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How to perform structural assessment of Victorian granite offshore lighthouses starting from 19th century archive drawings

This was another challenge of the STORMLAMP project. This was what we had to figure out in UCL and this post presents how we have done so.

The first step in creating a numerical model of the structure is to know its geometry. In this we were quite lucky, the archive drawings of the design and construction gave a pretty insightful view. The problem? We had to digitalise this information, creating CAD models based on these 19th century archive drawings (Figure 1). Wherever there were written dimensions, this was easy. But this was not the case everywhere. Measuring dimensions on the deformed 150 years old drawings (in digital version hence even more distortions) has not been a straightforward approach. But it was successfully carried out.

Initially, the “Finite Element Method” (FEM) software Abaqus 6.14 of Dassault Systèmes was used for the structural analysis. However, we soon discovered that a homogeneous continuous model is not able to capture realistically the structural response of such a complex structure. The lighthouses of this typology are built with an ingenious system of interlocked granite blocks. See the drawing below and the figures above.  The presence of dovetailing and vertical keys makes prohibits relative sliding between the blocks while it allows uplift and rocking between consecutive courses of stones. This highly non-linear behaviour is impossible to be captured by a continuous FEM model. 

Therefore, and after having tried different alternatives (more details here), we carried on with a more sophisticated approach. We modelled the structure as discontinuous but with contact interfaces between the courses (see Figure 3).  Such a model can reproduce the opening of horizontal joints, therefore simulating the rocking behaviour. However, there is a very big drawback: extreme computational cost. An analysis of 2 seconds of model time required around 48 hours of computing time in a workstation with 12 cores and 128 GB ram.

After having calibrated the material properties of FEM models, in terms of modes of vibration and modal frequencies (more details here), based on the experimental dynamic identification results, we were ready to run analyses for wave impact scenarios. This technique with contact interfaces, although very time consuming for creating and running the model, was able to reveal the stress levels, joint openings, vertical and horizontal displacements as a results of different wave impacts.

We analysed the scenario of the 250 year return period wave impacting the historic lighthouse of Fastnet Rock, south of Ireland (Figure 4). The structural analysis revealed modest values of maximum horizontal and vertical displacements and also low levels of principal stresses. Furthermore, the resulting displacements were in the order of magnitude of just 1 mm, while the compressive and tensile stresses remained rather low. This suggests that, even though the lighthouse was designed and built more than 100 years ago, the structure is not threatened by the current wave action.

We were very happy with the results of the discontinuous FEM approach, but then we wanted something even better. We wanted an analysis method that would require much less computational time, allow for an easier creation of the model, and enable modelling of more structural details like the vertical keys that prevent sliding. The solution was the “Distinct (often called Discrete) Element Method” (DEM). We used the DEM software 3DEC of Itasca Inc. Anyone who is familiar with DEM software programs could disagree that it allows for “easier creation of the model” . Indeed, they are right, 3DEC is was not the handiest software for creating a lighthouse model. Almost everything had to be coded. So, after writing “just” 500 lines of Python code, we had a script that can create the geometry of any lighthouse with the sole geometry inputs of external and internal diameter, keying dimensions, and height for each course of stones. See the Wolf Rock lighthouse DEM model in Figure 5.

We used the DEM model of Wolf Rock for the analysis of extreme impact, identification for 250 year return period wave using a Bayesian method taking also into consideration different scenarios regarding the prediction of the sea level rise due to climate change (scenarios of 2017 and 2067 sea level). The forces are applied between in similar areas, but 2 courses higher for the scenario estimated for the 2067 sea level (Figure 6).

In this structural analysis we mainly focused on the areas joint opening and the maximum level of joint opening, and the levels of horizontal and vertical displacements and velocities. The analysis revealed maximum horizontal and vertical displacement of around 83 mm and 27 mm respectively whereas the maximum joint opening fortunately remained lower than the height of the vertical keys. Note, that although these values do not sound extreme, they are almost a 100-fold increase compared to what we found for Fastnet Rock. This makes sense. Fastnet was constructed on an elevated rock and was also in contact with a rock mass which supports the structure laterally. Wolf Rock is completely exposed on the mercy of waves and its first course of stones lays under the sea level. Therefore, the impact height, measured from the base of the structure, is considerably bigger for Wolf Rock which results in higher overturning moments and more dramatic structural response.

So, what happens if we have to analyse a structure that really needs a FEM approach (e.g. a steel helideck) which is on top of a granite lighthouse (e.g. Wolf Rock, Eddystone, Bishop Rock, Longships, Les Hanois etc.) for which DEM is the best way to proceed? This will be another blog post but you can see our relevant publication here.

Author: Athanasios Pappas, UCL

This blog post was written by Prof Alison Raby for the EPSRC.

