Damage Stability

1 (14)
Workshop N4
Damage Stability
Ralf Eklund and Daniel Lindroth, Napa Ltd
Table of contents
1. Introduction ...................................................................................................................................2 2. Damage stability advice ..................................................................................................................2 2.1 Calculation possibilities and regulations ....................................................................................2 2.1.1 Pay attention ......................................................................................................................2 2.1.2 Verifying results using simulation .........................................................................................3 3. Probabilistic methods ......................................................................................................................3 3.1 SOLAS 2009 ...........................................................................................................................3 3.1.1 General ..............................................................................................................................3 3.1.2 Rule setup ..........................................................................................................................4 3.2 Subdivisioning for SOLAS 2009 ................................................................................................5 3.2.1 Partial watertight limitations.................................................................................................5 3.2.2 Structures strong enough to seriously restrict the flow of water ..............................................5 3.2.3 Piping arrangements ...........................................................................................................5 3.2.4 Incorrect or missing damages ..............................................................................................6 3.3 Special purpose ships 2008 (SPS 2008) ....................................................................................7 3.3.1 Complicated arrangement ....................................................................................................7 3.3.2 Aft ramps, recesses, moon pools and complicated shapes ......................................................7 3.3.3 Vertical extent of DAMHULL .................................................................................................7 4. Survivability beyond factor s ............................................................................................................7 4.1 Safe return to port according to SOLAS 2009, Ch. II-1, Reg. 8-1 .................................................8 4.2 GZ-based method ...................................................................................................................8 4.2.1 NAPA standard criteria.........................................................................................................8 4.2.2 NAPA macro to calculate a proposed index .......................................................................... 11 4.3 Time-domain flooding simulation ........................................................................................... 12 4.3.1 General ............................................................................................................................ 12 4.3.2 Damage scenarios ............................................................................................................. 13 5. End remarks ................................................................................................................................ 14 © 2010 Napa Ltd
NAPA User Meeting 2010
2
Workshop N4
Damage Stability
1. Introduction
This workshop focuses on damage stability calculations using NAPA. The solutions in NAPA for SOLAS 2009
and SPS 2008 are dealt with, and the survivability on a level beyond the rule-based subdivision index is
pursued. The workshop is intended for users with basic knowledge regarding the calculation of damage
stability.
2. Damage stability advice
2.1 Calculation possibilities and regulations
NAPA is a flexible program allowing the design of different ships and vessels for many purposes. Providing
support for this sometimes introduces options in the program that are not applicable to all regulations.
Therefore, it is important that the users make themselves familiar first with the regulation itself and secondly
also the calculation process in order to fully utilize the program. To produce the numbers desired it is
important to understand what is required as well as the logic of NAPA.
Some standardized applications have been introduced in NAPA to ease up the process for the user. However,
in some cases special options might be required and all of them are not necessarily covered by the Manager
applications at hand.
2.1.1 Pay attention
Options can significantly change the results in a way that the regulation does not allow. Examples of these
options are the progressive flooding options, CDISP and VDISP, or calculating the stability using a dry initial
condition compared to initial conditions loaded with liquids.
Some examples on these are:
SOLAS 2009 and progressive flooding – the regulation does not approve stepped GZ curves leaving only one
progressive option (apart from NOPROGRESSIVE which is the default) in NAPA that can be used, which is
the WEPROGRESSIVE option. If any kind of progressive calculation is used, it is highly recommended to
make oneself familiar with the effects different opening types have on the results in NAPA first.
SOLAS 90 and SOLAS 2009 initial conditions – both regulations use dry initial conditions. The initial
conditions cover a sufficient range of draft and trim of the floating positions the ship normally will face
during operation.
© 2010 Napa Ltd
NAPA User Meeting 2010
3
Workshop N4
Damage Stability
MARPOL and VDISP – as real loading conditions are used in the MARPOL damage calculation, the effect of
liquids flowing out from the loaded tanks is not taken into account in the GZ calculation unless the VDISP
calculation option is being used. The effect is clearly visible in the example shown below.
Table 1 Option CDISP
CDISP
Table 2 Option VDISP
VDISP
2.1.2 Verifying results using simulation
One thing to keep in mind is that the NAPA flooding simulation can also be used to further investigate a
single case. This is naturally subject to the approval of the authorities in case it should be submitted to
officials, but deeper knowledge of how water progresses and spreads out can be valuable for future designs.
