It
is necessary to distinguish between damage and failure apart from the fact that
the results of both incidents are usually described as “damage”.
1. THE DIFFERENCES BETWEEN FAILURE AND
DAMAGE
Definitions
Whilst
damage is “physical harm done to something”, failure is a mere “lack of success”
or “something that does not do what it is supposed to do”.
Accordingly
damage occurs by external forms of attack, influences, impacts etc. in
circumstances and/or ambient conditions causing harm or loss to the object.
Failure
occurs in a vessel when structural systems, components or elements become
defective before the end of their designed or required lifetime, when they
cannot endure the usual stresses or ambient conditions.
In
such cases a part, a component, a subsystem, or even a complete system may
break down and not necessarily reveal signs of failure as under the collapse
all usual signs of damage are visible in spite of the fact that the collapse
was the result of an internal fault only.
More
than often failure occurs in connection with severe outside conditions which
can also conceal the actual cause. It may then be difficult to locate properly
the attributable reasons for failure.
But
only if a failure is identified as such, proper corrective measures can be
taken to prevent further or progressing defects and to initiate failure
prevention on other similar structures or vessels.
When
a failure has been identified, calculations, operations, production methods
and/or materials can be reconsidered and parts of systems properly adapted and
cured thereafter. Without such detective work accidents may repeatedly recur
due to similar reasons and may damage be not avoided nor the system failures be
excluded.
1.2
Practical Aspects
For
damage by external forces a vessel is usually insured and in identifying the
external impacts protective means can be developed or provided.
As
causes for damage, human error or unfavorable natural conditions or a
combination of both influences may be traced.
As
failures may usually be classified as weakness of design or defective material,
or improper calculations, estimations, manufacturing or assembly processes,
material composition, faulty inspection or testing methods, a shipbuilder may
guarantee for the losses.
But
also undue operational procedures in loading or discharging, ballasting,
inerting, cargo handling, maneuvering and/or maintenance may contribute to
failure or premature defects. The latter may, however, be found under other
circumstances without an indication of wrong operations.
As
simultaneously induced vibration with detected or undetected response of local
structures may lead to failures, the matter can become rather complex and prone
to dispute.
On
the other hand such response may be induced by a damaged propeller, by
imbalance of the main engine or improper operation of the vessel in bad weather.
In
many cases only thorough investigations may lead to the roots of failure,
especially when this was found after external circumstances which actually
contributed to defects.
Load
and/or recalculation programmes might be required, based on actual in-service
measurements of hull deflection or excitation. In fact, however, recalculations
can only be as good as the service data presented.
Damage
on the contrary can always be related to external attack or impact of the
surrounding media. Also abnormal local influences in a ship compartment may
create adverse environmental circumstances leading directly to breakdown or
accelerate it by deformation, fracturing, corrosion or other forms of material
deterioration.
The
same may happen if maintenance is poor. Then similar signs of wastage and
defect will appear and may result in damage.
Grounding,
impact to quay, collision, wrong maneuvers, overloading etc. may directly lead
to hull damage with visible traces, fractures and indentations at the areas of
contact.
1.3
Risk Control
In
broader terms one may say that the risk of damage is limited by the system of
international safety regulations, IMO resolutions, guidelines and directives.
That does not exclude damage by nature, human error, negligence or similar
reasons, but the regulations make sea transport rather safe.
The
risk of failure is kept under control by the system of classification.
This also neither excludes failures
nor prevents damage in terms of 100% reliability, but the system provides a
general exchange of know-how in the shipbuilding and repair industry as well
as between sea-faring nations and shipping companies.
Ships are usually designed and delivered with appropriate
margins of strength and corrosion resistance as defined by the classification rules
and feasible by approved standards of technology.
Design and plan approval is carried out in due course with
dedicated scrutiny to every detail.
Materials, shipbuilding components, machinery and equipment
parts are examined before these parts can be built-in in a classified vessel. Many
of these have to satisfy safety requirements specified in SOLAS 74 as amended.
