## 5 Conclusions  
Monopile-foundation utilisation for OWTs plays an important role in supporting the development of green electricity alternatives. Further development of OWT farms is expected to include bigger and heavier turbines, to add more electricity, in addition to being moved to deeper water locations, hence increasing loads on foundations and requiring massive foundation elements. A typical foundation system for OWTs can consume up to 40% of the initial cost with monopile diameters and depths reaching 7.8 m and 80 m, respectively. Therefore, a critical review of foundation design is needed, and optimization and innovation in this field is necessary. Although there are plenty of studies on the lateral ultimate capacity of piles, few studies exist on determining the lateral ultimate capacity for monopile foundations for wind turbines, and, therefore, the aim of this paper was to close this gap in the literature by studying their lateral ultimate capacity in cohesive soils under eccentric loading that is typical for offshore locations.  
Different soil conditions were considered, and a validated FE model was employed using the hardening soil (HS) constitutive relationship with parameters calibrated and verified from field tests. Typical procedures involved creating a k\({}_{\rm o}\) geostatic stage followed by an installation stage, which was then used to apply displacement-controlled loading, from which generic curves and best-fit equations were established to obtain ultimate lateral capacity for the considered system. Several conclusions were drawn from the results and are discussed below:  
1. The normalized ultimate capacity versus normalized stiffness of MPs showed three distinct slopes, and the normalized ultimate capacity was observed to increase significantly for E\({}_{\rm P}\)*/E\({}_{50}\) values less than 984 for all L/D ratios. At normalized stiffness values of more than 2000, the effects of increasing the stiffness for a given L/D ratio was small for L/D ratios less than six and was considerable for higher L/D ratios.
2. The normalized lateral ultimate capacity increase with L/D ratio was dependent on the normalized stiffness (E\({}_{\rm P}\)*/E\({}_{50}\)) values. At large E\({}_{\rm P}\)*/E\({}_{50}\) values, the piles behaved as rigid piles and linear growth of the normalized lateral ultimate capacity was observed for the L/D ratios considered herein. As normalized stiffness (E\({}_{\rm P}\)*/E\({}_{50}\)) values decrease below 984, the effects of L/D ratio on the lateral ultimate capacity diminish around L/D values of 5-8. These findings are better understood through inspection of the bending-moment profiles for clay soils 1 and 6.
3. Higher L/D ratios had increased normalized lateral capacity for clays 1-5. In clay 6, no effects on the normalized lateral capacity were observed with changes in L/D.
4. For clay 1, increasing L/D ratio from 3.3 to 13.3 gave substantial increases in normalized lateral ultimate capacity of more than 700%. For clay 2, a similar observation was made, however, with a lower magnitude.
5. For clays 3-4, the effects of L/D ratio on the normalized lateral ultimate capacity were noticeable, amounting to 100%. However, the increase ceased after L/D values of 8.3.
6. For clays 5-6, the effects of L/D ratio on the normalized lateral ultimate capacity were negligible due to the flexibility of the monopiles.
7. Predictive polynomials showed excellent matches with the FE data, giving promising results and paving the way for more FE-based charts for preliminary estimates of monopile lateral ultimate capacity.  
Y.A.A.: Conceptualization, Interpretation, Writing- Original draft preparation. M.H.E.N.: Software and supervision. T.N.: Supervision, methodology, Reviewing and Editing. J.A.B.: Supervision. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.