BUREAU VERITAS ENVOLVEMENT IN NEW DESIGN OF LARGE LNG CARRIER EXAMPLE OF A 165000M3 LNG C PROJECT

Bureau Veritas / Marine Division, 92077 – Paris La Défense Cedex, France

Philippe CAMBOS Technical Management

Head of Oil & Gas Section

Mirela ZALAR Research Department

Head of Sloshing Assessment Section

Sime MALENICA Research Department

Hydrodynamic Engineer

Rina TANI-MORATALLA Technical Management Head of hydrodynamic

Gwendal BACHELOT Technical Management

Hull surveyor

ABSTRACT Bureau Veritas has developed a unique technical expertise in the LNG industries. In the LNG industry, Bureau Veritas has classified more than 60 LNG carriers and has provided a wide range of services (from design review to project certification) on over 10 LNG receiving terminals around the world. More than 20 LNGC are now under building with Bureau Veritas class. Methodologies have been developed to assist shipyards in the design phase of new projects . The paper will show how the latest research and development improvements are taken into account for the assessment of new designs of LNG Carriers . The methodologies are illustrated through the example of a 165000m3 recently ordered with BV class. The following different topics will be addressed in this paper: Ø Hydrodynamic analysis , Ø Structural analysis of ship hull by three cargo tank model and full ship analysis, Ø Spectral Fatigue analysis, Ø Buckling analysis, Ø Liquid motion analysis, Ø Bow impact analysis,

1. INTRODUCTION In the LNG industry, Bureau Veritas has classified more than 60 LNG carriers and has provided a wide range of services (from design review to project certification) on over 10 LNG receiving terminals around the world. More than 20 LNGC are now under building with Bureau Veritas class. Methodologies have been developed to assist shipyards in the design phase of a new project. This paper, based on an actual case list the tasks carried out by Bureau Veritas to help the ship design. The purpose of the present paper is to show how computerised calculations are applied to the design of a modern LNG carrier of large size. The subject is treated through the example of a 165000m3 membrane type LNG carrier recently classified by Bureau Veritas. The following different topics will be addressed in this paper: Ø Hydrodynamic analysis , Ø Structural analysis of ship hull by three cargo tank model and full ship analysis , Ø Spectral Fatigue analysis, Ø Buckling analysis, Ø Liquid motion analysis, Ø Bow impact analysis.

2. HYDRODYNAMIC ANALYSIS Hydrodynamic analyses in Bureau Veritas are performed using advanced in-house program HYDROSTAR. HYDROSTAR is powerful 3D diffraction/radiation potential theory 3-D panel software for wave-body interactions taking into account wind & current loads, multy-body interaction, effects of forward speed and internal liquid motions. Evaluation of 1st and 2nd order wave loads, motions, accelerations, relative motions, wave elevation is dedicated to all types of marine structures. HYDROSTAR is constantly improved by integrating the most recent theories and powerful algorithms, fully validated through the comparisons with semi-analytical studies, computation results from recognized numerical tools and experiments. Seakeeping analyses for 165 000 m3 LNGC project were carried out for two loading conditions, Ballast and Full Load. Examples from hydrodynamic computation are presented in Figure 1, demonstrating 3D panel model, vessel motion on the waves and wave load distribution on underwater hull.

Figure 1 - Hydrodynamic models, motion on the waves and wave loads

North Atlantic wave data, according to IACS URS 34 [1] is used for this analysis as shown figure 2. The spreading function given in the same recommendation was used.

Figure 2 - North Atlantic: Areas 8, 9, 15 and 16

The results of the hydrodynamic analysis are input data for the following analysis:

• Spectral fatigue analysis, • Sloshing analysis, • Bow impact analysis.

3. STRUCTURAL ANALYSIS The ship has received the notation VeriSTAR hull, the structural analysis is carried out within the scope of classification. The ship is completely modelled as shown on the figure 4 & 5.

Figure 3 - Complete Ship Model

Figure 4 - Half-view of the complete ship model

The structural analysis is carried out in two parts :

• Partial models extended over three cargo tanks loaded by rule waves, • Complete ship model loaded through hydrodynamic analysis.

