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Subsea Solutions Alliance Carries Out Azimuthing Thruster Overhaul Completely Underwater

Finding a suitable dry dock for your installation in a location where the vessel operates sometimes can be a daunting challenge. In many areas of the world there are no suitable dry docks for a semi-submersible vessel.

With the closest suitable dry docks thousands of kilometers away, the client evaluated many different methods to create a dry environment around the azimuthing thrusters for their overhaul onboard. The customer also wanted to avoid the excessive costs of floating crane barges, a heavy lift vessel and also wanted to develop a method of repair for emergency interventions if required.

After consultation with Subsea Solutions Alliance and discussions with the original equipment manufacturer (OEM), it was determined that an in-water overhaul was feasible and could be performed at the customer’s requested location. The recommended process of repair would not require any special ballasting of the vessel, no special vessels for lifting the semisubmersible out of the water and would not require a large atmospheric cofferdam. The engineering project commenced.

Critical drawings and detailed information was compiled by the client and the OEM and provided to the engineering department of the Subsea Solutions Alliance. The engineering department of the Subsea Solutions Alliance conducted a feasibility analysis and performed preliminary engineering to verify to the client and the regulatory bodies that the solution was feasible, safe, environmentally friendly and will meet the quality assurance requirements as outlined by the OEM. Detailed engineering commenced.

During the detailed engineering phase, the customer realized the significant benefits of the solution being developed by the Subsea Solutions Alliance. The client realized that the specialized hyperbaric cofferdam being designed by the engineering department of the Subsea Solutions Alliance could be designed for easy storage onboard the semi-submersible or in a local warehouse for emergency or other maintenance repairs to the azimuthing thrusters. The customer also wanted to design the hyperbaric cofferdam to be operated in areas where there maybe draft restrictions. As a result, the cofferdam was designed with a specialized ‘skirt’ to allow operation in areas of shallow water depth.

With these new design criterions, the engineering department of the Subsea Solutions Alliance enhanced the hyperbaric cofferdam design to make it a ‘build up’ design with sections of a proper size and material so that two people can easily manage the section. The design incorporated staging for personnel and incorporated lighting, hydraulics, pneumatic connections for tools for the upcoming azimuthing thruster overhaul and was also fitted with atmospheric control and atmospheric monitoring systems.

To remove any residual oil left in the thrusters gear box underwater after the unit is drained from the inboard system required the engineered cofferdam to have bulkhead threaded penetrations in the cofferdam panel corners to allow a leak proof passage of air, oil and vent lines. This alleviated excessive hoses from being passed through the bottom opening of the hyperbaric cofferdam creating a safe and organized work environment. In addition, the hyperbaric cofferdam was design reviewed by state-certified professional engineers and built to a very tight tolerance in order to minimize deviations.

By maintaining this very tight tolerance, the as-built sections would assemble easily each and every time it was used. Incorporating properly sized and located pad eyes and by performing a finite element analysis on the hyperbaric cofferdam, the engineering department of the Subsea Solutions Alliance engineered this specialized cofferdam for minimal deflection and even load distribution; thus making it safe and robust for the cyclical loading in which it may encounter through its lifetime.

Once engineering was completed, the entire aluminum structure was fabricated by the Subsea Solutions Alliance, a complete assembly manual, and storage / maintenance manual was developed ALL in accordance with the owner’s request. As with all new solutions, demonstrated performance relieves everyone’s concern. The entire hyperbaric cofferdam was shipped in a conventional short shipping container from its point of manufacture at the Subsea Solutions Alliance’s Long Beach California facility to the location of the repair.

Within a couple of days of shipment the entire hyperbaric cofferdam was assembled and ready for launch. Once waterborne, the hyperbaric cofferdam was quickly installed and dewatered. The overhaul commenced. The overhaul included underwater propeller removal (for topside inspection and maintenance), replacement of the propeller shaft seal, inspection of the lower gear box gear set, replacement of all anodes, replacement of the other critical gaskets and re-installation of the propeller in a dry environment.

The team also performed critical NDT measurements and tests on thruster gears, bearings and the propeller shaft, a key to performing the overhaul in accordance with the classification requirements. The azimuthing thruster overhaul incorporated all of the OEM required inspection and maintenance repairs as outlined by the OEM’s maintenance guide for a five-year class certified overhaul.

The entire process was performed together with the OEM’s service engineers and critical steps were witnessed by the attending classification society surveyor. Over the following weeks, all four azimuthing thrusters were overhauled completely underwater utilizing the specialized hyperbaric cofferdam and the highly trained diver / technicians from the Subsea Solutions Alliance.

Attention to detail and a highly skilled work force allowed the Subsea Solutions Alliance to complete the required thruster overhauls on time, on budget and without any safety or environmental incidents. It’s incredible what one little phone call can do. It can change the entire outlook of a project.

One year ago, the customer was not sure how the azimuthing thruster overhauls would be achieved. Now he sits back realizing that his thrusters are overhauled, backed by a full OEM warranty, certified by the classification society for at least five more years of continuous operation and the entire project was performed on time and without an incident. Making challenging problems look easy is what the Subsea Solutions Alliance is all about. By combining all of its resources from concept, design and then to performance, the Subsea Solutions Alliance demonstrates yet again why it remains a leader in underwater ship and equipment repair.

How many times in your life do you have the opportunity to dive in the crystal clear waters of the extreme southern oceans? As far...
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