No. 96    Double Hull Oil Tankers - Guidelines for Surveys,

(April    Assessment and Repair of Hull Structures

2007)

IACS

INTERNATIONAL ASSOCIATION OF CLASSIFICATION SOCIETIES

DOUBLE HULL OIL TANKERS

Guidelines for Surveys, Assessment and Repair of Hull Structures

2 CLASSIFICATION SURVEY REQUIREMENTS

2 Classification Survey Requirements

2.1    General

2.1.1    The programme of periodical surveys is of prime importance as a means for assessment of the structural condition of the hull, in particular, the structure of cargo and ballast tanks. The programme consists of Special (or Renewal) Surveys carried out at five-year interval with Annual and Intermediate Surveys carried out in between Special Surveys.

2.1.2    Since 1991, it has been a requirement for new oil tankers to apply a protective coating to the structure in water ballast tanks, which form part of the hull boundary.

2.1.3    From 1 July 2001, oil tankers of 20,000 DWT and above, to which the Enhanced Survey Programme (ESP) requirements apply, starting with the 3^rd Special Survey, all Special and Intermediate hull classification surveys are to be carried out by at least two exclusive Surveyors. Further, one exclusive Surveyor is to be on board while thickness measurements are taken to the extent necessary to control the measurement process. From 1 July 2005, thickness measurements of structures in areas where close-up surveys are required are to be carried out simultaneously with close-up surveys. Refer to I ACS PR 19 and PR 20.

2.1.4    The detailed survey requirements complying with ESP are specified in the Rules and Regulations of each IACS Member Society.

2.1.5    ESP is based on two principal criteria: the condition of the coating and the extent of structural corrosion. Of primary importance is when a coating has been found to be in a “less than good” condition (“good” is with only minor spot rusting) or when a structure has been found to be substantially corroded (i.e. a wastage between 75 % and 100 % of the allowable diminution for the structural member in question). Note, for vessels built under the IACS Common Structural Rules, substantial corrosion is an extent of corrosion such that the assessment of the corrosion pattern indicates a gauged (or measured) thickness between t_net + 0.5mm and W

Reference is also made to SOLAS 74 as amended regulation Part A-1/3.2 regarding corrosion protection system for seawater ballast tanks at time of construction.

2.2 Annual Surveys

2.2.1 The purpose of an Annual Survey is to confirm that the general condition of the hull is maintained at a satisfactory level.

2.2.2 Generally as the ship ages, ballast tanks are required to be subjected to more extensive overall and close-up surveys at Annual Surveys.

2 CLASSIFICATION SURVEY REQUIREMENTS

2.2.3 In addition, a Ballast Tank is to be examined at annual intervals where:

a.    a hard protective coating has not been applied from the time of construction, or

b.    a soft coating has been applied, or

c.    substantial corrosion is found within the tank at a previous survey, or

d.    the hard protective coating is found to be in less than GOOD condition and the hard protective coating is not repaired to the satisfaction of the Surveyor at a previous survey.

2.3 Intermediate Surveys

2.3.1    The Intermediate Survey may be held at or between the second or third Annual Survey in each five year Special Survey cycle. Those items, which are additional to the requirements of the Annual Surveys, may be surveyed either at or between the 2^nd and 3^rd Annual Survey. The intermediate survey contains requirements for extended overall and close-up surveys including thickness measurements of cargo and ballast tanks.

2.3.2    Areas in ballast tanks and cargo tanks found suspect at the previous surveys are subject to overall and close-up surveys, the extent of which becomes progressively more extensive commensurate with the age of the vessel.

2.3.3    For oil tankers exceeding 10 years of age, the requirements of the Intermediate Survey are to be of the same extent as the previous Special Survey. However, pressure testing of cargo and ballast tanks and the requirements for longitudinal strength evaluation of Hull Girder are not required unless deemed necessary by the attending Surveyor.

2.4 Special Surveys

2.4.1    The Special (or Renewal) Surveys of the hull structure are carried out at five-year intervals for the purpose of establishing the condition of the structure to confirm that the structural integrity is satisfactory in accordance with the Classification Requirements, and will remain fit for its intended purpose for another five-year period, subject to proper maintenance and operation of the ship and to periodical surveys carried out at the due dates.

2.4.2    The Special Survey concentrates on close-up surveys in association with thickness measurements and is aimed at detecting fractures, buckling, corrosion and other types of structural deterioration. See Figure 7.

