Results from Large Scale Ultimate Strength Tests of K-Braced Jacket Frame Structures

Abstract

ABSTRACT Phase II of the JIP Frames Project included four large scale collapse tests of K-braced frames in which both gap and overlap K joints were theoretical components. The results are presented in this paper. The local failure modes differed from typical isolated component tests, yet were representative of structural damage observed following Hurricane Andrew. The frame test results therefore provide important insight to theultimate response of offshore jacket structures. 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. These criteria are based on an experimental database of component capacities derived from laboratory tests on isolated tubular members or joints. However, within a redundant framed structure there is the potential for nonlinear structural interaction between components through plastic deformation and load redistribution, such that jackets may exhibit reserve strengths far beyond the required design resistance. Formal recognition of this is now contained in the new Section R addition to API RP 2A (1) which specifically permits the use of ultimate strength analysis techniques in the assessment of existing platforms, There are clearly enormous economic advantages to the Operator if, despite changes to the operating condition of a platform, it can be demonstrated that there is adequate structural resistance to assure safe operations without costly modifications, An obvious progression from this is the evolution of more efficient steel structure designs based on system strength evacuations in preference to elastic component based approaches (2). 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. Frames Project Phase I - Scope and Findings The Tubular Frames Project was established specifically to investigate the reserve and residual strength of offshore tubular framed structures, through both large-scale experiments and the development and calibration of advanced numerical techniques. The first phase included four tests of two bay X-braced frames, 15m long by 6m wide, which constituted thelargest controlled collapse tests undertaken to date. important results were obtained regarding the influence of tubular joint behaviour and framing redundancy on ultimate system strengthand these have been reported elsewhere (3, 4). The nonlinear collapse analysis software SAFJAC (for the strength analysis of frames and JAQkets) which was developed in the Project and was fully validated against the frame test results, is now used extensively world wide for the assessment of jacket structures(5). At the conclusion of Phase I it was recognized that the tests had afforded particular insight to the ultimate responses of frames in which tubular joints had been the critical components. No such data were known to exist outside the Frames Project yet the work had highlighted the considerable influence of frame continuity on joint behaviour both in termsof boundary constraint.