Wheatstone Reef Wave Transformation Studies
Authors: Santala Markku 
Published: May 2011

The Wheatstone central processing platform (CPP) is to be located on top of a submerged relic reef offshore Northwestern Australia in a water depth of 71m. This reef is a long narrow feature roughly 10km long but only about 1km wide (Fig 1). Existing wave hindcasts the project had access to had spatial resolutions of greater than 7km. As such, these models did not fully resolve the wave refractive effects of the reef and presented a biased view of the CPP site. While one approach to refining the hindcasts to be more representative of the CPP site to would be to fully re-hindcast the area with high resolution nested grid models; this paper describes alternate methods whereby wave transformation studies were used to establish extreme and operational wave criteria. The extreme wave climate on the reef is governed by tropical cyclones. In this case transformational mappings were applied to criteria developed along the slope. The aspect of the day-to-day wave climate of most immediate concern to installation planning was the occurrence of long period swells. For the operational criteria a transformation mapping was developed which could be applied on a time-step-by-time-step basis to a spectral archive point of the underlying hindcast. These differing approaches were driven both by the different types of governing storms and the different types of underlying hindcast products available. Using the transformation approaches described in the paper allowed derivation of results appropriate to the CPP site on a faster time schedule than would have been possible than if brute force re-hindcasts had been performed. This was a critical driver in under-taking this approach. Furthermore, since this approach required careful treatment of different aspects of the wave environment, it also allowed more insights to be gained into the wave transformation processes important in the region of the reef.

Introduction

The general meteorological conditions offshore Northwest Australia are governed by interaction between the southeast trade winds and monsoonal flows (Gentilli, 1971). Though shorter period local seas will vary in conjunction with the the seasonal changes in these winds, the long period wave energy of great concern for installation operations is dominated by the so-called Southern Ocean swells. These swells are generated by large storms in the Southern Indian Ocean and arrive from the southwest throughout the year. The left panel of Fig 1 shows the location of the CPP site on the Northwest Shelf of Australia. Owing its particular location and the predominant swell direction, swells at the CPP site differ from those offshore both due to the sheltering from the Northwest Cape and due to the refraction and shoaling effects characteristic of a modest depth site. Because swells are generated remotely, we can develop a transformational mapping to the site from deepwater considering just the sheltering, refraction and shoaling of long-period swell waves without reference to the local winds. This will allow us to develop the mapping using steady state wavefields which are numerically efficient to compute. To do this the mapping should start by considering the effects of swells somewhat to the west of the Northwest Cape to properly capture sheltering. Proper characterization of swells is critical since it is planned to install the CPP topsides with a float-over operation. Swell directionality is an important consideration in determining the orientation of the CPP base since it is desired that the installation barge will be aligned as nearly as possible with the predominant swell direction. Eight months of wave-rider buoy measurements from the site provided us with critical baseline data. A multi-year hindcast gives us insight into inter-annual variations in the operational swell environment. The goal of the work here was to reconcile these two sources of information to create a solid basis for operational planning.

Extremes are governed by tropical cyclones. Cyclones affect the region mainly from December to April and generally originate in convective areas of disturbance in latitudes 5oS to 15oS (Bureau of Meteorology, 2008). Though their paths show a high degree of variability, they tend to initially move west-southwest and then track more southerly as the pass through our region of interest (Fig. 2). With their clockwise rotation, the most severe side of the storm tends to be the left hand side and the result on the local slope is that the most severe cyclone generated waves tend to be towards southerly directions.