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By Carlos Molina

Tank settlement is one of the topics of the Body Of Knowledge for the API 653 exam. It is a very important subject for us tank inspectors, althought is also one of the most vague topics for a new inspector. In fact, the word “settlement” is mentioned more than 250 times in the API 653 standard. As an inspector, you should be able to determine the type  and extent of tank settlement, and decide if it can affect tank integrity. In the api 653 exam, maybe 2 questions will show up about this subject. And as complex as it may look in the api 653 standard, the limitations imposed by the BOK make it a really easy topic.


In new tanks, the API 650 standard doesn´t necessarily asks for a settlement measurement to be done during hydrostatic tests. If there is no settlement expected (for example,  a tank over a giant rock), it might not need settlement measurements, but that´s a decision that is entirely up to the owner.  In normal conditions, there will always be settlement. Anyway, you should design and construct foundations to limit any settlement at all, as impossible it is to eliminate it.

For the sake of information, you should know how setttlement measurements are made. During hydrostatic testing for new and old tanks, at least 6 sets of measurements shall be made.

1. When the tank is empty before hydrostatic testing

2. When the tank is 1/4 full

3. When the tank is 1/2 full

4. When the tank is 3/4 full

4. 24 hours after it is filled

4. With the tank empty again.

Shell elevation measurements shall be made at equally-spaced intervals around the tank circumference not exceeding 10 m (32 ft)


During operations, shell settlement measurements should be taken at a planned frequency, based on an assessment of soil settlement predictions. Bottom settlement monitoring is to be made during internal inspections, respecting the intervals given for inspections in API 653 4.4.6. Identify and evaluate any tank bottom settlement is one of the 3 key objectves of internal inspections, because it plays such an important role on many tank failures and floating-roof problems.

Settlement can be caused by the following:

  1. Lack of support under the base circumference affecting the cylindrical shell and the tank bottom. Parts or the concrete ring may be lost.
  2. Non homogeneous geometry or compressibility of the soil deposit (voids or crevices below the bottom plate)
  3. Non uniform distribution of the load applied to the foundation. Differential pressure during emptying and filling cycles
  4. Uniform stress acting over a limited area of the soil stratum
  5. Wrongly constructed foundations (deficient reinforcement of the concrete, bad quality cement, etc)
  6. Liquefaction phenomenon around the foundation generated by earthquakes. Consider the following excerpt:

The Niigata earthquake was also the first seismic disaster in Japan where the liquefaction of the ground attracted notice. Among the disaster incidents caused by the earthquake, five crude oil storage tanks in a refinery caught fire and continued burning for two weeks, spreading into the surrounding area and burning down a total of 286 adjacent houses. One of them was a 30,000kL floating roof type tank, 51,500mm in diameter, and 14,555mm in height, which was fully stocked with oil. The cause of the fire was ignition by sparks generated by the collision of the floating roof with the side wall, which in turn was caused by the movement of the crude oil by the sloshing phenomenon. Source.


Various forms of settlements could take place in tanks. The BOK considers 3 types of settlement and their evaluation.

1. Edge settlement

Edge settlement occurs when the tank shell settles sharply around the periphery, resulting in deformation of the bottom plate near the shell-to-bottom corner junction,  or the depth of the depressed area of the bottom plate. You can see a diagram for edge settlement below.


Edge settlement affects bottom parallel and perpendicular welds in different manners. It affects weld seams that are “parallel” to the shell in a more critical manner that the ones that run “perpendicular”.

How to evaluate edge settlement?

STEP 1. Annex B of API 653 separates Edge Settlement evaluations in two separate scenarios:

1. If edge settlement is in an area with a welded seam than runs parallel +-20°  to the shell, B turns into Bew

2. If edge settlement is in an area with a welded seam than runs perpendicular +-20°  to the shell, B turns into Be


STEP 2. With the value of R, B and the tank diameter, you can check the maximum allowable vertical settlement in figures B-11 of API 653 for Bew or B-12 for Be. A sample of that diagram you can see next.


See API 653 B-11 and B-12 for the whole details.

Welds in tanks with settlement greater than or equal 75 % of Bew or Be, and larger than 2 in., are to be inspected with magnetic particle of liquid penetrant method. Additionally, weld seams should be inspected vissually and if they show strains bigger than 2%, they should be repaired.  Any plate exceeding acceptable plastic strains (typically 2 % to 3 %) should be replaced.

2. Bottom settlement near the tank shell

This kind of settlement can be 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. That deformation is caused by stresses in the bottom plate that have to be evaluated.


