Abstract
The oil and gas industry in Ghana is required to conduct environmental monitoring in offshore areas where exploration, development and production activities take place. Operations marine environment discharges include produced water, bilge water, ballast water, drill cuttings, drilling fluids, treated sewage, food waste, chemicals and others may impact the quality of the marine environment over a protracted time period. Periodic monitoring is conducted to assess the detectable temporal changes of the marine environment as a result of continuous discharges from offshore activities.
Monitoring data is meant to support environmental management activities, assist in meeting environmental regulatory requirements and assess temporal and spatial trends within the marine environment. Offshore environmental monitoring currently involves the study of the water column and sediment compartments seaward of the intertidal zone. Generally, the objective of offshore monitoring is to gather information to support responsible oil and gas operations for maintaining local environmental quality and ensuring ecological conditions do not suffer significant and irreparable deterioration.
To fulfil this Ghana EPA regulatory environmental monitoring requirement, a Jubilee Field monitoring survey was successfully conducted in September/October 2015 using the Bourbon Liberty 200 vessel to collect water and sediment samples at pre-plotted stations as per the statistically defensible survey design. TGL contracted CSA Ocean Sciences Inc. (CSA), a marine survey specialist company based in Stuart, Florida, USA to execute the project work scope i.e. survey design, planning, survey logistics mobilization field sampling and analysis and reporting. The project was executed with success on time, on budget and without incident.
The Monitoring Survey consisted of four sampling components: field-wide sampling, sampling at the FPSO site, sampling at reference stations, and assessment of drill cuttings distribution within the oil and gas development area. A grid pattern design with 23 uniform cell sizes was superimposed on the entire Jubilee Field. One fixed sediment/infauna/water column sampling station was located at the midpoint of each cell. In cells with existing infrastructure, 125 m buffers were built into the survey design to provide protection for infield subsea oil and gas infrastructure. To prevent and minimize potential impacts from survey related activities (e.g. deployment of sampling equipment), a full risk assessment was carried using a multidisciplinary team comprising professionals in Environment, Marine, Subsea Engineering, EHS, Logistics and Operations to identify hazards and risks associated with the detailed work scope and provide appropriate mitigation measures for the identified risks.
Water column and sediment sampling were conducted at 39 stations including 12 stations around the FPSO, 23 stations within the Jubilee Field, and 4 stations from the reference area. Sediment Profile Imaging (SPI) sampling was conducted to assess the presence and distribution of cuttings at four wellsite locations—i.e. two wellsites drilled before 2012 and two wellsites drilled after 2012. SPI sampling of these wellsites provided information on temporal changes and potential recovery of benthic conditions specific to mud/cuttings thickness, sediment redox, and benthic community status.
ALS Environmental laboratories in Kelso, Washington, USA, conducted all water and sediment analyses, except for the hydrocarbon analyses. Water and sediment hydrocarbon analyses were performed by TDI Brooks laboratories in the College Station, Texas, USA.
Metals and hydrocarbon concentrations were compared to the USEPA sediment quality benchmarks to assess levels that may have a potential to cause adverse ecological effects. The infaunal analysis involved taxonomic identification, the evaluation of the population densities and the assessment of possible interactions between biotic communities within the benthic zones of the field.
Findings of the Monitoring Survey indicate some differences in sediment concentrations of organics and various metals (i.e., barium, copper, mercury, lead, and zinc) among study areas. Development activities have affected the concentrations of some individual parameters, but there have been no detectable effects on the infaunal assemblage, which serves as a biological impact indicator. The Monitoring Survey indicate that the water column is relatively homogenous concerning measured parameter concentrations. There was no detectable toxicity associated with any elevated levels of sampling parameters in water or sediment. The results of the SPI sampling did not detect any adverse impact to the benthos in response to disturbance caused by the drilling operation or discharge of drilling muds.
