t04bw - structural.pdf

8/14/2019 T04bw - Structural.pdf

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ABS TECHNICAL PAPERS 2008

Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 37

Structural Integrity Assessment of Cargo Containment Systems

in Arctic LNG Carriers under Ice Loads

JinChil Kwon, ByungYeong Jeon, JaeHyun Kim

Hyundai Heavy Industries Co., Ltd., Ulsan, Korea

Bo Wang, Han Yu, Roger Basu, Hoseong LeeAmerican Bureau of Shipping, Houston, USA

Claude DaleyMemorial University, St.Johns, Canada

Andrew KendrickBMT Fleet Technology Ltd., Ottawa, Canada

Presented at the Fourth Annual Arctic Shipping 2008 Conference held in St. Petersburg, Russia, April 8 - 11, 2008,

and reprinted with the kind permission of the organizers of Arctic Shipping 2008

Abstract

There has been an increased interest in shipping in ice-covered waters such as the Arctic Ocean due to the

efforts in recovering the large deposits of gas and oil in these areas. This circumstance leads to many

technical issues related to the structural strength of liquefied natural gas (LNG) carriers subject to intensive

ice loads. The hull structures of both the membrane tank type and the spherical tank type LNG ships have to

be designed by Polar Class Rules. However, the Rules are not available for cargo containment systems

(CCS) in LNG ships under ice impact loads.

In this paper, ship and ice interaction scenarios have been investigated in possible operation routes for the

finite element (FE) analysis. Also, simplified ice load models have been developed. For the membrane tank

type LNG carrier, finite element models have been developed to include not only hull structure but also

cargo containment systems (CCS) at the midbody and shoulder areas for analyses. For the spherical tank

type LNG carrier, finite element models including the hull structure and skirt structure at the midbody and

shoulder areas have also been developed. In FE simulations, linear buckling analyses have been performed

to determine the critical buckling load for ensuring the stability of hull structure. Nonlinear static FE

analyses have been conducted to compute the response of the cargo containment system. Based on FE

results and assessment criteria, the strength of the cargo containment systems in LNG carriers has been

evaluated. Finally, structure analysis procedures have been developed for assessing the strength of LNG

cargo containment systems under ice loads.

1 Introduction

Shipping in ice-covered sea such as the Arctic region is increasing because the gas and oil exploration is

moving to harsher, more northern environments such as in Russia area. In available literatures, some

fundamental research work has been done only focusing on the investigation of the ice properties and the

mechanism of ice-structure interaction [1-2]. So far, Baltic Rules has been widely used, since the majority of

new ice-strengthened tankers are built for the Baltic region. For example, the vessels traveling in the

Northern Baltic are required to be in agreement with the Finnish-Swedish Ice Class Rules (FSICR) [3]. Thus,

the FSICR has been widely adopted by all major classification societies such as ABS [4]. Additionally, ABS

has also developed a direct calculation procedure to determine a rational side shell thickness under ice loads

[5]. Recently, the IACS Polar Class Unified Requirements (UR) becomes effective in March, 2008, whichaddresses the ice strengthening of ships navigating in the Arctic region. However, there still have been no


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38 Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

experiences of large LNG carriers operations in Arctic waters. Both Baltic and IACS ice class rules do not

address the ice loading effect on LNG containment systems and the ice strengthening of LNG containment

systems beyond the hull structure.

With regards to LNG cargo containment systems, previous research work and guidance notes focused on

sloshing impact loads [6-8]. The interest in the present study is directed towards investigating the structural

integrity of cargo containment systems (CCS) in membrane and spherical tank type LNG carriers under

various ice loads. Objectives of this project are to develop practical ice loads based on ship and ice

interaction scenarios, and to develop the complete procedures for evaluating the structural integrity of cargo

containment systems of LNG carriers under ice loads. In this study, an LNG vessel designed based on 1A

class is selected as an example ship. Since ice loads for FE analysis are not available in the Baltic Rules, the

BMT ice load model is selected in FE analysis, which is consistent with that in Polar class rules. Accordingly,

1A class LNG ship is modified into PC7 class LNG ship for FE analysis under PC7 class. Consequently,

structural analyses on the hull structure and LNG containment systems are conducted under ice loads.

2 Ice-ship interaction scenarios and ice loads

2.1 Ice-ship interaction scenarios

The ice that may be encountered by an LNG carrier (or other vessel) comes in a wide variety of types

and sizes. Table 1 indicates a matrix of possible combinations of properties. This is a summary the World

Meteorological Organization (WMO) has a more detailed set of descriptors for ice characteristics, even

though this is insufficient to address all features of importance for ice navigation.

