Annex 11 - Guidelines for Technical Assessment in Conjunction with the Planning of Enhanced Surveys for Oil Tankers

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 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

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 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.

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 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, where available;
information with respect to types of cargo carried, use of different tanks for cargo/ballast, protection of tanks and condition of coating, if any.

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

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 and corrosion.

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 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 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 programme.

3.2.2 Corrosion

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 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 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.

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 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
TYPICAL DAMAGE		PROPOSED REPAIR

FACTORS CONTRIBUTING TO DAMAGE 1 Asymmetrical connection of flat bar stiffener resulting in high peak stresses at the heel of the stiffener under fatigue loading. 2 Insufficient area of connection of longitudinal to web plate. 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.
FIGURE 1	TANKER STRUCTURE CO-OPERATIVE FORUM		FIGURE 1
	SUBJECT: CATALOGUE OF STRUCTURAL DETAILS