Evaluation of Out-of-Plane Settlements Based on API 653 Annex B

Introduction

Settlement is a condition caused by a loss of compaction or support of the surface where a tank rests, which affects the plates of the bottom, shell, and roof due to plastic deformation. However, piping, tank nozzles, and attachments can also be affected by settlement, as well as the functionality of the floating roof. Therefore, settlement survey measurements and analysis are part of the tank integrity assessment, as described in API 653 Annex B, where the types of settlements are described, and an entire methodology is developed to determine the maximum allowable deflection [1]. However, as there are no calculation examples, given its complexity and for a better understanding and applicability of Annex B, we developed the following flow chart shown in Figure 1, where we will elaborate on three possible cases [1].

Figure 1 shows the step-by-step applicability of Annex B to determine the out-of-plane settlement described in points B.2.2.4 and B.2.2.5 [1]. These points are linked to the three graphs corresponding to Figures B.3, B.4, and B.5, as well as the equations described in sections B.3.2.1 and B.3.2.2, for a cylindrical, vertical storage tank resting evenly on a soil, concrete, or asphalt surface [2,3].