# Decision Support Models for Operations and Maintenance for Offshore Wind Farms: A Review
## 2 Operation and Maintenance: Influential Factors
### Transportation and Vessel Routing  
In addition to the availability of crew, spare parts and vessels, the transport of crew and parts to the offshore wind turbine must be organized. Therefore, a route must be established for the vessels to unload and pick-up crew and parts. Those tasks requiring a vessel or crane at the turbine need to be scheduled in the vessels route as well. [PERSON] and [PERSON] [46] analyzed the transport of crew and small parts for the DOWEC reference wind farm [51]. They combine five different vessel types and four different maintenance categories. Their simulation shows that high availability can be achieved for a hard to access farm, by optimizing the access system and maintenance strategy. Three different transport options are considered by [PERSON] and [PERSON] [10]. The simplest option is to always access the wind turbines with a boat if a repair needs to be conducted. Option two is to conduct the repair as soon as possible, using a helicopter if access with a boat is restricted due to waves. The third option is to do a risk-based analysis to determine the transport alternative. Here the cheapest solution, including production losses, is calculated based on a weather forecast that is assumed to be perfect. [PERSON] et al. [52] investigate fleet composition and vessel routing for offshore oil and gas industries. Their optimization model provides weekly routes and schedules for the vessels. Since offshore wind turbine maintenance vessels differ from offshore supply vessels and since the needs for corrective maintenance and immediate access are hard to plan very far ahead, the model needs to be modified to solve vessel routing problems for offshore wind. [PERSON] and [PERSON] [16] do not model the vessel routing but consider a fixed travel time for each vessel type from the maintenance base to the wind farm instead. In [PERSON] et al. [17] and [PERSON] and [PERSON] [21], vessels travel the shortest distance between their location and destination. A vessel specific speed is used to calculate the travel times. However, no optimization of the routing takes place. [PERSON] et al. [19] compare different access restriction for the vessel routing. They use both single- and multi-criteria restrictions and show that a single value limit (significant wave height) is sufficient, as long as the value is estimated in a correct way. [PERSON] et al. [20] consider three types of transportation, with a different routing plan each. For the crew transfer vessels and helicopters, the first mobilized transport system is routed such that the number of turbine visits is maximized, while minimizing the total number of vessels/helicopters used. The workers are dropped off at the turbines and later collected, such that the vessel/helicopter is free to travel within the wind farm during the shift. Tasks that can be completed within one shift are prioritized. The offshore access vessel visits the turbines sequentially and stays at the turbine while the maintenance is conducted, it is only limited by daylight and weather restrictions. Also, the jack-up vessel visits the turbines that need maintenance in sequence and is limited by weather restrictions. [PERSON] and [PERSON] [53] present two models for solving the offshore supply vessel routing and scheduling problem. As for the model in [PERSON] et al. [52], modifications are necessary to apply the models to offshore wind. [PERSON] et al. [54] present an accessibility analysis for the North Sea. They investigate two different vessel types and present the access restrictions for them. [PERSON] et al. [25] present a vessel routing and maintenance scheduling model. They compare different vessel fleet compositions with a commercial mixed-integer programming solver and two rolling-horizon heuristics. In their model [PERSON] et al. [26] consider a mobilization and a response time for the vessels as well as operational limits for the vessels and restrictions for overnight work. Therefore, they also include a model to calculate the sunrise and sunset times for each day.