An Analytical Study and Systematic Monitoring Procedure Developed for the Load-Out Operation of the North Rankin Jacket 'A'

Abstract

ABSTRACT The loadout of the 22,000 tonnes North Rankin Jacket 'A' onto a floating barge was successfully accomplished in April, 1982. During the loadout the barge ballast was continually adjusted to compensate for both jacket weight transfer onto the barge and full tide variation. The preparation for the loadout and the operation itself was characterized by newly developed integrated techniques. The techniques included:The development of a barge, jacket and quayside three-dimensional computer model to check the validity of conventional and simple ballast system software. The model was also used to evaluate the control parameters of the operation in a series of analyses which determine the sensitivity of critical steps of the operation to human or equipment errors.The development and operation of an integrated control system for jacket load transfer that relates jacket position to barge level and ballast pump requirements.The development and operation of a tide-expectation computer programme and associated ballast pump time scheduling software to compensate for differences between actual water level and that determined from standard tide tables, and to minimize the effect of short-term, local tide variations that are not forecast.The incorporation of fail-safe concepts and measures into the operation. INTRODUCTION In recent years the offshore search for hydrocarbons has extended into much deeper waters and hostile environments. The design of the considerably larger structures required to operate in these environments has become more comp1ex with emphasis during construction on total quality control for the primary elements of the permanent structure. In turn it has become essential to develop much more sophisticated analytical techniques and procedures for use during the construction, transport, and installation of the structures to ensure that their integrity is maintained until they are operational. A major risk to the structures is the stressing to which they are submitted during the hazardous loadout operation from land to ocean transport. This paper describes the techniques that were used to achieve a successful loadout of the North Rankin Jacket 'A'. The jacket is an eight-leg, barge launched steel tower weighing 22,000 tonnes at loadout. It measures 146.5m from the base of the legs to the top of the module support beams and has a plan dimension of 60m × 38m at the top and 83m × 6lm at its base. The jacket forms part of the first of two proposed for the development of the North Rankin Field, 135 kilometer of the northwest coast of Western Australia. The facilities are designed to handle a production rate of 1200 MMscfd of gas and associated condensate. The Operator for the development is Woodside Offshore Petroleum Pty. Ltd. Jacket 'A' was designed by Earl and Wright Consulting Engineers, and its fabrication was awarded to Nippon Kokan K.K. (NKK) in April 1980. In less than two years, the jacket was completed, loaded out onto the Heerema launch barge H-109, and transported to Western Australia, where it was successfully launched and installed in May 1982. The platform is scheduled to be on stream in 1984. Figure 1 shows the installed jacket and flare structure.