International Shipbuilding Progress 64 (2017) 41–50
DOI 10.3233/ISP-170136
IOS Press
41
Ship stability analysis during launching from
longitudinal sloping slipway by pneumatic airbags
L.S. Volenyuk and A.S. Rashkovskyi ∗
Department of Shipbuilding Technology, Admiral Makarov National University of Shipbuilding,
Nikolaev, 54025, Ukraine
Received 23 August 2016
Revised 25 April 2017
Accepted 1 May 2017
The changing forces acting on a ship during launching from a longitudinally sloping slipway can be
divided into four stages: the 1st stage – from beginning of ship movement until the launching runners
or the ship’s aft end contacts the water; 2nd stage – from the end of 1st stage until the ship begins to
float off the ways; 3rd stage – from the end of 2nd stage until the ship paces the slipway threshold and
is fully afloat; 4th stage – from the end of 3rd stage until comes to a full stop. The biggest danger for a
ship during the launch is the end of stage 2 and the beginning of stage 3, when tipping is possible as the
ship rotates around the slipway threshold; also as the ship pivots on the slipway’s threshold there are large
concentrated forces imparted on the forward section of the ship due to pivoting pressures (the difference
between the mass of the ship and launching devices and the rising buoyancy). This paper presents, for the
first time, a new analysis method of taking into account the possibility of a tanker tipping during launching
on pneumatic airbags. Launching calculations and diagrams for both the traditional and pneumatic airbags
launching are presented along with a comparative analysis of the two launching methods. The analysis
showed that during launching using pneumatic airbags, float off begins earlier than during a traditional
launch. This earlier float off does not increase the risk of tipping but does increase the pivoting pressure
due to the increased buoyancy from the airbags.
Nomenclature
S
WC
V
γ
xG , zG
L1
L2
the way passed by ship at the beginning of stage 2
launching weight of the ship and launching device
the volume of submerged hull part
specific gravity of the water
center of gravity coordinates
the length of forward runners part (from plane y O z to aft perpendicular)
the length of back runners part (from plane y O z to fore perpendicular)
* Corresponding author. Tel.: 380977594518; E-mail: alexrash@trion.mk.ua.
0020-868X/17/$35.00 © 2017 – IOS Press and the authors. All rights reserved
42
c
β
α
T0
λ
MW
MV
MW
MV
xO y
xOz
y O z
yOz
b
K1
g
R
Ld
N1
Tav
xC , zC
xf
R1
GM L
xf
Tf
Ta
M
s
l
a
Np
CB
Lpp
N
D1
L.S. Volenyuk and A.S. Rashkovskyi / Ship stability analysis during launching
the elevation of ship keel above slipway runners in plane y O z
the angel of runners inclined (slipway incline)
the angle of ship base plane inclined to horizon
the calculated water depth on slipway threshold
the length of underwater runners part
the moment of gravity force about back part of runners
the moment of buoyancy force about back part of runners
the moment of weight force about slipway threshold
the moment of buoyancy force about slipway threshold
the plane parallel to base plane of the ship
central plane of the ship
transverse plane, that passing through center of gravity – G-point (Fig. 1)
middle frame transversal plane
the length of each pneumatic airbag
safety factor
acceleration of gravity
load capacity per unit length of airbag
the length of contact between ship bottom and airbag at middle plane
the numbers of pneumatic airbags between slipway threshold and aft perpendicular
mean draft of the ship
center of buoyancy coordinates
x-coordinate of center of floatation waterline area
longitudinal metacentric radius
longitudinal metacentric height
distance between center of floatation of waterline and plane y O z
draft at the fore perpendicular
draft at the aft perpendicular
static moment about plane y O z
distance, that airbag pass on slipway surface at the beginning 2nd stage
distance between airbags
distance between center of gravity and slipway threshold
pivoting pressure
the block coefficient of launching ship
the length of the launching ship
the number of airbags
the external diameter of airbag
L.S. Volenyuk and A.S. Rashkovskyi / Ship stability analysis during launching
43
1. Introduction
Several methods of launching ships can be used. The principal methods are: floating off, gravity (end or side) launching, and mechanized methods. A longitudinal
sloping slipway (end launching) is the most complicated method of launching a ship
as it has a number of elements in the process which are largely uncontrollable. To
launch a ship on a longitudinal sloping slipway requires a ship specific launching
cradle to be mounted on hull above the slipway. The main disadvantages of this
launching method are: the impossibility to control all aspects of the launch; the necessity of using a large number of steel slipway runners; attachment of the launching
cradle to the ship’s hull; the large amount of wood required for both the slipway runners and the launching cradle; the complexity of ship transfer from the keel blocks
to the launching cradle; the preparation and application of grease on to the slipway
runners; pollution of shipyard’s water area by grease components; difficulty of dismounting and retrieval of the launching cradle after the ship’s launching; the long
lead time to prepare for the launch; and the large number of man hours required for
the entire process [2,4].