One example could be a questionable cross-flooding situation; whether the cross-flooding actually takes
place or not. Taking the time aspect into consideration might reveal information that the static approach
would not consider.
3. Probabilistic methods
3.1 SOLAS 2009
3.1.1 General
The harmonized revised SOLAS chapter II-1 was adopted and entered into force January 1st 2009.
By calculating the probability of the occurrence for each of the damage scenarios included and the
probability of surviving each of these damages with the ship loaded in the most probable loading conditions,
an overall probability of the ship in question surviving a collision can be evaluated. This probability is
referred to as the “attained subdivision index”.
© 2010 Napa Ltd
NAPA User Meeting 2010
4
Workshop N4
Damage Stability
3.1.2 Rule setup
Figure 1 Rule setup
Compliance with the
rules means:
Reg. 6
A≥R
Reg. 7
Reg. 8
Reg. 9
Figure 2 Formula setup
© 2010 Napa Ltd
NAPA User Meeting 2010
Workshop N4
Damage Stability
5
3.2 Subdivisioning for SOLAS 2009
The general approach when defining a subdivision should always be to include all watertight limitations into
the subdivision.
Below are some issues to consider in the subdivision.
3.2.1 Partial watertight limitations
Some structures in a ship are not completely watertight or they end before they reach the opposite side of a
space leaving the space partly watertight – until the end where the structure opens up to another space.
These “connections” between spaces can e.g. be modelled using openings to catch the occasions when
water flows around the watertight structure from one space to another. Regardless of how flooding around
the watertight structure is checked, the watertight structures should be included in the subdivision definition
to get any benefit from them at all.
3.2.2 Structures strong enough to seriously restrict the flow of water
Typically these structures are A-class steel bulkheads or structures of similar strength. The SOLAS 2009
explanatory notes require these to also be checked in the event that they would fail or remain intact. This
results in alternative damage case studies, where the compartments behind these limitations are either
broken or remain intact, from which the worst condition (smallest s) is taken for the index calculation. In
order to properly consider these alternatives these limitations should also be modelled in the subdivision
table, but additionally they also need to be included in the compartment connection definition with the
parameter CLASS = “A”.
3.2.3 Piping arrangements
Regarding pipes, there are a few different approaches in NAPA that can be utilized:
1) Stop the damage before the pipes
When calculating index, if the reserve (A compared to R) is great enough, you can just stop the damage
penetration before the pipes. By doing this, some contribution to the index is lost as the full penetration is
not checked, but on the other hand, if the damage does not reach the pipe(s), the pipes can reasonably be
assumed to remain intact. Limiting the maximum penetration used in the damage generation is done by
adding the columns PLIMIT and SLIMIT (PS and SB sides respectively) to the subdivision table if they are
missing (COL PLIMIT; COL SLIMIT), and by entering the limit for the penetration in the corresponding zone
to the column.
Figure 3 Penetration limit before B/2
© 2010 Napa Ltd
NAPA User Meeting 2010
6
Workshop N4
Damage Stability
2) Add an additional damage due to pipes
After having generated all one zone damages, it is possible to make a copy of a damage where the pipes are
located (as a lesser extent damage) and add the rooms connected by the pipes to this new damage. In the
PROB Manager this can be done in the “Edit / Insert / Remove damages” item after the one zone damage
generation. Just select the damage in question and insert a new one; the added damage will become a
lesser extent damage of the selected one. Then edit the new damage normally and add the additional rooms
connected to the pipes. Would the piping arrangement require more than one alternative damage, they can
be added by repeating the procedure described above. After this continue to generate the multi-zone
damages. In the damage description list there should now be new combinations with the added damage(s).
3) Model the pipes as rooms
This is perhaps the most difficult alternative, but if the pipes are modelled as rooms, they will naturally be
damaged when the damage penetration reaches them. It is suggested to try out the first two approaches
before attempting this one, the first approach (to stop the damage prior to the pipes) is usually the easiest
one to do.
Note that pipes cannot be modelled as openings unless the calculation is done in a progressive mode.
