STORMLAMP - STructural behaviour Of Rock Mounted Lighthouses At the Mercy of imPulsive waves

Lead Research Organisation: University of Plymouth
Department Name: Sch of Engineering

Abstract

Historic rock-mounted lighthouses play a vital role in the safe navigation around perilous reefs. However their longevity is threatened by the battering of waves which may be set to increase with climate change. Virtual navigational aids such as GPS are fallible, and reliance on them can be disastrous. Mariners will therefore continue to need the physical visual aids of these strategic structures. The loss of any reef lighthouse will be incalculable in terms of safety, trade and heritage.

Plymouth University has trialled the use of recording instruments to capture limited information on the loading and response of Eddystone Lighthouse, with the support of the General Lighthouse Authorities (GLAs) having legal responsibility to safeguard aids to marine navigation around the British Isles. The study evaluated the extreme logistical constraints of lighthouse operations and the feasibility of using instrumentation to understand the response of the lighthouse to wave loads, with results strongly encouraging a comprehensive study of the load and response environment.

Hence a full-scale project is proposed whereby field, laboratory and mathematical/computer modelling methods, novel both individually and collectively, will be used to assess six of the most vulnerable rock lighthouses in the UK and Ireland. Depending on the findings the investigation will then focus on extended full-scale evaluation of one lighthouse for the following two winters.

The field instrumentation run by University of Exeter, and which will include modal testing and long term instrumentation will require novel procedures and technologies to be created to deal with the challenging environmental and logistical constraints e.g. of access, timing power.

The modal test data will be used to guide the creation, by UCL, of sophisticated multi-scale numerical simulations of lighthouses that can be used with the data to diagnose observed performance in the long-term monitoring.

The numerical structural model will also be linked with advanced physical modelling at Plymouth University's COAST Laboratory, and numerical (computational fluid dynamic) simulations.

Finally, based on the structural and wave loading models, the long term monitoring will be used to characterize the wave loading in-situ at full scale.

Outcomes of the project will be used to inform the comprehensive structural health monitoring of other lighthouses both in the British Isles and further afield through the International Association of Lighthouse Authorities. This will lead to the identification of structural distress and reduction in the risk of failure through preventative measures. Methods developed will also be of relevance to other masonry structures under wave loads so the project team includes a number of industrial partners: AECOM, Atkins, HR Wallingford and the Environment Agency who have interests in this area. As the UK has a large number of ageing coastal defences whose vulnerability to wave load was demonstrated in the winter 2013/14 storms, the applicability of the STORMLAMP findings to these structures is an important additional benefit of the project.

Planned Impact

As an island trading nation experiencing some of the world's strongest storms, the UK is particularly vulnerable to maritime navigation failure, and loss of one strategic lighthouse will have incalculable effect on safety and trade. The impact of STORMLAMP technology will be to reduce the likelihood of structural failure by characterising the loading and performance so that operators can integrate objective performance information with subjective visual assessment.

Impacts are expected through lighthouse authorities and associated business in UK and worldwide, the community of seafarers, coastal engineering and heritage agencies and due to the fascinating extreme engineering and historical significance, to schools, the general public and heritage agencies.

For national lighthouse authorities: STORMLAMP will primarily impact the General Lighthouse Authorities of the British Isles, e.g. Trinity House, who have internationally sanctioned legal responsibility to maintain the visual navigational aids of which offshore rock lighthouses are a vital component. Modern and efficient tools will be researched and tested to help them continue this vital role and preserve the structures. STORMLAMP investigators have a working relationship the GLAs who are necessarily project partners and provide all possible means of support and access.

For international agencies: Beyond the British Isles, STORMLAMP has the support of the Chair of the International Association of Marine Aids to Navigation and Lighthouse Authorities. STORMLAMP thus has direct access to and can provide input to the technical committee that provide guidance to its members, who include overseas lighthouse authorities and associated businesses.

For mariners: The Nautical Institute provides a direct impact route to seafarers and practical maritime professionals, via its international journal Seaways as well as wide range of technical guidance notes and opportunities for lectures.

For the UK coastal engineering industry: STORMLAMP has capability to generate broader benefits to other types of marine structures where the nature of structure/seabed interface leads to enhanced wave loading. AECOM, Atkins, HR Wallingford and the Environment Agency have specific interest in STORMLAMP outcomes for this application.

Other routes to technical impact are through the EPSRC-funded Collaborative Computational Project on Wave Structure Interaction and a range of (journal and conference) publications and presentations targeting academia and industry.

