Annex 11 - Guidelines for
Technical Assessment in Conjunction with the Planning of Enhanced
Surveys for Oil Tankers
These guidelines contain information and suggestions concerning
technical assessments which may be of use in conjunction with the
planning of enhanced special surveys of oil tankers. 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
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 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, 2 and 3 respectively, of Annex B of resolution
A.744(18), which are, in all cases, to be complied with as a minimum.
As with other aspects of survey planning, the technical
assessments described in these guidelines should be worked out by
the owner or operator in co-operation 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 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,
- information with respect to types of cargo carried, use of different
tanks for cargo/ballast, protection of tanks and condition of coating,
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
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 basically an evaluation
of the risk based on the knowledge and experience related to design
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.
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 analysed, noted and marked on sketches. 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.
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. 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
In order to evaluate relative corrosion risks, the following
information is generally to be considered:
- usage of tanks and spaces
- condition of coatings
- condition of anodes
- cleaning procedures
- previous corrosion damage
- ballast use and time for cargo tanks
- corrosion risk scheme (see reference 2, Table 3.1)
- location of heated tanks
Reference 2 gives definitive examples which can be used for
judging and describing coating condition, using typical pictures of
conditions.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 as derived from the information collected
in order to prepare the survey programme. The various tanks and spaces
should be listed with the corrosion risks nominated accordingly.
3.2.3 Locations for close-up survey and
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 and
spaces where corrosion risk is judged to be the highest. The nomination
of tanks 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.
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 Planning Process Technical Assessment and the Survey
Figure 2 Typical damage and
repair example (reproduced from ref.1)
LOCATION: Connection of longitudinals to transverse webs
EXAMPLE No.1 Web and flat bar fractures at cut-outs for
longitudinal stiffener connections
FACTORS CONTRIBUTING TO DAMAGE
1 Asymmetrical connection of flat bar stiffener resulting in
high peak stresses at the heel of the stiffener under fatigue
2 Insufficient area of connection of longitudinal to web
3 Defective weld at return around the plate thickness.
4 High localized corrosion at areas of stress concentration
such as flat bar stiffener connections, corners of cut-out for the
longitudinal and connection of web to shell at cutouts.
5 High shear stress in the web of the transverse.
6 Dynamic sea way loads/ship motions.
|TANKER STRUCTURE CO-OPERATIVE FORUM
|SUBJECT: CATALOGUE OF STRUCTURAL DETAILS