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Lifetime optimisation case study

Case Study 2: Existing turbines with limited life left

Can new turbines still be approved?

A low design class neighbour park, a turbine with only 7.1 years of fatigue life left, and a layout under pressure. How windPRO found a solution and simplified it to 15 curtailment rules.

At a glance
Project type
Onshore wind - new development alongside existing park
Turbines
7 planned + 4 existing (design class IIA–IIC, 16–19 years remaining)
Critical finding
Turbine E2 projected at only 7.1 years fatigue life with new turbines added
Tool used
windPRO Lifetime Optimizer (LOAD RESPONSE + OPTIMIZE modules)
Key result
New turbines approved - entire park managed with 15 curtailment rules

The situation

Following the initial optimisation study (see Case Study 1), the project team obtained real data on the four existing turbines at the site. What they found raised the stakes considerably. Rather than the IIB design class assumed in the baseline analysis, the existing turbines were built to lower standards - IIC and IIA - with between 16 and 19 years of fatigue life remaining.

A lower design class means less tolerance for turbulence. Adding seven new turbines to the site would increase wake-induced loading on these machines significantly. The central question became unavoidable:

Can the planned turbines even be installed at all?

To answer it, the team incorporated the existing turbines' actual specifications into the lifetime optimisation - giving a much more accurate picture of what the new layout would mean for the whole park.

The approach

The optimisation parameters for the new turbines remained unchanged from Case Study 1: design class IA, 20-year target lifetime, IEC-standard binning across wind speed and direction. What changed was the input data for the existing turbines.

The table below shows what the team was working with:

Turbine ID
Turbine
Design class
Remaining lifetime
E1
NORDEX N117/3600 3600 116.8 !O! hub: 98,0 m (TOT: 156,4 m) (401)
IIC
18
E2
NORDEX N117/3600 3600 116.8 !O! hub: 98,0 m (TOT: 156,4 m) (402)
IIC
19 → 7.1 years projected comptab-infoalt7-icon
E3
NORDEX N117/3600 3600 116.8 !O! hub: 98,0 m (TOT: 156,4 m) (403)
IIA
16
E4
NORDEX N117/3600 3600 116.8 !O! hub: 98,0 m (TOT: 156,4 m) (404)
IIA
18

 

The IIC design class of turbines E1 and E2 is the core problem. These machines were not built to handle high turbulence loads. In their current position, relative to the planned new turbines, they would be exposed to significant additional wake loading - far beyond what their design class was intended to withstand.

Phase 1

The discovery: E2 has 7.1 years left

The optimisation revealed the extent of the problem immediately. Without curtailment, turbine E2 - already the weakest machine on site - was projected to have just 7.1 years of fatigue life remaining once the new turbines were operational.

Its IIC design class made it particularly vulnerable to wake turbulence from the planned machines nearby.

To extend E2's life to an acceptable level, the Optimizer determined that frequent shutdowns would be required across a wide range of wind conditions - limiting the turbulence it experiences from neighbouring machines.

Picture13

E2 projected fatigue life (no curtailment)

7.1 years

Down from 19 years remaining - driven by wake turbulence from the new turbines and its low IIC design class.

Average AEP loss across all turbines

22%

The shutdowns needed to protect E2 cascade across the park, with new turbines bearing up to 38% AEP loss individually.

Picture14
Curtailment matrix for Turbine 6

Surprisingly, a solution exists

Despite E2's low design class and the cascading shutdowns required, the Lifetime Optimizer was able to find a curtailment scheme that brings all turbines within acceptable fatigue limits - meaning the new turbines can be approved for installation, even within the highly restricted area.

Phase 2

The next problem: too complex to implement

The turbine manufacturer reviewed the optimised curtailment matrices and raised a practical objection: the original scheme involved too many decision points to implement reliably in the field. Each matrix contained a large number of wind speed and direction combinations, each with a different curtailment response. For a real wind farm control system, this level of granularity is unmanageable.

The Optimizer was re-run with two adjustments: higher thresholds for triggering curtailment, and a penalty for non-contiguous curtailment rules (favouring schemes that group conditions into broader, simpler blocks). This came at a cost of an additional 2.5% AEP loss - but the result was dramatically simpler.

Original scheme
Simplified scheme
Number of curtailment rules
Many - too complex to implement
15 rules total
Additional AEP loss vs. Phase 1
comptab-no-icon
+2.5%
Implementable by turbine manufacturer
No
Yes
Project approval
comptab-no-icon
Granted

What 15 rules looks like in practice

Each rule defines a wind speed range and direction sector that triggers a specific turbine response. For example:

Turbine 4 → SHUTDOWN if wind speed 4–25 m/s AND direction 45°–75°
Setup strategy lifetime optimisation
Optimisation settings (simplified scheme)
Curtailment matrix lifetime optimisation
New curtailment matrix for Turbine 6

The outcome

The new turbines received approval for installation. Despite a constrained site, low design class existing turbines, and a neighbour park that had almost eliminated any available fatigue margin for E2.

The total AEP loss under the final simplified scheme is higher than Case Study 1 - a direct consequence of the existing turbines' vulnerability.

But the alternative was no installation at all. The Optimizer did not just find a theoretical solution; it found one that could actually be handed to a turbine manufacturer and implemented.

Original scheme
Simplified scheme
Number of curtailment rules
Many - too complex to implement
15 rules total
Additional AEP loss vs. Phase 1
comptab-no-icon
+2.5%
Implementable by turbine manufacturer
No
Yes
Project approval
comptab-no-icon
Granted

What 15 rules looks like in practice

Each rule defines a wind speed range and direction sector that triggers a specific turbine response. For example:

Turbine 4 → SHUTDOWN if wind speed 4–25 m/s AND direction 45°–75°
Setup strategy lifetime optimisation
Optimisation settings (simplified scheme)
Curtailment matrix lifetime optimisation
New curtailment matrix for Turbine 6

Key takeaways

Low design class existing turbines (IIC) can become the binding constraint for a new project - even if the new turbines are designed to a higher standard

Without curtailment, turbine E2's fatigue life dropped from 19 years to just 7.1 years once new turbine wakes were accounted for

The Optimizer found a valid solution even in this highly constrained scenario - enabling project approval

A second optimisation pass simplified the scheme from an unmanageable number of rules to just 15 at a cost of only +2.5% additional AEP loss

Optimised curtailment is not just a fatigue tool - it is a project approval tool

Previous case study

Constrained layout, 20-year lifetime

How optimised curtailment delivered a net +13.6% lifetime energy gain on the same site and why annual AEP loss is the wrong metric to evaluate curtailment.

The modules you need

Key modules for Lifetime Optimization

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  • Introduction to windPRO
  • Advanced Wind Resource Assessment
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