Sunday, January 06, 2013

What to look for?

Where to look? Is the most difficult for a Marine Surveyor during their training. This skill is acquired through years of own experience.
This entry aims to provide guidance in the beginning training as a Marine Surveyor.

During the process, you will find difficult situations for decision making.
Refer to "Damage Repair" in the connections of the references below for different types of ships.

Condition questionable?
Recognition Resources
When there is a doubt for the condition are advised Marine Surveyor:
(1) Remove obstruction parts
(2) Close-up inspection
(3) Measures of deformation and clearances
(4) Test temperature hose for tightness
(5) Test pressure water or air tightness
(6) Measures thickness with ultrasound
(7) Non-destructive testing for hidden fractures
(8) Evidence of inclination for stability

Video What to look for?: 


video


Look, you have to have in consideration as follows:



Longitudinal strength
When a vessel is floating in still water, there are two forces acting on the hull; buoyancy, acting upwards (which is more evident in the fuller sections of the hull), and weight acting downwards. The resultant force is zero (Archimedes’s principal).

However, the weight distribution along the length varies. The unevenness in the weight distribution acting downwards and the buoyancy force distribution acting upwards, causes a resultant, “still water bending moment”. This causes the hull girder to bend.

If the weight distribution is higher in the mid ship region than the buoyancy distribution, this causes "sagging", if the buoyancy in this region is higher, it causes "hogging".

In addition to the weight and buoyancy forces, the wave forces also act on the hull girder at sea. This causes further deflection of the hull due to the “wave bending moment”.

The sum of the still water bending moment and the wave bending moment is the “total bending moment”.

The hull girder also experiences shearing forces due to the static and dynamic forces mentioned above. The shearing force at any position of the ship’s length is that force which tends to move one part of the ship vertically to the adjacent portion.

Effect of hogging and sagging on hull girder

Hogging
Deck level - deck plating, longitudinal stiffeners, longitudinal hatch coamings, sheer strake are in tension (maximum around mid ships) stress increase at corners of deck openings, brackets of longitudinal stiffeners, longitudinal hatch coaming brackets - any transverse crack can propagate rapidly.

Side shell - Tensile and compressive stresses increase on side shell plating and longitudinal stiffeners, towards the deck and bottom. High shear stresses on side shell plating and attached stiffeners around the neutral axis. Stress increase at corners of side shell openings.
Bottom level - bottom plating, bilge plating, longitudinal stiffeners are in compression (maximum around mid ships) - any wastage in plating or stiffeners can cause increase in compressive stresses hence buckling.
Sagging
Deck level - deck plating, longitudinal stiffeners, longitudinal hatch coamings, sheer strake are in compression (maximum around mid ships) - any wastage in plating or stiffeners can cause increase in compressive stresses hence buckling.
 
Side shell - Tensile and compressive stresses increase on side shell plating and longitudinal stiffeners, towards the deck and bottom. High shear stresses on side shell plating and attached stiffeners around the neutral axis. Stress increase at corners of side shell openings.
Bottom level - bottom plating, bilge plating, longitudinal stiffeners are in tension (maximum around mid ships) - stress increase at corners of bottom openings, brackets of longitudinal stiffeners – any transverse crack can propagate rapidly.

 
The longitudinal hatch coaming on the sketch above partially contributes to the longitudinal strength as it is not continuous.
In general, longitudinal structural members, including plating and stiffeners, contributing to the longitudinal strength are continuous and not interrupted when crossing transverse members.

Hull girder bending is not considered as very significant for conventional vessels less than 65 m in length.

What to look for

Stress concentrations
Stress concentrations occur at abrupt change of section, sharp corners and openings. The degree of stress concentration is a function of the shape of the discontinuity.

The shape therefore is very important.

In some cases, the local stress levels will cause failure and fracture will advance across the plate.

 
Hard points
Hard points are caused when a load is transferred from one structural member to another through a limited (concentrated) area. Hard points cause stress concentrations and eventually fractures and must be avoided.
Opening
Openings in the structure are potential sources of fractures.
The edges must be smooth and well shaped. Openings must not generally be too close to the end of the structure.

Cut-outs in primary members for secondary stiffeners
A potential area for fractures is the intersection of primary supporting members and ordinary (secondary) stiffeners. Normally the ordinary stiffener is uninterrupted and traverses the primary supporting member. 

Cut-outs on the primary supporting member to allow this arrangement can raise stress concentrations and fractures. Also cyclic loading can result in fatigue fractures in these areas.

Photo above shows stress corrosion on the web frame stiffener/longitudinal connection. Other similar connections should be checked to see if there is a trend. Repairing by replacement only may not solve the problem.

Cut-out edges should be smooth with rounded corners; the edges should be checked for potential fractures or buckling.
In areas of high shear stresses, cut-outs are supported by collar plates. See photos below.

Discontinuity
The stresses passing through the structure have to continue to the surrounding structure, if there is no continuity provided the stresses concentrate at the location of discontinuity, possible causing fractures.

Check areas where there is a change of section for potential fractures. Bracket toes are areas particularly vulnerable.

 In another context we have to consider a very common damage, this is the pitings, fractures etc..
Pitting repair
The maximum acceptable depth for isolated pits is 35% of the as-built thickness.
For areas having a pitting intensity of 50% or more, the maximum average depth of pits is 20% of the as-built Thickness. For intermediate values between isolated pits and 50% of affected area, the interpolation between 35% and 20% is made according to the table below.


Pitting intensity
(%)
Maximum average pitting depth
(% of the as-built thickness)
Isolated
35.0
5
33.5
10
32.0
15
30.5
20
29.0
25
27.5
30
26.0
40
23.0
50
20.0

Welding of pitting corrosion
The general requirements for welding have to be complied with.
Shallow pits may be filled by applying coating or pit filler. Pits can be defined as sallow when their depth is less than 1/3 of the original plate thickness.

 
Extent/Depth
Standard:
Pits/grooves are to be welded flush with the original surface.
Limit:
If deep pits or grooves are clustered together or remaining thickness is less than 6 mm; the plate should be renewed.
Cleaning
Heavy rust to be removed.
Welding sequence
Reverse direction for each layer.
NDE
Min. 10% extent
Preferably MPI




Pitting Intensity Diagrams:


Fractures
Fractures are mostly found at locations where stress concentrations occur. These could be due to:
Discontinuity
Cuts in highly stressed areas
Abrupt changes in continuity
Fabrication problems
Poor welding
Rough plate edges

Misalignment of structures.
Fracture initiating at latent defects.
Fractures in welding more commonly appear at the beginning or end of a run welding, or rounding corners at the end of stiffeners, or at an intersection.
Fracture may also be initiated by undercutting the weld in way of stress concentrations.





 References:

Materials and Welding IACS UR W 11, W13 Y W17