Annex 9 - Guidelines for Technical
Assessment in Conjunction with the Planning of Enhanced Surveys for
Bulk Carriers
1 Introduction
These guidelines contain information and suggestions concerning
technical assessments which may be of use in conjunction with the
planning of enhanced special surveys of bulk carriers. As indicated
in new 5.1.5 of Resolution A.744(18), the guidelines are a recommended
tool which may be invoked at the discretion of an Administration,
when considered necessary and appropriate, in conjunction with the
preparation of the required survey programme.
2 Purpose and Principles
2.1 Purpose
The purpose of the technical assessments described in these
guidelines is to assist in identifying critical structural areas,
nominating suspect areas and in focusing attention on structural elements
or areas of structural elements which may be particularly susceptible
to, or evidence a history of, wastage or damage. This information
may be useful in nominating locations, areas, holds and tanks for
thickness measurement, close-up survey and tank testing.
2.2 Minimum Requirements
These guidelines may not be used to reduce the requirements
pertaining to thickness measurement, close-up survey and tank testing
contained in annexes 1 and 2 and paragraph
2.7 respectively, of annex A of Resolution A.744(18) which
are, in all cases, to be complied with as a minimum.
2.3 Timing
As with other aspects of survey planning, the technical
assessments described in these guidelines should be worked out by
the owner or operator in cooperation with the Administration well
in advance of the commencement of the renewal survey, i.e., prior
to commencing the survey and normally at least 12 to 15 months before
the survey's completion due date.
2.4 Aspects to be Considered
Technical assessments, which may include quantitative or
qualitative evaluation of relative risks of possible deterioration,
of the following aspects of a particular ship may be used as a basis
for the nomination of holds, tanks and areas for survey:
- design features such as stress levels on various structural elements,
design details and extent of use of high tensile steel.
- former history with respect to corrosion, cracking, buckling,
indents and repairs for the particular ship as well as similar vessels,
where available.
- information with respect to types of cargo carried, protection
of tanks, and condition of coating, if any, of holds and tanks.
Technical assessments of the relative risks of susceptibility
to damage or deterioration of various structural elements and areas
should be judged and decided on the basis of recognized principles
and practices, such as may be found in reference 3.
3 Technical Assessment
3.1 General
There are three basic types of possible failure which may
be the subject of technical assessment in connection with planning
of surveys; corrosion, cracks and buckling. Contact damages are not
normally covered by the survey plan since indents are usually noted
in memoranda and assumed to be dealt with as a normal routine by Surveyors.Technical
assessments performed in conjunction with the survey planning process
should, in principle be as shown schematically in Figure 1 which depicts, schematically,
how technical assessments can be carried out in conjunction with the
survey planning process. The approach is based on an evaluation of
experience and knowledge basically related to:
The design should be considered with respect to structural details
which may be susceptible to buckling or cracking as a result of vibration,
high stress levels or fatigue. Corrosion is related to the ageing
process, and is closely connected with the quality of corrosion protection
at newbuilding, and subsequent maintenance during the service life.
Corrosion may also lead to cracking and/or buckling.
Figure 1 Planning Process Technical Assessment and the Survey
3.2 Methods
3.2.1 Design Details
Damage experience related to the ship in question and similar
ships, where available, is the main source of information to be used
in the process of planning. In addition, a selection of structural
details from the design drawings should be included. Typical damage
experience to be considered will consist of:
- number, extent, location and frequency of cracks.
- location of buckles.
This information may be found in the survey reports and/or the
owner's files, including the results of the owner's own inspections.
The defects should be analyzed, noted and marked on sketches.In addition,
general experience should be utilized. For example, Figure 2 shows typical locations
in bulk carriers which experience has shown may be susceptible to
structural damage. Also, reference should be made to reference 3 which
contains a catalogue of typical damages and proposed repair methods
for various bulk carrier structural details.Such figures should be
used together with a review of the main drawings, in order to compare
with the actual structure and search for similar details which may
be susceptible to damage. An example is shown in Figure 3. The review of the
main structural drawings, in addition to using the above mentioned
figures, should include checking for typical design details where
cracking has been experienced. The factors contributing to damage
should be carefully considered.The use of high tensile steel (HTS)
is an important factor. Details showing good service experience where
ordinary, mild steel has been used may be more susceptible to damage
when HTS, and its higher associated stresses, are utilized. There
is extensive and, in general, good experience, with the use of HTS
for longitudinal material in deck and bottom structures. Experience
in other locations, where the dynamic stresses may be higher, is less
favorable, e.g. side structures.In this respect, stress calculations
of typical and important components and details, in accordance with
relevant methods, may prove useful and should be considered.The selected
areas of the structure identified during this process should be recorded
and marked on the structural drawings to be included in the survey
programme.
Figure 2 Typical locations susceptible to structural damage or
corrosion
| AREA 1
|
Structural item
|
Side shell
frames and end brackets
(Seperate bracket
configuration)
|
EXAMPLE 1
|
| Detail of
damage
|
Fractures
on brackets at termination of frame
|
|
|
| Notes on
possible cause of damage/repair
1 This type
of damage is due to stress concentration.
2 For small fractures, e.g. hairline fractures, the
fracture can be 'veed' out, welded up, ground and examined by
NDT for fractures.
3 For
larger/significant fractures consideration should be given to
cropping and partly renewing/renewing the frame brackets. If
renewing the brackets, ends of frames can be shaped to soften
them.
4 If felt prudent, soft toes
should be incorporated at the boundaries of the bracket to the
wing tanks.
5 Attention to be given to
the structure in wing tanks in way of the extended bracket arm
i.e. reinforcement provided in line with the bracket
arm.
|
Figure 3 Typical damage and repair example (reproduced from ref.3)
3.2.2 Corrosion
In order to evaluate relative corrosion risks, the following
information is generally to be considered:
- usage of tanks, holds and spaces
- condition of coatings
- condition of anodes
- cleaning procedures
- previous corrosion damage
- ballast use and time for cargo holds
- risk of corrosion in cargo holds and ballast tanks
- location of ballast tanks adjacent to heated fuel oil tanks
reference 2 gives
definitive examples which can be used for judging and describing coating
condition, using typical pictures of conditions. For bulk carriers, reference 3 should be used as
the basis for the evaluation, together with the age of the ship and
relevant information on the anticipated condition of the ship as derived
from the information collected in order to prepare the survey programme.
The various tanks, holds and spaces should be listed with the corrosion
risks nominated accordingly.
3.2.3 Locations for Close-Up Survey and Thickness
Measurement
On the basis of the table of corrosion risks and the evaluation
of design experience, the locations for initial close-up survey and
thickness measurement (sections) may be nominated. The sections subject
to thickness measurement should normally be nominated in tanks, holds
and spaces where corrosion risk is judged to be the highest. The nomination
of tanks, holds and spaces for close-up survey should, initially,
be based on highest corrosion risk, and should always include ballast
tanks. The principle for the selection should be that the extent is
increased by age or where information is insufficient or unreliable.
4 References
1 TSCF, “Guidelines Manual for the Inspection
and Condition Assessment of Tanker Structures, 1986.”
2 TSCF, “Condition Evaluation and Maintenance
of Tanker Structures, 1992.”
3 IACS, “Bulk Carriers: Guidelines for Surveys,
Assessment and Repair of Hull Structures, 1994.”
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