3.1. 3 cargo tanks models The calculation has been performed with VeriSTAR Hull software developed by Bureau Veritas. This software is a powerful integrated finite element software carrying out structural assessment with respect of Bureau Veritas rules [2]. Each typical cargo tank model is calculated separately. An example of a three cargo tank model is shown on the figure 6.

Figure 5 – Model for Center Cargo tank (Coarse Mesh Half Model)

Figure 6 – Model for Cargo Tank No 1 (Coarse Mesh Half Model)

Figure 7 – Model for Cargo Tank No 4 (Coarse Mesh Full Model)

Beyond the 3D coarse mesh, several refined analysis of selected structural members have to be performed on the basis of 3D fine mesh models, namely:

• Cargo tank transverse web frame, as shown figure 9, • Tank longitudinal girders in way of the cofferdam bulkhead, as s hown figure 10, • Cofferdam between tank 1 & 2, as shown figure 11, • End of trunk deck, as shown figure 12, • Connection of Trunk Deck structure with Deck House, as shown on figure 13, • Connection of Pump Room on Trunk Deck, shown on figure 13.

Figure 8 – Fine Mesh Model for Transverse web

Figure 9 – Web Frame Section for Cargo Hold Figure 10 – Cofferdam between Tanks No1 and No1

Figure 11 – Forward End of Trunk Deck Figure 12 – connection of Trunk Deck with Deck House

Figure 13 – Connection of Pump Room on Trunk Deck Calculations have been carried out for the most severe conditions, as given in the loading manual, with view of maximizing the stress in the primary supporting structure of the ship. Beyond the current homogeneous and alternate loading conditions, alternate loading conditions have been taken into account. For each internal loading condition, appropriate external loading corresponding to head sea, beam sea or quartering sea have been taken into account. These conditions have been summarised in the table 1 hereafter. They combine the various dynamic effects of the environment on the hull structure, i.e. external sea conditions (hull girder wave loads and wave pressures ) and internal dynamic cargo pressures in accordance with Bureau Veritas rules. The specific densities considered are 0.5 for LNG and 1.025 for sea water.

N° Description Draught SWBM Ship

upright

Inclined

ship Harbour

A1 A2 B C D

LC1 Homogeneous loading conditions T MSW,S X X X

LC2 Ballast conditions TB MSW,H X X

LC3 Alternate loading conditions 0.9 T 0.9 MSW,H X X X

LC4 Alternate loading conditions 0.75 T 0.9 MSW,S X X X

Table 1 - Load cases applied to 3 cargo tanks model

The results are analysed according to the following rules criteria:

• Combined stress, • Buckling criteria.

Several reinforcements have been recommended based on this analysis as for example:

• Floors at side in typical cargo tank and fore cargo tank, • Cofferdam bulkhead for buckling, • Cofferdam between tanks 1 & 2, • Additional brackets at connection of tank 4 and accommodations.

3.2. Complete ship model In addition to the previous analysis, the full ship was analysed using VeriSTAR CSM (Complete Ship Model) The methodology used by this software was described in a separate paper [3]. This software is used as a complement of the VeriSTAR 3 cargo tanks, which is based on the assumption that the warping of the structure is negligible. This assumption is generally reasonable for membrane type LNG carriers. This calculation is generally necessary for ships with large opening on decks as for example, container vessels or Moss type LNG carriers. However the calculation, requested by the ship owner was carried out in order to identify any additional hot spot area on the vessel. The boundary conditions on the model are limited to one point, in order to avoid rigid body motion. It is verified that the reaction at this node is negligible. The model is to be balanced under the internal loads, including self weight and accelerations, and the external sea pressures on the hull. The software is based on the method of equivalent wave: an hydrodynamic software is used to define the ship motions due to a regular wave, and the pressures due to the wave and the accelerations are applied to the structural model. The wave is chosen to apply on the structural model, the effects of the sea states as: hull girder bending moment, maximum acceleration, relative wave elevation as given by Bureau Veritas rules. This balancing method has been developed in VeriSTAR CSM, with many automatic routines which make the calculation user friendly, despite the complexity of the method. 7 load cases were chosen as shown in the table 2.

