DPO Materials Selection 3 — Questions and Answers
Question 1: In DP vessel thruster design, why is nickel-aluminum bronze (NAB) preferred over carbon steel for propeller blades?
- It is cheaper than carbon steel
- It offers superior corrosion resistance and antifouling properties in seawater (Correct answer)
- It has higher density for better thrust
- It is easier to weld than carbon steel
Correct answer: It offers superior corrosion resistance and antifouling properties in seawater
NAB resists seawater corrosion and de-alloying, has natural biofouling resistance, and maintains strength at marine operating temperatures, making it ideal for propeller applications.
Question 2: When a DP class 3 vessel requires redundant hydraulic systems, which seal material is selected for hydraulic cylinders to handle both mineral oil and fire-resistant fluid compatibility?
- Nitrile (NBR) seals only
- EPDM rubber seals
- Fluorocarbon (Viton/FKM) seals (Correct answer)
- Silicone seals
Correct answer: Fluorocarbon (Viton/FKM) seals
FKM (Viton) seals are compatible with both mineral-based hydraulic oils and fire-resistant fluids (HF-DR, HF-DU types), making them the standard choice for DP hydraulic systems.
Question 3: Which standard governs the material requirements for structural steel used in the hull of DP offshore vessels built to Class notation?
- ASTM A36 only
- Classification society rules (e.g., DNV, ABS, Lloyd's) referencing IACS UR W11 (Correct answer)
- ISO 9001 quality management
- NACE MR0175
Correct answer: Classification society rules (e.g., DNV, ABS, Lloyd's) referencing IACS UR W11
IACS UR W11 and individual classification society rules specify notch toughness grades, chemical composition, and testing requirements for hull structural steel.
Question 4: For DP reference sensor housings mounted below the waterline, which composite material is preferred to avoid interference with acoustic positioning systems?
- Carbon fiber reinforced polymer (CFRP)
- Glass fiber reinforced polymer (GFRP) (Correct answer)
- Stainless steel 316L
- Titanium alloy
Correct answer: Glass fiber reinforced polymer (GFRP)
GFRP is acoustically transparent and non-conductive, preventing signal attenuation or reflection that would impair USBL/LBL acoustic positioning sensor performance.
Question 5: What is the primary reason copper-nickel (Cu-Ni 90/10) alloy is selected for DP vessel seawater piping over austenitic stainless steel?
- Lower cost per kilogram
- Resistance to microbiologically influenced corrosion (MIC) and biofouling in seawater (Correct answer)
- Higher pressure rating
- Better weldability with carbon steel
Correct answer: Resistance to microbiologically influenced corrosion (MIC) and biofouling in seawater
Cu-Ni 90/10 naturally resists biofouling and MIC in seawater systems, reduces the need for chemical dosing, and performs reliably across a wide seawater velocity range.
Question 6: Which property must be verified when selecting lubricating grease for DP azimuth thruster slewing ring bearings exposed to seawater ingress?
- Flash point only
- Water washout resistance per ASTM D1264 (Correct answer)
- Dropping point only
- Viscosity index only
Correct answer: Water washout resistance per ASTM D1264
ASTM D1264 water washout resistance ensures grease remains in bearings despite seawater intrusion, preventing lubricant loss and accelerated bearing wear in DP thrusters.
Question 7: For DP vessel dynamic cables connecting turret-moored buoys to the vessel, which armor wire material is selected for resistance to hydrogen embrittlement?
- High-carbon steel wire
- Galvanized steel wire
- Stainless steel 316 wire (Correct answer)
- Martensitic stainless steel wire
Correct answer: Stainless steel 316 wire
Austenitic stainless steel 316 wire is resistant to hydrogen embrittlement unlike high-carbon or martensitic grades, which are susceptible under cathodic protection-induced hydrogen generation.
DPO Materials Selection 3
In DP vessel thruster design, why is nickel-aluminum bronze (NAB) preferred over carbon steel for propeller blades?