Guidance Manual for Tanker Structures - Consolidated Edition 2022

This revision includes IACS Recommendation No. 72: Confined Space Safe Practice – Rev.3 Dec 2018 and IACS Unified Requirements (UR) Survey and Certification: Z10.4 Hull Surveys of Double Hull Oil Tankers (Rev.16 May 2019), where changes introduced in Rev.16 are to be uniformly applied by IACS Societies for surveys commenced on or after 1 July 2020.

Introduction to the 1997 Edition

This manual combines in one document the various sources of information necessary for assessing the condition of ballast and cargo tank structure of tankers in service. The manual is based on the experience and practices of members of the Tanker Structure Co-operative Forum* consisting of Owners, Operators and Classification Societies.

The following organisations were members of the Tanker Structure Co-operative Forum at the time of compiling this revision:

A.P. Møller Gotaas Larsen Limited American Bureau of Shipping Keystone Shipping Company Amoco Transport Company Lloyd’s Register of Shipping Arco Marine Incorporated Maritime Overseas Corporation

Bergesen d.y. A/S Mobil Shipping and Transportation Company

BP Shipping Limited Nippon Kaiji Kyokai

Bureau Veritas NYK Line

Chevron Shipping Company Shell International Trading and Shipping Co. Ltd. Det Norske Veritas Stena Rederi AB

Exxon Company, International Stolt Parcel Tankers Inc. Germanischer Lloyd Texaco Marine Services Incorporated

Guidance is given first on survey preparation, including a review of requirements, safety aspects, equipment, and details of carrying out and reporting different types of surveys. The analysis of data follows with the interpretation of wastage and structural defects in terms of the effects on local strength or overall structural integrity. Basic maintenance and repair guidelines are provided along with sketches of experienced structural failures and proposed repairs. The manual incorporates, where appropriate, the results of Forum activities in areas of corrosion, defects and repairs, ultrasonic gauging and the strength of corroded structural items, as well as corrosion and maintenance evaluation of tanker structures.

The International Association of Classification Societies (IACS) has co-operated in the updating and revision of the Manual.

* Appendix I contains the background to the Forum activities.

Guidance Manual for Tanker Structures

Introduction to the 2022 Consolidated Edition

This consolidated edition combines in one guidance manual the various sources of information necessary for assessing the condition of ballast and cargo tank structure of tankers in service, while the original manual mainly addressed the conventional single hull tanker, this revision includes IACS Recommendation No. 72: Confined Space Safe Practice - Rev.3 Dec 2018 and IACS Unified Requirements (UR) Survey and Certification: Z10.4 Hull Surveys of Double Hull Oil Tankers (Rev.16 May 2019), where changes introduced in Rev.16 are to be uniformly applied by IACS Societies for surveys commenced on or after 1 July 2020.

Phase-Out of Single Hull Tankers

Since 2016, single hull tankers (SHTs) have been phased out of international trading. Since 1993, double hull structures are the mandatory standard for tanker design, as detailed in the 1996 requirements of MARPOL Annex 1, Regulation 19.

Phase-Out Process

Following a series of high profile tanker accidents and spills in the 1990s, MARPOL was amended to begin the process of a gradual phase-out of the SHT. Timings were based on tanker category.

Due to the impact of the sinking of the ‘MV Erika’, the IMO adopted an accelerated phase-out scheme, which entered into force on 1st September 2003, under the ‘Regulation 13G’ amendments of MARPOL Annex 1. The phase-out process was further accelerated by additional amendments that entered into force on 5th April 2005 (note that Regulation 13G is now incorporated as Regulation 20 in the revised MARPOL Annex I, which itself entered into force on 1 January 2007).

Under MARPOL, the final phasing out date for Category 1 tankers (pre-MARPOL) was 2005. The final phasing out date for category 2 and 3 tankers (MARPOL tankers and smaller tankers) was brought forward to 2010, from 2015. In addition, a new Regulation (Regulation 21) for vessels carrying heavy grade oil (HGO) banned the carriage of HGO in SHT of 5,000 tons dwt and above after 5th April 2005, and in SHT of 600 tons dwt and above, but less than 5,000 tons dwt, by not later than 2009.

Exceptions from 2010

The 2010 date remained the phase-out date for most of the global SHT tonnage. However, four exceptions permitted the use of SHTs after 2010, which were:

1. If the vessel was engaged in cabotage trade i.e. trading locally and not internationally (the IMO extended this option to allow for some member States to achieve renewal for their domestic fleets, where the vessels were confined to operate only within the member State)

2. Theoretically, if the vessel was engaged in trade between States where neither were signatories to the MARPOL 1973/1978 Convention (although this is effectively nil, as 99.1% of world tonnage is under a Contracting State that has ratified MARPOL)

3. If the vessel was not engaged in the carriage of oil (provided that the ship was in service on 1 July 2001, fitted with only double bottoms or double sides that extend to the entire cargo tank length or double hull spaces, not meeting the technical minimum distance protection requirements and that the flag State was satisfied. Again, such continued operation cannot go beyond the date on which the ship reaches 25 years of age after the date of its delivery)

4. If the vessel operated under the Condition Assessment Scheme (CAS), as approved in IMO Resolution 94(46). However, such operation is now phased out.

