Wolf Rock lighthouse: past developments and future survivability under wave loading

Abstract

Lighthouses situated on exposed rocky outcrops warn mariners of the dangers that lurk beneath the waves. They were first constructed when approaches to wave loading and structural response were relatively unsophisticated, essentially learning from previous failures. Here, we chart the evolution of lighthouses on the Wolf Rock, situated between Land's End and the Isles of Scilly in the UK. The first empirical approaches are described, followed by design aspects of the present tower, informed by innovations developed on other rocky outcrops. We focus on a particular development associated with the automation of lighthouses: the helideck platform. The design concept is described and the structure then scrutinized for future survivability, using the latest structural modelling techniques of the entire lighthouse and helideck. Model validation data were obtained through a complex logistical field operation and experimental modal analysis. Extreme wave loading for the model required the identification of the 250-year return period wave using a Bayesian method with informative prior distributions, for two different scenarios (2017 and 2067). The structural models predict responses of the helideck to wave loading which is characterized by differential displacements of 0.093 m (2017) and 0.115 m (2067) with associated high tension forces and plastic strain. This article is part of the theme issue ‘Environmental loading of heritage structures’.

Environmental loading of heritage structures

“…The Special Issue features comprehensive investigations undertaken on the Wolf Rock lighthouse [1], located eight nautical miles from Land's End at the southwest tip of England, and one of the most exposed lighthouses in the British Isles (figure 1).…”

“…Tony Martinez, who had served on Wolf Rock in the early seventies, described the sound of an impacting wave like being in an oil drum and banging it with a hammer. Interestingly, he reckoned that the period of vibration of the tower had increased following the installation of the helideck; however, the structural modelling of the tower and helideck [1] do not support this particular observation. Jument), castellated (Les Pierres Noires) and even church-like (Tévennec), but also because they experience some of the most extreme wave climates of any lighthouses in the world.…”

“…In this light, Bayesian inference can be successfully used to deal with this unavoidable uncertainty of the results, incorporating different sources of prior information/knowledge providing a rigorous methodology for quantifying the involved uncertainties in the problem. The versatility of the Bayesian method is reflected in the applications present in this issue: Bartoli et al [13] use a Bayesian model updating framework with probabilistic structural analyses leading to fragility curves for seismic loading and Raby et al [1] use a Bayesian method with informative prior distributions to consider how the wave climate may change 50 years into the future at the Wolf Rock and show the potential implications on the lighthouse. Finally, a time-dependent contemporary probabilistic design approach is applied to the Luarca breakwater in Spain by Lara et al [10], showing how the structure serviceability and stability have been evolving over time in a context of uncertainty.…”

“…Regarding the structural response to environmental loading, several of the contributions to this special issue undertake structural modelling with finite-element modelling (FEM) or discreteelement modelling (DEM). Raby et al [1] describe a combined FEM and DEM approach to investigate the structural response of the Wolf Rock helideck finding that it could present localized damage due to the vibration of the tower caused by extreme wave impacts. Validating a model is not without logistical challenges, as described by Raby et al [1] and Denarié & Fady [4], but the rewards include the possibility of conducting inverse analysis to determine the actual load, for a given monitored response.…”

“…Raby et al [1] describe a combined FEM and DEM approach to investigate the structural response of the Wolf Rock helideck finding that it could present localized damage due to the vibration of the tower caused by extreme wave impacts. Validating a model is not without logistical challenges, as described by Raby et al [1] and Denarié & Fady [4], but the rewards include the possibility of conducting inverse analysis to determine the actual load, for a given monitored response. FEM was also undertaken by Carter et al [12] to determine likely failure modes on the transporter bridges and by Guedes et al [11] to assess the wind and earthquake loading effect on brick masonry industrial chimneys.…”