This new method of ship launching using pneumatic airbags eliminates the above
mentioned disadvantages [1,3,5]. The research which began in [6] are continued in
the present paper.
The application of such method can minimize the cost for production and mounting launching cradle, simplifies preparation work to launch a ship, and eliminates
pollution of a shipyard’s water area. In order to launch a ship using airbags it is sufficient to build a flat inclined plane slipway made of concrete, crushed stone or coarse
sand [8,9]. The most of modern vessels engine room and superstructure are located at
the stern part of the ship. That increases the possibility of the ship tipping, bouncing
and hitting the slipway threshold during the 3rd stage of launching on a longitudinal
sloping slipway. Such researches have never been done before.
The object of the article is to explore a tanker’s launching on a longitudinal sloping
slipway using pneumatic airbags in order to determine the possibility of tipping, to
determine the pivoting pressure and to conduct the analysis of the airbags influence
on the launching process.
2. Method of calculation
For these ship launching calculations on a longitudinal sloping slipway it is assumed that the ship speed is low as it moves along slipway thus, the influence of
hydrodynamic forces are negligible. Therefore a static analysis has been carried out.
The main aim of the static analysis is to find the position of the vessel when the
angular displacement occurs – tipping or floating off. Also of interest, are all the
positions along the ways where high concentrated forces occur [10,11].
44
L.S. Volenyuk and A.S. Rashkovskyi / Ship stability analysis during launching
Fig. 1. Forces acting on ship during launching.
In order to determine the distance traveled by the ship before she begins to emerge
and the value of pivoting pressure, as well as to determine the possibility of tipping,
it is sufficient to consider the data from a static analysis of the ship launching at 2nd
stage. The ship motion takes place parallel to the launching runners and the gravity
forces, slipway reaction and water pressure are acting on her [2,4]. The buoyancy
calculations are carried out by using Bonjean curves and the results are presented as
a launching diagram – the complex of graphics presenting the dependence of values
, M on the distance S the ship moves at 2nd stage (Fig. 1)
W C , γ V , MW , MV , MW
V
[1].
It is admitted that ship launching is carried out astern. The coordinate system xy z
is determined for the ship during launching. The coordinate planes are main plane
xO y, central plane xOz , transverse plane y O z which is located at a distance xG
from midship plane [4].
The abscissas of the ship’s center of buoyancy and center of gravity at calculated
waterline area are measured from plane yOz and y O z . Let’s mark them as xC , xf
and xC , xf respectively. Thus xC = xC − xG and xf = xf − xG .
The following symbols of launching device’s parts are used in the calculation:
stationary part β, T0 , λ and movable part α, L1 , L2 , b, c.
Angle α is slightly different from angle β and therefore it is accepted that tg α ≈
sin α ≈ α and cos α ≈ 1.
As an example, the tanker with the below characteristics shown in Table 1 is used
in the calculations.
