USE OF SATELLITE MEASUREMENTS FOR THE PREPARATION AND OPTIMISATION OF MARINE OPERATIONS : A CASE STUDY OF SEA-STATE TIME SERIES AND EVENT DURATIONS OFF NIGERIA

Demonstration Case Study (Ref: 345129)

I Name:

USE OF SATELLITE MEASUREMENTS FOR THE PREPARATION AND OPTIMISATION OF MARINE OPERATIONS : A CASE STUDY OF SEA-STATE TIME SERIES AND EVENT DURATIONS OFF NIGERIA

II Keywords:

Primary Keywords: Marine operations, Sea-state

Secondary Keywords: Waves, Site selection, Oceanography

III Summary:

The aim of the study performed for Bouygues OffShore was the determination of the best workable period between mid-May and end-June, for the achievement of an operation on Nigeria. This determination relied upon the elaboration of offshore statistics of significant wave heights (Hs) and above all upon statistics on events duration, according to an Hs operational threshold defined by Bouygues OffShore.
These latter statistics, for instance allowing the knowledge of the probable durations of stand-by or workable periods during the scheduled operation, relied upon the determinant construction of Hs time-seies offshore Nigeria.
Faced with the lack of field measurements, satellite based procedures now bring dramatically new possibilities for such studies , relying upon the construction of environmental parameter s time series.
In order to answer to Bouygues OffShore technical requirements, MétéoMer implemented an operational procedure, based upon the determinant use of altimeter significant wave heights (Hs) measurements provided by Geosat, Ers1 and Topex-Poseidon.

IV Description:

1 Customer Need:

The preparation and optimization of a marine operation is an essential prerequisite, regarding both technical and economical criteria.
One aspect of the preparation - and as a consequence the optimization - of marine operations corresponds with the definition of the best workable period, according to operational criteria, among which metocean conditions have a significant contribution. Regarding the environment criteria, a marine operator can define workable sea-states, beyond which the conditions become too critical to allow the beginning or the continuation of an operation ( the so-called stand-by period ). A metocean study for the preparation and optimization of marine operations can include the following steps :

• classical statitics of sea-states conditions, for instance on a monthly or lower basis, as a first criteria to select the best period, regarding significant wave heights ( and/or period, and /or direction ) workable thresholds,
• statistics on events duration, according to the same thresholds. For instance, these statistics allow the knowledge of the probable durations of stand-by or workable periods, according to the considered period, once started an operation. These statistics rely upon the construction of local temporal series of the required sea-states or winds parameters, which will be the basis of the events duration counting according to given thresholds,
• refined simulations of the different procedures steps, with their own respective thresholds, on these time series.

As an example of such operation preparation studies, the aim of the study performed for Bouygues Offshore was the determination of the best workable period between mid-may and end-June, for the achievement of an operation on Nigeria. This determination relied upon the elaboration of 15 days- period offshore statistics of significant wave heights ( Hs ) and of sea-states events durations according to an Hs operational threshold.

2 Method:

The process of the estimation of such operational statistics for a given location, be it offshore or near-shore, first relies upon the constitution of a database pertaining to the point of interest.

For this aim, field measurements are a very reliable source of data, and can be carried out on the precise operation location. 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 based procedures bring dramatically new possibilities for such studies of operation preparation and optimization. In order to answer to Bouygues Offshore technical requirements, and also time constaints ( a two weeks delay for the results provision ), MeteoMer implemented an operational procedure, 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, and 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 definition of a local climate coherent area, offshore the location study,
• Selection of the altimeter measurements available on this local climate coherent area. This area has a sufficient extension in order to ensure a satellite measurement repetitivity coherent with the final aim of temporal series construction,
• For every altimeter track - and as a consequence measurement time - in the study area, calculation of the corresponding Hs mean and standard deviation values. These values give an information about the sea-state spatial ( or temporal variability ) in the selected area,
• Temporal interpolation between the different altimeter measurements dates within the area, in order to get the final 6 hours step time series of offshore Hs, allowing the final calculation of event duration statistics according to Bouygues Offshore operational threshold.

3 Result:

Through this study, Bouygues Offshore was provided with a final report including a brief description of satellite measurements and procedures used. For every 2 weeks-period between mid-may and end-June, offshore Nigeria, MeteoMer drew the corresponding 6 hours - step Hs temporal series, and the resulting statistics of events durations , from 6 hours to several days, lower than an operational threshold defined by the customer.

Satellite measurements - Gulf of Guinea

This figure shows the altimeter ground tracks for Topex-Poseidon, on the gulf of Guinea.
This coverage permits the selection of altimeter measurements avalaible on a local climate coherent area with a sufficient extension in order to ensure a repetitivity coherent with the final aim of temporal series construction .

4 Deliverables:

MeteoMer supply a report incuding graphics of Hs 6 hours step time series, and tables of statistics of event durations over several thresholds.

5 Customer Opinion:

For the estimation of sea-state time series 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.

With this study, Bouygues Offshore benefited from three dramatic new aspects of metocean studies yeld 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 offshore Nigeria, and above all direct information about their temporal sequences, which allowed to Bouygues Offshore an efficient determination of the best workable period for the operation achievement,
• on an operational hand, MeteoMer developed some satellite measurements handling procedures, allowing the provision of these duration statistics in accordance with operational time constraints : the whole results and the final report were provided with a 2 weeks delay.
• on a financial hand, and considering the final aim of offshore sea-states duration estimations, satellite measurements instantaneously offered to Bouygues Offshore the equivalent of a several years duration field measurement campaign.

6 Cost / Benefit:

The total cost of this study was 5 KECU, VAT excluded, including data and report.

As a remark, this study was a first-level one, since its aim was rather simple : the estimation of sea-state duration statistics,offshore and only according to Hs thresholds.
Operational procedures exist in MeteoMer for more complex studies of preparation and optimization of marine operations. For instance, such basic time series can be determined on a precise offshore or near-shore location, and can include several simultaneous sea-states ( heights, periods, directions ) and winds parameters.

As a consequence, MeteoMer can answer to most marine engineers specific needs for preparation and optimization of marine operations, from the simple aforementioned study to near-shore events duration statistics and procedure simulations on simultaneous environmental time series. Corresponding studies costs vary from about 5 to 50 KECU, and delays from 1 to 6 weeks.
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 ( Aden Gulf, Guinea Gulf, North Sea, Mediterranean area,... ) based upon such use of satellite measurements, both for oil and harbours customers.

V References:

1. Customer References

   Customer

   Name: Mr Marc

   Company: BOUYGUES OFFSHORE

   Address: 3 rue Stephenson - 78180 Montigny le Bretonneux - 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