INTRODUCTION

The settlement of the tank bottom, in most cases, such a monitoring program isinitiated during the construction.During operations, settlement measurements should be taken at a plannedfrequency, based on an assessment of soil settlement predictions. For existingtanks that do not have initial settlement data, aprogram of settlement monitoring should be based on prior service history.

If at any time settlement is deemed excessive, the tanks should be emptied andreIeveled. Releveling of a sizable tank is expensive and rather difficult to achieve.Thus, a decision to relevel a tank is a crucialone, and relies very much on the proper interpretation and evaluation of themonitored settlement data.

There are several methods to the improvement of the shell and settlement in thebottom, such as: repairs in the plates of the bottom, partial leveling in the peripheryof the tank and releveling complete of the tank.

When it occurs tank bottom settlement?

When the settlement adhere strongly to the periphery of the shell of thetank, resulting in deformation of bottom plate close to the shore betweenshell-bottom.

Placing a straight edge on the unsettled floor as shown in Figure B-5, andobserving where the floor separates from the straight edge will help definethe break over point.

When it occurs tank bottom settlement?

When it occurs tank bottom settlement?

If the bottom is concave or convex, the settlement bottom, should bemeasured from an outstanding deformation from the bottom withoutsettlement and not the level of settlement.

Settlement bottom

Maximum allowable settlement Bew is shown in Figure B-10 for settledareas that include floor lap welds essentially parallel to the shell (± 20degrees). In settled areas where the measured settlement B exceeds 75percent of allowed settlement Bew, all shell-to-bottom welds and floorwelds should be inspected visually and with magnetic particle examinationor liquid penetrant examination. All indications should be repaired, orevaluated for risk of brittle fracture, and/or fatigue failure prior to returningthe tank to service.

Settlement bottom

Settlement bottom

Maximum allowable settlement Be is shown in Figure B-Il for areas ofedge settlement with no welds, butt welds, or lap welds in the floor thatare essentially radial to the shell (± 20 degrees).In settled areas where the measured settlement exceeds 75 percent of theallowed settlement, all shell-to-bottom welds and floor welds should beinspected visually and with magnetic particle examination or liquidpenetrant examination. All indications should be repaired or evaluated forrisk of brittle fracture and/or fatigue prior to returning the tank to service.

Settlement bottom

Settlement bottom

In general, settlement occurs slowly, and for most existing tanks, themajority of settlement is presumed to have occurred in the first few yearsof service. Significant additional settlement willnot be expected after the initial inspections. Therefore, typical practice isto compare the measured edge settlement with the maximum allowableedge settlement Bew and Be' and not include allowance for additionalsettlement during subsequent operation. Note that erosion of the padadjacent to the tank may cause local settlement. In this case thesettlement will continue unless the pad is repaired and future erosionprevented. For cases where significant additional settlement is expected,an engineer experienced in tank settlement evaluation should evaluatethe settlement expected at the next inspection with the limits in B.3.4. Thisis analogous to a corrosion allowance for components expected tocorrode.

Typical images in a evaluation of tank bottom settlement