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

Transcription

1 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 Lee American Bureau of Shipping, Houston, USA Claude Daley Memorial University, St.John s, Canada Andrew Kendrick BMT 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, which addresses the ice strengthening of ships navigating in the Arctic region. However, there still have been no Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 37

2 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/ Floe Size Ridging Pressure Grounded Mass First year Thin/small Small Light Light Yes Multi-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 Sverdrup Basin Siberian Trans-polar Yamal - GoM Primorsk Fig. 1 Possible Operation Routes 38 Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

3 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. BOW MIDBODY SHOULDER STEM STERN TURN OF BILGE BOTTOM 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 up analysis, 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. Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 39

4 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 Shoulder Reflected on shoulder : This is 2 nd oblique shoulder collision with an ice edge. Case 9 Wedging on shoulder : This is a symmetrical shoulder collision with 2 ice edges. Case Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

5 Position Loading scenario Case Picture Glancing impact on midship : This is an oblique midbody collision with an ice edge. Case 11 Midship Close pack pressure (Pressured ice load): This is midbody static contact an ice edge. Case 13 Ice floe impact pressure Case 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]: KE e = IE + PE (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 ζc : ζm IE = F d n ζ (2) 0 c 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 force Fn on the recoverable displacement ζe: ζ PE = F d n ζ (3) 0 e 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. Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 41

6 y α x Gravity Center (midship) z γ β x Gravity Center (midbody) Fig. 5 Hull form parameters Fig. 6 Sample spreadsheet for ice load calculation Region Midbody Shoulder Table 3 The detailed ice patch loads with area for six selected scenarios Load case Width (m) Height (m) Area (m 2 ) Pressure (MPa) Force (MN) Case 11 Glancing impact on midbody, Accident Case 13 Close pack pressure condition Case 21 Ice floe impact pressure condition Case 7 Glancing impact on shoulder, Accident Case 9 Reflected on shoulder, Accident Case 10 Wedging on shoulder Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

7 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 is 150,000 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 Membrane type (MarkIII) Item Spherical type Cargo Volume 150,000 CBM 140,000 CBM Length O.A m m Length B.P m m Breadth 44.2 m 48.0 m Depth 26.0 m 26.5 m Draught (design) (scant.) m (12.35 m) 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 Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 43

8 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) LNG type and region Forward region (No.1 hold, Sp.=660mm) Grade ICE 1A (Baltic) Scantling Shell plate 39.0 HT HT32 Longitudinals 425x20 HT x18 HT32 F.B(T) 500x35 HT32 F.B PC7 (Polar) Midbody region Shell plate 29.5 HT HT32 (No.3 hold) Longitudinals 425x12+125x18 F.B(T) 450x x18 F.B(T) Membrane tank type Aft region Shell plate 26.0 HT HT32 Longitudinals 350x100x12/17 I.A 400x x16 F.B(T) Bow bottom Shell plate HT32 (Sp.=660mm) Longitudinals - 500x35 HT32 F.B Bow intermediate Shell plate lower Longitudinals - 500x x17 F.B(T) Spherical tank type Bow intermediate bottom Shell plate Midbody lower Shell plate Stern lower Shell plate Forward region Midbody region Aft region Bow bottom Bow intermediate lower Shell plate 36.5 HT HT32 Longitudinals 425x17 HT x14 HT32 F.B(T) 625x20 HT x12 HT32 F.B(T) Shell plate 29.5 HT HT32 Longitudinals 425x x18 F.B(T) 700x x20 F.B(T) Shell plate 26.0 HT HT32 Longitudinals 350x100x12/17 I.A 400x x14 F.B(T) Shell plate HT32 Longitudinals - 625x20 HT x12 HT32 F.B(T) Shell plate HT32 Longitudinals - 400x16 HT x13 HT32 F.B(T) Bow intermediate bottom Shell plate HT32 Midbody lower Shell plate HT32 Stern lower Shell plate Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

