surface elevation map calculated by inversion of the image.

The left side of Figure 2 shows_{a WaMoS Il image obtained from Ekofisk with}

clearly visible sea clutter at the peak of the storm (May, 29, 2001, 10: 10 UTC). The

colour-coding corresponds to the image intensity which

_{is related to the radar}

backscatter strength. The waves are modulating

_{the radar backscatter and are}

represented in the image as striped patterns of high (bright, red) and low (dark, blue) image intensity. The lower right part of the image is blanked due to shadowing by platform constructions. The standard wave parameters as determined by WaMoS II are given, the peak wave propagation direction O is indicated by the grey arrow.

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A directional wave finding algorithm (DWFA) _{was developed to identif' single} waves in inverted radar images. Waves are detected in wave propagation direction (± 20°) in a distance of 4.5 times the peak_{wave length (Ai).}

The DWFA corresponds to the zero up-crossing method which is generally used for the wave height analysis of time series. The point between wave crest and trough at which the rising wave slope is crossing the mean sea level ( = 0) is defined as

zero up-crossing. The horizontal distance between two adjacent _{zero up-crossing}

points defines the wave period, while the vertical distance between the highest and

lowest point of two adjacent zero up-crossing points is defined as the zero

up-crossing wave height (H).

The right side of Figure 2 shows the sea surface elevation map for the radar

image on the left side. The wave crests are visible as red areas while wave troughs marked blue. The individual waves, wave crests and troughs as obtained by DWFA, are localised and indicated as lines with respect to the wave propagation. The white

lines are indicating up-crossing waves (crests and troughs in wave propagation

direction), black lines respectively down-crossing waves (against wave propagation).

The black box encloses the

_{area in which the highest wave was found. In this}

example N0 = 706 individual _{waves were determined by DWFA, with a maximal} wave height Of Hm = 9.0 rn. The significant wave height is calculated from the mean

of the upper third waves to H _{= 4.9 m whereas the mean wave length estimated to}

= 140 m. These values agree very well with the spectral

_{wave parameters,} H 4.7 m and A1, = 145 m measured by the standard WaMoS li.

Figure 3 (left) shows the surrounding of the maximum wave in the sea surface

elevation map. To prove the consistency of the results the spatial and temporal

evolution of the maximum wave are compared in Figure 3 (right). The upper panel shows the temporal transect at the point with the maximum surface elevation. The lower panel shows the spatial transect along the propagation line of the wave to the

time when the maximum wave occurred. Due to the characteristics of the radar

images, the spatial transect has

_{a much finer resolution (Ax = 8.4 m) than the} temporal transect (At = 2.55 s). besides this difference both transects show the same

characteristics. The individual wave period of Ti _{10.2 s and the wave length}

A = 126.6 m of the maximum wave were estimated from the plots. These values _are

in the same range as the spectral

_{wave parameters T1., = 9.7 s and A = 145 m} measured by the standard WaMoS II.

6 4 --4.. 6 5.ls 5.ls -2 -4: - -200 -100 0 100 200 o 100 _xImj dstance[m]

Figure 3: Left: Sea Surface elevation in the area with maximum wave height.

Right: Temporal and spatial transects in this area.

t - 2.55 4.4 rs 8.9 m -4.5 m 4OO 3OO .

2

44m - _{.. O}

W W

9.0 m -2OO

__

.4. 8.4 -4.6 rs s io 15 o time [s] -lo -5

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Hs = 38m Tp = 9.6s Up = 284 2p 130m 1km O xm 1 2

r

O 5 102 153 204 255

rader backscatter irtensty

Hs = 38m Tp =9es Op = 284° = 130m 1 km o -4.50 _2.25 _0.00 ₂₂₅ _4.50 surce elevation '11m)

Figure 4: Left: WaMoS II radar image (FINO, January, 14, 2004,00:15 UTC). Right: Sea surface elevation map calculated by inversion of the image.

VALIDATION OF SINGLE WAVE DETECTION

For the validation of the DWFA its results are compared to independent surface

elevation records from a buoy. At the WaMoS II station Ekofisk the closestwave

rider buoy is moored outside the radar range. Therefore data measured on board the

offshore wind test platform FINO was used. The station is placed in

an area of

about 30 m water depth in the Southern North Sea (54°0.86N, 6°35.26 E). Since

August 2003 the German Maritime and Hydrographic Agency (BSH) runs a

WaMoS II connected to the stations X-Band radar (Furuno FR 2125 B)as well as a

nearby wave rider buoy, which measures the sea surface elevation with a temporal resolution of At = 0.78 s. The buoy is moored within the field of view of WaMoS II.