It is necessary to calculate the number of airbags and the distance between
them for ship launching using pneumatic airbags [7,10]. The numbers of pneumatic
airbags for ship launching could be taken from the following formula
N = K1
WC · g 2
+ N1 ,
C B · R · Ld
(1)
where K1 = 1.2 ÷ 1.3; WC = 12035.11 t; g = 9.81 m/s2 ; CB = 0.79; R =
11.54 t/m (see Table 2); Ld = 15 m; N1 = 1.
According to (1) N = 53 pcs are required.
L.S. Volenyuk and A.S. Rashkovskyi / Ship stability analysis during launching
45
Table 1
The tanker characteristics
Characteristics
Value
Length between perpendiculars Lpp , m
Ship breadth B, m
Ship depth D, m
Ship draft T , m
Block coefficient CB
Cargo density ρ, t/m3
174.6
31.1
17.1
12.2
0.79
0.75
Carrying cargo capacity W , t
Launching weight of ship and launching device WC , t
Lightship abscissa of center of gravity xG , m
Light ship application of the center of gravity zG , m
59610
12035.11
−15.12
10.93
Table 2
Ship launching airbags characteristics
Characteristics
Value
External diameter D1 , m
Effective height (airbag height during the descent), m
Load capacity per unit length, t/m
Maximum length, m
Effective length, m
Weight, kg
Volume, m3
1.8
0.9
11.54
18
15
581
48.5
The effective airbag length is 15 meters which is half the launched ship’s breadth.
Therefore it is necessary to mount two rows of airbags under the ship (Fig. 2). Such
airbag arrangement provides the required ship stability during launching. Thus, the
total number of airbags is N = 106 pcs.
The longitudinal distance between the centers of two neighbor pneumatic airbags
should meet the required strength of hull structure.
The distance between airbags centers l can be calculated by
l=
Lpp
;
N −1
l⩽6
and l ⩾
πD
+ 0.5,
2
(2)
where Lpp = 174.6 m; N = 106 pcs; D1 = 1.8 m.
The distance between airbags is taken as l = 3.3 m.
Taking into account a contact length Ld of ship’s bottom and airbags at midship
the airbags with characteristics shown in Table 2 [3,5] are taken. Choosing the value
of Ld in the catalog (assortment) of airbags, depending on width of the ship, airbags
circuit placement. The length of the working airbags body should correspond to the
width of the vessel at midship.
46
L.S. Volenyuk and A.S. Rashkovskyi / Ship stability analysis during launching
Fig. 2. Airbag arrangement.
In order to determine the possibility of the ship tipping during launching at 2nd
stage the launching diagram has been built. The most suitable is English diagram.
Since the ship’s center of gravity moves parallel to slipway plane during the 2nd
stage of longitudinal launching the beginning of emerge. tipping and maximal pivoting pressure can be determined. A static analysis of ship launching during the 2nd
stage is made by using Bonjean curve. The point at which the aft perpendicular is
over water boundary line is taken as a beginning of the 2nd stage [10].
The following are the input data for calculations: WC ; ship’s center of gravity coordinates xG and zG (Table 1); L1 = 72.2 m; L2 = 102.4 m; c = 0.9 m; β = 0.05;
α = β = 0.05; T0 = 3.6 m; λ = 68.7 m; water specific gravity γ = 1.025 t/m3 .
The approximate values of emergence waterline and pivoting pressure are determined during preliminary calculations for the ship. By using hydrostatic curves
for displacement WC (Table 1) the following characteristics of the launched ship:
T = 2.98 m; xC = 7.088 m; zC = 1.561 m; xf = 7.195 m; R1 = 646.3 m.
xg −xC
GM L = R1 + zC − zG = 639.93 m and trim is ψ = GM
= −0.0349 are
L
calculated and preliminary value of pivoting pressure is:
Np =
wC · GM L
(α + ψ) = 1678.2 t,
Lz − x f
(3)
where xf = xf − xG = 22.4 m.
