The Hibernia Platform: A New Topsides Structural Concept

Abstract

Abstract The Hibernia Platform uses a new deck structural concept based on the integration of very large modules, i.e. "Super Modules", to form a single primary deck support structure. The paper describes the structural concept, and how it was designed and constructed. The new concept was advanced from the initial idea to a fully developed design, the Super Modules constructed, transported around the world to the assembly site in Canada, and finally assembled into one structure. The design effectively makes use of the structural steel frame of the individual nodules to act as the overall deck support structure, thereby eliminating the need for a separate module support frame. Because the modules are the full height of the deck, module lifting operations were nor required, and hook-up work was kept to a minimum. Extra light strength steel was used extensively for the Primary structure which resulted in significant cost and weight savings. The feasibility of integrating very large modules using multi-wheeled trailers was demonstrated. Tight dimensional control was essential to a successful integration. The Super Module deck concept is applicable to other Platforms where minimizing cost and topsides weight is important for the installation and operating phases. Concept Development The Hibernia Topsides structure was initially conceived to be similar to several other GBS platforms such as Statfjord "B" and "C". These platforms have an open truss type module support frame(M SF) about 19 meters deep supporting one layer of individual modules. The modules were designed as separate structures for lifting and for the operating phase. In 1986 the first detailed study of the Hibernia Topsides was performed b y Stone and Webster Canada Limited 1. One of the restraints put on the design at that time was that the Topsides should not extend out beyond the perimeter of the GBS so that any icebergs that hit the GBS would be highly unlikely to damage the Topsides. This resulted in a very tall Topsides with multiple levels of individual modules. The final arrangement arrived at required 25 modules weighing up to 2200 tonnes, which the limit of most semi-submersible crane vessels (SSCV's) at that time, for the height and reach required. Mobil. the Hibernia operator at that time, decided to further study the Topsides configuration to reduce the cost and weight and in 1987 Aker Engineering a .s. of Oslo, Norway were asked to see what alternative approaches were possible. In the first phase of their study, they retained the original concept of a MSF but proposed to use fewer, larger modules which achieved significant cost savings. However, in the second stage of their study they proposed a completely new concept that departed from the MSF concept. Aker proposed that there would be only five "Super Modules" which would be the full height and width of the deck. By joining the individual nodules side by side, an overall primary deck structure 30 meters deep was achieved.