This section discusses the following topics concerning tank settlement: • Shell settlement
• Bottom settlement
• Correcting settlement problems
In spite of all attempts to prevent or minimize settlement during tank foundation design and construction, tank shell and/or bottom settlement may still occur over a period of time after the tank has been placed in service. Therefore, shell and bottom settlement must be evaluated as part of the periodic tank maintenance activity to determine if any corrective action is required. The types of shell and bottom settlement that may occur must first be understood in order to make these evaluations. The sections that follow review the principal types of settlement and describe how they are evaluated.
Shell Settlement Types
The three types of shell settlement that may occur are as follows:
• Uniform
• Planar tilt • Differential
Uniform Shell Settlement - Uniform shell settlement and the problems that it may cause are
illustrated in Figure 21. In uniform shell settlement the shell remains level as it settles. This type of settlement does not introduce significant stresses or distortions in the tank shell or bottom and does not necessarily require correction. Problems that can be caused by uniform shell settlement and possible corrective actions are as follows:
• Blockage of surface water drainage from the tank pad into the diked area may result in water retention at the tank shell. Water retention can be corrected by regrading the tank pit such that water cannot accumulate near the tank. If this problem is not corrected, it can cause localized tank corrosion in the lower
Figure 21. Uniform Shell Settlement
Planar Tilt Shell Settlement - Planar tilt shell settlement is when the shell tilts as it settles
and the bottom of the shell remains in a plane. If the shell elevations are plotted on a linear scale, true planar tilt settlement produces a sine or cosine curve as illustrated in Figure 22. As the shell tilts, stresses are introduced that tend to change the shape of the shell. The top of the shell tends to become elliptical. Shell out-of-roundness can be determined by checking top diameters and floating roof seal clearances around the circumference of the tank. Figure 22 also illustrates the effect that planar tilt settlement can have on a tank. Typical problems that may be caused by planar tilt are as follows:
• Distortion or support problems in connected pipe • Poor surface drainage near the tank
• Malfunction of floating roof seals
• Other interference with floating roofs travel • Buckling in flanges or webs of wind girders
Differential Shell Settlement - Differential shell settlement is when the bottom of the shell is
no longer in either a level or tilted plane. API-653 also refers to differential shell settlement as out-of-plane deflection. With differential settlement, the shell undergoes different amounts of settlement at different points around its circumference. This settlement usually does not damage the tank structure as long as the settlement is minor and there is adequate support under the shell. The amount of differential settlement is defined as the deviation between the actual shell settlements and the sine or cosine curve that represents true planar tilt.
A plot that describes differential settlement is shown in Figure 23. The inherent stiffness of the shell tends to concentrate shell support at the points with the least amount of settlement. As with planar tilt settlement, the top of the shell tends to become elliptical. Differential settlement can cause the same problems as planar tilt settlement. In addition, differential settlement may cause the shell to buckle or cause the shell-to-bottom area to become overstressed. Figure 24 illustrates the potential problems that may result from differential shell settlement.
Evaluation
32-SAMSS-005 requires that shell settlement measurements be made before, during, and after hydrostatic testing of newly constructed tanks. The purpose of these measurements is to determine if the settlement that occurs during the initial filling of the tank is within acceptable limits. Shell elevation measurements will then be made periodically during the life of the tank to determine if any unexpectedly large settlements occur. The interval between elevation measurements is determined based on the results of these measurements. If no settlement problems are indicated, elevation measurements will typically be made during each T&I. Shorter settlement measurement intervals are used if initial measurements indicate that there might be settlement problems. Tank elevation measurements will not disrupt operations because the measurements can be made with the tank in service.
The shell settlement readings are made relative to the elevation of a permanent bench mark (See Figure 25). The bench mark must be installed in such a manner that it will not be affected by future ground settlement due to the tankage. This permanent bench mark permits an accurate measurement of tank shell settlement over a period of years.
Reference points are established on the tank shell by welding nuts or similar steel objects to the tank shell. The reference points are located 100 mm (4 in.) above the bottom edge of the bottom shell course at equal distances around the circumference of the tank. One reference point is located at the catch basin. The minimum number of reference points depends upon the diameter of the tank. API-653 requires that at least 8 reference points be used, and that the reference points be spaced no more than 9.1 m (30 ft.) apart.
The elevation measuring instrument should be set up at least 1-1/2 tank diameters away from the tank shell. The elevation readings should be accurate to within 2 mm (1/16 in.).
Appendix B of API-653 contains a basis that may be used for the evaluation of differential shell settlement (i.e., out-of-plane deflection). The API-653 evaluation basis is contained in Work Aid 4 and is based on the following parameters:
• Arc length between shell elevation measurement points
• Tank height
• Modulus of Elasticity and yield strength of the tank shell plate
In order to use the API-653 basis, the shell elevation measurements that are made must first be converted to out-of-plane deflections around the tank circumference. This data conversion is typically done using a computer program and subtracts the uniform and planar tilt settlement components from the total settlement measurements.
