ASSESSMENT PROCEDURE WITH TODAY S LATEST CALCULATION TOOLS OF MODERN AND EXISTING DESIGNS OF LARGE LPG TANKERS.

Transcription

1 ASSESSMENT PROCEDURE WITH TODA S LATEST CALCULATION TOOLS OF MODERN AND EISTING DESIGNS OF LARGE LPG TANKERS. Bruno Dabouis, Product Manager Tankers Philippe Cambos, Head of Tanker Structure Department, GASTECH 2000 HOUSTON, November Dabouis

2 ASSESSMENT PROCEDURE WITH TODA S LATEST CALCULATION TOOLS OF MODERN AND EISTING DESIGNS OF LARGE LPG TANKERS. Bruno Dabouis, Product Manager Tankers Philippe Cambos, Head of Tanker Structure Department, BUREAU VERITAS, MARINE DIVISION. INTRODUCTION The world fleet of Liquefied Petroleum Gases (LPG) tankers is accounting more than 950 ships representing a total capacity of about 13 million cubic meters. This fleet is consisting of a wide range of ships, different in size, in design, in cargo capabilities and in trading patterns. Type of ships is spreading from the coastal fully pressurised butane and propane tanker of a couple of thousand cubic meter capacity, to the very large fully refrigerated LPG tanker of 80,000 m3 involved on long haul traffics, and also includes more sophisticated small and medium size tankers, semi-pressurised and fully refrigerated, able to carry a wide range of more delicate substances such as ethylene or chemical gases in addition to the current LPG products. The purpose of this paper is to focus on the segment of large LPG carriers designed with atmospheric type A cargo tanks, in accordance with the definition of the IMO IGC code (International Code for the Construction and Equipment of Liquefied Gas Carriers), and to review in particular the design principles, methods and tools, for the hull and cargo containment structures of these ships. For memory, there are today about 100 fully refrigerated VLGC on operation worldwide. In accordance with the IMO IGC code, independent type A cargo tanks are self-supporting tanks designed primarily using recognised ship structural analysis procedures. They are usually gravity tanks made of plane surface and their design vapour pressure is limited to a maximum of 0.7 bar. They in principle differ from the independent type B cargo tanks in so far as their design does not call for refined analysis to determine stress levels, or for fatigue life and crack propagation assessments. As a counter part, independent type A cargo tanks are required to be duplicated with a complete secondary containment system (secondary barrier) able to preserve the integrity of the ship s structure in case of leakage of the cargo tanks and to safely keep the cryogenic cargo into the ship during 15 days. However, calculation tools implemented by shipyards, design offices and classification societies have largely progressed since the first large independent type A LPG carriers were built in the late 60s and early 70s. FEM analysis tools have been more and more user-friendly, easier and faster to run, and, as a consequence, more and more frequently used. Thanks to this evolution in the technology available for calculation of structures, shipyards have been able to develop more refined designs and classification societies have introduced requirements in their Dabouis 1 / 12

3 Rules for direct calculations of the ships structures. Besides classification societies have also introduced minimum fatigue requirements into the classification checkpoints. The question we will address in the paper is therefore the following: to what extent would it be possible to credit the improvements in today s current design practice of independent type A tanks to bring their design closer to type B concept and subsequently, what is the remaining gap between the two? 1. DESCRIPTION OF THE SHIPS STUDIED Two cases have been investigated: - one existing design of a series of ships built in the 80 s, reviewed with the latest tools for design assessment, within the scope of a condition assessment after 20 years of service, - one design recently developed and implemented for a new building. A particular attention was paid to the interaction between the structures of the cargo tanks and of the ship s hull in way of the supports and keys. The two series of ships are of similar dimensions (LBP~210m, B~35m, D~22.5m) and capacities, have both cargo tank design temperature around 48 C, but their hulls are of significantly different designs: - the existing ships have a capacity of 72,000 m3, are designed with a double hull (double bottom and double longitudinal side bulkheads enclosing water ballast capacities), fitted with four cargo tanks insulated with perlite located in the hold space surrounding the cargo tanks and with polyurethane foam panels on top (figures 1a & 1b), - the recent ships have a capacity of 78,500 m3, are designed with single longitudinal side bulkhead, double bottom and upper wing tanks (the latter spaces being used as water ballast tanks), fitted with four cargo tanks insulated with polyurethane foam panels (figures 1a & 1c). Fig. 1a: Typical profile of an independent type A LPG tanker Dabouis 2 / 12