Table 1 Ice condition matrix

Ice Type Thickness/

Mass

Floe Size Ridging Pressure Grounded

First year Thin/small Small Light Light YesMulti-year Medium Medium Medium Medium No

Glacial Thick/large Large Heavy Heavy

There are many current and potential LNG carrier routes through ice-covered waters. These link the gas

reserves in Russia and in the Canadian Arctic with markets in Europe, Asia, and North America. Fig. 1

provides an overview of the possible operation routes. They comprise three Arctic routes, two sub-Arctic

routes, and one Baltic route, where carriers traveling these will encounter various types of ice.

Sakhalin 5

Siberian Trans-polar

Yamal - GoM

Primorsk

Sverdrup Basin

Fig. 1 Possible Operation Routes


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Most types of ice-ship interaction scenario are only possible for certain hull areas. As examples, seven

areas in a ship are considered, shown in Fig. 2. Each area may experience the following types of scenario:

Area 1 Stem: Icebreaking and Ramming (any large feature)

Area 2 Bow: Icebreaking, Glancing impact (any large feature), and Reflected impact

Area 3 Shoulder: Glancing impact, Reflected impact, and Wedging impact

Area 4 Mid-body: Glancing impact (especially towards aft quarter during manoeuvres) and

Pressure loads

Area 5 Turn of Bilge: Impact with submerged pieces broken during icebreaking

Area 6 Bottom: Beaching loads and Impact with submerged pieces broken during icebreaking

Area 7 Stern: Backing loads (conventional), Icebreaking loads (double acting),

Appendage impact loads, and Propeller-induced impact loads

These scenarios do not take into account those incidents considered to be avoidable accidents; such as

reflected impacts in which the ship may yaw violently off an impact with one large floe and into another.

The accidental scenarios can impose very high loads on shoulder or midbody structure, due to the

combination of moderately high impact velocities and very unfavorable impact angles.

STEM

SHOULDER

BOW

BOTTOM

TURN OF BILGE

MIDBODY

STERN

Fig. 2 Seven areas for ice-ship interaction

2.2 Selected ice loading scenarios

In terms of each ice-ship interaction scenario, there is a corresponding load case for an LNG carrier.

Each load case should be investigated for the strength evaluation of cargo containment systems in membrane

and spherical type LNG carriers. However, this task will be very time-consuming work. Therefore, some

critical ice loading scenarios need to be selected for the practical analysis. One task in this project is the

completion of a hazard identification (HAZID) exercise. The purpose of conducting the HAZID is to provide

input into an initial Hazard Register to be used as a log and as a screening utility for more specific and

detailed analyses to be conducted. Based on the HAZID and the focus of containment system, six ice loading

scenarios in shoulder and midbody areas in LNG ships have been selected for a more detailed follow upanalysis, as listed in Table 2.

During the advancement of the ship in the ice-covered sea, the ramming frequently occurs at the

shoulder (or bow) area and sometimes occurs at midbody area in the ship. For idealization purposes, only

static pressure at the local area will be considered in nonlinear static FE analysis. In this case, the ice load is

simplified as a static patch load applied to the shoulder (or bow) area. Ice loading scenarios at the shoulder

such as glancing pressure, reflected impact pressure, and wedge ramming pressure, are illustrated in Table 2.

Midbody loads from the glancing scenario can result from a reflected impact, as shown in Fig. 3. The

impact can either be immediate or more likely somewhat delayed.


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Fig. 3 Shoulder (or Bow) - glancing impact of midbody sequence (a) immediate, (b) delayed

In this scenario, the impact velocity in midbody will be a function of the initial bow impact magnitude

and location, and of the time lag between the first and second impacts, depending on the channel width or

distance between floes. The combined yaw and sway velocities mean that impact severity will tend to be

higher forward of amidships. The glancing impact can also result from turning in a channel, shown in Fig. 4.

In this case, the impact velocity will depend on turning rate, and will be higher aft of amidships.

Fig. 4 Glancing impact of midbody during a turn

Table 2 Summary of selected ice loading scenarios

Position Loading scenario Case Picture

Glancing impact on shoulder :

This is an oblique shoulder collision

with an ice edge.

Case 7

Reflected on shoulder :

This is 2nd

oblique shoulder collision

with an ice edge.

Case 9

Shoulder

Wedging on shoulder :

This is a symmetrical shoulder collision

with 2 ice edges.

