Let’s start with basic dimensional controlling. How to measure Perpendicularity of Tubulars.
First, we have to specify the reference length between 2 circles or rings. Then, continue with defining circularity dan centerline of each circle. The surveyor will put reflector prism on any points (8 points per circle) inside the tubular shell. Refer to Construction Specification for Offshore Structures in the North Sea, EEMUA no. 158-1994, number of survey points should be 24 points (or every 15 degrees). This code is often used as a basis for many project specification. For practical reason, our DC guys just measure 8 points. I think this is acceptable, since 24 points is a requirement for Ring Stiffened Tubulars Only to measure circularity.
Once collecting data is completed, using software from the total station, we can get the optimum centerline of each circle. By connecting both centerlines, then we get (imaginary) centerline of tubular. Yup, it’s an imaginary line. My mentor often emphasize this ‘term’ to make me really understand. 
Surveyor also take the survey points at tubular edge at the same time taking the circularity. By the time, they calculate circularity (as above), we will get the error on tubular edge comparing to ideal perpendicularity ring.
See illustration below and I hope you understand what I mean.
Ilustration of Perpendicularity Measurement
This steps is applied if using Total Station. I’m using this as the opening of my next post.
Posted on October 13, 2011 by [Wan]
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