• Underwater repairs: case studies

    Underwater repairs: case studies


    Hydrex is able to perform a wide variety of operations with its flexible mobdocks. These enable us to create a dry environment underwater for our divers to work in. There is no need to send the vessel to drydock. We will bring the drydock to you and all operations can be carried out alongside or at anchorage. Normal commercial activities can therefore continue without interruption. The lightweight mobdocks allow for a fast mobilization and the necessary adaptability.

    Hydrex was the first company ever to use a prefabricated cofferdam, introduced as early as 1979. It was used to carry out repairs to the m/v Lunar Venture. By 1983 the technology was in use to perform insert repairs on double bottom tanks from the inside.  The company has advanced this concept extensively over the last 30 years, along with the technology to ensure that fast, professional and high quality work can be done while the vessel stays afloat and even while continuing normal ship operations.

    Initially these techniques were applied mainly to the repair and replacement of bow thrusters. Using steel cofferdams as a mobdock to seal off the thruster tunnel, with an access shaft protruding above the water, work teams accessed the tunnel and from there could work on the thruster in complete safety.

    We has since then developed this technology further using lightweight flexible mobdocks (mobile mini drydock). These were first used in 2002 and have been further developed by our in-house R&D department ever since. Hydrex constantly invests in the research necessary to evolve repair techniques and procedures. It is now possible for our diver/technicians to perform permanent repairs on seals, thrusters and any other part of the underwater vessel without the need to go to drydock.

    For many of these operations we work together with OEMs. The most common type of mobdock operations are seal and thruster repairs or replacements. Mobdocks needed for underwater repairs on the standard sizes of seal assemblies and thruster tunnels are available in our fast response centers for immediate transportation. Tailor-made mobdocks can also be designed be our R&D department.

    Dry shaft seal renewal underwater

    Damaged stern tube seals will cause an increasing amount of oil leaking or water ingress as the damage worsens. By replacing the seals when the damage is first discovered Hydrex keeps the down time low. Oil leakages can also produce a potential or actual liability when for instance going to the United States or other sensitive areas. When they have a leaking stern tube, ships are often not given permission to enter ports, they can receive fines or are even not allowed to sail on.

    By using a flexible habitat our divers create a drydocklike environment around a seal assembly. This enables our teams to perform seals replacements or other work on the housing. These repairs or replacements can be performed on a large variety of seal applications.

    It is not always straightforward to replace seals because there can be quite a bit of variation in the configurations of the stern tube itself. There can also be complications with the liners, which can be worn down and show running marks. All this is routinely handled by the teams on the jobs. We usually supply the equipment and the owner is free to supply his own seals. We can handle all type of seals from all original manufacturers.

    Case study: Underwater stern tube seal repair under challenging circumstances

    Oil was leaking from the stern tube seal assembly of a 157-meter dredger. A team of Hydrex diver/technicians therefore mobilized to the vessel’s location in Uruguay, together with all the needed equipment.

    A thorough underwater inspection revealed that the entire housing of the assembly was severely corroded and needed to be replaced. Because the housing consisted of split shells, this could easily be done by the Hydrex team.

    Next they installed the flexible mobdock. The team then removed the three damaged seals one by one and replaced them with new ones.

    Because the existing running area was worn out, the diver/technicians installed a spacer ring to create a new running area for the seals. Thanks to a newly developed method, this could also be done inside the flexible mobdock.

    The stern tube seal repair was carried out in less than optimum conditions. There was almost no inwater visibility at Montevideo at the time of the repair. It made the diving operations a lot more challenging than expected. However, our divers are trained to be flexible and adapt to constantly changing working conditions. This made it possible for Hydrex to perform the stern tube seal repairs under the strictest safety regulations, to the highest quality standards and without any unnecessary delay. The dredger was able to leave Montevideo in time to sail to its next operation.

    Bow thrusters

    Hydrex diver/technicians can perform a wide range of repair or maintenance work on all types of thrusters. An entire unit can be overhauled, propeller blades and seals can be replaced or repair work on another specific part of a thruster can be performed on-site. These repairs are performed in cooperation with OEMs and can be carried out while the vessel stays afloat with minimum impact on its schedule.

    Bow thruster operations are carried out using our flexible mobdocks that close off the thruster tunnel on both sides. These lightweight mobdocks can quickly be transported to any location around the world.
    In most cases a thruster overhaul is planned during a scheduled drydocking. This usually means that the unit is removed in drydock. The ships then has to wait for the repaired thruster to return and be reinstalled before the vessel can leave drydock. This means a longer drydock time and consequent cost. Hydrex can however remove the unit while the ship is still afloat so it can already be brought to the manufacturer to be overhauled. When the vessel enters drydock the overhauled unit will be ready for reinstallation without any delay. The reverse procedure is also possible. If the thruster is removed in drydock, Hydrex can reinstall it in dry conditions underwater at a later date. In this way the ship can already leave drydock while the unit is still with the manufacturer.

    Case study: Fast response keeps vessel on project in Australia

    A Hydrex team removed the bow thruster of a large offshore supply vessel and reinstalled the unit after it was overhauled.  By carrying out both parts of the operation underwater while the vessel was at anchorage in Dampier,  Australia, the ship did not have to go to drydock and could stay on the project.

    Despite the remote location of the vessel, our technical department was able to make all logistic arrangements and arrange a mobilization of the equipment swiftly. Recently we have carried out several operations in Australia, all of which were done with a limited window of opportunity available.
    After our divers removed the bow thruster unit it was brought to a local workshop where it was overhauled by a technician team of the OEM. The Hydrex team remained on stand-by so that they could immediately start the dry reinstallation when the bow thruster arrived back on location.


    If you have any questions regarding a possible mobdock repair, do not hesitate to contact us.

    Animations of the procedures used can be found on our website. For more information on mobdock or other underwater repairs, please contact one of our offices. We are at your disposal 24/7 and ready to mobilize almost immediately.