SACS-MOSES INTEROPERABILITY:
MAKING THE CASE FOR
TRANSPORTATION ANALYSES
By Spiro J. Pahos, MOSES Application Engineer
Keywords: SACS, MOSES, interoperability, transportation, tow, stochastic modelling
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Scope of Work
This document explains how to set up a transportation analysis in SACS where the load cases are created in
MOSES.
MOSES v10.14 was used in generating results for this work.
Analysis Background
It is known that structures with significant dynamic response require stochastic modelling of the sea surface. There
are many examples in transportation where some elements are uncertain, the interoperability between SACS and
MOSES has come to address stochastic modelling in a streamlined way to return a more pragmatic representation
of structural response, motion statistics and stability. The present whitepaper focuses on showing the workflow
of a transportation analysis in SACS-MOSES synergy. It is assumed that the reader is familiar with the
transportation analysis in SACS, although a brief discussion is made first to remind the workflow in SACS.
Knowledge of MOSES syntax is considered an advantage for this type of analysis although a brief explanation is
given. The strengths of the SACS-MOSES interoperability and the differences between the two methods are
highlighted with analysis insights at the end of this work.
SACS Transportation Analysis
Traditionally, the transportation analysis in SACS is done in four steps as shown in Figure 1.
1. Gravity
2. Inertia
•Type: Static
•Type: Loading
•Subtype: Basic Static w. Gap •Subtype: Transportation
Elements
Inertia Loading
Input: sacinp, seainp
Input: sacinp, towinp,
Output: seaoci, gapcsf,
gapinp
psvdb, gap.runx
Output: gapcsf, towtran,
towlst
3. Combine
•Type: Utilities
•Subtype: Combine Solution
File
Input: cmbinp, gapcsf.dead,
gapcsf.tran
Output: gapcsf.tow,
cmbtow.runx
4. Code Check
•Type: Post Processing
•Subtype: Code Check
•Subtype: Tubular
Connection Check
Figure 1 Transportation analysis sequence in SACS
1.
Gravity
The jacket model is modelled natively in SACS to create the sacinp file together with the accompanying seainp file
where the deadweight definition is also prepared. In this linear static analysis, only dead loads are considered,
and the jacket is fully supported by its vertical supports (can members).
It is perhaps conservative, to assume that all vertical can members are effective during jacket loadout and
transportation, and that all tie-down members are only effective during transportation. The vertical cans and tiedown members need to be included in the sacinp file.
One needs to be wary and define the vertical can members group as Gap Elements and select Member Type as
“No Load”. This action will create a Gap common solution file (gapcsf).
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2.
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Gravity
In this step a gap input file is used to redefine no-load, tie-down members into standard members for all tie-down
members to take the transportation forces. The gap input file (gapinp.tran) contains the load cases that include
the transportation inertial forces. The load cases are manually created and are often based on industry practices,
or default motion criteria when site-specific data are not available. Typical deterministic load cases are similar to
the ones shown in Table 1.
Load Case
+R+H
-R+H
+R-H
-R-H
+P+H
-P+H
+P-H
-P-H
Description
+ve Roll +ve Heave
-ve Roll +ve Heave
+ve Roll -ve Heave
-ve Roll -ve Heave
+ve Pitch +ve
-ve Pitch +ve Heave
+ve Pitch -ve Heave
-ve Pitch -ve Heave
Load Case
+0.8R+0.6P+1.0H
+0.8R+0.6P-1.0H
+0.8R-0.6P+1.0H
+0.8R-0.6P-1.0H
-0.8R+0.6P+1.0H
-0.8R+0.6P-1.0H
-0.8R-0.6P+1.0H
-0.8R-0.6P-1.0H
Load Case
+0.6R+0.8P+1.0H
+0.6R+0.8P-1.0H
+0.6R-0.8P+1.0H
-0.6R-0.8P-1.0H
-0.6R+0.8P+1.0H
+0.6R+0.8P-1.0H
+0.6R-0.8P+1.0H
+0.6R-0.8P-1.0H
Table 1 Typical load cases studied in transportation analyses
𝜃𝜃
SACS uses the motion data and converts it into angular acceleration through 𝜃𝜃̈ = 2 , where θ is the roll/pitch angle
𝑇𝑇
and T is the period. The roll, pitch and period used here are deterministic values.
The load cases in the gap input file do not include dead loads but only the transportation inertial forces.
The solution of this step will return the gapcsf.tran which will be combined with the one created in the first step.
3.
Combine
In this step, load cases contained in the solution files from Step 1 and Step 2, will be combined into one solution
file. The two files used in this step are called Primary and Secondary common solution file.
4.
Code Check
Post processing of member forces and code compliance is done as in any other analysis for the preferred industry
requirements.
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SACS-MOSES Transportation Analysis
The SACS-MOSES transportation analysis involves six steps although alternative workflows may be possible. This
whitepaper is adopting the working sequence shown in Figure 2. The necessary files for each step, with the
corresponding output files is also highlighted.
1. Geometry
•Convert SACS
model to MOSES
input file.
