Offshore Environmental Assessment

Offshore Environmental Assessment

CHLOROPHYLL MAXIMA

Chlorophyll is the green pigment that is used by plants to produce sugars from sunlight. The microscopic phytoplankton are very variable and frequently occur in patches with sizes ranging from a few 100 metres to a few kilometres. Over a range of deepwater sites, the chlorophyll concentration reached a maximum just above the thermocline. The dead and dying microscopic plants (phytoplankton) sink slowly and become concentrated at the junction between the less dense surface water and the more dense deeper water.

INFAUNA IN THE SEDIMENTS

Infauna were assessed at each site using a Smith Macintyre grab. Samples were seived on site and then sorted and identified in the laboratory.

At this Oil based drilling mud site, biodiversity was still severely depleted twelve years after drilling operations. The central drill site location has reduced total numbers, (size of the circles) and reduced numbers of different organisms (different segments of pies).

Note the reduction in infaunal numbers is also seen in the direction of the prevailing current. The figure is actually a section of a GIS map with North upwards. Water currents were not measured at the time of drilling but because current data is known for the time of year we can be fairly certain that the oil based cuttings pollution spread from the central well site towards the Northeast. The biological data agrees well with the chemical data of total petroleum hydrocarbons and other pollutants in the sediments.

At a Petrofree site, biodiversity was severely depleted in the cuttings pile but little affected outside the area.

WATER CHEMISTRY

Water chemistry was measured with a Hydrolab and confirmed known trends in water quality.

Turbidity varied from inshore to offshore and muddy sediments in shallow waters were very turbid close to the seabed. Temperature, Oxygen, pH, and Redox were normal within the water column. These data are in contrast to the variations found in the sediment.

Salinity varied predictably from inshore to offshore.

ROXANN

RoxAnn is an innovative and unique hydro-acoustic processor which, when connected to a standard echo-sounder, will discriminate between seabed material types and output the data acquired in a quantitative format ready for computer analysis. The RoxAnn groundmaster, a portable system which can be transferred between vessels. was used for this project. The system consists of a Furuno DGPS (Differential Geographical Positioning System) and a Furuno echosounder with a pole mounted, over-the-side transducer. This requires no additional work through the hull of the vessel and may be operated at speeds of 15 knots in underwater surveying. In simple terms, RoxAnn compares the front and rear portions of the echosounder return signal to give electrical values to texture and hardness of the seabed. Unambiguous numerical RoxAnn data are objective and processed directly into colour-coded results against geographical position, profiled in either 2 or 3D.

Using powerful new RoxMap Scientific software for collection, display modeling, and mapping, data can be displayed in real-time, saved onto disk, or exported to Geographical Information Systems.

The results from the RoxAnn during this survey were groundtruthed with observations of seabed type from the grab, corer, and video and were found to be extremely accurate. There was also consistent repeatability of seabed characterisation outputs from RoxAnn when specific locations were revisited. An example is presented below.

Worldwide, RoxAnn has been successfully used in a wide variety of industries to discriminate many types of seabed including: corals; sea grasses; burnt oil residue; drilling cuttings; gravel; lobsters; patch reef; pipelines; sand; clay; mud; and rock. With the RoxAnn system for this project, we were able to identify a unique hard bottom area from a chance traverse made as the Armada Hydro traveled between stations.

RoxAnn was generally left on the whole time the Armada Hydro was steaming between stations. It was also used specifically as the first diagnostic approach around platforms, existing wellsites, and future wellsites. For platforms and existing wellsites, the first activity at the sampling location was completion of concentric rings with RoxAnn, ranging from 1-2 km from the platform or wellsite to about 100-200 m from the centre. At future wellsite locations, RoxAnn was used in a cross pattern centred on the site. The RoxAnn output on seabed type and water depth was then used to adjust the final position of sampling stations. RoxAnn was also used to track lander drifts and the towing of the epibenthic dredge.

DEEPWATER REEF

The Armada Hydro chanced upon a deepwater reef during a NE to SW station to station track. The RoxAnn clearly shows that the reef is relatively narrow (from about 115 to 135 m water depth) with typical muddy bottom both in shallower and deeper water.

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