Structural and wave measurements have been recorded on FRIGG DP 2 Platform. Several months of data have been analysed in order to describe the correlation between platform response and waves excitation for a wide range of significant wave heights taking into account the waves direction.
Experimental transfer function between the waves input and the response output show that natural movements are not excited as such, by sea surface elevation. The influence of the waves direction appears to be very important with regard to fatigue accumulation. Structural stiffness is larger than predicted. Platform response is, as predicted, non-linear to the waves input but, on the contrary, linear to the "internal" inputs. Displacements have remained relatively low assessing the structural integrity with regard to wave loads.
The oil production in North Sea Fields in deeper waters and rougher environmental conditions has stressed the need for a better knowledge of the platforms behaviour under wave loads both for dynamic response and fatigue. The decision to install instrumentation on DP 2 platform was the result of a continuous effort in understanding platform behaviour and improving design methods. It followed theoretical platform studies 1-2. Both short term and long term objectives were fixed to be met by the instrumentation.
Short term objectives were :
Verification of "actual" dynamic behaviour and comparison with "design" behaviour.
Obtaining of a clear description of "actual" behaviour under the wave loads to improve design. methods.
Long term objectives were :
Verification of fatigue.
Reestimation of the design extreme wave and computation of "actual" extreme response.
A third objective was to fulfill the Norwegian Petroleum Directorate criteria and regulations, for monitoring structural responses of the platform (E & P data).
Finally, a fourth type of objective was introduced during the course of the project. It was decided to use the instrumentation results as a valuable complement to the in-service inspection system of the platform. This was considered as an optimization of the programme. The objective set was to obtain from the instrumentation sufficient information to detect severe structural or foundation damages and to supplement traditional Non-Destructive-Techniques when they are not applicable, i.e. during the winter period.
The instrumentation was designed to achieve the above four objectives. To give a sound interpretation of platform responses, measurement of the major load input i.e. waves, was necessary. It was decided to measure waves at the platform location in order to measure the true input, considering that waves do not propagate conservatively. Consequently, a measurement at a distance from the platform would have led toobscured results and loss of correlation in input output relationships. At the same time, measurement of the waves' direction was felt to be necessary to understand probable variations in behaviour according to the rectangular shape of the cross-section of the platform which makes the inertia plane-symmetrical. The outputs had to be measured in terms of accelerations or displacements, and structural stresses.
