Modal testing of offshore rock lighthouses around the British Isles

Abstract

Given that 95 per cent of the UK's international trade is transported by sea, and as a vital complement to fallible virtual navigational aids such as GPS, rock-mounted lighthouses constructed in the 19th century have a crucial role to play in safe navigation. However the longevity of these historical structures is threatened by extreme weather so in the UK, the General Lighthouse Authorities comprising Trinity House, the Northern Lighthouse Board and the Commissioners of Irish Lights are supporting three British universities in a program of linked experimental and analytical investigations of full-scale performance under extreme wave loading. The aim is to use structural models calibrated by modal testing to deduce wave loading from response recorded by long-term monitoring. The paper describes the procedures for modal testing, taking into account the constraints on access, logistics, unfamiliar layout and time. The test program sequentially covered Les Hanois, Wolf Rock, Longships and Bishops Rock lighthouses over summer 2016 followed by Fastnet Rock in December 2016. Some conventional techniques of forced and ambient vibration testing were used along with some unusual excitation methods. Results from the measurements and observations on the particular challenges associated with testing two of these iconic structures are presented.