Fatigue Design of a Shuttle Tanker for the North Atlantic ... 3.3 - (only PPT...Fatigue Design of a Shuttle Tanker for the North Atlantic Operation October 23 rd, 2013 Byung-Ki Choi*,

Fatigue Design of a Shuttle Tanker for the North Atlantic Operation

October 23rd, 2013

Byung-Ki Choi*, Byoung-Hoon Jung, Young-Ho Seo, Byeong-Rok Lee, Wha-Soo Kim

Hyundai Heavy Industries Co., Ltd.

2/28 Hyundai Maritime Research Institute Shipbuilding Division

Introduction

In general, shuttle tankers are built for the purposes such as

- Frequent loading/unloading

- Specialized for harsh offshore conditions : dynamic positioning(DP) system, redundant propulsion, bow loading systems(BLS)

HHI has designed and built the shuttle tankers for safe and efficient operation with low environmental exposure


Key Requirements

Winterization in accordance with DNV cold notation

DP 2 positioning over the Goliat extended offloading sector in weather vane and AutoPos modes as applicable to achieve close to 360 degrees operation

Bow loading system (BLS) for cold climate operations

Compatible with the North Sea offloading standard as defined in Statoil’s requirements for shuttle tankers

Optimization on energy consumption and atmospheric releases (NOx and CO2)

Focus on safety and environmental improvements


Principal Dimensions

Length : 256 m - Class : DNV

Breadth : 46 m - Notation : ICE-1C, PLUS,

Depth : 22.7 m WINTERIZED-COLD,

Draft : 15 m CSA-FLS2

DWT : 123,000 tons


Characteristics of Structural Design

Based on design of SUEZMAX class tanker

Design basis for new buildings with requirement of 30 years of fatigue life on the North Atlantic, Norwegian Sea and Barents Sea

DNV notations of PLUS & CSA-FLS2

Deck trunk structure on the main deck


Requirements for Fatigue Design

Design fatigue life of 30 years on the North Atlantic

DNV PLUS Notation

- To confirm the fatigue strength of main stiffener-frame connections in cargo hold region based on the Rule loadings

DNV CSA-FLS2 Notation

- To confirm the fatigue strength of all the critical details in the hull structures based on the direct calculations, which includes hydrodynamic analysis and spectral analysis Fatigue of longitudinal end connections and frame connection in cargo hold area.

Fatigue of bottom and side-shell plating connection to frame/stiffener in the cargo hold area.

Fatigue of critical details in all the cargo hold area: knuckles

discontinuities

deck openings and penetrations.


PLUS Calculations

General Assumptions

- The vessel is expected to be 42.5% of its lifetime in full load condition and 42.5% in the ballast condition & cargo holds are assumed to be in non-corrosive environment.

The hot spot points & the geometric stress concentration factors KG are applied as specified in CN 34.2.

The maximum principal stress within ±45˚ of the normal to the weld is used for the analysis.

Simplified fatigue analysis is carried out to calculate the fatigue damage for each hot-spot point


Flowchart for stiffener-frame connections

*Refer to CN 34.2


F. E. Calculation for PLUS

The cargo tank finite element model for fatigue assessment is equivalent with the model used for CSR

Global analysis for aft, mid and fore cargo hold region are carried out to cover entire cargo holds. In each model, 1/2 + 1 + 1/2 cargo tanks are modeled to represent the behavior of the ship structures.

Totally six load cases are applied to the models. The two static load cases are applied in order to calculate the mean stress effect.

< Loading cases >


F. E. Calculation for PLUS

the semi-nominal element mesh(50x50mm) that is used to calculate the stress concentration factors

< Mid holds>

< Fore holds>


Reinforcements from PLUS Notation

Modification to the closed-typed collar plate

Normal scallops to keyhole or soft toe & back brackets

About 1,000 pieces added to enhance the fatigue strength after global screening


Spectral Fatigue Analysis for CSA-FLS2

All calculations are based on direct calculated wave loads using a 3D hydrodynamic program including effect of forward speed. The pressures and inertia loads from the hydrodynamic analysis are transferred to the FE-models maintaining the phasing definitions.

For CSA-FLS2, two principal fatigue calculation methodologies are used to comply with CSA requirements:

- Full stochastic (spectral) fatigue analysis

- Component stochastic method

It requires a number of numerical calculations related to F. E. calculation and processing of the numeric works

Hard to use the commercial software(SESAM package) due to the limited design schedule


Procedure of Spectral Fatigue Analysis

Sea State (Hsi, Tzj) (Wave Spectrum, Sw(ω|Hsi, Tzj))

Wave Spreading

Wave Scatter Diagram

Probability Density Function (Rayleigh Distribution)

Short-Term Statistics (m0, m2, m4, fijk , εijk)

Stress Range Response Spectrum (Ss(ω|Hsi, Tzj, θk)

Stress Transfer Function (Stress RAO, Hs(ω|θk))

Partial Damage

Weighting & Summing All Damages

Closed-Form Integration

Total Damage

S-N Curve

Palmgrens-Miner Rule

F. E. Structural Analysis Hot-spot stress, s(ωn,θk)

Ship Motion Analysis for given (ωn,θk)

L/C, Ship Speed(V)


Development of HDSafe

HFM (Hyundai FEA Manager)

- Handling of loads from ship motion analysis

- Global to local mapping (boundary & loading)

- Automatic job submission (each heading, frequency, phase)

- Manipulation of the FE analysis results

- Hotspot stress extraction

- Generation of the stress transfer function data for HSM

HSM (Hyundai Spectral Manager)

- Database of environmental(wave) data & fatigue data (SN curve)

- Statistical processing (wave spectrum, stress transfer function etc.)