Public and educational: Lighthouses feature prominently in British literature, art, and film as well as in children's books and television, and the STORMLAMP project will be used to promote the engineering and cultural heritage aspects and the context as a maritime nation. This includes international coverage e.g. through National Geographic documentaries.

Schools: The Institution of Civil Engineers is interested to base educational material for school and other public engagement activities, to be delivered by ICE STEM Ambassadors in the format of talks and workshops.

Heritage: UK's Victorian era lighthouses are of great historic significance, so the project would provide interesting case studies to international heritage agencies. STORMLAMP will also work with Historic England and Historic Scotland on publicity on protecting this heritage.
 
Title Creative Associates STORMLAMP project video 
Description Obtained funding to produce a video based upon the STORMLAMP project 
Type Of Art Film/Video/Animation 
Year Produced 2018 
Impact None yet. 
URL https://www.youtube.com/watch?v=X4FEXSUQ9rk
 
Description Modal tests/monitoring:
Forced vibration using a 'shaker' enables identification of at least the fundamental modes of vibration.
Where there is a helideck, there are up to 4 'fundamental' modes, in two directions and with different phase with respect to the masonry structure.
Helidecks modify the modal mass, a key parameter controlling the response to external horizontal loads.
For Wolf Rock, the identified modal parameters provide a means to estimate the breaking wave modal impulses.
Surprisingly, forced vibration testing during strong winds provided more usable data than ambient vibration testing.
Vibration modes are aligned weakly with specific directions, which there was some success at identifying.
Simple simulations explain directional affects on frequency response functions and mode splitting due to a helideck.
Monitoring data facilitated characterising the extreme impulsive wave loads, some at similar levels to a large earthquake.
Monitoring data for the 2017/2018 winter storm season corroborate wave tank data on extreme loads.
Bayesian operational modal analysis (BAYOMA) yields the probability density function of modal properties
Les Hanois: software was used, initially using a broadband random excitation, as is conventionally used with shakers, but this proved to be a poor choice for Les Hanois, with poor signal to noise ratio; swept sine (sweeping up then down a limited band) proved to be more viable
Modal mass is a vital parameter for inverse identification of wave loading from response data, but it cannot be directly estimated using OMA
There are at least four modes having a zero-node cantilever character in the masonry tower, partly due to the symmetry resulting in mode pairs with approximately orthogonal direction when seen in plan, and partly due to the helideck which behaves to some extent as a lumped mass on a spring and which splits what might be a single mode in a simple tower into two modes.
The mode pairs have alternate phase (0° or 180°), a behaviour rather similar to that of a structure with a tuned mass damper
The strongest response was observed during Storm Brian (21st November). The free decay is strongly non-linear and would not be amenable to OMA that assumes linear behaviour. The response to this and other storms will be 'decoded' to interpret wave loads based on the results of both forced and ambient testing

Hydrodynamics:
Fastnet's extreme wave climate shows no statistically significant effects of due to the climate change.
Lighthouses are not affected by direct breaking wave loading but by broken wave impacts - intensity is reduced but duration is increased, compared to unbroken wave.
Wave load does not vary linearly with breaking distance - depends on whether the wave can regenerate after it first breaks, so depends on precise bathymetry.
Impulsive force is always larger than the quasi-static force.
Force laboratory data can be effectively analysed by means of seismic analysis techniques such as deconvolution and Duhamel integral.
Considered 250 year wave but displacement maxima ~0.7mm
Site-specific wave description from directional extreme wave analysis using Ifremer's hindcast model data

