An impermeable and thermally insulated tank built into a load-bearing structure, the tank wall comprising: a thermally insulated barrier attached, to a load-bearing wall and made of insulating blocks, juxtaposed in parallel rows separated from one another, by gaps, an impermeable barrier supported by the thermally insulated barrier and made of welded metal sheets.

Each insulating block carries, on the face of same opposite the load-bearing wall, two metal connecting strips arranged in parallel to the sides of the insulating block. The sheets of the membrane carried by the insulating block are welded to the strips. The connecting strips are rigidly connected to the insulating block carrying same. The sheets each have at least two orthogonal folds parallel to the sides of the insulating blocks, the folds being inserted into the gaps formed between two insulating blocks.

A flexible fluid storage tank and method for assembling said tank, with specific emphasis on a system that may be easily road transported and field assembled with little or no tools, while preserving the flexibility of being tailored in size, shape and configuration to the user's needs with minimal customization and disassembly capabilities.