How to evaluate bulges in tank bottoms?

STEP 1. As per API 653 B3.3, measure the bulge or depression in its entire lenght. The half of that measure is radius R of the bulge.

STEP 2. The maximum dimension for bulges or depressions is given by the following equation:



B is the maximum height of bulge or depth of local depression, in inches;

R is the radius of an inscribed circle in the bulged area or local depression, in feet.

3. Localized bottom settlement remote from the tank shell.

Localized bottom settlement remote from tank shell are depressions (or bulges) that occur in a random manner, remote from the shell. The same equation (B3.3) used for bottom settlement near the tank shell can be used for the evaluation of this kind of settlement, granted the bottom has single-pass welded joints.


  1. Leave plenty of free space under any nozzle, to prevent any contact with the floor if there is settlement.
  2. Settlement occurs to every tank, and it can be different in practice from the measured settlement during and after hydrostatic testing.
  3. If there is uniform settled expected (If foundations werent well built), you can use flexible joints or maritime hoses that can absorb those misalignments.
  4. Edge settlement often can be predicted in advance, with sufficient accuracy from soil tests. Anyway, piping (especially buried piping) should be designed with adequate consideration to prevent problems caused by such settlement


See that it is pretty easy? I think this is a good explanation of what you will have to learn in order to answer correctly questions regarding tank settlement in the API 653 examination. If you liked this article that will help you in your inspections, then subscribe to my mail list, and you will receive a weekly article about equipment integrity and how to pass your exams.

Cheers and good luck


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By Carlos Molina

Hi friends. Today´s article is really short, but is to let you know of the new version of the API 653 exam Simulator. This 2.0 Version has 100 questions total, an improvement of 52 from the previous version. The new simulator has some bug fixes from the former one, given that it did convert percentages to decimal points, an issue we discovered late.


This version still has to improve, and I promise that I will make my best to update it within a week to the subscribers of my blog. I have to improve the result screen and allow the user to go back to solve the test. Remember that I have a dayjob and little time, jeje.

What is the objective of this simulator? To get you accustomed to the test environment, mimicking the conditions of the actual exam. As far as I know, this simulator is one of its kind.

I took special consideration to the fact that some 70% of questions in the actual exam are from the API 653 standard, so the most of the questions in this simulator were taken from API 653. By the other side, there are new questions that aren´t in my questions series.

To receive your copy, write your mail to me and I well send it to you in the span of 1-2 business days. If you write your mail address in the form below, you will not receive any more mails regarding any other subject. I won´t collect any personal data.

Be aware that this form will work like a subscribing list, so you will have to confirm by entering to your mail account and answering an automatically generated mail.

If you would like to receive weekly updates of the articles I write here in APIEXAM, you can type your mail address next.  I promise to never sell or giveaway your email address to anyone – and you are allowed to unsubscribe at anytime

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There are 5 days lacking to begin with the testing window for the API 653 exam. Study until the last minute. When you sit in front of that computer, you will be given 13 minutes, some paper and some pencils before your exam to take a “learn how to use a computer” tutorial (The same is summarized in the simulator). Obviously, you know how to do this already, so take this time to do a “braindump” and write anything you’d like as fast as you can that you think will help you during the test. I want you to pass in the first try.


Thanks and good study.


By Carlos Molina

According to API 653 3.14, Hot tapping identifies a procedure for installing a nozzle in the shell of a tank that is in service. This means that a tank can continue to be in operation whilst maintenance or modifications are being done to it. This is in complete compliance with the API 653 standard, but following some rules. First of all, for all of you students, let´s review what the BOK of the API 653 March 15 exam has to say.

a) The Inspector should be familiar with the Hot Tapping requirements. (API-653, Paragraph 9.14)
b) The inspector should be able to calculate the minimum spacing between an existing nozzle and a new hot tap nozzle. (API-653 Paragraph 9.14.3)

Hot tapping is more common in pipelines, althought the principles are the same that for tanks. In a normal pipe hot tapping operation, you wish 2 or 3 things.

1. You want flow in the pipe so you can cool the welded zone, given that the liquid works as a heat sink.
2. You want no gases or vapors in the pipe
3. You want to weld nozzles and reinforcements to the pipe without penetrating too much in the base metal, because of pressure.

With tanks it is the same, with the diferrence that flow conditions in tanks are close to stagnant.


