Joint Tanker Project Development of Common Structural Rules for Oil Tankers

Joint Tanker Project – next generation of rules for tankers

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Joint Tanker Project

Development of Common Structural Rules for Oil

Tankers


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• Overview

• Technical aspects of the JTP Rules

• Relationship with IACS (JBP)

• The Future


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Aim

To develop a set of unified Rules and Procedures for the determination of the structural requirements for oil tankers


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Objective

• To eliminate competition between class societies with regard to structural requirements and standards

• To employ the combined experience of all three societies to develop a single agreed standard, or set of Rules.

• To ensure that a tanker meeting this new standard will be recognised by industry as being at least as safe and robust as would have been required by any of the existing Rules

• To embrace the intentions of the anticipated IMO requirements for goal based new construction standards.


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Organisation

• The project is being led by a Steering Group, reporting to the CEOs of each society.

• A team drawn from ABS, DNV and LR manages the technical work packages, using technical experts drawn equally from the three societies.

• An External Review Group of senior, experienced technical experts is providing an independent industry input throughout the project.

• The Technical Committees of ABS, DNV and LR will be given the opportunity to comment on the Rule proposals.


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Deliverables from the project

• A new complete Rule set covering the structural requirements for oil tankers for new construction and for those ships subsequently in service.

• Supporting guidance to amplify the Rules, including the procedures for carrying out direct calculations and for fatigue life assessment.

• A background document explaining the implicit safety levels, design principles and assumptions on which the Rules are based


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Scope

• The Rules will cover the entire hull structure in a consistent and logical manner, including fore and aft ends.

• The Rules will be limited to oil tankers, for crude and products, of length greater than or equal to 150m.

• The Rules will cover the hull envelope, transverse and longitudinal bulkheads, main supporting members and local structure.

• Simplified approaches for determination of fatigue life and sloshing loads will be included.


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Schedule

• The draft set of Rules will be available for comment by the Technical Committees of ABS, DNV and LR by end May 2004.

• The draft set of Rules will also be available to all IACS members at the same time

• Final Rule version approved by ABS, DNV and LR will be published 1 January 2005 and become mandatory on 1 July 2005

• Withdrawal of equivalent existing Rules by ABS, DNV and LR with effect from 1 July 2005


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Approach

• The new Rules have been developed under a coherent development framework

• The new Rules include some new approaches, where there is sufficient confidence and justification for their adoption.

• The best and most transparent methods are selected for the new Rules.

• The new Rules will be subjected to a major programme of testing and calibration to ensure that the Rules are internally consistent and reflect service experience.


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Fundamental principles of the Standard

• To ensure that the overall safety of the hull structure is equivalent to or better than that currently achieved in relation to life, environment and property.

• To ensure sufficient durability throughout the operational life in terms of corrosion margin and fatigue strength.

• To define the minimum state of the structure at which steel renewal is required in order to continue safe operation.


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Objective of RulesFramework for the JTP Rule Development

JTP safety level

Reliability

Time

Current safety level

Durability (fatigue + corrosion)


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Design Basis• Design Life – 25 years

• External Environment – North Atlantic wave environment– Air temperature : -15 degree C– Sea temperature: 0 degree C

• Internal Environment– Specific gravity cargo: Min 1025 kg/m3 (900 for

fatigue)– Specific gravity ballast: 1025 kg/m3– Max cargo temperature: 80 degree C– Min cargo temperature: 0 degree C– Corrosion margins based on 25 years design life


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Principal new elements

• Net scantling presentation is used throughout the rules

• Inclusion of new procedures for the assessment of buckling and ultimate limit state of the hull girder

• Inclusion of a new method of describing dynamic loading

• Development of a Rule format that provides transparency and ease of use


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Principles for Prescriptive Rules

• Similar to existing Rules but based on first principles with respect to loads

• Loads based on the North Atlantic sea area with 10-8 probability level of occurrence

• Presented in a working stress format

• Based on net thickness philosophy


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Software

• The new set of Rules must be supported by software tools.

• At the time of the launch of the common Rules for oil tankers suitable rule check software will be available to designers.

• ABS, DNV and LR will incorporate the requirements of the new Rules into their existing software solutions.


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Benefits to industry

• The outcome of this project will be a single, clearly recognised standard that has been developed, with active input from industry, using the combined research and experience of ABS, DNV and LR.

• The costs of dealing with a number of similar but different Rules sets will be eliminated.

• The efforts of ABS, DNV and LR will be focussed on establishing and maintaining a single Rule set.


