Abstract
Abstract WaveRadar REX (SAAB) instruments are widely used in the offshore oil and gas industry for wave, tide and air gap observations. This study extends the application of WaveRadar REX to the estimation of storm surge and reservoir subsidence from long-term water level measurements. AirGap data from several offshore fixed steel platforms offshore Borneo island (South China Sea) were analysed, yielding waves, astronomical tides, Sea Surface Anomalies (SLAs), which included storm surge and changes to water level due to long-term climate variability. Vertical platform movements, which can be attributed to reservoir subsidence and Vertical Land movements, were also derived. The storm surge estimates were consistent with values reported for the region, with positive storm surge (+0.3m) peaking in November and December, at the onset of winter western North Pacific Monsoon and northeasterly winds; negative storm surges (-0.2m) were recorded from March to July, coinciding with the boreal summer East Asian Summer Monsoon and southwesterly winds. Locations further offshore showed marginally lower storm surge compared to the coastal stations. Along the coast of Borneo, there was a shift in the timing of the positive storm surge arrival offshore Sabah, with establishment of positive storm surge shifted forward by about a month. Lastly, trends of SLAs derived from WaveRadar REX AirGap can be used to monitor long-term Vertical Land Movements and offshore platform subsidence. The comparison of AirGap subsidence estimates against independent dGPS measurements of vertical platform movement showed excellent agreement for several offshore platforms, all results within 0.05m accuracy. The new technique offers a cheap and reliable method of continuous vertical subsidence monitoring of fixed offshore platforms using instrumentation routinely installed offshore, thus offering an alternative to dGPS subsidence monitoring.
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