Comparative Study of Structural Reliability Assessment Methods for Offshore Wind Turbine Jacket Support Structures

Abstract

Offshore wind turbines (OWTs) are deployed in harsh environments often characterized by highly stochastic loads and resistance properties, thus necessitating the need for structural reliability assessment (SRA) to account for such uncertainties systematically. In this work, the SRA of an OWT jacket-type support structure is conducted, applying two stochastic methods to predict the safety level of the structure considering various design constraints. The first method refers to a commercial finite element analysis (FEA) package (DesignXplorer© from ANSYS) which employs direct simulations and the six sigma analysis function applying Latin hypercube sampling (LHS) to predict the probability of failure. The second method develops a non-intrusive formulation which maps the response of the structure through a finite number of simulations to develop a response surface, and then employs first-order reliability methods (FORM) to evaluate the reliability index and, subsequently, the probability of failure. In this analysis, five design constraints were considered: stress, fatigue, deformation, buckling, and vibration. The two methods were applied to a baseline 10-MW OWT jacket-type support structure to identify critical components. The results revealed that, for the inherent stochastic conditions, the structural components can safely withstand such conditions, as the reliability index values were found acceptable when compared with allowable values from design standards. The reliability assessment results revealed that the fatigue performance is the design-driving criterion for structural components of OWT support structures. While there was good agreement in the safety index values predicted by both methods, a limitation of the direct simulation method is in its requirement for a prohibitively large number of simulations to estimate the very low probabilities of failure in the deformation and buckling constraint cases. This limitation can be overcome through the non-intrusive formulation presented in this work.

Structural reliability assessment of offshore wind turbine support structures subjected to pitting corrosion‐fatigue: A damage tolerance modelling approach

“…The TP is 7 m in length and joins together the jacket and the tower 10,11 . The OWT support structure was modelled using ANSYS, which is a widely used FEA software 6 (see Table 1 and Figure 3A).…”

“…This study focuses on FLS because according to several studies fatigue is the design driving criteria for offshore structural members 3,12 . For a detailed description of the ULS and FLS, the readers may refer to other studies 3,6,18,50 …”

“…A solution is to deploy wind turbines at sea thus minimizing sound and visual impact, with additional capacity factor benefits obtained from the improved wind domain due to unobstructed fetch. This has led to the increasing development of offshore wind industry, especially in Europe where the offshore wind energy capacity is expected to reach 64.8 GW by 2030 1,3–7 …”

“…The fatigue damage contribution for the multiplanar welded tubular joint from wind load effects can be high (exceeding 60% of the total), and the number of hot spot stress cycles associated with wind and wave loads in a year can exceed 7 × 10 7 . Hence, the fatigue performance of members is a design driving criterion for numerous structural details of OWT foundations 6,12 …”

“…Practically, this refers to methods that use Karhunen–Leove expansion (K‐L) and polynomial chaos expansion (PCE) to directly modify the stiffness matrix of a FEA procedure, such as spectral stochastic FEM (SSFEM) and stochastic Galerkin FEM. On the other hand, non‐intrusive formulations, such as stochastic RSMs, 6,17,48 represent uncertainties in a nonexplicit way and treat the analysis code as a ‘black box’ without requiring access to the source code. Surrogate models, also referred to as meta‐models or response surface models (e.g.…”