Figure 4 NAPA Manager application MGR*PROB, additional damage
3.2.4 Incorrect or missing damages
The best way to resolve the problem with incorrect or missing damages after the damage generation is to
remodify the subdivision definition. The subdivision is the primary source for the damage generation and
usually by modifying it the problems with incorrect or missing damages will be resolved. If due to limitations
needed somewhere else, a single damage gets multiple occurrences with different probabilities, the sum of
these probabilities should equal a single damage generated to the full extent anyway.
If editing the subdivision proves to be insufficient, the “Edit / Insert / Delete damages” item in the Manager
can also be utilized when trying to resolve this. Editing the subdivision is however recommended first.
© 2010 Napa Ltd
NAPA User Meeting 2010
7
Workshop N4
Damage Stability
3.3 Special purpose ships 2008 (SPS 2008)
The SPS 2008 code assumes that the subdivision and damage stability of special purpose ships in general is
in accordance with SOLAS 2009 where the ship is considered a passenger ship and special personnel are
considered passengers with an R value calculated as instructed in the SPS 2008 code. The R value is
automatically calculated by the NAPA Manager application MGR*PROB.
Below are some issues to especially consider when dealing with special purpose ships.
3.3.1 Complicated arrangement
Very often these ships have a complicated arrangement with a large number of small tanks with a complex
geometry. This together with circular shaped cement and mud tanks requires a detailed and thoroughly
designed subdivision. The risk of overlapping volumes is also higher in complicated and detailed
arrangements.
3.3.2 Aft ramps, recesses, moon pools and complicated shapes
Recesses like aft ramps and moon pools often complicate the automatic calculation of b-values and the
subdivision length. Therefore, these numbers should always be checked in the final calculations.
Note that a recess can in some cases also prevent a successful automatic B/2 surface generation in
MGR*PROB.
Figure 5 Recesses, ramps etc. complicating the probabilistic calculation
3.3.3 Vertical extent of DAMHULL
These ships are often much higher in the forward part and it is easily forgotten how index sensitive the
DAMHULL can be. A forgotten compartment at the forward end of DAMHULL can influence the subdivision
index significantly.
4. Survivability beyond factor s
As factor s is based on a simplified survival calculation it does not always reflect the real survivability of a
damaged ship, especially in grounding cases and cases where high sea states are involved. Still, as the Aindex originates from a comparative measure of the merits of a ship’s subdivision, it offers a feasible tool for
comparing a ship’s safety levels.
Further development of the survivability evaluation beyond the rule-based subdivision index is included in
the Formal Safety Assessment scope, e.g. GOALDS (GOAL based Damage Stability). GOALDS deals with the
formulation of a new probabilistic damage stability concept for ROPAX and cruise ships, incorporating
© 2010 Napa Ltd
NAPA User Meeting 2010
8
Workshop N4
Damage Stability
collision and grounding damages, along with an improved method for the calculation of the survival
probability.
Survivability beyond the requirements of SOLAS 2009 is currently relevant because of the activities around
the coming safe return to port requirements.
4.1 Safe return to port according to SOLAS 2009, Ch. II-1, Reg. 8-1
Passenger ships constructed on or after July 1st 2010, with a length of more than 120 m or designed with
more than three main vertical fire zones, have to comply with additional requirements included in regulation
II-2/21 dealing with casualty threshold, safe return to port and safe areas. This affects in general system
redundancies.
Safe return to port is to be ensured after:
-
loss of one watertight compartment or
loss of one space bound by ”A” class fire boundaries (if the space of fire origin is protected by fixed fireextinguishing system)
For this an assessment is to be made on which components are lost in the space concerned.
Ship’s stability during the flooding of any single watertight compartment has to allow the operational
reliability of essential systems. The vessel itself is in general considered to be the safest place for persons
onboard and the conclusion is that a safe return to port is the best alternative. It is, however, not specifically
required that the floating position of the vessel and the stability are assessed. This was especially
accentuated by SLF 52 where it was decided to concentrate on the development of operational information
for the masters of passenger ships for a safe return to port. Still, there are opinions that also suitable
stability criteria should be developed for a safe return to port.