Case 10


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Position Loading scenario Case Picture

Glancing impact on midship :

This is an oblique midbody collision

with an ice edge.

Case 11

Close pack pressure (Pressured ice

load): This is midbody static contact an

ice edge.

Case 13

Midship

Ice floe impact pressure Case 21

2.3 Ice load models

The problem under discussion is one of impact between two objects. It is assumed that one body is initially

moving (the impacting body) and the other is at rest (the impacted body). This concept applies to a ship

striking an ice edge, or ice striking an offshore structure. The energy approach is based on equating the

available kinetic energy with the energy expended in crushing and potential energy [9]:

PEIEKEe (1)The available kinetic energy is the difference between the initial kinetic energy of the impacting body and the

total kinetic energy of both bodies at the point of maximum force. If the impacted body has finite mass, it will

gain kinetic energy. Only in the case of a direct (normal) collision involving one infinite (or very large) mass

will the effective kinetic energy be the same as the total kinetic energy. In such a case all motion will cease at

the time of maximum force. The indentation energy is the integral of the indentation force Fn on the crushing

indentation displacement c:

m

0

cndFIE (2)

The potential energy is the energy that has been expended in recoverable processes, which can be either rigid

body motions (pitch/heave) or elastic deformation (of either body). The potential energy is the integral of the

indentation forceFn on the recoverable displacement e:

0

endFPE (3)

These equations are the basis of all solutions. Equation (1) can be solved for Fn provided that the required

kinematic and geometric values are known. Table 3 shows detailed ice loads for six selected loading scenarios,

which will be employed in this study.

The several parameters of tangent angle by hull surface (plan, elevation and section view) for calculation of

ice patch loading are shown in Fig. 5. The spreadsheet program for ice loading has been developed for the

easy calculation. A sample screen is shown in Fig. 6.


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x

y

z

x

Gravity Center (midship)

Gravity Center (midbody)

x

y

z

x



x

y

z

x



Fig. 5 Hull form parameters

Fig. 6 Sample spreadsheet for ice load calculation

Table 3 The detailed ice patch loads with area for six selected scenarios

RegionLoad case

Width(m)

Height(m)

Area(m2)

Pressure(MPa)

Force(MN)

Case 11 Glancing impact on midbody, Accident 3.89 0.277 1.076 5.700 6.133

Case 13 Close pack pressure condition 4.37 0.610 2.666 0.560 1.493Midbody

Case 21 Ice floe impact pressure condition 0.89 2.973 2.647 2.012 5.326

Case 7 Glancing impact on shoulder, Accident 2.665 2.213 5.898 2.319 13.677

Case 9 Reflected on shoulder, Accident 3.627 3.013 10.93 2.766 30.23Shoulder

Case 10 Wedging on shoulder 2.278 3.096 7.052 1.176 8.293


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3 Configuration of Target LNG Carriers

3.1 General arrangement and midship section drawing

General arrangements of the target vessel are shown in Fig. 7 and 8. The cargo volume of the vessel is150,000 cubic meters of membrane and 140,000 cubic meters of spherical type in total 4 tanks. Ice belt zone

is strengthened in accordance with PC 7 and Ice 1A requirement. Ice patch load, ballast loaded and/or full

loaded condition was simulated because inertia force induced by LNG cargo would cause higher response to

CCS and skirt structure. Main dimensions of the considering ships are shown in Table 4.

Fig. 7 General arrangement of 150K membrane tank type LNG carrier

Fig. 8 General arrangement of 140K spherical tank type LNG carrier

Table 4 Main dimensions of LNG carriers for the analysis

Ship type

ItemMembrane type (MarkIII) Spherical type

Cargo Volume 150,000 CBM 140,000 CBM

Length O.A. 288.0 m 288.7 m

Length B.P. 275.0 m 274.0 m

Breadth 44.2 m 48.0 m

Depth 26.0 m 26.5 m

Draught (design)

(scant.)

11.35 m

(12.35 m)

11.15 m

(12.30 m)

3.2 Scantling comparison between PC7 (Polar Class) and Ice 1A (Baltic Class)

The scantling calculations of LNG carrier have been performed by Ice 1A and PC7 respectively. The

comparison of hull scantling for membrane and spherical tank type LNG carrier between PC7 grade and Ice

1A grade is shown as Table 5. The scantlings based on IACS PC7 grade are higher than those of Ice 1A

grade except the shell plate of forward region. According to the result, the strengthened range of shell plate

and longitudinals by PC7 has been extended, which include the bow bottom, bow intermediate lower, bow

intermediate bottom, midbody lower and stern lower part and increased the scantling of longitudinals. In


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summary, the total hull weight base of PC7 for LNG carrier is about 4~6% much heavier than that of Ice 1A

grade. The scantlings based on IACS PC7 (Polar class 7) have been applied for FE analysis. It should be

addressed that the issue of weight and efficiency probably deserves a lot of attention and refinements of the

Polar Rules. The current rules make use of hull area factors, and so generally apply the same trends to all

structural areas. This doubtless might lead to over-design in some areas.