During construction the vessels are inspected in various
stages of assembly and undergo delivery trials. During these processes the builder
convinces himself of the production quality achieved, the buyer checks if in
all respects the product is according to specifications and contractual
submissions, and the classification society examines if their requirements have
been fully met and if the vessel corresponds accordingly during final delivery
tests.
As ships are usually not fabricated by mass production
systems, they are individually assembled and tested before commissioning.
The system of class examination adopted for construction
provides safe and reliable vessels, not only for their specific purposes, but
al so for the protection of environment and life at sea.
But in maintaining a class the vessels are not only subjected
to initial examination and testing, but al so to subsequent control by occasional
and periodical surveys.
This inspection system of classification inspection ensures
that controls are carried out at defined maintenance periods so that unexpected
failures and unknown damage can be detected and dealt with before serious
breakdowns occur.
In addition the classification societies provide by means of
their survey data collection and evaluation that proper steps of early warning
are realized and that the resulting experiences of various repairs are
introduced in maintenance steps, properly advised and applied early enough to
similar vessels so that catastrophic failures can be avoided.
In this way risk control is established and provided by proven
methods of plan approval, construction examination and following up survey procedures.
Attentive awareness of risk control is integrated in the
central brain work of safety engineering, as provided and collected over the years
by the staff teams in the classification headquarters and their committees
working in exchange of opinions and experiences in all major industrial
engineering institutions including those of standardization, development and
research.
1.4
A Surveyor’s Role
In
the inspection system of classification a decisive role is allotted to the
surveyors on the scene, not only during construction, but also in due course
when vessels are subsequently examined.
The
surveyor's capabilities, job dedication, experience and ability to cooperate
with the shipboard management, repairers and others can only lead to the
required success if he is guided by objectivity, sincerity and impartiality. That
may also require an honest self-control which will call for further and/or
specialized assistance, should the problems become too complex or exceed one’s
own experience or knowledge.
The
following explanations on causes and modes of failure are deemed to initiate
subsequent analysis.
2.
CAUSES OF FAILURE AND/OR DAMAGE
Some
major causes of failure and damage of the hull structure are discussed below;
it is, however, not intended to encompass all relevant issues. Attention is
focused on major aspects and circumstances only.
2.1
Material Defects
Well
advanced and closely controlled production methods in combination with material
certification by class requirements have to a great extent excluded material
defects in plates and shapes used in hull structures during shipbuilding.
Improved non-destructive testing procedures and quality assurance methods with
internal shipyard purchase examinations have also contributed to the fact that
flaws and hidden defects are to a great extent avoided in steel structures.
So
the only major source of material defects are usually attributable to
deficiencies owing to wrong welding material s or ambient temperatures, inner
welding failures, welding stress concentration, excessive shrinkage from
welding heat, welding sequence or change of the crystalline structure along
seams or butts.
It
follows that especially the welding preparations and procedures, as well as
welders’ qualifications, have to be closely supervised by a builders’ quality
assurance program and examined in accordance with the classification
requirements.
2.2
Weakness of Design
In the last 30 years major steps
have been made in ship design and construction technology. Vessels have been
built of ever larger tonnage and simultaneously the power requirements for this
tonnage have substantially increased. High tensile steel has progressively been
used for main girders, side plating, hatchcovers etc., resulting in saving of
weight. With the use of refined computer design methods, more rationally
calculated stress investigations became possible, enabling a theoretical
consideration of the yet inexperienced behavior of large structures.
Occasionally, applied design criteria have not been
completely successful in actual conditions, even when using reliable calculation
programmes, owing to interactions from propulsion systems, wave behavior, and
inertia of the huge mass movements which are difficult to predict precisely.
Additionally, even in the modern design software programmes, although quite
capable, may not necessarily reproduce the actual conditions a ship encounters
in its lifetime.
As a result, local structural components have occasionally
been subjected to unexpected stress levels and excessive operational strains.
Also, owing to inadequate detail construction, stress concentrations at
particular points occurred.
