USE OF SATELLITE BASED METHOD FOR THE DESIGN OF OFFSHORE STRUCTURES AND HARBOURS : A CASE STUDY ON CAPE VERDE ISLANDS

Demonstration Case Study (Ref: 345127)

I Name:

USE OF SATELLITE BASED METHOD FOR THE DESIGN OF OFFSHORE STRUCTURES AND HARBOURS : A CASE STUDY ON CAPE VERDE ISLANDS

II Keywords:

Primary Keywords: Structure design, Extreme conditions, Sea-state

Secondary Keywords: Oceanography, Waves, Wind

III Summary:

The aim of this study was to provide BCEOM with operational design sea-state parameters, offshore Fogo and Brava Cape Verde islands, for an harbour installaton project. The technical specifications were the determination of most probable extreme sea-states significant wave height (Hs) parameters, and associated periods and directions, on various project duration lives (ranging from 1 to 100 years), per typical meteorological sector.

For this aim, field measurements are a very reliable source of data. However, they are very expensive, and, in many cases, sufficient data, which usually amounts to several years duration, can not be accumulated in the short delays before a decision has to be taken.
Fortunately, satellite measurements can now provide an efficient and cost-effective alternative to part or all of these data gathering processes. In order to answer to BCEOM specifications for the offshore design sea-states determination offshore Cape Verde islands, MétéoMer implemented an operational procedure, here based upon the determinant use of altimeter significant wave heights (Hs) measurements, provided by Geosat, ERS1 and Topex-Poseidon, combined with metocean models.

IV Description:

1 Customer Need:

The aim of this study was to provide BCEOM with operational design sea-state parameters, offshore Fogo and Brava Cape Verde islands, for an harbour installation project. The technical specifications were the determination of most probable extreme sea-states significant wave height ( Hs ) parameters, and associated periods and directions, on various project duration lives ( ranging from 1 to 100 years ), per typical meteorological sector.

2 Method:

The process of the estimation of a design wave for a given location, be it offshore or nearshore, first rely upon the constitution of a database pertaining to the location of interest.
For this aim, satellite measurements can now provide an efficient and cost-effective alternative to part or all of these data gathering processes. In order to answer to BCEOM specifications for the offshore design sea-states determination offshore Cape Verde islands, MeteoMer implemented an operational procedure, here based upon the determinant use of altimeter significant wave heights ( Hs ) measurements, provided by Geosat ( from sept.86 to nov.89 ), ERS1( from dec.91 ) and Topex-Poseidon ( from august 92 ).
After application of specific correction formula and qualification processes, these measurements, as well as all other wind and sea-state satellite ones, are already stored in MeteoMer, on a world-wide scale. They are immediately accessible for metocean studies needs, through an internal dedicated archiving and interrogation system.

The achieved study included the following steps:

• Meteorological analysis of the area, resulting in the identification of the main storms direction sectors, and in the definition of a local climate coherent area, offshore the location study,
• Selection of the altimeter measurements available on this local climate coherent area,
• Due to the satellite measurements spatial and temporal samplings, one can realise that some storms may be missed on this climate coherent area. On following individual storms histories, one realises that most of them were however measured by the satellite, but not at the exact time when they were affecting the climate-consistent area. In order to enhance the Hs previous distribution, an ' influencing ' supplementary area is considered . Measured storms in this influencing area are individually identified and analysed. If they affect the climate-consistent area at a time when no satellite is over it, then the satellite information recorded over the influencing area is propagated onto the location of interest, and added to the database,
• Application of a high-level prediction procedure on the resulting local offshore Hs resulting database, and provision , for the different project duration lives,of the statistical distribution of the expected maximum Hs and thus associated probabilities, as well as the associated confidence intervals,
• Determination of mean and peak sea-states periods and directions associated with these Hs extreme values, through the hindcast of higher storms directional spectra on the area, from the combined use of metocean models and satellite measurements.

3 Result:

The results includes a brief description of satellite measurements and metocean models used, as well as procedures implemented, combining these data sources. A final synthesis table was given, including the following results for each offshore directional sector and project duration life : Most probable ( Hs ) with the 90% confidence intervals, the associated Mean and Peak periods and directions.

Satellite tracks over Cape-Verde Islands :

This figure shows the altimeter ground tracks for Topex-Poseidon for the area of interest.
It allows to select the Hs measurements available on the local climate coherent area and thus to determine the statistical distribution of the maximum Hs with the associated confidence intervals.

