The Impact of Tropical Cyclone Olivia on Australia's Northwest Shelf

Abstract

Abstract Tropical cyclone Olivia was the strongest storm of the 1995/96 season on Australia's Northwest Shelf. The storm caused significant damage to oil and gas facilities in the region. A broad spread of metocean measurements is available for this storm, enabling careful calibration of numerical simulations of associated winds, waves and currents. Together with tropical cyclone Orson (1989), this storm has led to a reassessment of metocean design criteria for the region. Introduction Tropical Cyclone Olivia was the most severe storm of the 1995/96 tropical cyclone season on Australia's Northwest Shelf. Wind gusts recorded on Barrow Island are believed to be the strongest ever measured anywhere in the world. Characteristics of the storm which contributed to its severity included:a central pressure as low as 925 hPa,a fast forward speed of up to 8 m s−1, andthe elliptical shape of the storm, contributing to elongated fetches. The storm was monitored by remote wind and wave buoys in the deep water off the Northwest Shelf, and by a comprehensive real-time metocean monitoring system (REMS) on North Rankin (NRA) platform. Winds were also measured on Barrow and Thevenard Islands. It is believed that waves generated by Olivia were responsible for damage incurred by the Campbell monopod in 40 m of water northeast of Barrow Island. Currents caused shifting of existing pipelines in the shallow water east of Barrow Island, and winds devastated facilities on Barrow and Varanus Islands. Data from the deep water wave buoys provides important information on wave spreading under severe storms. Wave profile data from North Rankin demonstrates the variability of maximum single wave to significant wave height ratios. The wind data indicate that very extreme gust factors may apply in the eye wall of some severe tropical cyclones. To provide answers to questions about actual storm loadings on various structures and pipelines, the winds, waves, currents and storm surge have been carefully modelled. This paper describes some of the more important aspects of the measured data obtained under tropical cyclone Olivia, and illustrates the results of the numerical simulation of the storm. Together with the recent occurrence of severe tropical cyclone Orson (April 1989), Olivia has caused a reassessment of design conditions on Australia's Northwest Shelf. The wind measurements on Barrow Island are separately addressed in a companion paper (Black et al.)1. Geographic Setting The southern portion of Australia's Northwest Shelf presently accommodates 24 marine oil and gas production facilities. The largest facility, North Rankin A platform, was also the first installed in 1984. It lies towards the northern periphery of the hydrocarbon domain, in 125 m of water. Further developments lie shoreward of North Rankin, and along the shelf break extending from North Rankin southwest to the Griffin Venture FPSO, in about 130 m of water. There are clusters of shallow water (40 m depth and less) developments to the northeast and south of Barrow Island.