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News 9 January 2014

 

Monitoring our impact on the seabed offshore Tanzania

We are exploring in three blocks offshore Tanzania that cover an area of some 20,000 square kilometres. Within this area we have drilled 14 wells, all in line with Tanzanian regulatory requirements and our own HSSE standards.

The absolute environmental impact of our drilling in such a large area is limited, but our team wanted to understand better what was happening in the water column and on the seabed when wells were drilled. The aim was to increase our understanding, and so be able to mitigate any negative impacts of future drilling. But their work has succeeded in deepening our understanding of the entire sub-sea ecosystem.

Partnership with the National Oceanography Centre

BG Tanzania carried out Drilling Discharge Modelling to measure the impact of the campaign on the marine environment. Though the results showed that the environmental impact of the drilling campaign was likely to be insignificant, the team wanted to improve their understanding by taking samples from the seabed.

With this in mind, Dr Lodewijk Werre, BG Tanzania’s environmental manager, approached Dr Andrew Gates from the UK’s National Oceanography Centre. They set up a plan to collaborate under a project scheme known as the Scientific and Environmental ROV Partnership using Existing iNdustrial Technology – SERPENT for short.

This project allows marine biologists from the centre to operate the cutting-edge remotely operated vehicle (ROV) on board the drill ship, using it to explore the seabed and gain greater understanding of our operating environment.

Mapping the disturbance footprint

Dr Andrew Gates from the UKís National Oceanography CentreDr Gates joined the Deep-sea Metro 1 drill-ship at BG Tanzania’s Mzia, Ngisi and Pweza sites. His first task was to map the distribution of drill cuttings around the well. Using the ROV’s video camera to record the appearance of the seabed, transects are taken moving slowly away from the site. Typically, by the end of a 200 metre survey line, the seabed is undisturbed.

Dr Gates used the ROV’s to operate push cores to sample the top layers of the seabed. He took samples which were then measured for chemicals found in drilling mud. Sediment grain size and the amount of organic carbon on the sediment’s surface were also evaluated.

This data is currently being used by BG Tanzania to evaluate their drilling disturbance models and to provide insight for future impact mitigation strategies.    

“As a marine biologist this work is very exciting, with each dive we are exploring seabed that has never been seen before. ROVs are important tools for studying the deep sea but they are limited resources so it is great that we can make use of the industry infrastructure in this way. It is so important that we understand the impacts we have in deep water and this way we can make in-situ observations as the drilling is happening” 

Dr Andrew Gates UK National Oceanography Centre

Understanding the deep-sea ecosystem

In addition to Dr Gates’s research into the sediments, he evaluated the fauna which make up the deep-sea ecosystem in the Ruvuma basin.

In the warm surface waters there is no shortage of life. As the ROV team launched the Millennium ROV for a dive, they saw large dorado chasing smaller fish or schools of tuna on transit to the seabed. At first glance the seabed in water depths of over 1600 metres seems like a desert. Evidence of life is revealed only occasionally as a tiny jellyfish drifts past or when the vehicle comes across small burrows or tracks in the vast expanses of deep-sea mud. Moving away from the well, as the pool of light from the ROV illuminates small areas of seabed, a diverse community of animals gradually becomes clear.

Rocks provide a completely different habitat. They are covered with animals that feed on suspended particles or tiny animals in the water. Barnacles, starfish, sponges and anemones compete for the highest point with the strongest current flows.

Sea creatures