Computer Design Of Cylindrical Petroleum Storage Tanks Information Technology Essay

Since the equations involved in the design of petroleum are tedious to work with and the design of petroleum storage tanks are still a relatively cumbersome iterative processes that involves the use of various data charts and tables from various design standard codes, thus leading to time consuming and less accurate results. Therefore there is the need to develop a computer software application that will be able to design the petroleum storage tank making work less tedious and cumbersome and with very high accuracy. This project shows how the computer – aided design of a petroleum storage tanks can be achieved and the project sheds more light on the advantages of computer – aided design in engineering.

As one of the largest oil producers in the world, Nigeria in the past decades has invested a lot into the petroleum sector of the economy. Due to the growth of the petroleum industry, and thus the vast increase in demand of petroleum products worldwide (as a result of increasing world energy demand and oil being a major source of this energy), there has been the need for the production of very large quantities of petroleum and thus the long term storage of petroleum products to meet the high energy demand in Nigeria (basically crude oil which is exported out of the country and general refined products stored to be used in the country).

From the engineering point of view the problem of storage led to the design and construction of storage tanks (which are pressure vessels). These tanks should be able to hold fluids at high pressure and protect them from explosion, bursting or leakage.

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Many standard design codes for the design of petroleum storage tanks have being made by researchers, an example is the American Petroleum institute (API) 650. Design of a petroleum storage tank can be done by 1st principles of pressure vessel design or by the use of a standard design code to calculate the various loads, stresses and thicknesses where necessary. All these forms of design stated above are still carried out on pen and paper, which make them so cumbersome because engineers will have to spend long hours trying to calculate values of shear stresses, plate thicknesses, number of items, etc, from series of equations and after knowing these values, there is still a need for producing drawings of the structure. The question being asked is – what happens now that we are in the computer age? The only answer is “Computer Aided Design (CAD)”: where the computer replaces the pen and paper and design can be done at the click of a mouse or the touch of a button.

This paper is aimed at applying a computer software programme (written in C# language) in the design of cylindrical petroleum storage tanks according to API 650 design code. The software is called “Tank Design Software”.

2.0 Methodology

The steps/procedure for the CAD of the petroleum storage tank are grouped to three major stages. These are going to be employed in developing the computer software application package that has been proposed in the objectives of this project. The three major stages are:

Theoretical Design Process.

Software Design Process.

Software Publishing.

2.1 theoretical design process.

2.1.1 the design process.

Design has attracted many definitions in engineering and science as a whole. Khurmi et al (2005) defined design as the creation of new and better machines and improving the existing ones. Adejuyigbe (2002) stated “a good design is hard to define. Every product that is new, starts with an idea from the inventor”. He also stated that design is iterative in practice (having feedback loops) and that there is n o rigid procedure in design. The two figures below compares the procedures of design from two different sources.

The design of petroleum storage tanks is done to adhere to a standard design code. The API 650 design code will be adopted for this project due to its frequent use by Nigerian Engineers who design tanks and it is also accepted by the Department of Petroleum Resources (DPR). According To API 650, certain factors must be considered before beginning the design, they are summarized below:

TANK PARAMETERS

Tank height – H (metres)

Tank diameter – D (metres)

Volume – V (m3)

Base area – A (m2)

DESIGN FACTORS

Design ambient temperature 18oc – 40oc (Nigerian Climate)

Design specific gravity 1.0 (water is assumed)

Corrosion allowance 1.4mm – 2.0mm

Design wind velocity 160km/hr (average value in coasts)

PROTECTIVE MEASURE

Outside, the corrosion allowance of 1.4mm – 2.0mm will be provided on each shell course. This is to guide against atmospheric corrosion. The product to be kept could either be AGO, DPK, or PMS which is not corrosive to steel, hence, corrosion is not imminent on internal portions of the tank.

FOUNDATION DESIGN

The civil engineer will design the foundation of the tank base based on the overall dead and live loads of the tank gotten from the mechanical tank design. The dead load consist of the weight of the tank filled with products.

