Vibration survey of Fastnet Lighthouse, December 2016

 


 

 

On Monday  5th December a team of researchers for the EPSRC STORMLAMP were flown from Castletown-Bearhaven, Ireland to Fastnet Rock, 6 miles off the Irish mainland.

James Bassitt, Karen Faulkner and Ian Moon from University of Exeter, along with Alessandro Antonini from Plymouth University and Athanasios Pappas from UCL planned to stay overnight after carry out a modal test of the lighthouse, while James Bassitt installed a remote acceleration monitoring system.

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Test team on Fastnet Rock. From left: Athanasios, Alessandro, Karen, Ian, James

Following a series of modal tests on lighthouses at Les Hanois (Guernsey), Wolf Rock (Cornwall), Longships (Cornwall) and Bishop Rock (Scilly Isles), James, Ian and Alessandro had developed skills in modal testing using an array of accelerometers and an electrodynamic shaker. While these lighthouses are accessed via a helideck constructed above the lantern, Fastnet is accessed by a helipad on a utility building constructed on the cramped space of the rock with stone steps up to the lighthouse. A team of five was used at Fastnet to manage the equipment transfer to and from the upper level of the lighthouse and to allow James Bassitt to install and commission the monitoring system.

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Unloading the helicopter
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View of helipad from lighthouse lantern room

First measurements started at 2PM on Monday and continued until 9.30PM, with the APS shaker providing a narrow-band swept sine signal to excite structural modes that are typically in the range 4-6 Hz for this type of structure. The accelerometer array was left recording data overnight as storm winds and occasionally waves battered the lighthouse.

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Shaker and bi-axial accelerometer pair in lighthouse kitchen.

 

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Lighthouse acceleration and shaker force data used for modal identification.

While on site James Bassitt deployed a drone for aerial footage at medium and close range.

Weather conditions of wind and sea spray overcame the drone after 11 minutes of flying the machine was returned to base with full manual control after which it was unserviceable. The short flight provided spectacular views of the rock and lighthouse.

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James Bassitt operating Phantom drone from helipad

Fastnet Rock is notorious for bad weather, particularly fog, hence its existence. From Tuesday 6th until Friday 9th fog was severe enough to prevent helicopter landing, stranding the team. While having more than the mandatory supply of emergency rations the team were very happy when a break in the fog allowed the pilot to make two trips to pick up crew and equipment. The Exeter team, who travel with the equipment by van and ferry arrived in Exeter late on Saturday 10th.

Without a retrofitted helideck, Fastnet Lighthouse is a relatively ‘clean’ structure for which to create a mathematical model.  Using the experimental modal test data and masonry stiffness estimates obtained on site using a Schmidt Hammer UCL researchers on the STORMLAMP project will be able to create a high-fidelity model which is intended to be used to estimate wave loads using response data provided by the monitoring system.

Two more lighthouse modal tests are lined up for 2017; one is likely to be Dubh Artach off the Scottish West coast and Eddystone Lighthouse off Plymouth.

 

 

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Modal test of Bishop Rock Lighthouse

STORMLAMP researchers from Exeter and Plymouth concluded a sequence of measurements on an offshore rock lighthouse in the South West with a modal test of Bishop Rock lighthouse.

Bishop Rock is a massive structure, built on a rocks four miles south west of the Scilly Isles, that sheer from 45m deep in the Atlantic Ocean. The first structure, completed in 1858, was strengthened in 1881 by building a new structure around the existing lighthouse and extending the height by 12m to 44m above mean high water.

The present structure is by far the largest of the the four lighthouses studied in the STORMLAMP project, which also include: Les Hanois ,Wolf Rock and Longships.

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On a very wet and windy September Monday, James Bassitt, Ian Moon (Exeter ian_james_selfie.jpg) and Alessandro Antonini (ale_daq.jpg) flew from St Just (Land’s End) airport to carry out a sequence of vibration measurements using accelerometers aligned horizontally at the top of the lighthouse.

