Ringing loads and their associated non-linear responses may be important in connection with the design of fixed installations. In 2007, MARINTEK carried out a comprehensive series of tests to determine the extent of ringing on the Troll A platform in connection with plans to increase its deck load. Tests of this kind require an elastic model with appropriately scaled stiffness and mass. The soil stiffness under the foundation of the platform is also of great importance for the platform’s dynamic response, and the model must therefore be set up on an elastic foundation.
Troll A has four concrete shafts that are linked by a concrete “riegel” about half-way between their base and the still-water level. This connection makes the load-bearing characteristics of the structure rather complex, and for the connection itself torsional stiffness, bending stiffness and shear stiffness must be scaled correctly if the load-bearing characteristics are to be correctly represented. The point of departure for the design of the physical model was a numerical finite element model that best represents the platform as it exists today. Based on studies of the dynamic properties of the numerical model, a scale test model with equivalent properties was designed. With support provided by a detailed ABAQUS model, the properties of the individual components were identified and a complete model built.
The physical model was instrumented to measure moment- and shear force distributions for all four shafts. Axial forces in the shafts under the riegel and where the shafts meet the deck, accelerations at various levels and wave elevation in the area around the deck were also measured. The total number of channels measured was about 200.
In order to validate the stiffness, dynamic properties and instrumentation of the physical model, the individual components and the complete model were tested both numerically and in the laboratory, using static and dynamic tests.
A comprehensive programme of tests of various combinations of deck loads and foundation soil stiffness was carried out, including a total of 290 three-hour wave tests and more than one hundred validation tests. A great deal of effort has since been put into analysing the test data and models in order to establish design loads for the structure.