Annex 11 – Guidelines for Technical
Assessment in Conjunction with the Planning of Enhanced Surveys for Oil Tankers Other Than
Double-Hull Oil Tankersfootnote
1 Introduction
These Guidelines contain information and suggestions concerning
technical assessments which may be of use in conjunction with the planning of
enhanced renewal surveys of oil tankers. As indicated in 5.1.5 of the
Code, these 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 and tanks
for thickness measurement, close-up survey and tank testing.
2.2
Timing
As with other aspects of survey planning, the technical assessments
described in these Guidelines should be carried 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.
2.3
Aspects to be considered
2.3.1 Technical assessments, which may include
quantitative or qualitative evaluation of relative risks of possible
deterioration, of the following aspects of a particular ship should be used as a
basis for the nomination of tanks and areas for survey:
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.1 design features such as stress levels on
various structural elements, design details and extent of use of
high-tensile steel (HTS);
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.2 former history with respect to corrosion,
cracking, buckling, indents and repairs for the particular ship as well as
similar vessels, where available; and
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.3 information with respect to types of cargo
carried, use of different tanks for cargo/ballast, protection of tanks and
condition of coating, if any.
2.3.2 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 references 1 and 2.
3 Technical assessment
3.1
General
3.1.1 There are three basic types of possible failure
which should 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.
3.1.2 Technical assessments performed in conjunction
with the survey planning process should, in principle, be as shown schematically
in figure 1. The approach is basically an evaluation of the risk, based on the
knowledge and experience related to design and corrosion.
3.1.3 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.
3.1.4 Corrosion is related to the ageing process, and
is closely connected with the quality of corrosion prevention systems fitted at
new building, and subsequent maintenance during the service life. Corrosion may
also lead to cracking and/or buckling.
3.2
Methods
3.2.1
Design details
3.2.1.1 Damage experience related to the ship in
question and sister and/or similar ships, where available, should be the main
source of information used in the process of planning. In addition, a selection
of structural details from the design drawings should be included.
3.2.1.2 Typical damage experience which should be
considered will consist of:
3.2.1.3 This information should 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 analysed, noted and marked on
sketches.
3.2.1.4 In addition, general experience should be
utilized. For example, reference should be made to reference 1, which contains
a catalogue of typical damages and proposed repair methods for various tanker
structural details.
3.2.1.5 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 2.
3.2.1.6 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.
3.2.1.7 The use of 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, is 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 favourable, e.g. side structures.
3.2.1.8 In this respect, stress calculations of
typical and important components and details, in accordance with relevant
methods, may prove useful and should be considered.
3.2.1.9 The selected areas of the structure
identified during this process should be recorded and marked on the structural
drawings which should be included in the survey programme.
3.2.2
Corrosion
3.2.2.1 In order to evaluate relative corrosion
risks, the following information should generally be considered:
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.1 usage of tanks and spaces;
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.2 condition of coatings;
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.3 cleaning procedures;
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.4 previous corrosion damage;
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.5 ballast use and time for cargo tanks;
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.6 corrosion risk scheme (see reference 2,
table 3.1); and
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.7 location of heated tanks.
3.2.2.2 Reference 2 gives definitive examples
which may be used for judging and describing coating condition, using typical
pictures of conditions.
3.2.2.3 The evaluation of corrosion risks should
be based on information in reference 2, 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.
3.2.2.4 The various tanks and spaces should be
listed with the corrosion risks nominated accordingly.
3.2.3
Locations for close-up survey and thickness
measurement
3.2.3.1 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 (areas and sections) should be
nominated.
3.2.3.2 The sections subject to thickness
measurement should normally be nominated in tanks and spaces where corrosion
risk is judged to be highest.
3.2.3.3 The nomination of tanks and spaces for
close-up survey should, initially, be based on where the corrosion risk is
judged to be highest, and should always include ballast tanks. The principle
for the selection should be that the extent is increased with age or where
information is insufficient or unreliable.
References
1 TSCF, Guidance Manual for the Inspection and
Condition Assessment of Tanker Structures, 1986.
2 TSCF, Condition Evaluation and Maintenance of
Tanker Structures, 1992.
Figure 1: Technical
Assessment and the Survey Planning Process
Figure 2 – Typical damage
and repair example (reproduced from reference 1)
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