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Tow Route Studies

Study Approach

Typical Project

The client usually requires preliminary MetOcean environmental design criteria for tow route(s)  from their fabrication site(s) to their production site.

 These studies enable the client to decide:


Study Deliverables

The requirements vary depending upon the client's naval architects, designers and their insurers; but typically these studies provide:

Seasonal or monthly values are usually supplied for all of the above items.


History

MetOcean  has been providing tow route studies for 18 years. Initially historic ship observations were used, however, these were not always reliable due to data transmission errors. Additionally, with the advent of ship routing in the 1960,  vessels began to avoid areas of severe weather and ceased to report the extreme conditions.


MetOcean over the last 13 years have relied on meteorological wind models and ocean wave models to provide reliable data which is more spatially coherent.


Method

The client needs to identifie all possible routes from the fabrication site to the production site; Tropical cyclone criteria can be included for their design, or they may rely on ship routing to avoid tropical cyclones. 

An experienced meteorologist at MetOcean identifies a number of points along each route (as shown above) through which, the winds and waves can be considered to be homogeneous.

Data at each point is used in the analysis of the ambient conditions and the extreme analysis.

All points along each route are used in the calculation of the risk for each route.


Data Sources

Period Selection

To obtain reasonably reliable 10-year return period wind and wave values by month, it has been found that at least 7 to 8 years of modelled data is required and preferably 10 years. Too few extreme events are likely in any less number of years.

In the tropical regions it is important to try to ensure the period selected contains 'extremes' within the three Southern Oscillation index phases; 'El Nino',  'La Nina' and 'Neutral'.

Modelling

Non-Tropical Cyclone Modelling

MetOcean maintain a number of gridded wind and wave databases and wind and wave data for each grid point along the route is extracted.

If a tropical cyclone was located within a specified distance of a grid point the wind and wave data were excluded from the subsequent extreme and risk analyses for the ambient conditions.

These occurrences are noted and reported as an indication of possible wait on weather situations.

Tropical Cyclone Modelling

If tropical cyclone extreme conditions are to be considered, these are treated as a separate population and analysed separately. Tropical cyclone track data is held by MetOcean in a separate database.

These tracks and the 'Holland' tropical cyclone wind field model and a 2 nd Generation wave model, ADFA, are used to calculate extreme conditions at various grid points along the route.


Data Analysis

Extreme Analysis

The extreme analyses are conducted using the Conditional Weibull analysis technique. The 10 year return periods for both cyclonic and non-cyclonic conditions at each grid points for both winds and waves are provided where requested.

Wave Direction and Period

Correlations of the various ambient wave periods T m, Tp, and Tz and Hs are provided for each grid point and detailed directional information can be provided for both winds and waves.

Fatigue Analysis

Individual wave count by direction octant and for all directions combined can be supplied for the Fatigue analysis calculations.

Risk analysis

For each tow route, the risk of exceedence analyses can be conducted at levels of 0.01, 0.02 and 0.1 for various towing speeds (usually two).


Costs & Times

The cost of this type of study is dependant upon, the number of years modelled, the number of grid points, the number of routes and the number of towing speeds and can vary from US$20,000 to US$100000. The study may take 1 to 3 months to complete.