A single WaMoS II measurement consists of 32 subsequent images with a spatial

resolution of 7.5 m and a temporal resolution of At = 2.5 s. For the validation a

storm event from January 13 to 15, 2004 was analysed. This storm was particularly

suitable for the validation because of an almost constant peak wave direction of

Northl North-West and its broad range of significant wave heights which increased from 1.5 up to about 4.0 m.

Figure 4 (left) shows a radar raw image with typical wave patterns from the

peak of the storm (January

14, 2004, 00:15 UTC). The range of the radar

measurement is about 2 km from the antenna. The lower left area of the image is

blanked due to shadowing by platform constructions. The wave parameters were

determined by WaMoS il. The sea surface elevations of the inverted radar images are compared with the according time series of the moored buoy, therefore the position of the buoy has to estimated. Only in calm sea conditions the exact buoy position is detectable in WaMoS 11 radar images by averaging several images to

localise its signature. In Figure 5 (left) a part of an averaged radar image at FINO is shown (January 13, 2004, 03:12 UTC; IL = 1.6 m). In this figure the signature of the buoy is clearly visible as an area of high (red) image intensity. It is located 275m off

the radar antenna. The cross labeled 'analysis' represents the reference point in the WaMoS II image, in 50 m distance to the buoy which is chosen for the further

comparison. This localisation is assumed to be valid also for heavy sea states when the high radar backscatter of the sea surface covers the buoy's signature.

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To identify the differences due to errors in localising the buoy the sea surface elevation at two WaM0S II measuring points are compared in Figure 5 (right). The

time series of a measuring cycle of 32 images at both locations is plotted in the upper panel. The black line represents the sea surface elevation at the determined

buoy position, the dotted grey one at

a position 50

m away. Due to the wave

propagation, the time series are shifted about half a wave period against each other. Besides this time shift the curves are in good agreement. Therefore a displacement of the buoy can be compensated by temporal shifting.

2 E 0 32 64_{radar bckctL intenty}96 128 159 191 22 Buoy WaM0S Il -200 0 200 x [ml

Figure 5: Left: Averaged set of radar images (FINO, January 14, 2004,

00.15 UTC). Right: Comparison of buoy measurements and WaMoS II data. Furthermore, the different sampling rates of buoy and WaMoS II have to be

matched by averaging the higher resolved buoy data to the

coarser WaMoS Il temporal resolution, as shown in Figure 5 _{(right) in the lower panel. The blue dotted}

line represents the buoy measurement with the original temporal resolution of At = 0.78 125 s. The light-blue line depicts the buoy measurement with adjusted

temporal resolution of At = 2.5 s.

The comparison of the sea surface elevation obtained by WaMoS li and the

buoy measurement are demonstrated in Figure 6 for the example from January 14,

2004, 00:15 UTC. The black line in the figure represents the WaMoS II data, the

blue line is related to the adjusted wave buoy data set. The general shape of the two

curves is rather similar. The periods are in good agreement, only the amplitudes

show smaller differences. This example demonstrates that the sea surface elevation of WaMoS II is comparable to buoy measurements.

i.5

os:

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0,0-. sea sUrface olovation

- at buoy position inaçJistanceof59m 0 20 tfs] 40 50 sea-suraóe ..evatíòn -

-:

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t0.78125s - .\t25sj 0 20 _ts) 40 60 60

sea surface elevation

20 40 60 80

t [s]

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INDIVIDUAL WAVES- HMAX Ills RATIO

A possible application of the single wave detection is the identification

of extremes. The common definition for extreme waves is based on the ratio of the

maximum wave height

_{and the significant wave height H, In time series}

extremes are identified by the condition H,,,uJ-L >2 (Kjeldsen, 1993 and 1997),

Ochi (1998) suggested a threshold of Hm,uI-f, > 2.15 _{- 2.35. This condition is not} proven to be valid for 2D wave fields. To examine the behaviour of this ratio in the 2D case, WaMoS II data from both stations Ekofisk and FINO were analysed.

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s 29!May,

11O

.2.56

I

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0:00

5:00

10:00

15:00

20:00

Time [Hours of May 29, 20011

Figure 7: Development of H (diamonds),

_{(dots) and HJllm. (lower panel)}

measured at Ekofisk.

Sea surface elevation maps of 64 data sets from the storm event at Ekofisk

(May, 29. 2001) were analysed. Here, _{is defined as the maximum individual} wave height in the sea surface elevation map, regardless of its location, whereas I-f was taken from the spectral parameters of the standard WaMoS 11 measurements. In Figure 7 the upper panel shows the time series of Hn,ax (red diamonds) and H(black

dots), the lower panel gives the ratio H,QX/HX. A vertical line in the upper panel

marks the time at which the maximum of H,na is found. The dotted line in the lower panel marks the threshold I-Ç,0.1/I-i = 2, the vertical line marks the location of the

maximum ratio H,nax/HS. The maximum individual wave height H0,, = 10.2 m was observed on May 29, 15:10 UTC. At that time H1 was measured with H1 = 4.0 m, yielding a ratio

of

H,,,,V/H.S = 2.55, whereas the maximum ratio

of

HII-J, = 8. 7rn /34m

= 2.56 was detected around one hour later on May 29,

16:15 UTC.

