Wandoo Concrete Gravity Substructure - Economy Through Design
Authors: Wallis Rob , Jackson Gordon 
Published: October 2008

This paper describes the design innovation adopted for the Wandoo concrete gravity substructure (CGS). The entire project process is covered, from initial concept screening and design, through to offshore hook-up and commissioning (HUC). It shows how substantial value can be created through design during the early stages of a project, and how the value can then be captured and improved in the detailed design, documentation and implementation.

The specific features of dry-built concrete gravity substructures (CGSs) are identified, together with the economies which flow from this substructure concept.

Key features and techniques of the concrete design are discussed. These include the benefits of simple and repetitive structural geometry, low reinforcement and prestress densities, fast-tracking the design schedule, and the approaches to construction detailing, documentation, coordination and specification. All contributed to achieving substantial overall economies to the project.

The advances in outfitting design are similarly described. These encompass the oil storage / installation ballasting systems, and other major outfitting components such as deck-connections, risers, caissons and conductor frames.

In conjunction with the other participants in the Wandoo Alliance, the Substructure Design Team successfully delivered a development that was substantially quicker than industry norms and under the project budget. The Wandoo Full Field Development has become one of the offshore industry's benchmarks, against which subsequent developments are now compared.

Introduction

Engineering design is a creative process. Few professional people would disagree that of all the many fields of engineering, offshore is one of the most challenging and consequently, for most of the time at least, one of the most enjoyable.

Offshore design demands creative solutions to deal with the unpredictability of nature, whilst delivering projects that are safer, cheaper, faster, more reliable and, perhaps most importantly, that improve the way that our industry cares for the environment. The trend in all of these matters has been positive for decades and, because we all enjoy the challenge and understand our responsibilities, it will, no doubt, continue.

Substructure design is no exception to these demands. The focus of this paper is on initial project Capex. Nonetheless, all the challenges of our industry had to be addressed in the design of the Wandoo CGS. Mention is made as to how this was successfully achieved.

The Onshore Dry-Built CGS

The onshore dry-built CGS concept was originally developed for the Ravenspurn North CGS in 1989 (Figure 1). The concept was conceived to open the market to experienced civil engineering constructors, to allow them to bring their skills to bear in a commercially competitive environment, without the specific need for prior experience in the more demanding and, to many civil constructors' view, closed environment of the offshore industry.

Up until Ravenspurn the predominant perception of both the offshore oil & gas and the civil engineering industries was that CGSs were the domain of only specialised offshore companies, with experience in wet dock construction for deepwater developments. Ravenspurn demonstrated the viability of dry-built CGSs (References 1 & 2). It was further confirmed by the NAM F3/FB and Harding substructures in following years. Optimisation ideas developed from all these projects and concept designs for other developments were brought to bear, with great effect, on Wandoo.