Table 5 Scantling of membrane and spherical type LNG carrier for Baltic Ice 1A and Polar PC7 (unit:mm)

ScantlingGradeLNG type and region

ICE 1A (Baltic) PC7 (Polar)

Shell plate 39.0HT32 36.5HT32Forward region(No.1 hold,

Sp.=660mm)Longitudinals 425x20

HT32+125x18

HT32F.B(T) 500x35

HT32F.B

Shell plate 29.5HT32 33.5HT32Midbody region(No.3 hold) Longitudinals 425x12+125x18 F.B(T) 450x20 + 125x18 F.B(T)

Shell plate 26.0HT32 30.0HT32

Aft region Longitudinals 350x100x12/17 I.A 400x18 + 125x16 F.B(T)

Shell plate - 36.5HT32Bow bottom(Sp.=660mm) Longitudinals - 500x35 HT32 F.B

Shell plate - 41.0Bow intermediatelower Longitudinals - 500x20 + 125x17 F.B(T)

Bow intermediate

bottomShell plate

- 30.0

Midbody lower Shell plate - 29.5

Membrane

tank type

Stern lower Shell plate - 29.5

Shell plate 36.5HT32

34.5HT32

Forward region

Longitudinals 425x17HT32 + 125x14HT32 F.B(T) 625x20HT32 + 100x12HT32 F.B(T)

Shell plate 29.5HT32 33.5HT32Midbody region

Longitudinals 425x13 + 150x18 F.B(T) 700x20 + 125x20 F.B(T)

Shell plate 26.0HT32

30.5HT32

Aft region

Longitudinals 350x100x12/17 I.A 400x18 + 125x14 F.B(T)

Shell plate - 34.5HT32

Bow bottom

Longitudinals - 625x20HT32+ 100x12HT32 F.B(T)


Bow intermediatelower

Longitudinals - 400x16HT32

+ 100x13HT32

F.B(T)

Bow intermediatebottom


Midbody lower Shell plate - 27.0HT32

Spherical

tank type

Stern lower Shell plate - 29.5


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Figs. 9-11 show that the scantling difference between 1A class and PC7 class for both membrane type

and spherical type LNG carriers.

(a) Baltic 1A class (b) Polar PC7 class

Fig. 9 Shell thickness of midship section for membrane type LNG carrier (unit: mm)

(a) Baltic 1A class (b) Polar PC7 class

Fig. 10 Shell thickness of midship section for spherical type LNG carrier (unit: mm)

Fig. 11 Strengthened zone by Baltic 1A and Polar PC7 in membrane type LNG carrier

29.5

33.5

29.5

27.

33.5

29.5

: Area of increased thickness by Batic Ice 1A

: Additional area of increased thickness by Polar PC7


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4 Finite Element Analysis

The main focus of this study is the strength evaluation of the cargo containment system with hull

structure of membrane type and spherical type LNG carriers. At the first step, the eigenvalue analysis has

been performed and the thickness of web and stringer plate has been increased slightly which corresponds to

the buckling mode. In the next step, nonlinear static analysis has been performed by patch loading for thepreviously selected load cases.

The ranges of the finite element model of midship and shoulder part and the FE details of cargo

containment systems for membrane type and spherical type LNG carriers are described in Fig. 12 and Fig.

13, respectively.

Fig. 12 FE model for membrane tank type LNG carrier

The hull structures are made of steel, which behave in elastic-perfectly-plasticity for membrane and

spherical type LNG carriers. The containment system for the membrane type LNG carrier is made of

No.3 hold model No.1 hold model Simplified No.1 hold model

Cargo containment system model

Midbody Shoulder


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different materials such as mastic, plywood and polyurethane foam (R-PUF). These materials behave in

isotropic elasticity (mastic) and orthotropic elasticity (plywood and R-PUF), respectively.

