In the last decades off-shore industry has extended its field of activities in very deep waters up to more than 2000 m. The concurrent effects of different forcing terms, water column stratification and momentum transfer across the vertical levels results in rather complex current structure that should be properly captured (as an example to predict failure stress levels and fatigue damage to offshore risers).
Over the last decade Empirical Orthogonal Function (EOF) have been proposed as an analysis tools to identify mode of response of the water column and hopefully account for them in statistical extrapolation for extremes analysis.
The present paper proposes a simplified methodology to analyze deep water current profile extremes using Complex Empirical Orthogonal Functions (CEOF), thus keeping both the intensity and direction of the current response modes as module and phase of the orthogonal basis of the EOF decomposition. The extreme profile is evaluated by extrapolating the EOF extreme amplitudes by means of Inverse First Order Reliability Method (I-FORM), and combining with the associated eigenvectors (so called modes). The structure of the current profile is derived by i) a no-correlation and ii) correlation approach among principal modes. The directionality of the current is preserved by searching into the time series for contemporary occurrence of large values of marginal and dependent variables from the respective considered directions.
The proposed methodology was applied to hindcast data of a current profile (TAMOMS dataset) in the deep ocean region of the Mozambique Channel. The results reveals that CEOF technique is a successful application to derive extreme profiles.
The assessment of extreme current spanning the whole water column is necessary for the design of export riser connecting wells to the SSFU (Ship Shaped Floating Units) against extreme stress levels. The traditional approach used to derive design current profiles, i.e. to consider current velocities at each depth independently, is too conservative and physically unrealistic. At the same time the use of field data is impractical due to the large amounts of data to establish full-field joint distributions of current velocity at different depths.
The application of EOF techniques has been discussed among others by Forristall and Cooper, (1997), Jeans and Feld, (2001), Meling et al., (2002), Jeans et al., (2003). EOF are very effectively to capture the current profile coherent structures across the water column, however their physical interpretation is an hard task. Empirical modes do not necessarily correspond to true dynamical modes or modes of physical behavior.