Generally, jack-up structures are used for production drilling and exploration of hydrocarbons. The combination of mobility and the behavior as a fixed structure in operational conditions has made it an important structure in the offshore industry over the last 40 years. When a jack-up structure has been in operation for a great part of its original design-life and intention is there to extend the usage of this structure at a specific location, an investigation on fatigue degradation of the structure is an essential factor that has to be carried out before taking any decision. Fatigue is the process of damage accumulation in material due to stress fluctuation caused by variation of loads in service time. The fatigue failure occurs when accumulated damage has exceeded a critical level. In this paper, the remaining fatigue capacity of the jack-up structure is considered as an indicator for adequate use of the structure. It can be specified based on the difference between design-fatigue and fatigue experienced by the structure. The design-fatigue can be determined based on fluctuation of loads during the lifetime of the structure and experienced fatigue is specified by the load conditions, which the structure has experienced during its service time. When the information on the load conditions which the structure has experienced in its service time is available or known precisely, determination of the remaining fatigue capacity could be carried out by using the Palmgren–Miner’s rule. In practice, uncertainties are present in loads and characteristics of material. Hence it will be reasonable to determine the remaining fatigue reliability of the structure by the reliability methods. In this paper, based on a crack propagation approach and achieved information from inspection, it is shown that the remaining fatigue reliability of jack-up structures could be determined and updated by using a Bayesian procedure in the duration of the service time.
This paper presents an analysis of a model investigation on a semi-sul:xnersible platform in a wind tunnel. Both forces and overturni~manents on the part above water and the part below water are investigated. Furthermore full-scale anchorline force observations as reported earlier are re-analysed and the results are compared to those of theoretical calculations and of the model tests.
Due to the collision with a supply boat one of the bracings of the semi submersible drilling rig "West venture" was badly damaged calculation of residual strength indicated the necessity of repair 1. The size of the damage was such that almost one third of the member needed to be removed and replaced by a new section. This paper describes the establishment of an adequate procedure to be followed during the repair. Main objective was to ensure that the diagonal after the repair would have the original system length. In that case no residual stresses or deformations would be generated in the structure after the repair. The procedure to be used was developed by analyzing the intact damaged geometry and the cutter geometry. Since the member is in compression during normal loading Conditions measures were evaluated to obtain the longest stress level possible in the member before repair started. Calculation of the effect of ballast rearrangement resulted in optimal loading conditions before and airing the repair. During repair work strain gage measurements were made and adequate survey was required. This enabled the comparison of calculations with actual events during repair. The various steps during repair are being illustrated. Introduction In this paper the repair of a side diagonal of the semi submersible drilling rig "West Venture" is being described. The diagonal was badly damaged due to the collision with a supply boat and almost one third of the member needed to be repaired. Though the rig is a five column-floater design with bracings forming a rigid space frame with sufficient redundancy, figure 1, it was felt that attention had to be paid to the effect of removing part of the bracing This resulted in a structural analysis of possible effects and complications that might be expected Also measures to be taken in case of difficulties during the repair were evaluated. A detailed procedure for repair and survey of the repair was established based on the objectives that no excessive residual stresses or deformations of the original gentry were introduced in the structure and still be able to perform the job in as little time as possible. Necessity Of Repair The damage to the diagonal with 2 m. diameter was reported to be severe local denting although no axial deformation occurred. In this case the damage is sufficiently described with dent size, relative depth and location along the member. The following data were established on the dent which is presented in figure 2 after removal.–dent size : length overall 5.0 m, width 1.2 a 1.5 m.–dent depth: approximately one fourth of diameter which results to 0.58 m.–location : centre of dent 14.0 m from lower member end. Using a procedure developed by Taby, Moon band Rashed1 the necessity of repair was investigated. Comparing the original intact capacity for axial compression 1000 with an estimated value for the dented member a reduction of 60 a 70% was determined which was regarded unacceptable. Struciural Evaluation Of Repair Before the actual work started structural analysis was performed of damaged, cutted and repaired geometries under the ballast and load conditions during repair.
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