Please, find the comments in the attachment

Lifetime extension of wind turbines is a very important topic of high industrial relevance. Using a probabilistic approach is also very relevant. Therefore, the paper is of interest to be published. However, there are a number of unclear sections and missing explanations, see below. A major revision is recommended.

Detailed Comments:

Line 79

’ One must consider that the material properties are calibrated such that the target reliability level of 3.7 ISO-2394 (2015) is reached’: this is an annual reliability index? And why 3.7? which components are considered?

Line 80

‘the levels are not’: unclear – reformulate

Line 161

Which ‘exponent’? wind shear?

Table 2

Is full (Weibull) distribution of turbulence used as specified in IEC 61400-1:2019? And if not add a comment on the potential influence

Line 174

Rayleigh

Line 174 + 178

Why use two different editions. Use ed 4 in order to obtain up-to-date comparisons?

Line 190

Explain equation

Eq (4)

Missing m in eq?

Eq (6)

k ?

M to power m ?

for composites the mean stress level is important. How is that accounted for?

Line 213-215

Unclear – reformulate

Eq (8)

How is lifetime damage obtained from 10-min damage?

Line 218

‘probability of turbulence’: which turbulence (ambient, effective, …) is the probability linked to?

Line 221

Conditional probabilities?

Line 235

Explain why ‘log’ is used

Line 240

Describe what is 30-year return loads’. Is it 30-year extreme loads to account for the extreme loads being important due to the high Wohler exponent?

Line 240

‘Forming a database based on the distribution’: unclear – explain which distribution. If the realizations follow the distribution function how is new information obtained?

Eq (10)+(11)+(12)

Explain the probabilistic assumption behind eq (10)

Eq (1): Lr is not included in the right hand side of the equation?

Probabilities in eq (11) always between 0 and 1?

Explain reference times for the probabilities

Are the loads obtained ‘random point in time’ loads or maximum loads with a certain reference period?

Section 2.4.4

The Frandsen and IEC turbulence models together with partial safety factors are intended for deterministic design and not for probabilistic design and reliability analysis. This link should be included in the probabilistic formulations.

Line 268

‘probability of failure at time t and can be stated as the probability of exceeding a certain level’: this probability is the probability of failure at time t and not the accumulated probability of failure up to time t and also not the annual probability of failure?

Figure 2

The uncertainty of DEL is modelled by log(DELlifetime) ? add description of the uncertainty modelled by DELlifetime . How is this uncertainty quantified and does it include model uncertainty in estimating the stress ranges (obtained from a validation process)? This stochastic modelling assumes that strain gauge measurements are available for the fatigue detail considered?

Eq (14)

Where does the time t enter in the limit state equation?

Line 288

Explain how R=10 is used and why R=10 to account for mean stress level?

Table 3

How is the mean value calibrated?

Mean and standard deviation of log(DELlifetime) are missing in the table?

Line 295

‘based on survival in the year before’: not correct – reformulate

Figure 2

Add explanation of all symbols in the figure

Figure 3

Could a Weibull distribution (as used in IEC 61400-1:20+29) fitted to the upper tail be as representative as the distributions considered?

Line 336

‘extrapolate the distribution to a 30-year return load’: figure 3 shows random point in time observations of the turbulence level. Is this distribution used to estimate the load with a return period of 30 years? Or is the load with a return period of 30 years estimated using e.g. a peak-over-threshold technique considering the extreme, statistical independent loads observed during the measurement period (as in DLC 1.1 load extrapolation)? More explanation is needed. 

And how to use the load with a return period of 30 years for fatigue assessment?

Line 357

The target annual reliability index in IEC 61400-1 Annex K is 3.3 (and not 3.7 as indicated in some DNV standards – assuming a ductile failure mode)

Figure 7

As mentioned above the IEC and Frandsen models are intended for deterministic design with safety factors, not for reliability analyses. Recommendation: use the same approach for reliability analysis as in papers and reports related to fatigue of welded steel details in wind turbines.

Table 4

How is sensitivity defined?