Development Of Jackup Assessment Criteria Using Probabilistic Methods

ABSTRACT An international joint industry study has been in progress since 1987, managed by Noble Denton, to address the performance assessment of jack-ups in the elevated condition. One of the key questions with which the JIS committee has been wrestling has been the identification of rational safety factors which would provide acceptable levels of safety. In common with recent code developments, advanced first order reliability methods have been used to describe the implications of various safety factors on the notional probabilities of limit state exceedance. A method has been developed to incorporate time domain simulations of jack-up behaviour and applied to a number of jack-ups in water depths ranging from 55m to l18m to derive the safety factors. The detailed numerical results are confidential to the participants in the joint industry study and therefore cannot be presented in this paper. However, the paper demonstrates the philosophy behind the methodology which has been applied to the development of jack-up assessment criteria and discusses some of the key findings. INTRODUCTION Self-elevating mobile units, commonly known as jack-ups, operate in water depths up to about 120 metres. These rigs are primarily used for exploration drilling but have also been used for production and accommodation purposes. In their role as drilling rigs, jack-ups operate at different locations during their design life with typical duration of operation in any one location being 3–12 months. Because of the variety of environmental conditions to which a jackup is exposed while operating at different locations, site specific assessment is frequently carried out to establish the fitness for purpose of the jack-up. In 1987, there was considerable discussion within the industry as to the limits of capability of jack-ups. This culminated in the launch of a major international Joint Industry Study to evolve/define/specify analysis methods for jack-ups which is discussed in Jones et.al.1. One of the key questions of this project has been the identification of rational safety factors for the various checks of performance such as overturning, soil bearing strength and leg strength. In this paper, the approach used for the derivation of these safety factors is presented. PROBLEM DEFINITION When a jack-up is offered for operation at a marginal location, the following issues are normally assessed:Overturning StabilitySoil Sliding/Bearing CapacityLeg Strength Other issues are often assessed in addition to these, such as hull displacement, jacking system capacity, etc., but here, we will concentrate on the above issues. The overturning stability of a jack-up is checked primarily to see whether the windward leg is likely to slide, as it is unlikely that under the dynamic wave loading regime, the instantaneous exceedance of the gravitational righting moment by the environmental loads would lead to overturning per se. However, the instant of overturning load equalling the righting moment is also the instant of zero vertical reaction on the windward leg.