SATELLITE BASED METOCEAN STUDIES FOR PREPARATION AND OPTIMIZATION OF TOWED STRUCTURES TRANSPORTATION : THE NKP FLOATING UNIT CASE

Demonstration Case Study (Ref: 345113)

I Name:

SATELLITE BASED METOCEAN STUDIES FOR PREPARATION AND OPTIMIZATION OF TOWED STRUCTURES TRANSPORTATION : THE NKP FLOATING UNIT CASE

II Keywords:

Primary Keywords: Marine operation, Structure design,Sea state

Secondary Keywords: Extreme conditions, Waves, Wind

III Summary:

Through this metocean study, ELF required some operational sea-state statistics along an around 42 days transportation of the NKP floating unit case, from the Mediterranean location of Fos/Mer to the N'Kossa field in Congo. The final aims were to :

• define the best towing period, between January and April, through statistics on sea-states significant wave heights ( Hs ) conditions and on sea-states events durations according to Hs operational thresholds,
• define the most probable extreme Hs conditions along the routes, for a further design by ELF of the towing of the NKP floating unit.

Common practice is to construct these sea-state Hs distributions from ship observations or hindcast models. However, these data sources show limitations, particularly for the transportation design (extreme prediction purpose). Fortunately, satellite measurements now provide a dramatically more efficient and cost-effective alternative to these classical data sources gatherings.
In order to answer to ELF specifications for the NKP floating unit towing, MétéoMer implemented an operational procedure, here based upon the determinant use of altimeter significant wave heights ( Hs ) measurements, provided by satellites Geosat, ERS1 and Topex-Poseidon .

IV Description:

1 Customer Need:

The preparation and optimization of shipping routes and of towed structures transportation includes several aspects, among which :

• the definition of the optimum route ( following a whole set of technical and economical criteria ) : choice of a route and of a period , mainly guided by metocean inputs,
• the design of a vessel, or the design in transportation of a platform during its towing.

Through this metocean study, ELF required some operational sea-state statistics along an around 42 days transportation of the NKP floating unit case, from the Mediterranean location of Fos/Mer to the N'Kossa field in Congo. The final aims were to :

• define the best towing period, between January and april, through statistics on sea-states significant wave heights ( Hs ) conditions and on sea-states events durations according to Hs operational thresholds,
• define the most probable extreme Hs conditions along the routes, for a further design by ELF of the towing of the NKP floating unit.

2 Method:

Such platform towing preparation, optimization and design require the use of a reliable, here sea-state Hs database, along the route. Common practice is to construct these sea-state Hs distributions from ship observations or hindcast models. However, these data sources show limitations, particularly for the transportation design ( extreme prediction purpose). For the former, we may mention the uncertainties related to visual observations of sea conditions and the poor statistical sampling due to the avoidance of stormy areas by shipmasters; for the latter, when used alone and routinely for a database construction, we can outline that the modeling imperfections, particularly for storm conditions, may influence the results in a way that is difficult to quantify.

Fortunately, satellite measurements now provide a dramatically more efficient and cost-effective alternative to these classical data sources gatherings : they offer dense, homogeneous, world-wide, all-weather, reliable measurements of winds and sea-states conditions.

In order to answer to ELF specifications for the NKP floating unit towing, MeteoMer implemented the following 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 april 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 :

• division of the whole route in six climate coherent segments, covering the Mediterranean area, the North-Atlantic route, and the Guinea Gulf zone,
• definition of the temporal resolution of the transportation simulation : routes with a 2 weeks departure shift were studied, from January to april,
• for each considered global route between Fos/Mer and Congo - selection of the corresponding Hs altimeter measurements in every climate coherent sub-segment according to the crossing period of the towed platform in this area ; construction of corresponding Hs distributions and of the 6 hours step time- series in every segment, with a thorough analysis on storm measurements,
• for each considered global route between Fos/Mer and Congo , application of a high-level prediction procedure on every previous Hs distribution, and provision , for the different project duration lives,of the statistical distribution of the expected maximum Hs and thus associated exceedance probabilities, as well as the associated confidence intervals,
• determination of statistics on events duration met along each considered route, according to operational Hs thresholds defined by ELF.