2.4.3    Thickness measurements are to be carried out upon agreement with the Classification Society concerned in conjunction with the Special Survey.

The Special Survey may be commenced at the 4^th Annual Survey and be progressed with a view to completion by the 5^th anniversary date.

2.4.4    Deteriorated protective coating in less than good condition in salt water ballast spaces and structural areas showing substantial corrosion and/or considered by the

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Surveyor to be prone to rapid wastage will be recorded for particular attention during the following survey cycle, if not repaired at the special survey.

2.5 Drydocking (Bottom) Surveys

2.5.1    There is to be a minimum of two examinations of the outside of the ship’s bottom and related items during each five-year special survey period. One such examination is to be carried out in conjunction with the special survey. In all cases the interval between any two such examinations is not to exceed 36 months. An extension of examination of the ship’s bottom of 3 months beyond the due date can be granted in exceptional circumstances. Refer to IACS Unified Requirement Z3.

2.5.2    For oil tankers of 15 years of age and over, survey of the outside of the ship’s bottom is to be carried out with the ship in dry dock. For oil tankers less than 15 years of age, alternative surveys of the ship’s bottom not conducted in conjunction with the Special Survey may be carried out with the ship afloat. Survey of the ship afloat is only to be carried out when; the conditions are satisfactorily and the proper equipment and suitably qualified staff are available.

2.6 Damage and repair surveys

2.6.1    Damage surveys are occasional surveys, which are, in general, outside the programme of periodical hull surveys and are requested as a result of hull damage or other defects. It is the responsibility of the owner or owner’s representative to inform the Classification Society concerned when such damage or defect could impair the structural capability or watertight integrity of the hull. The damages should be inspected and assessed by the Society’s Surveyors and the relevant repairs, if needed, are to be performed. In certain cases, depending on the extent, type and location of the damage, permanent repairs may be deferred to coincide with the planned periodical survey.

Any damage in association with wastage over the allowable limits (including buckling, grooving, detachment or fracture), or extensive areas of wastage over the allowable limits, which affects or, in the opinion of the Surveyor, will affect the vessel’s structural watertight or weathertight integrity, is to be promptly and thoroughly repaired. Areas to be considered to are to include:

-    bottom structure and bottom plating;

-    side structure and side plating;

-    deck structure and deck plating;

-    watertight or oiltight bulkheads.

2.6.2    In cases of repairs intended to be carried out by riding crew during voyage, the complete procedure of the repair, including all necessary surveys, is to be submitted to and agreed upon by the Classification Society reasonably in advance.

2.6.3    IACS Unified Requirement Z13 “Voyage Repairs and Maintenance” provides useful guidance for repairs to be carried out by a riding crew during a voyage.

3 TECHNICAL BACKGROUND FOR SURVEYS

3 Technical Background for Surveys

3.1    General

3.1.1    The purpose of carrying out a structural survey of any tank is to determine the extent of corrosion wastage and structural defects present in the tank. To help achieve this and to identify key locations in the tank that might warrant special attention, the Surveyor should be familiar with the service record of the tank and any historical problems of the particular vessel or other vessels of a similar class.

An experienced Surveyor will be aware of typical structural defects likely to be encountered and some knowledge of the contributing factors to corrosion (including the effectiveness of corrosion control systems) will assist him in assessing the corrosion patterns he finds.

3.2 Definitions

3.2.1 For clarity of definition and reporting of survey data, it is recommended that

standard nomenclature for structural elements be adopted. A typical midship section is

illustrated in Figures 4 to 6. These figures show the generally accepted nomenclature.

The terms used in these guidelines are defined as follows:

(a)    A Ballast Tank is a tank, which is used solely for the carriage of salt water ballast.

(b)    A Combined Cargo/Ballast Tank is a tank, which is used for the carriage of cargo, or ballast water as a routine part of the vessel’s operation and will be treated as a Ballast Tank. Cargo tanks in which water ballast might be carried only in exceptional cases per MARPOL 1/13(3) are to be treated as cargo tanks.

(c)    An Overall Survey is a survey intended to report on the overall condition of the hull structure and determine the extent of additional Close-up Surveys.

(d)    A Close-up Survey is a survey where the details of structural components are within the close visual inspection range of the Surveyor, i.e. normally within reach of hand.

(e)    A Transverse Section includes all longitudinal members such as plating, longitudinals and girders at the deck, sides, bottom, inner bottom and longitudinal bulkheads.

