Very occasionally the response of a structure to static loading cannot be successfully found in a static analysis. This is normally due to the fact the structure is highly sensitive to changes in configuration from iteration to iteration, and a static analysis may tend to diverge rather than converge to a stable configuration. One of the most commonly occurring cases of this sensitivity is in systems which include significant water surface piercing, such as for example a buoyancy tank used during a riser tow-out. If an initial approximation to the equilibrium position of the structure actually has an excess of, say, buoyancy over gravity, in the next iteration the structure may be largely or wholly out of the water, and subsequent iterations may find alternately increasing portions of the structure exposed or submerged, and the solution rapidly diverges. In such circumstances, a quasi-static approach is more suitable, and the model will tend settle in a dynamic fashion, typically under the influence of damping, towards its static equilibrium configuration. Refer to Quasi-Static Analysis for further discussion on quasi-static analysis, including some helpful guidelines.
