In my office the majority of the subsea structures are designed to the 'normal' structural design codes. We use a variety depending on where the project is, who the client is etc. You will know the codes. We use API RP2A both LRFD and WSD, AISC again LRFD and WSD, BS5950, NORSOK, NS3472.
In addition to these we often have to met the requirements of classification societies such as DNV or Lloyds, and warranty surveyors such as Noble Denton. They also publish structural design 'codes' which we work too. I use the word codes lightly because the generally specify the types of load cases , factors, and other things you must consider and refer you back to bonafide codes. The are often not just considered with the permenant works, but also the installation and transportation (seafastening, lift analysis, rigging, padeyes etc)
There is a lot more in terms of subsea specific codes when you look at subsea piping (Norsok U-001, PD8010 etc).
Some of the things we look at that may not be considered in Topsides stuff is fishing loads (making structures overtrawlable), anchor loads, dropped object loads, ROV Loads, hydrostatic collapse,
