This policy aids inspection and safety case assessment, and guides operational enforcement policy, as well as providing information for the UK offshore industry. The projects will provide support and specific technical input into HSE’s ongoing Metocean themed structural integrity inspection programme that aims to sustain the UK offshore infrastructure in the national interest.
The extreme weather major accident hazard for sea-bed supported structures is controlled by ensuring operational decks are above the waves. This is an important safeguard as it has been shown that wave loading increases substantially once significant deck immersion occurs. This mechanism is believed to have caused a number of platform collapses in the Gulf of Mexico during hurricanes.
The first study aims to identify which computer generated wave, tide and surge data sets are acceptable to HSE when used in combination for the purpose of establishing fixed platform deck elevations. Several sources are available to UK users for waves, tides and surges. The project aims to establish in quantitative terms the degree of similarity/difference between alternative wave, tide and surge models in different regions of UK waters, e.g. Northern, Central and Southern North Sea, West of Shetland and West of Ireland. For selected locations, comparison will be made between model output and measurements from the field, either as direct comparison between the time series, or by comparison of frequency distributions.
The second study aims to achieve an improved understanding of the relationship between wave crest, tidal and surge elevation necessitated by the recently introduced ISO Standard 19901-1 entitled “Petroleum and natural gas industries – Specific requirements for offshore structures – part 1: Metocean design and operating considerations”. However, the ISO standard does not address ways in which this might be achieved, and the study proposed here will specifically address some important gaps in present understanding of these issues and thereby provide us with the ‘good knowledge’ required by the standard.
The third study is looking at extreme total sea surface elevation in UK waters. The statistical argument put forward in favour of the ISO compliant technique is that small probability contributions from all sea states are taken into account when the short and long term distributions are convolved. The indication suggested by an initial phase of this research is that the low probability events in the tail of the wave crest distribution may not accumulate in the manner predicted. However, it can be argued that longer data sets are required in order to provide sufficient data to allow some manifestation of rare waves in the tails of the distribution. Therefore the study will repeat the work required in order to see whether a longer data set provides a similar output, or whether it tends to results closer to those suggested by ISO compliant analyses.
The results from these projects will underpin HSE’s ability to satisfactorily assess safety cases, and supporting information, by providing a detailed understanding of the traps and pitfalls associated with different base data sets from which various duty holders may have calculated criteria presented with a safety case. In particular, HSE wishes to ensure that the smaller/newer operators do have access to adequate data from which to derive operating and design parameters and that data that are being used in this respect are, in fact, fit for purpose, and that appropriate ISO compliant methods are being applied.
Contact:
Malcolm Birkinshaw, HSE Offshore Division
Tel: +44 (0) 20 7717 6775