Mating Of Large Integrated Decks To Concrete Gravity Structures
Authors: Bjordal B.N. , Hagen Øyvind 
Published: October 2008

In 1993 Norwegian Contractors performed the mating of the topside structures with the Gravity Base Structures for the Draugen and Sleipner A platforms.

The mating of the deck is an important milestone in any project for a variety of reasons. The deck but especially the GBS have to reach a certain degree of completion before the operation can commence. Thus, when the deck mating milestone is reached the confidence in meeting overall project schedule is considerably enhanced.

From a structural point of view the mating operation is of great importance for both the topside and the concrete substructure. The GBS is submerged to a very small freeboard and is exposed to the largest hydrostatic pressures in its lifetime. Lift-off, transportation and mating of the deck is equally critical for the loadbearing structure on the topside, Correct positioning of the topside is critical for the interaction between GBS and topside during the lifetime of the platform.

The deck mating operation also means important changes to project organisation. The topside fabrication contractor and the GBS contractor meets physically and the results of interface coordination is seen in a full-scale format. Construction activities on both deck and GBS is reduced to carry-over activities, and the hook-up contractor is brought into the center of attention. The platform's permanent staff for the production phase is also engaged to follow up and perform commissioning activities and to familiarize with their new work place.

All these organizational changes implies relocating a large number of people, establishing new working relations and interfaces, coordination of activities and priorities of different contracts and organizations and implementing new lines of responsibility, reporting and command.

This paper describes how the mating operations for Draugen and Sleipner was successfully planned and performed, with focus on the technical aspects.

INTRODUCTION

The mating operation comprise transfer of the topside structure from dummy pillars at its construction site to the GBS moored at the mating site. The number and size of the barges which are used for this transfer vary with the weight and size of the structure to be transferred as well as the size and shape of the receiving structure - the GBS.

The Draugen and Sieipner topside and base structures varied considerably in these respects, the most prominent differences being the Sieipner carrying the topside on four shafts versus Draugen's monotower support and the topside weight of Sieipner at mating being abt. 37 000 MT and Draugen a mere 18 500 MT.

For the Draugen GBS project the scope was to transport the deck structure (already loaded onto the barges) from its quayside mooring to the GBS and mount the deck onto the top of the platform's single shaft. Supply of barges, grillage design and production as well as lift-off from pillars and transport to the temporary mooring was performed by others.

The scope of work for the Sleipner GBS project included in addition lift-off from the dummy pillars; design of barge arrangement and design and production of grillage was performed by others.