3 Result:

ELF was finally provided with a final report, including a brief description of satellite measurements and procedures used, as well as the following graphics and results :

• computer graphics showing the different climate coherent segments along the global route,
• definition of the different studied routes, according to their departure times - with a 2 weeks shift - from January to April,
• for every route, and for each spatial segment along this route, provision of :
  • the Hs statistics ( histograms, mean and maximal values ),
  • the corresponding Hs extreme values, with associated 90 % confidence intervals and periods, on various project duration lives,
  • the corresponding statistics on events duration according to operational Hs thresholds,
• MeteoMer comments about the best transportation period and Hs towing design values to be selected.

Satellite tracks over the NKP route :

This figure shows the spatial coverage of the Topex-Poseidon altimeter for the Mediterranean part of the NKP route.
These altimeter measurements allows to build the Hs distributions and the time-series in every climate coherent sub-segment according to the crossing period of the towed plateform in this area.

4 Deliverables:

MeteoMer provides to ELF a report including tables of Hs statistics, extreme values and statistics on event durations for every segments of each studied routes.

5 Customer Opinion:

With this study, ELF 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, leading to study spatial scales much more narrow than those used with classical atlases.
  On one hand, the quality of Hs altimetric measurement, especially in storm conditions, is a prerequisite for a reliable estimation of design parameters.
  On the other hand, the density and homogeneity of satellite measurements allow the study of transportations with very refined temporal ( departure scenarii ) and spatial ( division into climate coherent segments ) resolutions. Particularly, this study led to an important re-evaluation of the sea-states design parameters, that were formerly determined through the use of ships observations. As a technical result, this study brought refined criteria for the choice of the best departure time, and led to an increased reliability and confidence for the final design of the NKP floating unit towing ,and as a consequence to a better definition of risks,
• 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 4 weeks delay,
• on a financial hand, and considering the final extreme prediction aim, satellite measurements instantaneously offered to ELF the equivalent of several years duration field measurement campaigns in many locations along the towing traject !

6 Cost / Benefit:

The total cost of this study was around 17 KECU, VAT excluded, including data and report.

As a remark, this study was a first-level one, as its aim was rather simple : the estimation of Hs sea-state statistics and extreme values along a route, and of sea-states events duration statistics only according to Hs thresholds. More complex studies can be achieved for the preparation and optimization of shipping routes - including the vessel design - and of towed structures transportation. For instance, basic time series can include several simultaneous sea-states ( heights, periods, directions ) and winds parameters. These procedures not only rely 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. Complex simulations can also be achieved on such time-series.

As a consequence, MeteoMer can answer to most marine engineers specific needs for preparation and optimization of shipping routes and towed structures transportation, from the simple aforementioned study to simulations on simultaneous environmental time series. Corresponding studies costs vary from about 6 to 50 KECU, and delays from 1 to 6 weeks, according to the transportation length and duration, and to the engineering specific requirements.

For metocean preparation and optimization of marine transportation, the combined use of satellite data, metocean models, as well as specific tools for events duration counting, procedures simulation and prediction of extreme values, are now implemented in procedures answering to most engineers specific needs.

Furthermore, the dense, homogeneous and world-wide coverage of satellite measurements enables the achievement of such route studies, which are typically on a global scale ( including several seas and oceans ), and thus including many otherwise poorly investigated locations. These studies can be carried out 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, based upon such use of satellite measurements, all over the world, both for industrial ( towed structures transportation ) and sailing racing customers.

V References:

1. Customer References

   Customer

   Name: Mme Lebas

   Company: ELF

   Address: 26 Av. des Lilas - 64018 Pau - 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