The following diagram summarizes the requirements for hot tapping found in API 653 9.14


Requirements for hot-taps in tanks

1. Hot taps are not permitted on shell material requiring thermal stress relief
2. Welding shall be done with low hydrogen electrodes.
3. Hot taps are not permitted on the roof of a tank or within the gas/vapor space of the tank.
4. Hot taps shall not be installed on laminated or severely pitted shell plate. As an inspector, you have to make sure that thickness measures are taken in the proposed area for a hot tap.
5. Hot taps are not permitted on tanks where the heat of welding may cause environmental cracking (such as
caustic cracking or stress corrosion cracking).
6. Minimum spacing in any direction (toe-to-toe of welds) between the hot tap and adjacent nozzles shall be equivalent to the square root of RT where R is the tank shell radius, in inches, and T is the shell plate thickness, in inches.
7. Minimum distance between the toe of the hot tap weld and a vertical seam should be 12in
According to API RP 2201 (Remember this number very well as could be a question of the exam), the hazards for a hot tapping operation in tanks are the following:

a. Tank venting, with vapors reaching the exterior area where welding is taking place.
b. Product within the tank rising and overflowing.
c. Inadvertently allowing the liquid level within the tank to fall below the point of welding, exposing the vapor space within the tank to an ignition source.

Welding on the exterior of tanks in service shall not be conducted unless controls are established and in place to prevent flammable vapors from reaching the area of welding. Work must be stopped immediately should flammable vapors be detected in the welding area.

When hot tapping or welding on a rank in service maintain liquid in the tank at a level at least 3 feet (1 meter) above the area where the work is being performed. No attempt should be made to hot tap or weld above this liquid level in atmospheric pressure petroleum storage tanks because of the potential danger of an explosive atmosphere inside the tank vapor space. Measurements of the tank level should be made by a hand tape gauge to verify the accuracy of automatic or remote reading gauges.


Welding should never be allowed on the decks of floating roof tanks, as they are subject to flammability hazards in several locations:
a. Inside the pontoons.
b. Between the deck and liquid surface near the tank roof gauge float compartment
c. Near the roof seal vent.
d. Near the floating roof lift leg vent.
e. Between the primary and secondary seal.
f. Near the roof drain.

Thanks for reading this article and consider the following warning



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By Carlos Molina

Hi friends. I am longing to make it in the integrity field. I mean, I want to make it in the tanks, in the pipes and in the pressure vessels and static equipment.

Tank level seen with Infrared camera

Yo can see tank levels at once with infrared cameras

Here is a gift for you

Today I am writing you about the API 580 certification, the same I talked about in this article

On february 2, I received this mail from the guys of prometric that contained a pdf file. I opened up the PDF file and read that I  achieved a passing grade and i am now certified to conduct rbi inspections. I dont have the experience yet though. Here is a screenshot of the document.


But as for the exam, I actually just studied some 3 days total. I really didn’t have much time in the previous months to study, and just one month before the exam, the API 580 course I was due was cancelled. I thought I had no chance. In the last week, I read the RP on the bus while in my conmute, I read the RP while I was having my lunch, and I reviewed some questions and answers I had in an spaced repetition software (in a very disorderly way).
I almost flunked. I got 50 questions right, just to a little over the 71% needed. What helped me was that I can understand english well and generally have been good in multiple choice exams troughout my life.

During the exam, all you had to do is make sure that you get the 49 questions you need in order to pass. Be sure of 49, and you can do anything to the others, from guessing to discarding to whatever. And if you passed, congratulations on this significant professional acomplishment.

And continuing with my goal to completely prepare you for these exams, here are more questions for the API 653 Certification examination.


I am bringing you today 25 questions about Microbiologically induced corrosion (MIC). You can download them Here. Microbial corrosion, also called bacterial corrosion, bio-corrosion, microbiologically influenced corrosion, or microbially induced corrosion (MIC), is corrosion caused or promoted by microorganisms, usually chemoautotrophs. It can apply to both metals and non-metallic materials (Wikipedia)

For the exam, I can´t tell you how many questions about this subject will show up. Maybe none. Anyway, I prepared these questions with the intention to make you really knowledgeable to the subject.

More pages on the API 653 questions series

1. Beginning the path: API 653 Questions

2. General and definitions. #2 in the API 653 questions series.

3. In the core of API 653. Path # 3.

4. Damage mechanisms for API 653 inspection. Path #4

5. Corrosion Under Insulation 

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Thanks and good study.


By Carlos Molina

Following with the subjects that you should study for the exam, today we will have a short explanation of the reason behind some of weld spacings in tank elements. Boy, I have to tell you, thinking for this article was difficult, because it involves many figures and tables that are present in the API 650 and API 653 standards. And writing it was more difficult, because of the little time I have (As most of you in the Oil and Gas Field).