For the FINO platform, surface elevation maps for 26 sets of radar images from

the storm event (January 13/14, 2004) were analysed and compared to buoy

measurements. Figure 8 shows the time series Of Hm, (red diamonds), H(black dots)

and in the lower panel the ratio Hm,/H. The maximum individual wave height

7.1 m was observed on January 13, 23:02 UTC while the significant wave

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height of H, = 4.2 m was measured. This results in a ratio of Hm/Iis = 1 .69. The

maximum ratio was detected on January 13, 14:0 1 UTC with = 2.80

In addition to the parameters gained from radar images the buoy_{values of H,,} _(blue diamonds) and H, (black squares) are displayed in the upper panel of Figure 8. The blue squares in the lower panel show the ratio Of HmJH,. as obtained from the buoy

data. Compared to values of the radar images, the HZC,X/HV ratio_{calculated from buoy}

measurement is lower in most cases while the general appearance of the curves in the upper panel is similar for both measurement techniques.

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iDooDD

Go

₀₀

LP

gOo0

-.. D.D1DDDDD

S 0DJj

DOD

.

Time [Hours of January 13/14, 2004]

Figure 8: Development of H (buoy: triangles, WaMoS

_{II: diamonds), Hm}

(buoy: squares, WaMoS II: dots) at FINO; Lower panel: Ratio

_{HmU.,/HS (buoy:}

squares, WaMoS II: dots).

These examples show that in general the ratio _{H,,,,X/HS seems to be a useful}

parameter to indicate extreme wave events. But a threshold value of 2.0 to 2.3 leads to the detection of more extremes than in temporal wave records. Therefore, a more

detailed analysis needs to be carried out to understand the qualily of the extreme

wave event criterion when applied to 2D wave measurements. Validation and _{use of}

statistics for 2-dimensional data must be included in further_{considerations and}

investigations.

SUMMARY AND CONCLUSION

A statistical analysis of a large X-Band radar data set was presented with the aim

to identif' and validate individual wave events from nautical radar images. The

radar images were inverted to derive sea surface elevation maps applying the method of Nieto et aL, 2004. The new DWF algorithm identifies single waves in space and time and was applied to WaMoS Il data from the offshore platform Ekofisk and the offshore wind test platform FINO. The identification of maximum wave heights and extremes from spatial radar data sets is described.

These preliminary findings were verified with data from an in-situ sensor at the offshore wind test platform FINO. First comparisons of sea surface elevation maps derived from WaMoS Il to data of a wave buoy show a good agreement.

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The results of the individual wave detection were applied to the extreme wave event criterion: _{2. This investigation pointed out that further validation and} an extension of extreme statistics for 2-dimensional data is needed. The aim of this

further investigation is to receive a warning criterion for offshore platforms and

navigation.

ACKNOWLEDGMENTS

The research work presented here is partly supported by the EU Project

MaxWave, N°: EVK:3-2000-00544 and partly by the German Project SinSee FK.Z 035X145B. The Ekofisk data were kindly provided by ConocoPhillips and DNMI.

The data of the offshore wind test platform FINO were kindly provided by the

Federal Maritime and Hydrographic Agency (BSH).

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Evaluation of WaMoS H Wave Data. Proc. of the 4th _{International Symposium}

Waves 2001, September 2-6, 2001, San Francisco, _{CA, 221-230.}

Kjeldsen S.P. 1997: Examples of Heavy Weather Damages caused by Giant Waves. Bulletin of the Society ofNaval Architects ofJapan. Vol. 10, 24-28.

Kjeldsen, S.P., 1993: The Wave Follower Experiment. Proc. of Symposium on the Air-Sea Interface, Radio and Acoustic Sensing. Turbulence and Wave Dynamics,

Marseilles, France.

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12, No 4-16.

Nieto, J.C., Rodríguez, G.R., Hessner, K., and P.1. González, 2004: Inversion of Marine Radar Images for Surface Wave Analysis. Journal of Atmospheric and

Oceanic Technology, Vol.21, 1291-1300.

Ochi, M.K, 1998: Ocean Waves - The Stochastic Approach. Cambridge Ocean

Technology Series, Cambridge University Press, Cambridge, _{United Kingdom,}

1998.

WMO, 1998: Guide to wave analysis and forecasting.

_{World Meteorological} Organization, WMO-No.702.