Subsea Global Solutions: Double Bow Thruster Removal Afloat

[By: Subsea Global Solutions]

Problem 
A valued client contacted Subsea Global Solutions (SGS) requiring underwater support to remove two of its existing bow thrusters during the vessel’s port call in Long Beach, California. The client indicated that they would not be replacing the thrusters at this time as no spares were available, as such SGS was asked to remove the damaged thrusters and install class approved blind covers over the hull openings. All work needed to be done during the vessel’s cargo operations and in a dry environment—underwater—to avoid potential oil spills and sea water ingress through the propeller hub. Considering SGS has had a history with this vessel and its owners, and having performed hundreds of similar thruster removal and installations afloat in the past, we already had the related equipment and special tooling ready to execute the work as well as the means to palletize and ship the damaged thrusters out of our work shop in Long Beach.

Solution 
The SGS centralized Technical Repair Department and SGS Long Beach worked closely with the bow thruster OEM and vessel owners to outline our unique thruster removal procedure afloat and once the procedure and risk assessment were approved by the relevant parties, we planned to complete the project working around the clock in 11 x 12-hour shifts with a team for each thruster unit. Each team was composed of a topside technical team with 3 commercial diver technicians, 2 commercial diver/welder technicians, and 1 dive supervisor along with a floating project manager from SGS Long Beach.

The project began with the dive team installing 10-ton pad eyes in and above the thruster tunnels using underwater welding procedures in accordance with the Class A requirements of AWS D3.6M: Underwater Welding Code in the overhead (4F) and vertical (3F) positions. In conjunction with the main rigging points, 1-ton pad eyes were installed in the thruster tunnels for the thruster blade removal. All four blades needed to be removed prior to removing the thruster unit. The divers installed thruster habitat doors on both sides of the thruster tunnels and de-watered the tunnels with compressed air so the thruster blades could be removed and the blind cover plates could be installed on the blade openings.

In parallel, our internal team was disconnecting shaft couplers and rigging the thruster E-motors to pre-fabricated cradles to gain access to the top of the thruster mounting flange for each thruster. They installed an onsite SGS designed thruster dome/cofferdam that incorporates a unique center cabling system with a pressure safe packing gland that enables a rigging cable to be shackled to the lifting eye of the thruster pinion shaft. This allows the internal team to lower the full weight of bow thruster unit safely with no water ingress in the bow thruster room. Once the bow thrusters were lowered enough to clear the pinion shafts, the loads were transferred from the center cable to the underwater tunnel rigging, and then transferred to a 10-ton lift bag, and finally to a pier side crane that could safety retrieve the bow thruster units.

With both thrusters safely on the pier, the commercial dive teams prepared the sealing surfaces in both tunnel thruster openings by cleaning the mounting flanges and installing new O-ring seals. They rigged each blind cover plate into position with alignment pins using the center rigging cable through the thruster dome so the internal team could raise the blind covers into position and seal off each opening with 5-tons of internal pressure. The internal team could now safely remove the thruster dome and install the original thruster mounting bolts to the OEM torque specifications.

Conclusion 

The vessel was able to sail with no restrictions after a final inspection with Class. The benefits of removing a damaged bow thruster afloat are many. The customer was able to prevent further damage to the bearings and gears caused by the presence of sea water in the gear housing and the procedure allows the units to be overhauled prior to the vessels next dry-dock—or, they can be re-installed afloat by SGS, which would help to eliminate the need for additional tug cost. Having a centralized Technical Repair Department makes a difference when managing complex underwater repairs on a global scale, as it allows SGS to ensure both our Clients and Class receive the same familiar experience from SGS they are used to, regardless of where the project is taking place. Stay tuned for our second part of this article related to the installation of the overhauled thrusters.