Offshore Environmental Assessment

In 2000, Brunei Shell Petroleum commissioned an offshore EA to assess the effects of past drilling operations on the seabed.  The project emphasis was to collect high quality data on the effects of the various drilling muds and make management recommendations to minimise future environmental impact.  Baseline locations were also chosen to increase the available biological, physical and chemical knowledge on the offshore environment.  This was very useful because during the offshore fieldwork, a unique, previously unknown, deepwater reef environment was found. 

Survey sites were chosen in a modified x shape to correspond to the prevailing water currents and to include outer sites which were far from the impacted sites.  The outer sites at impacted locations and the baseline locations enabled assessments to be made of the environmental effects of offshore drilling.  A variety of impacted locations were chosen, in various water depths, each with different durations since drilling operations.  The impacted locations had been drilled with three mud types, oil based drilling muds, Petrofree ester based muds and water based muds.  The project was conducted over a three week fieldwork period and a variety of data collected. 

The physical and chemical data from undisturbed and previously drilled sites were compared and the data correlated to the biota found at each site.  Traditional grab samples were taken and these were compared with the data from a video transect at each site.  All the data was prepared on a Geographical Information System and maps prepared to present the data in its geographical context. (All maps shown are geo-referenced with North upwards.)




The underwater video was successfully deployed in water over 800m deep and collected the first known images of a Petrofree ester based drilling mud cuttings pile at a depth of 750m.  The video transect across the site showed the small size of the pile and the active bacterial decomposition of the cuttings.  

Other cuttings piles (shown right) showed increased levels of chemical pollutants, massive changes in redox and anoxic sediments.  The biological data corresponded well with the chemical data and significant differences were evident with increasing distance from the cuttings pile.

The footprint from drilling operations at individual wells was measured at nearly 1km 2 and as part of the management recommendations, the impact of all wells on the continental shelf was estimated and mapped.   The management recommendations were synthesised into a series of flow chart action plans, each applicable for drilling operations in different parts of the offshore environment.




Water current data like this is essential for responsible environmental impact reduction.  By drilling during the monsoon season shown in this example, sediment pollution would be kept away from sensitive areas to the south and west.



Example of water current during the Southwest monsoon (May - Sept)


This deepwater site had a well mixed surface layer with constant temperature and salinity.  From 85-90 metres both the temperature and the salinity change rapidly with the warm, less saline water and less dense surface water floating on top of the cooler, more salty and more dense deep water.

The importance of data like these for management is that fine sediments discarded by drilling operations above the thermocline will tend not to sink through the thermocline but will remain in the surface water and travel a long distance.  On the other hand, sediments discarded below the thermocline will sink to the bottom and travel much shorter distances. This concentration of slowly sinking particles is clearly demonstrated by the chlorophyll maxima; dead and dying microscopic plants (phytoplankton) rarely sink through the thermocline and so chlorophyll levels are much grater just above the thermocline.