Structural verification of the pioneering Litus Lux lighthouse

Litus Lux

Litus Lux installed onshore. It can also be installed on an offshore foundation.

The Norwegian Coastal Administration has developed a new AtoN (Aids to Navigation), called Litus Lux. This new Aton is designed according to new international standards for safe sailing. The Aton will be installed from the Swedish boarder in South of Norway all the way up North, including Spitsbergen (Svalbard). This AtoN is specially designed as a maintenance free structure using solar energy as power source, strong enough to withstand all types of weather. Shown in Figure 1, Litus Lux is a 6 m tall solar powered lighthouse made completely from glass fiber reinforced polymer (GRP). cDynamics has performed a structural verification.

 

How much wind can it take?

Induced velocity pressure on the Litus Lux lighthouse during wind speed of 52 m/s.

As Litus Lux is placed in some of the harshest coastal environment on the northern hemisphere, it is inevitable that it will experience some unpleasantly high wind loads. But exactly how extreme weather can we expect that the Litus Lux can withstand, and where will the first failure occur? These are the questions the designer of the AtoN, Per Erik Ose at Kystverket asked the prototype manufacturers EasyForm, who kindly passed the task along to us at cDynamics.

To answer these questions we used the Eurocode methodology for wind loads, and applied a wind induced velocity pressure on each surface of the structure, as shown in Figure 2. This methodology is not as accurate as using computational fluid dynamics (CFD) to estimate the pressure field, but it is fast, conservative, and good enough for static structures. In cases where the elasticity of the structure significantly influences the loads, such as slender wind turbine blades or Wavefoils, a coupled CFD and finite element analysis (also known as fluid structure interaction, FSI) can lead to significantly lower loads. The wind speed applied in the following analysis is, according to the national annex of eurocode, the highest that can occur in Norway.

 

Tsai-Wu utilization of Litus Lux, values blurred.

Singleskin construction with internal stiffeners

The structural model is shown in Figure 3, with the Tsai-Wu response plotted. All the wall-, roof-, and foundation-panels of the lighthouse are singleskin GRP. Four of the six wall panels have vertical stiffeners, while the panel with the doors and hatch, as well as the one opposite, are without stiffeners. There are also three ring stiffeners at different heights.

Vertical strain component in the un-stiffened panel in the lower part of the lighthouse.

Structural response

The widely accepted failure criterion Tsai-Wu is used to assess the strain-utilization of the GRP-panels. Although all the panels are within the acceptable limit for the 52 m/s wind speed by a significant margin, Figure 3 shows that there are some potential hot spots around the door frame. Buckling is assessed by computing a panel-specific strain limit which is smaller for bigger panels. Since the un-stiffened panel opposite the door is both the largest panel, and also one that experiences high vertical forces, buckling is a relevant failure mode here. Figure 4 shows the vertical strain component in the lowest part of the lighthouse. However, if need be, one can increase the buckling capacity of this panel by introducing a vertical stiffener.

Total displacement (in millimeters) of Litus Lux during 52 m/s wind load.

Litus Lux can withstand wind gusts of 52 m/s

The structural verification confirmed that Litus Lux is a robust construction that is made for nature. The analyses made by cDynamics checked a range of possible failure modes, and found that none of them become critical before wind gusts exceed 52 m/s. Even more, in this extreme wind condition, the top of the roof will only move 4 mm compared to initial position, and the lantern will point more or less completely horizontal as it is supposed to. We therefore conclude that Litus Lux not only will survive, but also successfully operate, during the most extreme Norwegian winter storms.

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cDynamics AS was founded in 2015 as an engineering consultancy supplying technical expertise in CAD, CFD and Structural Analysis (FEA) to the composite, shipbuilding and renewable industries of Norway.