Reliability-Based Code Calibration: Application to Jackup Structures

Abstract

Abstract A recommended practice for site assessment of jack-ups was published by SNAME in their T&R Bulletin 5-5A1. Although experience indicates that jack-ups have been safely operated over the past 40 years, the acceptance criteria given in Bulletin 5-5A1 result in unfavourable assessments in cases where the track record suggests otherwise. The present work aims at examining the central issues related to jack-up reliability and propose a way forward concerning assessment criteria. 1. Introduction Assessments leading to safe but not excessively expensive operations are of primary importance for the continuing success of jack-up operations. The debate on the applicability of Bulletin 5-5A1 is increasingly important as ISO TC67/SC7/WG7 is developing a worldwide standard for mobile drilling units (MODUs) including jack-ups. The North Sea is an area of particular concern where jack-ups may be operated in relatively deep waters (up to 120m) and harsh environments. In such a mature region there is a trend towards marginally economic developments where overly conservative criteria are an unacceptable liability. In the general context of jack-up safety, accident data2 indicate that structural failure in general is a minor contributor to rig damage when compared to other causes such as blowout, collision, explosion and fire. Foundation failure is an important risk for jack-ups but mostly during the preload phase (potential for punch-through). Advantage can be taken from hull buoyancy by preloading under more favourable weather conditions and with smaller air gap thus minimizing punch-through risks. However, some rigs may have limited hull buoyancy and in addition spatial variability in soil properties may lead to weaker soil layers not being detected in the soil borings. Improving soil sampling in a practical and economical manner remains one of the areas that can significantly improve jack-up safety. Another important area of potential improvement relates to human and organizational factors. Jack-up assessment criteria such as given in Bulletin 5-51, however, do not tackle these major risks in a direct manner. These criteria relate mainly to exceedance of given safety margins against structural and foundation failure under storm loading based on conservative models of the rig physical behaviour. Acceptance criteria take the form of load and resistance factor design (LRFD, as in Bulletin 5-5A1) as well as working stress design (WSD). A comparison is not simple due to the different formats but in rough terms, a recent review3 indicated that Bulletin 5-5A1 leads to safety factors that are about 10 – 25% more conservative than the safety factors used in more traditional WSD assessments. Structural or foundation failures attributable specifically to exceedance of a safety factor against extreme storm loading tend to be small contributors to failure rates. For new jack-up designs, the increase in safety factor will simply tend to reduce these already small risks even further and, overall, is likely to have only a minimal impact on the safety of the rig. For existing rigs, the increase in safety factors leads to a downgrading in terms of operational water depth.