Introduction to the 2020 Consolidated Edition

Phase-Out internationally complete

The CAS allowed individual flag States to permit continued operation of Category 2 and 3 tankers beyond 2010, albeit subject to satisfactory inspection and results. However, the continued operation was not allowed to go beyond the anniversary of the ‘date of delivery of the ship’ in 2015, or the date on which the ship reached 25 years of age after the date of its delivery, whichever was earlier (note that the CAS process was later implemented under Annex B, part B of the Enhanced Survey Programme (ESP Code)). This time limit was passed in 2016, meaning that it is not permissible for an SHT to operate internationally between MARPOL member States. Therefore, most of

the global SHT fleet has now been scrapped. In addition, the IMO has amended documents to reflect the phase out of the SHT; for example on 1 March 2018, amendments to MARPOL Annex I entered into force which removed references to SHT designs and arrangements from Form B of the International Oil Pollution Prevention (IOPP) Certificate.

Only very limited numbers remain

The number of SHT vessels remaining in the world is, therefore, very low. Those that do remain are strictly limited in their trading and operational abilities to within the national boundaries of their member State, and to those not carrying oil cargoes (subject to the other requirements specified above).

The few member states still operating SHTs, have already begun the process of full phase-out in their national legislation. Therefore, it is likely the few remaining operational SHTs will be phased out within the next few years. Double hulled vessels are, therefore, effectively considered the universal standard for new and existing tankers in international maritime operations.

Introduction to the 1997 Edition v

Introduction to the 2022 Consolidated Edition vi

Chapter One – Survey Preparation Guidelines 3

1.1 Survey Requirements 3

1.1.1 Class and Statutory Requirements 3

1.1.2 Owner’s Requirements 4

1.2 Safety and Access 5

1.2.1 Safety During Surveys 5

1.2.2 Access to the Structure 7

1.3 Ultrasonic Thickness Determination 8

1.3.1 Survey Team Qualifications 8

1.3.2 Basic Equipment Types 9

1.3.3 Thickness Measurement Accuracy 10

1.3.4 Measurement Procedures 11

1.4 Technical Background for Surveys 12

1.4.1 Nomenclature 12

1.4.2 Structural Load Descriptions 13

1.4.3 Structural Defects 13

1.4.4 Fatigue 14

1.4.5 Typical Corrosion Patterns 15

1.4.6 Factors Influencing Corrosion 15

1.4.7 Corrosion Trends in Tank Spaces 18

1.4.8 In-Service Corrosion Rates 19

1.4.9 Corrosion Control Systems 19

Chapter Two – Survey Execution Guidelines 33

2.1 Introduction 33

2.2 General Planning Requirements 33

2.2.1 General 33

2.2.2 Data Collection and Reporting — Databases 33

Guidance Manual for Tanker Structures

2.3 Structural Aspects 34

2.3.1 Tank Bottom Structures 34

2.3.2 Side Shell, Longitudinal and Transverse Bulkheads 35

2.3.3 Deckhead Structures 35

2.4 Risk Categories 35

2.4.1 Corrosion 35

2.4.2 Structural Elements 35

2.5 Classification Society Surveys/IACS Unified Requirements for Hull Surveys of Double Hull Oil Tankers 37

2.5.1 General 37

2.5.2 Special Survey 37

2.5.3 Annual Survey 37

2.5.4 Intermediate Survey 38

2.5.5 Preparation for Survey 38

2.6 Owner’s Surveys 42

2.6.1 General 42

2.6.2 General Condition Survey 42

2.6.3 Detailed Condition Survey 43

2.6.4 Corrosion Rate Survey 44

2.6.5 Repair Specification Survey 44

Chapter Three – Survey Data Analysis Guidelines 49

3.1 Method of Assessment 49

3.1.1 Steel Condition 49

3.1.2 Coating Condition Assessment 49

3.1.3 Cathodic Protection System Assessment 50

3.2 Structural Integrity 50

3.3 Acceptance Criteria 50

Chapter Four - Maintenance and Repair Guidelines 57

4.1 General 57

4.2 Type of Repair 57

4.3 Maintenance and Repair Methods 57

4.3.1 Repairs of Existing Coated Areas in Segregated Ballast Tanks 57

4.3.2 Repairs of Uncoated Areas in Cargo Tanks 58

Introduction to the 2020 Consolidated Edition

4.3.3 Pitting and Grooving Repair 58

4.3.4 Steel Renewal 58

4.3.5 Steel Reinforcement 58

4.3.6 Steel Design Modification 58

4.3.7 Repairs of Structural Defects 58

4.4 Optimum Maintenance and Repair Strategy 59

4.4.1 General 59

4.4.2 Short-Term Maintenance and Repair Program 59

4.4.3 Long-Term Maintenance and Repair Program 59

Bibliography 61

Appendix I Background to Forum Activities 63

Appendix II IACS Unified Requirements UR Z10.4 69

Appendix II IACS Rec No. 72: Confined Space Safe Practice 157

Appendix IV Catalogue of Structural Detail Failures 177

Appendix V In-Service Corrosion Rate Studies 261

Appendix VI Structural Inspection Guidelines 267

Appendix VII Assessment of Existing Surface Coating Systems 281