Typhoon Wind, Wave, And Current Directionality In China Sea
Authors: Driver D.B. , Borgman L.E. , Bole J.B. 
Published: October 2008

The paper describes the development of directional wind, wave, and current extremes for the Liuhua area in the northern South China Sea. Hindcast model results from nine contiguous grid points were used to produce univariate and associated extreme values for eight direction sectors, including average angle differences between coincident wind, wave, and current values. Empirical simulation was utilized to estimate confidence bounds for the extreme values.

The results show a strong directional bias, with the eastern sectors experiencing significantly more severe conditions than the western sectors. This asymmetry in the directions/ distribution of metocean extremes has been applied in the design of the two Liuhua mooring systems. It is believed that the use of directional information has provided a more realistic description of the environment that has resulted in a more thorough and efficient design than would have been achieved using the standard "omni-directional" approach.

INTRODUCTION

Since 1986, Amoco Production Research (APR) has worked to provide meteorological and oceanographic design and operational criteria for the Liuhua development in the South China Sea. The Liuhua site is located approximately 150 nautical miles east-southeast of Hong Kong in the Pearl River Basin. The water depth is about 305 meters. The area is relatively immature in terms of offshore activity and as a result there was little information available to derive required criteria. It was recognized early on that our understanding of the environment and our ability to develop the site would require a multi-year program involving extensive measurements and state-of-the-art numerical modeling work. This work, detailed in subsequent sections, evolved into a 5-year multinational effort that included 18 months of measurements and an in-depth wind, wave, and current hindcast study.

The Liuhua reservoir will be produced using a Floating P roduction S ystem (FPS) semisubmersible and a F loatingP roduction S torageand O ffloading (FPSO) tanker, each permanently moored by a multi-leg, wire rope/chain combination catenary mooring system (Figure 2). Traditionally, mooring systems for floating platforms have been designed assuming that the design event can approach from any direction. While this may be a valid assumption in many nstances, for areas exhibiting a clear bias in the directional distribution of extreme conditions, it can result in an over designed, overly expensive mooring system. Given the expense associated with the fabrication and installation of a deepwater mooring system, significant savings may be realized by taking advantage of the directional bias.

The climatology of the South China Sea is dominated by two principal weather systems: monsoons and tropical cyclones. Each is responsible for certain aspects of system design and operation and both have been studied in detail. The monsoons, with their predictable and persistent winds, generally define the day-today operational criteria. During the period October to March, the northeast monsoon prevails, with average wind speeds of 6–12 m/see. The southwest monsoon dominates during the period of May to August, but the accompanying winds are significantly weaker, with average speeds of 3-6 m/see.