= 2.5 m and x
By using hydrostatic curves the values of average draft Tav
f2 =
1
3
7.28 m for volume V2 = γ (WC − Np ) = 10104.31 m are calculated.
The parameters of highest calculated waterline taken with 20% reserve:
h = 1.2 Tav
+ xf · α ,
(4)
where h = 4.41 m.
The parameters of lower calculated waterline is determined assuming the ship’s
center of gravity is at the slipway threshold h = T0 − c = 2.7 m.
L.S. Volenyuk and A.S. Rashkovskyi / Ship stability analysis during launching
47
The lower calculated waterline is determined taken drafts at fore Tf and aft Ta
perpendiculars:
L
− xg α = −2.67 m;
2
L
+ xg · α = 6.48 m.
TA = h +
2
Tf = h −
(5)
The position of lower calculated waterline is determined by
h = h − h /n = 0.342 m.
(6)
where n is numbers of waterlines (n = 5).
The distance 0.35 m between calculated waterlines is decided during calculations.
The waterlines with step h = 0.35 m from the highest calculated waterline parallel
to it downward on Bonjean curve are drawn.
The values of frames’ areas ω are taken from waterlines in Bonjean curve. The
buoyancy force γ V and its static moment M relatively to plane yOz are calculated
for each waterline. Thus static moment M relatively to plane y O z is M = M −
γ V xg (Table 3).
The buoyancy forces of pneumatic airbags and actuating momentum are determined in the same way as for additional pontoons which sometimes are fixed to the
hull for increasing of buoyancy force.
During the calculations of the distance the airbags pass the slipway it’s necessary
to take into account that when the ship passes distance S, the airbags pass distance s.
In order to determine this distance it’s considered that airbags have inextensible shell
[6]. Thus s = 1/2S. Based on this statement the number of airbags that act on the
ship launching process is determined as
N =
S
2 +l
c + π2 (D1 − c) + l
.
(7)
The distance the ship passes from the beginning of the 3rd stage is calculated by
formula (8)
S = L1 +
h+c
β
(8)
The abscissa of buoyancy forces is calculated by formula (9)
xC =
M
γV
(9)
48
L.S. Volenyuk and A.S. Rashkovskyi / Ship stability analysis during launching
Table 3
The calculation of displacement and static moment for 1st waterline
1
No. frame
2
multiplier
3
4
Frame areas, sq.m
fore
aft
10
9.11
8.12
7.13
0
1
2
3
50.838
37.897
25.088
12.631
6.14
5.15
4.16
3.17
2.18
1.19
4
5
6
7
8
9
0.2
5
1
2
γV
M
M
5
(3) − (4)
6
(2) × (5)
63.799
76.135
87.803
50.838
−25.903
−51.047
−75.172
0
−25.903
−102.094
−225.515
0.869
0
0
0
0
0
97.904
101.812
96.116
76.746
45.363
11.605
−97.0356
−101.814
−96.116
−76.746
−45.363
−11.605
−388.142
−509.068
−576.696
−537.219
−362.904
−104.441
0
127.322
0
657.283
0
0
−2831.98
784.6
−2831.98
7020.83
−221229.2
−115074.2
The buoyancy force moments relatively to the runner’s aft end (10) and relatively
to slipway threshold (11) are calculated:
MV = γ V L2 − xC
MV = −γ V a + xC ,
(10)
(11)
where a = L1 + λ − S (Fig. 1).
The calculation is performed for 1st waterline and presented in Table 3. The same
calculation is used for each waterline. The calculation results are presented in English
diagram (Fig. 3).
Based on the calculation taking into account the distance S which the ship passes
at 2nd stage, next lines are built in the diagram: WC ; γ V ; MV ; MW = WC · L2 =
is a line between two points: point with abscissa S =
123.24 · 104 t*m; MV ; MW
= W · a = 0 t*m and point with abscissa S = 173.54 m
140.9 m and ordinate MW
C
and ordinate MW = WC · a = −32.16 · 104 t*m [10].
