Comparison Between Hibernia and the West Bonne Bay Concrete Gravity-Based Structure Concept
Authors: Blanchet D. , O&apos;Connor P. , Kovalsky K. , Nyborg A. , Munkeby J. 
Published: October 2008

As part of an early engineering study, a conceptual design of aproduction and drilling concrete gravity-based structure (GBS) for the West Bonne Bay (WBB) field offshore Newfoundland is compared with Hibernia's as-built results. The optimization of the WBB structure shape and dimensions allowed moderate steel reinforcement to be used and more than 75% of the concrete volume to be slip formed. Cost estimates were derived. The total scoping cost estimates for the construction and delivery of the WBB GBS concept have been estimated at about a third of Hibernia's cost. Savings resulted from the following areas:Lower ice and wave design criteriaEngineering and design of a single concept with a simple structural geometryImproved civil construction operations due to slip forming and moderate steel reinforcementSingle union contract for Bull Arm construction facilities in NewfoundlandIntegrated client and contractor team and common project management processesReduced mechanical outfitting quantities based on a lower number of wells and utility decks

Introduction

In 1995, Amoco Canada acquired two leases, East Flying Foam and West Bonne Bay, offshore the East Coast of Newfoundland in the Grand Banks area1. The WBB field is located 365 km ESE of St. John's, 20 km NE of Terra-Nova and about 50 km SE of Hibernia in 95 to 105 m of water (Fig. 1). Amoco Canada drilled its first well at that location in 1997. In 1996 and 1997, an engineering study2 was conducted to evaluate and appraise innovative alternatives to Hibernia's GBS3,4 and Terra-Nova's steel FPSO5. One concept selected amongst those alternatives, the concrete GBS, was designed6 using Hibernia's experience7,8,9. The results of a visit to the Bull Arm construction site in 1996 and data obtained from other GBS projects in the North Sea6 also provided vital information to this study. This paper presents the results obtained from the comparison of Hibernia and the WBB GBS concept. A second paper10 (OTC 11021) describes in detail the design of the WBB GBS.

Design Criteria

Terra-Nova11 ice, geotechnical and wave design criteria12 were used for this study. The WBB specific data were not available at the time of the study. The use of WBB specific criteria in the future may provide additional optimization and savings. For example, probabilistic ice loads derived for the WBB site were found to be lower than the deterministic loads estimated for Terra-Nova. In addition, if the distance between the top of the WBB GBS storage cells and waterline could be increased, a significant reduction in the global wave load could be achieved.

Global Wave Loads.

A maximum wave height (Hmax) of 30 m and wave period varying between 17 seconds to 21 seconds were used. For monolithic structures with a large water line diameter, maximum wave loads are sometimes sensitive to the wave period. While the expected value is about 19 seconds, variations on either side were also checked in order to determine the maximum load. Hibernia's Metocean conditions were similar to those at Terra-Nova. Hibernia's 100-year wave load of 1,500 MN was higher by about 10% due to the WBB GBS narrower waterline diameter (25.4 m vs 106 m) as shown in Figs. 2 and 3. It is possible that further reduction can be achieved with more advanced analysis techniques and further design optimization6.