The requirement emerged following a report of water ingress in the area of the ship's internal shaft seal. A subsequent inspection revealed that the pipe work, which feeds cooling water to the shaft seals and the shaft bearings, required modification.

To allow this work to be undertaken safely afloat, avoiding the risk of catastrophic flooding, there was a requirement to seal, underwater, the gap surrounding each shaft as it exits the hull.

To satisfy this requirement UMC worked with the MoD's Warship Support Agency MPS and MW Integrated Project Teams to design and manufacture a temporary underwater shaft seal and develop an associated procedure and risk assessment for the seal's installation and removal. The aft face of the shaft's stern tube was selected as the location for the temporary seal. However, direct access to this face was barred by an outer bossing (installed to enhance hydrodynamic efficiency as the shaft exits the hull). To gain access to this face it was therefore necessary to first unbolt and removed a large portable GRP fairing plate that forms part of the outer bossing.

With the stern tube aft face exposed, the temporary sealing system, which had been designed by UMC's Design and Consultancy Division specifically for this task, was quickly installed.

The sealing system consisted of a blanking plate, which fitted and sealed against the stern tube aft face, a gland ring, which formed a seal against the shaft, and a clamp ring, which was used to compress both sets of seals and hold the complete assembly together. To further compress the seals and to drain the water within the stern tube area, a hose was connected to the seal arrangement and to a pumping system operated from dockside.

The underwater temporary sealing system proved a great success. The assurance that UMC's on-site project engineer was able to demonstrate in the equipment's design, fabrication and installation was sufficient for the ship to allow the seal to remain in place, without undue concern, for a number of days.

This provided sufficient time for each shaft's inboard sealing system to be dismantled and modified in a controlled, safe environment. In due course, once the required modifications were complete, the removal of the temporary shaft seal and the reinstallation of the outer bossing by UMC was essentially a reverse of the installation procedure.

During the last few years UMC has designed and installed similar systems for the external shaft blanking afloat of other classes of warship, as well as commercial vessels. On one occasion, an external temporary shaft seal was fitted in dock .

This allowed the ship to be undocked early and maintenance to the internal shaft seal system that had been started in dock, to continue, uninterrupted, afloat.

Meanwhile, the dock was made available for use by another waiting vessel, which minimised the interference to each ship's programme and optimised the dock's usage.

MJ Information No: 19533