Modular Topsides for Large Floating Production Platforms

1. ABSTRACT Topsides for floating production systems are traditionally built as integrated decks. This paper describes the results of study work, conducted to determine the relative merits of an integrated deck and a modular topsides for a tension leg platform (TLP). The studies relate to a concrete hull TLP, but the results are equally valid for a steel hull TLP. The main conc1usion is that the 1esser weight of an integrated deck should not by itself make it an automatic choice. The modular topsides has a number of significant advantages which could well outweigh the points in favor of the integrated deck. It is essentia1 that all factors and their advantages and disadvantages for the entire project are taken into account when deciding between the two concepts. This study concludes that the modular approach for TLP platforms can provide a number of advantages over an integrated deck for 1arge topside structures, such as flexibility in design and fabrication. 2. INTRODUCTION Floating platforms for drilling, production or other purposes, are typically constructed with integrated decks as topsides. The advantages of an integrated deck are the lower centre of gravity, and the lower weight of the structure. "Integrated" in this context means that the deck structure will be assembled as a single unit, completed with equipment and outfitted. In many cases the hull and the integrated deck will be built separately, and mated after completion. To facilitate the transport, the deck and substructure will often be built in the vicinity of each other. For larger floating platforms, especially production platforms, the logistics of building and assembling the two units is complex. The choice of yards capable of fabricating and assembling these large units is limited and probably involves prefabrication at numerous sites. "Modular" decks are used mostly on fixed platforms and consist of a module support frame, supporting a number of separate modules. Each package can be fabricated in a different yard, and after completion, transported to the assembly site, either inshore or offshore, depending on the type of substructure. After module installation by derrick barge/SSCV lifting, the platformwill be completed by hooking-up the MSF and modules, followed by commissioning and start-up. Norwegian Petroleum Consultants performed studies for Conoco Norway Inc. for the Heidrun Field off the coast of Mid-Norway, in several stages from 1987 to 1989. The studies focused on utilizing a modular topsides on a concrete hull TLP. The concept development is described below. 3. DEVELOPMENT OF THE MODULAR TOPSIDES CONCEPT In 1987 the Heidrun licensees chose a concrete hull TLP platform for the development of the Heidrun Field, and several topsides alternatives were evaluated. For the modular topsides, an existing conceptual design of an outfitted module support frame (MSF) for a three-shafted gravity base structure, with six large modules on top, was converted to suit the main dimensions of the concrete hull. This alternative provided a number of advantages, but the size and weight of the steel MSF and the installation limitations of the heaviest modules did not provide the required flexibility for further design development.