*2.1. Manual Installation and Maintenance Maneuvers for First Generation TECs*

We define the term *manual* for installation and maintenance maneuvers for TECs in an open loop. These sorts of maneuvers are currently used in the first generation tidal technologies which are, at present, in a pre-commercial stage [38]. An example of the gravity-based TEC described in this section and designed by the GIT-ERM research group is illustrated in Figure 3. The manual installation of the TECs can be divided into the installation sequence for the support structure of the TECs and the installation sequence for the gondolas. The following steps have, therefore, been defined in order to perform the installation tasks:

**Figure 3.** Example of TEC used for manual installation and maintenance maneuvers. Additional details about this TEC design can be found in [38].

	- **–** The special vessel transports the complete TEC and the equipment required (support structure, ballasts, gondola, etc.) simultaneously (see Figure 4a), and moves from the base

port towards the TEF. When it is at the TEF, it uses its dynamic positioning system to place all the necessary items in the exact position in which the TEC will be installed.


**Figure 4.** Installation of the structure of the TECs: (**a**) position required to install the base; (**b**) connection of the umbilical cables to the TEC structure; (**c**) controlled descent of the TEC structure; (**d**) fixing the TEC structure to the seabed; (**e**) placement of the concrete ballasts; and (**f**) installation of the TEC structure once the process has been completed.

	- **–** Once the support structure has been completely installed, the special vessel is placed on the TEC structure and the guide cables of the gondola are recovered by means of an acoustic signal (see Figure 5a).
	- **–** In order to work with the gondolas, a specific tool equipped with a hydraulic system will be used, whose objective is to wrap itself around the gondola that is to be installed or recovered. Its operation is similar to that of a clamp (see Figure 6).
	- **–** The guide cables are connected to the tool used to lower the gondola (see Figure 5b). These cables facilitate the descent of the gondola and the insertion of the gondola into the structure.
	- **–** The gondola initiates its descent with the guide cables thanks to its own weight and without oscillations until the gondola has been inserted into the structure. Figure 5c illustrates the descent process of the gondola and Figure 5d depicts the gondola-structure insertion process.
	- **–** The final step is that of removing the tool used to install the gondola and the retrieval of the guide cables. Figure 5e illustrates the removal process. When the tool is on the deck of the vessel, the guide cables are removed from the tool and are submerged in a safe location by means of a ballast and a buoy in order to recover them during the next intervention. Figure 5f shows the installation of the whole TEC once the process has been completed.

**Figure 5.** Installation of the gondola of the TECs: (**a**) cable-recovery process; (**b**) connection of the cables to the tool in charge of lowering the gondola; (**c**) controlled descent of the gondola; (**d**) process of inserting the gondola into the TEC structure; (**e**) tool and cable removal process; and (**f**) end of gondola-installation process.

**Figure 6.** Tool used for manual installation and maintenance maneuvers (installation and recovery of the gondola).

The maneuvers that are necessary to perform the maintenance tasks (recovery of a submerged gondola) follow the inverse order of that described for the installation of the gondola. The procedure for maintenance maneuvers, therefore, shares a lot of similarities with the procedure of installing the gondolas, and the steps required to perform maintenance maneuvers are the following:

	- **–** The starting point is that of locating the special vessel above the gondola to be recovered. The first step is the recovery of the cables from the seabed. The ends of the cables are released from the seabed by means of an acoustic signal (see Figure 7a) and these cables are connected to the tool used to recover the gondola.
	- **–** The tool starts its descent, following a trajectory with an inclination angle that permits the tool to wrap itself around the back of the gondola (see Figure 7b).
	- **–** When the tool is ready to perform the grip, the cables are tightened and placed completely vertically (see Figure 7c). The hydraulic system of the tool is activated in order to close it and fix it to the gondola (see Figure 7d).
	- **–** The process of raising the gondola begins. As the cables are tightened, the displacements are very small and the operation is carried out under safe conditions (see Figure 7e).
	- **–** When the whole system (gondola + tool) is outside the water, the cables are removed from the tool and are submerged again by means of a ballast and a buoy in order to recover them in the future (see Figure 7f).

**Figure 7.** Maintenance operations of the gondola: (**a**) positioning the special vessel above the gondola; (**b**) descent process of the tool; (**c**) coupling process between the tool and the gondola and cable tensioning process; (**d**) activation of the hydraulic system of the tool to fix it to the gondola; (**e**) gondola lifting process; and (**f**) gondola recovered.