If the measured differential shell settlement exceeds the API-653 acceptance basis, CSD should be contacted before any action is taken to relevel the tank. Further evaluations are typically made to determine if the settlement has caused any damage or operational problems to the tank. Experience has shown that excessive differential shell settlement will typically cause shell distortion before any failure will occur. A detailed stress analysis may also be done to help make a decision. Releveling a tank can be expensive and could cause more problems than it solves if it is not done properly.
Bottom Settlement Types
The three types of bottom settlement that may occur are as follows:
• Localized
• Center-to-edge
• Combined bottom and shell
Localized Bottom Settlement - Localized depressions in the tank bottom are normally due to
a soft spot or void in the foundation. Voids in the foundation may occur when settlement has occurred and the tank has been jacked for repairs. After the jacking operation, the foundation must be refilled with a grout material to fill in the vacant spaces. However, no technique can guarantee that the vacant spaces are entirely refilled. Therefore, after jacking operations, it is not unusual for voids to exist in the foundation. Tunneling under a tank to inspect bottom plates, or leakage through a bottom plate that softens or disperses pad material, are other mechanisms that can also cause voids in the foundation.
The bottom plate is not designed to support the tank contents without being uniformly supported from underneath by the foundation. Therefore, a localized weakness in the foundation soil can cause overstress in the bottom plates and result in a bottom plate weld failure. If the foundation in the area of the weld failure is unstable or poorly drained, the resulting leak can wash out a considerable portion of the foundation and lead to a major tank bottom failure. Figure 26 illustrates localized bottom settlements that may result from soft spots or voids in the foundation.
Figure 26. Localized Bottom Settlement
In addition to localized bottom settlement that can occur away from the tank shell, localized settlement can occur near the shell of a tank. Localized bottom settlement that occurs near the tank shell is normally accompanied by shell settlement, and the two settlements should be
Center-to-Edge Bottom Settlement - Center-to-edge bottom settlement is illustrated in
Figure 27. A relatively large center-to-edge bottom settlement over the entire bottom may be accommodated without overstressing the tank bottom because the bottom plates act as a thin membrane and are flexible. However, extreme cases can occur when the bottom settlement takes up all slack in the bottom plate and exerts an inward pull on the shell, as illustrated in the detail in Figure 27.
On tanks that are less than about 50 m (150 ft.) in diameter, excessive bottom settlement is likely to buckle the shell. On tanks that are over about 50 m (150 ft.) in diameter, frictional drag is a bigger factor and excessive settlement is more likely to overstress the bottom plates before noticeable shell buckling occurs.
In tanks that are built on poor foundations, the failure of a bottom weld can lead to catastrophic foundation washout. However, if the tank foundation was preloaded and complies with Saudi Aramco design requirements, center-to-edge settlement should not be a problem.
Combined Bottom and Shell Settlement - Bottom settlement will normally occur in
combination with one or more types of shell settlement. Differential settlement of the shell of a large diameter tank relative to its bottom can result in significant radial pull on the bottom plates by the shell. This type of settlement is illustrated in Figure 28. The difference in settlement between the shell and bottom must be absorbed over a very short distance in the bottom plates at the tank edge. The resulting excessive distortion of the bottom plates, that must accommodate all of the stretching, may crack a bottom fillet weld in the distorted region. The cracked fillet weld could lead to a failure of the bottom.
Evaluation
Although excessive bottom settlement occurs less frequently than shell settlement, bottom settlement can result in greater damage and much higher releveling costs. At the same time, it is more difficult to determine bottom plate settlement patterns while the tank is under hydrotest or in service.
Because of the greater risks associated with bottom settlement, bottom elevation patterns are sometimes monitored while the tank is in service in locations where sub-soil conditions are doubtful or unsatisfactory. In these situations, important data points can be checked by dropping a sounding line through roof openings, such as manholes and support leg openings, before and during hydrotest and while the tank is in service. Warped roof plates in a cone roof tank are a strong indication that excessive bottom settlement may have occurred. In addition, excessive shell settlement indicates a strong possibility of excessive bottom settlement as well.
In most situations, bottom elevation measurements to determine settlement patterns will only be made when the tank is taken out of service for a T&I. API-653 contains recommended locations for bottom settlement measurements, as shown in Figure 29. Closer measurement spacing
(75 - 150 mm [3 - 6 in.] apart) should be used in areas where the bottom elevation changes rapidly, especially close to the shell.
Appendix B of API-653 contains a basis that may be used for the evaluation of tank bottom settlement. This basis is included in Work Aid 4. The API-653 criteria is based on the following parameters:
• Depth of depression (or the height of the bulge) in the tank bottom. Note that local areas of the bottom may be bulged up rather than depressed down. Bulges are evaluated using the same basis as depressions.
• Radius of the largest circle that may be inscribed within the depressed (or bulged) area.
• An assumption that the bottom plate lap welds are made with a single weld pass.
If the measured bottom plate settlement exceeds the API-653 acceptance basis, CSD should be contacted before any action is taken to repair or relevel the bottom. The API-653 evaluation basis is relatively conservative, and it may be worthwhile to do a detailed stress analysis to determine the actual situation if extensive repair or releveling is required. The API-653 basis is especially conservative if the bottom plate lap welds are made with two or more weld passes rather than the one pass that API-653 assumes.
