ship construction Q& answer thickness terms

    13  explain Each of the following with regards to thickness measurements and acceptance critiria   

   a) Gauged thickness   

b)      Reserve thickness
c)       Renewal thickness
d)      Voluntary thickness adition
e)      Substantial corrosion

      

1.       Measured thickness at the location by a certified technician with a calibrated machine under the instruction of a surveyor.
2.                wastage allowance in reserve for corrosion occurring in the two and a half years between  Intermediate and Special surveys
3.          Renewal thickness; Minimum allowable thickness, in mm, below which renewal of structural
4.                members is to be carried out  t_ren = t_as-built – t_corr – t_{own  .}Thickness of a material after deducting the maximum diminution allowed is the renewal  thick ness where the replacement of the plate has to be carried out.
5.            owner/builder specified additional wastage allowance, if applicable, in mm Owners if decides to increase the thickness of a material beyond designed thickness to achieve better safety or to reduce renewal of steel during operational life
6.      measured thickness indicates a wastage in excess of 75% of the allowable margins, but within the acceptable limits. If not renewed annual inspection has to be carried out to accertain the condtion of the plating

ship construction Q&Answer 2016 Jan

Describe possible  faults which may be found during a dry dock inspection of the following.

1. sren frame
2. sea chest
3. bilge keel
4. anodes
5. rudder

Stern frame  defect may differ according to the construction of the stern frame most of the ships currently constructed are fitted with a welded fabricated stern frame .stern frames also may take different shapes according to  type of rudder and rudder stock .

If a rudder horn is welded sharp changes in shape  at the curvature may lead to cracks

If the stern frame consist of a sole piece  that may get deformed due to loading of the aft end has tobe checked similarly bending  of the sole piece on sideways could occur 

Sea chest

Faults at sea chest

a)      Fouled sea grating- poor performance of the MGPS system or

      the anodes of the MPGS system installed in the sea chest are fully consumed

b)      Corrosion in the sea chest – zn anodes provided are wasted

c)       Steam outlet pipe section inside the sea chest corroded and wasted

d)      Pipe opening are thinned down or erroded (stub pieces are thinned down0

e)      Usually vent pipe is fiond chocked due to the small pipe diameter  the pipe get chocked

f)       In high sea sucion  upper area corrosion due to the accumulated air in the sea chest may find if the vent holes are not provided on the top

g)      Sea chest top plates from the engine room side may get thin down due to the water leakages of glands and accumulated water.

Bilge keel

Possible faults

a)      Deformation of the bilge keel due contact

b)      Intermittent weld seam detached from the doubler plate attached to the hull

c)       Damage to the welding seam of the doubler plate and the ships hull

d)      Partial or fully missing bilge keel

In the event of above the repair or renewal of the bilge keel done according to the time available

However the doubler plate and the shell plate attachment has to be ensured by carrying out a suitable crack detection test of the hull after removing the paint from the welding seams renewal of the bilge keel remains a discretion of the owner

Anodes 

Has to be divided into to categories

 Sacrificial anodes  and impress current system  

For Sacrificial anodes 

a)      Anodes are fully waisted

b)      No wastage of the anodes while corrosion appear on the paint damaged area of the hull

Impress current system

Coating damage in area around the anodes

Protective shield in the vicinity of the anodes are detached

Waitige or mechanical damage of the anodes

Rudder

a)      Excessive pintle clearance

b)      Excessive pintle clearance or missing juming bar welded to the rudder

c)       Cement sealing of the palm bolts are broken,

d)      Cracks appear at the rudder especially at radius area of the pintle  area

e)      Cracked or leakey welding seams at the slotted weld areas

f)       Wastage and thinning of the plates at the leading edge of the rudder specially propeller turbulat water area

g)      Water leaking into the pintle pin taper area

h)      Excessive clearance at the top bearing

i)        Increased rudder drop due to wear at rudder carrier bearing

j)        Misalignment due to grounding damage

k)      Leaks

 Sea chests and their gratings, sea connections and overboard discharge valves and cocks and their fastenings to the hull or sea chests are to be examined. Valves and cocks need not be opened up more than once in a special survey period unless considered necessary by the Surveyor.