As part of EPSRC-funded project STORMLAMP (STructural behaviour Of Rock Mounted Lighthouses At the Mercy of imPulsive waves), the Vibration Engineering Section carried out a modal test of the Les Hanois lighthouse (photo1) off the coast of Guernsey. This was the first of a planned sequence of modal tests of offshore lighthouses around the British Isles.
Modal tests are used in the aerospace and automotive industries to validate designs through direct measurements on prototypes. They are also used (particularly by VES) to evaluate the dynamic performance and structural condition of bridges, buildings and their components, as well as ‘special structures’ – and this is believed to be the first such exercise ever carried on an offshore lighthouse. Test equipment and procedures were carefully organised to fit within airlift capacity (photo 2) and manhandling constraints onsite, and to maximise chance of success with uncertain time constraints and on-site conditions.
The VES team for Les Hanois, comprising James Brownjohn (photo 3a), Professor of Structural Dynamics, Research Fellow Emma Hudson (photo 3b) and Laboratory Technician Ian Moon (photo 3c) were flown to the lighthouse courtesy of Trinity House to join a maintenance crew already on board. Due to flight limitations, time on board was only four hours, which included two hours for unloading and loading the helicopter and for setting up and packing up the test equipment. However, through meticulous pre-planning and intense on-site activity, the VES team carried out the entire exercise successfully.
During the test, orthogonal pairs of accelerometers were placed in sequences at the ten levels of the lighthouse, from entrance (for boat landing), through engine room, bedroom, lantern level (photo 4) and helideck (photo 5). As well as using the sensors to measure the minute wind-induced sway of the lighthouse, structure vibrations were also forced, using an electro-dynamic shaker located outside the lantern room (photo 6). On-site analysis of the data was used to refine the test procedure in real time, and the results show the test to have captured all the structural information needed for use by other members of the STORMLAMP consortium at Plymouth University and University College London.
Emma Hudson who managed the data acquisition and on-site modal analysis said: “The testing provides an indication of how much movement we can expect from the lighthouses. Based on these modal tests, we can identify which of the lighthouses is especially problematic, then focus on that lighthouse in particular for the rest of the project.”
“We will then leave a set of accelerometers running to log data, so we can measure the response of the structure during stormy weather, when the sea waves apply large forces to the lighthouse. When these measurements are combined with the dynamic model derived from the initial modal test, we can then infer what sort of forces these waves are generating.”
She added: “It was a useful exercise because, in VES, we often deal with footbridges, offices, road bridges and stadia. Lighthouses are such narrow, enclosed spaces, with limited access. So, it was a challenge in terms of placing the equipment, and interesting to see the different dynamics of the structure.”
The next data gathering exercise will take place at Wolf Rock lighthouse, off the coast of Cornwall, in July 2016.