4 Fig. 1b : Typical midship section of an Fig. 1c : Typical midship section of an independent type A LPG tanker independent type A LPG tanker with perlite insulation with foam panels insulation The independent cargo tanks of these two series of ships are supported and maintained in position within the ship s hold thanks to various types of supports and keys. Wooden chocks ensure the contact: - Four rows of vertical supports are arranged in way of the transverse frames of the cargo tanks. They are intended to support the cargo tanks and to transfer the loads from the cargo tanks to the underneath structure while enabling the thermal shrinkage of the tanks versus ship s structure. There are currently different types of such supports able to sustain loads of various ranges from about 400 t to 1,000 t. Those fitted in way of the corners of the cargo tanks are stronger than those located either at mid span, in the longitudinal direction, or close to the centre line, as they have to account for a higher stiffness of both cargo tank structure and ship s double bottom structure in this area. - Anti-rolling keys, preventing from lateral motion of the cargo tanks and enabling them to expand in the transverse direction, are fitted along the centre line of the cargo tanks both in way of the upper and lower parts of the cargo holds. They have to support loads from some 250 t to 600 t. - Anti-pitching keys, preventing from longitudinal motion of the cargo tanks and enabling the tanks to expand in the longitudinal direction, are fitted along a transverse line of the cargo tanks. On one series of ships they are located at the bottom aft of the tanks. On the other, they are located at mid span of the tanks both in the lower and upper parts of the cargo holds. Depending of their arrangement the load they are subjected to varies from about 150 t for upper keys to more than 1,000 t for lower keys. - One row of anti-flotation keys, preventing from lifting of the cargo tanks in case of flooding of the cargo holds, is located on the top of the cargo tank on each side. These keys are supporting loads from some 300 t to about 450 t. Dabouis 3 / 12

5 2. CALCULATION PROCEDURE AND TOOLS The analyses have been performed with VeriSTAR Hull software (version 2.3) developed by Bureau Veritas. This computer programme has already been introduced to the marine Industry. It is a powerful integrated finite element analysis software enabling to automatically generate and load the model of a ship and to obtain in a directly legible format (thanks to 2D & 3D figures with colour coded stress ratios, corrosion ratios, etc.), the assessment of the ships scantlings with respect to Bureau Veritas rules requirements. It includes facilities to perform local refined analysis and fatigue assessment of critical details directly from the coarse mesh model. The following procedure (figure 2.0) has been applied to the structural analyses performed on these ships in accordance with Bureau Veritas rules as relevant for large LPG carriers fitted with independent type A cargo tanks. The assessment was based on a direct 3D FEM analysis of the ship and cargo tank structures along the length of the cargo area. Demand Capability of the Hull Structure Loading Cases 3D FE Model of Ship & Thickness Longitudinal Strength Cargo Tank Structure Measurements Boundary Conditions Light Weight Still Water Loads Wave Loads Determination of:. Deformations. Stresses Strength criteria:. ielding. Buckling. Fatigue Adjustment of Scantlings es No Local Design of Structural Details Fig. 2.0: Procedure for Direct Structural Analysis Dabouis 4 / 12

6 The main objectives of the structural analyses are: - to determine the stress distribution in the cargo tank structure, in the ship structure, and in the keys supporting the tanks ; - to verify that the rule strength criteria are complied with ; - to identify, and draw the attention on, the possible critical area in term of stress levels or fatigue life expectancy. 2.1 Modelling principles of the structural analysis: coarse mesh and fine mesh models. The global 3D FEM model integrates cargo tanks and holds primary members and supports as follows (figure 2.1.a): - Cargo tank shell plating, bottom plating, bulkhead plating, top plating, transverse web frames, horizontal stringers, girders ; - Hold inner (when relevant) and outer shells, inner bottom, longitudinal and transverse bulkhead plating, double bottom longitudinal girders, horizontal stringers, deck longitudinal girders, transverse web frames or main frames, primary members of transverse bulkheads; - Vertical supports (-axis), anti-pitching supports (-axis), anti-rolling supports (-axis) and antifloating supports (-axis). In the coarse mesh, they are modelled by specific elements flexible mounts, whose stiffness is nil when in tension. The number of nodes and elements is so defined as the stiffness and inertia of the model represent properly those of the actual hull structure. In particular, secondary stiffeners, which contribute to the hull girder bending, are accounted for. g g ( Fig. 2.1.a: Detail, over three cargo holds, of the coarse mesh model. Dabouis 5 / 12

