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The helicopter transport represents one of the major hazards in North Sea oil and gas production. Seven oil companies have therefore initiated a helicopter safety study, carried out in 1997–1999. Main objectives were to assess the present risk level of this helicopter traffic, and to predict the effect of possible operational and technical changes (e.g. risk reducing measures). Introduction The first Helicopter Safety Study was carried out in 1989–1990, and was reported in Ref. 1. The present paper presents some results of the Helicopter Safety Study 2 (HSS-2), carried out in 1997–1999, see Ref. 2. The main objectives of HSS-2 were "to estimate the present (1998) risk level associated with civil helicopter transport in the North Sea (i.e. the Norwegian and British Sector) and to give expedient recommendations regarding how to improve helicopter flight safety in this area for the next decade." The offshore helicopter transport represents a complex operation, and the risk is affected by a number of factors. Thus, a risk model based on the use of Risk Influencing Factors (RIFs) has been developed. The RIFs are organised in three levels, i.e. operational risk factors, organisational risk factors and regulatory and customer related risk factors, and are further arranged in influence diagrams. The influence diagram communicates the interrelations and importance of the various RIFs. It gives a visual impression of the influences between RIFs and assists the identification of the RIFs with the highest improvement potential. The present paper focuses on RIFs for accident frequency. However, the RIFs for accident consequence are handled in a similar way. The risk is first estimated on the basis of accident and incident statistics from both British and Norwegian offshore helicopter transport. These data, together with expert judgements, are used to assess the effects of the various RIFs, and hence the effect of various risk reducing factors. This paper first presents some results obtained from the analysis of the historical (accident) data. Then some main results of the RIF modelling is presented. Historical data on the risk of helicopter traffic The risk level is here given as number of fatalities per million person flight hours. The accident data for the period 1990–1998 have been analysed. There were 15 relevant accidents which resulted in 29 fatalities, see Table 1. In Table 2 and Fig. 1 the accidents are split into eight Incident/accident categories (for definition of these see below). The statistical analysis of accidents for 1990–98 gave the following main results concerning risk to crew/passengers of the helicopter transport in the North Sea: Accident frequency: 0.96 accidents per million person flight hrs Accident consequence: 1.93 fatalities per accident Risk: 0.96 × 1.93 =1.85 fatalities per million person flight hrs1 Fig. 2 also presents a comparison with the periods 1966–90 (first helicopter study) and 1990–98. In order to make this comparison correct two fatalities in 1990–98 on British Sector has to be included, even if they do not affect passengers or crew. There is considerable statistical uncertainty related to these figures, and we should not draw strict conclusions from these results. In particular, it should be stressed that the result of comparing present period (1990–98) with the previous period (1966–90), the result is heavily dependent on the chosen "cut-off" point, 1990. Quite different results could be obtained if for instance 1988 or 1986 was used instead of 1990. However, the figures clearly indicate that there has been a risk reduction compared to the period prior to 1990.

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