Case LC Wave Target Effect Targeted Rule Value

1 Full load Head Sea Wave Induced Vertical Bending Moment in Hogging condition

Maximum Bending Moment

2 Full load Quartering Sea Heading 120°

Wave Induced Horizontal Bending Moment

in Hogging condition

Maximum Bending Moment

3 Ballast Head Sea Wave Induced Vertical Bending Moment in Sagging condition

Maximum Bending Moment

4 Alternate Beam Sea Wave Induced Vertical acceleration Maximum Torsion Moment

5 Alternate Beam Sea Wave Induced Horizontal acceleration Maximum Torsion Moment

6 Alternate Head Sea Relative Motion at Midship Maximum

Angle of Rolling

7 Alternate Beam Sea Relative Motion at Sides Difference of height at sides

Table 2 - Load cases for complete ship model

The figures 14 and 15 show results of the complete ship model with VeriSTAR CSM for two load cases.

Figure 14 - Hydrodynamic analysis in head sea condition

Figure 15 - Equivalent stress for Full model

Figure 16 - Equivalent stress for Cargo area

This analysis has conformed that there was no additional hot spot area or needed reinforcement after the 3 cargo tank model calculations. This analysis is also used to verify that the stress level in the structure supporting the membrane is in accordance with the membrane designer requirements. 4. SPECTRAL FATIGUE ANALYSIS Since the use of computers for design leading to structural ship size increase, scantling reductions and of high strength steels to reduce hull weight, the fatigue cracki ng becomes a major failure mode for LNG carriers . A failure in the double hull may have significant consequences:

• Water may leak from the ballast to the insulation leading to serious damage, • The rewelding of the double hull is difficult due to the presence of the insulation (risk of fire)

This question was addressed by the publication in 1987 of a guidance note "Cyclic Fatigue of Nodes and Welded Joints of Offshore units" followed the following year 1988 by another guidance note "Cyclic Fatigue of Welded Joints on Steel Ships". Then an updated guidance note introducing the probabilistic approach was edited in 1988 "Fatigue Strength of Welded Ship Structures" and compulsory rules were introduced in 2000 classification rules [2]. During the same time, the enormous progresses in IT allowed the development of tools to compute the ship behaviour on regular and irregular waves, the resulting static and cyclic loads, and finite elements modelling to compute the stresses at the designer desired detailed level. The in-house developed software and gained experience, in particular for 120.000 m 3 LNG carriers, but also for in-service tankers and FPSO, and the numerous participations to European and international R&D projects place Bureau Veritas as one of the major actors in ship and offshore fatigue method development. Bureau Veritas has developed a class notation “VeriSTAR Hull DFL 40 years” which may be granted to LNG carriers which have been subject to a fatigue analysis. It is the case of the 165000m3 LNG carrier taken as example of this section.

4.1 Rule requirement The verification of fatigue strength of the structure of seagoing ships has been developed and based on a deterministic methodology. In the early 1980s, this methodology was introduced with the purpose to assure that the seagoing ships could respect a fatigue life of 20 years, in North Atlantic sea conditions, with a cumulative damage ratio of 1, taking into account the SN curve at minus two standard deviations. At that time, a guidance notes for fatigue assessment have been developed by BUREAU VERITAS. Fatigue assessment became mandatory, for ships more than 170 m in length, in 2000. In the case of membrane type LNG carriers, fatigue calculations are manda tory for structural details as

Ø Connection of longitudinals with transverse web frames,

Ø Hopper knuckles ,

Ø Stringer connection ,

Ø Tank dome (particularly for some containment systems which request large dome openings). The rules developed by BUREAU VERITAS [4] comply with IACS recommendation No56 [5], the IIW documents, and the results of the JIP fatigue [6]. Different methodologies may be applied to take into account the wave load in the fatigue analysis:

• Deterministic calculation The stress ranges are calculated based on loads at a probability level of 10- 5. The loads take into account the hull girder ending moment and shear forces, the wave pressure on the shell and the internal tank pressure due to the accelerations, which are taken from rule formula. The distribution of long term stress range assuming a Weibull distribution, and rule formula are used to define the shape parameter. The deterministic methodology has been calibrated using the spectral methodology, described hereafter, as well as return of experience.

• Spectral calculation The spectral fatigue analysis includes the following steps:

Ø An hydrodynamic cal culation in the frequency domain, using 3D diffraction radiation, by which the vessel motions, accelerations and wave pressures on the hull are obtain.

