The structural integrity of FPSO's and FSU's is clearly vital to the safe operation of such installations. Current UK Health & Safety law requires that an offshore installation at all times possesses such integrity as is reasonably practicable. In a goalsetting regime as exists in the UK, it is necessary that information is available in order to help to judge what is 'reasonably practicable'. In the case of the structural integrity of FPSO's and FSU's, there is a significant experience base available from the shipping industry, although not all of this is directly relevant to the requirements for fixed offshore installations. Thus, there are areas where more information is required, and some of these have been highlighted by incidents occurring to installations. The paper describe issues that are believed to be of significance to the structural integrity of FPSO's and FSU's, and how these are being addressed by the HSE, Offshore Division (OSD), through research and dissemination.
The paper is concerned with the structural integrity of FPSO's and FSU's in the UK, but will have relevance to such installations in other areas of the world and possibly to other types of floating structures.
FPSO (Floating Production Storage and Offloading) and FSU (Floating Storage Unit) installations of a monohull form (i.e. ship-like) are being increasingly used, particularly for marginal fields and for deep water locations. Fifteen installations of this type are currently in operation on the UK Continental Shelf (UKCS) including West of Shetland (WoS), with several more currently under consideration for future projects. Most of these installations have been built in the last five years, although some have been converted from tankers more than 17 years old. It is likely that many of these installations will continue to be operated for several years to come. The distribution of these installations throughout the area covered by the UKCS is shown in Figure 1.
Much research effort has been expended by the offshore industry in assessing the effects of a wide range of hazards (including structural hazards) on the more conventional types of installations. However, the Industry soon recognised at an early stage of developments that further work was required in assessing the effects of certain hazards for the small but growing number of monohull installations.
The use of monohulls as offshore installations has been mainly supported by the use of existing oil tanker Classification and international ship standards (e.g. IMO), for design, construction and in-service inspection. However, there are some significant differences between the operational and environmental conditions for trading ships and FPSO's/FSU's. The Classification Societies have made considerable efforts to produce more specific guidance and rules for floating offshore installations. Nevertheless, knowledge is still developing and it is important that marine standards are not too readily adopted by FPSO/FSU owners and operators without careful consideration of their applicability of these standards offshore and their verification.