Best Practices for Isometric Drawings, Scan Data & PE Lined Pipe Fabrication

From the owner of the largest pipe and pressure vessel rotational lining factory in China

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.

Scan Data & Point Clouds: What Really Matters

When an engine room or other structure is scanned, the first step is to create a point cloud – a 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, you cannot be confident in the position of a pipe tie‑in if there is 3mm of rust and paint on the pipe you are scanning. 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 poke above the deck plates.

I am a big believer that the person who does the actual scan should be the same person that creates the isometric drawings. There will be many judgement calls, and only a knowledgeable and experienced technician who did the scanning will know how to make them. For example:

• Was the vessel built in a dockyard with CNC fit‑up of the spools, so you can count on all pipes being parallel and fabricated according to best practices?
• Or was it built in a dockyard without those resources – all pipes are “spaghetti runs” where machinery was placed and pipes just welded to fit?

In the first case, I am much more confident we can create tie‑in spools directly from the isometrics. In the second case, I will specify more calibration spools.

My First Pet Peeve: “Fit Up In Situ”

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 low 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. If the scan data and the designer are good, even those calibration spools will likely fit.

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

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 will 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 this way is probably more interested in making a sale than being the right partner for a successful docking.

Design for Practical Fabrication & Coating

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).
• If you cannot grind an interior weld, then do not 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 talking to a project manager who acts as a go‑between.

When I get a set of drawings that includes a note like “If you have any questions or would like to suggest any changes, you can reach the designer at...” – that tells me I am working with an experienced company that does not want to waste time or money.

Many excellent designers, following a 15‑minute drawing review with the fabricator, will make simple changes that result in thousands of dollars in savings. Example: replacing 1D elbow fittings with a 3D bent elbow for smaller diameter pipes – a bent pipe is cheaper, faster to fabricate, and easier to coat.

Being a stickler for quality drawings is the best way a fabricator can prevent disputes.

Always Include a 3D Rendering

Use a software package that will create a 3D rendering for each isometric. I have one expert on my staff who 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 looking at thousands of drawings). Include a 3D rendering, and then everyone becomes an expert at reading the drawings.

Never Spec an Existing System Pipe to Be Modified In Situ

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 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: the answer will be “NO”. If you are planning on minimizing docking time, you need a new factory‑coated pipe that will be connected via a flange connection to the existing pipeline.

If a Pipe Does Not Fit – Never Delay

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:

1. Let the vessel sail with an uncoated fitted spool.
2. Send the second spool for coating, then after coating send it to the vessel to be fitted.

We make the calibration spools small – so they are easy to fit, 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.