1.1.5. Objectives

The O&M programs and models rely on condition monitoring (CM)-based technologies such as dynamic load characteristics, oil analysis, strain measurements, physical condition of the materials, acoustic monitoring, performance monitoring, etc., which are helpful for monitoring wind turbines. The primary research goal is oriented around the condition monitoring of wind turbines and CM data is used to decide on maintenance planning and strategies/alternatives to be implemented, as well as to define deterioration models and develop mathematical models. The second objective is operational and maintenance (O&M) cost reduction coupled with less downtime. Due to offshore wind farm locations being much further from shore, new challenges will emerge which may interfere with reducing O&M costs.

The third objective shall be to overcome such challenges to minimize O&M expenditure.

#### 1.1.6. Condition Assessment and Condition Indicators

The degradation speed curve of the technical condition of a component/element is an on-going process from an "as new" condition until failure happens, as illustrated in Figure 3.

**Figure 3.** Generic continuous degradation curve of a component.

Very few, if any, condition monitoring methods give a direct and accurate description of the actual technical condition of the component/element. Methods used for the condition monitoring normally result in an indication of the technical condition of the components/elements. Energy companies have also carried out condition monitoring using visual inspections. As we know, visual inspections have higher uncertainty when

giving precise knowledge of the point in time and momentum in space on an on-going deterioration curve. Besides O&M activities by power companies, visual inspection in modern industrial manufacturing plants has applied condition monitoring, based upon specific software solutions installed in each piece of equipment (for their respective production machines), which incorporate a tracking system for their technical condition.

Above all, O&M demands four key principles [11]:


Most publications have focused on quantifying the limitations of the three key O&M variables [10]:

	- -Service personnel stationed at an onshore site to service offshore platforms;
	- -Service needs (e.g., vessels and helicopters);
	- - Adequate safe access to vessels for operational needs (e.g., replacing or transferring large components) [10].

#### *1.2. Scope Work*

This document reviews O&M management research on OWT operations and maintenance, including strategies, critical challenges and proposed solutions, on-site operations, and endpoints. Capable solutions are recognized with regard to the future development of O&M strategies. In addition, the negative effects of weather conditions, weather delays, repair times, and accessibility on offshore maintenance are presented. This analytical review presents a comprehensive overview of the OWT maintenance literature and provides a basis for improving O&M strategies and alternatives (1 vs. 2) in the future for offshore wind power installation facilities. To solve the information gaps, the comparison of scientific publications, technical reports and projects, and open databases has been used. The analysis is organized as follows. In Section 2, the research methodology, vessel data, personnel data, maintenance data, and online health monitoring are introduced and discussed, as well as the case studies (O&M Strategy 1 and O&M Strategy 2). Based on the designated maintenance methodology adopted, optimal maintenance direction-finding and scheduling are analyzed in Section 3. Several characteristics of the associated cost optimization problem are analyzed, including their advances, challenges, and targets. O&M strategies and alternatives, namely, O&M Alternative/Strategy 1 and O&M Alternative/Strategy 2 and their respective assumptions, are highlighted. A life cycle cost (LCC) analysis is conducted to evaluate both O&M alternatives/strategies (1 vs. 2) and determine which one is better. In Section 4, conclusions are drawn and discussed regarding operational and maintenance related issues from the outcomes obtained from the O&M alternatives-strategies analyzed, such as that a long-term life cycle (25 years) is more suitable for implementing Alternative 1, as it is more cost-effective. In contrast, it is more suitable to switch to Alternative 2 in order to guarantee major capabilities, as well as to have the advantage of achieving the access levels need to efficiently operate.

#### **2. Methodology for Detailed Maintenance—Parameters Analyzed**

#### *2.1. Vessel Data*

The O&M tasks to be carried out involve a fleet of diverse vessels. A standardized vessel consists of a vessel with a pre-established access system; therefore, maintenance technicians can easily access the OWTs. Some boats have additional capabilities (for example, cranes for lifting elements). There are questions about "high climate dependency" to access the OWT and "specific functional ship climate requirements" for the operational restrictions, in terms of the maximum possible, to access an OWT. Offshore vessels will not be able to participate in the maintenance tasks if the height of the waves or the speed of the wind exceeds their own meteorological limits, so they are not capable as such.
