PT Pertamina Hulu Energi West Madura Offshore (PHE WMO) Offshore Drilling Rig is located near most populated island in the world (Java Island). It is important to control environment impacts from PHE WMO drill cutting and mud discharge to prevent pollution to surrounding area. The scope of this paper is to determine dispersion of Total Suspended Solid (TSS) and Total Petroleum Hydrocarbon (TPH) concentration in water column from discharged materials to sea. A modeling software is used to simulate TSS & TPH dispersion around the rig.

Data used in this study are sea bathymetry, wind, tides, waves, currents, temperature-salinity and drilling waste (drilling mud and cutting). Simulation software used is a 3-dimensional modeling with an approach based on cell-centered finite volume method including Hydrodynamics and Transport Module to simulate water current circulation and sediment contaminants when entering waters. The simulation duration is 15 days in four different seasonal conditions. Simulation scenario depends on drilling pipe casing size and discharge duration. The simulation result of TSS and TPH concentration in water is then compared to national regulation. Additional sediment depth simulation is also predicted to measure environment impacts.

PHE WMO drill plan to discharge (Option 1) 5686 m3 WBM Mud and Cutting or (Option 2) 1668 m3 SBM Cutting with discharge duration of 703 hours. This data is simulated with environment data obtained from Indonesia Geospatial Information Agency, Center for Hydro-Oceanography of the Indonesian Navy (PUSHIDROSAL), European Centre for Medium-Range Weather Forecasts (ECMWF) and PHE WMO internal data. Based on the results of the model simulation, it is concluded that TSS dispersion pattern of drill mud and cutting movement mostly follows the dominant current pattern, which is moving eastward (Dec-Feb), dominant to the west (Mar-Sep) and move west to southwest (Oct-Nov). The increase of TSS concentration in sea water could reach 32 mg/L within 50 m radius from rig. Outside radius 100 m, the TSS drop below 10-12 mg/L and reduced to below 1 mg/L at 500 m radius for all seasons. Referring to National TSS standard (20 mg/L), it is concluded PHE WMO drilling activity does not exceeds national TSS standard outside 100 m radius from rig and therefore still comply with national regulation. The TPH concentration from discharge activity only reach 0.018 mg/L) which is still below national TPH standard (0.02 mg/L). Additional sedimentation thickness at discharge point is only 1.6 cm additional thickness of sediment.

This study help to determine a guideline for environmental management and monitoring of drilling discharge activities of PHE WMO. This study is a crucial analysis to predict condition of after drill cutting and mud discharge to help company to mitigate impacts of discharge material which has potential to increasing TSS and TPH concentrations and additional sediment thickness in waters. Based on this study, it is concluded that PHE WMO drilling discharge activity has a minor impact to environment and still comply with Indonesian national regulation.

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