7 Beyond the 3D coarse mesh analysis, several refined analyses of selected local structures have been performed on the basis of 3D fine mesh models, namely: - Cargo tank transverse web frame and horizontal stringer, - Hold typical floor, girder below the transverse bulkhead, main frames of the side shell (figure 2.1.b), vertical stiffeners of the transversal bulkhead, knuckles in the double hull (figures 2.1.c & 2.1.d). The standard size of the finite elements used in these local models is at least based on the spacing of secondary stiffeners. Boundary conditions for these fine mesh models are directly obtained from the results of the coarse mesh models with VeriSTAR Hull system. The fine mesh model is thus forced to the same displacement pattern as the coarse mesh model corresponding to the ship s global deformation. Meanwhile, the effects of pressure loads are taken into account to evaluate the local loads and subsequent combined stresses. Fig 2.1.b: Fine mesh model of main frames Fig 2.1.c: Fine mesh model of a double of a single hull side shell. hull knuckle in fore hold. Calculation results are illustrated (figures 2.3.d) by the following prints out of VeriSTAR Hull related t o the inner hull knuckle of a typical hold n 1, for different loading cases in term of Von Mises stresses and in term of resulting stress ratio with respect to the various Bureau Veritas criteria. Dabouis 6 / 12

8 Figs. 2.1.d: 3D Fine Mesh Results Knuckle Line, Hold N 1 Von Mises Stress Model : Holds 1-2-fore - Full load State : Survey state DSA Top Down : DSA Knuckle Line STRESS COMPONENTS LOAD CASE 2 TR/PLATE SVMM QD/PLATE SVMM 2.28E E E E E E E E E > Sorting elements ---- Maximum stress ratio, knuckle line Model : Holds 1-2-fore - Full load State : Survey state DSA Top Down : DSA Knuckle Line VeriSTAR STRESS RATIO ALL LOAD CASES GEN/BEAM PerLong TR/PLATE PerComb QD/SHELL PerMax BAR PerLong TR/MEMB PerComb Von Mises Stress Model : Holds 1-2-fore - Full load State : Survey state DSA Top Down : DSA Knuckle Line VeriSTAR STRESS COMPONENTS ALL LOAD CASES TR/PLATE SVMM QD/PLATE SVMM 2.64E E E E E E E E E > Sorting elements ---- VeriSTAR Dabouis 7 / 12

9 2.2 Assessment of supports Each type of supports has been subjected to a detailed FEM analysis. The structural elements surrounding the supports have been included on both sides (cargo tanks & hull structure) into the relevant models. The selected size of the current finite element edge is based on a quarter of a longitudinal spacing Model : Hold 234 Full load Ho oge D A eous Top cargo Dow (11/ : Fig. 2.2.a: Fine mesh models of a vertical support with its associated structure iwo cargo tanks and ship s double bottom area close to the inner hull hopper. Dabouis 8 / 12

10 - Vertical supports (e.g. figure 2.2.a) Friction forces (horizontal) have been taken into account and combined with the maximum vertical force obtained from the global analysis. Generally, each vertical support is fitted in way of an intersection between a longitudinal girder with a floor. But some vertical supports in the fore in the aft holds may be fitted in way of an intersection between a floor with an ordinary longitudinal stiffener. In such a case, this disposition is subject to a specific fine mesh analysis. - Anti rolling supports (e.g. figure 2.2.b) The transversal force is determined, taking into account a static roll angle of 30. The total weight of the cargo and the structure is considered. The lump distribution of the resulting transversal forces between the upper and lower supports, is of 50% to the upper supports, and of 80% to the lower supports. These forces are generally considered equally distributed on the anti rolling supports along the length of the hold. The transversal force may be reduced by a force corresponding to the friction between the tank and the vertical supports. In this case, account should be taken of the minimum values of dynamic friction coefficient guaranteed by the manufacturer of the wooden chocks. The force generated by the friction on the vertical supports reduces only the part of the transversal force applied on the lower supports. Fig. 2.2.b: Anti rolling keys hull side - Anti pitching & anti collision supports If upper and lower anti pitching supports are fitted, both are to be refined. In such a case, the lump distribution of the longitudinal force is 50% for the upper supports and 80% for the lower supports. Dabouis 9 / 12

11 The longitudinal force corresponding to one half the weight of the tank and the cargo in the forward direction and one-quarter the weight of the tank and the cargo in the aft direction, as defined by the IMO IGC code 4.6.4, is currently the most stringent loading case. This force may be considered as equally distributed on the different anti pitching supports over the hold breadth. The longitudinal force may be reduced by a force corresponding to the friction between the tank and the vertical supports. In this case, account should be taken of the minimum values of dynamic friction coefficient guaranteed by the manufacturer of the wood. If upper anti-pitching supports are fitted, the force generated by the friction on vertical supports, reduces only the part of the longitudinal force applied on the lower supports. - Anti flotation keys The model is to be loaded by a vertical force generated by the buoyancy of the cargo tank in the hold flooded to the summer load draught. This force may be considered as equally distributed on all the anti floating keys. 2.3 Loading conditions Calculations have been carried out for the most severe loading conditions as given in the loading manual, with a view to maximising the stresses in the longitudinal structure and primary resistant members. Beyond the current homogeneous and alternate loading conditions at scantling draught, relevant partial loading conditions and ballast conditions, the three following loading conditions have also to be considered: sequential loading cases in harbour conditions, flooded conditions of one hold up to 85% of the depth (corresponding to hydrostatic tests of the internal bulkheads), flooded conditions of one hold up to scantling draft (for the assessment of the anti-flotation arrangement). For each internal loading condition, an appropriate external loading corresponding to head sea conditions, beam sea condition or harbour condition, is applied. These basic conditions are summarised in the table 2.3 hereafter. They combine the various dynamic effects of the environment on the hull structure, i.e. external sea loads (hull girder wave loads and wave pressures) and internal dynamic cargo pressures in accordance with the Bureau Veritas Rules, Section The specific densities considered are 0.62 t/m 3 for cargo and t/m 3 for seawater ballast. Dabouis 10 / 12

