Examining Variations in West of Shetlands Current Profile Shapes in Light of Regional Oceanography
Authors: Santala Markku 
Published: April 2012

The West of the Shetlands environment is an area where a significant amount ofhistorical exploration and development activity has taken place. It has beenstudied in fairly great detail from an oceanographic perspective due to thewater mass exchanges between the Atlantic and Nordic Seas which occur throughthe Faroe-Shetland Channel. The oceanographic literature tends to treat watermasses in different levels. Such treatment does not lead to much insight intothe characteristics of instantaneous profiles of interest for riser design. Also, while industry has significant experience and knowledge of the highcurrents along the continental slope, the various deepwater measurementscollected in the past have not been analyzed in a systematic way to gainunderstanding as to how current profile shapes change when moving from theslope into deeper waters. Recent oceanographic literature is reviewed with aview to summarizing how the knowledge of different water masses can be used toanticipate the qualitative changes that should be expected in current profilesas one moves from the slope into deeper waters. Data from three moorings inwater depths varying from 500m to over 1000m are then quantitatively analyzedwith trends in profile shapes compared to the expectations one would developfrom the oceanographic literature. The main analysis tool used in doing this isan EOF analysis method based on one proposed in the late '90s as a technique tohelp summarize data from current meter moorings as input to VIV analyses. Theprocedure used in this analysis is modified slightly from what would be useddirectly as input for a VIV analysis so that the results can be more clearlyused to interpret the underlying oceanography and yet to retain a result whichcan directly demonstrate changes in profile characterization withdepth/location relevant to offshore engineering. Deepwater currents West ofShetlands are quite strong and vary in character over a relatively shortdistance from the slope to deeper water. Understanding the extent of differentcurrent regimes and the type of conditions to be expected in each regime isimportant to the engineering of safe and efficient development concepts.

Introduction

The West of Shetlands environment is known to have among the most challengingwind, wave and current conditions in the world from the standpoint of offshoredevelopment. A general summary of conditions along the slope, where there is along history of development activity, has been presented by Grant (1995). Assites deeper into the Faroe-Shetland Channel (FSC) are considered the wind andwave conditions will be quite similar to those on the slope but currentconditions will change as different water masses and current regimes areencountered. Because current models have not been able to adequately describethe dynamic currents in the West of Shetlands area it has been necessary tomake use of measured data to develop criteria for deepwater prospects. Indeveloping measurement-based criteria one would like to make use of data fromnearby measurement sites to help improve the reliability of criteria values. But first, one must ensure that data used from nearby sites are representativeof the site of interest. Without an adequate knowledge of the oceanography of aregion and the spatial scales over which conditions vary, adopting data from" nearby" sites could actually introduce biases rather than improvereliability.