Deterioration Of Offshore Structure

Question 1

Categories of deterioration

Answer 1

Structural damage 
Corrosion or erosion 
Fouling defects
Coating defects
Scour
Metal and weld defects

Question 2

Gross structural damage

Answer 2

Deformation due to impact
Missing bolts
Missing members or anodes

Question 3

Corrosion

Uniform

Pitting

Answer 3

Uniform
Metal is removed all over the surface of the material reducing the member or pipelines thickness

Pitting. Localised which takes place in otherwise corrosion free material creating a put in the surface.

Question 4

Corrosion

Will be accelerated by

Answer 4

Erosion 
Increase in temperature 
Increase in oxygen content 
Chemical attack 
Loading known as stress corrosion

Question 5

Fouling defects

Both marine growth and debris

Debris is a type of foreign object or body onto near the structure.

Effects of debris

Answer 5

Structaral damage
Galvanic corrosion
Overloading of the co system
Safety hazard for divers

Question 6

Coating defects

Answer 6

Blistering of the coating
Flaming
Sagging or wrinkling.

Question 7

Scour to structure

Answer 7

When the seabed from around the structure moves and rubs against the structure with the movement of the water. This creates a scour.
This effects the welds. Metal and is serious when dealing with concrete structures.

Question 8

Fatigue

Local failure caused by cyclic loading. Cracks may form and grow from flaws in the material such as weld defects and accidental damage

Fatigue cracks may also start pite created by corrosion

Answer 8

Fatigue is difficult to predict this is due to a number of variables

Number of load cycles 
Stress and strain 
Environment 
Amplitude 
Mean stress level 
Material 
Temperature

Question 9

Embrittlement ( brittle failure)

This is when material changes it’s properties from ductile to brittle.
Brittle materials fail due to crack propagation so they as susceptible to fatigue and brittle failure.

Answer 9

Brittle failure is most commonly know around low temperatures

Question 10

Four stages of structures life span

Answer 10

Production of materials
Fabrications
Installation and in service
Decommissioning

Question 11

Raw material casting defects

Answer 11

Splashes
Inclusions
Incorrect cooling forms laminations

Question 12

Weld defects

Answer 12

Lack of root penetration
Slag inclusions
Porosity due to air contamination or dirt in the weld
Hydrogen induced cold cracking

Question 13

Hydrogen induced cold cracking

?

Answer 13

Cracking that is formed in the heat affected zone (HAZ)

This can occur right after the weld or a few hours later. NDT is carried out 48 hours after fabrication for this reason