Analysis Of Dynamic Transient Response (Ringing) In A Gravity-Base Structure
Authors: Ahilan R.V. , Klaas Øyvind Munthe , Langen Ivar 
Published: October 2008

Troll Gravity Base Structure (GBS)will be located in about 300m water depth and Will be the tallest GBS when installed in 1995. The natural periods of the structure in surge and sway are of the order of 5-5Yi seconds. Model tests carried out showed that greater responses than expected from "classic" design practice were likely under steep wave conditions.

Recent model tests on other structures have also shown that loads on large diameter surface piercing cylinders in steep wave conditions are considerably larger than those expected from classic design practice. In irregular waves, this has contributed to a dynamic transient response, a phenomenon commonly referred to as "ringing". The challenge was to explain the physics behind these additional loads and quantify these to such a degree that structural reinforcement carried out on the Troll GBS for this specific reason during construction could be justified.

This paper describes the work that was performed to verify the integrity of the Troll GBS in the in-place conditions focusing upon the resolution of the above challenge.

INTRODUCTION

Gravity base structures have been successfully deployed in the production and storage of offshore hydrocarbons since 1973. Since then, the design of GBS's has undergone a steady evolution which has resulted in considerable optimization of the strength of these structures. This optimization has gone hand in hand with increases in production water depths. Figure 1 shows the first and the most recent(Troll)(Available in full paper) Condeep GBS's with their basic particulars illustrating the significant advances which have been made on all fronts of GBS technology.

Design optimization and increasing water depths have contributed to the lengthening of the natural periods of these structures which started off rather stiff with natural periods less than 2 seconds, increasing to 5½ seconds for the Troll design.

In early 1992, model tests on two other concrete structures, Heidrun TLP and Draugen Monotower, had indicated that the loads experienced during the model tests under 100-year storm conditions were in excess of those which would be expected from calculations. Thus, preparations were put in place to consider whether similar increase in loads might be expected on the Troll GBS, The issue was being carefully studied by Norske Shell's EPC contractor, NorwegianContractors, via their consultant, SINTEF. In view of the uncertainty surrounding the evaluation of these additional loads, Norske Shell appointed Noble Denton to verify the work via independent analyses.

Norske Shell is the Development Operator and Statoil will subsequently take over as Production Operator of the Troll Phase I gas production. Other partners are Norsk Hydro, Saga Petroleum, Elf Aquitaine, Norsk Conoco and Total Marine.

This paper describes the work performed by Noble Denton to verify the calculations carried out on behalf of Norske Shell's EPC contractor. The work was performed in two key stages: Initially, efforts were focused upon developing a hydrodynamic loading model which would capture the characteristics of the dynamic transient response (ringing) which was causing the additional loads. The developed loading model was verified against model test results.