Picture yourself in a 150-year-old lighthouse on a rock outcrop miles from dry land during a force nine gale. You’re sitting in a small kitchen 30 metres above cauldron-like waves that occasionally rear up and smash against the tower, causing the crockery in the cupboards to rattle. Welcome to the world of a 21st-century Lighthouse Engineer!

You won’t be on station to keep the lights on, as the lighthouses have all become automated in recent decades. Instead, as maintenance crew, you’ll be staying for up to two weeks to look after anything from the central heating that keeps damp from the structure, to the sophisticated communication systems operating around the clock to keep mariners safe.

Longing for winter storms

Trinity House, a charity and statutory General Lighthouse Authority dedicated to protecting shipping and seafarers, approached us at the University of Plymouth in 2011 to join in research on these rock lighthouses. Trinity House was getting disturbing accounts from engineers on station during storms, and were concerned that the situation might only worsen with climate change.

The famous Eddystone Lighthouse was the focus of the pilot project, being the closest lighthouse to the university campus. Line-of-site views of the tower meant that we could establish control of our video cameras, linking the incoming waves with the tower vibration recordings.

The team were probably in a minority of people in the south-west of the UK longing for more storms in the winter of 2013/14, with a particular requirement that the storms arrived at high tide and in daylight.

This combination of factors meant that the waves were as big as they could be before breaking, and we could film them in all their glory. Storms Petra and Hercules did not disappoint and we ended the season with about 3,000 impact events to analyse and some preliminary structural modelling.

Not for the faint-hearted

Fast forward to 2016 and EPSRC funded a comprehensive study on lighthouses. We teamed up with the University of Exeter for field-testing and UCL for structural modelling. At Plymouth we kept our focus on the hydrodynamics, using extreme wave analysis, the COAST Laboratory, and Computational Fluid Dynamics (CFD). The two other UK General Lighthouse Authorities (Northern Lighthouse Board and Irish Lights) joined Trinity House, which enabled us to consider the geographically diverse lighthouses of Les Hanois, Wolf Rock, Longships, Bishop Rock, Fastnet, Dubh Artach and back to Eddystone.

Constrained only by aviation rules, the team flew by helicopter in all conditions to conduct vibration tests of these lighthouses. This was not for the faint-hearted: heavy equipment had to be hauled up and down steep spiral staircases (or in some towers only ladders). The work occasionally meant staying overnight, and on one occasion being stranded for the best part of a week when the fog rolled in at Fastnet.

Trouble-shooting equipment problems so remote from the lab poses particular challenges and requires a resourceful character. On two of the lighthouses (Wolf Rock and Fastnet), monitoring equipment has been installed to capture the structural response to extreme storm loads.

Untangling the ‘slamming force’

The structural modelling has moved apace, with vibration data validating the finite and discrete element models. Extreme wave analysis has provided the characteristic load for the models and is revealing some helpful findings about the best approach to represent the real structure behaviour.

Interestingly, a quick tool for structural assessment has been devised which can be applied on all these towers to show how the size and extent of the wave loading affects the stability of the structure. By combining the measurements with the modelling, we also found how the modern helideck structures affect lighthouse vibration characteristics.

In the lab we’re untangling the wave front slamming force, from the more slowly varying component which follows. We’ll combine understanding from these tests with CFD findings to give a fuller picture of wave loading on lighthouses, which may be applicable to other offshore structures like the giant wind turbines currently being deployed in relatively shallow water.

It’s a real privilege to be involved in the latest chapter of these iconic structures, and we hope that insight gained through the project keeps the lighthouse engineers, and the mariners, safe for generations to come.

Lighthouses are essential for the safety of mariners in navigating at sea but are they fit for purpose? This film looks at the undergoing research of STORMLAMP project into the impact of strong waves on lighthouse and how the structure changes.

You can enjoy some breath-taking drone aerial views and videos of the on-site work on rock mounted lighthouses, in peaceful or not so peaceful weather. Alongside with these shots, the film presents the core aspects of the STORMLAMP project and explains how the expertise of University of Plymouth, University of Exeter and University College London is combined for protecting our lighthouses and the safety of mariners.

Congratulations to Alessandro Antonini, indispensable member of STORMLAMP project for his promotion to Lecturer.  Alessandro joined Plymouth University in 2016 for working in STORMLAMP as Research Fellow. His work on analytic estimation of wave impacts, hydrodynamic lab-testing, on-site testing and data treatment has been of significant importance. He will keep contributing to the project from the position of Lecturer in Mechanical Engineering of Plymouth University.

 

Dr Alison Raby and Dr Alessandro Antonini this weekend drove model test equipment up to Oban, Scotland in advance of tests on Dubh Artach lighthouse.

Van packed and ready to go.

  

From van to helicopter, the transport gets ever more impressive.