Fortunately,
in such cases, all “weak” point’s usually announced themselves early by small
cracks and buckling, which were detected in time and dealt with. Stress release
and reinforcements were then necessary and carried out, but the system of risk
control by classification surveys had excluded catastrophic failures.
Simultaneously
with the increasing ship sizes increased hull flexibility was experienced. In
large ore carriers it had occasionally led to problems of "bagging"
(bulging of side shell under the inner pressure of the cargo). Also during
loading and discharging or ballasting operations in a quasi static environment,
and not only in adverse weather, an increase of “hogging” and “sagging” conditions
was observed.
Therefore
calculators have been introduced, to an increasing extent, in shipboard
management systems. They carry out cargo distribution and hull bending
calculations and can simulate the performance of proper loading and discharging
sequences; thus undue hull stresses can be avoided.
Today
it appears that we have to live with relatively flexible hull structures and
consider shorter lifetime expectations for a vessel. In addition we have to pay
more attention to operational methods which, using pre-calculated and
rationally controlled handling and managing systems, avoid local overstresses.
In
conclusion, where failure may have been attributed to “weakness of design”, it
might turn out that lack of proper operation of the vessel is the cause.
The
above does not exclude local cracking and deformation being traced to
structural deficiency, especially at joints where local discontinuities exist.
2.3 Assembly Imperfections
Defective assembling processes will
practically be excluded by strict supervision of ship construction and section
assembly, protected working locations, proper working sequences, adequate
machines and qualified personnel as well as increased shipyard experience at
all levels. But fabrication of structural components in large modules and
assembly of sections in different locations can occasionally lead to the
following imperfections:
-
misalignment
of internals,
-
displacement
of seam crossings,
-
displacement
of vertical or angled cross joints, or
-
excessive
welding butt gaps,
-
excessive
heat distortion of parts,
-
seam
sagging,
-
plate deformations.
The majority of the above imperfections can
be detected visually and measured for evaluation, comparison and eventual
corrections.
Not all imperfections can be avoided and
approved tolerances are the acceptable standard. The aim is to reduce the
volume of acceptable allowances as far as possible.
Again, improper welding work can be
considered as a major source for failures, especially at points of interruption
and at joints. Assembled structures are therefore especially checked at joints
by non-destructive testing methods.
2.4
Vibration
The
phenomenon of vibration in structural systems is experienced in small and big
vessels. Huge tankers and bulk carriers of moderate propulsion outputs are
likewise prone to vibration failures, as well as large open container and small
multi-purpose carriers with high service speeds and high-powered diesel
engines.
The
majority of modern vessels, that have engine rooms aft, with short stiff shaft
lines at the extreme end of the hull, have rather flexible hull structures.
Even coasters suffer from similar design features.
Hull
vibrations can usually be induced and/or excited by
-
propeller;
-
main engine and/or auxiliary
machinery; and finally
-
forces of the sea (rolling,
pitching, slamming, green water).
Defects
to structures or machinery and disturbance of human behavior are, however, only
caused when the frequencies of excitation are either close to or directly
coinciding with the natural frequencies of the whole ship, the main structure
or plate sections or panels.
Hull
response and/or local component resonance may then frequently lead to various
failures in hull parts and machinery components.
2.5
Defective Material Protection
Where
structures and hull components are coated to allow for reductions in thickness,
these areas can only render suitable strength as long as the coating is not
impaired by poor quality or application or by general wastage or mechanical
abrasion in exposed locations.
Catholic
protection in tanks can only work as long as the anodes are submerged and alive
in their capacity.
As
a general experience, coatings in tanks or holds and at outer shell sides are
firstly prone to deterioration along their welding seams and butts.
A
secondary area of wastage is normally found under decks, where mechanical
stresses develop as a result of moving liquids and where coating layers are
especially subjected to temperature differences between inside and outside
surfaces.
The
areas prone to erosion and corrosion of welding are forward and aft at outside
underwater parts of a vessel, more specifically:
-
forward in way of bottom
sections where slamming and anchor chain chafing is experienced,
-
aft in way of stern section
where cathode ion exchange is always imminent and the flow of water
concentrated;
-
also in way of the rudder blade
where welding and slot welding is abundant and wake action, corrosion and
erosion are high.