Ø A structural analysis, also in the frequency domain, using a 3D FE model. BV generally make full ship model.

Ø From RAO’s of stress range, in the short term stress distribution is obtained by the technique of Spectral analysis. The figure 3 shows an example of RAO of stress.

Ø Calculation of the long term stress distribution by summation over the wave scatter diagram, of the short term distribution. Generally North Atlantic sea conditions from IACS are used.

Ø Calculation of the fatigue damage ratio by the Miner sum. BV uses HydroSTAR software and VeriSTAR hull, full ship. A interface between the two software have been developed . In the case of the 165000m3, the Spectral fatigue calculation was applied.

The following load cases were taken into account: • 25 frequencies, • 2 drafts (ballast and full load) • 7 headings ( 0°, 30°, 60°, 90°,120°,150°,180°)

For the 165000m3, taking into account the 2 load cases for intermittent wetting, the total number of load cases is 702. Structural models The figure 17, 18 and 19 show some fine mesh models for the fatigue analysis of the knuckles, the foot of cofferdam bulkhead and the tank dome.

Figure 17 - Liquid Dome Opening

Figure 18 – Fine mesh for fatigue of knuckles 1 & 2

Figure 19 - Foots of cofferdam bulkhead

This analysis has led to significant structural detail improvements, in knuckles, cofferdam bulkheads, dome, bottom longitudinal stiffeners, ends of trunk deck… The reinforcements were performed by local increased thickness and scantlings, addition of inserts, grinding, improve of bracket shapes… 5. BUCKLING ANALYSIS Two parts of the structure were analysed through linear buckling analysis, in order to assess the strength of these connections. These two parts are:

• Knuckle in trunk deck at side, • Bilge.

Concerning the trunk deck, the loads are hull girder stress, when for bilge, there in addition to hull girder the lateral sea pressure was taken into account. Boundary conditions were applied at the border of the model to allow longitudinal displacements. Simple supports were prescribed in way of transverse frames.

The analysis has shown a safety coefficient of the structure. In particular it was shown, in both cases, that the most critical area was not the bent plate, but the flat plate stiffened by longitudinal stiffene rs, adjacent to the bent plate. Consequently, there was no modification of the structure. The figures 20 & 21 show the models and the results of the analysis.

Figure 20 – Buckling analysis of Knuckle in trunk deck at side

Figure 20 – Buckling analysis of bilge 6. LIQUID MOTION ANALYSIS Bureau Veritas is extensively involved in studies dedicated to the new -generation large LNG vessels, relying on the comparative approach supported by the competence gained through the almost 40 years of experience in LNG Carriers. One of the key issues for the design of the large size LNG carriers is the effect of the LNG flows in the large tanks, on the following members:

• Containment system (membrane), • Hull supporting the membrane, • Pump support tower.

Common operation of membrane type LNG Carriers is carried out with the cargo tank fully laden or with minimum cargo contents during the return ballast voyage. A large number of sloshing studies carried out since early 70's resulted with the conclusion that severe sloshing effects can be mitigated by accommodation of large chamfers in the upper part of the tanks, enabling the extension of upper filling limit to 80% and then even to 70% of filling height for conventional LNG Carriers.

Up to now, the conventional membrane type LNG vessels were approved for conventional filling levels (below 10% and above 70%) for ship with a capacity less than 155 000 m3. As the subject 165 000 m3 vessel is beyond standard capacity and therefore beyond conventional limitations, particular attention was given to the investigation of sloshing flows occurring at various fillings and their effects on different structural members. To assess the strength of the different structural parts of LNG vessels, a comprehensive methodology has been developed in Bureau Veritas which may be summarised on the Figure 21.

Figure 21 – Overall Methodology for Sloshing Assessment in Bureau Veritas Overall sloshing assessment methodology incorporated in basic principles of Bureau Veritas procedure for qualification of containment system resistance and verification of double-hull scantlings against sloshing loads, employs a complex set of information, tools and methods, such as:

• Seakeeping analysis (basin model test and hydrodynamic computation), • Sloshing analysis (small-scale experiments and CFD computation), • Structural examination (material and mechanical tests of structural properties in static,

dynamic and cryogenic conditions), • Fluid-structure interaction (hydro-elastic FEM analysis), • Experience feed -back (analysis of sloshing-induced damages and full -scale measurement).