Input: sacinp.file
Output:
mod0001.txt
2. Read
Geometry
•MOSES
Input: sacinp.file,
tow_auto.cif,
tow_auto.dat
Output:
tow_auto.ans,
tow_auto.dba,
towinp.jacket
3. SACS
Transportation
Analysis
•Type: Loading
•Subtype:
Transportation
Inertia Loading
(Tow)
Input: sacinp.file,
gapinp.tran,
towinp.jacket
Output: towoci,
gapcsf, psvdb,
towtran.runx,
towlst
4. SACS Dead
Load Analysis
5. SACS
Combine
Analysis
6. Post-Process
Results
•Type: Static
•Type: Post
•Type: Utilities
•Subtype: Basics
•Subtype: Combine Processing
•Subtype: Code
Static Analysis w/
Solution File
Gap Elements
Input: cmbinp.tow, Check
Input: sacinp.file,
gapcsf.tran,
seainp.dead
gapcsf.dead
Output:
Output:
gapcsf.dead,
cmbcsf.check,
gapdead.runx,
cmbcheck.runx,
gaplst.dead
cmblst.check
Figure 2 SACS-MOSES interoperability sequence
1.
Geometry
The geometry file is created in SACS Precede, or Datagen, as in any other analysis with no specific requirements
for the downstream steps. The SACS geometry can be converted into MOSES input with the &CONVERT command
through a set of user-defined MOSES files.
The two files, the data (.dat) and command input file (.cif) with their contents in MOSES syntax are shown here.
The model conversion is done with the following commands:
SACS_Convert.dat
&convert sacs
&insert sacinp.sample09
The command input file should contain the following commands:
SACS_Convert.cif
&emit -sacs
inmodel
&finish
Executing the command file creates the converted MOSES model in the answers directory; i.e. sacs_convert.ans.
The text format of the converted geometry, mod0001.txt, is written in MOSES language.
Note: Ensure that the sacinp.sample09 geometry file resides in the same directory as the sacs_convert.cif prior to
execution.
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Note: For a detailed discussion of MOSES commands please see MOSES Reference Manual.
2.
Read Geometry
Two additional files are needed in this step. In this work we call these two files Tow_Auto.dat and Tow_Auto.cif,
but users can define any other name. This new set is written in MOSES syntax and the contents of the two files
are discussed briefly here.
The data file defines analysis settings like the barge definition, water depth and the jacket support nodes. An
example of the data file is:
Tow_Auto.dat
use_mac install
&dimen -save -dimen feet kips
&title Sample of Jacket Transportation
i_set wdepth 91
use_ves tow_brg
&set port_nod = *J|2101 *J|2201 *J|2301 *J|2401
&set stbd_nod = *J|2105 *J|2205 *J|2305 *J|2405
model_in Jacket ./sacs_convert.ans/mod00001.txt 0 0 0 \
-port_nod %port_nod% \
-stbd_nod %stbd_nod%
&dimen -remember
Where the *J support nodes can be seen in SACS Precede and shown in Figure 3 for the cargo model.
Figure 3 Cargo support node definition in Precede
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Note: The jacket does not have to be rotated for positioning on the barge. MOSES will do this automatically as part of the
developed automation with the MODEL_IN command.
Note: The tow_brg used in the USE_VES command is merely an example. Alternatively, users can take advantage of the
barges found in MOSES library.
The x, y, z variables used in the MODEL_IN command define the location of the origin of the structure on the
barge with X being the location aft of the bow, Y the distance off the centerline and Z the height above barge deck.
This definition positions the jacket on the barge as shown in Figure 4.
Figure 4 Jacket positioning on the tow_brg barge
The command file contains the necessary instructions to carry out the transportation analysis based on sea spectra
this time. An example of the command file could be the following input:
Tow_Auto.cif
&dimen -dimen feet kips
&device -oecho no -g_default file -fig_num yes
inmodel -offset
setup
inst_transp -s_cond
h 8.7 12.5 \
s 7.3 8.90 \
v 6.2 8.00 \
-type_spect issc \
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-period 4 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 13 15 20 \
-heading 0 45 90 135 \
-wind 100 50 40 100 -draft 10 -trim 0 -no_struct \
-mo_points *CEN1 -sacs_tow snapshot
i_finish
A number of analysis options are detailed in the INST_TRANSP command. The sea states are defined with a onecharacter sea identifier and specified by a wave frequency spectrum with a significant wave height and a
representative period with the -s_cond option. Perhaps the important options to comment on are the
mo_points and sacs-tow.
-
-
The mo_points option will return additional cargo G-force statistics at *CEN1 point. By default, cargo
G-force statistics will be returned at the jacket CG. This is an easy way to determine the motion
accelerations at specified locations on the cargo.
The sacs_tow option returns motion RAOs, or acceleration load cases for the subsequent SACS analysis
for all requested periods in a separate file, towinp.jacket, found in the .ans directory. The snapshot
instruction will return a time synthesis and report the extremes for each degree of freedom. The extremes
are the minimum and the maximum event.