First let me ask you a question: In which plane are tank shell stresses higher, in a vertical or an horizontal plane?

[click to continue…]


By Carlos Molina

Today Apiexam is receiving  requests for a simulator of the API 653 exam. It was designed by me and built by Bareta. It features 48 API 653 questions in a multiple choice question format.

It runs on Windows Vista, 2008, 7, 8 and 2012. It should be preferablly used in operative systems of 64bits, although it can work in 32bits environments.

This software mimics the experience you will have in the real exam, which I think is the most important use of the software at this stage in your study: getting familiar with the computer based testing environment. The software will show you a simplified tutorial screen in the likes of the actual tutorial (the real one has several pages, while this has only one). You can mark the questions where you have any doubt for later review. if you notice you have problems with any specific question, go open your book and answer it, and put that question into your flashcard software.

It has only 50 questions in multiple choice format, because it is a beta version. It will have an upgrade the fastest possible to the most possible quantity of questions. Remember I have by now created 240 questions from the subjects in the BOK. To receive your copy, write your mail to me and I well send it to you in the span of 1-2 business days. If you write your mail in the form below, you will not receive any more mails regarding any other subject. I won´t collect any personal data.

Be aware that this form will work like a subscribing list, so you will have to confirm by entering to your mail account. (I haven´t mastered in internet skills yet) But it will only be used once for the software.

These are 2 screen captures of the software.

API 653 exam simulator

API 653 exam simulator


API 653 exam simulator

API 653 exam simulator

In the actual exam, API is the sponsor, which has invested other enterprise with the task of administering the exam. This other enterprise will have then facilities and expert personnel to watch over the test. Be ready to be frisked, at least visually, as you are not permitted to ingress foods, pens or electronic equipment (only eyeglasses are allowed). The test lasts 4 hours, which is much more of the time you actually need if you study enough. They will let you go to the bathroom or let you go retrieve your cellphone, but they may be overseeing your call.

Lastly, I would like you to consider an option. I am for the first time using a mail list subscription form. If you would like to receive weekly updates of the articles I write here in APIEXAM, you can type your mail next.  I promise to never sell or giveaway your email address to anyone – and you are allowed to unsubscribe at anytime

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Thanks and good study.


By Carlos Molina

Hi friends.

I keep studying about API 653. I have to be a re-certified API 653 Inspector before september 2015.  I obtained my certification back in september 2012, with only one year experience in tank repair and, as I did with my english language skills, most of it was self-taught (I took a course, though). Now I have to regain all the knowledge again, which is really easy if you know how to learn it.

I hope you are studying well for your own exam, taking advantage of what I or others offer, and if you are not, remember it is scheduled for March 15, 2015. So you better  wash out of the holidays inertia and start studying right now.

Today we are going to talk about hydrostatic testing in API 653, more specifically about hydrostatic height calculations. Some of the questions of the exam are from this subject.


Hydrostatic testing is (or a least should be) done in every new welded tank for oil storage and it is mandatory for any tank that has been under a major alteration, according to API 653.

And as we are students,  let´s remember the definition of hydrotest of API 653.

Definition of hydrotest per API 653



As it says, for new or repaired tanks, the purpose of hydrostatic testing is to demonstrate the tank´s fitness for service, and the better you test, the less risk you have once the tank is in operation.

If you are planning to take the exam, you should be familiar with all of the requirements of API 650 and 653 regarding hydrotesting. But right now we will concentrate in calculating hydrostatic test height.


For a tank that has been in operation, several things can happen that may highlight the need for an hydrostatic test

*If the tank is going to be used for a new, more sever service. That means, when the liquid that will be stored has a higher specific gravity than the current stored  product.

*When there have been repairs. Maybe your repairs are perfect, but some corrosion remains in other areas of the tank.

Determination of hydrostatic test height Ht, when you have calculated a controlling thickness for an entire shell course, can be achieved solving for the following equation, where Ht is the height from the bottom of the shell course under consideration to the hydrostatic test height

Hydrotest height over an entire shell course

Over entire shell course

Determination of hydrostatic test height Ht, when you have calculated controlling thickness for a locally thinned area (I haven´t treated the subject of controlling thicknesses here so far, my bad) can be achieved solving for the following equation, where Ht is the height from the bottom of the length, L, (see for the most severely thinned area in each shell course to the hydrostatic test height in feet

Hydrotest height over locally thinned area

Over locally thinned area

So, hydrostatic test height depends on 4 variables, which are.

