Conceptual Design of a Concrete GBS for East Coast Canada

Abstract

Abstract A concrete gravity based structure (GBS) which supports production and drilling facilities, over 1 million barrels of oil storage, and has been designed to withstand iceberg and 100 year return period wave loads in the Grand Banks offshore Newfoundland. A comprehensive structural analysis and design has resulted in moderate reinforcement and post tensioning quantities. Simple construction techniques, where as much as 75% of the total concrete volume is based on the cost effective slip form technique, has been utilized. This paper describes the design premise, design basis, and analysis leading to the development of a cost effective concrete GBS for the Grand Banks. Introduction Prior to the drilling of the first exploration well at West Bonne Bay, Amoco Canada engaged in a Front End Loading (FEL) study to consider several possible options for development of the field. West Bonne Bay (WBB) field which is located in 95 to 100 m of water approximately 50 kilometers southeast of the Hibernia field. Several different drilling and production systems were considered including subsea, disconnectable floaters, and gravity based structures. Conceptual designs for both steel and concrete GBS were completed. A part of the FEL study a conceptual design of a concrete GBS was developed. This paper describes the conceptual design of a concrete GBS. Given their extensive CONDEEP experience in the NorthSea and the Hibernia GBS, Amoco engaged Aker Norwegian Contractors to perform the conceptual design. To provide further confidence in the design, two independent structural design audits were performed by the Offshore Division of Lloyd's Register1 and by Bernt Jakobson2. Considerations for the design of a concrete GBS included: iceberg and hydrodynamic force resistance, a narrow neck to reduce wave loads, structural components which maximize the advantages of concrete, oil tanks surrounded by ballast water, an outer ice wall, local constructability, towing, installation, and schedule. This paper briefly describes the conceptual design of the concrete GBS for West Bonne Bay. Interested readers should note that a detailed comparison of the WBB GBS described herein and the Hibernia GBS is given in another paper in this volume3. Design Premise and Basis The design premise for the WBB field development required a manned drilling, production, and quarters system for year round operations in the harsh environment of the Grand Banks. Amoco Canada estimated the need for 28 wells having a maximum production rate of 125,000 BOPD with produced gas reinjected into the reservoir. Minimum internal oil storage requirements were 750,000 bbl of 40°C oil while a generic operational topsides load of 35,000 tonnes was assumed for the conceptual design. An operating life of 25 years was assumed. A tow out topsides weight of 30,000 tonnes was assumed. The concrete structure was designed according to Norwegian Petroleum Directorate (NPD) Regulations for the Structural Design of Load Bearing Structures Intended for the Exploitation of Petroleum resources and Norwegian Standard NS 3473: Concrete Structures - Design Rules. As part of the conceptual design, a comparison was made of the Norwegian4 and CSA5 codes and were found to be largely compatible with slight differences in minimum reinforcement quantities. The environmental design basis for the GBS was adopted from the nearby Terra Nova field. The basic criteria are given in Table 1 while the pressure versus area curve for iceberg loading is given in Table 2. It is important to note that the Grand Banks location has some of the harshest environmental conditions in the world. An offshore