The theoretical design is divided into 3 parts:

2.1.2 design of bottom plates

Estimated load on bottom plates:

Weight of tank product F = ρ x V – – – – (i)

Pressure at tank base P = F/A – – – – (ii)

Where ρ = density of stored liquid (water is used in the design),

A and V are the tank area and volume respectively.

After the load per unit area has been determined, the value of the thickness is taken from tables on in the API 650 manual, where a pressure range has a corresponding base plate thickness.

2.1.3 design of the annular bottom plates

These plates carry the shell plates directly

Design annular thickness:

From table 3.1 (section 3.6,12) of API 650, the recommended thickness of the annular plates for a base plate 10mm thick is 18mm depending on the value of thickness of the shell plate.

Radial width of the annular plates:

Applying the formula (according to API 650)

– – – – (i)

Where tb = thickness of the annular plates in mm,

H = maximum design liquid level in m.

G = Design specific gravity of the liquid to be stored.

2.1.4 shell design

ALLOWABLE STRESS

The design stress basis, Sd, shall be either two thirds the yield strength or two – fifths the tensile strength, whichever is less. There are different grades of steel and for those grades of steel, the values of Sd will vary. For the purpose of the software design, I am going to adopt designing with the three available steel grades in the Nigerian market, which are accepted by the API 650 manual. They are ASTM A36, A573, and A131

According to API 650, there are two methods of calculating the shell thickness of the storage tanks, they are:

The one – foot method and,

The variable point method.

The one – foot method will be adopted in this project, because of its simplicity and it will be easy to modularise it while programming.

CALCULATION OF SHELL THICKNESS BY THE ONE – FOOT METHOD.

This design method calculates the thickness required at design points 0.3m above the bottom of each shell course.

From API 650 section 3.6.3.2 – the required minimum thickness of shell plates shall be the greater of the values computed by the following formulae:

In SI unit

Td = – – – (i)

Tt = – – – (ii)

Where Td­ = Design shell thickness, in mm,

Tt = Hydrostatic test shell thickness,

D = Nominal tank diameter, in m,

H = Maximum design liquid level, in m,

G = Design specific gravity of the liquid to be stored in the tank,

CA = Corrosion allowance,

Sd = Allowance stress for design condition, Mpa, and

St = Allowance stress for the hydrostatic test condition, Mpa.

After a successful theoretical design and thus gathering all the equations, assumptions and conditions that will be needed to develop the computer software, the next stage of the design process is carried out

2.2 software design process

Adejuyigbe (2002) divided the software design process into four parts which will be adopted in this project, they are:

Analysis;

Algorithm design;

Coding; and

Testing.

They are considered broadly below:

Analysis

According to Adejuyigbe (2002) “A proper identification of objective is only possible through analysis”. An analysis will; clearly examine the specification of the software and select the appropriate data structure. Adejuyigbe, S. B. (2002) also stated that “Analysis is the key to a well designed software; A good analysis minimizes coding difficulties”.

The software application must be able to achieve the following:

Calculate the thickness of the tank shells, base plates and annular plates.

Determine the number of Shell plates, base plates and annular plates needed.

Determine the cost of all the plates needed to fabricate the tank.

The software package must also have the following characteristics:

It should be user friendly and easily accessible (a graphic user interface “GUI” is preferred).

The software will work based on the input of certain parameters (for this project the input parameters are the tank height and volume).

The software should be able to accurately manipulate the input parameters through the written codes (which are the design equations) to determine various results (which is are the values of sizes, numbers and cost of items).

Since all the tank parameters and even the results to be determined at the end of the design are all numbers, the data type will be real numbers and in some programming languages they are referred to as “DOUBLE”. The programming language to be used for this project will be C# with a “Visual Studios 2008” IDE, this is because C# language is very good for handling mathematical expressions and can be used to draw. The testing of the program will be done by running the software and comparing its results with that of a practical design which has been done successfully in the past.

The chart below shows the pictorial representation of the analysis stage of the software design.

Fig 2.1 A typical result of the analysis stage in a software design process (Source Adejuyigbe 2002).

Algorithm Design

An Algorithm is a logical sequence of steps for solving a problem, often written out as a flow chart, that can be translated into a computer program.

