Structural monitoring is increasingly becoming everyday business in the offshore industry. The monitoring may target the strain estimation or focus on tracking the changes in the dynamic properties of the structure in order to predict damages at remote / or possibly subsea locations. This paper will show that by monitoring the structural response, it is also possible to indirectly estimate the wave loading acting on the system. This information can be used to increase confidence in the load probability models for the structural design or aid the health monitoring procedure. During ambient vibration, the principles of operational modal analysis (OMA) are applied to harvest the dynamic properties of the structure. Successively, a dynamic model is formulated and used to calculate the loading from a random sea state using the response of the structure. A laboratory experiment is conducted in a wave flume at LASIF, Marseille, France, where a scaled offshore model is equipped with accelerometers to monitor the structural response during a random sea. The study shows that it is possible to use the structure as a dynamic load cell and monitor the loads occurring in actual conditions. Both the short time variations and the load spectra can be computed successfully using the structural response.
In the field of offshore structures, an increase is seen in the subject of monitoring. Recently, TOTAL announced that as for the redevelopment of the Tyra field, the platform Tyra East will be equipped with no less than 100000 sensors (Beck, 2018). Most of these will, of course, target the production processes, but the monitoring scope will also include the structural performance. The aim of structural monitoring may be plentiful, for instance with regards to operational limitations such as heading, static deformation or vibration level. The vibration pattern can be used for health diagnostics, and since offshore structures are prone to fatigue damages, monitoring their well-being is essential for ensuring safety and reliability.
