The Frames Project was established to provide large scaie test data on the collapse performance of frames representative of offshore jacket structures. Four two-bay X-braced frames were tested in Phase / and the results were re/eased from confidentiality in late 1993. With a variety of configurations, the frames exhibited different sequences of member and joint failure providing important insight into potential jacket responses. A relatively weak X joint typical of an older structure, was shown to impart significant ductility, mobilizing alternative Ioadpaths and giving a high frame capacity albeit at large global deformation. Members with low utilisation under moderate frame loads, were shown to play an essential role in redistributing internal forces once other components failed, offering lessons for modem jacket weight reduction strategies. Material tests indicated an appropriate correlation between the minimum specified and actual yield properties and the rate of tensile coupon tests and stub column performance. In addition to the testing, a comprehensive and efficient program for nonlinear structural collapse analysis was developed and validated and now provides a powerful tool for jacket assessment. As the work continues with K-braced frame tests and a new 3D test programme, the additional benefit of the results from benchmarking other pushover analysis software packages, on which the industry is becoming increasingly reliant is demonstrated. INTRODUCTION Design of a steel offshore jacket has traditionally been based on an elastic skeletal frame analysis to determine the distribution of forces through the structure when subject to specified design environmental loads. Checks are then performed on a component basis to ensure that no element of the jacket fails to meet the governing criteria, Advancement of knowledge leading to revisions of these codified loading and resistance criteria, changes to topsides loading or structural deterioration, may mean that older installations fail to satisfy current design requirements. However the potential for nonlinear structural interaction between components through plastic deformation and load redistribution, means that jackets may in fact exhibit reserve strengths beyond the required design resistance. In addition, altering the bracing configuration and disposition of steel within a frame can significantly influence the ultimate resistance and failure mode. Indeed, it has been shown (1) that, without incurring a weight penalty, a structure can be designed not only to meet the governing elastic design criteria but also to provide a reserve strength beyond the design requirement as insurance should extreme events or unforeseen operational changes arise in the course of the platform life, The need to understand and predict the ultimate response of jacket structures is therefore of considerable importance for the economic exploitation of hydrocarbon reserves from both new and existing installations. The Frames Project was initiated in 1987 specifically to address this need by generating data representative of offshore jacket structures from collapse tests on large scale tubular frames.

From an ul timate strength standpoi nt, research to date on tubul ar joi nts has concentrated on testing work.However, the high costs associated with testing has directed recent efforts towards non-l i near anal ys is techni ques to generate data and information. This paper gives a background to these efforts and descri bes the current status.The manner of application of numerical techniques is described and discussed by reference to various separate investigations which have been carried out, or are currently underway.New strength data generated using the non-linear finite-element program FINAS are presented and discussed. These data, together with others generated el sewhere, are compared where appropri ate with existing experimental data and API RP2A recommendations.Areas where future effort needs to be directed are identified and presented.The non-linear techni~ues (and the results) discussed herein demonstrate their applicability to generate, new information in a much more cost-effective manner than the high-cost and difficult experimentation that has been undertaken to date. Further, the flexibil ity offered by such techniques can be applied in practical areas where experimentation becomes prohibitively expensive and difficult, ego for complex joints and joints with realistic load scenarios (hydrostatic loading for example). Notwithstanding this aspect, it is recognised that non-linear numerical techniques represent a developing technology, and the need for experimental data will remain for a variety of reasons, not 1east the need to have rel evant test results to assist in the all-important calibration of the numerical techniques.References and illustrations at end of paper. 399

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This paper concerns the integrity reassessment of offshore wellhead platforms damaged by a typhoon, the damage consisting of cracking at joints between the conductor support frames and the main jacket structures. These structures were reassessed to establish the stability and growth rates of the cracks, and the effect of the cracks on the overall stability of the conductor frames and the structures as a whole. The structures were reanalysed using models that included joint flexibility which had a significant effect on load levels at the critical crack locations. Non-linear ultimate load analyses were carried out at selected joints in order to develop plastic collapse values for a fracture mechanics assessment of crack fracture criticality and growth rates. A series of analyses was carried out to investigate the potential progressive failure of the conductor frame supports and the effect of this on overall structural stability. Remedial measures were considered in detail including crack grinding, chord grouting, member removal and clamping. The results of the reassessment showed that the analysis including joint flexibility predicted more accurately the crack growth rates. The flexibility considerably relieved member end moments albeit increasing mid span moments and deflections. The progressive collapse analysis showed that a rapid failure in the event of a complete member severance is unlikely and that therefore a planned programme of inspection and remedial grinding can be instituted. This paper describes the overall assessment strategy and the detailed assessment methods used. It demonstrates the useful role of non-linear finite element analysis and fracture mechanics techniques in minimising the need for expensive underwater structural repairs.

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