St is the smaller value between fractions of yield strenght or tensile strenght, or the maximum allowable hydrostatic test stress.

Stress values for hydrostatic height calculus

Stress values for hydrostatic height calculus

E is the joint efficiency. Look how to find joint efficiency here

Tmin is the controlling thickness

D is the diameter of the tank.

Let´s see an example of height calculation. I am assuming you have some background on the formulas  of API 650 and API 653.


A tank will be subjected to hydrostatic testing after repairs. After some study, the inspector decides to run calculations for hydrostatic test height over the first shell course and over a locally thinned area close to a vertical seam, 36 inches high in the 4th shell course. Steel is A36 with Y = 36000psi and T=58000psi. Shell courses are 6ft high and the tank is 48ft diameter. First two courses were welded before 1980, and the other courses were added recently. The controlling thickness for the first shell course is 7mm and for the locally thinned area is 6,35mm. What should be the hydrostatic test height?


Well, for the equations, values are these

Case 1. Lower shell course

E=0,85. See Table 4-2

St = smaller of 0,88Y or 0,472Y, then St = 27376psi

D= 48

Tmin= 7mm = 0,275in

Hydrotest height for entire shell course

So, hydrostatic test height will be 15,93m.

Case 2. Locally thinned area in 4th course

=1. See Table 4-2.

St = smaller of 0,9Y or 0,519Y, then St = 30102psi

D= 48

Tmin = 6,35mm=0,25in

Hydrotest height for locally thinned area

Hydrostatic test height over the locally thinned area can be 16,7m. (A total height of 23,01m)

Then the tank has a maximum fill height  is 15,93m.


You have to be aware that variables in these and other equations of the standards can have different values depending on year of fabrication, purpose of calculation (design or hydrostatic loads), etc. In the exam, you have to be very careful to avoid mistakes, as they always trhow some confusing questions.

You can notice that the presence of different defects in tank shells implies than more than one value of Tmin or Ht should be calculated in a real tank inspection. This kind of dual example analysis is not common in other literature.

And that´s it for now.

As always, I am open to all kind of comments about the information in this site. And lastly, take a look at the following Ad.

ad for the simulator


By Carlos Molina

To download 38 more questions for your own study, click here


Corrosion Under Insulation (CUI) is the corrosion of piping, pressure vessels and structural components resulting from water trapped under insulation or fireproofing.

Probability of failure in Corrosion Under Insulation is drove at least by 10 critical factors, making CUI one of the most common types of damage mechanisms. It attacks more agressivelly pipes of 4″ diameter or less. That is why in some cases, if possible, designers will choose stainless steel over carbon steel when there is probability of CUI in piping. Besides, the older the piping or equipment and insulation system, CUI is more likely to appear.

Corrosion under insulation

Some locations where CUI is more likely to occur

CUI is a potentially harmful form of corrosion, difficult to predict with certainty and difficult to find without 100% insulation removal. So the best idea is to have a good insulation from the start. As a designer, you should ake sure your tank and piping has the best insulation design, given that maintenance of insulation is also costly. A good summary of strategies to prevent Corrosion Under Insulation can be found here

As a tank inspector, you should know that the defense against CUI is using appropriate coatings under well maintained insulation barriers. It can be present at the lower portions of tank shells, and in protrusions of the isolation barrier, as many other locations. As for the API 653 exam, I don´t remember any specific questions about CUI in my exam back then in 2012, but you should study it, because it is in the BOK.

Now you have downloaded a new pdf with 38 more questions taken from parts described in the BOK for the March 2015 API 653 exam. With this, we reached 240 questions in the API 653 questions series.


More pages on the API 653 questions series

1. Beginning the path: API 653 Questions

2. General and definitions. #2 in the API 653 questions series.

3. In the core of API 653. Path # 3.

4. Damage mechanisms for API 653 inspection. Path #4


By Carlos Molina

Hi friends. I hope you are all doing well.

Today we are going to review 3 relevant damage mechanisms you should know in order to pass the API 653 certification exam. All of them appear in the BOK for the March 2015 exam. Download this pdf and you´ll find all of the 52 questions, based in API 571 (2 more questions than other pdfs, because I was so into it)

These are Brittle Fracture, Mechanical Fatigue and Atmospheric Corrosion. [click to continue…]


By Carlos Molina

Hi folks. I hope you are doing well studying for your API 653 certification examination on March 20, 2015.

Here at Apiexam.com, we continue with the explanation of concepts neccessary for passing the API 653 exam. One of the topics you should study in order to pass is calculation of joint efficiencies for shell joints. [click to continue…]