Describe and elaborate upon the internal and external pressures that your equipment is exposed to.

Introduction

Brazil has significant reserves of oil in deep waters. The deep-water flow lines are getting high pressures and high temperatures (HP/HT). Floating production unit had been considered for pumping oil, since FPSO are particularly effective in deep-water locations where seabed pipelines are not cost effective and water depths present no limitation to FPSO. FPSOs can provide an economically attractive solution for smaller oil fields, which can be exhausted in a few years and once the field is depleted, the FPSO can be moved to a new location. In our hypothetical field the Electrical Submersible Pump (ESP) will be used since they are most efficient method in lifting large amounts of oil and since we are pumping heavy oils and presents low gas/oil ratio that’s favourable to use of ESPs. The ESP is highly efficient making distant fields economic to exploit. [5]

1. Describe and elaborate upon the basic forces in a vertical 21¨ drilling riser with.75 wall thickness for your field.

To be able to describe and elaborate upon the basic forces we have to look at the “Effective tension concept”, which was introduced by C.P. Sparks at “Fundamentals of Marine Riser Mechanics” book. C.P. Sparks has derived the relation between effective and true tension in a simple equation:

Where:

To be able to calculate effective tension, we need to consider top tension of the riser, weight of the riser with influence of internal and external pressures.

C.P. Sparks promotes an illustration for subject submerged in a fluid. Figure 1 shows us how external pressure behaves with a positive effective tension element equal to . [2]

To understand the model, the Archimedes Law must be applied.

“Archimedes’ Law states that when a body is wholly or partially immersed in a fluid, it experiences an upthrust equal to the weight of fluid displaced.” [1]

“There are some considerations must be made before to apply the Archimedes’ Law such as: the pressure field is have to be completely closed. Since the pressure at the surface is zero, the field is closed.” [1]

Figure 1: C.P. Sparks’s illustration of acting forces on a submerged body [3]

When the pressure and weight of submerged body are acting in a fluid:

Where:

Where:

The effective tension is then:

The tensile force acting on a body has to be added to the True Tension and the sum of we can see from the Fig. 1. [2]

Now we will take a look at another case where pipe is subjected to internal fluid pressure .

“The pressure field acting on the internal fluid column is closed and in equilibrium with the weight of the internal fluid. The lateral pressures acting on the pipe wall are equal and opposite of those acting on the internal fluid. Hence, by superposition and addition of the two force systems, those lateral pressures are eliminated. However, the axial “tension” in the fluid column remains. This leads to the equations for the effective tension and apparent weight of the equivalent system:” [1]

Equation for the effective tension :

Describe and elaborate upon the internal and external pressures that your equipment is exposed to.

We need to establish an effective force diagram for suspended, closed bottom end pipe which one is exposed to external and internal fluid and pressure.

Figure 2: Top suspended, closed bottom end pipe exposed to external and internal fluid and pressure [2]

Table 1: Refers to Figure 2: Presents five cases to show the difference between the true and effective tension. [2]

1. Dry pipe	2. Pipe with internal fluid	3. Pipe with external fluid	4. Pipe with internal pressure	5. Pipe with external presseure

We will take a close look on each case to understand the difference between the true and effective tension.

Case 1: In a case 1 we have no fluid, that leads that true and effective tension are the same. [2]

Case 2: In this case fluid is presented inside the pipe. Pipe is subjected to internal fluid pressure . Due that weight of the fluid are remains on a bottom of our pipe, the true tension line moves to the right. The effective tension line is established from the relation between effective and true tension equation, which was derived by C.P. Sparks. [2]

Case 3: In this case our pipe is submerged in a fluid. Compression force acting on a bottom of the pipe and that leads to the true tension line moves to the left. The effective tension line matches with the Archimedean line it is established from the relation between effective and true tension equation, which was derived by C.P. Sparks. [2]

Case 4: This case is identical to Case 2. The effective tension line is not affected and remains identical to the true weight line. [2]

Case 5: This case is identical to Case 3. The effective tension line is not affected and remains identical to the true weight line. [2]

Once we have reviewed all the cases, we can say that internal and external pressure alone doesn’t give any effective tension. The true and effective forces are strongly dependent that mean that one being needed to define the other. [2]

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