Fatigue of Ship Shaped Production and Storage Units
Authors: Lotsberg lnge , Thomsen Tom 
Published: October 2008

The side longitudinals of production ships permanently installed in hostile North Sea environmental conditions are subjected to a significant dynamic loading. During the last years a considerable effort has been made to reduce the possibility for fatigue cracking of the connections between the side longitudinals and the transverse frames by improvement of the local geometry to reduce the stress concentrations. This paper describes the background for this work and our recommendations for new designs are given.

Introduction

The welded connections of side longitudinals to bulkheads and transverse frames have for a long time been considered as difficult areas with respect to fatigue cracking. A significant effort was made during the seventies to arrive at more optimal local geometry in terms of strength, fabrication and operation, ref. Tiegten et al. (1974), Broelman et al. (1975), Haslum et al. (1976), and Satsangi (1979). Design standards for these connections were developed such as NS 2558 (1983). With the introduction of high strength steel it was realized that cracking of these areas was still a problem and it needed more attention as well as improved design requirements and rules ref. Yoneya (1993), and Lotsberg et al. (1997). As oil production has moved into more marginal fields and deeper water ship shaped production units (FPSO) are being used in harsh environment on a permanent basis. These units are planned to be on the same location for a rather long lifetime without scheduling of any shutdown for maintenance and repair. This implies more strict requirements to a reliable fatigue design than that earlier has been considered as a standard procedure by the ship industry with inspection and possible repair within each 5 year interval. Furthermore, an FPSO can typically produce oil for 2–3 MUSS per day, leaving possible production stops as a consequence of repair of fatigue cracks as a highly undesirable event. This has implied that effort has been made to achieve a more reliable fatigue design of these units than that has earlier been tradition for tankers. During the last couple of years DNV has through more detailed analyses experienced that these details may still be marginal in terms of fatigue life and that design improvement is wanted. With this background a research and development project has been carried out to improve the details further by a slight modification of local geometry. The main findings from this work are reported in the following.

Design alternatives

For present design of floating production vessels two principal different methods for transfer of side longitudinals through transverse frames are used:Longitudinals going through the transverse frame with buckling stiffener in transverse frame welded to the flange (bulb in case of bulb section) of the longitudinal. In addition brackets may be installed by welding to the stiffener and the longitudinal in order to reduce effective span length between transverse frames and to reduce stress concentration factors at the flange of the longitudinal, ref. Figure 2.