Structural modelling:
Structural response of lighthouses due to waves is governed by uplift and rocking. DEM and FEM with contact interfaces between discontinuous blocks successfully used for modelling extreme wave impacts.
Limit analysis used for assessing failure using Python code used on 6 lighthouses - size, shape, density, and height above the sea level were found to be crucial.
The vulnerability of the lighthouses, in combination with extreme wave hazard can be used for prioritising the detailed analysis and structural assessment.
Uplift of contact joints in FEM achieved with discontinuous model with contact interfaces between courses (m=0.8)
Elevation of wave impact area important.
The combination of bigger diameter and greater weight near the bottom makes the lighthouse able to resist significantly bigger forces if the impact area is near the bottom.
The structural response of the lighthouse resembles a quasi-linear elastic vibration for small impacts and becomes highly nonlinear with intense rocking and joint separation for stronger impacts.
The 50 years wave impact that was calculated based on the climatic wave conditions for Wolf Rock which has a steep bottom topography, causes intense rocking and opening of the horizontal joints but it is not expected to cause collapse.
Great importance of the vertical keying to the stability of the lighthouse. Wolf Rock lighthouse would not be able to survive the impact of a 5 years return period wave without the presence of vertical keying
The continuous FE models do not yield satisfactory results. The elastic model underestimates the level of motion, especially for strong impacts, and the nonlinear model manifests unrealistic deformations.
The discontinuous FE model which allows uplift and detachment of the horizontal joints was the most reliable approach.
The structural response (i.e. horizontal displacement at control points) of the lighthouse impacted by dynamic wave forces is not proportional with the magnitude of these forces. The horizontal displacement near the top of the lighthouse increases drammatically for forces that exceed the threshold calculated by the limit analysis.
Limit analysis curves are found to be a reliable approach and a tool for quick structural assessment.
Exploitation Route Findings from STORMLAMP will be applicable to all lighthouse authorities and other marine structures users. We have just established contact with PIANC who have expressed an interest in our findings. This is an on-going area of activity which will be forming the basis of an Impact Case Study.
Sectors Construction,Digital/Communication/Information Technologies (including Software),Environment,Culture, Heritage, Museums and Collections,Transport

URL https://stormlamp.org.uk/
 
Description This project will hopefully underpin a REF2021 Impact Case Study for the University of Plymouth. We are anticipating that the UK and Irish General Lighthouse Authorities (GLAs) will be changing their approach to the long-term assessment of their rock lighthouses. The final work package of the STORMLAMP project is the development of guidelines for the GLAs and we hope that details from this document will be incorporated into the International Association of Lighthouse Authorities updated Guideline on Extreme Environmental Conditions (I was contacted by the relevant IALA committee in February 2020 to see if I could provide information).
First Year Of Impact 2020
Sector Culture, Heritage, Museums and Collections,Transport
Impact Types Cultural,Economic

 
Description Appearance of COAST laboratory tests on local news 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Media (as a channel to the public)
Results and Impact BBC Spotlight, the regional news programme filmed a piece in the University of Plymouth COAST Laboratory, while the 1:40 scale-model of the Wolf Rock lighthouse was being tested in the Ocean Basin.
Year(s) Of Engagement Activity 2019
 
Description Article published in Seaways magazine 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact We produced an article for the Nautical Institute Seaways magazine (erroneously ascribed to Communications Officer at UCL, though Alison Raby wrote it). It appeared in the April 2019 issue.
Year(s) Of Engagement Activity 2019
URL https://www.nautinst.org/resource-library/publications/seaways.html
 
Description Hong Kong lighthouse talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited to give a talk at the First International Lighthouse Symposium in Hong Kong
Year(s) Of Engagement Activity 2017
 
Description Invited presentation entitled: STORMLAMP Guidance on structural condition assessment of rock lighthouses, to Trinity House Engineering and Operations Annual Meeting 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Invited to update this Trinity House committee with the findings of the STORMLAMP project
Year(s) Of Engagement Activity 2019
 
Description Invited presentation entitled: Structural condition assessment of rock lighthouses, to IALA ENG Committee in Paris 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Presentation of the STORMLAMP project findings to the International Association of Lighthouse Authorities ENG Committee
Year(s) Of Engagement Activity 2019
 
Description Lighthouses, Bridges and Breakwaters talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact The STORMLAMP project was invited to take part in the Institution of Civil Engineers 200th anniversary celebrations in conjunction with the Plymouth Museum. We gave a talk entitled 'Wave loading on heritage lighthouses in the British Isles'
Year(s) Of Engagement Activity 2018
 
Description NI evening talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Professional Practitioners
Results and Impact Presented the lighthouse research to an audience of the Nautical Institute at an evening meeting
Year(s) Of Engagement Activity 2019
 
Description On-going blogs posted to the STORMLAMP website 
Form Of Engagement Activity Engagement focused website, blog or social media channel
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact On-going blogs posted to the STORMLAMP website have provided a useful shop window for the project. For example, we were recently filmed by a production company working for Channel 5 who were able to find out general information about the project from our blogs which focus on various aspects of the project. The company are producing 3 programmes on lighthouses, and we provided the engineering knowldege.
Year(s) Of Engagement Activity 2017,2018,2019,2020
URL https://stormlamp.org.uk/blog
 
Description Trevithick evening talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Other audiences
Results and Impact Presented the lighthouse research to an audience of the Trevithick Society at their weekly meeting in Liskeard, Cornwall
Year(s) Of Engagement Activity 2019