Methods for Correcting Settlement Problems
If the shell or bottom settlement is excessive, corrective action must be taken before unacceptable damage and tank leakage occurs. Because significant time is required to properly plan, evaluate, and execute settlement corrections, the decision to relevel cannot be taken lightly. Improper releveling can cause as much or more damage to the tank than the settlement that it is meant to correct. The paragraphs that follow describe the primary considerations and techniques for correcting shell and bottom settlement.
Shell Releveling Considerations and Techniques
Considerations - Three forms of shell releveling may be considered: releveling only a part of
the shell, releveling the entire shell, or releveling both the entire shell and the tank bottom as well. The extent of releveling should be minimized consistent with fixing current problems and minimizing the probability of needing future releveling.
In many cases, releveling only part of the shell is necessary. In these cases, the low points of the shell are jacked by amounts that range from 50 mm (2 in.) to 175 mm (7 in.). In addition, when the entire shell must be releveled and settlement is expected to continue, local overjacking of the shell in areas of poor soil may be desirable. When overjacking is specified, it should be done so that the resulting radial displacements of the shell will not cause floating roof binding or gaps between the floating roof and shell. Calculation procedures are available that predict the amount of radial shell displacement for specified changes in shell elevation.
Techniques - The most common technique for releveling a tank shell is to lift the shell with
hydraulic jacks and pack selected materials beneath the bottom and annular plates. Two basically different procedures for tank jacking are widely employed: jacking against the tank shell and jacking from beneath the bottom or annular plates.
When jacking against the tank shell, jacking lugs or brackets are welded to the tank shell around its periphery as close to the bottom or annular plate as possible. Figure 30 illustrates the details for typical jacking lugs. A compact hydraulic jack (or a pair of jacks) is placed on a timber footing at each bracket location, and the tank is gradually lifted to a height that is equal to the jack stroke (generally 100 mm [4 in.]). Timber beams or steel shims are then used to temporarily support the tank shell while the jack is released, and additional timber beams are placed between the jack and the foundation. The entire process is repeated until the tank shell is level and at the required elevation.
Depending on the jacking system, the jack spacing, and the shell thickness, externally mounted jacks can impose significant stresses on the tank shell. Therefore, it is important that the shell stresses be checked and any necessary strengthening measures carried out before jacking is begun.
The major advantage of jacking against the shell when compared to jacking from beneath the bottom or annular plates is that disturbance to the existing foundation is minimal. Because the jacks are not beneath the shell, placement and compaction of select backfill is more uniform and results in less potential for differential shell settlement in the future.
The disadvantage of jacking against the shell when compared to jacking from beneath the annular plates is the welding that is required to attach the brackets to the shell and to provide any necessary shell reinforcement. This welding can also build up residual stresses in the shell and possibly cause brittle fracture, particularly in older tanks where steels with poor fracture toughness were often used. On newer tanks that are constructed of high strength steel, the weldability of the bracket to the shell may be a problem.
There are at least two methods for jacking from beneath the bottom or annular plates: using a jacking frame, or excavating pits for the placement of hydraulic jacks.
The preferred method for jacking from beneath the bottom annular plates is to use a jacking frame. A typical jacking frame is illustrated in Figure 31. The frame is equipped with slender jacking shoes that are shaped so that they can be easily slipped beneath the tank shell. The frames are spaced every 3 to 4.5 m (10 to 15 ft.) around the tank periphery. The tank can be jacked to a maximum height of 300 mm (12 in.) with this method. No welding to the tank shell is required, and the jacking frames are reusable. Due to the size and shape of the jacking shoe, there is little disturbance to the existing foundation. The disadvantages of this method are the initial fabrication cost of the frames and the limited jacking height of 300 mm (12 in.). The frames also cannot be used to lift and relevel the entire tank bottom; therefore, this method cannot be used for all tank releveling needs.
Figure 31. Typical Jacking Frame
Figure 32 illustrates pits that may be excavated beneath the bottom or annular plates to provide space for placing hydraulic jacks. Jack spacing depends on the size of the tank and the thickness of the shell. Jacks are typically spaced 6 to 7.5 m (20 to 25 ft.) apart; however, a stress analysis should be made for the specific tank to be jacked in order to determine the required jack spacing. After placing the jacks, the tank is lifted to the desired height by utilizing timber cribbing. The jacking pits are then backfilled after the tank is lowered onto the newly releveled foundation pad.
Localized settlement over the jacking pit areas can cause additional stresses in the annular plates and shell. Therefore, it is important to minimize these settlements and resulting stresses by keeping the size of the jacking pits as small as possible. Only high quality fill such as crushed stone should be used, and the fill should be properly compacted by means of rams
Figure 32. Typical Jacking Pit
The actual jacking operation should be accomplished using a predetermined procedure that considers the following factors:
• All tank elevation changes should be gradual to minimize stresses in the overall tank structure.
• Localized sections of differential settlement should first be jacked to a planar