What type of tank do not form part of the vessel structure?

Independent tanks are designed to be self-supporting and are not integrated into the structure of the vessel itself. These tanks are often used for transporting various types of cargo, including liquids, and are built to withstand the forces acting upon them independently of the ship's hull. This design allows for greater flexibility in terms of placement and maintenance since the tanks can be installed or removed without affecting the hull integrity or requiring structural modifications to the vessel.

In contrast, integral tanks are part of the ship's structure and contribute to the overall strength of the vessel. Double hull and single hull tanks are also integrated into the ship's design, with double hulls providing an additional layer of protection, particularly in the case of oil tankers, while single hulls are more straightforward in construction but less safe compared to double hulls. Thus, independent tanks stand out because they are not part of the vessel’s structure, which is the key distinction emphasized in the question.