(f)    Representative Tanks are those, which are expected to reflect the condition of other tanks of similar type and service and with similar corrosion prevention systems. When selecting Representative Tanks account is to be taken of the service and repair history onboard and identifiable Critical Structural Areas and/or Suspect Areas.

Note: Critical Structural Areas are 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 (if available) to be sensitive to cracking, buckling or corrosion, which

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Contents

Introduction

Classification survey requirements

2.1    General

2.2    Annual Surveys

2.3    Intermediate Surveys

2.4    Special Surveys

2.5    Drydocking (Bottom) Surveys

2.6    Damage and Repair Surveys

Technical background for surveys

3.1    General

3.2    Definitions

3.3    Structural Load Descriptions

3.4    Structural defects, damages and deterioration

3.5    Structural detail failures and repairs

Survey planning, preparation and execution

4.1    General

4.2    Survey Programme

4.3    Survey Planning Meeting

4.4    Conditions for survey

4.5    Access arrangement and safety

4.6    Use of Boats and Rafts

4.7    Personal equipment

4.8    Thickness measurement and fracture detection

4.9    Survey at sea or at anchorage

4.10    Documentation on board

4.11    Reporting and Evaluation of Survey

Structural detail failures and repairs

5.1    General

5.2    Actions to be taken by the Classification Society when Fatigue Failures have been Identified

5.3    Catalogue of structural detail failures and repairs

1    Bilge Hopper

2    Wing Ballast Tank

3    Bottom Ballast Tank

4    Web Frames in Cargo Tanks

5    Transverse Bulkhead in Cargo Tank

6    Deck Structure

7    Fore and Aft End    Regions

8    Machinery and Accommodation Spaces

1 INTRODUCTION

1 Introduction

The International Association of Classification Societies (IACS) is introducing a series of manuals with the intention of giving guidelines to assist the Surveyors of IACS Member Societies, and other interested parties involved in the survey, assessment and repair of hull structures for certain ship types.

This manual gives guidelines for a double hull oil tanker which is constructed primarily for the carriage of oil in bulk and which has the cargo tanks protected by a double hull which extends for the entire length of the cargo area, consisting of double sides and double bottom spaces for the carriage of water ballast or void spaces. Figures 1 & 2 show the general views of typical double hull oil tankers with two longitudinal bulkheads or one centreline longitudinal bulkhead respectively.

Figure 1 General view of a typical double hull oil tanker (150,000 DWT and greater)

INTRODUCTION

The guidelines focus on the IACS Member Societies’ survey procedures but may also be useful in connection with survey/examination schemes of other regulatory bodies, owners and operators.

The manual includes a review of survey preparation guidelines, which cover the safety aspects related to the performance of the survey, the necessary access facilities, and the preparation necessary before the surveys can be carried out.

The survey guidelines encompass the different main structural areas of the hull where damages have been recorded, focusing on the main features of the structural items of each area.

An important feature of the manual is the inclusion of the section, which illustrates examples of structural deterioration and damages related to each structural area and gives what to look for, possible cause, and recommended repair methods, when considered appropriate.

This manual has been developed using the best information currently available. It is intended only as guidance in support of the sound judgment of Surveyors, and is to be used at the Surveyors' discretion. It is recognized that alternative and satisfactory methods are already applied by Surveyors. Should there be any doubt with regard to interpretation or validity in connection with particular applications, clarification should be obtained from the Classification Society concerned.

Surveyors dealing with single hull oil tankers should be encouraged to read the “Guidance Manual for Oil Tankers” by Tanker Structure Co-operative Forum.

IACS Common Structural Rules for Tankers implemented from April 2006 have been

INTERNATIONALASSOCIATION OF CLASSIFICATION SOCIETIES_

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developed in response to a consistent and persistent call from industry for an increased standard of structural safety. This has been achieved through enhancing the design basis and applying engineering first principles. The development of the CSR for Tankers included review of existing Rules, new development using a first principle approach, application of the net thickness philosophy, an enhanced design environment and a longer life i.e. 25 years North Atlantic. These Rules are applicable to double hull oil tankers exceeding a length of 150 metres.

Note:    Throughout    this    document    reference    is    made    to    various    IACS    Unified

Requirements (UR), Procedural Requirements (PR) and Recommendations. All URs and PRs and key recommendations are available from the IACS website (http://www.iacs.org.uk).

INTRODUCTION