Hydrex: Fast Underwater Bow Thruster Repairs Keep Ships Out of Drydock

Hydrex Fast Underwater Bow Thruster Repairs Keep Ships Out of Drydock

Hydrex can assist shipowners with almost any problem they encounter with their vessel’s thruster. A wide range of repair or maintenance work can be carried out on all types of thrusters. An entire unit can be overhauled, propeller blades or seals can be replaced or repair work on a specific part of a thruster can be performed by Hydrex diver/technicians on-site. All of these repairs can be carried out without the need to drydock the vessel.

Hydrex was the first company to show that it was possible to remove and then replace thrusters by creating a dry environment underwater. Using the Hydrex-developed steel mobdocks to seal off the thruster tunnel, with an access shaft protruding above the water, work teams accessed the thruster tunnel and removed or repaired the thruster within the tunnel in complete safety. This was done in conditions similar to those above water.

Hydrex has since then developed this technology further using lightweight flexible mobdocks. These modernized mobdocks, which are designed to be easily transported around the world, are used to close off the thruster tunnel on both sides. This allows divers to work in a dry environment around the unit.

Hydrex has also developed a permanent thruster repair and replacement system. This system has been developed so it can be tailored to most vessels. It can be included in the planning for a newbuild, installed on a unit going to drydock or constructed and brought onboard at any other suitable time. With such a system on standby, any repair work to the thruster that may arise can be dealt with much faster and more easily.

There is no need to send the vessel to drydock as all operations can be carried out in port or while the vessel is stationary at sea. Normal commercial activities can therefore continue without disruption.

This article gives an overview of some of the more important recent bow thruster repairs carried out by Hydrex.

Fast bow thruster operation in Congo and Gabon avoids drydocking

Recently an 86-meter research vessel needed the stainless steel belt in one of its thruster tunnels replaced. A Hydrex diver/technician team therefore flew to Pointe-Noire, Congo to perform the repairs.

The stainless steel belt is installed around the perimeter of a thruster tunnel at the location of the thruster blades. There the impact of the cavitation caused by the movement of the blades is the most severe. Extra protection against cavitation damage is therefore essential. When the stainless steel belt in the thruster tunnel of the research vessel suffered cracks, the underlying steel was exposed to cavitation. The belt needed to be replaced as soon as possible to prevent the thruster tunnel from getting damaged too severely. The owner of the vessel would have had to take his vessel to drydock if no on-site solution was found.

A tailor-made open-top cofferdam was designed by the Hydrex technical department. It was constructed in a local workshop in Pointe-Noire under the supervision of Hydrex diver/technicians. At the same time a regular shaped second cofferdam was also built.

Infrastructure and dredging work in the port of Pointe-Noire brought the visibility down to almost zero. The safety of the divers could not be guaranteed. For this reason Hydrex proposed a new location for the operation.

The owner gladly accepted the proposal to move the research vessel to Port Gentil, Gabon. Thanks to the sheltered environment of the bay the swell is limited. This makes it an excellent location to carry out repair or maintenance work on a ship, barge or rig and this in ideal conditions.

After a short trip the ship arrived in Port Gentil with the Hydrex team and all the equipment on board. A diving station was set up and the diver/technicians started the installation of the cofferdams. Next they emptied all water from the thruster tunnel, descended into it and carried out the repair. The old damaged belt was removed and replaced with a new stainless steel belt.

The actual operation was finished in only five days. The team removed the cofferdams and the ship was ready to continue its schedule with its thruster tunnel fully protected against cavitation once more.

Fast underwater bow thruster blade replacement in Rotterdam during commercial activities

Hydrex carried out bow thruster blade replacements on three 366-meter container ships belonging to the same owner. The vessels were given the same speedy treatment during unloading in Rotterdam.

The owner of the ships had the luxury of being able to schedule the blade replacements well in advance. This allowed the Hydrex technical department to get a team on the road before the vessel arrived. As a consequence, the diver/technicians were ready to start the operation as soon as the ship was berthed.

The same procedure was used during each of the operations. The new blades were lowered from the deck onto the workboat with the ship’s crane. Meanwhile the rest of the Hydrex team prepared the bow thruster tunnel for the operation.

