Monitoring wave-induced shocks at Eddystone Lighthouse

The winter storms were some of the most severe recorded in South West England. Coincidentally, during this period Plymouth University’s School of Marine Science and Engineering were using vibration monitoring equipment from Caption Data limited, to measure the dynamic structural response of the historic Eddystone Lighthouse’s tower under various stress loads. This equipment is more commonly found monitoring construction and ground works near sensitive buildings or structures, around tunnelling or pipelines; but is also in use on other iconic structures such as the Thames Barrier and the Ta’ Bistra catacombs in Malta.

Eddystone Lighthouse (Fig.1 above) was built on a dangerous rocky reef 13 miles south west of the city of Plymouth over one hundred and thirty years ago. The current structure is the fourth to be built on the site. The first and second were destroyed by storm and fire and the third is best known for its influence on future lighthouse designs. Before the first lighthouse was built on Eddystone Rocks, sea merchants would sail around the Channel Islands or to the French coast for fear of being wrecked on the reef.

Research focus

Plymouth University, in collaboration with the General Lighthouse Authorities (GLA) of the UK and Ireland and specifically Trinity House, have been engaged in research to characterise the environmental loads on the granite structure of the Eddystone Lighthouse, as the first of a wider study with implications across the population of rock-based lighthouses in the UK.

This is an area of research that has been neglected for generations and even today research is still made difficult by the remoteness and low power availability.

Research has followed a triple sequence approach with Finite Element Analysis models, then a 1:100 physical scale model used in the wave flume of the COAST laboratory of Plymouth University, and finally field measurements on the Eddystone Lighthouse itself using RDL//Vibe and geophone sensors supplied by Caption Data Limited (Fig.2).

In addition, CCTV cameras have been developed with this research project in mind by Plymouth University to monitor wave impacts and “wave run up” at the lighthouse with the ability via date and time stamping to synchronise images of waves to specific vibration data from the RDL//Vibe to validate results.

Field Measurements

The RDL//Vibe produced by Caption Data Limited was chosen because it is autonomous, robust (including an IP66 rated enclosure), reliable, simple to install, and yet sensitive enough to monitor wave induced vibration in the structure.

The RDL//Vibe is optimised for low power usage and is battery powered by a Lithium D-Cell, in addition it can also accommodate an external power supply. It constantly monitors vibrations and shocks at user selected frequencies from 10Hz to 500Hz.

It has inputs from two tri-axial geophone sensors typically with low frequency response down to 4.5Hz. Minimum trigger levels are 0.18mm/s while maximum sensor levels are 43.4mm/s. Alarm thresholds can be set to alert any number of recipients by email or text when predefined limits are exceeded. An alarm “event” is captured at high speed for a pre-set number of samples. The RDL//Vibe uses a fully roaming SIM to lock onto the most appropriate GSM network at any given time.

Data is available online from Caption Data Limited’s dedicated web portal, where the settings of the RDL//Vibe are managed, and which is accessible on any internet enabled device using standard web browsers, i.e. with no software to load. As well as receiving graphical interpretation of events, raw data can be downloaded for further analysis, and derived parameters such as peak particle velocity, displacement and acceleration are available on-line.

An on-line, un-editable audit trail logs every event and every alarm that is ever sent.

Winter Storms 2013/14

The winter storms of 2013/14 saw a catalogue of wave-induced damage along the coast of south west Britain, from the washing away of rail lines at Dawlish to the demolition of a Victorian shelter on the promenade in Aberystwyth, Wales. Nearer to Eddystone Lighthouse, the famous Kingsand Clock Tower was left facing demolition but has since had £100,000 of repairs. Eddystone Lighthouse of course stood firm throughout, but with the benefit of remote monitoring from Caption Data Limited, the research team from Plymouth University were able to see exactly the effect on the structure of these immense natural forces.

During the period from 15/12/2013 to 28/2/2014 there were 3090 “events” logged by the remote monitoring system; typical data from an “event” is shown in Fig.3 and represents velocities from the tri-axial geophones installed 28m above sea level on the structure.

Associate Professor Alison Raby from Plymouth University’s School of Marine Science and Engineering, who is leading the research, commented “The ability to remotely access live motion data from Eddystone lighthouse, and to be able to change the threshold at which we acquire such data has been a tremendous asset for our research project.”

During daylight hours it was possible to correlate vibration data with CCTV images of wave strikes (Fig.4)..

Future Research

Plymouth University has purchased further RDL//Vibes and geophones in order to equip the Eddystone Lighthouse with measuring points at differing heights on the structure, in addition further research is also about to commence using geophones with a low frequency response of 0.3 Hz. Research is also being planned to monitor a number of other rock-based lighthouses around the UK in the future.

Acknowledgements: The research work has been funded through a Plymouth University School of Marine Science and Engineering PhD studentship and from the GLA which has provided equipment and helicopter access. As well as Dr Raby, PhD student Davide Banfi and a host of experienced technicians at Plymouth University, Ron Blakeley Principle Civil Engineer at Trinity House and Martin Bransby at the GLA have had pivotal roles in creating and supporting the team involved in this research work.

For further information about the RDL//Vibe please contact:
Jon Penn, Managing Director at Caption Data Limited ([email protected])
Paul Sanders, Technical Director at Caption Data Limited ([email protected])

Fig.1: The Eddystone Lighthouse sits on a treacherous rocky reef South West of Plymouth (Photo: Trinity House)

Fig.2: The RDL//Vibe and a single tri-axial geophone

Fig.3: Graph data relates to the event pictured by CCTV footage in Fig.4.

Fig.4: Images capture wave run up at Eddystone Lighthouse using a CCTV system specifically designed by Plymouth University with this research project in mind.

Fig.5: From left to right: Dr Alison Raby, Ron Blakeley and Prof Geoff Bullock