We were being approached to conduct a servicing work on an important component of a diving vessel: Diving bell trunk. It is the short man-way that connects the entry locks (wet pots) to diving bell which is used when subsea diver entering or leaving the diving bell to transit to a subsea work site. The trunk can be sealed at both ends when the bell is connected to a diving system. As this structure interconnect between Diving Bell and Chambers in deep sea, the connecting flange surfaces must in good condition at all time to prevent any possible leakage when operating in highly pressurized environment.
The focus of this task is the flatness and dimensional check on bell trunk’s flange (labelled in Yellow) and gear box seating faces (labelled in Green and Red) as shown in Figure 1.
The API laser tracker – OT2 was chosen to fulfill the “10:1” rule to factor in the measurement uncertainty. With Maximum Permissible Error (MPE) of 0.042mm from OT2 laser tracker at a 2m range setup, it is well within the 0.5mm tolerance allowance given by our client. The physical form factor of API OT2 is advantageous for this application as it can also easily accommodate the constrictive site condition (as shown in figure 2) and operate with in-build battery power supply if needed.
To achieve repeatability and reproducibility (R&R) of the readings taken at site, positions of placing the Spherically Mounted Retroreflector (SMR) were also marked on the structure.
By using the Spatial Analyzer software, a best-fitted plane was constructed easily using the collected points to determine the flatness runout as illustrated in Figure 4. The coordinate reference for analysis was also setup in the software with respect to the engineering drawing as shown in Figure 2.
With the high accuracy readings obtained via the laser tracker, an in-situ linear milling machine was able to setup closely with respect to the reference points of the Bell Trunk to correct the runout and offset distance with minimum material removal.
The whole process of data collection and analyzing was completed within a day whereas the in-situ corrective machining was accomplished within 2 days to achieve client’s requirements.