Fig. 13 FE model for spherical tank LNG carrier

Ice impact loads at midbody areas of membrane and spherical type LNG carriers are located near the

No.3 & 4 stringer of No.3 hold and plan/section drawing are shown in Figs. 14 and 15. The shoulder areas of

membrane and spherical type LNG carriers are located near the No.3 & 4 stringer of No.1 hold and

plan/section drawing are shown in Figs. 16 and 17. In nonlinear FE analysis, applied loads will include the

ice patch load and/or seawater pressure, liquid cargo loading pressure, and inertia gravity.

Fig. 18 shows all of these three pressure loads in FE model of midbody. The ice patch load is developed

and calculated by using BMT ice load model (see Table 3) and others are the seawater pressure and the

liquid cargo loading pressure, respectively. Fig. 19 shows the critical location where the ice pressure isapplied. From preliminary FE results, six loading location cases to the side shell plating have been

investigated for each ice loading scenario and the critical loading location in the side shell plating has been

found near the connection between the stringer and the web.

(a) Membrane tank type

Midbody Shoulder

No.3 hold model No.1 hold model

Skirt and skirt foundation deck


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(b) Spherical tank typeFig. 14 Position of patched ice pressure in No.4 stringer plan of midship

(a) Membrane type (b) Spherical type

Fig. 15 Position of the patched ice pressure in midship

(a) Membrane tank type

(b) Spherical tank type

Fig. 16 Position of patched ice pressure in No.4 stringer plan in shoulder


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(a) Membrane tank type (b) Spherical tank type

Fig. 17 Position of the patched ice pressure in shoulder

(a) ice patch pressure (b) sea pressure (c) liquid cargo pressure

Fig. 18 Configuration of applied loading for analysis

Fig. 19 Ice patch load at critical loading location

Side shell plating

Stringer

Web

CCS

Ice patch load

ice patch pressure


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5 Assessment Criteria

Nonlinear static FE analysis on the local model is to be performed to obtain stress fields. In the case of

membrane type LNG carrier, the main focus is the strength of the cargo containment system in membrane

type LNG carriers. So the maximum normal tensile/compressive stress and shear stress are to be evaluated

with the ultimate strength of material in each orientation for foam and plywood layers, and maximum vonMises stress is to be evaluated with the ultimate strength of material for mastics. Minimum specified ultimate

strengths of materials are listed in Table 6.

Maximum Normal Stress Criterion.Maximum normal tensile/compressive stress in each orientation is to satisfy the following condition:

maxc, where c = Smyor Smuis permissible normal stress, yis minimum specified yield

strength of materials, uis ultimate strength of materials, Smis the strength reduction factor (SRF),

which is recommended as 0.50 for foam and 0.67 for plywood, respectively.

Maximum Shear Stress Criterion.Maximum shear stress is to satisfy the following condition: max c , where c = Smu is

permissible shear stress, uis minimum specified ultimate shear strength of materials, Smis strength

reduction factor (SRF), which is recommended as 0.67 for foam and plywood.

Von Mises Stress Criterion.Maximum von Mises stress is to satisfy the following condition: eqmax c, where c= Smyor

Smuis permissible normal stress, yis minimum specified yield strength of materials, uis ultimate

strength of materials, Sm is strength reduction factor (SRF), which is recommended as 0.50 for

mastic.

Table 6 Ultimate strengths of polyurethane foam, plywood, and mastic [6]

Material(20C) Orientation or Grade

Strength(MPa)

Horizontal Tension 2.4

Horizontal Compression 1.2

Vertical Tension 1.2

Vertical Compression 2.0

Polyurethane Foam(PUF)

Shearing 1.77

Horizontal Tension 40.

Horizontal Compression 80.

Plywood

Shearing 2.8

Mastic 15.

Steel(Mild) 235.Steel

Steel(Hiten-32) 315.

6 Evaluation of Results

6.1 Membrane tank type LNG carrier

For a membrane type LNG ship, one local model named No. 3 hold model for midbody and another local

model named simply No. 1 hold model for shoulder part are employed for FE analysis. In these two local

models, one individual piece of CCS is attached in the inner hull of side structure, shown in Fig. 12.

Nonlinear static FE analyses have been conducted on these two local models under ice patch loadscorresponding to six loading scenarios.


