As the owner of the largest pipe and pressure vessel rotational lining factory in China, I have been recently looking at many drawing packages related to BWTS and EGCS retrofits. Specifically, I look at many sets of isometric pipe drawings for fabrication and PE lining of the spools. The drawings are all over the place in terms of quality and information so I hope to share some of the best practices. Since scanning technology and the associated software packages have really only come into their own in the last few years, many otherwise technically qualified individuals are not sure what they can expect from a quality set of drawings taken from scan data.
When an engine room or other structure is scanned, the first step is to create a point cloud. A point cloud is bunch of 3D coordinates collected by the scanner. Software then crunches this data, smooths it out and filters it to create a dimensioned space that other dimensioned objects can be placed (like that new pipe run for your BWTS). While there are differences in accuracy between different systems and scanning modes, the theoretical accuracy of a scanner has very little to do with the ultimate accuracy of the drawings. For example, if you have a scanner that can scan to 0.5mm accuracy, your cannot be confident in the position of a pipe tie in, in that scan, if there is 3mm of rust and paint on the pipe you are scanning and using as a reference. You definitely cannot hope to have that accuracy if the deck plates were never lifted and the pipe run in the drawing is just an extrapolation of the parts that poked above the deck plates. I am a big believer in the person that does the actual scan should be the same person that creates the isometric drawings since there will be many judgement calls that need to be made when creating the isometrics and only a knowledgeable and experienced technician, that did the scanning, will know how to make those judgement calls. For example, was the vessel built in a dockyard with CNC fit up of the spools so you can count on all the pipes in the vessel being parallel and fabricated in accordance with best fabrication practices or was this vessel built in a dockyard without these resources and all the pipes are spaghetti runs where it looks like machinery was placed and they just welded up the pipes to make it fit? In the first case, I am going to be much more confident we can create tie in spools directly from the isometrics. In the second case, I am going to specify more calibration spools.
This brings me to my first pet peeve about isometric drawings. If you have quality scan data, I never ever want to see a large spool specified “fit up in situ” or “leave fittings loose” or “for site fabrication.” This indicates a low level of professionalism by the designer or a lack of confidence in the scan data or excessive risk aversion. I do not care if the pipes are GRE/GRP and everyone knows the fittings will have to be bonded in situ, I do not want to see a single large pipe specified for site fabrication in the isometrics. A professional designer with quality scan data should only specify small calibration spools to ensure proper tie in tolerances and if the scan data and the designer is good, even those calibration spools will likely fit. Having worked on projects and fabricated and coated spools with excellent drawings and excellent scan data, I can tell you it is possible to get the tie ins spot on, the first time. We have previously fabricated and coated DN1000 main condenser lines with an S elbow, branches and all fixed flanges that was over 6 meters long with a drawing taken from a scan and it fit spot on. If your engineering company says this is impossible, then it is time to interview a new engineering company. Remember that every designer wants to minimize risk, but this should not be at the expense of the customer. Specifying more spools for site fabrication may increase the chance certain spools fit and reduce the designer’s risk, but at the owner’s expense of requiring more time and cost at the dockyard.
Never use scan data from a sister vessel to create isometric drawings. Sister vessels may be sisters, but they are not identical twins! The minimal amount of money that could be saved by using data from a sister vessel is going to be crushed by the additional time and cost incurred by not having optimal drawings. An engineering company that tells you about all the money you will save by using data from a sister vessel is probably more interested in making a sale than being the right partner to contribute to a successful docking.
Rotational lining is a robust process that can apply a seamless lining to difficult geometry pipes, but do not use our ability as a crutch to make stupid designs. If a pipe goes in the engine room, it should not be more than 3 meters long. If a pipe has an elbow or branches, it should not be more than 3 meters long. No more than two elbows or one elbow and one branch on any single pipe (manifolds are the exception), regardless of length. If you cannot grind an interior weld, then do make the pipe that way. Flanges save money, they do not cost money.
The fabricator needs to talk directly to the designer. Even the best designers, will on occasion, create a design that is not optimal for fabrication, coating or fitting. The fabricator is the expert on fabrication so let the fabricator collaborate with the designer. This does not mean talk to the project manager that then acts as a go between for the designer. When I get a set of drawings that includes a note that says, “If you have any questions or would like to suggest any changes, you can reach the designer at….” tells me I am working with an experienced company that does not want to waste time or money. There are many excellent designers that following a 15 minute drawing review with the fabricator will make simple changes that will result in thousands of dollars in savings. For example, can we replace the 1D elbow fittings with a 3D bent elbow for smaller diameter pipes. A bent pipe is cheaper and faster to fabricate and easier to coat. Being a stickler for quality drawings is the best way a fabricator can prevent disputes.
Use a software package that will create a 3D rendering for each isometric. I have one expert on my staff that is really good at reading isometrics and when I cannot figure out which way something should be fabricated, I have to ask him and this is after I have looked at thousands of drawings. Include a 3D rendering and then every one are experts at reading the drawings.
Never specify an existing system pipe to be modified in situ to accommodate a retrofit unless you have a plan on how to coat that pipe. Spray applied “galvanizing” and most two part epoxies that can be used for in situ coating are junk. Even if you have a good product, can the surface to be coated be properly prepared? Is the repair coating compatible with the pre-existing coating. If you are still wondering about the right answer, let me help you out and tell you the answer will be “NO”. If you are planning on minimizing your docking time, you need a new factory coated pipe, that will be connected via a flange connection to the existing pipeline.
Never delay anything if a pipe does not fit. Remember, if you have good drawings, the only pipes that will not fit are the calibration spools. Immediately ask the dockyard to make two identical spools that do fit, let the vessel sale with an uncoated fitted spool and then send the second spool for coating and then after coating send it to the vessel to be fit. We make the calibration spools small so they are easy to fit and cheap to ship, fabricate and coat.
Coating allowances are important. We typically machine the PE on the flanges to 2mm. Sometimes vulcanized rubber coatings can be up to 6mm thick. I always want to see a coating allowance specified on the fabrication drawings because this tells me it has been considered in the design. If a designer calls me up and asks how to properly specify the coating allowance, then I know I am dealing with a good designer.