Bureau Veritas sloshing assessment methodology has been subject of a paper presented in GASTECH 2005 conference [7]. It should be underlined that the current state of the art in sloshing assessment is essentially a comparative one, due to the limited knowledge of real physical models. As regard to particular BV sloshing assessment procedure, response-based sea-states are introduc ed in order to avoid penalisation of ship scantlings due to the non -realistic operation conditions. Limitation of excessive motions has been performed through the comparison of absolute ship response on the extreme sea-states to the selected operability parameters (as roll angle and accelerations) according to BV Rules recommendation. Ship absolute response to the environmental conditions has been determined by means of spectral analysis, combining ship RAOs (from hydrodynamic computation) and selected sea -states (from operational conditions).

According to the results of hydrodynamic computation and subsequent spectral analysis, set of representative cases was selected and dominant motions and liquid flows identified. Representative tank for the sloshing study has been selected according to the criteria of the largest capacity being the most agitated on-board the vessel. For the subject 165,000 m3 LNG Carrier, selected tank is Tank N°2, due to it's furthest disposition measured from the ship centre of gravity. Liquid motion analyses have been carried out for several different filling levels of Tank No2:

• Low filling level: R = 10%L • High filling levels: R = 70%H, 80%H & 95%H

Using the most severe states identified in study of Tank No2, additional examination of sloshing loads has been performed on Tank No1, to verify influence of its particular geometry on generated LNG flows. Various scenarios related to the type and natures of liquid flow have been set for each filling level, governed by the contemplati on of the following conditions:

→ Resonance condition, since sloshing is a typical resonance phenomenon occurring when the ship motion contains energy in the vicinity of the highest tank natural period,

→ Maximum motion condition, following the consideration of vessel design requirement to sustain extreme environmental loads.

→ Intermediate condition, for screening of fluid flow variation in function of wave headings and periods.

Numerical analyses have been performed using HydroSTAR (for sea-keeping part) and CFD software FLOW3D (for liquid motion part). As for it concerns CFD applicability for LNG sloshing problem, it should be noted that pressure calculated in each mesh cell does not consider impact pressure. Impact pressure is strongly related to both, liquid and gas compressibility and hydro-elasticity effects that are not taken into account in actual CFD model. Thus, we prefer evaluating kinetic energy of the liquid and “quantify” impact only by the impact velocity, impact angle and geometry of the jet before the impact. Nevertheless, pressures or forces of quasi-static nature are also calculated and provided for verification of double-hull back-up structure. For each studied filling level, a period scanning analysis has been carried out to determine the numerical resonance period, which is needed to accurately simulate resonant liquid flows. Moreover, a Full Zone Approach (FZA) is performed to analyse spatial and temporal distribution of sloshing loads in predefined hot-spot zones inside the cargo tanks. Tanks have been meshed using Volume of Fluid technique (VOF), and models of Tank No2 and Tank No1 are presented on Figure 22 and 23, respectively.

Figure 22 – Tank No2 VOF model Figure 23 – Tank No1 VOF model

Throughout all sloshing numerical analysis carried out for 165 000 m3 project, different types of fluid flows have been observed for different filling levels.

In the case of very moderate excitation, free surface remains flat moving in a succession of static-equilibrium states. Contrary, when th e excitation velocity increases, two quite different kinds of fluid flows will appear according to the filling rate and tank proportions. For the shallow filling depth, hydraulic bores and travelling waves appear moving back and forth between tank walls. At high fillings, standing waves appear moving upwards and downwards with one or two nodes depending on excitation period. Some examples of captured instants from sloshing simulation for different tank geometry and different fillings are displayed on Figures 24 to 26 below.

Figure 24 – Tank No2

70%H filling level Figure 25 – Tank No2

10%L filling level Figure 26 – Tank No1

70%H filling level Maximum quasi-static pressures and impact velocities were obtained for 10%L filling level. But significant impact values have also been witnessed for high filling levels. Finally, liquid motion analysis of carried out for 165 000 m3 project has led to the following :

• Confirmed feasibility of 165 000 m3 LNGC design with 4 -tanks arrangement from liquid motion point of view,

• Confirmed model tests results, • Many reinforcements in the double hull, particularly the ordinary stiffeners in the midship

section, • Local reinforcements of the containment system, • Strength assessment of the pump mast, by verification of the pump mast designer.