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6.1.1 Shoulder No. 1 hold model

Under the ice patch load, 2.319 MPa in glancing impact case, 1.176 MPa in wedging impact case and

2.766 MPa in reflected impact case (see Table 3) at the shoulder area, local large plastic deformation takes

place in the side shell plating, web frames and stringers near the loading area. The maximum deformation in

the inner hull is 3.72 mm in glancing impact case, 2.24mm in wedging impact case and 4.47mm in reflected

impact case, which occurs near the connection between the web frame and the stringer. These all maximum

values are less than the allowable value, 4.6 mm, in ABS Guide for LNG vessels [10]. From nonlinear FE

analysis, von Mises stresses of membrane are shown in Fig. 20(a) and mastics are shown in Fig. 20(b) in

glancing impact, and Fig. 21(a) and Fig. 21(b) are in wedging impact, and Fig. 22(a) and Fig. 22(b) are in

reflected impact case. FE results are summarized in Table 7~9.

(a) Stringer and web structure (b) Mastics structure

Fig. 20 Von Mises stress in glancing impact case

(a) Vertical stress of R-PUF (b) Horizontal stress of plywood

Fig. 21 Axial stress in glancing impact case



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Fig. 22 Von Mises stress in wedging impact case


Fig. 23 Axial stress in wedging impact case


Fig. 24 Von Mises stress in reflected impact case


Fig. 25 Axial stress in reflected impact case

These tables show that all usage factors are much less than 0.5. It can be concluded that the cargo

containment system in this ship is safe in these ship-ice interaction scenarios.


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Table 7 Strength evaluation for each component of CCS at shoulder in glancing case (Case 7)

Layer OrientationMaximum Stress

(MPa)

Reference Stress

(MPa)Usage Factor

Mastic Von-Mises 1.30 15 0.087

Vertical Ten. 0.23 1.4 0.164Vertical Comp. 0.11 2.0 0.055R-PUF

Shear 0.20 1.4 0.143

Horizon Ten. 0.95 40 0.024

Horizon Comp. 0.41 40 0.010Plywood

(bottom)Shear 0.48 2.8 0.179

Table 8 Strength evaluation for each component of CCS at shoulder in wedging case (Case 10)


(MPa)

Reference Stress

(MPa)Usage Factor


Vertical Ten. 0.14 1.4 0.100Vertical Comp. 0.08 2.0 0.040R-PUF

Shear 0.11 1.4 0.079

Horizon Ten. 0.61 40 0.015


(bottom)Shear 0.31 2.8 0.111

Table 9 Strength evaluation for each component of CCS at shoulder in reflected case (Case 9)


(MPa)

Reference Stress

(MPa)Usage Factor


Vertical Ten. 0.25 1.4 0.179

Vertical Comp. 0.14 2.0 0.070R-PUF

Shear 0.22 1.4 0.156

Horizon Ten. 1.08 40 0.027


(bottom)Shear 0.55 2.8 0.196

6.1.2 Midbody - No 3. hold model

Under the ice patch load, 5.7 MPa in glancing impact case, 0.56 MPa in closed pack ice pressure case and

2.012 MPa in ice floe impact pressure case at the midship area (see Table 3), Nonlinear static analyses withrelated static pressure of sea and/or cargoes have been carried out.

Von Mises stress distributions in the web and stringer structure at patch load case is shown in Fig. 26(a) and

the mastics is shown in Fig. 26(b). The axial stress of R-PUF at patch load case is shown in Fig. 27(a). The

axial stress of plywood is shown in Fig. 27(b) in glancing impact case. Fig. 28(a), Fig. 28(b), Fig. 29(a) and

Fig. 29(b) are in closed pack ice pressure case, and Fig. 30(a), Fig. 30(b), Fig. 31(a) and Fig. 31(b) are in ice

floe impact pressure case.


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(a) Vertical stress of R-PUF (b) Horizontal stress of plywoodFig. 27 Axial stress in glancing impact case

(a) Stringer and web structure (b) Mastic structure

Fig. 28 Von Mises stress in close pack pressure case


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Fig. 29 Axial stress in close pack pressure case


Fig. 30 Von Mises stress in ice floe impact pressure case

(a) Vertical stress of R-PUF (b) Horizontal stress of plywoodFig. 31 Axial stress in ice floe impact pressure case

Table 10 Strength evaluation for each component of CCS at midbody in glancing impact (Case 11)

Maximum Stress (MPa) Usage FactorLayer Orientation

Patch Ballast Full load

Reference

Stress (MPa) Patch BallastFullload

Mastic Von-Mises 1.45 1.46 3.05 15 0.100 0.100 0.204

Vertical Ten. 3.57E-2 2.96E-2 3.17E-2 1.4 0.026 0.021 0.023R-PUF

Vertical Comp. 1.19E-2 7.84E-3 1.86E-1 2 0.006 0.004 0.100

Horizon Ten. 2.04 1.39 9.4 40 0.053 0.036 0.233Plywood

(bottom) Horizon Comp. 1.63 0.95 5.8 40 0.042 0.024 0.145


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Table 11 Strength evaluation for each component of CCS at midbody in close pack pressure (Case 13)


