ABSTRACT

The paper summarizes the experiences and the technological development folio wing the construction and operation of25 concrete platforms installed over the last 20 years in the North Sea, Over the period there have been significant improvements in the concrete qualities which have permitted realization of new platform concepts for demanding conditions. The in service performance of the concrete structures has been very good. Concrete platforms are anticipated to play a major role in the future, mainly for deeper waters, floating production, smaller platforms and arctic conditions.

INTRODUCTION

After the installation of the pioneering Ekofisk Tank in 1973, the North Sea has been endowed with another 25 large platforms. The majority of these platforms have been of the caisson and tower configuration, however, to meet pertinent functional requirements and metocean, bathymetric and foundation conditions a variety of different platform designs have been realized.

The platform types include the DORIS concepts, designed to reduce the wave forces by the perforated JarIan wall. Examples are the Ekofisk tank, the Frigg CDP1 and Ninian Central.

DORIS Engineering have also designed compliant concrete towers such as the Maureen offloading buoy and the two piece barrier protecting the Ekofisk tank.

Tower and caisson platforms are designed to minimize the wave force and overturning moment by utilizing slender shafts and a wide stabilizing caisson. Different designs have individual particularities such as the CONDEEP caisson composed of circular cells and the SEA TANK and ANDOC caissons with square cells. Smaller concrete platforms were recently installed in shallow waters at the Ravensburn North Field in the UK and at F3 in the Netherlands.

New types of platforms are presently being constructed for very deep waters. These platforms include the Troll East platform where the 300 meters high shafts are stiffened by framing, and the pioneering Heidrun and the Troll West floating platforms for 345 meters/330 meters depths respectively.

Concrete has also been chosen for platforms in other waters. The Hibernia platform, designed to resist impacts from icebergs of several million tonnes, is under construction at Newfoundland, Canada for installation on the Grand Banks. Two concrete platforms have been installed in the Baltics (Schwedeneck), two platforms in Brazil (PUB2 and PUB 3) and two platforms in the severely ice infested part of the Beaufort Sea. In Australia two concrete platforms are under preparation for installation in the Bass Strait. Most of these platforms have been smaller in size.

CONCRETE PLATFORM CONSTRUCTION
Construction

To date, construction of all the major concrete platforms was started in a dedicated graving dock. Upon removal of the dock gate the base structure was towed out. Several platforms have been completed in the dry dock, but more commonly the upper parts of the substructure including the shafts are constructed while floating at a sheltered deep water site. The deck is normally transferred to the platform after immersing the substructure. Only the upper few meters of the shafts project above water during this operation, which in effect constitutes an inshore pressure test prior to the offshore installation.

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