9 Figs 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) : Area of increased thickness by Batic Ice 1A : Additional area of increased thickness by Polar PC7 Fig. 11 Strengthened zone by Baltic 1A and Polar PC7 in membrane type LNG carrier Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 45

10 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 the previously 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. Midbody Shoulder No.3 hold model No.1 hold model Simplified No.1 hold model Cargo containment system model 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 46 Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

11 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. Midbody Shoulder No.3 hold model No.1 hold model Skirt and skirt foundation deck 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 is applied. 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 Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 47

12 (b) Spherical tank type Fig. 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 48 Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

13 (a) Membrane tank type (b) Spherical tank type Fig. 17 Position of the patched ice pressure in shoulder ice patch pressure (a) ice patch pressure (b) sea pressure (c) liquid cargo pressure Fig. 18 Configuration of applied loading for analysis Ice patch load Web CCS Stringer Side shell plating Fig. 19 Ice patch load at critical loading location Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 49

14 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 von Mises 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: σ max σ c, where σ c = S m σ y or S m σ u is permissible normal stress, σ y is minimum specified yield strength of materials, σ u is ultimate strength of materials, S m is 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: τ max τ c, where τ c = S m τ u is permissible shear stress, τ u is minimum specified ultimate shear strength of materials, S m is strength reduction factor (SRF), which is recommended as 0.67 for foam and plywood. Von Mises Stress Criterion. eq Maximum von Mises stress is to satisfy the following condition: σ max σ c, where σ c = S m σ y or S m σ u is permissible normal stress, σ y is minimum specified yield strength of materials, σ u is ultimate strength of materials, S m 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 (20 C) Polyurethane Foam (PUF) Plywood Orientation or Grade Strength (MPa) Horizontal Tension 2.4 Horizontal Compression 1.2 Vertical Tension 1.2 Vertical Compression 2.0 Shearing 1.77 Horizontal Tension 40. Horizontal Compression 80. Shearing 2.8 Mastic 15. Steel Steel(Mild) 235. Steel(Hiten-32) 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 loads corresponding to six loading scenarios. 50 Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

15 ABS TECHNICAL PAPERS Shoulder No. 1 hold model Under the ice patch load, MPa in glancing impact case, MPa in wedging impact case and 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 (a) Stringer and web structure (b) Mastics structure Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 51

16 ABS TECHNICAL PAPERS 2008 Fig. 22 Von Mises stress in wedging impact case (a) Vertical stress of R-PUF (b) Horizontal stress of plywood Fig. 23 Axial stress in wedging impact case (a) Stringer and web structure (b) Mastics structure Fig. 24 Von Mises stress in reflected impact case (a) Vertical stress of R-PUF (b) Horizontal stress of plywood 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. 52 Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

17 Layer Table 7 Strength evaluation for each component of CCS at shoulder in glancing case (Case 7) Orientation Maximum Stress (MPa) Reference Stress (MPa) Usage Factor Mastic Von-Mises Vertical Ten R-PUF Vertical Comp Shear Horizon Ten Plywood Horizon Comp (bottom) Shear Table 8 Strength evaluation for each component of CCS at shoulder in wedging case (Case 10) Layer Orientation Maximum Stress (MPa) Reference Stress (MPa) Usage Factor Mastic Von-Mises Vertical Ten R-PUF Vertical Comp Shear Horizon Ten Plywood Horizon Comp (bottom) Shear Table 9 Strength evaluation for each component of CCS at shoulder in reflected case (Case 9) Layer Orientation Maximum Stress (MPa) Reference Stress (MPa) Usage Factor Mastic Von-Mises Vertical Ten R-PUF Vertical Comp Shear Horizon Ten Plywood Horizon Comp (bottom) Shear 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 MPa in ice floe impact pressure case at the midship area (see Table 3), Nonlinear static analyses with related 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. Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 53

18 (a) Stringer and web structure (b) Mastics structure Fig. 26 Von Mises stress in glancing impact case (a) Vertical stress of R-PUF (b) Horizontal stress of plywood Fig. 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 54 Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