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Joint Tanker ProjectCommon Scantlings

Oil tanker structural requirements will not be a competitive issue for ABS, DNV and LR


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• Overview


• Relationship with IACS

• The Future


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• Net thickness approach

• Ultimate Limit State

• Buckling

• Loads

• FEM

• Fatigue

• Prescriptive Rules


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Net thickness approach and wastage allowance

• The Rules are presented on the basis of determination of the net scantlings required for adequate strength, with a design corrosion margin.

• This approach will result in a unified methodology for the determination of renewal scantlings, which will provide a more transparent estimate of the corrosion margin available before renewal.

• To have a direct link between minimum thickness accepted during vessel operations and the thickness used for strength calculations at newbuild.

• Strength calculations during newbuilding are based on the net thicknesses of plating and structural members.

• Newbuilding gross thickness requirement is calculated by the addition of expected wastage during the vessel design life to the required net thicknesses.


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Net Thickness

tcorr 2.5

tgross required

tnet requiredtSiO renewal

tcorr

tw

Owner’s Extra

Corrosion allowances based on published IACS data, in mm based on 25 year design life


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Net Thickness Definitions

Wastage between inspection, local structure

The assumed wastage during inspection intervals is referred to as t corr_2.5 and links the corrosion addition and the allowable wastage thus:

t corr = t waste + t corr_2.5

The 2.5 suffix indicates inspection periods of 2.5 years as in existing survey schemes.

The JTP Rules take t corr_2.5 as 0.5 mm


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Corrosion Wastage Allowance (mm)

To 1.5 m Below Deck

Deck 3.5

Sheerstrake 3.5

Bottom 3.5Keel 4.5

Long.bhd 2

Long.bhd 2.5

Hopper 2.5

inner.bottom 3.5

Long. Girders 2.5Long stiff. 2.5

Side shell 2

Long bhd stiff 2

Inner skin 2.5

Stringer 2.5

Deck trans Web 2.5

Face plate 3.0

Long stiff. 2.5 Web plate 2.5

Face plate 3

Face plate cross-tie 3

Web cross-tie 2

Internals in upper portion of WBT 3.0

Long stiff. 2.5


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Net Thickness Philosophy for Simplified Fatigue Calculation

Simplified fatigue assessment

Local stresses calculated with average dimunition of the stiffener web, plate flange, and face plate.

t fatigue = t gr offered – 0.5 * t corr

t fatigue = t net offered + 0.5 * t corr


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Net Thickness Philosophy for FE Fatigue Calculation• Corrosion deduction used for cargo hold FE model is based

on average corrosion which is less than full corrosion designated t corr

t corr_FE cargo = 0.5 * t corr

• The cargo hold FE model is therefore built up based upon

t = t gr offered – 0.5 * t corr

• For the modelling of structural elements that act as single components (e.g. a cross tie) the full deduction t corr is to be used since the “large area reduction” does not apply.

• For local model areas with fine mesh the full deduction t corr is to be applied.


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Net Thickness Philosophy for Buckling Calculation

• The buckling capacity is based upon

t net offered = t gr offered – t corr

This ensures consistency with the approach to buckling due to pure longitudinal stresses.


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Hull girder - ultimate limit state

• Prescriptive requirements for single failure of local elements based on IACS Unified Requirements

• Introduction of a simplified ULS assessment procedure based on method under consideration within IACS

• Consideration will be given to a more comprehensive approach later


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• New buckling procedure

• Strength assessment of stiffened thin plate elements in ship structures

Buckling


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Loads

• Clear description of load definition and load application

• Load combination factors are used

• Rule design wave loads based on current IACS practices

• Load formulations have been validated by direct calculations

• Sloshing, slamming and bow impact loads use available current methods


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Head Sea Conditions


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Bow Quartering Sea


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Beam Sea


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Basic Concept of JTP load model

• Green sea loads

• Sloshing pressures

• Bottom slamming pressures

• Bow impact pressures


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Basic concept of JTP dynamic load model

Rule load at 10-4

Stress combination factors

Strength Assessment

(ULS/SLS)

Most severe load expected

Equivalent design wave

approach

Rule load at 10-8

*Load combination factors

Selected load cases for maximised responses

Long-term distribution approach

Fatigue Assessment

(FLS)

Expected load history

Weibul distribution for

total stress range


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FE Analysis Procedure

To verify the ship structure meets the required standard:

• Strength assessment

Coarse mesh FE cargo tank analysis

Fine mesh FE local structure analysis

• Fatigue assessment

Very fine mesh FE fatigue analysis


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FE Analysis Procedure Strength assessmentCargo tank FE model