There exists a variety of proposals how to deal with the stability component of safe return to port and even
if the stability elements are not included in the amendments of II-1/8-1 there is a need for an assessment of
the residual stability of a vessel in damaged condition in order to ensure operational reliability of the
essential systems for safe return to port. Two approaches are elaborated here: 1) A purely static GZ-based
method assessing the residual stability according to intact criteria for a set of damages originating from the
SOLAS 2009 damages; 2) Time-domain simulation for a set of damages selected from the SOLAS 2009
damages or generated by the Monte Carlo method. As the latter usually includes the sea state it also covers
the survivability in design operational area.
Below are short descriptions of how these methods can be applied by using the NAPA software.
4.2 GZ-based method
4.2.1 NAPA standard criteria
The range of the level of survivability criteria lies between the unity for the SOLAS 2009 factor s (s2009=1)
and full intact stability criteria. Therefore, the SOLAS 2009 damage cases with s2009≥1 are considered. The
residual stability beyond SOLAS 2009 requirements is calculated by checking the righting lever curve
properties against the 2008 IS code criteria. The criteria elements to be included depend on the design at
hand and the desired level. A desired level close to the intact stability requirements would need to engage
most of the requirements of the 2008 IS code.
(The weather criterion is not included below.)
© 2010 Napa Ltd
NAPA User Meeting 2010
Workshop N4
Damage Stability
9
DES CRI 2008IS-A2.2
CRIT,
TYPE,
REQ,
RANG,
OK
V.AREA30, 'Area under GZ curve up to 30 deg'
MINAREA
0.055
0, 30
CRIT,
TYPE,
REQ,
RANG,
OK
V.AREA40, 'Area under GZ curve up to 40 deg.'
MINAREA
0.09
0, MIN(40, FAUN)
CRIT,
TYPE,
REQ,
RANG,
OK
V.AREA3040, 'Area under GZ curve between 30 and 40 deg'
MINAREA
0.03
30, MIN(40, FAUN)
CRIT,
TYPE,
REQ,
RANG,
OK
V.GZ0.2, 'Min. GZ > 0.2'
MAXGZ
0.2
30, FAUN
CRIT, V.MAXGZ25, 'Max. GZ at an angle > 25 deg.'
TYPE, POSMAX
REQ, 25
OK
CRIT, V.GM0.15, 'GM > 0.15 m'
TYPE, MINGM
REQ, 0.15
OK
Below is a short example of the calculation flow for a case where the starting point is the result table
(SDSD1RES2-TR0), which is the result of a SOLAS 2009 calculation round for TRIM=0 performed with the
Manager application MGR*PROB. The example handles one draught, i.e. the deepest subdivision draught
(DS).
NAPA Damage Stability task:
LQ PROB CASE INIT STAGE PHASE ZONE NZONE SFAC
LIS PROB PTAB=SDSD1RES2-TR0 TAB=SRTP
© 2010 Napa Ltd
NAPA User Meeting 2010
10
Workshop N4
Damage Stability
Table 3 Subset of the SOLAS 2009 result table
The selected damages are gathered into a damage group by referring to the table SRTP:
DGR SRTP
TAB SRTP
OK
INIT,
T,
TR,
GM,
OK
DS, 'Deepest subdivision draught'
4
0
1
LIST DMGM gives the MINGM-based results:
RCR 2008IS-A2.2
LQ
DMGM, CASE, STAGE, PHASE, SIDE, MINGM, GM, DCRI, REQ, ATTV
LIS
DMGM DS/SRTP STAGE=*LAST PHA=EQ SORT=-MINGM TAB=SRTP2
Table 4 LIS DMGM to a NAPA table
© 2010 Napa Ltd
NAPA User Meeting 2010
11
Workshop N4
Damage Stability
4.2.2 NAPA macro to calculate a proposed index
A more comprehensive approach is presented by the German IMO proposal SLF 52/8/1, (refers also to SLF
51/11/1), where the residual stability beyond the damage stability requirements of SOLAS, chapter II-1 is
calculated in the format of a survivability index sSRtP according to the formula below. The formula contains
some general parameters of the GZ curve according to the requirements of the IS code and an additional
heel and range requirement. Only the final stages of flooding with s2009 = 1 according to SOLAS chapter II-1
contribute to the calculation of sSRtP.
where:





GZmax is the residual maximum GZ from equilibrium up to flooding and it is not to be taken as more
than 0.2 (in metres);
GM is the actual GM at equilibrium and it is not to be taken as more than 0.15 (in metres);
Area is the area of the GZ curve up to flooding and it is not to be taken as more than 0.09 (in radian
metres);
Heel is the heeling angle and it is not to be taken as less than 5 (in degrees);
Range is the GZ curve range up to the flooding angle and it is not to be taken as more than 30 (in
degrees).