- Component based fatigue analysis and full stochastic fatigue analysis

- Short term and long term fatigue damage prediction

Gap analysis between SESAM & HDSafe was carried out and HDSafe is confirmed to be equivalent & acceptable


Handling of F. E. Model

Load/boundary interface for global & local model

Definition & extraction of Hot-spot stress


Damage Calculation

Damage calculation & visualization

Case study based on the databases on wave & S-N data


Hydrodynamic analysis

Linear wave load analyses are performed for the two loading conditions based on a 2/3 of design speed. The hydrodynamic loads have been calculated using a seakeeping computation program WASIM. FLS design loads for the vessel to be based on:

- North Atlantic wave environment

- Pierson-Moskowitz wave spectrum

- Equal probability of wave heading

- Cos2 wave spreading

- Vessel speed is 2/3 of design speed

Long-term analysis was carried out for assessment of hull girder loading & local pressures based on the 30 years of operation on the North Atlantic


Results of Long-Term Analysis

0.00E+00

5.00E+05

1.00E+06

1.50E+06

2.00E+06

2.50E+06

3.00E+06

3.50E+06

4.00E+06

0.00 50.00 100.00 150.00 200.00 250.00 300.00

HBM

(kN

m)

x (m)

HBM

2P

4P

6P

8P

20R

DNV_Rule

0.00E+00

1.00E+06

2.00E+06

3.00E+06

4.00E+06

5.00E+06

6.00E+06

7.00E+06

0.00 50.00 100.00 150.00 200.00 250.00 300.00

VBM

(kN

m)

x (m)

VBM

2P

4P

6P

8P

20R

DNV_RULE(sag)

19 % higher than CSR Rule value

6 % lower than CSR Rule value


External Wave Pressures

0

20

40

60

0 5 10 15 20

CSR

CSA

0

5

10

15

20

25

0 20 40 60 80 100

CSR

CSA


Results of Component Method

The component stochastic fatigue calculation procedure is based on combination of load transfer functions calculated by the wave load analysis program and stress response factors representing the stress per load ratio.

Hot-spot points to be checked

- (a) all the stiffener end connections at ordinary frames and at transverse bulkheads.

- (b) all the plate welds towards longitudinals(long edge) and frames(short edge)

(a) (b)


Comparison of Fatigue Damage

ID

CSR CSA End

connection

End connecti

on

Short edge

Long edge

SSL28 0.16 0.58 1.14 1.00 SSL32 0.11 0.83 0.38 0.71 SSL35 0.19 0.59 0.27 0.38 SSL38 0.21 0.43 0.22 0.28 SSL42 0.13 0.31 0.21 0.25 SSL43 0.16 1.49 0.19 0.10 SSL44 0.12 1.16 0.58 0.66 SSL45 0.10 0.22 0.51 0.45 SSL50 0.15 0.17 0.45 0.00 UDL26 0.13 0.17 0.52 0.00 UDL16 0.11 0.49 0.40 0.00 UDL01 0.14 0.18 0.48 0.04


Full Stochastic Analysis

To be performed using a global structural model and local fine-mesh sub-models.

It is required that the wave loads are transferred directly from the hydrodynamic analysis to the structural model. The load transfer ensures that the loads are applied consistently, maintaining load-equilibrium.

From local stress concentration models the geometric stress transfer functions at the hot spots are determined by the t x t elements that pick up the stress increase towards the hotspot.

The global screening analysis is carried out to calculate allowable stress concentrations in the trunk deck & the upper deck and also to find the most fatigue critical detail from a number of similar or equal details.


Free Surface Effect on Hydrodynamic Load

How to take into account the free surface effect in the cargo or ballast tank correctly in carrying out the hydrodynamic analysis and load generation on the global model

- Modification of GM by re-distribution of the weight or modifying the stiffness matrix(C44) directly in the potential code

- This modification causes an unfavorable hull girder loading such as the torsional moment and horizontal bending moment.

0.00E+00

1.00E+04

2.00E+04

3.00E+04

4.00E+04

5.00E+04

6.00E+04

0 0.2 0.4 0.6 0.8 1 1.2

RAO of Torsional Moment

with Additional C44

without Additional C44


Treatment of the Unbalancing Forces

Deviation of still water loading caused by how to consider the limited data of weight

- Loading manual vs. hydrodynamic model

Large forces and moments at the reaction points are normally caused by errors in the load transfer but inevitable in some cases, i.e. for taking into account the free surface effect and effect of intermittent wet surfaces in waterline region.

- No clear guidance on usage of inertia relief method to remove the unbalancing forces generated during load transfer of hydrodynamic loading to F. E. model


Typical Connections

HS1

HS2

HS3

HS1

HS2

HS1

HS2

HS1 , F-1


Special Details


Results of Global Screening


Conclusions

HHI has carried out fatigue design for the shuttle tanker which should be operated in the harsh environment using HHI’s own analysis system and all the critical details satisfied the requirement of fatigue strength.

It is necessary that the clearer guidance for carrying out the spectral fatigue analysis in order to complete a huge amount of the calculations without any human errors within the limited schedule of design & construction of the special-purposed vessels.

It is recommended that the some cooperative studies would be proposed to investigate the uncertainty of the direct calculation and to develop the unified methods for the spectral method.

Documents

Fatigue Design of a Shuttle Tanker for the North Atlantic ... 3.3 - (only PPT...Fatigue Design of a Shuttle Tanker for the North Atlantic Operation October 23 rd, 2013 Byung-Ki Choi*,