Patch BallastFull

load

Reference

Stress

(MPa) Patch BallastFull

load

Mastic Von-Mises 0.34 1.73 2.88 15 0.023 0.115 0.192Vertical Ten. 8.7E-3 5.4E-3 4.2E-2 1.4 0.006 0.004 0.030

R-PUFVertical Comp. 2.7E-3 2.3E-2 8.2E-2 2 0.001 0.011 0.042

Horizon Ten. 0.47 0.79 9.7 40 0.012 0.020 0.244Plywood

(bottom) Horizon Comp. 0.38 0.56 6.71 40 0.001 0.014 0.167

Table 12 Strength evaluation for each component of CCS at midbody in ice floe impact (Case 21)


Patch BallastFull

load

Reference

Stress

(MPa)Patch Ballast

Full

loadMastic Von-Mises 0.999 1.58 3.2 15. 0.067 0.105 0.213

Vertical Ten. 3.39E-2 3.07E-2 3.14E-2 1.4 0.024 0.022 0.023R-PUF

Vertical Comp. 1.07E-2 1.98E-2 2.10E-1 2. 0.005 0.010 0.105

Horizon Ten. 1.67 1.67 10.8 40. 0.042 0.042 0.270Plywood

(bottom) Horizon Comp. 0.69 0.60 6.5 40. 0.017 0.015 0.161

All FE results of maximum stresses in the CCS components are shown in Table 10~12. This table shows

that all usage factors are much less than 0.5. It can be concluded that the cargo containment systems in this

ship is safe in these ship-ice interaction scenarios.

6.2 Spherical tank type LNG carrier

6.2.1 Midbody - No. 3 hold model

Under the ice patch load, 5.7 MPa in glancing impact case, 0.56 MPa in close pact ice pressure case and

2.012 MPa in ice floe impact pressure case (see Table 3) at the midship area, the deformed shapes of skirt

structure are shown on Fig. 32(a) and Fig. 32(b). According to the results of nonlinear FE analysis, von

Mises stress distributions of skirt structure at patch load case are shown in Fig. 33(a) at glancing impact case,

Fig. 34(a) at closed pack ice pressure case, and Fig. 35(a) at ice floe impact pressure case. Von Mises stress

distributions of skirt foundation deck are shown in Fig. 33(b) at glancing impact case, Fig. 34(b) at closed

pack ice pressure case, and Fig. 35(b) at ice floe impact pressure case.

All FE results of maximum stresses are shown in Table 13. All stress levels of the skirt and the skirt

foundation deck are located within yielding criteria.


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(a) Patch load case (b) Full load case

Fig.32 Displacement of skirt structure in glancing impact case - Plan view

(a) Skirt structure (b) Skirt foundation deck


(a) Skirt structure (b) Skirt foundation deckFig. 34 Von Mises stress in close pack ice pressure case


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Fig. 35 Von Mises stress in ice floe impact pressure case

Table 13 Strength evaluation and displacement at midbody of spherical tank type LNG carrier

ShellInner Longl.

Bhd.

SkirtSkirt foundation

deckPosition

Load case (Mpa)

Disp.(mm)

(Mpa)

Disp.(mm)

(Mpa)

Disp.(mm)

(Mpa)

Disp.(mm)

Patch load 65 3.3 20 2.2 15 2 92 2.9Glancing impact(Case11) Full load 69 10 26 7.3 196 16.2 150 13.6

Patch load 33 1.1 5 0.5 3 0.5 19 0.7Close packpressure(Case13) Full load 38.6 7.2 25 7.6 186 12.2 158 9.7

Patch load 151 4.1 19 2.5 12 1.7 75 2.3Ice floe impactpressure(Case21) Full load 149 4.8 28 5.1 188 8.8 158 6.4

6.2.2 Shoulder - No. 1 hold model

Under the ice patch load, 2.397 MPa in glancing impact case, 2.766 MPa in reflected impact case at

shoulder part (see Table 3), according to the results of nonlinear FE analysis, von Mises stress distribution in

the side structures are shown in Fig. 36(a), Fig. 38(a) and Fig. 40(a). Transverse webs are shown in Fig. 36(b),

Fig. 38(b) and Fig. 40(b). The skirt structures are shown in Fig. 37(a), Fig. 39(a) and Fig. 41(a). The skirt

foundation decks are shown in Fig. 37(b), Fig. 39(b) and Fig. 41(b) at each load cases.

