Displacement and frequency response measurements of a ship using GPS and fibre optic-based sensors

© 2020, Società Italiana di Fotogrammetria e Topografia (SIFET). GPS and more recently multi-GNSS carrier phase measurements have been used to measure the dynamic displacements of large structures, such as long-span bridges, in both the time and frequency domains. Such measurements can be used as part of a structural health monitoring system. Additionally, fibre optic-based systems have been used to measure long-term deformations of structures, such as tunnels and roads. The research presented in this paper brings together the ideas and technologies in the two aforementioned areas of research, resulting in dynamic displacement measurements of the hull of a ship at high frequencies, and indoor environments. Field trials using kinematic GPS and FBG sensors on the 138-m long Smyril passenger and vehicle ferry operating in the North Atlantic Ocean on the Faroe Islands are presented. FBG sensors were in the bow and engine room of the ship, gathering data at 1 kHz. The configuration of the surveys and the results from the FBG and GPS sensors are presented, in both the time and frequency domains. Various frequencies were measured, due to the movements of the ship in the ocean as well as the vibrations caused through the ship mainly due to the engine. One dominant frequency was of 12.25 Hz, measured at all the FBG locations, due to the engines’ operating speed of 735 RPM. Common frequencies were evident for both the FBG and GPS results for lower frequency displacement, caused by the movements in the sea. Such measurements could be used to monitor the long-term displacement characteristics and changes, in both the time and frequency domains, and used to help understand the health characteristics of the ship. Further, such measurements could be used to analyse the noise characteristics of the ship for both operational and environmental reasons.