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.

This paper describes spatial and seasonal variability of metocean design criteria in the southern South China Sea. Non-stationary extreme value analysis was performed using the CEVA approach (Covariate Extreme Value Analysis,[1]) for a 59-year long SEAFINE hindcast of winds and waves, estimating metocean design criteria up to 10,000-year return period. Wind design criteria are mostly driven by large-scale monsoonal events; at higher return periods infrequent cyclonic events have strong influence on the tail of the extreme value distribution but confined to a limited geographical area. The CEVA analysis of waves showed much less dependence on the tropical cyclone events; the spatial metocean design criteria were smoother, mostly influenced by the monsoonal wind strength, fetch and local bathymetry. Return value estimates illustrate the strong seasonality of metocean design criteria, with boreal winter (December-February, Northeasterly monsoon) contributing most to the extremes, while April and May are the mildest months. Estimates for the ratio of 10,000/100-year return values are also presented, both for winds and waves. There is empirical evidence that the range of “typical” values of generalised Pareto shape parameter observed for Hs is different to that observed for wind speed. For this reason, an upper bound of +0.2 for generalised Pareto shape was specified for wind speed analysis, compared to 0.0 for Hs. In some cases, increase of upper bound for waves to 0.1 is justified, leading to slightly more conservative Hs values. We confirmed that the upper end point constraint was not too influential on the distributions of generalised Pareto shape parameter estimated. Nevertheless, it is apparent that specification of bounds for generalised Pareto shape is a critical, but problematic choice in metocean applications.

Response Based Analysis (RBA) aims at prediction of the long term distributions of critical responses such as motions, accelerations, wave loads, which have significant impact on the design of a floating system. As compared with the conventional analysis, which predicts the responses of the facility to the N-year return period metocean conditions, RBA provides directly the N-year return period responses by analysing the statistics of their long term time histories. Another outcome of RBA is the Design Metocean Conditions (DMCs) which are combinations of sea state, wind and current causing the corresponding N-year response. The knowledge of the DMCs enables the more detail time domain analysis and model tests to be performed for a set of critical combinations of metocean parameters. It also enables all the associated responses to be determined. The RBA framework is generally well addressed in the literature, but the DMC identification methods are not necessarily clearly established. The objective of this paper is to present the theoretical background, numerical method and an example for the determination of the N-year return period responses and the associated DMCs for a floating facility. The method includes prediction of the N-year response, identification of the metocean combinations within the available time history which produce this response, determination of the required percentile of the short term response to match the N-year response, and the search for the most probable DMC within the joint probability density of the metocean parameters. Features of the method are discussed and results are presented for several critical responses of a weather vaning vessel.

The objective of this paper is to derive representative vertical extreme current profiles for a location offshore Borneo continental shelf using statistical methods. Firstly, 95th percentile was used on one-year through the water column current measurements in 140 m of water to identify extreme current profiles. Principal Component Analysis (PCA) was then applied to the extremes to retain fundamental modes of the current dataset. The statistical approach was followed by K-means clustering analysis to objectively group the extreme current profiles. Lastly, actual weather and sea state conditions are examined to validate and explain each extreme current profile. Extreme currents are typically observed during the Northeast monsoon in South China Sea (boreal winter). From the statistical analysis, there are two prominent ocean current profiles observed at the location, (1) sheared profile with strong surface currents and (2) mid-depth current maxima. Type 1 profile is typically observed during strong winds, which lead to the formation of a wind-driven sheared profile with maxima at the surface. In contrast, Type 2 current profile has relatively weak near sea surface currents, while strong currents are observed at 60-100 m water depth. This profile is accompanied by a formation of mixing layer within a thermocline region and might be associated with the formation of basin-wide Kelvin wave in response to north-easterly monsoon.