Adejuyigbe (2002b) defined Algorithm Design as “an unambiguous set of instructive or executable actions or steps that must be taken in order to solve a particular problem”.

A simplified form of the algorithm for the software is shown below.

START

Input Tank Diameter, Height etc

STOP

STOP

Module 2

No

yes

Use advance design window?

STOP

Module 1

No

yes

Use simple design window?

Fig 3.2 Simplified Flow Chart for the Software Design.

The terms Module 1 and Module 2 will be discussed under the coding aspect of the software design.

Coding

This is the program writing phase of the software design. In any computer – aided design where a software application is to be developed, the coding stage of design is the major and delicate aspect since an accurate and efficient coding system will give rise to the software working properly. If the designer knows all the theoretical design equations and has done a perfect design analysis with a detailed flow chart, and the designer fails to code these variables properly, the program will fail to execute.

While making use of the IDE to compile the program, the designer can check for errors by debugging each stage of the code. Debugging will help to avoid too many un traceable errors.

The C# “pronounced see sharp” language will be used for this software design. To design a software that will be used by another user on a computer, a Windows Form Application has to be created. Windows form implies that the user will manipulate and operate the software with his keyboard and mouse click; unlike a Console Application which runs the software on the background of the computer.

The following steps are to be taken when creating a Windows Form Application using a Visual Studios IDE:

Open the IDE,

Click on file menu and then new project,

Click on windows form application and name the application to suit your taste,

After these you can start designing the software and writing the programs for each stage.

The various Modules for the program are shown in the appendix to this report and are all written with the syntax of Microsoft Visual Studios IDE 2008 on C# language platform.

Testing

Testing is used to determine if the software is working properly. A software used to perform some mathematical operation might give results as numbers etc, but these values of numbers might be wrong if there has being a logical error while compiling the software, i.e. putting a negative sign where a positive sign is meant to be. So a software is said to be working properly when it gives the accurate result not just any output. In testing the tank design software, a practical design case will be executed by the software and the result will be compared with that of the practical design since that of the practical design has already being accepted as being correct.

The testing of this software will be carried out fully in chapter four “results and discussion”.

2.3 software publishing

Software publishing does not imply publishing literarily. While coding, the software will only run on the IDE or any computer with a similar IDE, but this software is not designed for programmers alone, because only programmers have IDEs on their computers; Therefore the software must be published. Publishing is the process where the software is converted from a program code to an executable file extension “.exe”, all applications and software we use on our computers always have an executable file, this file helps the normal computer interface to run the software instead of an IDE. You may have noticed some program file on your system ending with .exe e.g. AutoCAD.exe. The publishing stage is the penultimate stage of the Computer – Aided Design of petroleum storage tanks and it will be done at the end of all the coding, testing etc.

3.0 Results and Discussions

3.1 output of the software

After a successful publishing of the software, which successes the mathematical modelling( theoretical design process) and programming (software design process), the output of the software application can then be analysed. The analysis of these outputs can only be appreciated when the user runs/executes the program, this is because the program was designed to have a graphic user interface (GUI) which means that the true beauty and details of the output can be best appreciated while making use of the software which appears on the computer screen in form of a user friendly graphics environment. The graphics environment consists of all the menu, tools, buttons, windows etc. Which helps a user to access the software with ease and which makes the software user friendly.

The major Windows in the Tank Design Software are highlighted as follows:

Welcome Screen: This is shown in plate 1, it shows the title of the project and has a button “Click To Begin”. When this button is clicked the software begins to run fully.

Simple Design Window: This is shown in plate 2, it opens after clicking the simple design menu of the MDI. The user can only choose between two grades of steel to design for and design is done by entering the values of the diameter and height and clicking the design button.

Advance Design Window: The difference between the advance and simple design window if that in the former, the user has a choice to impose to design based on the plate dimension available in the market, the corrosion allowance (depending on the location of the tank), and the grade of steel. Plate 3 shows the various design factors which the user can choose from. This form of design is more flexible since the user can vary the plate dimensions or corrosion allowance for different grades of steel to know that with an appropriate thickness for the job at hand. The user can choose from either a 10m X 2.5m, 10m X 1.8m or 10m X 1.5m steel plate to design for and also choose a corrosion allowance of either 1.4mm, 1.6mm, 1.8mm or 2.0mm for 3 different grades of steel. Finally the Advance design interface is capable of designing the tank based on the product stored, other than using only water to design.

