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
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.
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
- 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.
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.
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:
- 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
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”:
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
3. abrasive corrosion with
- erosion, or
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.
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”.
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.