Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Scantling Study for FLNG Convex Deck Design
Hu Shengjun, Chen Gang, Wang Chao, Yuan Hongtao, Wu Bin
Shanghai Waigaoqiao Shipbuilding Co., Ltd.
Shanghai, China

In this paper, the studied unit is selected as FLNG, the mid-crosssection with the convex deck is for rule calculation, finite elementcomparative analysis, the strength characteristics of the convex deck
structure are analyzed, and the direct processing method of rulecalculation for the convex deck structure is discussed, so as to provide
technical thinking for the designer.

ABSTRACTThis paper proposes that the convex deck should be directly calculated
and checked in the design stage, which can not only meet therequirements of the Rule, but also ensure the strength and stiffness ofthe convex deck and save the workload of later design and fine element
analysis, and reduce times of calculation iteration. In this paper, taking
the FLNG project as the research object, the mid-cross section with aconvex deck is designed according to the characteristics of the FLNG
independent LNG tank compartment layout and the requirements of theABS Rule. The ABS FLGT software is used to perform the rulecalculation of the mid-cross section, and the section modulus at deck, atbottom and the neutral axis are obtained, and the finite element modelis established by Patran/Nastran to check the stress distribution at themid-cross section, and their results are almost consistent through thecomparation of the Rule calculation and the finite element analysis. Theresults show that other structures meet the relevant requirements when
the direct calculation of the convex deck strength meets the rulerequirements. Therefore, the design method of directly calculating thesection modulus of the convex deck directly and separately in thedesign stage is more reasonable.
KEY WORDS: Convex Deck; FLNG; Rule Calculations; Section
Modulus; Independent LNG Tanks; Midship Section

THEORY FROM ABS RULE:
In the process of operation, the ship has to withstand various externalforces, such as gravity, buoyancy, inertia force during movement, waveimpact force, etc. When analyzing the strength of the hull, the hull isusually regarded as a hollow beam, and the emphasis is on the uneven
distribution of gravity and buoyancy along the direction of the captain,
so that the hull produces a total longitudinal bend.
According to ABS rule (American Bureau of Shipping 2021),
under the action of the total longitudinal vertical bending moment of
the hull beam, the section modulus at the strong deck needs to meet two
conditions:
Condition 1: Euler-Bernoulli beam theory，it was discovered (EulerBernoulli, 1970). According ABS rule, the required hull girder section
modulus for 0.4L amidships is to be the greater of the values obtained
from the following equation:

INTRODUCTION

SM = Mt / fp cm2-m

The global supply and demand situation of LNG is affected by thecombined factors of the international energy market. Against thebackdrop of increasing global attention on environmental issues, themedium- and long-term outlook for the LNG trade market is very
optimistic. Whether it is the LNG mining liquefaction front end, theLNG receiving station storage rear end, or the LNG transportation,
many project hull cargo compartment designs use convex decks tosolve the LNG storage problem.
Because the convex deck does not extend to the side, the convex deck
is not a strong deck in ABS rule, the main deck belongs to the strong
deck component, the ABS rule only puts forward the requirements of
total bending moments and the minimum section modulus for thestrong deck, the convex deck is not a part of strong deck, and no clear
direct design method is used for this deck, so the designer is easy toignore in the early design, and it is easy to cause the strength of theconvex deck to be greater than the strength of the code when designing,
resulting in an increase in the workload of later design and calculation,and the design cycle is lengthened.

(1)

whereMt = total bending moment；
fp = nominal permissible bending stress；= 17.5 kN/cm2 (1.784 tf/cm2)
The total bending moment, Mt, is to be considered as the maximum
algebraic sum of still-water bending moment and wave-inducedbending moment。
Condition 2: Minimum Section Modulus (SMmin ). This is therequirement from ABS rule. The minimum hull girder section modulus
amidships is not to be less than obtained from the following equation:
SM = C1C2L2B (Cb + 0.7) cm2-m
where
C1 = Wave Parameter;
C2 = 0.01;3855(2)