Cost Analysis Window: This is the window that the user uses to estimate the number of plates and their total cost based on the specified dimensions of the tank and a single plate sample. The unit price of each type of plate should be known in order to know the total price of either the tank shell, annular or base plates.

PLATE 1 welcome screen PLATE 2 simple design window

PLATE 3 advance design window PLATE 4 cost analysis window

3.2 testing

The software will be tested with a practical tank design to check for its validity and accuracy. The practical design used is the design of two identical tanks of 35m diameter and 20m in height.

TANK PARAMETERS

Tank height – 20m

Tank diameter – 35m

Volume – 19,250m3

Base area – 962.5m2

Circumference – 110m

Corrosion allowance – 2.0mm

The result from the practical design (after applying equations i and ii) is shown in the table below,

SHELL COURSE

THICKNESS (mm)

1st

23.12

2nd

20.44

3rd

17.76

4th

15.10

5th

12.40

6th

9.72

7th

7.04

8th

4.36

Table 3.0 Design shell thickness (Source K.O.K 2008)

The result obtained when Tank Design Software is used is show on the plate below

PLATE 5 shell thickness results window

4.3 ADVANTAGES OF TANK DESIGN SOFTWARE OVER MANUAL DESIGN

4.3.1 DESIGN CONSTRAINTS

The various design constraints stipulated by the API has been introduced to the software and if your design deviates from them, the software alerts you with a warning suggesting an alternative step to take, unlike the manual method where the designer might forget to use such constraints which are very important to avoid failure of the tank. A typical example is the Diameter to Height ratio, if this ratio is greater than 1 i.e. D>H then the tank will not fail by buckling but if DPLATE 6

4.3.2 QUANTITY AND COSTING OF MATERIALS

The Tank Design Software is capable of calculating the number of plates that will be needed for the construction of the tank based on a specified dimension of the plate. The 3 dimensions suitable in petroleum storage tank construction are 10m X 2.5m, 10m X 1.8m and 10m X 1.5m. these plates can be rolled to tank shells, cut to the shape of annular plates and laid directly as tank base plates. After the number of quantities are known, the software automatically estimates the cost based on the unit price cost of materials. The Estimation Tools Window is used for this process, when the window is opened, the following steps are taken to estimate the cost and quantity.

Select the dimension of Steel to be used.

Enter the values of the tank diameter and height and the unit prices of the shell plates, annular plates and base plates.

Click the “Estimate” button and the results will appear on the screen.

The results are shown below for a hypothetical design case and unit price.

PLATE 7

PLATE 8

4.3.3 DRAWING THE TANK DEVELOPMENT AUTOMATICALLY WITH AUTOCAD.

The software is capable of drafting the tank shell development on AutoCAD automatically by just a click of a button. This is done immediately after calculating the shell thickness from the software by clicking o the “AutoCAD” menu. A program module shown in the appendix called “AutoCAD event module” was written for this process. The development of the tank designed in section 4.2.1 is shown in plate 12 below. N.B. any user of the Tank Design Software is expected to have a recent AutoCAD software version pre – installed on their computer i.e. AutoCAD 2009 and above.

PLATE 9

PLATE 10

4.3.4 3D MODEL VIEW OF THE CYLINDER

The software is capable of producing a 3D view of the cylinder which is just a sketch. The procedure and output for the design in 4.2.1 is shown in plate 13 and 14 respectively.

PLATE 11

PLATE 12

4.0 conclusion

The necessity of petroleum and the importance of tank design for storage has been stressed in the beginning chapters, but the simplification and improvement of this design process has been shown by the application of Computer – Aided Design to the design stage of petroleum storage tank manufacture. The CAD of petroleum storage tank has not only shown tremendous advantage based on the speed and accuracy in with the design is done but also its flexibility and multi – functionality and its user friendliness.