4classifying structural failures

To provide general guidance about criticality of affected structural members

Review

  • USCG guidance (CAIP,…)
  • IACS
  • ABS in-house guidance
  • Additional risk-based analysis
  • Propose/refine/develop a scheme for risk ranking structural members That are dependent on vessel type, compartment, location
  • Using risk assessment concept

Task is led by ABS Technology Lifecycle Support (LCS)

4.1General

Generally, classification societies require fractures be fixed.

There are, however, provisions that provide decisions of design, maintenance, inspection or repair based on criticality of affected structures.

The criticality of ship structures can be determined based on:

  • Consequences of structural failure of the affected structure,
  • Likelihood of structural failure of the affected structure, or
  • Risk of structural failure in the structure

The consequences of structural failure can be:

  • Safety of ship and personnel
  • Environment
  • Economy

Traditionally, the criticality scheme has decided mostly based on the potential consequences or likelihood of potential structural failures. A more recent trend is to use a risk score in determining the criticality.

4.2USCG Classification of structural failures

The USCG Navigation and Vessel Inspection Circular (NVIC) No. 15-91: Critical Areas Inspection Plans (CAIP’s) has the following definition of classification of structural failure:

Class / Description
1 / A fracture that occurs during normal operating conditions (i.e. not as the results of a grounding, collision, allision or other casualty damage), that is:
  • A fracture of the oil/watertight envelope that is visible and of any length or a buckle that has either initiated on or has propagated into the oil/watertight envelope of the vessel; or
  • A fracture 10 feet or longer in length that has either initiated in or propagated into an internal strength member.

2 / A fracture less than 10 feet in length or a buckle that has either initiated in or propagated into an internal strength member during normal operating conditions.
3 / A fracture or buckle that occurs under normal operating conditions that does not otherwise meet the definition of either a Class 1 or Class 2 structural failure.

With the following definitions:

  1. Oil/watertight envelope – the strength deck, side shell and bottom plating of a vessel, including the bow and stern rakes of barges.
  2. Internal strength members – the center vertical keel; deep web frames and girders; transverse bulkheads and girders; side, bottom and underdeck longitudinals; longitudinal bulkheads; and bilge keels.
  3. Buckle – any deformation in the oil/watertight envelope whereby the adjoining internal structural members are also bent to such an extent that structural strength has been lost.

Apparently, these definitionsare based on consequences of structural failures.

4.3Critical Areas Defined by TSCF

The Tanker Structure Co-operative Forum defines Critical Areas as follow:

“Critical areas within the tank structure of double hulled tankers can be defined as locations that, by reason of stress concentration, alignment/discontinuity and corrosion will have a higher probability of failure during the life of the ship than surrounding structures.”

In the Guidance Manual For Tanker Structures, typical locations of defect types, such as corrosion, buckling and cracks, at longitudinal material, transverse web frames, transverses bulkheads and swash bulkheads are listed. These are results from the data collection from operational experience with a variety of tanker designs, elements prone to defects.

4.4Critical Areas Defined by Classes

Classification societies define “critical areas” for guiding inspection of hull structures. These definitions are based on the likelihood of structural failure.

Table 4.1. Critical Areas Defined in Class Rules

Class and publication / Definition of critical areas
IACS / Critical Structural Areasare locations which have been identified fromcalculations to require monitoring or from the service history of the subject ship or from similar ships to be sensitive to cracking, buckling or corrosion which wouldimpair the structural integrity of the ship. (See Section 4.5 below for IACS Risk Ranking Scheme for CSR Tankers)
ABS SVR/Part 5C/Appendix 1:
Guide for SHCM program 2008 / The term critical area, as used in this Guide, is defined as an area within the structure that may have a higher probability of failure during the life of the vessel compared to the surrounding areas, even though they may have been modified in the interest of reducing such probability. The higher probability of failure can be a result of stress concentrations, high stress levels and high stress ranges due to loading patterns, structural discontinuities or a combination of these factors.
ABS SHCM-Plan Review (EWZ-001-05-p05-w055) / The term “critical joint” is defined as a joint within the critical area that should be carefully designed and constructed with high workmanship. Special attention shall be paid on critical joints for alignment, details of welding, soft end toes, etc., as applicable.
LR
2008 DNV Rules / Areas in way of critical load transfer points and large stress concentrations where a failure will endanger the safety of the ship, such as :
-stress concentrations in rudder or intersection between rudder structure and hull
-for twin hull vessels stress concentrations in way of connections between hull and wet deck
-deck beams in open hatch container ships
-strength deck plating at outboard corners of cargo hatch openings in container carriers and other ships with similar hatch opening configuration
-other areas where the likelihood of occurrence of detrimental defects is considered to be extra high
NK Guidance for Condition Assessment Scheme 2004 / Locations which have been identified from calculations to require monitoring or from the service history of the subject ship or from similar or sister ships to be sensitive to cracking, buckling or corrosion which would impair the structural integrity of the ship.
RINA
BV
GL
KR

4.5Risk Ranking Scheme in IACS CSR Tankers

IACS “Principles for the Development of Common Rules for Double Hull Tanker Structures”introduced a systematic reviewto identify the hazards on the structure due to operational and environmental influences as well as the likely consequences these hazards have on the ship structure. The process comprises three main stages;

  • Hazards: Ship in a System (SIS)
  • Consequences: Ship as a Structural System (SAS)
  • Critical Hazard Management (CHM)

The types of structural failures were categorized into one of the following three groups:

  1. Loss of Containment

- Boundary no longer oil-tight, water-tight or gas-tight. (Leakage of tank, tank or ballast space contaminated, sea water leaked into ship)

  1. Loss of Strength

- Structure component failed its function (Load carrying element)

  1. Loss of Containment and Loss of Strength

- Applies to element higher up the hierarchy. (Stiffened panels and major components)

The importance of each structural component and its acceptance criteria are decided based on the Criticality Class assignment to failure modes of the component. During the process of the approach, three categories of risk were established:

  • LIFE – control risk of human injury and fatality
  • ENVIRONMENT – Control risk of leakage leading to environmental pollution or explosion or similar
  • PROPERTY – Control risk of structural failure leading to structural damage and subsequent off-hire and repair

Each category of risk is influenced by each structural failure mechanisms differently.