Figure 27 shows the area of the midship section candidates for reinforcements against sloshing loads.

Figure 27 – Area of the midship section candidates for reinforcements against sloshing loads

6. BOW IMPACT ANALYSIS The purpose of this analysis is to assess the strength of the bow against slamming impact and to specify structural reinforcements if necessary.

The strength of the bow was assessed against rules criteria prior to this analysis.

The zones of interest which were identi fied as critical and representative for the ship fore part are shown in Figure 28.

Figure 28 - Representative zones in which the slamming loads are evaluated. The overall computational scheme for local slamming calculations is shown in Figure 29. This methodology was subject of a article [8]. This methodology includes :

• Sea keeping calculations in frequency domain using the 3D panel code HYDROSTAR in order to determine the global ship motions and relative ship motions at the bow.

• Time domain simulations (frequency domain reconstruction for this case) in order to determine the impact occurrences and impact conditions (relative geometry and velocity).

• Slamming calculations for specified ship sections in order to calculate the time history of the pressure distribution.

• Structural assessment of bow structure against impact pressure.

Figure 29 - Computational scheme. The slamming code is based on 2D generalized Wagner approach. The boundary value problem at each time step is solved using the singularity distribution method.

Once the RAO’s are calculated, spectral analysis on relative motions and velocities is performed in order to identify the most critical conditions from the slamming point of view. In Figure 30, the maximum relative motions and velocities are given for different loading conditions and for the sea states defined by the North Atlantic wave envelope as previously discussed.

Figure 30 - Maximum responses in terms of relative motions and velocities at the bow, for

different loading conditions.

Once the critical conditions identified, the time domain simulations can be performed and the exact impact conditions identified. This procedure is briefly illustrated in Figure 31. The history of the relative motion and velocity is calculated at the same time. In that way we are able to identify the instant when the relative motion exceed the draught and subsequently the instant when the section hits the water i.e. impact occurrence. For that particular time instant we know also the relative velocity which is used as input to the slamming code.

Figure 31 - Determination of the impact conditions.

The above described procedure is valid for any particular point on the ship hull. Here below we first apply it to the bow stem. The state of the art in slamming calculations do not allow for 3D calculations, so that the usual procedure employs the so called strip approach which is believed to be conservative since the 3D effects tend to reduce the slamming loads The figure 32 shows typical pressures results.

Figure 32 - Maximum pressure at different points

The calculation is carried out in North Atlantic sea conditions with reduced speed for maximum Hs and for reduced Hs for maximum speed.

The structural assessment was carried out taking into account the obtained pressures, using rule formula for scantlings of stiffeners and plating. The analysis led to some local reinforcements of plating and stiffeners in the upper part of the bow.

7. CONCLUSION

During the last years Bureau Veritas has developed several methodologies and tools to assess the ship hull against different loads. These tools take into account the l atest developments made by shipping and offshore industries, by the mean of research groups, JIPs…

The paper shows how this means may be used by shipyards, shipowners or charters to improve the new designs of LNG carriers, on a rationally basis.

8. REFERENCES

[1] IACS, Recommendation 34: “Standard Waves Data for Direct Wave Load Analysis”, IACS Blue book.

[2] BUREAU VERITAS , Rules for steel ships, 2006 .

[3] M. FRANCO, F. BIGOT, VeriSTAR Hull, un systéme integer de suivi de l’état structurel des navires, ATMA, Number 101, 2002.

[4] BUREAU VERITAS , Spectrale Fatigue Analysis Methodology for Ships and Offshore Units, 2006.

[5] IACS, Recommendation No56, “Fatigue assessment of ship structure”, IACS Blue book.

[6] BUREAU VERITAS, DNV , Fatigue Design Recommendations for FPSOs, revision 2, 2003.

[7] M. ZALAR, P. CAMBOS, P. BESSE, B. Le GALLO, Z. MRAVAK, Partial filling of membrane tank LNG carriers, GASTECH, Bilbao, 2005.

[8] J.F. SEGRETAIN , “Structural damages due to slamming”, Boxship, London, Oct. 2003.