12 Table 2.3: Elementary Loading Conditions (Bureau Veritas Rules Chapter 3.5) Description Draught SWBM Head Sea Beam Sea Harbour HS1 HS2 HS3 BS1 BS2 Homogeneous loading conditions T M cs Ballast conditions T b M ch Alternate loading conditions 0.9 T 0.7 M ch Loading 0.5 M ch * Flooded conditions (test of long. bulkhead) T 0.5 M ch Flooded conditions (test of antiflotation) T b 0.5 M ch With : HS1 : Head Sea : Internal Load : Dynamic External Load : Static HS2 : Head Sea : Internal Load : Static External Load : Dynamic (Crest) HS3 : Head Sea : Internal Load : Static External Load : Dynamic (Trough) BS1 : Beam Sea : Internal Load : Dynamic External Load : Static BS2 : Beam Sea : Internal Load : Static External Load : Dynamic (Rolling) M CS : M CH : Design SWBM in Sagging Condition Design SWBM in Hogging Condition * The case Loading may be calculated only in harbour conditions if not specified in the loading manual. 2.4 Strength criteria: stress, buckling and fatigue. - Stress levels are analysed according to the Von Mises criteria. - Buckling strength is assessed with respect to the requirements of Bureau Veritas Rules, Section 5.12, that encompasses four criteria: uni-axial compression, bi-axial compression, shear, and combined flexural, compression & shear. Buckling calculations performed with VeriSTAR Hull consider a usage factor of 1.15 for the as built state and 1.0 for the state at special surveys. - Fatigue requirements integrated in the Rules of the classification societies. Dabouis 11 / 12

13 3. CONCLUSION : CONSIDERATIONS ON TPE A AND TPE B CONCEPTS In accordance with the IMO IGC code, cargo tanks may be qualified as type B if they are satisfactorily assessed, on the basis of a 3D FEM or equivalent analysis of the cargo tanks, supports and hull structure, for their suitability with respect to plastic deformation, buckling, fatigue failure and crack propagation. But today, the situation appears different to this of the original time of the shipbuilding of large type A LPG tankers: the works performed by classification societies on fatigue of structural details have enabled them to introduce quantitative assessment criteria in their rules, the account of the construction tolerances of structural details is nowadays part of the class societies rules for prevention of buckling and fatigue, the design assessment tools by FEM analysis and direct calculations of the ship s structures are largely available to the designer, the materials used for the cargo tanks (carbon-manganese steels for 48 C) may easily be characterised in term of fracture mechanics properties. Additionally, the return of experience of more than 100 ships over a 30-year period, shows a very limited number of cases where damages were reported to the cargo tanks themselves. Reportedly, the particular problems were mostly due to stress concentrations and fatigue in way of the supports. Besides, the use of the equivalent wave method has now been substantially validated using a number of long-term analyses for different ships hull lines. It may therefore be considered to apply this method to validate the structural design as required by the IGC code paragraph , and this would have the great advantage of offering the possibility to establish a direct link between calculation results and rule design criteria. These arguments are speaking in favour of a possible up-grading of type A designs at the level of type B standard with only few additional work to be done on top of the nowadays current design and building practices of type A cargo tanks. We expect that this paper may contribute to further address this issue with the marine industrial partners. References & bibliography (1) Clarkson World Shipyard Monitor, July (2) Gastech Conference Proceedings (Gastech 90, 93 & 94). (3) Lloyd s Register World Fleet Statistics (4) IMO International Code for the Construction and Equipment of Ships Carrying Liquefied Gas in Bulk. (5) Bureau Veritas Rules for the Classification of Steel Ships, Edition September 1998 & February (6) Bureau Veritas NI 393 Fatigue Strength of welded ship structures. (7) Bureau Veritas NR 418 Implementation of VeriSTAR Hull & VeriSTAR Machinery (8) Bureau Veritas Rules for the Classification of Steel Ships, Edition February 2000, Part B, Chapter 7, Appendix 3, Analyses based on Complete Ship Models. Dabouis 12 / 12