4 Deliverables:

Through this study, BCEOM was provided with a final report including tables given for each offshore directional sector and project duration life : Most probable ( Hs ) with the 90% confidence intervals, and the associated Mean and Peak periods and directions

5 Customer Opinion:

For the estimation of a design wave for a given location, field measurements are a very reliable source of data, and can be carried out on the precise location of interest, which is particularly important on complex coastal areas. However they are expensive and, in many cases, sufficient data (which usually amounts to several years duration) can not be accumulated in shorts delays before a decision has to be taken.
Thus, common practise is to construct the sea-state distributions from ship observations or hindcast models. However, these data sources show limitations for an extreme prediction purpose.
For the former, we may mention the uncertaincies related to visual observations of sea conditions and the poor statistical sampling due to the avoidance of stormy areas by the shipmasters. For the latter, when used alone and routinely, we can outeline that the modeling imperfections, particularly for storm conditions, may influence the results in a way that is difficult to quantify.

With this study, BCEOM benefited from three dramatic new aspects of metocean studies brought by the basic use of satellite measurements :

• on a technical hand, once applied some high-level analyses processing, satellite now offer reliable wind and sea-state measurements, with a world-wide, homogeneous and dense coverage of oceans. Here, satellite brought direct reliable sea states measurements on the Cape Verde Islands, which otherwise was a very poorly investigation area. Particularly, this study led to an important re-evaluation of the sea-states design parameters, that were formerly determined on a other location on these islands, through the use of ships observations. This study thus led to an increased reliability and confidence for the final design of harbours structures,
• on an operational hand, MeteoMer developed some satellite measurements handling procedures, allowing the provision of these elaborated engineering metocean parameters in accordance with operational time constraints : the whole results and the final report were provided with a 3 weeks delay,
• on a financial hand, and considering the final extreme prediction aim, satellite measurements instantaneously offered to BCEOM the equivalent of a several years duration field measurement campaign.

6 Cost / Benefit:

The total cost of this study was 18 kecu, VAT excluded, including data and report.

As a remark, this study aim was the determination of offshore sea-state extreme prediction. Operational procedures exist in MeteoMer for the design sea-states prediction on coastal areas. These procedure rely not only upon the previous use of Hs offshore altimeter measurement, but also benefit from the determinant construction of an associated offshore directional wave height spectra database, issued both from SAR in wave mode ( Synthetic Aperture Radar ) ERS1 inversed spectra and metocean models.

As a consequence, MeteoMer can answer to most marine engineers specific needs for design parameters, from the simple offshore annual extreme conditions to nearshore directional ones, through operational procedures which corresponding costs vary from about 6 to 50 kecu, and delays from 1 to 6 weeks.
The combined use of satellite data, metocean models and methods for the sea-state analysis and prediction, are implemented in procedures, which different steps can be adapted to the area conditions ( open or closed seas, offshore and nearshore metocean conditions, bathymetry,...).

Furthermore, the dense, homogeneous and world-wide coverage of satellite measurements enables the achievement of such studies for many otherwise poorly investigated locations anywhere on the oceans, with significantly lowered investments and delays, as well as improved ( results guarantee / cost ) ratio, compared to all classical data gathering processes.

MeteoMer achieved numerous similar studies all over the world ( Asia, Guinea Gulf, South America, North Sea, Mediterranean area,... ) based upon such use of satellite measurements, both for insurance, oil and harbours customers.

V References:

1. Customer References

   Customer

   Name: Mr Merrien

   Company: BCEOM

   Address: Place des Freres Montgolfier - 78286 Guyancourt cedex - France

2. Provider References

   Name: Mr Lasnier

   Company: MeteoMer

   Address: Quartier les Barestes - RN7 - 83480 Puget sur Argens - France

   Tel: +33 (0)4 94 45 66 11

   Fax: +33 (0)4 94 45 68 23

   E-mail: 4meteomer@meteomer.fr

   Web: www.meteomer.fr www.meteomer.com

3. Scientific References

   Name: Mr Lasnier

   Company: MeteoMer

   Address: Quartier les Barestes - RN7 - 83480 Puget sur Argens - France

   Tel: +33 (0)4 94 45 66 11

   Fax: +33 (0)4 94 45 68 23

   E-mail: 4meteomer@meteomer.fr

   Web: www.meteomer.fr www.meteomer.com