19 (a) Vertical stress of R-PUF (b) Horizontal stress of plywood Fig. 29 Axial stress in close pack pressure case (a) Stringer and web structure (b) Mastics structure Fig. 30 Von Mises stress in ice floe impact pressure case (a) Vertical stress of R-PUF (b) Horizontal stress of plywood Fig. 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 Factor Reference Layer Orientation Patch Ballast Full load Stress (MPa) Full Patch Ballast load Mastic Von-Mises R-PUF Vertical Ten. 3.57E E E Vertical Comp. 1.19E E E Plywood Horizon Ten (bottom) Horizon Comp Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 55

20 Table 11 Strength evaluation for each component of CCS at midbody in close pack pressure (Case 13) Maximum Stress (MPa) Reference Usage Factor Layer Orientation Full Stress Full Patch Ballast load (MPa) Patch Ballast load Mastic Von-Mises Vertical Ten. 8.7E-3 5.4E-3 4.2E R-PUF Vertical Comp. 2.7E-3 2.3E-2 8.2E Plywood Horizon Ten (bottom) Horizon Comp Table 12 Strength evaluation for each component of CCS at midbody in ice floe impact (Case 21) Maximum Stress (MPa) Reference Usage Factor Layer Orientation Full Stress Full Patch Ballast load (MPa) Patch Ballast load Mastic Von-Mises R-PUF Vertical Ten. 3.39E E E Vertical Comp. 1.07E E E Plywood Horizon Ten (bottom) Horizon Comp 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 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 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. 56 Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

21 (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 Fig. 33 Von Mises stress in glancing impact case (a) Skirt structure (b) Skirt foundation deck Fig. 34 Von Mises stress in close pack ice pressure case Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 57

22 (a) Skirt structure (b) Skirt foundation deck 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 Inner Longl. Skirt foundation Position Shell Skirt Bhd. deck Load case σ (Mpa) Disp. (mm) σ (Mpa) Disp. (mm) σ (Mpa) Disp. (mm) σ (Mpa) Disp. (mm) Glancing impact Patch load (Case11) Full load Close pack Patch load pressure(case13) Full load Ice floe impact Patch load pressure(case21) Full load Shoulder - No. 1 hold model Under the ice patch load, MPa in glancing impact case, 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 Fig. 36 Von Mises stress in glancing impact case 58 Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

23 (a) Skirt structure (b) Skirt foundation deck Fig. 37 Von Mises stress in glancing impact case (a) Shoulder model (b) Transverse web Fig. 38 Von Mises stress at patch load in reflected impact case (a) Skirt structure (b) Skirt foundation deck Fig. 39 Von Mises stress at patch load in reflected impact case Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 59

24 (a) Shoulder model (b) Transverse web and longl. Fig. 40 Von Mises stress at patch load in wedging impact case (a) Skirt structure (b) Skirt foundation deck 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 Load case Glancing (Case7) Reflected (Case9) Wedging (Case10) Position σ (Mpa) Side shell Disp. (mm) Inner Longl. Bulkhead σ (Mpa) Disp. (mm) σ (Mpa) Skirt Disp. (mm) Skirt foundation deck σ (Mpa) Disp. (mm) Patch load Patch load Patch load 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 60 Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads

25 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 I. J. Jordaan, Mechanics of Ice-Structure Interaction, Engineering Fracture Mechanics, 68, 2001, pp FMA, Finnish-Swedish Ice Class Rules, ABS, Rules for Building and Classing Steel Vessels, American Bureau of Shipping, ABS, Guidance Notes on Ice Class, ABS, Guidance Notes on Strength Evaluation of Membrane-Type LNG Containment Systems under Sloshing Loads, 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, 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, C. G. Daley, Energy Based Ice Collision Forces" - POAC '99, Helsinki Finland, August, ABS, Guide for Building and Classing Membrane Tank LNG Vessels, Structural Integrity Assessment of Cargo Containment Systems in Arctic LNG Carriers under Ice Loads 61

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