The fate of a Maui condensate spill

Abstract

Natural gas and condensate are produced from the Maui A platform some 30 kilometres off the west coast of the North Island of New Zealand. As part of a study of engineering and environmental aspects of the offshore operation, 'The Fate of a Maui Condensate Spill' is an attempt to provide an informed prediction of the fate of a condensate spill, with an emphasis on the lifetime of a condensate slick and the possible impact of a condensate slick on the New Zealand coastline. Literature on the movement of oil slicks suggests that with a 40 knot onshore wind, it is possible for a condensate slick originating from the Maui A platform to reach the Taranaki shoreline within l0 to l2 hours. A review of literature on oil spills revealed that evaporation of the volatile components of condensate would be the principal initial weathering mechanism. However, the rate of the condensate evaporation could not be estimated. The present preferred method for quantifying evaporation rates in oil spi11s is the evaporative exposure approach suggested by Stiver and Mackay (1984). In this approach the evaporative behaviour of an oil is related to the amount of evaporative exposure an oil is exposed to. Small scale laboratory wind tunnels were used to obtain the evaporative behaviour of a sample of Maui condensate. The evaporation of some individual condensate components were measured to obtain representative gas phase mass transfer coefficients for the wind tunnels. The gas exchange behaviour of some of the Maui condensate hydrocarbon components was also measured using radiolabeled tracer techniques. Gas exchange is related to the half-life of residence of the dissolved hydrocarbons, allowing the subsurface fate of Maui condensate to be estimated. The evaporative exposure method suggested that about 80% of the condensate would be removed by evaporation from a surface slick in the first hour following an accidental discharge. Only hydrocarbons heavier and less volatile than the C9, C10 alkanes would remain in liquid form. Under 40 knot winds it is unlikely that a condensate slick would travel more than 4 or 5 kilometres before most of the condensate had evaporated. Within 10 to l2 hours, only those hydrocarbons less volatile than C14 alkanes would remain in liquid form. Even at lower wind speeds, dispersion due to wind and wave activity would probably ensure that no coherent slicks or even large patches of condensate would reach the New Zealand coastline from an accidental spill at the Maui A platform. It is also unlikely for mousse to be formed during a condensate spill. Dissolved and dispersed hydrocarbon plumes in the water column would probably have a maximum half-life of residence of 1 to 2 days. The speed and direction of the subsurface ocean currents in the offshore Maui region would determine the area and volume of the subsurface impact and contamination. In winter, or following discharges from the wellhead or underwater pipeline, half-lives of 3 to perhaps 7 days could be expected. These conclusions are only relevant to condensates with physical and chemical characteristics that are similar to those of the Maui condensate sample. If the composition of the Maui condensate being produced at present changes or if new condensates or oils are brought into production, then the evaporative behaviour of these hydrocarbon mixtures must be determined in order to update or modify the predictions of this study.