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After an energetic exercise laying out, connecting and configuring the sensors, horizontal accelerations were measured in the lower entrance level, engine rooms, bedroom, kitchen, service and battery rooms, lantern level and helideck (ian_helideck.jpg).

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Response due to ambient conditions of strong winds and relatively benign waves was measured, as well as the effect of artificial excitation using a 140 N electro-dynamic shaker (shaker.jpg). Measurements started at around 1PM and were completed within two hours.

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Unfortunately, the strong winds were quickly followed by low visibility on Monday afternoon, shutting down Trinity House helicopter flights and stranding the team overnight. Happily the fog horn no longer operates at Bishop Rock, dining facilities were available (dinner.jpg) and the eerie effect of the powerful twin lantern beams

provided some entertainment. The team escaped when the fog lifted on the Tuesday afternoon.

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The modal test data are being used to evaluate the structure and provide the means for inverse identification of wave loads. The next STORMLAMP mission will be to Fastnet Lighthouse in December.

The video is by Peter Cox a wonderful landscape photographer based in Ireland.

http://www.petercox.ie

VES team completes another successful data gathering exercise, at Wolf Rock

Following a successful modal test at Les Hanois lighthouse, off the coast of Guernsey, the Vibration Engineering Section embarked on a data gathering exercise at Wolf Rock lighthouse 15km southwest of Land’s End, Cornwall. This was the second in a planned sequence of modal tests of offshore rock mounted lighthouses around the British Isles, as part of the EPSRC-funded project STORMLAMP (STructural behaviour Of Rock Mounted Lighthouses At the Mercy of imPulsive waves).

After fears that the testing would be delayed, due to a dense blanket of fog over the sea at Sennen Cove, the team for Wolf Rock, comprising James Bassitt (photo 5), VES Experimental Officer, Research Fellow Emma Hudson (photo 7), Laboratory Technician Ian Moon (photo 4) and Alessandro Antonini, Research Fellow in the School of Marine Science and Engineering at the University of Plymouth (photo 8) set off for the lighthouse from St Just Airport by helicopter.

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Photo 2

 

Due to the bulk of the 350kg equipment, including a horizontal shaker, amplifiers, laptops, cabling, cameras and accelerometers, it took two helicopter journeys, courtesy of Trinity House, to transport the Wolf Rock team to the lighthouse where, on arrival, they decanted the equipment into the service room at the top of the lighthouse and installed the computers in the battery room, lowering equipment throughout the levels by rope.

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Photo 3 – inside the lighthouse

As well as carrying out modal tests in real time, the team installed accelerometers throughout the levels of the lighthouse, to capture data and measure the response of the structure during stormy weather.

Despite high winds coming off the land across the sea, the modal tests were successful, and the team left the lighthouse after seven and a half hours.

James Bassitt, who managed the data acquisition and on-site modal analysis, said: “The whole exercise was logistically harder than at Les Hanois, because the lighthouse was narrower, taller and access to the helideck was restricted.

“It was difficult to carry out the modal testing, because the background noise from the wind was far outweighing the forces we were generating. But we persevered and completed the testing successfully.”

The next data gathering exercise will take place at Longships Lighthouse, off the coast of Lands End, in August 2016.

 

World experts converge in Exeter to tackle structural vibration as ‘global challenge’

Millenium BridgeExperts in vibration serviceability of civil engineering structures from across the globe met at the University of Exeter over two days to discuss the growing challenge of vibrating structures. The best known example of this problem was the ‘wobbly’ Millennium Bridge in London, a case that brought the field of vibration serviceability from obscurity into the limelight 16 years ago.

To read more, see

Transverse Load Distribution characteristics of Exe North Bridge

The Exe North Bridge (Figure 1) is one of the two almost identical adjacent bridges crossing River Exe and forming a big roundabout in Exeter, UK. It is 60m long and consists of three spans, resting on two wall type pier structures in the river and abutments at the ends. It was constructed in 1969, so it is very close to its 50 years of designed service life.