Note that this is only a proposal, which most probably still will be discussed at IMO. It is handled in this
paper only for the sake of an example of how such a calculation can be performed in NAPA.
The sSRtP index can be calculated in NAPA by using a short RESULTOF macro in LQ DRES. Note that the
formula for the sSRtP index can easily be modified in the macro if one wants to change the GZ curve
requirements.
Figure 6 NAPA Text Editor
The same initial condition and damage case as in the previous example are considered:
LQ DRES CASE(F=15) STAGE PHASE SFACSOL (GZMAXR) (GMACT) (AFA) (HEEL),
(RANGEF) SSFAC(SSRTP F=10.5)/'RESULTOF("SSRTP" GZMAXR GMACT AFA HEEL RANGEF)'
LIS DRES DS/SRTP STAGE=*LAST PHA=EQ SEP=INI TAB=SRTP3
© 2010 Napa Ltd
NAPA User Meeting 2010
12
Workshop N4
Damage Stability
The results of the index calculation are included in the table named SRTP3.
Table 5 LIS DRES to a NAPA table
4.3 Time-domain flooding simulation
4.3.1 General
This NAPA feature enables direct calculation and further evaluation of damage studies. The survivability of a
ship in different damage and flooding scenarios including operational aspects such as sea state can be
evaluated by investigating how the ship survives a collision. The time component enables the determination
of the capsizing time. In safe return to port calculations the systems (especially the ones needed for fire
fighting) need to be capable of operation at least three hours, which outlines the survivability range. This
means that the vessel has to survive without capsizing for at least three hours in different flooding
scenarios. The percentual rate of scenarios surviving at least three hours is a measure of the ship’s
survivability.
© 2010 Napa Ltd
NAPA User Meeting 2010
13
Workshop N4
Damage Stability
Figure 7 NAPA Flooding Simulation Manager application for calculating time to capsizing
4.3.2 Damage scenarios
Manual and NAPA generated
The damage cases to be used in the simulation can be chosen quite freely. Manual definition works for
single case studies but in extensive calculations the cases can preferably be chosen from the ones generated
by NAPA for SOLAS 2009 calculations. Different selection criteria based on the length and penetration of the
damages can easily be included in NAPA table calculation.
Table 6 SOLAS 2009 damages generated by MGR*PROB
© 2010 Napa Ltd
NAPA User Meeting 2010
14
Workshop N4
Damage Stability
Automatic generation
A quite common method to generate damages is the Monte Carlo simulation, which is a method for
iteratively evaluating a model by using random numbers as input. A random number generator chooses a
value between zero and one. The chosen random number is considered as a probability and the
corresponding value is selected from a distribution (see the figure below), which in this case is taken from
the SOLAS 2009 background material.
C(b/B)
C(b/B) = 1/5[-12(b/B)2 + 16(b/B)]
b/B
Figure 8 Cumulative distribution for the non-dimensional penetration b/B
The distributions for damage location, damage length, damage penetration (see the figure above) and the
height of the upper damage limit lead through the random generator to damage extents, which relate to a
unique compartment combination.
The advantage of this method is that it is completely automatic and that the frequency of different cases is
easily determined.
A Monte Carlo simulation can be done by using spreadsheet applications but there are obviously advantages
by using NAPA for the task as the entire ship model is immediately available. There is a newly created
random generator service function called in.random(), which preferably can be used. The distributions are
stored in tables, which are easily accessed by NAPA macros.
5. End remarks
More information can be obtained from the NAPA Online Manual: “Damage Stability” and “Selected User
Meeting Workshops” (chapter “5 Damage Stability Calculations” and chapter “6 Probabilistic Damage Stability
Calculations”).
© 2010 Napa Ltd
NAPA User Meeting 2010