Past Paper Questions - Dec 2017

Question 1

Define the term “toughness” of a material. How is the toughness related to the stress strain curve of the material? Illustrate your answer with a suitable diagram.

Answer 1

• Toughness is defined as the energy to break a unit volume of material (not to be confused with the stress required to break a specimen)
• Related to the area under the stress-strain curve, ductile and brittle cross section.

Question 2

What is the Drop Weight Tear Test and how is it used to set toughness levels for pipeline materials?

Answer 2

DWTT is a material characterisation test aimed at avoiding brittle fracture and ensuring crack arrest in pipelines.

Defines a specification below which brittle fracture propagation cannot occur.

To control brittle fracture: specify the minimum DWTT shear area

Question 3

Describe the Charpy test and indicate how it is used in the specification of toughness for pipeline materials.

Answer 3

The Charpy test determines the amount of energy absorbed by a material during the fracture.

This absorbed energy is a measure of the materials notch toughness and acts as a tool to study temperature dependant ductile-brittle transition.

To control ductile fracture: specify minimum Charpy Upper Shelf Energy

Question 4

Define the terms formation water and condensed water indicating the source of each type of water.

Answer 4

Formation water is naturally present in the rock formation that is mobilised with the oil or gas.

Condensed water is in the gas phase, it condenses where steel surface temperature falls below dew point

Question 5

What is the effect of pH on the corrosion rate for sweet corrosion? Explain the differences in pH and therefore corrosion rate for condensed water and formation water.

Answer 5

In general the corrosion rate decreases as the pH increases. Sweet corrosion occurs where free water is present also known as C02 corrosion.

Formation water: pH influenced by CO2 concentration but also ionic species present in the rock.

Condensed Water: Relatively pure and therefore pH determined by partial pressure of C02. During corrosion becomes saturated with carbonate, pH increases and corrosion rate falls

Question 6

What is the effect of temperature on the corrosion rate for sweet corrosion? Illustrate your answer using a plot of corrosion rate against temperature.

Answer 6

Below 60°C at low levels of pH, solubility of FeCO3 is high and stable film formation is difficult.

Above 60-80°C, solubility of FeCO3 decreases and more adherent semi-protective scale is formed. Leads to a reduction in corrosion rates To provide protection, scale must be adherent and cover entire pipeline surface.

Graph of corrosion rate vs temperature: steady increase and decrease after 60-80 degrees.

Question 7

List five factors which the marine pipeline designer has to consider when planning a marine pipeline route.

Answer 7

• Seabed Terrain
• Shore Approach
• Other Users
• Construction Limitations
• Local environment and politics

Question 8

Explain the meanings of the symbols 𝑃𝑐

, 𝑃𝑒𝑐𝑟 and 𝑓0. in the buckling formula.

Answer 8

Pc = characteristic collapse pressure, the resistance to external pressure.

Pecr = elastic collapse pressure, pressure before elastic collapse of the pipe.

𝑓0 = Ovality, from a circular shape to ovality as external pressure increases until collapse.

Question 9

How do you calculate the pressure difference? State the other formulas you need for this calculation.

Answer 9

deltaP = (2row𝑓(V^2)L)/D

𝑓=m/4
Q=VA
Re = rowVD/mu

Question 10

How do you calculate the outlet temperature of a pipe?

Answer 10

Tout = Ts2 + (Tin - Ts2) exp (-hpiD/CprowQ)

Question 11

How do you find the average temperature of the inner surface?

Answer 11

Ts1 = (Tin - Tout)/2

Question 12

How would you find the thickness of the insulation knowing all the temperatures (Tin, Tout, Ts1 and Ts2)

Answer 12

ln(r2/r1) = (Ts1-Ts2) * 2piL*k / Q

Q given, Ts1 from Tin-Tout/2

Rearrange so r2 = exp (number) * r1

thickness of the insulation = r2-r1

Question 13

For multi-phase pipe flow, what is the Flow Regime Map ? With an increasing of gas flow rate, for a horizontal pipe, write down the main flow regimes and describe their features. What are the main parameters affecting the multi-phase flow regime map?

Answer 13

The flow regime map depends on a number of parameters:

• Pipe diameter
• Pipeline pressure
• Liquid velocity
• Gas velocity
• Water Cut
• Pipeline Inclination

For increasing gas flow rate in a horizontal pipe, the main flow regimes are:

• Bubbly - high gas flow rate
• Slug - frothy waves
• Annular - ringed flow
• Wavy - ripple flow
• Stratified - layered flow

Question 14

List the main factors that an engineer has to consider when designing a multiphase flow pipeline that are different from a single-phase pipeline.

Answer 14

• Different flow regime and flow pattern
• Pressure loss and heat loss estimation is different. (Friction pressure drop with two phases is much larger than single phase flow)
• Physical and thermodynamic properties are different.
• Hydrate and wax formation management different.