Article: Fatigue and Tidal Energy

Xi Engineering

The environment in which tidal turbines operate in is harsh. Exposed to fluctuating and often cyclical loads, tidal turbines are subject to repetitive stress, which can result in cumulative damage at the microscopic scale.  The accumulation of this damage over a prolonged period of time can occur at forces far below that of the structure’s strength and can lead to failure from fatigue.

The location of tidal turbines on the ocean floor, exposed to the tidal stream, make maintenance considerably more challenging than for wind turbines. The deployment and removal of tidal turbines is expensive, driving manufacturers to design systems which are robust and require minimal maintenance. The work Xi perform on tidal turbines aims to ensure that the structure can fulfil a design life of up to 20 to 25 years and minimise the need for maintenance interventions. In working with tidal turbines, determining the risk of fatigue failure is a critical part of the successful design and running of rotational machines and their supporting structures.

Tidal turbines experience significantly greater loads than wind turbines. Although tides move at slower speeds than wind, the density of water is several orders of magnitude greater than air resulting is vastly greater energies and related loads.  As a result, the foundations and supporting structures used to anchor tidal turbines have to be really very strong.  Additionally, the loading is not regular due to waves-tides interactions as well as large scale turbulence, and this exposure to irregular loads can result in sharp spikes in vibration, making modeling these structures difficult.

To manage the challenges of representing tidal flow over a period of time, Xi apply data from a number of scenarios. Xi uses a cumulative damage approach where a range of load cases, representing several of months of tidal flow, are fed into the model and used to calculate events when the most damage occurs. Using COMSOL Multiphysics software to assess the structure, Xi can determine what stresses are going to result in damage and over its lifetime how much damage the structure can endure before it fails. Through assessment of the cumulative damage, using a rain-flow counting algorithm, Xi are able to identify the positions where most damage occurs and whether there is risk of fatigue failure. Xi can then make design recommendations regarding geometries and materials to withstand fatigue. Through providing an overview of the whole design, Xi can determine any modifications the structure requires.

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