In oil and gas pipeline projects, the welding of fatigue-sensitive steel catenary risers (SCR) and flowline pipes to tight specifications is critical.
Counterboring the pipes might not be a viable method of controlling pipe geometry due to cost considerations or to limitations on pipe wall thickness. So how can pipeline contractors ensure that pipe fit-up, welding and pipelaying processes run smoothly with minimal interruptions?
Managing risk is key here. On occasions, a pipeline contractor can receive ‘free issue’ pipe for a project from the project operator. Normally, these pipes will have been sourced by the project operator from a pipe broker and if this is the case, the specific tolerance and geometric details of the pipes are unknown.
So how does the pipeline contractor know that all the pipes for a certain project are within the manufacturing specification? And, if they are, how easily will the pipes fit together prior to welding and pipelaying in order to minimise disruption and delays?
To prevent these kinds of bottlenecks and to minimise project risk, oil industry owners, pipelay contractors and welders need to capture, record and analyse pipe end geometry quickly and accurately.
This measurement data, if used correctly, can then help to ensure that pipes delivered into the bead stall will fit together easily and within the welding specification requirements. End dimensioning and fit-up typically involves two steps: measuring and fit-up.
Measuring involves actually collecting pipe measurement data. Automatic, laser-based measurement tools can be used to measure geometrical features of pipe ends, normally performed onshore, although this process sometimes needs to occur on a cargo barge.
Laser-based measurement tools can be used to measure the IDs, ODs and WTs of pipe ends in rapid time.
Typically, several thousand IDs and ODs of a pipe can be measured simultaneously in less than 10 seconds, enabling hundreds of pipe ends to be measured in one shift. This means less time on site, minimal delays and costs for the pipelay contractor. Laser measurement tools are also very accurate (typically to 0.05mm).
Data from laser measurement tools can be made available to pipe optimisation software, which will include some sort of simulation or sequencing software. SmartFit, for example, uses pipe measurement data to predict and control the fit-up, before the pipes are brought into the bead stall for welding. This averts production issues relating to poor fit-up and manages the assembly of problem pipes so as to maximise the welding productivity.
Measure, mark, fit-up
Each pipe end is measured, identified and entered into the software. The software analyses the fit-up of pipes and allows the operator to mark the best rotational position on each pipe end. In the bead stall, these marks are aligned to immediately achieve the best rotational position so that misalignment is minimised and the project HiLo is achieved.
Any problem pipes that won’t fit at a specified HiLo are also indicated and are re-sequenced or removed completely so that fit-up problems do not occur in the bead stall. Production delays due to mismatched pipes are avoided.
Experience shows that with typical flowline HiLo limits and using typical seamless line pipe that has not been counterbored, fit-up issues can occur regularly depending on the HiLo requirement. For a HiLo of around 1.0 to 1.2mm, problems are likely to occur every 10 to 20 pipes.
Using pipe optimisation and simulation software enables the required HiLo’s to be achieved in the bead stall without trial and error. But when pipes will not fit, this will be indicated and the problem pipe can be taken out of sequence.
Barzan Gas Project
OMS recently completed a pipe measurement survey and pipe fit up project for RasGas, a Qatar-based joint venture between Qatar Petroleum and ExxonMobil. OMS was contracted by RasGas and its pipeline contractor to perform an onshore pipe end dimensioning survey of more than 2,000 pipe ends destined for use on the Barzan Gas Project offshore from Qatar.
Two OMS engineers were deployed to the pipe-coating yard in Ras Al Khaimai, UAE, to measure the pipes. The 32” diameter pipe ends were long seam welded pipes with a 30mm concrete coating. OMS measured the pipe ends (in batches of 400 at a time) in order to identify 2,000 pipes that were ‘fit for purpose’ i.e. within the project’s specified HiLo of 1.0mm.
Certain weld seam restrictions were also placed on the pipes. First, the long seam welds located in the upper quadrant of the pipe had to be within +/- 45 degrees of the vertical. Second, successive long seams were to be rotated in such a way that they were not less than 30 degrees from the previous weld seam. OMS performed several different scenarios in order to ascertain the optimum pipe fit up.
As the batches of pipe were end dimensioned and fit for purpose pipes marked up accordingly, the measurement data was made available to OMS’ SmartFitTM system back in the UK.
Overnight, the data was processed and the pipes were sequenced by the software, so that the most closely matched ends could be brought together for welding.
This information was sent back to OMS engineers at the coating yard, who then managed the process of physically locating the correct pipe ends in the yard, adding suitable rotation marks to the pipes (which would allow the welders to immediately align the pipe ends in order to achieve the best rotational position, thus minimising misalignment and achieving the project HiLo).
OMS also managed the loading of these pipes onto the pipelay barge in the planned sequence. Overseeing this process was absolutely key to the success of the project.