As a rule of thumb, the snapshot instruction can possibly return a maximum number of load cases equal to:
𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 = 𝑁𝑁𝑁𝑁. 𝑆𝑆𝑆𝑆𝑆𝑆 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 × 𝑁𝑁𝑁𝑁. 𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 × 6 𝐷𝐷𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝑜𝑜𝑜𝑜 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 × 2(+/−)
Note: The 2(+/-) designation accounts for positive, or negative motion.
Note: Any load cases that occur twice are filtered out automatically to avoid calculating the same event twice.
As a result of this automation, MOSES will return additional model information and figures in the .ans directory.
Namely:
-
The jacket properties (mass, CoG and radii of gyration) and the buoyancy properties (submerged
buoyancy, center of buoyancy and reserve buoyancy) are reported in doc0001.txt.
The righting arm, wind arm and the area ratio of the barge-jacket system will be plotted for stability
purposes.
Vessel RAOs for all requested headings and frequencies.
Translational and angular accelerations at the jacket GoG for all requested headings and frequencies in all
six degrees of freedom. This data can later be used for sea-fastening design purposes.
As with any MOSES analysis, all output data is found in text format for further post-processing.
An example of the acceleration values at jacket CoG for quarter seas is shown in Figure 5.
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Figure 5 Jacket transportation accelerations for quarter seas
3.
SACS Transportation Analysis
Based on the -sacs_tow snapshot option MOSES creates a SACS Tow file with a time synthesis where angular
and translational accelerations in X, Y, Z axes are reported. Alternatively, if -sacs_tow rao is used, the SACS
Tow file will contain displacement RAOs for all requested periods with reported motion amplitudes and phases in
all six degrees of freedom.
An excerpt of the towinput.jacket file is shown in Figure 6 for -sacs_tow snapshot. Each load case is an
applied angular (deg/sec2) and translational (m/sec2) acceleration on the cargo CoG. As a result of the input in the
tow_auto.cif discussed above, 115 load cases were created.
With the Tow file available, the transportation study is carried out in SACS as in any other analysis.
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Figure 6 Excerpt of the SACS Tow file created in MOSES
4.
SACS Dead Load Analysis
The dead load effects have not been addressed so far. A user-defined SACS file (seainp.dead) where gravity is
acting in the -z direction is used to take care of this. The contents of this file in SACS syntax could be of the following
form:
LDOPT
FILE B
LOAD
LOADCNDEAD
INCWGT
DEAD
DEAD
END
NF+Z1.0280007.849000
GLOBMN
NPNP
K
MASS
-Z
M
The solution of this analysis will return a gap common solution file (gapscf.dead) to be used in the next step.
5.
SACS Combine Analysis
In this step the dead load and the inertial load effects are combined. The Combine Input File and two files,
Primary and Secondary common solution files are used. The Primary and Secondary files have been created
previously, these are the gapscf.tran and gapcsf.dead respectively.
The combine input file is a user-defined file where the combine options are included in SACS syntax and could be
of the following form:
COMBOPT
LCOND
LIN
COMP P
COMP SDEAD
END
1.0
1.0
1.0
ALL
1.00
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Post-process Results
This step is optional when additional steps are needed. In this white paper it is merely a step that separates the
files from the previous step. With the SACS combined solution file (cmbcsf.check) available one can interrogate
the jacket members and code check the structure for all load cases MOSES created earlier. Post processing takes
place in SACS with all benefits of SACS interface and documentation capabilities.
Error! Reference source not found.Figure 7 depicts the combined UC member values as per WSD 9th/API-RP 2A
21st Ed.
Figure 7 Combined UC member values
Conclusions
It is evident that a spectral analysis can be comprehensive in terms of considering a far greater number of load
cases. The studied load cases generated in MOSES can be of two forms, namely motion RAOs, or acceleration
load cases, generated from the defined sea spectra if desired.
Even if project managers realize the existence of uncertainty in marine operations, deterministic models are still
in use; the reason for this is that stochastic models are typically more difficult to solve. The present whitepaper
shows the steps in SACS-MOSES transportation interoperability and hopefully helps to highlight the degree of
automation.
SACS-MOSES interoperability can also address the longitudinal integrity of the barge, although not discussed
here. This capability allows for a holistic integrity assessment on both the jacket and the barge where project
liabilities might lie.
The most salient differences in the two approaches are given in Table 2.
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SACS Transportation
Barge longitudinal integrity cannot be studied
Deterministic load cases are based on
recommended practices or industry codes
The barge geometry is simplified. Users cannot
model complicated hulls
No stability assessment
SACS-MOSES Interoperability Transportation
Barge longitudinal integrity can be addressed
Stochastic load cases created by MOSES are
based on displacement RAOs or accelerations
Users can consider different barges from the
MOSES library, or model their own
Intact and damage stability assessment is
available
The wave phase is considered when searching for
maximum response in 6 d.o.f.
Cargo G force statistics are available in 6 d.o.f.
MOSES includes gravity in the generated load
cases by default
The wave phase cannot be considered
Statistics are not available
Users need to define how gravity is considered
Table 2 Comparison of the two methodologies
Bentley Systems advocate the use of stochastic models in marine operations where uncertainty and risk need to
be contained. It is believed that stochastically-generated load cases provide a holistic picture of the load cases
likely to be encountered in transit.
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