CHIEF ENGINEER Unit 12 Classification Q4

A classification survey of a vessel in dry dock is a comprehensive inspection conducted by a classification society surveyor when a ship is out of the water and resting on blocks in a dry dock facility. The primary purpose is to examine and assess the underwater portions of the hull and other areas that are normally inaccessible while the ship is afloat.  

1. A Guide to Ship Surveys and Inspections – VLS Environmental Solutions

2. Classification surveys – Ask a Question

Key objectives of a classification survey in dry dock:

• Assess Hull Condition:
  • Thoroughly examine the hull’s external plating, welds, seams, and underwater appendages like bilge keels, sea chests, and propellers for signs of corrosion, damage, or deterioration.   1. Classification surveys – Ask a Question rs-class.org
  • Take thickness measurements of the hull plating using ultrasonic gauges to determine if there’s any significant wastage or thinning that may require repairs or renewals.
  • Inspect the condition of the ship’s paint and anti-fouling coatings, which protect the hull from corrosion and marine growth.
• Examine Underwater Fittings and Sea Valves:
  • Check the condition and operation of sea valves, through-hull fittings, and overboard discharge valves to ensure they are watertight and functioning correctly.   1. Classification surveys – Ask a Question rs-class.org
  • Inspect and test the stern tube seals and bearings to ensure they are not leaking or worn.   1. Stern Tube Repair & Surveys | AtZ Marine atzmartec.com
• Inspect Propeller and Rudder:
  • Examine the propeller for damage, cracks, or signs of cavitation or erosion.   1. Classification surveys – Ask a Question rs-class.org
  • Inspect the rudder and its associated pintles and bearings for proper operation and wear.   1. Typical Dry Dock Activities – AtZ Marine Technologies atzmartec.com
• Check Anodes:
  • Inspect and replace sacrificial anodes as needed to maintain the cathodic protection system that prevents corrosion of the hull.
• Internal Tank Inspections:
  • Depending on the survey scope, internal tanks like ballast tanks or cargo holds may also be inspected for corrosion, leaks, or structural issues.
• Verify Compliance:
  • Ensure the vessel continues to comply with the classification society’s rules and relevant international conventions like SOLAS and MARPOL.   1. Classification surveys – Ask a Question rs-class.org

Types of Surveys Conducted in Dry Dock:

• Special Surveys: These are major surveys conducted every five years to renew the ship’s class and involve a thorough inspection of the entire vessel, including the hull, machinery, and equipment.   1. Class Surveys – Capt.S.S.Chaudhari captsschaudhari.com
• Intermediate Surveys: These are less extensive surveys conducted between Special Surveys to verify specific aspects of the ship’s condition.   1. Do you know the various types of survey onboard? – SAFETY4SEA safety4sea.com
• Docking Surveys: These surveys focus primarily on the underwater portions of the hull and related systems. They may be conducted in conjunction with Special Surveys or separately.   1. Classification surveys – Ask a Question rs-class.org

Importance:

• Safety and Seaworthiness: Dry dock surveys are crucial for maintaining the safety and seaworthiness of a vessel by identifying and addressing potential structural or mechanical issues that could compromise its integrity.   1. 10 Key Benefits of Regular Dry Docking for your Commercial Vessel – Gulf Copper www.gulfcopper.com
• Regulatory Compliance: They ensure that the ship continues to meet the stringent safety and environmental standards required by international conventions and classification societies.   1. Classification surveys – Ask a Question rs-class.org

In conclusion, a classification survey in dry dock is a comprehensive and essential inspection of a ship’s underwater hull and related components. It is critical for maintaining the vessel’s safety, seaworthiness, and regulatory compliance, ultimately contributing to the overall safety and sustainability of maritime operations.

During a classification survey of a vessel in dry dock, the surveyor would meticulously examine numerous separate items of the hull and associated equipment to ensure the vessel’s structural integrity, safety, and compliance with regulations. Here’s a breakdown of some key areas and components that undergo scrutiny:

Hull Structure:

• Shell Plating: The surveyor will closely inspect the entire external hull plating for signs of corrosion, pitting, cracking, deformation, or any other damage that could compromise its strength and watertightness.
• Welds and Seams: The welds and seams joining the hull plates are critical for structural integrity. The surveyor will examine them for any signs of cracking, corrosion, or inadequate penetration.
• Frames and Bulkheads: The internal structure of the hull, including frames, bulkheads, floors, and other structural members, will be inspected for signs of corrosion, buckling, or other damage.
• Double Bottom and Tank Tops: If applicable, the double bottom and tank tops will be examined for any signs of corrosion, leaks, or structural weaknesses.

