I can't stress enough how important this step is. In the past, I usually jumped on a final piece without spending too much time looking for ref. The result, most of the time, was a painfully slow painting process. No matter what you think, you probably don't know how that particular object is made, how it reacts to light, what its real color is. Reference are useful for details, but they're also important to keep your imagination spinning. One thing to be aware of thought: don't limit your research to the subject you have in mind. Obviously, in this case, a good amount of “refinery” or “factory” pictures is important, but it's also a good idea to look for “lens flare” “fog” “blu light” “city lights” etc. One last thing about reference: try to look at movie stills. If you see a particular shot in a movie that sparks your interest, grab a still.


After the references gathering, it's time for some thumbnails. This is the fun part. Thumbnails shouldn't be too detailed...actually, they shouldn't be detailed at all. Try to limit the size to 2000 px and paint them without zooming in too much. Think about this stage as the “big shapes” one. All you really want to define is composition, value structure, colors. This is where you want to explore different ideas, angles, perspective. Once on the final piece, you really want to have all these aspects defined. In this case, I explored different ideas even though almost all of them have the “giant pipes” element. That was definitely something I want to use in my final piece.


out of all the thumbnails, n3 was the one I liked the most. All I had to do was scale it up (3000 px wide) and start painting.
Since my idea was to make a concept and not a final matte painting, I could be a little looser than usual. I browsed through the references I collected earlier, looking for shapes that were close to what I imagined this environment to look like. Once I found something, I pasted it in the file and color corrected it to match my colors. Nothing too advanced here, just curves and painting on top. When I had some basic elements in the scene, I started painting over to fill the rest of the picture.


I’ll admit it, I'm not a “brush guru”. I have Jaime Jones' brush palette loaded in my photoshop, but I usually use just a few of them. This doesn't mean you should do the same. If you look at Jamie's work, you will see what a good use of different brushes could do. 
Below are the main ones I used in this particular picture:


The painting process in this picture was pretty straightforward.
Two elements that required a slightly different approach were tubes and smoke.
To paint the tubes I used a mix of path, layer style and smudge tool:

The smoke was painted with two brushes (one as paint, one as eraser) and smudge tool in spots:


Strong silhouettes make the image readable at a glance. Used them to drive the attention to your focal points. If two elements are too close in value, paint something between them to better define their position in space. Smoke, light, fog are usually a good solution.


Always check your value structure and use it to create the illusion of depth in your environment.
To help you with this, put a b/w adjustment layer on top of your painting and turn it on every now and then to check your values. If something doesn't look right, it's probably wrong. Fix it before jumping on anything else.
The picture below shows you a value breakdown of the painting.


Try to think about “contrast” every step of the way. Big vs small, dark vs light, horizontal vs vertical, hard edge vs soft edge. If you have to much of one, put some of the other. This rule applies to brush strokes as well, play with different sizes and directions.