Jacket Ductility Design
Authors: Piermattei E.J. , Ronalds B.F. , Stock D.J. 
Published: October 2008

The Goodwyn A steel jacket, located on the North West Shelf of Western Australia and operated by Woodside Offshore Petroleum, is designed with specific requirements for resistance to progressive collapse under extreme or accidental events.

Jacket structure reliability is established using static nonlinear pushover analysis for intact and damaged configurations to achieve specified reserve strength criteria. The paper describes the ductility design process which starts with a jacket design that satisfies the working stress requirements of API RP 2A (Ref. 1). Two dimensional pushover analyses are used in the conceptual design phase to confirm a unique framing configuration prior to detailed three dimensional investigation.

The paper presents results for a four legged extended base jacket and a eight legged expanded bay jacket, each with an MSF. The results of establishing satisfactory reserve strength show the benefit of early pushover studies and the small additional steel required to significantly improve structure reliability. In addition to satisfying individual member and joint reliability, overall system reliability must be confirmed separately.

INTRODUCTION

The Goldwyn platform is located on the North West Shelf of Western Australia, in 131m water depth, and produces 80,000 barrels of oil condensate and 900 MMscfd of gas from thirty wells. The structure consists of a separate module support frame (MSF) and expanded bay eight legged jacket. The foundation is made up of four corner pile groups with a total of twenty driven primary piles (with a maximum batter of 1.5 degrees) and sixteen drilled and grouted second stage insert piles. The four remaining primary plies are provisional. The substrate formation is calcareous sand.

The jacket design incorporates ductile behavior by using a configuration of frames and elements which achieve satisfactory: redundancy, individual element ductility and excess system load capacity, while satisfying component serviceability criteria.

The jacket structure was initially designed to satisfy the working stress design practice defined in API RP 2A. In addition, the jacket vertical framing configurations and the primary diagonal bracing element slender nesses satisfied the 'Ductility Requirements' recommendations of paragraph 2.3.6d of API RP 2A.

The discussion herein covers the process of ductility design, and is intended to show the implementation of static fully nonlinear analysis methods and the results from two distinctly different jacket concepts for the same location. The intention of this paper is to present a practical implementation of nonlinear offshore jacket design. The desired structure reliability is assumed to be achieved using the static pushover method, and limitations due to this assumption and the adequacy of the nonlinear analysis procedure are outside the scope of this paper (Ref. 2).

DUCTILITY DESIGN REOUIREMENT

Ductile resistance requirements against progressive collapse have been established using reliability based evaluations of structural capacity as described in (Ref. 2, 3). The target level of structural reliability for the Goodwin platform is approximately the same as the present generation of conventional central North Sea (UK sector) platforms.