The choice for a rear-mounted generator is explained by O&M-preference for a modular design with high degree of ‘plug and play’ valid to all main and most minor components. Also at built in systems for separating and joining drivetrain components without requiring a jack-up vessel before commencing actual hoist operations and for securing components alignment.
The drivetrain selection process considered multiple aspects including Cost of Energy, time-to-market, expected customer preferences, technology maturity and market acceptance, and supply chain capacity and risk.
The evaluation’s baseline was a high-speed geared up scaling of the V112-3.0MW incorporating a four-stage gearbox. However, assessment showed it unlikely that the design would achieve preset reliability targets and was therefore deselected.
Second option was a direct drive system with separate main shaft, integrated bearing housing, and rear-mounted generator. Basic motivation was reducing the number of mechanical parts, but ‘to the expense’ of more electrical parts.
The third medium-speed geared concept design features a three-stage planetary gearbox with flanged connections between main shaft housing, gearbox and generator. This layout proved much more compact compared to baseline, by meeting comparable targets with direct drive regarding modular design and ease of main components exchange.
For these remaining options, direct drive and medium speed, a decision was made for a rear-mounted generator. This choiceis explained by O&M-preference for a modular design with high degree of ‘plug and play’ valid to all main and most minor components. Also preferred were built in systems for separating and joining drivetrain components without requiring a jack-up vessel before commencing actual hoist operations and for securing components alignmen.
The final business case is represented by a graph showing customer internal rate of return on investment (IRR; %) versus drivetrain replacement rate for direct drive versus medium-speed. The Vestas study main conclusions favour of medium speed, claiming:
- Medium speed offers the highest IRR if drive train replacement rates are assumed zero for both options;
- The difference in slope follows the assumption that a larger direct drive failure would require full generator exchange, whereas a medium-speed failure only requires replacement of the generator or gearbox;
- Only with a very high number of medium-speed replacements, direct drive offers higher IRR.