As turbine sizes and demands on the foundations continue to increase, foundation designers are looking at options to optimize designs.
The long-term performance of the soils supporting wind turbine foundations under cyclic loading (loads that vary in magnitude with time) has not historically been well investigated nor incorporated in design codes and standards for onshore wind turbine foundations. This limitation is widely acknowledged in the wind industry and efforts are underway to build an industry consensus on best practice to assess soil deterioration for wind turbine foundations.
In recognition of this need, the American Clean Power Association (ACP) Wind Turbine Standards Committee authorized a sub-committee to develop a report on U.S. wind turbine foundation geotechnical design practice relative to cyclic degradation of the soils below a foundation. The report is motivated by the need to ensure satisfactory foundation performance during service life, particularly when the foundation-soil interface is subjected to a high number of cycles.
I’ve had the privilege of Chairing the sub-committee which consists of an industry wide group including foundation designers, developers, turbine manufacturers, Independent Engineers and other stakeholders. The sub-committee includes a Technical Review panel of distinguished Professors and industry subject matter experts on various aspects of geotechnical and structural foundation behavior. The sub-committee work has been on-going for about 18 months and report preparation is nearing completion for publication in 2022.
Foundation uplift and deformation from a numerical model indicating uplift (Demonstrative only)
Initial discussions on the scope of the report were focused on what is referenced as the no-gapping criterion for design of gravity base (mat) foundations which are commonly used for support of wind turbines. The typical requirement is for foundations to remain in full contact with the supporting ground materials during normal power production of the wind turbine. The no gapping criterion is primarily to ensure that the foundation reliably provides the minimum stiffness for the turbine to operate within the design frequency envelope, and to limit potential for cyclic loading-induced failure of the soils supporting the foundation. It is unclear to most engineers how the no-gapping requirement was incorporated into design practice, but appears to have originated in Europe. It has been my privilege to have one of my European colleagues provided some interesting background on its beginnings. The requirement is apparently based on historical design standards that were intended to simplify designs for tall slender structures (e.g. coal chimney stacks) in some European countries. The rule was empirically derived and has been applied for more than 70 years prior to development of current methods for assessment of soil cyclic degradation. By sticking to this design approach, one could therefore easily design a sufficiently robust foundation for power plants, steel mills, chimneys, etc. based on a simple rule. Later, the no gapping criteria was adopted as a design rule worldwide for wind turbine foundation design as discussed in the Germanischer-Lloyd (GL) Rules that have been widely used in the industry (prior to the DNV-GL merger).
As turbine sizes and demands on the foundations continue to increase, foundation designers are looking at options to optimize designs. This optimization has included relaxing (or completely ignoring) the no gapping requirement. Contemporary international design standards (e.g. IEC 61400-6) include guidance on specific aspects of ground soil behavior that should be evaluated if foundations are designed to allow gapping during normal wind turbine power production. Some of the requirements in IEC 61400-6 specify quantitative assessment of soil deterioration due to cyclical loading potential and such testing is not typically performed in routine geotechnical practice in the US and Canada for onshore wind turbine foundations. An advancement in the standard of practice is therefore necessary and will inevitably be required to reliably evaluate the potential risks due to foundation gapping. Cyclic soil testing during geotechnical investigations will be a necessity for on-shore wind farms for reliable evaluation of foundations designed to incorporate gapping rather than relying on judgement. The primary mechanism that contributes to cyclic degradation is development of excess pore pressures within the soil which can result in a reduction in strength and stiffness. Laboratory testing is required to gain a reliable understanding of the behavior of soils under cyclic loading and the report under development by the ACP sub-committee will provide recommendations on specific testing that can be performed to quantify cyclic degradation risk.
Based on the recognition that guidance on assessment of ground soil deterioration due to cyclic loading is lacking for commonly utilized foundation types for onshore wind in the US and Canada, the ACP sub-committee scope includes assessment of the cause of soil deterioration (aka geomaterial degradation mechanisms) for various foundations types. The report will address shallow gravity base (mat foundations), conventional deep foundations (e.g., drilled or driven piles), monopiles and rock anchors. One important aspect the committee is currently discussing is regarding the load cases that should be utilized in the cyclic degradation assessment of ground soils. Loading information currently provided by turbine manufacturers does not appear to have sufficient information for assessment of soil cyclic degradation, as the Markov matrices typically provided for structural fatigue analysis may be insufficient for geotechnical applications. An important distinction appears to be that while the no-gapping load is a normal power production load case, the Markov matrices are constructed from fatigue load cases per IEC 61400-1. In addition, Markov matrices do not provide information on loading levels that can be expected within a finite time interval, nor the order in which the cyclic loads are applied, both of which can impact cyclic behavior of soil. Efforts are on-going to develop recommendations for loading information necessary to facilitate sufficiently robust and reliable assessment of ground soil deterioration for wind turbine foundations.
To learn how DNV can help you address foundation uplift and potential for cyclic degradation, please reach out to Eric Ntambakwa.
The author is thankful to colleagues and industry peers on the ACP TR-2 subcommittee who are volunteering their time and efforts to fill the knowledge gap to contribute to the transition to a safe and sustainable energy future.