Numerous Acts of Parliament and Statutory Instruments which apply to Floating Production, Storage and Offloading (FPSO) developments in the United Kingdom Continental Shelf (UKCS), covering a wide range of issues including health, technical safety, work place safety, lifting operations, environmental protection and pollution prevention and control, are described. A comprehensive study of system safety evaluation of a typical turret-mooring system used on FPSOs is described in this paper. A safety assessment method suggested using approximate reasoning and evidential reasoning approaches is proposed in this study. Subjective safety modelling at the bottom level in a hierarchical framework is carried out using an approximate reasoning approach. The evidential reasoning method is used to combine or aggregate safety estimates at lower levels to produce the safety estimate at the system level. The four main sub-systems (Turret (T), Fluid Transfer System (FTS), Turret Transfer System (TTS) and Interfacing System (IS)) are thoroughly examined in order to perform a subjective safety assessment of the turret-mooring system.