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 Production Executive Dave Bleyenberg talks about the history of on-site thruster operations and mentions the benefits this can have for shipowners in the shipping as well as the offshore industry.
Hydrex: How did it all start?
Dave Bleyenberg: “We started off with small repairs on bow thrusters: welding anodes, performing bow thruster blade repairs in the wet, repairing the steel belt,… We then began looking into ways to carry out on-site repairs in dry circumstances. Emptying the thruster tunnel of water was the most logical next step.
In 1995 we started doing this by closing off both sides of the thruster tunnel and blowing air into it. This created a dry environment in which our diver/technicians could remove a blade or perform a seal repair. For its innovative work in this field Hydrex won the 2002 Lloyd’s List SMM Award in the category of “Innovation in Naval Shipbuilding and Marine Technology.”
Hydrex: But it did not stop at these smaller repairs, did it?
Dave: “The next step was to perform complex repairs to the thruster unit itself. The easiest way to do this was to remove the unit in its entirety. The difficult part was to find a way of lowering the unit into the thruster tunnel without water coming in through the hole connecting the tunnel to the engine room. If the air in the tunnel escapes through the hole, the tunnel will fill up with water very fast. We needed to find a way the close off the engine room.
After lots of brainstorm sessions around the drawing table, we found the solution. Things went very fast after that and we started putting the idea into practice soon after. We visited engine rooms to get a good idea of how we needed to develop the concept. A universal solution was not possible and we had to adapt it to different sizes. Good drawings and ideally a pre-inspection (of the vessel or a sister ship) is required to perform a full thruster unit replacement.”
Hydrex: What are the typical problems encountered with a thruster unit that result in a need for it to be overhauled?
Dave: “There can be a number of causes for a thruster’s malfunction. The gearing wheels can be worn down so that the unit does not function as it should, one or more hydraulic tubes can be severed so that the correct pitch cannot be established or the feedback cable can be broken, preventing correct readings. If a thruster does not function correctly, the vessel has to use an extra tugboat when entering port and this raises the cost of entering and leaving considerably.
If possible we perform on-site repairs inside the tunnel, to prevent a complete removal of the thruster unit. We recently did a repair of the stainless steel belt on a vessel in Port Gentil (Gabon). An 86-meter research vessel needed the stainless steel belt in one of its thruster tunnels replaced. The 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 suffered cracks, the underlying steel was exposed to cavitation. The belt needed to be replaced as soon as possible to prevent the 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 our technical department. It was constructed in a local workshop under the supervision of Hydrex diver/technicians. At the same time a regular shaped second cofferdam was also built. This allowed us to empty all water from the thruster tunnel and descend into it to carry out the repair. Our diver/technicians removed the old damaged belt and replaced it with a new stainless steel belt. The actual operation was finished in only five days.”
Hydrex: Does the size of the thruster matter?
Dave: “No. We have also performed several offshore thruster operations. We only need a crane with a large enough lifting capability. For instance, we have replaced one of the 40-ton, swing-up, azimuth thrusters of a crane barge stationed in Gabon with the spare on several occasions. One of these operations had to be carried out in a very short time frame because the crane barge was scheduled to leave for an operation in Nigeria. All repairs and other servicing work needed to be carried out before the start of this operation. For this reason going to drydock was not an option, as the nearest suitable location was South Africa and this would have taken the repairs far beyond the available time frame. When the first azimuth thruster was replaced, a large mobdock (measuring 9 x 6 x 2 meters and weighing over 25 tons by itself) was constructed under Hydrex supervision in Belgium and transported to Gabon. There it was stored after the repair to be used at short notice whenever future repairs were required on thrusters. This allowed for a very fast mobilization and thruster replacement on the next two occasions.
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 on board 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.”
Hydrex: Despite the difficulty of these operations, they can be performed very fast.
Dave: “That is correct. The removal of the unit can be done in the wet, because it is overhauled. We only need to remove the blades and close the unit off with flanges. The reinstallation needs to be done in the dry because you need to reinstall the blades without water ingress. Removal can be done within 24 hours. Reinstallation is usually done in 48 hours.
We have now taken it even a step further. On several occasions in 2013 we have removed the old bow thruster unit and reinstalled a new one in a single operation. This saves the customer extra time because we do not need to mobilize our divers and equipment twice.
A vessel can sail after we remove the unit and close off the tunnel from the engine room. The reinstallation can then be done at a later date on-site or during the next drydock visit if required. This saves time in dock because they do not need to remove the old unit and they can have the overhauled or new unit ready when the ship comes in.
For bow thrusters unit removals or blade replacements in Western Europe we have two workboats available at our headquarters in Antwerp. The transport of the unit to the manufacturer can also be arranged by us if needed. This is a package we offer to customers. If a job comes in we ask if the thruster needs to be overhauled and where it needs to go. Spare parts or thruster blades can be stored at our offices so that they are immediately available when we start the operation.
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.”
Hydrex: Thank you for the interview.
Press Release, December 20, 2013