Jack-up sliding probability in the context of current assessment practice

Abstract In order to establish the significance of the leg sliding `failure mode’ a non-linear time-domain dynamic finite element analysis procedure has been developed which allows the sliding phenomena to be simulated. The first application of the technique was the study of a model of the Santa Fe Monarch, a Friede & Goldman L780 Mod V class jack-up, for elevated operational conditions applicable to a location in the BP ETAP area of the Central North Sea. This paper describes the analytical techniques and results for a case where a routine assessment to the SNAME `Recommended Practice for the Site Specific Assessment of Mobile Jack-Up Units’ (RP) indicated a sliding utilisation check of unity. The paper includes results showing the structural response for different levels of load factor. These results are used to determine the load factors at which sliding would initiate and at which sufficient sliding would occur to cause the onset of structural collapse. By relating load factor to wave height and thus return period the annual probabilities of sliding initiation and onset of structural collapse are estimated. Significantly, the results show that the nominal annual probability of occurrence associated with an RP sliding utilisation check of unity is in excess of one order of magnitude greater than that associated with the dynamic initiation of sliding and about one and a half orders of magnitude greater than that associated with the onset of structural failure after sliding has occurred.