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Principles for Fatigue Procedure

• Simplified fatigue procedure for end-connection of longitudinals

• Simplified fatigue procedure based on FEM for selected critical details; e.g. hopper knuckle

• Principles

• Minimum design life 25 years in North Atlantic

• Classic Palmgren-Miner cumulative fatigue damage

methodology

• Long-term stress distribution


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Fatigue Assessment Procedure

• Representation of long term distribution of stress ranges

• Design S-N curves

10

100

1000

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08

N

Str

ess

Ran

ge

(MP

a)

B

C

W

D

E F F2

G


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FE Analysis - Fatigue assessment

Dynamic fatigue loads

• Tuned on 10-4 probability level, North Atlantic

• Load combination factors - relative phase relationship between loads accounted for

• Cargo density, SG > 0,9

Fatigue loading conditions

• Full load and ballast used for dynamic loads

• Static loads included for mean stress effect


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FE Analysis - Fatigue assessment • Hot spot stress calculated using very fine mesh FE analysis (mesh

size t x t)

• Model based on average (over life) corroded thickness, tgross – 0,5 tcorr

• All potential fatigue sites to be investigated


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Rules Document Overview• The JTP Rules package:

Design Principles

Appendices

Rules

Background Document

Consequence Document


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The Rules Structure

Underpinning Principles:

• Transparent and modular

• Easy to use and identify requirements

• Logical flow core structure from design principles to design verification

• Self-containment where possible

• Inclusion of direct calculation procedures

• Comprehensive Rules but with detailed procedures in appendices

• Rules will be supported by a Background Document


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The Rules Structure

Rules Background Document:

• Contains more information or background

• Explains safety objectives

• Explains design principles

• Rules sub-section by sub-section approach

• Specific version for external use

• Useful for training new users of the Rules


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The Rules Structure

Consequence Document:

• Incorporates impact on scantling requirements applying the JTP Rules to current designs

• Covers all technical aspects but focuses on new requirements

• Continuously compiled during development, testing and hearing phases

• Available to industry


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• Overview



• The Future


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Relationship with IACS

• The new Rules take cognizance of all relevant IACS Unified Requirements

• The new Rules developed by LR, ABS and DNV have been offered to IACS and accepted as a “pilot project” with the expectation of adoption within IACS Common Structural Rules

• ABS, DNV and LR continue to work with other IACS members and to contribute to the development of the Joint Bulker Project, Unified Requirements and Unified Interpretations through the various Working Parties and other groups.


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Relationship with IACS

• Last week the JTP participants gave a presentation on common oil tanker rules to IACS in DNV offices in Oslo on Thursday 18th.

• The JBP participants carried out a similar exercise on common rules for bulk carriers in GL’s offices in Hamburg

• The JTP participants are of course LR/DNV ABS

• The JBP participants are the 7 other societies of BV/GL/NK/RINA/KR/CCS/RS

• These presentations have highlighted some basic fundamental differences in philosophy which IACS Council will now seek to resolve and harmonise prior to adoption


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• Overview



• The Future


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Rules Document Overview

Status of Preliminary JTP Rules

Joint Tanker ProjectJoint Tanker Project

Common Structural RulesCommon Structural Rules

For Oil TankersFor Oil Tankers

Preliminary DraftPreliminary Draft

JTP Preliminary Draft, 05 March 04_V1.0JTP Preliminary Draft, 05 March 04_V1.0

Appendices

Rules


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Status of Preliminary Rules

• General:

• Currently includes about 75% of the anticipated

content

• Preliminary Rules are still evolving but moving rapidly towards completion

• Calibration and internal testing ongoing


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• Current Focus:

• Development or review of outstanding text and formulations

• Further review of all incorporated text and formulations

• Ramifications of calibration and internal testing

• Integration of technical aspects

• Consistent style and presentation

• Technical and editorial proof-reading


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• Summary of Activities Leading to a Complete Rule Set by 1st May 2004

• Delivery of outstanding 25% of Rule proposals

• Major editorial and technical review of each section Review of text and formulations in the light of feed back from calibration and internal testing

• Review of the complete Rule set


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• Future Targets:

• Migrate from Preliminary Draft Version to First Draft Version by 1st May 2004

• Editorial review and preparation May 2004

• Deliver to the JTP Members’ Technical Committee hearing processes in June 2004

• Incorporation of any amendments following IACS Rules harmonization/consistency process after June 2004

• Incorporation of any amendments following TC hearings


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Joint Tanker Project Development of Common Structural Rules for Oil Tankers

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