Presenting PIANC The World Association for Waterborne Transport Infrastructure

Presentation by Professor Alison Raby

at the occasion of

AGA 2021 / 26 & 27 May 2021 Wave Loading on Rock

Professor Alison Raby, University of , STORMLAMP project

AGA 2021 / 26 & 27 May 2021 UK UKGeneral General LighthouseLighthouse Authorities Authorities

UK GLAs:  – , , the and  Northern Board – Scotland and the Isle of Man  Irish Lights - Ireland  Remit is to deliver an aid to navigation service to assist the safety of all classes of mariners in navigation.  UK GLAs are committed to keeping physical aids, including several rock lighthouses situated offshore.

But why keep the rock lighthouses in an era of satellite navigation and modern light beacons?

AGA 2021 / 26 & 27 May 2021 Why keep the rock lighthouses? geosynchronous Earth orbit

low Earth orbit

There are 500,000 pieces of debris the size of a marble or larger.

There are many millions of pieces of debris that are so small they can’t be tracked. NASA

AGA 2021 / 26 & 27 May 2021 Why keep the rock lighthouses?

Debris impacts on Mir's solar panels

2009 collision between the Iridium 33 communications satellite and the derelict Russian Kosmos 2251 spacecraft, resulted in the destruction of both satellites

AGA 2021 / 26 & 27 May 2021 Motivation for GLAs concern?

Eddystone Lighthouse Principal keeper’s log: - particularly violent storm caused “considerable motion of the cylinder glasses fixed in the lamps”… - tower appeared to “jump as if resting on an elastic body”. 1800s

Bishop Rock Lighthouse report: …awoken by severe wave impact accompanied by a loud booming noise …oscillation of tower similar to minor earthquake …accompanying groaning noise with a rattling …chinking of glasses in cupboards …when seated, motion felt like gentle rocking.

5th February 2014 AGA 2021 / 26 & 27 May 2021 Motivation for GLAs concern?

• Climate change predictions of increasingly frequent and intense storms (IPCC, 2011). • Contacted University of Plymouth in 2010 via AECOM, due to expertise in wave impact loading e.g. BS6349 Maritime Works. • Pilot project on , 14 miles offshore of Plymouth: Wave Loading on Rock Lighthouses.

AGA 2021 / 26 & 27 May 2021 STORMLAMP project

Prof Prof D’Ayala Brownjohn Dr Pappas Mr Bassitt Expertise in FEM/DEM Expertise in of masonry structures field testing of structures

Prof Raby Expertise in wave Industrial Partners: Prof Greaves hydrodynamics Dr Ransley (statistical, physical & Roland Pyzer, AECOM Dr Antonini numerical) Andrew Wareing, Atkins Dr Dassanayake Nick Ely, Environment Agency William Allsop, HR Wallingford (rtd.) AGA 2021 / 26 & 27 May 2021 STORMLAMP investigations

1. Field modal 2. Long-term 3. Structural 4. Hydrodynamic testing monitoring analysis modelling

AGA 2021 / 26 & 27 May 2021 Investigation 1 - Field modal testing

AGA 2021 / 26 & 27 May 2021 Field modal testing: overview (1/2)

• System identification of seven lighthouses, two without helidecks, 2016- 2019. • Single day or overnight (sometimes enforced) stays. • Technology of forced vibration testing (FVT) and ambient vibration survey (AVS).

AGA 2021 / 26 & 27 May 2021 Field modal testing: overview (2/2)

AGA 2021 / 26 & 27 May 2021 Field modal testing: key findings

• Helidecks modify the modal mass, and hence the response to impulsive breaking wave loads. • Modal identification is extremely useful for inverse analysis of monitoring data, allowing estimation of wave loads.

Implications

• Analytical studies and sample extra measurements suggest that targeted campaign on helidecks would be useful for supporting inspections.

AGA 2021 / 26 & 27 May 2021 Investigation 2: Long-term monitoring

AGA 2021 / 26 & 27 May 2021 Long-term monitoring: overview

Triaxial 3/4G mobile accelerometer communication

AGA 2021 / 26 & 27 May 2021 Long-term monitoring: camera system

• Plymouth sub-contract designed a ruggedised stereo camera system with sophisticated control features e.g. the system ‘sleeps’ until woken for storm measurements to preserve power. • Input from from Dr Alvise Benetazzo from ISMAR – CNR. • Communicates via SMS messages.

AGA 2021 / 26 & 27 May 2021 Cameras

AGA 2021 / 26 & 27 May 2021 Long-term monitoring: key findings

• Accelerations due to wave impacts occasionally exceeded 0.2 g. Implications • On-going monitoring of structural response at to yield valuable data to estimate wave loading, useful for future design work. • Operational decisions by considering vibration thresholds.

