**2. Description of Installation and Maintenance Maneuvers for Gravity-Based First Generation TECs**

In this section, we provide details on the installation and maintenance maneuvers for gravity-based first generation TECs. It should be noted that the information regarding these sorts of maneuvers is very limited owing to the fact that these technologies are currently at an initial stage of development (pre-commercial stage), signifying that real data about these maneuvers is not yet available [30]. The implementation of improved procedures for installation and maintenance maneuvers will actively influence their successful future commercialization [31], and this is one of the most important aspects studied by the GIT-ERM research group. The vessels used to perform these maneuvers should have the following characteristics: (i) *Dynamic positioning*, which allows redundancy in order to ensure work under extreme conditions and to guarantee security and reliability while these maneuvers are being carried out. These kinds of vessels have a high level of technology and are very costly to acquire/rent [32,33]; (ii) a *Heavy lifting crane*. Any cranes operating with these gravity-based first generation TECs must have a lifting capacity of around 250 tons [34], and (iii) the special vessel needs to have a high area on its deck on which to transport the structure, gondolas, auxiliary tools, etc. The aforementioned considerations allow us to conclude that the characteristics of the vessels required to carry out these maneuvers are not typical since the number of specialized vessels is not currently high and they are not easy to find on the market. They are, at present, used in the installation and maintenance of offshore wind energy farms and in the oil and gas industry, but the cost of hiring them is currently very high and oscillates according to the market (thus causing a high economic dependence). Furthermore, other sorts of vessels, such as remotely operated vehicles (ROVs), cable-laying vessels or tug vessels, among others, will be necessary to provide these special vessels with support when performing the installation and maintenance maneuvers, and these are very costly [35]. The following subsections deal with the definition of the installation and maintenance maneuver methods for gravity-based first generation TECs using, in the first case, manual and, in the second case, automated control. However, before performing the TEC installation and maintenance maneuvers, several stages have to be carried out on the TEF, which are graphically illustrated in Figures 1 and 2 and explained below:

• *Installation sequence at the tidal farm level*: The first elements to be installed are the transformation platform and the converters. Bearing in mind the depth and the composition of the seabed on which the TEF is installed (around 40 m), the use of a jacket platform is recommended owing to the fact that it is very safe, in addition to being highly adaptable and reliable [36,37]. The following element to be installed is the exportation cable, which requires the use of a cable-laying vessel (Figure 1a). The cable-laying vessel transports the umbilical cable from the transformation platform to the special vessel in charge of transporting the TEC (Figure 1b), and the connection between the base structure and the transformation platform is, therefore, achieved (Figure 1c). The cable-laying vessel waits until the base TEC has been installed (Figure 1d), after which it is

possible to install the base structure on the seabed by means of gravity (the procedure employed to install the base structure will be described below and is illustrated in Figure 2). Once the base support has been installed, the cable is extended in order to connect it to the adjacent TEC. During this procedure, the installation vessel has sufficient time to return to the base port and then return to the TEF with a new device. The cable-laying vessel waits to be given the end of the cable in order to perform the connection between the end of that cable and the new TEC structure and to repeat the cable connection process that will join it to the next TEC. This process is repeated until the TEF is completely installed.

• *Installation of the submarine cables*: It should be noted that it is fundamental to provide the interconnection cables and the exportation cables with adequate protection in order to avoid possible natural damage (resulting from earthquakes or movements caused by waves and currents) or damage caused by human activities (anchors or fishing artifacts, among others). The protection usually employed is that of burying the cables to a sufficient depth (from 0.5 m to 1 m) [38]. The following equipment is required to install the submarine cables: (i) a cable-laying vessel with its auxiliary equipment; (ii) ROVs to perform the trenching and burial processes; (iii) tug vessels with cranes and a diving team; and (iv) ground equipment, such as excavators, winches, trucks, etc. The procedure employed is the following: the cable-laying vessel is in charge of depositing the cables on the seabed following the most homogeneous path in order to avoid zones with rocks (Figure 2a). The trenching process is carried out in the opposite direction to the cable-laying process and is performed by a ROV-trencher (Figure 2b). This device is in charge of removing the cable, making the trench and placing the cable inside the trench [39]. The same ROV (but using a different tool) then performs the burying process in the opposite manner to the trenching process, thus leaving the cable completely covered (Figure 2c,d).

**Figure 1.** Installation sequence at tidal farm level: (**a**) joining the umbilical cable to the platform by means of a cable-laying vessel; (**b**) umbilical cable-laying process; (**c**) connection of the umbilical cable to the TEC (Tidal Energy Converter) structure; and (**d**) cable-laying process of the next umbilical cable.

**Figure 2.** Installation of the submarine cables: (**a**) installation of the base; (**b**) ROV (Remotely Operated Vehicle) performing the trenching process of the first umbilical cable; (**c**) ROV starting umbilical cable burial process; and (**d**) ROV finishing the burying process of the umbilical cable.