Underwater Appendages:

• Rudder: The surveyor will inspect the rudder for any signs of damage, cracks, or misalignment. The rudder pintles, bearings, and hydraulic systems will also be examined.
• Propeller: The propeller blades, hub, and shaft will be inspected for cracks, erosion, cavitation damage, or any other defects that could affect its performance or safety.
• Bilge Keels: These stabilizing fins attached to the hull will be checked for damage or corrosion.
• Sea Chests and Sea Valves: All sea chests (inlet and outlet openings in the hull) and sea valves will be inspected for tightness, proper operation, and corrosion.

Other Equipment:

• Anchors and Chains: The anchors, anchor chains, and associated equipment will be examined for wear, corrosion, and proper securing arrangements.
• Mooring Equipment: Mooring lines, bollards, fairleads, and other mooring equipment will be inspected for their condition and ability to withstand the forces exerted during mooring operations.
• Cathodic Protection System: Sacrificial anodes and impressed current systems, which protect the hull from corrosion, will be inspected and replaced as needed.
• Through-Hull Fittings: All through-hull fittings, such as overboard discharge valves and underwater sensors, will be checked for tightness, corrosion, and proper operation.

Additional Checks:

• Thickness Measurements: Ultrasonic thickness measurements will be taken at various locations on the hull plating to assess its thickness and remaining strength. This helps determine if any areas require repairs or renewals.
• Non-Destructive Testing (NDT): In areas of concern or high stress, NDT techniques like magnetic particle testing or dye penetrant testing may be used to detect cracks or other defects that are not visible to the naked eye.
• Internal Tank Inspections: Depending on the survey scope, internal tanks such as ballast tanks or cargo holds may also be inspected for corrosion, leaks, or structural issues.

Documentation:

• Surveyor will review relevant documentation, such as previous survey reports, repair records, and the ship’s drawings, to ensure compliance and track the history of the hull and its components.

The classification society surveyor’s meticulous inspection and assessment of these various hull components and associated equipment are vital to ensure the vessel’s continued seaworthiness, safety, and compliance with regulations. By identifying and addressing potential issues during the dry dock survey, the surveyor helps prevent future problems, minimize downtime, and contribute to the overall safety and sustainability of maritime operations.

Here are several ways in which the thickness of a steel hull can be determined:

1. Ultrasonic Thickness Measurement (UTM):

• Principle: This is the most common and preferred method. It utilizes high-frequency sound waves to measure the time it takes for the sound to travel through the material and reflect back from the inner surface. This time is then used to calculate the thickness.
• Advantages:
  • Non-destructive, meaning no damage is done to the hull.
  • Can be performed with the vessel in or out of the water.
  • Can measure through coatings, eliminating the need to remove paint in many cases.
  • Provides accurate and reliable results.

2. Mechanical Gauging:

• Principle: This involves physically measuring the thickness using tools such as calipers, micrometers, or depth gauges. It requires access to both sides of the hull plating.
• Advantages:
  • Simple and inexpensive.
  • Can be used in areas where access for UTM probes is limited.
• Disadvantages:
  • Destructive, as it often requires removing paint or coatings.
  • Less accurate than UTM, particularly for thicker plates.
  • Not suitable for underwater measurements.

3. Drill Test:

• Principle: A small hole is drilled through the hull plating, and the thickness is measured directly using a depth gauge. The hole is then plugged and welded.
• Advantages:
  • Provides a direct measurement of thickness.
  • Can be used to verify UTM readings or in areas where other methods are not feasible.
• Disadvantages:
  • Destructive, as it creates a hole in the hull.
  • Requires welding and subsequent inspection to ensure watertightness.
  • Time-consuming and labor-intensive.

4. Radiography:

• Principle: X-rays or gamma rays are passed through the hull plating, and the resulting image is used to assess the thickness and identify any internal defects.
• Advantages:
  • Can detect internal flaws or corrosion that may not be visible on the surface.
  • Non-destructive, but requires careful radiation safety measures.
• Disadvantages:
  • Expensive and requires specialized equipment and trained personnel.
  • Not commonly used for routine thickness measurements.

5. Other Methods:

• Laser Thickness Measurement: Emerging technology using laser beams to measure thickness, potentially offering non-destructive and remote measurement capabilities.
• Electromagnetic Thickness Measurement: Another non-destructive method, but its accuracy can be affected by coatings and material properties.

Choice of Method:

• The choice of method depends on factors such as:
  • Accessibility to the hull (in dry dock or afloat).
  • Presence of coatings.
  • Required accuracy and level of detail.
  • Cost and time constraints.

UTM is the most widely used method for routine hull thickness measurements due to its non-destructive nature, accuracy, and ability to measure through coatings. However, other methods may be employed in specific situations or to verify UTM readings.