As
the condition of the built-in material protection is a structural feature of
fitness, its status should be regularly examined and noted. Where material
protection is an integral part of hull integrity, as in vessels with reduced
scantling thickness due to the application of a coating system or other
protective means, a proper level of material protection must be maintained.
2.6
Poor Maintenance
Maintenance
is a process of keeping an object in good condition by regular checking and
doing necessary repairs. The periods between checking and corrective means of
repair can usually be established in considering the characteristics of the respective
object.
As
long as a system does not fulfill the designed duties within the designated
periods, one would consider that the system failed (unless special and unusual
circumstances caused the deficiencies).
If
system is properly constructed, applied and functioning as initially expected
but gradually deteriorating due to lack of maintenance, the efficiency,
strength and suitability of the system become “damaged”.
Proper maintenance is a major contribution
to general safety and it is a decisive factor in hull and structural systems
where scantling thicknesses are reaching limits, or where weathertight
integrity is impaired.
2.7
Maloperation
If
a vessel is hit by a tug or when contacting other vessels or berthing
facilities, touching ground etc., a major indentation may result. The “damage”
will usually be visible and vary in intensity, extent or seriousness.
Although
such casualties, malfunction or mishandling effects can never be excluded, the
resulting damage is usually evident.
Maloperation
of a vessel can, however, also be exercised without producing direct or
corresponding visible signs of deficiency. It may take repeated actions of
similar events before signs of damage announce latent or inherent danger of
defect.
Such
actions can be misjudgment or ship mismanagement with regard to:
-
sea and wave forces,
-
weight distribution in the
ship,
-
ballasting and/or
-
tank cleaning, inerting,
-
loading and discharging,
-
anchoring or berthing.
Heavy
sea waves and swells which a vessel meets may cause unsound
"breathing"/variations of hull displacement. The hydrodynamic
pressures to which the ship's body as a whole or single side shell sections may
be exposed, can lead to a notable induced vibration of the hull girder or of single
components, especially if the frequency of swell is close to the natural
frequency of the hull.
If
a vessel is pitching into waves, slamming may occur with whipping of the fore
body. Green seas may twist the hull or throw tons of hydrodynamic masses over
deck. Shock vibrations occur, and, even if dampened within seconds, vibrations
with changing nodes will run through the ship, exciting responsive elements.
The
harmful effects of vibration induced damage may be avoided if properly met by
counteractions as indicated below:
-
change of speed or engine
revolutions,
-
changes of course and
occasionally
-
changes in trim or ballast
distribution.
Small
changes are usually enough to smooth the excessive effects of free or forced
hull vibration and the resultant overstresses of local hull parts.
Wrong
loading sequences, excessive localized loads (on deck with cargo) and
non-uniform load distribution (container stacks, ore in holds, etc.) may have
similar effects in overstressing certain hull components and leading to
premature fatigue.
Dynamic
loads by liquid motions can also be created by steaming with part-filled tanks
and cargo holds or wrong ballasting. Such measures will not necessarily
directly effect damage, but if repeatedly done the contribution to early
defects will suddenly become evident.
In
tankers, where crude oil washing and ballasting or inerting cycles are
frequent, unprotected hull structures are especially exposed to pronounced
corrosion. Accordingly, the trade of a vessel, the methods of operation and
utilization are decisive factors in determining structural lifetime.
2.8
Fatigue
Fatigue
in steel or other materials, occasionally called “aging”, is the decline of
resistance against changing stresses. It may also be described as exhaustion of
strength caused by numerous variations of stress.
Such
stresses may be cyclic, non cyclic, heat or vibration induced, or otherwise
exercised.
The
stresses can be exerted in different directions and by various external or
internal forces as a result of:
-
tension,
-
pressure,
-
shearing, or
-
bending, or
-
torsional forces.
The
stresses may have varying values as a result of changing amplitudes and regular
or irregular periodicity and direction.