(a) Shoulder model (b) Transverse web



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(a) Shoulder model (b) Transverse web

Fig. 38 Von Mises stress at patch load in reflected impact case


Fig. 39 Von Mises stress at patch load in reflected impact case


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(a) Shoulder model (b) Transverse web and longl.

Fig. 40 Von Mises stress at patch load in wedging impact case


Fig. 41 Von Mises stress at patch load in wedging impact case

Table 14 Strength evaluation and displacement at shoulder of spherical tank type LNG carrier

Side shellInner Longl.

BulkheadSkirt

Skirt foundation

deckPosition

Load case

(Mpa)

Disp.

(mm)

(Mpa)

Disp.

(mm)

(Mpa)

Disp.

(mm)

(Mpa)

Disp.

(mm)

Glancing

(Case7)Patch load 108 15 61 10 57 9 199 12

Reflected

(Case9)Patch load 189 39 93 29 128 26 355 33

Wedging

(Case10)Patch load 56 9 30 7 31 6 122 7

All FE results of maximum stresses are shown in Table 14. All stress levels of the skirt and the skirt

foundation deck are located within yielding criteria.

7 Conclusions

In this study, ship and ice interaction scenarios have been investigated in possible operation routes and

appropriate loading scenarios have been selected through the HAZID analysis together with JDP members to

evaluate the strength of LNG cargo containment systems. Ice patch loads and patch load areas corresponding

to selected loading scenarios have been determined using the ice load model.

For structural analysis, scantlings of 150K cubic meters LNG carriers for PC 7 (Polar Class) and Ice 1A

(Baltic Class) are described in detail. According to the comparison results of scantling calculation of


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captioned LNG carrier, the hull weight based on PC7 (Polar Class) is about 4~6% much heavier than that of

Ice 1A (Baltic Class). More study is needed for the issue of weight increase and efficiency.

A membrane type LNG ship and a spherical type LNG ship under PC 7 (Polar Class) have been examined

under the same ice loads, respectively. The FE models at midbody and shoulder part have been developed

for linear eigenvalue analysis and nonlinear static analysis. For a membrane type LNG carrier, an individual

panel of cargo containment systems (CCS) is attached to the inner hull in FE model, which consists of

corrugation membrane, top plywood, top polyurethane foam, triplex, bottom polyurethane foam, bottom

plywood, and mastics. For a spherical type LNG carrier, the skirt structure and the skirt foundation deck has

been considered in FE modeling.

The strength of LNG CCS for the midbody and shoulder range has been evaluated based on FE results and

assessment criteria. Assessment criteria and the complete structure analysis procedures have been developed

for strength evaluation of cargo containment systems in LNG carriers under the ice loads. According to FE

results, all stresses of the CCS structure of the membrane type and the skirt and hull structure in way of the

spherical tanks are satisfied with the stress criteria. It can be concluded that the strength of the CCS of

membrane type LNG carrier and the strength of skirt and hull structure of spherical type LNG carrier are

strong enough under the design ice loads.

REFERENCES

1. J. P. Dempsey, Research trends in ice mechanics, Int. J. of Solids and Structures, 37, 2000, pp 131-153.2. I. J. Jordaan, Mechanics of Ice-Structure Interaction, Engineering Fracture Mechanics, 68, 2001, pp 1923-

1960.

3. FMA, Finnish-Swedish Ice Class Rules, 2002.

4. ABS, Rules for Building and Classing Steel Vessels, American Bureau of Shipping, 2005.

5. ABS, Guidance Notes on Ice Class, 2005.

6. ABS, Guidance Notes on Strength Evaluation of Membrane-Type LNG Containment Systems under

Sloshing Loads, 2006.

7. B. Wang, J. Kim, and Y. Shin, Strength Assessment of Membrane-Type LNG Containment System,

International Conference on Ship and Offshore Technology, RINA, Busan, Korea, 2006.

8. B. Wang and J. Kim, Strength Evaluation of LNG Containment System Considering Fluid-Structure

Interaction under Sloshing Impact Pressure, 26th International Conference on Offshore Mechanics and

Arctic Engineering, USA, 2007

9. C. G. Daley, Energy Based Ice Collision Forces" - POAC '99, Helsinki Finland, August, 1999.

10. ABS, Guide for Building and Classing Membrane Tank LNG Vessels, 2006.


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