@ayy i watched this ad/documentary by and about norwegian contractors, the company who built the majority of the condeep platforms in the norwegian north sea sector. https://www.youtube.com/watch?v=uNKNitYvDJA the platform legs are built with a slip form, which basically means that the concrete is added at the same pace as it hardens, making the structure have very homogenous properties. of course concrete hardens slowly, so it takes several years to build the platform legs. but it also allows them to use relatively light equipment since relatively little concrete is needed at a time.