Deflection Monitoring and frequency response of a Ship using GPS and Fibre Optic based sensors

Kinematic GPS and GNSS has been used as a tool to monitor the deflections and natural frequencies of large structures, and hence Structural Health Monitoring. In addition, fibre optic based measuring systems have been used to measure the long-termdeformations of natural and engineered structures. In this paper, we discuss the use of both GPS and fibre optic based measuring systems to measure the deflections and natural frequencies of specific locations on a ship. Field tests were conducted on the 138 m long Smyril ship on the Faroe Islands. The one-wayjourney time between Tórshavn and Suðuroy is 2 hours, with a distance of approximately 68 km.Surveys were carried out on the 1 August 2017 and on the 12, 13 and 14 February 2018. Fibre optic sensors of three different configurations were placed within the ship, one at the bow and two in the engine room, gathering data at 1 kHz. Two GPS antennas were placed on either ends of the roof of the ship’s bridge, and a third at the stern of the ship, all gathering data at 1 Hz. This paper presents details of the configuration of the surveys andshows results from both the GPS and fibre optic units. A fundamental frequency of around 0.11 Hz was extracted from the GPS data, illustrating this as being the frequency of the motion of the ship at sea. Higher frequencies due to the vibration of the ship mainly due to the engine were too small for the GPS to pick up. The fibre optic sensors, however, were much more sensitive and could pick up a variety of frequencies, and changes in frequencies due to the ship movement and engine operation.