# LCOE trend in a RAFT Optimization

**URL:** <https://forums.nlr.gov/t/lcoe-trend-in-a-raft-optimization/7932>\
**Category:** Computer-Aided Engineering Software Tools\
**Created:** [December 3, 2024, 1:22pm UTC](https://forums.nlr.gov/t/lcoe-trend-in-a-raft-optimization/7932 "2024-12-03T13:22:38Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Vincenzo.Auriemma](https://avatars.discourse-cdn.com/v4/letter/v/57b2e6/32.png) [@Vincenzo.Auriemma](https://forums.nlr.gov/u/Vincenzo.Auriemma)\
**Post date:** [December 3, 2024, 1:22pm UTC](https://forums.nlr.gov/t/lcoe-trend-in-a-raft-optimization/7932/1 "2024-12-03T13:22:38Z")

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Dear all,

I was exploring a constrained optimization problem in WEIS, using the level1 (RAFT) option for a single floating turbine ([15MW](https://github.com/WISDEM/WEIS/blob/main/examples/06_IEA-15-240-RWT/IEA-15-240-RWT_VolturnUS-S.yaml)).  
I have defined the following modeling and analysis options:

modelling options:

> WISDEM:  
> RotorSE:  
> flag: True  
> spar\_cap\_ss: Spar\_Cap\_SS  
> spar\_cap\_ps: Spar\_Cap\_PS  
> te\_ss: TE\_reinforcement\_SS  
> te\_ps: TE\_reinforcement\_PS  
> TowerSE:  
> flag: True  
> DriveSE:  
> flag: True  
> FloatingSE:  
> flag: True  
> BOS:  
> flag: true
> 
> Level1:  
> flag: True  
> potential\_model\_override: 0  
> trim\_ballast: 0  
> heave\_tol: 1  
> save\_designs: True

analysis options:

> constraints:  
> control:  
> rotor\_overspeed:  
> flag: False  
> min: 0.0  
> max: 0.25  
> Max\_PtfmPitch:  
> flag: True  
> max: 5.5  
> Std\_PtfmPitch:  
> flag: True  
> max: 2.  
> Max\_Offset:  
> flag: True  
> max: 30.  
> floating:  
> stress:  
> flag: True  
> global\_buckling:  
> flag: True  
> shell\_buckling:  
> flag: True
> 
> merit\_figure\_user:  
> name: financese.lcoe
> 
> driver:  
> optimization:  
> flag: True # Flag to enable optimization  
> solver: COBYLA # Optimization solver. Other options are ‘SLSQP’ - ‘CONMIN’  
> tol: 1.e-6 # Optimality tolerance  
> max\_iter: 100 # Maximum number of iterations (SLSQP)

The optimization seems to be successfully completed, converging to a final geometry of the platform. Nonetheless, I have noticed that during the process a single iteration shows a lower LCOE value compared to the final converged result, seemingly with no violation of the imposed constraints.

 ![Optimization_path](https://europe1.discourse-cdn.com/nrel/original/2X/2/21b3aa3eaaacbe9481faf1ee47cac85d49a38aac.jpeg)

May you please help me to understand the trend followed by the optimization process and the possible issues related to the singular lower LCOE value observed along the convergence path.

Thanks in advance  
Best regards,

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**Author:** ![Daniel.Zalkind](https://avatars.discourse-cdn.com/v4/letter/d/5daacb/32.png) [@Daniel.Zalkind](https://forums.nlr.gov/u/Daniel.Zalkind)\
**Post date:** [December 3, 2024, 9:23pm UTC](https://forums.nlr.gov/t/lcoe-trend-in-a-raft-optimization/7932/2 "2024-12-03T21:23:09Z")

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Hi Vincenzo,

Thanks for the question.

I don’t know of any WEIS studies using the merit figure you’ve selected, so my response will contain some level of conjecture.

I am guessing that some constraint has stopped the LCOE from decreasing further. Otherwise, your optimization would not have converged. I recommend finding which constraints are violated or nearly violated. It’s possible that you have found another local minima in those points. The COBYLA solver constructs linear approximations of the merit figure and constraints within a trust region.

We typically use the `LN_COBYLA` solver (the NLOpt implementation) because the standard scipy solver is known to violate constraints. Also, it’s known that the stress and buckling constraints may not be adequately modeled in WISDEM for floating substructures, so I would verify those constraints against another method, as well.

I hope this helps.

Best, Dan