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Figure 1. Exe North Bridge spanning the River Exe

The Vibration Engineering Section supported The University of Exeter undergraduate engineering student project “Analysis of transverse load distribution of Exe North Bridge superstructure”.

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Figure 2. Undergraduate engineering student Nick Trump installing strain transducers on the deck soffit.

The north span of the test structure was instrumented with 12 strain transducers, which made it possible to study the load shedding characteristics of the deck structure under moving load. As a test vehicle, a four-axle, 32 tonne lorry was used to obtain a quasi-static strain response. The load test was performed overnight to avoid disturbing traffic. The truck made several passes in each lane, stopping every time for 30-45 seconds to record static strain (Figure 2.).

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Figure 3. 32 tonne, 4-axle lorry remaining stationary over the bridge to record static strains.

The load test revealed that, although the structure is nearing its 50 years of designed life, it still retains significant strength reserves.

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Figure 4. VES researchers Zandy Muhammed and Farhad Huseynov supervising undergraduate student Nick Trump.

 

Successful modal test of Les Hanois Lighthouse by Vibration Engineering Section

As part of EPSRC-funded project STORMLAMP (STructural behaviour Of Rock Mounted Lighthouses At the Mercy of imPulsive waves), the Vibration Engineering Section carried out a modal test of the Les Hanois lighthouse (photo1) off the coast of Guernsey. This was the first of a planned sequence of modal tests of offshore lighthouses around the British Isles.

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photo1_Les Hannois Lighthouse

Modal tests are used in the aerospace and automotive industries to validate designs through direct measurements on prototypes. They are also used (particularly by VES) to evaluate the dynamic performance and structural condition of bridges, buildings and their components, as well as ‘special structures’ – and this is believed to be the first such exercise ever carried on an offshore lighthouse. Test equipment and procedures were carefully organised to fit within airlift capacity (photo 2) and manhandling constraints onsite, and to maximise chance of success with uncertain time constraints and on-site conditions.

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photo2_equipment ready to load

The VES team for Les Hanois, comprising James Brownjohn (photo 3a), Professor of Structural Dynamics, Research Fellow Emma Hudson (photo 3b) and Laboratory Technician Ian Moon (photo 3c) were flown to the lighthouse courtesy of Trinity House to join a maintenance crew already on board. Due to flight limitations, time on board was only four hours, which included two hours for unloading and loading the helicopter and for setting up and packing up the test equipment. However, through meticulous pre-planning and intense on-site activity, the VES team carried out the entire exercise successfully.

During the test, orthogonal pairs of accelerometers were placed in sequences at the ten levels of the lighthouse, from entrance (for boat landing), through engine room, bedroom, lantern level (photo 4) and helideck (photo 5). As well as using the sensors to measure the minute wind-induced sway of the lighthouse, structure vibrations were also forced, using an electro-dynamic shaker located outside the lantern room (photo 6). On-site analysis of the data was used to refine the test procedure in real time, and the results show the test to have captured all the structural information needed for use by other members of the STORMLAMP consortium at Plymouth University and University College London.

Emma Hudson who managed the data acquisition and on-site modal analysis said: “The testing provides an indication of how much movement we can expect from the lighthouses. Based on these modal tests, we can identify which of the lighthouses is especially problematic, then focus on that lighthouse in particular for the rest of the project.”

“We will then leave a set of accelerometers running to log data, so we can measure the response of the structure during stormy weather, when the sea waves apply large forces to the lighthouse. When these measurements are combined with the dynamic model derived from the initial modal test, we can then infer what sort of forces these waves are generating.”

She added: “It was a useful exercise because, in VES, we often deal with footbridges, offices, road bridges and stadia. Lighthouses are such narrow, enclosed spaces, with limited access. So, it was a challenge in terms of placing the equipment, and interesting to see the different dynamics of the structure.”

The next data gathering exercise will take place at Wolf Rock lighthouse, off the coast of Cornwall, in July 2016.