Gibdock completed a complex propulsion system job, following the recent docking of the 57-ton bollard pull capacity Boluda group harbour tug Siroco.
The 2001-built Siroco, which operates in the port of Algeciras, arrived in Gibraltar in early June for a one month docking that included the complete removal and overhaul of the two Voith Schneider Propeller (VSP) units onboard.
As soon as the vessel was drydocked, it was jet washed with high pressure water and hand-scraped to remove marine growth in preparation for the VSP overhaul. The next task was to crop four access plates and two upright supports from the protective structure to allow access for Gibdock engineers so they could remove the VSP blades.
“This was a time consuming task and it took around two shifts to remove all ten blades and transport them to the mechanical workshop for overhaul," said John Taylor, Gibdock production director. "This included disassembling, cleaning, calibrating and remounting the bearing assembly with new spare parts and all the blades had to be inspected and polished.”
Gibdock engineers then removed the two rotor casings and transported them to the workshop. The main cover plate nuts were removed using a hydro torque, and all internal components, including pistons, gears, actuating rams and the thrust shaft were similarly transported to Gibdock’s mechanical workshop for overhaul. Using chain blocks the VSPs rotor casings were lowered from the vessel and, with the assistance of the yard’s shore cranes, these were lifted out from the bottom of the dock and transported to the workshop.
As part of the Siroco’s refit, Gibdock wet grit blasted and painted the tug’s hull and decks. This required considerable preparation work to ensure that exposed areas were unaffected. Mr Taylor said: “Once the Voith Schneider units and rotor casings had been removed, the main deck fixtures were completely protected and the two openings were blanked off before blasting.”
Throughout the wet grit blasting, the overhaul of the VSP equipment continued in the workshop. Following the dismantling and inspection of the rotor casing, the bearing flange assembly and protective ring was found to be extremely corroded. It was decided to fabricate and fit steel inserts to the two bearing flange assemblies.
Taylor said: “Although this was unexpected extra work, we incorporated a night shift to complete the repairs as soon as possible, in order not to affect the undocking date, which was crucial to the owners.” The new inserts were machined to an external diameter of 1,285mm, internal diameter of 1,065mm and a 60mm thickness. These inserts were then fitted to the existing bearing flange assembly.
While in the Gibdock workshop all the mechanical parts of the VSP units were cleaned, calibrated and the data recorded, and all the seals, packing and bushes were renewed, with Gibdock engineers working closely with service specialists from Voith Turbo Schneider. The reassembly of the VSP units in the workshop continued while a second full coat of paint was applied to the hull. Furthermore, Gibdock staff also applied a non-slip compound onto the tug’s main decks.
Once the Voith Schneider systems were completely reassembled onboard and the access plates refitted and welded, a final full topcoat of silicone paint was applied to the tug’s hull. After a 24 hour period to allow for the paint system to cure, the dock was flooded and the vessel left for sea trials.