Dimensional Control on The Volantis

This project was split into three separate parts. Part 1: the Carousel which was built in Halifax. Part 2: When the ship was docked in Sweden Part 3: Final stages when the ship was located in Norway.

Instruments used:
Sokkia 1” Total Station Sokkia SDL30 Electronic Digital Level & Bar Code Staff
Digital Surveys also used calibrated tapes and mini reflective targets to carry out the job accurately and quickly.

The Task:

The deck of the ship accommodates a large cable laying drum which sits in a large rotating carousel unit. To enable smooth operation of the cable laying procedure the Carousel unit, A-Frame, Umbilical Winch and a Firing Line had to be positioned with millimetre accuracy.

These units had to individually stand on pads, which were sunk into the deck. Digital Surveys were asked to position and level the pads according to the relative structures / design drawings. Digital Surveys also helped in the fabrication of the Carousel unit at Fabtech Halifax, where we were brought in as third party surveyors and our job was to ensure accuracy of the structure during fabrication.

Steel supports were set out on the floor of the factory using the Sokkia Electronic total Station The supports were positioned to support the grillage in the design position. Once the supports were positioned, they were then leveled to a datum.

They were all leveled accordingly to the accuracy of less than 1mm, using the Electronic Digital Level & Bar Code Staff. The main members of the carousel were then set out relative to the centre point. These were set out using a coordinate system from the centre point by using the Sokkia Total Station.

This was a long process, as the welders could only position a certain number of steel members at one time. When they were positioned and checked, we had to then make sure that the surface was level, using the SDL30 Digital Level. This process was repeated a number of times until all of the steel members were in place. During fabrication the work was monitored to ensure minimal movement had taken place during this process.
Digital Surveys also carried out dimensional checks on the pads where the male and female cones used to locate the carousel attached.

As the carousel was split into two parts,(due to the size and space limitations of the fabrication building) once the first part was complete it was then repeated exactly the same for the second part. Finally, a set of pads were fitted on the underside of the grillage. These were all different thickness due to varying load points. These were positioned and leveled using the Sokkia total Station with an accuracy of +-1mm. The pads were then re-levelled and liquid shims were inserted to pack them to suit. All pads were levelled using the SDL30 Digital Level and the datum was in this case was the highest pad.

Most of the results were given on site as soon as we worked them out. If results couldn’t not be given on site, an excel spreadsheet was produced with levels, or an AutoCad dwg was produced after the data collected on site had been downloaded and processed through our survey software. The results were then sent to the client.

Landskrona – Sweden

Sweden – the centre line of the ship had to be checked, as all of the information for the carousel and other structures were related to this. This was checked using a bearings on the Total Station. This was checked in both X and Y axis.
Once this was checked we were then instructed to check the work which had already been marked out on the deck, by the on site construction team. This was checked by Total Station. After all positions were logged, the instrument was downloaded and data processed.

An AutoCad dwg was then produced and overlayed with design drawings. We could then check the differences and record them into an excel spreadsheet, which was given to the client. These points were also checked using a calibrated tape for any gross errors. When the results were satisfactory the supervisor carried on marking out the rest of the pads. The next stage was then to burn and cut out the relevant marks in the deck. The pads were then installed and welded.

Level Checks

Digital surveys then had to carry out a level check on and next to the pads during welding. This enabled monitoring of movement due to heat distortion.
After completion of the pad installation, The ship was then moved to Norway.
The surveyors would work together to position the bolt holes with an accuracy of 0mm. The deck was marked with a “punch hole” and then circled in white paint. Once all the points were set out, they were then crossed checked using steel tapes by the client and the Surveyors.
The points on the A-Frame and Firing Line were for templates, the points on the Umbilical Winch pads had no templates so every bolt hole had to be set out, and the centre points of the carousel pads were set out for the machining process.

All pads were then levelled, taking a number of levels on each pad. These levels were taken for the shims and the machining process which was decided by the designer. The levels were taken from the datum and then the declivity was applied in both planes, then the results were inputted into an excel table and sent to the client.

Norway Dimensional Control Survey
During the machining process Digital Surveys had to monitor all positions and levels as they were being machined. All levels were passed onto the client. After the machining process was complete a final sweep of levels were taking using the SDL30 digital level and the PCD Orientation was checked as instructed by the design team.

Digital Surveys also undertook a survey of the second deck where the life boats were being attached. Positions and levels were taken with the Sokkia Total Station, and related to the datum. The upper decks were also surveyed and overlaid. This survey was for the clearance of the A-Frame when it was in operation. A second survey was then taken for the umbilical which where it attached the second deck. This was for the alignment of the wheels. The co-ordinates were taken off the design drawing and then set out on the floor and the kick plate of the railings. These marks were “punched” then painted. A final deck survey “as-built” was then produced showing all deck openings and pipes that were on the main deck.

Digital Surveys ensured that all dimensional checks were recorded and approved to the accepted dimensional tolerances. During 2008 the process was repeated for the second ship “Deep Cygnus”

Rissa Norway

First of all the declivity of the ship was found. To ensure the readings were accurate, we set the electronic level (SDL30) up in the centre fame/pad, (R19) and took a level reading to common points. Once we took the level and position we could then work out the difference in level per metre. This would then be applied to every set of results which were taken by ourselves. The datum was always the centre pad.

Norway
It was then our job to set out relevant points given to us by the design team. These points were mainly bolt holes on the pads. To do this we again set up on the centre pad and back sighted our common point on the ship centre.

The dimensions of all the relevant points were transferred into co-ordinates on site by the surveyors, and then were inputted into the total station. When set up on a known point the instrument would calculate a bearing and distance for each point.