Moodboard

L. Diebold, N. Moirod & Ph. Corrignan

ABSTRACT: The objective of the Task 3-4 (WP3) of the Improve Project was to provide (through a calculation module) quasi-static pressures to be applied on the inner hull structure supporting membrane cargo containment system, to account, at preliminary design stage, for the additional loads generated by liquid sloshing in the tanks of Liquefied Natural Gas Carriers. These quasi-static pressures denote the representative design pressures (acting on stiffeners and platings) which are to be taken into account for structural verification according to BUREAU VERITAS Rules (Bureau Veritas, 2007 and 2004) .

Four LNGC tank capacity ranges were to be considered in this task of the Improve Project: <125 000 m3 / 125 000 to 140 000 m3 / 140 000 m3 to 155 000 m3 / 155 000 m3 to 180 000 m3. Some reserves are given for the capacities larger than 155 000 m3. Standard fiilng ratios were considered (ie less than 10%H and above 70%H) and ship service conditions were defined as world-wide. In addition, within the Task 6-2 (WP6) Bureau Veriats carried out a complete liquid motion analysis for a STX Europe 220,000 m3 LNGC in order to provide at preliminary stage the quasi-static loads to be applied on the inner hull structure

1

INTRODUCTION

Sloshing phenomenon represents one of the major considerations in the design of vessels carrying liquid cargo, and in particular for vessels operating LNG. Sloshing may be defined as a violent behaviour of the liquid contents in tanks that are subjected to the external forced motions.

The present work exhibited within Improve Project is focused on the hydrodynamic part of sloshing impact, i.e. evaluation of the sloshing loads on the structure, involving BV long experience in LNGCs and the existing sloshing data base from LNGCs under BV Class.

2

BUREAU VERITAS METHODOLOGY

The sloshing analysis of a LNGC consists of 2 main steps. First, the hydrodynamic analysis which allows to calculate the motion of the LNGC, once the environmental data is given. Second, the sloshing analysis itself which consists in experiments (called also small scale sloshing model tests) and numerical calculations using numerical tools such as Computational Fluid Dynamics. BUREAU VERITAS overall methodology for sloshing assessment of LNG vessels (Bureau Veritas, 2005) is essentially based on the comparative approach based ont the LNGC reference case 138,000 m3.

Finally, quasi-static preesure loads to be apllied on the inner hull structure are derived from the obtained sloshing loads.

2.1 Hydrodynnamic & Spectral Analysis

The purpose of hydrodynamic analysis (HydroSTAR, 2009) is to evaluate range of wave first order motions in order to determine sloshing excitation for either numerical or small-scale model tank. After having obtained the transfer functions, the motions in irregular waves of a given wave energy spectrum are obtained by performing spectral calculations. The results include significant magnitude and average period of the motions.

Figure 1. Examples of roll transfer function and roll spectral calculation cartography for a LNGC in the range [120k:140k]

Beacuse ship service conditions for subject LNGC in the Improve project have been defined as world-wide, the environmental data for sloshing analysis refer to North

0 0.5 1 1.5 2 2.5 3 3.5 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 R o ll a t r e fe rence poin t ( deg/ m)

Wave frequency (rad/s) Range [125k:140k], LC Ballast, Speed=0.0kn, Roll RAO at COG

roll.rao( 180.0deg) roll.rao( 195.0deg) roll.rao( 210.0deg) roll.rao( 225.0deg) roll.rao( 240.0deg) roll.rao( 255.0deg) roll.rao( 270.0deg) roll.rao( 285.0deg) roll.rao( 300.0deg) roll.rao( 315.0deg) roll.rao( 330.0deg) roll.rao( 345.0deg) roll.rao( 360.0deg) WAVE PERIOD Tz (s) WA V E H E A D IN G (° ) 4 5 6 7 8 9101112131415161718 180 195 210 225 240 255 270 285 300 315 330 345 360 24.32 21.88 19.45 17.02 14.59 12.16 9.73 7.29 4.86 2.43 0.00 ,

LC BALLAST - V=0.0 kn - IACS NA 40-y

AMPLITUDE (dg) 10%L 1 0 % L 10%L 10%H 10%H 10 %H 10%H 2 14.65 1 11.75 RESONANCE CONDITION FILLING TR,T (s) 10%H 10%L

Sloshing Loads to be Applied on LNG Carriers’ Inner Hull Structure

Atlantic trade route with 40-years return period wave height envelope (Bureau Veritas, 2005).

2.2 Sloshing Analysis – Model Tests

The small scale sloshing model tests consist in moving a model tank (scale 1/70 for the BV tests) with water at ambient conditions, in order to measure pressures at various locations for a given case (filling ratio, heading, ship speed, wave period). Sloshing small-scale model tests provide identification and confirmation of the most critical cases. Because impacts pressures depend on many parameters like (density ratio, hydro-elastcitcity, cryogenic environment with free surface condition at boiling point of gas etc...) which are difficult to reproduce at model scale, sloshing model tests are used in a comparative manner.

Figure 2. Example of the Test Rig (Ecole Centrale de Nantes) used for BV sloshing model tests. Tank (Courtesy of GTT).

2.3 Sloshing Analysis – CFD Calculations

Numerical sloshing simulations provide overall evaluation of fluid kinematics and independent verification of sloshing effects on cargo tank walls, and overall for the Task 3-4 (WP3) of the Improve Project) evaluation of representative design loads on ship inner-hull structure.

Figure 3. Normalized representative quasi-static pressure loads for a LNGC in the range [155k:180k]

Present sloshing analyses have been carried out using numerical CFD software FLOW3D (currently used in BV) whose mathematical formulation is based on Navier-Stokes equations (mass and momentum conservation), Volume of Fluid (VOF) modelling technique and Finite Volume discretization.

3

SLOSHING LOADS

3.1 Sloshing Loads Module

The objective of the Task 3.4 (WP3) related to sloshing loads was to provide trough a calculation module quasi- static pressures to be applied on the inner hull structure for four LNGC tank capacity ranges: <125 000 m3 / 125 000 to 140 000 m3 / 140 000 m3 to 155 000 m3 / 155 000 m3 to 180 000 m3. Some reserves are given for the capacities larger than 155 000 m3.

The input data describe the ship’s cargo capacity, the number of tanks and the reference tank defined as the tank of biggest capacity with the furthest location relative to the centre of gravity of the considered ship. This reference tank is described through its dimensions: length, breadth, height, lower chamfer, upper chamfer.

The output data represent the representative design pressure pw (Bureau Veritas, 2007) on one quarter of the

tnak for symmetry reasons.

3.2 Slsoshing Loads for STX Europe LNGC

The BV objective of this Task 6-2 (WP6) was to provide quasi-static pressures generated by sloshing to be applied on the inner hull structure of a STX Europe 220,000 m3 LNGC membrane tank in order to perform its structural optimization (only for standard fillings; below 10%H and abiove 70%H).

Thus, a complete liquid motion analysis (hydrodynamic, spectral and liquid motion analysis) was performed and leaded from one hand to the prelimnary sloshing feasibility which should be confirmed by some dedicated model tests and from the other hand to the representative design perssure loads to be applied on the inner hull strcuture.

REFERENCES

Bureau Veritas Rules for Steel Ships, July 2007.

Bureau Veritas Guidance Note, 2004, “Guidelines for structural analysis of membrane LNG Carriers”.

Bureau Veritas Preliminary Guidance, 2005, “Sloshing Assessment for Membrane Type LNG Vessels & Offshore Units”.

EU FP6 project IMPROVE-Final Conference IMPROVE 2009, Dubrovnik, CROATIA, 17-19 Sept. 2009