Structural Design of the lceberg Resistant Hibernia Reinforced Concrete GBS
Authors: Huynh T.L. , Clark W.J. , Luther D.C. 
Published: October 2008

This paper deals with some of the unique and unusual aspects of the structural design of this GBS, which is the first offshore oil structure designed to resist iceberg forces, and will support a Topsides weighing 39000 tonnes at towout, increasing to 58000 tonnes in operation. There were significant challenges in developing a structural solution with adequate strength which was also constructible. In addition, unusual design situations resulted from the construction methods and the structural components used.

Introduction

The Hibernia Oil Field is located 310 km south-east of St. John's, Newfoundland, on the Grand Banks, off the east coast of Canada. Recoverable oil reserves are estimated at 615 million barrels. The project is managed by a specially formed operating company, the Hibernia Management and Development Company (HMDC), on behalf of the partners of the Hibernia Field.

The main platform selected to develop the Hibernia reserves is a concrete gravity base structure (GBS), located in 80 m water depth. It is designed to include the following facilities and functions:drilling for 64 wells, enclosed in two shafts;collecting point for production from subsea manifolds;production, separation and treatment of oil (150 000 BOPD);storage of 1.25 million barrels of crude oil (200 000 m3);crude oil off-loading (at a rate of 8 800 m3/hr);gas and water injection;living quarters for 280 personnel.

In September 1990, HMDC awarded a contract which included the GBS design to NODECO (Newfoundland Offshore Development Constructors). The detailed design was subcontracted to DDC (Doris Development Canada Ltd.). The GBS construction took place at Bull Arm Site and was completed in October 1996. The platform installation is scheduled for mid 1997.

Predictably, the design of this first of its kind iceberg resistant structure, weighing approximately 450 000 tonnes, involved a large amount of structural analysis and detailing. Much of that work was challenging, and part of the challenge was to maintain constructibility.

It is not the intent of this paper to discuss the generalities of the structural design, but rather to deal with some of the unique and unusual aspects of that work.

Iceberg Loads and Conceptual Development

The Hibernia GBS is the first offshore platform designed to withstand iceberg loads. This aspect dictates the basic structural form of the GBS (Figs. 1 and 2).

The iceberg data was provided by HMDC and included:number of icebergs annually crossing the Grand Banks 48N parallel ;exceedance probability distributions of iceberg mass, speed, height, shape, etc.

The methodology to calculate the iceberg impact loads1 is deterministic and is used in conjunction with a probabilistic procedure in which return periods can be associated with specific force levels.

The basic steps of the methodology are as follows:For the simulation period (200 000 years), determine the number of icebergs that will impact the structure.For each impacting iceberg, calculate and save the impact force, overturning moment, torsion moment and other parameters, defining the impact.