Tank Settlement (chap 9 / A 6.7)

Question 1

How are shell elevation measurement made? (Different thank settlement measurements)

Answer 1

During hydro test for new and old tanks, at least 6 sets of measurements shall be made. 
1-empty
2-1/4 full
3-1/2 full
4-3/4 full
5-24 hours after filled
6-when tanks is empty again
should be made at equally spaced intervals around the tanks circumference not exceeding 32ft (10m).

Question 2

When are shells settlement measurements made?

Answer 2

planned frequency based on an assessment of soil settlement predictions.
Bottom settlement monitor is to be made during interval inspection.

Question 3

The type of settles?

Answer 3

1. edge settlement
2. Bottom settlement near the tank shell
3. Localized bottom settlement remote from the tank shell

Question 4

Edge settles affect?

Answer 4

Affects bottom parallels and perpendicular welds

Question 5

How to evaluate edge settlement?

Answer 5

If edge settlement is in an area with a Lap welded seam than runs parallels +/- 20 , B turns in to Bew
Run Perpendicular +/- 20, B turns into Be

Question 6

What is Be and Bew?

Answer 6

Maximum allow vertical settlement.

Question 7

How to calculate Be/Bew?

Answer 7

With the value of R, and B measurement obtain max allowable vertical settlement Be/BeW from figure B11 / B12

Question 8

When is MP and LP needed?

Answer 8

If B > .75 Be/Bew and larger than 2”

Question 9

What if B is less than .75% of Be/Bew

Answer 9

May return back to service

Question 10

What happens if weld seams show strains >2%

Answer 10

they should be repaired

Question 11

If plate exceeding acceptable plastic strains (2-3%)

Answer 11

be replaced

Question 12

Should weld seams be VI

Answer 12

yes

Question 13

Bottom settlement near the tank shell

Answer 13

present in the bottom or in the annular ring zone (if there is one). It occurs when the bottom deforms showing a depression or a convexity in relation with a flat plane bottom.

Question 14

How to calculated bottom settlement near the tank shell?

Answer 14

Bb=0.37R, where R is half the budge or depression in its entire length. Bb= max allowed height of budge or depth of the local depression.
B

Question 15

What happens when B>Bb

Answer 15

Repairs or rigorous assessment is needed

Question 16

Localized bottom settlement remote from the tank shell

Answer 16

are depressions/budges that occur in a random manner remote from the shell. Same equation of bottom settlement near the tank shell can be used near the tank shells (bottoms has single pass welded joints)

Question 17

How to improve tank reliability incase of settlements. (settlement do occurs)

Answer 17

1. leave plenty of free space under nozzles
2. if there is uniform settlement (in case foundation not built well) , you can use flexible joints or maritime hoses to absorb misalignments.
3. Piping should be designed with considerations to prevent problems caused by settlement.

Question 18

How to calculate the number of survey points for determining tank settlement

Answer 18

N = D/10
N=number of required settlement points
D= Diameter (ft)

• max spacing between settlement measurement points shall be 32ft
• no less than 8 measurement points

Question 19

How to obtain elevation measurements?

Answer 19

1. min of 4 equally spaced diametrical lines that make a min of 8 measurement points ( like a pizza), with equal angles
2. max of 32 ft spacing for circumferences - for external settlement. (where pizza crust is)
3. max 10ft of spacing between measurement points for internal settlement (at the diametrical lines across the tank)

Question 20

max allowable differential settlement between 2 consecutive stations that may have severe consequences?

Answer 20

a. out of plane displacement are induced in the shell in the form of building under a displacement controlled mechanism
b. high stress developed at the based of the shell and in the region of the settlement
c. high stresses develop in the tank bottom