Material
behavior under cyclic stress is demonstrated in the WOEHLER CURVES. These
curves show the dynamic strength or the endurance limits of a material as a
function of stress with constant and/or changing amplitudes against the number
of cycles (N) before the lifetime of the metal is considered to be exhausted.
With
mild steel the WÖHLER CURVE runs out in a range between 106 and 108
cycles, aluminum alloy may resist up to 1010 cycles.
Fatigue (material) http://en.wikipedia.org/wiki/Fatigue_%28material%29
Fatigue (material) http://en.wikipedia.org/wiki/Fatigue_%28material%29
 |
| Micrographs showing how surface fatigue cracks grow as material is further cycled. From Ewing & Humfrey (1903) |
The
life expectation (N) of material s against vibration stresses is influenced by
a number of factors of which the following are the most important:
A
minute fatigue crack may start at a sensitive location under cyclic or
non-cyclic but repeated stresses. Such locations are often called crack raisers
and are normally due to:
a)
Changing thicknesses (causing stress increase at joints)
b)
surface roughness (reducing endurance strength by existing
unevenness)
c)
coarse cut-outs (at manholes, penetration holes etc.)
d)
welded joints (causing stress increase e.g. at weld toe).
Ambient
conditions as temperature levels, a corrosive medium, or humid air are factors
contributing to reduce resistance against cycle stress.
Crack
initiation and crack propagation are to be distinguished. With tough materials
approximately 10% of a component's lifetime (N) may be required to initiate
crack formation, while approximately 90% N may be necessary for the crack to
propagate into a failure of the material.
With
brittle materials as cast iron the above is usually reversed.
Stress
induced cracking does not need corrosion as a starter. But corrosion actively
assists the process. The strain which causes an inter-crystalline plastic micro
flow at an inner point of stress concentration can be smaller if the root of
the crack raiser is subjected to a corrosive medium.
Fatigue
is, however, not a phenomenon which can be described as a general decay or
overall loss of strength. Close to the fatigue fracture, the material can be as
sound as new. The “ageing” starts only at locations of stress concentration
under the effects of cyclic or non-cyclic loads. These locations should be
designed to withstand these loads, especially within the component's lifetime.
Summarizing
the above explanations, cracking/fracturing due to fatigue can be expected and
found in steel structures under. the following circumstances:
-
low designed limits of
lifetime,
-
wear and tear, especially by
corrosive media,
-
stress raisers in assembled
parts,
-
in high tensile steel
components, where corrosion has a greater influence than in other steel parts.
Apart
from machinery or propeller induced vibrations, the stresses which cause
fatigue are usually initiated by the action of sea and the movements of the
vessel in heavy weather or different swell conditions. These stresses are
induced in the structural components as a result of movement of inertia masses,
in or on or outside of the ship, as the weight of the cargo or the free
surfaces of ballast and/or other liquids moving or only “breathing” with the
motion of the sea.
Under
the influence of these moving masses, shell sides, decks and bulkheads tend to
vibrate in various degrees.
The
effect of moving masses and induced structure vibration depends to some extent
on experience in shiphandling by the officers on board. Gradual changes of
course, reductions in speed or alterations in trim or ballast influence the behavior
of steaming at sea and increase or decrease the "laboring" of a
vessel in bad weather.
Good
navigational practice and careful cargo handling procedures can essentially
contribute to a vessel's lifetime, especially with huge bulkers, tankers and
container carriers.
3.
MODES OF FAILURE AND DAMAGE
Defects
in steel structures, components or plates show certain modes of breakdown.
Indicators of impending breakdown may initially announce themselves separately
(for instance with the break-up of a coating), but can quickly combine with
other forces to produce deterioration. Whilst origin and cause(s) can vary and
the result be either failure or damage, the modes of defect have common causes
normally present under what is generically labeled as “wear and tear”:
-
corrosion
-
deformation
-
cracking
The
characteristics of these will be summarized below.