AGA 2021 / 26 & 27 May 2021 Investigation 3: Structural analysis

AGA 2021 / 26 & 27 May 2021 Structural analysis : overview

The structural response of the Lighthouses to wave impacts is assessed with:

• Limit Analysis • Finite Element Method (FEM) • Discrete Element Method (DEM) (+ combination between DEM and FEM for the assessment of the helidecks)

Limit analysis FEM DEM

AGA 2021 / 26 & 27 May 2021 Limit Analysis

Calculate the critical overturning and sliding loads

Resultant force >> Resultant force >> Sliding limit Overturning limit But... the interlocking prevents Uplift is expected ! any sliding !

INTENSE ROCKING

17.32 m

AGA 2021 / 26 & 27 May 2021 FEM

Software: Abaqus 6.14 (Dassault Systèmes) Different FEM modelling approaches tested

MODEL MODEL MODEL #1 #2 #3 Homogeneous continuous material Discontinuous structure Linear Non-linear

Unsuitable Unsuitable Suitable for strong for anything... for strong impacts impacts •Unrealistic for dovetailed •Too stiff (-) structures (-) •Opening of horizontal joints (+) •Very small •Rocking behaviour (+) deformations (-) •Extreme computational cost (-) •Very high stresses (-) AGA 2021 / 26 & 27 May 2021 DEM

Software: 3DEC 5.0 (Itasca Inc.)

• Allows detachment and separation of blocks • Essential for this non-continuous

structure Max • Detailed geometry is needed opening

Vertical keying Max opening

Max opening

AGA 2021 / 26 & 27 May 2021 Helideck assessment

DEM analysis for the masonry tower and FEM analysis for the steel helideck 250 year return period wave (2017 - 2067) with sea level rise and future wave climate

Plastic strain Most problematic areas: tops of the vertical gallery posts

Plastic strain levels around 7% and 12% of the Damage is expected assumed plastic strain capacity (2017 and 2067 but not immediate scenarios respectively) collapse AGA 2021 / 26 & 27 May 2021 Structural analysis : key findings

• 250-year return period wave impact for Wolf Rock causes intense rocking and opening of the horizontal joints which it would not be able to survive except for the presence of vertical keying. • Wolf Rock helideck experiences large axial forces on vertical gallery posts, enough to damage bolts at the base of the helideck and the encastrated bars. Implications • Structural verification can be prioritised as potential failure mechanisms have been identified and damage prone areas (e.g. helidecks, pointing between courses) highlighted.

AGA 2021 / 26 & 27 May 2021 Investigation 4: Hydrodynamic modelling

AGA 2021 / 26 & 27 May 2021 Hydrodynamic modelling: overview

• Wave climate predictions 50 years into the future. • Creation of physical and numerical models of the wave interacting with the reef then tower. • Investigations provide: • insight into the wave transformation process to understand the wave runup and wave load distributions; • method for assessing wave loading on specific and generic structures; and • input data for structural models.

AGA 2021 / 26 & 27 May 2021 Hydrodynamic modelling: field and numerical details

Wave loading estimations using Bayesian extreme analysis at different locations.

Computational Fluid Dynamics using varied parameters: Hs, Tp, g, shoal shape, shoal height and area, water depth.

AGA 2021 / 26 & 27 May 2021 Hydrodynamic modelling: tank testing details

• Flume (35m) tests with generic cylinder at 1:81 scale.

• Coastal basin (15.5m x 10m) with generic cylinder tests at 1:81 scale. +++ SCALE +++ +++ COMPLEXITY • Ocean basin tests with Wolf Rock lighthouse replica (35m x 15.5m x 3m) at 1:40 scale.

AGA 2021 / 26 & 27 May 2021 AGA 2021 / 26 & 27 May 2021 Time-lapse video of Ocean basin

AGA 2021 / 26 & 27 May 2021 Ocean basin instrumentation

• Total force from 6 DOF load cell • 12 pressure transducers • Optical tracking system • Surface elevation gauges • High speed camera

AGA 2021 / 26 & 27 May 2021 AGA 2021 / 26 & 27 May 2021 Hydrodynamic modelling: key findings

Wave climate: • Wave climates at Fastnet and Wolf show no dramatic increases in wave height but even modest changes in wave loading has implications for structural response, combined with sea level rise. Individual wave impacts: • Lighthouses are affected by broken wave impacts - intensity is reduced but duration is increased, compared to directly breaking wave. • Wave runup can be 3 x wave height. • Wave runup and total force have dependence on the spectral content of a storm.

AGA 2021 / 26 & 27 May 2021 Guidance for lighthouse authorities

Procedure for investigating the structural behaviour of rock lighthouses under wave loading

AGA 2021 / 26 & 27 May 2021 Where you can find out more stormlamp.org.uk

AGA 2021 / 26 & 27 May 2021 Where you can find out more

AGA 2021 / 26 & 27 May 2021 Other lighthouse investigations

AGA 2021 / 26 & 27 May 2021 THANK YOU

AGA 2021 / 26 & 27 May 2021 Q&A

AGA 2021 / 26 & 27 May 2021