Floatover Deck Installation for Spars
Authors: Maher J.V. , Prislin I. , Chao J.C. , Halkyard J.E. , Finn L.D. 
Published: October 2008

The spar floating production system offers an attractive alternative for drilling, producing and storing oil in deep water depths. Of the spars installed to date, conventional derrick barge methods have been used to install the top deck in multiple lifts and to perform hook up and commissioning operations offshore. Development and engineering have been carried out in developing economical alternatives to install top decks. One such scheme is the float-over deck installation onto a pre-installed spar hull with the deck supported by two barges. Using this method, the top deck will be hooked up and commissioned onshore and installed in one piece to reduce offshore operation time and to avoid the use of a large, expensive derrick barge. The development of such a floatover method is presented in the present paper.

Introduction

Several spar-based drilling and production platforms have been installed in the deep water of the Gulf of Mexico (GOM) [1,2,3]; a number of other spars are currently being fabricated and designed for the GOM and other deep water regions (Fig 1). The deep draft, intrinsic stability, and low motions of the spar make it an attractive option in many deep water regions throughout the world.

All the spars that have been installed to date have utilized dynamically positioned large crane vessels to install the top decks in single or multiple lifts followed by hook up and commissioning at the deepwater site. Day rates of such crane vessels with large lift capacity are high and these vessels are not available at all offshore locations. Furthermore, hook up and commissioning of deck facilities offshore are inherently expensive.

CSO Aker Engineering, Inc. (AEI) has been conducting research and engineering in developing alternative methods to install top decks onto a spar hull without using a large expansive crane vessel. One of the methods developed is a self-floating deck and has been described in an early presentation [4]. In this method, all the required drilling and/or production facilities are installed on a barge and hooked up and commissioned inshore; the barge is then towed to the spar site to mate with the pre-installed spar hull (Fig 2). The second method being developed is the float-over deck in which the integrated deck is supported by two small-waterplane-area pontoon barges for mating with the spar hull (Fig 3). The advantage of this method is its reduced motions and higher operable seastates. This method was also presented in an early paper [5].

An alternate float-over deck method is to use, instead of two small-water-plane-area pontoon barges, two ships or barges to support the deck. Again, the top deck is fabricated in one piece with all facilities hooked up and commissioned onshore. The top deck is transported by two support barges to the spar site and positioned over the spar for mating with the spar hull.

AEI has carried out theoretical studies and model tests for all the above schemes. In the present paper, the second floatover deck method using two ships or barges will be discussed.