The Hydrex flexible mobdocks were then installed on both sides of the thruster tunnel. Next the team could remove all water from the tunnel. In this manner a dry working environment was created. The diver/technicians then removed the first blade of the bow thruster and brought it to the surface. A replacement blade was then lowered into the water and taken to the thruster tunnel. The team positioned the new blade on the bow thruster and secured it with bolts. This procedure was repeated for the other blades.

During each of the replacements a local representative of the bow thruster manufacturer was present. He supervised the operations and gave his approval.

Underwater bow thruster removal using Hydrex workboats

Also in Rotterdam, a Hydrex diver/technician team removed the bow thruster of a 300-meter container vessel. The unit needed to be overhauled. Hydrex performed the operation underwater using its own workboats based at the Antwerp depot. This made it possible for the owner to keep his vessel out of drydock.

The team mobilized from the Hydrex headquarters in Antwerp after all basic preparations had been made and the needed equipment was loaded onto one of the Hydrex workboats. The Hydrex catamarans are fully equipped as dive support stations with hydraulic cranes, winches, nautical and communication equipment and a dive control room. They can be used for a wide range of operations in Belgium, the Netherlands, the United Kingdom and France, permitting even more rapid deployment from the Antwerp depot. This increases flexibility of operations and helps to keep costs down for the client.

After they arrived in Rotterdam, the divers installed flexible mobdocks on both sides of the thruster tunnel and emptied all water from the tunnel. This created a dry working area around the bow thruster unit. The team could then detach the bow thruster blades one by one.

Next the diver/technicians removed the flexible mobdocks again, concluding the first part of the operation.

The following step was to secure the gearbox with hoisting equipment. The team then disconnected the bow thruster unit from the engine room and lowered it onto a cradle that can be adjusted to the size of the unit..

The bow thruster was then brought onboard the Hydrex workboat, ready to be overhauled. Next the team securely sealed off the engine room by positioning a flange over the space connecting the thruster tunnel to the room. This made it possible for the vessel to sail until the overhauled unit is reinstalled.

With the bow thruster unit on deck, the team sailed back to the Hydrex headquarters. From there the unit was transported to the manufacturer to be overhauled.

Underwater bow thruster reinstallation in Tacoma, U.S.A.

Three months after Hydrex diver/technicians removed the bow thruster of a 294-meter container vessel in Tacoma, a Hydrex team once again mobilized to this location to reinstall the overhauled unit underwater with the use of the Hydrex flexible mobdock.

The superintendent of the ship was very satisfied with the first part of the operation. The job was completed well within the available time frame thanks to good team work of the Hydrex divers, the ship staff and the floating crane operator. For this reason the customer asked Hydrex to take care of the reinstallation as well, which was carried out last month.

Together with all the necessary equipment, the team mobilized from the Hydrex office in Clearwater, Florida to the vessel’s location. After they set up a monitoring station on a workboat, the team positioned the bow thruster onto a cradle as described in the previous case. The Hydrex divers could lower the unit into the water and maneuver it inside the thruster tunnel in one operation. The team positioned the bow thruster and secured the unit.

The team then used the lightweight flexible mobdocks developed by Hydrex to close off the thruster tunnel on both sides. This allowed the diver/technicians to evacuate all the water from the tunnel and create a dry working environment around the bow thruster. The thruster propeller blades were then reinstalled one by one. The team completed the operation by reconnecting the thruster unit to the engine room.


Performing jobs like these on a tight schedule takes a lot of planning. This can only be done successfully by staff who have familiarity with such operations and the relevant know-how and equipment. Hydrex has a technical department capable of executing all the required planning. Our diver/technicians are trained and qualified to perform the full range of required class-approved repair procedures in even the harshest conditions. Hydrex also has very well-equipped rapid response centers including customized workboats, ready to mobilize directly to the job site.

In cases like these, timing is also of the highest importance. Hydrex team members are trained to carry out the approved procedures within a short time frame. The operations described were concluded well before the end of these commercial activities. This allowed the vessels to leave the port again perfectly on schedule.

Press Release, September 13, 2013