Structural Analysis of Tension Leg Platforms for Floating Wind Turbines
1, Markus Starr1
1DNV GL, Hamburg, Germany, 2University of Applied Sciences Flensburg, Flensburg, Germany
The work discussed in the presentation lays its main aim in the development of structural analysis methodologies for tension leg platforms (TLP) for floating wind turbines (FWT).Its purpose is to serve as preparatory work for the development of a new design standard for floating wind turbines. The fundamentals of wind and wave theory, wind and wave induced loads to offshore structure, as well as relevant topics in the field of structural engineering are introduced as a theoretical basis. TLP in their application to FWT are introduced with their advantages and disadvantages in relation to other FWT concepts and their specific characteristics as a floating offshore structure.
The verification process of a FWT is identified by three work processes, the definition of the environmental conditions, the arrangement and realization of a load calculation, and the structural analysis of the support construction. The focus of the work is the structural analysis, with limitation to Ultimate Load States.
Based on a given open source TLP design, the support structure is fragmented in its components. Each component is categorized, verifiable by parametric equations or verifiable by computational methods only.
For the components verifiable by parametric equations, the relevant standards are examined and example calculations are performed for the required verification.
One component is verified using the Finite Element Analysis. Considering the difficulties of FEA for FWT, a methodology study is performed comparing various simulation methods.
The analysis of the decomposed support structure shows that existing standards can be applied for most of the components. Performed example calculations supported the general feasibility of the analyzed design and most stress and stability requirements given by standards and recommended practices are met.
The methodology study results display that for a ULS consideration, all load driven simulation methods are generally applicable. In the prospect of fatigue analyses however, further developments and improvements are considered crucial. This especially includes a better coherence of the stiffness representation between global load simulation model and local structural analysis model.
The analysis indicated that the dynamic coupling between wind and wave loading is generally an assignment mainly to be considered in the load estimation, not the structural analysis. Once the sectional loads are generated the structural analysis can basically use many of the already existing standards and recommended practices. Parametric equations given in the standards however do fail to be applicable due to singularities in the geometric design of floating wind turbine support structures. With the necessity of a particular analysis of new, unique designs the challenges coming with computational methods of such structures have to be met.
A correct stiffness representation between different modelling methods in load calculation and structural analysis appears to be a major requirement that has to be met during future verification processes of floating wind turbine structures.