Results from Ultimate Load Tests on 3D Jacket-Type Structures
Authors: Bolt H.M. , Billington C.J. 
Published: October 2008

Abstract
The capacity of steel structures to yield and redistribute loads can contribute significantly to their ability to tolerate damage and to survive extreme events. This system reserve strength, beyond the design capacity of components, has recently been examined in a series of large-scale ultimate strength tests of a jacket type structure. As described in the paper, the 3D constraint of the frame has contributed to significant differences in the response mode and capacity of components in comparison with isolated test data on which current design practices are based. The project included an industry-wide exercise to benchmark analytical strength predictions against three of the eight test cases. Important guidelines for future best practice are now being formulated through the Structural Systems Group described.

Introduction

Reserve Strength Technology.
For structures which fail to satisfy current component-based design criteria, reliance is often placed on their inherent 'reserve strength' to demonstrate the safety of continued operations. The concept of reserve strength provides a measure of ultimate system capacity as a multiple of the design load. By identifying the extreme load levels that would cause collapse these can be related to rare environmental events, for example, with associated return periods, from which probability of failure and hence reliability measures can be derived. Design practices embody intentional conservatism, for example in the derivation of component resistance formulations and provision of explicit safety factors. However, beyond this, redundant steel structures offer the inherent capacity for plasticity and load redistribution. In addition resistance criteria are largely based on laboratory tests of planar components under single loading modes, whereas jacket structures provide three dimensional continuity and load interactions.
These 'system' contributions to reserve strength vary with overall structural form, local component configurations, utilisation of loadpaths and, of course, the distribution and direction of loading. Therefore in order to exploit these inherent sources of reserve strength, it is important that the nonlinear response modes at component and system levels are fully understood and that there are reliable analytical techniques available to quantify the ultimate strength of platforms on a case by case basis.

Frames Project Tests.
It was on this basis that the 'Frames Project' was first established with the support of nine oil companies and regulators in 1987. Through two phases (I and II), large scale tests of two dimensional (2D) X-braced and Kbraced frames were undertaken, as reported at previous Offshore Technology Conferences1,2. Great care was taken to ensure that the scale of testing was adequate for the results to be applicable to offshore jacket structures; design ensured that non-dimensional geometric proportions reflected offshore practice and materials were selected to give a nonlinear response typical of offshore tubulars. Joints as well as members were the critical components providing response data relevant to existing installations as well as to modern structures with strong joint cans. Important conclusions concerned:The role of redundant members in contributing to reserve strength and particularly to residual strength in the event of damage