3.1
Corrosion Modes
Corrosion
is caused by chemical or electrochemical reactions. Corrosion is oxidation,
i.e. exchange of electric particles directly by contact from metal to environment
or metal to metal or through an electrolytic medium.
Corrosion
generally appears in the form of
-
surface corrosion (oxidation,
rust),
-
pitting corrosion (needle hole
formation, deep local penetration of surface),
-
cracking corrosion (hair
cracking in and under the surface).
Surface corrosion is easy to detect and
results in a general deterioration of the steel thickness.
Pitting formation by local needle points
and holes under the surface is initially difficult to detect; it may not cause
heavy structural damage, but isolated leaks.
Corrosive cracking is also dangerous as
it is difficult to detect and because cracks progress under the surface and may
reach a wide spread pattern.
According
to its origin, corrosion can normally be the result of the following:
1.
corrosion by contact,
2.
stress induced corrosion due to
- pressure/tension,
- vibration,
3.
abrasive corrosion with
- friction,
- erosion, or
- cavitations,
4.
corrosion by hydrogen formation and diffusion.
The
resulting defects are structural loss by either material wastage over wide
steel surfaces or weakness by deep pitting concentration or extensive creeping
cracks, especially in those locations where high stress levels exist. More than
often, especially in tankers, all corrosion phenomena appear in combined forms,
apart from the extremely specific feature of hydrogen corrosion.
3.2
Deformation
Deformation
of steel structures is usually the result of local overload. A plating profile
or structure moves under load and expands in a direction where resistance is
less. A certain amount of elasticity enables the material to buckle and waive.
When the elastic limit is surpassed, a plastic flow of the crystalline material
structure occur leading to permanent deformations.
Fractures
may occur where buckling is contained before the stresses reach high levels,
otherwise buckling will absorb energy and possibly avoid cracking. Either
buckling or cracking, however, are undesirable effects.
Toughness
and elasticity in new steel structures may accept deformation stresses rather
easily, but components already reduced in thickness by corrosion will tend to
lose their inherent stiffness and elasticity.
Generally,
existing deformations only increase if the loads are increased. Under high
constant loads an increase of deformation may be observed called “creeping” or “relaxation”
(as in pretension screws or springs etc.). After settling of some of the
stresses one may find that the material strength may not be impaired. This is
why after a certain period of observations minor plate deflections and buckling
may remain in hull structures without impairing structural strength.
3.3
Cracks and Fractures
A
crack is “a mark or very narrow opening caused by breaking, tearing or
splitting without dividing into separate parts”[1].
A
fracture is “the act or the final result of cracking or breaking apart”.
In
our terms cracking of material is the beginning of tearing under strain (strain
= excessive stress).
There
are various forms of cracking:
-
hair cracks: extremely thin
surface cracks, usually not detected
-
with the naked eye, but with
dye penetrants;
-
creeping cracks: extremely fine
fissures in the material progressing between or through the crystalline
structure;
-
heat cracks: fissures on and under
the surface of a metal usually due to local overheat. It may be anticipated by
"blue" discoloration of steel, but detected only with ultrasonic or
magnetic particle testing.
A
prolonged cracking process can finally lead to complete fracturing. Fractographic
inspections may disclose indications of the possible origin or cause by the characteristic features of a crack, as
follows:
-
mode of stress or strain
(tension, shear or torsion),
-
period of development
(vibration induced or rupture by impact),
-
influence or existence of
notches, as crack raisers.
Macroscopic
signs of fractured surfaces (seen with the naked eye or hand lenses) can
indicate sources of defect, direction, mode and level of stresses, influence of
notches and material behavior.
If
macroscopic fracture analysis does not reveal the nature of cracking, microscopic
inspection may finally determine the mode of crack and fracture.
However,
whilst such explanations are in many cases helpful for repair and
reconditioning, the actual cause of the defects may still remain unidentified
and may require additional material analysis.
Such
investigations can be hardness testing, tensioning or vibrating tests, or
chemical analysis of the material used.
Methods
of metallography may finally explain defective heat treatment procedures or
alternations of crystalline structures.
