Answer Construction Question 12
Question 12: With reference to the underwater surface of a ship’s hull;
A. Describe a hull plate roughness analyser system;
B. State the significance of the roughness profile and compare the typical roughness values for a new ship and a ship eight years old;
With reference to the application of self-polishing paint in dry dock –
C. Describe the plate preparation necessary;
D. State the defects that may occur in the paint coating if it is not correctly applied.
Answer: A. The Hull roughness Analyzer (HRA) has a portable microprocessor with a digital print out and display. The computer is connected to a hand held carriage having a stylus measuring head. The head is moved over the hull surface and the stylus traces the hull roughness. The shortwave roughness 'cut off is the diameter of the stylus tip, whilst the long wave cut off has been selected as 50mm. One traverse of the head at any pint on the hull will gather information from approximately ten sample lengths of 50mm. For each 50mm sample, the microprocessor will assess the mean gradient through the peaks and valleys and give the highest peak to lowest valley measurement in that sample. This measurement is called $\displaystyle \small \mathrm{R_{t(50})}$ .
Roughness survey needs to cover about 100 selected locations around bow, stern, mid ship and boot topping. At each of the stations, a number of traverse of the carriage will give 'n' readings of $\displaystyle \small \mathrm{R_{t(50})}$ . The Mean Hull Roughness (MHR) at that station is the average of the 'n' readings.
i.e. $\displaystyle \small \mathrm{MHR=\frac{\sum R_{t(50)}}{n}}$
There will be 'm' stations around the hull and the 'average hull roughness' (AHR) will be the average of MHR at each station,
i.e $\displaystyle \small \mathrm{AHR=\frac{\sum MHR}{m}}$
A. Describe a hull plate roughness analyser system;
B. State the significance of the roughness profile and compare the typical roughness values for a new ship and a ship eight years old;
With reference to the application of self-polishing paint in dry dock –
C. Describe the plate preparation necessary;
D. State the defects that may occur in the paint coating if it is not correctly applied.
Answer: A. The Hull roughness Analyzer (HRA) has a portable microprocessor with a digital print out and display. The computer is connected to a hand held carriage having a stylus measuring head. The head is moved over the hull surface and the stylus traces the hull roughness. The shortwave roughness 'cut off is the diameter of the stylus tip, whilst the long wave cut off has been selected as 50mm. One traverse of the head at any pint on the hull will gather information from approximately ten sample lengths of 50mm. For each 50mm sample, the microprocessor will assess the mean gradient through the peaks and valleys and give the highest peak to lowest valley measurement in that sample. This measurement is called $\displaystyle \small \mathrm{R_{t(50})}$ .
Roughness survey needs to cover about 100 selected locations around bow, stern, mid ship and boot topping. At each of the stations, a number of traverse of the carriage will give 'n' readings of $\displaystyle \small \mathrm{R_{t(50})}$ . The Mean Hull Roughness (MHR) at that station is the average of the 'n' readings.
i.e. $\displaystyle \small \mathrm{MHR=\frac{\sum R_{t(50)}}{n}}$
There will be 'm' stations around the hull and the 'average hull roughness' (AHR) will be the average of MHR at each station,
i.e $\displaystyle \small \mathrm{AHR=\frac{\sum MHR}{m}}$
B.
With reference to the underwater surface of a ship's hull, the
roughness profile signifies the degree of frictional resistance, the
hull offers to the movement of the vessel.
It is found that, the
power requirement is higher for a rougher hull due to increased
frictional resistance than smoother hull for the same speed. Increased
power requirement necessitates increased fuel consumption. The benefits
of smoother surface is that - the adhesive secreted by marine organisms
is a low i viscosity compound that can flow into surface cavities. The
adhesive then cures to a high modulus material creating a secure
mechanical lock at every surface imperfection. Typical roughness value
(AHR, Average Hull Roughness) for a new ship is approximately 120 μm. Since deterioration is around 20 to 40 μm per year, then a ship eight years old will have a AHR of approximately 300-400 μm.
C.
Self polishing paints are based on a copolymer binder. Methacrylate is
the main chemical which is mixed with a binder which releases a
non-toxic element by hydrolysis with sea water and keeps the fouling
away. The copolymer maintains a polished surface and the frictional
resistance of the ship's hull is kept to a minimum. Thus the
effectiveness of paint protection is largely dependent upon the plate
preparation necessary prior to the application of the self-polishing
paint. The best preparation is grit blasting which cleans the hull to
the bare steel and provides a surface finish for the paint to 'Key' to.
D. The defects that may occur in the paint coating, if it is not correctly applied are :
(a) Blistering - These are unbroken bubbles or swelling of the paint caused by (1) moisture on the surface, (2) poor adhesion of the paint leading to gas or liquids being collected between the paint and the surface, and (3) painting a thick coat at a high temperature so that solvent gets trapped below the paint skin.
(b) Curtaining, Sagging, Runs - The paint may sag or droop under its own weight if applied in a thickness more than recommended or if it is thinned to such an extent that it flows excessively.
(c) Dry Spray - When the particles dry before they reach the surface, they do not flow into a continuous film but remain as globules giving a sand paper effect. This happens when (1) the temperature is high, (2) the spray gun is held for from the surface or (3) the solvent evaporates too quickly.
(d) Peeling and Flaking - Peeling is when the paint curls away from the surface, while flaking is detachment from the substrate in flakes. These faults can occur even between coats. They could be caused by -
(1) Moisture, dist, oil, grease, rust, mill scale or previously failed coatings on the surface.
(2). Poor application giving rise to dry spray or pin holing.
(3). Inability of the primer to hold up subsequent coats due either to its incompatibility with the top coats or it is being affected by solvents in the topcoat.
(4) Too great film thickness leading to stress between the paint and the steel due to the weight of the paint.
(5). Incompatiability between coats, severe chalking, intercoat contamination and incomplete Curing
(e) Cissing and Crawling - When the paint contracts immediately on application leaving part of the substrate bare. It happens when the paint or the substrate is contaminated with oil, water or grease.
(a) Blistering - These are unbroken bubbles or swelling of the paint caused by (1) moisture on the surface, (2) poor adhesion of the paint leading to gas or liquids being collected between the paint and the surface, and (3) painting a thick coat at a high temperature so that solvent gets trapped below the paint skin.
(b) Curtaining, Sagging, Runs - The paint may sag or droop under its own weight if applied in a thickness more than recommended or if it is thinned to such an extent that it flows excessively.
(c) Dry Spray - When the particles dry before they reach the surface, they do not flow into a continuous film but remain as globules giving a sand paper effect. This happens when (1) the temperature is high, (2) the spray gun is held for from the surface or (3) the solvent evaporates too quickly.
(d) Peeling and Flaking - Peeling is when the paint curls away from the surface, while flaking is detachment from the substrate in flakes. These faults can occur even between coats. They could be caused by -
(1) Moisture, dist, oil, grease, rust, mill scale or previously failed coatings on the surface.
(2). Poor application giving rise to dry spray or pin holing.
(3). Inability of the primer to hold up subsequent coats due either to its incompatibility with the top coats or it is being affected by solvents in the topcoat.
(4) Too great film thickness leading to stress between the paint and the steel due to the weight of the paint.
(5). Incompatiability between coats, severe chalking, intercoat contamination and incomplete Curing
(e) Cissing and Crawling - When the paint contracts immediately on application leaving part of the substrate bare. It happens when the paint or the substrate is contaminated with oil, water or grease.
Comments
Post a Comment