The criticality classes are as defined in Table 4.2 and Table 4.3.

Table 4.2. IACS CSR Tankers Criticality Class Definitions

Criticality Class / Description
High / For structural components where failure implies high risk of human injury, significant environmental pollution or very high economics consequences.
Medium / When failure implies medium risk of human injury, medium environmental pollution or high economical consequences.
Low / When failure implies low risk of human injury, minor environmental or economic consequences.

Table 4.3. Typical Descriptions of the Criticality Class

Risk / Criticality Class
Low / Medium / High
Life / Injuries: Some
Fatalities: None / Injuries: Many
Fatalities: Few / Injuries: Many
Fatalities: Many
Environment / <0.5t spill
Film on the surface, minimal clean-up / 35-350t
Clean-up necessary / >350t
Clean-up Necessary
Property / $
Temporary repair or postponement until next scheduled docking / $$
Immediate Repair necessary. Out of service less than 2 weeks / $$$
More than medium

For each structural item, it is easier to establish a single combined criticality class. This is presented by using a ranking score system.

Table 4.4. Ranking Combinations for Overall Criticality Class

Ranking Score / Physical Meaning / Combined Criticality class
1,2 / No more than one medium / Low
3, 4, 6 or 8 / All medium; two medium one low; one of each / Medium
9, 12, 18 or 27 / Two or more high; two medium and one high / High
Note
The ranking score is given by the Life x Environment x Property using High = 3, Medium = 2, Low = 1

Figure 4.2 and Figure 4.3 illustrates the combined criticality class of a double hull tanker. The levels in the structural arrangement shows the hierarchy of the structure reflecting how each structure relate to one another. (See Figure 4.1)

Figure 4.1. Structural hierarchy described in the
background document of IACS CSR for Double Hull Tankers

Figure 4.2. Criticality Class for structural component described in the
background document of IACS CSR for Double Hull Tankers

Figure 4.3. Risk-based structural criticality described in the
background document of IACS CSR for Double Hull Tankers

The Criticality Class for each element reflected the assessment with regard loss of containment and loss of strength. Criticality Class Low is assigned for elements where:

  • Redistribution of loads is possible following loss of strength
  • Loss of containment does not lead to pollution or an increased chance of explosion
  • Temporary or postponement of repair is acceptable.

Fatigue cracks for Local items that may lead to through thickness cracks in the tank boundary were given a higher criticality class than other Local items.

Based on the assignment of the criticality classes done in this assessment,

  1. Loss of strength is found generally not critical at the Local (plate/ stiffener) level because of the redistribution of the loads through adjacent structure. Hence, a Criticality Class of Low is appropriate as long as the failure does not lead to loss of containment,
  2. Local (plate/stiffener) level failures that could lead to loss of containment are more critical at the cargo tank boundary (inner side, inner bottom) than at longitudinal or transverse tank boundaries.
  3. Repairs that require tank cleaning are associated with higher costs than repairs to clean or ballast spaces.
  4. Failures at stiffened panel level and above are more likely to lead to both loss of strength and loss of containment.

The criticality class of the structural component represents its severity for the Systematic Review. It will then contribute to the risk assessment procedure during the Critical Hazard Management stage where hazard is identified and controlled.

4.6Experiences of Offshore Floating Production Units

After.

4.7Enhanced Surveys for ESP Vessels

After.

4.8Summary and Recommendations

Different organizations and classification societies have different definitions of critical areas. Some are based on calculations while the others based on industrial experiences. Identifying the critical structural areas on a vessel can bea challenging task.

Table 4.5 tabulated the basis of the publications considered and the practice/approach that have been adopted. The criticality of an affected structure member is should be determined by using a risk based approach, taking into consideration of not only the consequences of the defect, but also the probability of the defect leading to structural failure.

Table 4.5. Criticality of Ship Structures

Organization and publications / Basis for criticality assignment
USCG
(CAIP) NVIC 15-91 / Consequences of structural failure
TSCF
Guidance Manual for Tanker Structures / Areas of high risk based on operational experience of a variety of tanker designs
IACS
Principles for the Development of Common Rules for Double Hull Tanker Structures – Systematic Review / Risk based

Table 4.6. shows the similarities in structural failure definitions between the USCG NVIC No. 15-91 and IACS CSR Tanker Risk Ranking Scheme.

Table 4.6. Classification/Categories of Structural Failure

USCG
(CAIP) NVIC 15-91 / IACS
Principles for the Development of Common Rules for Double Hull Tanker Structures – Systematic Review
Structural Failure Class 1 / Loss of Containment
Structural Failure Class 2 / Loss of Strength

With the definitions stated by the two organizations being similar and the consequences of failure were taken into account during the process, the criticality class illustrated in Figure 4.3 was able to be adapted into the USCG CAIP.