L.S. Volenyuk and A.S. Rashkovskyi / Ship stability analysis during launching
49
Fig. 3. British diagram of ship launching: —— – with airbags; – · – · – – without airbags.
3. Conclusions
Based on the above drawn diagram the following conclusions can be made:
• the ship’s tipping during her launching using airbags will not occur at 2nd stage
. The critical position can occur at S
because MV > MW
emer. = 152.75 m when
ship is launched using airbags and at Semer. = 159.12 m when ship is launched
using standard launching device;
• the ship floating off starts at distance S = 152.75 m from the beginning of the
2nd stage (MV = MW ) when the ship is launched using airbags and at distance
S = 165.22 m when ship is launched using standard launching device;
• the largest value of pivoting pressure is at this time (the moment of MV = MW )
Np = WC − γ V = 4507.1 t during launching using airbags and Np = 2028.1 t
during launching using standard launching device;
• the highest depth of launching device’s front end is determined by formula
T1 = S · β = (λ + L1 + L2 ) · β = 12.75 m.
(12)
Based on the analysis of the ship stability during launching from longitudinal sloping slipway as explained above the following conclusions have been made:
Based on obtained results a comparison has been made, which showed that during ship launching using pneumatic airbags the ship floating off earlier, tipping
and leap does not happen. However, additional buoyancy of airbags increases
the pivoting pressure, but within a tolerable range.
50
L.S. Volenyuk and A.S. Rashkovskyi / Ship stability analysis during launching
References
[1] H. Akyildiz, Balonla ve standart denize indirme yöntemlerinin ekonomik ve teknik yönden incelenmesi, Gemi ve Deniz Teknolojisi Sayi 185 (Temmuz 2010) (Assessment of launching with balloon
and standard procedures in economical and technical aspects), Ships and Marine Technology 185
(2010), 25–30 (in Turkish).
[2] V.L. Aleksandrov, G.V. Bavykin, A.V. Dogadin, A.S. Rashkovskiy and V.F. Sokolov, in: Basics of
Shipbuilding Technology, Shipbuilding, Saint Petersburg, Russia, 1995 (in Russian).
[3] Z. Cheng, Ship launching technology by airbag and design of its ramp, Transportation Science &
Technology 1 (2011), 039.
[4] A.A. Kurdyumov, V.V. Kozlyakov, V.Y. Leyzerman and V.F. Sokolov, in: Ship Launching, Shipbuilding, Saint Petersburg, Russia, 1969 (in Russian).
[5] S. Qian, Y. Mao and X. Sun, Measurement and research on ship pitch change during process of ship
launching by gasbag, Ship & Boat 6 (2008), 013.
[6] S.A. Rashkovskiy and A.S. Rashkovskyi, Rolling resistance of marine airbags at the longitudinal
inclined slipway during ship launching, International Shipbuilding Progress 61 (2014), 41.
[7] A.S. Rashkovskyi and L.S. Smirnova, Improvement of ship launching form slipway at public company «Chernomorskyi shipbuilding yard», Visnyk of National University of Shipbuilding 1 (2012),
25 (in Russian).
[8] A.S. Rashkovskyi and L.S. Smirnova, Researches for creation of flat plane on longitudinal sloping
slipways for ship launching by pneumatic airbags, Zbirnyk Naukovykh Prac Natsionalnogo Universytetu Korablebuduvannya 1 (2013), 11 (in Russian).
[9] J. Sun and M. Zhu, Research report of safe countermeasures to apply the airbag launch and upgrade
technology to large ships, China Shiprepair 5 (2007), 011.
[10] L.S. Volenyuk, Researches for ship launching by pneumatic airbags, Commission of Motorization
and Energetic in Agriculture 16 (2014), 189 (in Russian).
[11] L.S. Volenyuk, Static analysis of ship launching by pneumatic airbags, in: Proceedings of All
Ukrainian Scientific-Technical Conference, Mykolaiv, NUOS, May 15–20, 2014, p. 129 (in Russian).