Environmental Engineering - Sable Offshore Energy Project - Water quality modeling

Prior to beginning trenching and hydrotesting operations, the pipeline must be flooded with chemically treated seawater to preserve the pipe from interior corrosion. A water quality study was carried out to determine the effects of releasing the water and liquid chemicals (a corrosion inhibitor, an oxygen scavenger, biocide and fluorescent dye) from the pipeline.

Near-field modeling of the initial jet plume, buoyancy and mixing processes indicated that the chemicals remain within 11 m of the surface after release, but mixing may occur after the first 1 or 2 km. Far-field modeling was carried out using a depth-averaged finite element program to simulate subsequent spreading and movement of the chemical constituents in the water column, due to diffusion and advection by ocean currents.

Finite element models were set up using coastlines and bathymetry near the Venture, Thebaud and North Triumph platforms. The hydrodynamic flows driving the simulations are hourly current fields, consisting of a uniform residual current superimposed on the tidal currents (M2, K1, O1, N2 and S2 tidal constituents). The mass flow rate for each chemical is based on the assumption that the water is released over a four-day period. The vertically integrated nonlinear finite element model solves the convection-diffusion equation to determine the concentration of each constituent. Contour plots of constituent concentrations for dissolved oxygen, biocide and fluorescent dye indicate that it is possible to meet the ‘No Observed Effect Concentration’ values at a distance well within the 500 m compliance distance at all three platforms.