analyses of ship structures using ansys

13 Pages·2008·4.1 MB·English

Analysis of ship structures using ANSYS SeaTech Solutions International (S) Pte Ltd
ANALYSES OF SHIP STRUCTURES USING ANSYS
Suman Kar, D.G. Sarangdhar & G.S. Chopra
SeaTech Solutions International (S) Pte Ltd
E-Mail:[email protected]
Abstract

This paper describes the use of ANSYS Structural in the simulation of the complex ship structure and the
various loading conditions that a ship experiences during its operation.
The ship, during operation experiences complex loading conditions, which is generally divided into few
categories: Linear Static and Dynamic Loads, Thermal Loads and Complex Non-Linear Dynamic Loads.
Generally the loading on the ship is a combination of some or all of the load categories mentioned above,
depending on the type of the ship.
The first part of the paper is dedicated to Static and Linear Dynamic Analysis and its combinations. This is
used to analyze merchant and Naval ships. Static Analysis is usually done to find the overall strength of the
structure. For this, in addition to hydrostatic and hydrodynamic loadings from exterior, the local loads due
to ballast tanks; cargo loads etc are also considered. In the domain of Dynamic Analysis, Vibration
Analysis (Free and forced) is done for all ships/components to check if the structure is dynamically stable
or not. The natural frequencies of the structure are compared with the forcing frequency to check for
resonance.
The second part of the paper deals with Complex Non- Linear Static and Dynamic Analysis, which is either
used for research or for analysis of Ship local components / regions. These analysis includes analysis of
Forward part of the ship due to slamming loads, Analysis of Floating structures due to underwater
Explosions, transient dynamic analysis of heli-decks due to crash landing of helicopters etc and Ultimate
strength Analysis of stiffened ship panels and Midship sections of various ships. Both Geometric and
Material Non-Linearity are extensively incorporated to analyze these components/regions, which gives near
accurate results. Ships, which carry LNG, require a Thermal Analysis to be done to check the structural
integrity of these LNG Carriers. Hence for these ships ANSYS is used for Thermal Analysis.
Keywords: Ships, ANSYS Mechanical, Static Analysis, Vibration Analysis, Transient Dynamic Analysis,
Ultimate Strength Analysis, Thermal Analysis
1. Introduction
Provision of ‘adequate’ strength in a ship at a reasonable cost, has always been one of the most challenging
task for the ship designers. Over the years the classification societies have been providing the necessary
standards to ensure the adequacy of strength against all demands that can be envisaged during service life
of the ship. Earlier, these formulations were largely based on experience of good ships and the scantling
requirements were given in simple tabular forms based on few basic parameters like main particulars of the
ship. Subsequently, from around 1970, there was significant change in the class rule formulations, wherein
the requirements were specified by various formulae for environmental loads and allowable stresses based
on simple principles of structural mechanics. Though these formulations were more scientific, the designer
did not explicitly know the safety margins in the formulation and inherent assumptions made, so it was
difficult to use such formulations for a novel design.
In the last two decades there has been a rapid development in the fields of hydrodynamics and structural
analysis. The advanced methods developed now provide a better understanding of environmental loads
(Demand) and strength of ship (Capacity). As these methods became more and more mature, providing
reliable tools for structural optimization, it was only logical that they were incorporated in the classification
rules for ship and components design and analysis.
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Analysis of ship structures using ANSYS SeaTech Solutions International (S) Pte Ltd
2. Advanced methods for assessment of structural strength
Behavior of ship in a seaway has been traditionally assessed by means of sea keeping model tests. The
procedures for model testing and subsequently scaling of the results for actual ship size are now well
established and are quite reliable. However one of the drawbacks with the experimental methods is the
large cost associated with it. With the advancement in the structural and hydrodynamics theories today it is
possible to predict the ship behavior to an acceptable reliability by means of numerical simulation.
Once the sets of various realistic loads applicable to a vessel are determined, the combined effects of these
loads are assessed to determine the overall strength of the ship structure. Advanced methods based on
Finite Element Method are best suited to analyze complex structures in three dimensions. With the
advancement of Numerical analysis methods and availability of computing facility at affordable costs, use
of such methods provides more reliable and direct assessment.
3. Ship and Allied Structures
Ships and its components are generally assessed for Strength and Vibration under normal operation and
Strength assessment under critical loading condition like collision or underwater explosion etc. Assessment
of strength is now a days based on ‘Direct Strength Analysis’ which eliminates any error in judgment,
which may arise out of several assumptions that otherwise have to be made regarding the interaction
between several structural members. The method takes into account the effect of bending, shear, and axial
and torsional deformations all together. Thus it is very useful for larger Ships with complex structural
arrangements and large variety of cargo / ballast loading conditions. There major aspects of structural
analysis are (i) Creation of structural model for FE analysis (ii) Calculation of loads and transfer to the
structural elements (iii) FE analysis (iv) Extraction of results, checks and report.
3.1 Modeling
The complex ship structure, which may vary from 100 to 300 meters in length: 7 to 30 meters in breadth
and 5 to 30 meters in depth, is modeled completely using ANSYS Pre processor. The plates are modeled
using Shell 63/ Shell 43/Shell 181 Elements. The stiffeners are modeled using Beam 4/Beam 44/ Beam 188
Elements. All the pillars are modeled using Pipe elements and all other structural masses are modeled using
Mass elements.
The modeling of the entire ship structure with all details consumes lot of time and is usually cumbersome.
Hence ANSYS macros and APDL’s are extensively used in modeling. This reduces lot of modeling time
and manual errors.
Ones the model is completed, each of the plates, Stiffeners, Pipes and masses are individually selected and
the real constants calculated and inputted in the tabular form to model each component of the structure with
complete accuracy. This process is also automated at times with the use of ANSYS macros and APDL’s
3.2 Meshing
After the complete structure is modeled, the plates, stiffeners, pipes and masses are individually meshed.
The last step to be completed before meshing the model is to set the meshing controls, i.e. the element
shape, size, the number of divisions per line, etc. Selecting the various parts of the model, one by one finite
element mesh is generated. The critical portions are plates with sharp corners, curvature etc. These areas
can be remeshed with advance mesh control options. "Smart element sizing" is a meshing feature that
creates initial element sizes for free meshing operation.
Proper care has to be taken to have the control over the number of elements and hence the number of
degrees of freedom associated with the structure. This is done to have a control over the solution time.
However, no compromise is made on the accuracy of the results.
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Analysis of ship structures using ANSYS SeaTech Solutions International (S) Pte Ltd
3.3 Loading
Loads are generally estimated using the classification rules or by direct hydrodynamic calculations. The
loads that a ship experiences during its voyage can be roughly divided into two parts.
Static Loads – These consists of loads, which do not vary with time, or even if they vary, the effect of
time could be neglected. The hydrostatic pressure, Weights of the ship components, Cargo and Ballast
loads come under this category. In addition to these wave moments and forces coming due to ship
components are also considered as static loads.
Dynamic Load – These are the loads, which vary with time, and the variation is substantially large
because of which a dynamic analysis is generally required. The hydrodynamic Pressure due to waves,
Wind Loads and other operational Loads like loads due to underwater Explosion, Machinery
operational loads etc., are the loads, which are considered as dynamic loads.
Both the above categories of loads, would act on ship and its components from time to time. Hence it
becomes essential to consider the loads correctly and analyze the structure accordingly. Use of ANSYS
makes the process of application of load very simple and manageable., also the chances of errors in
combining the loads is eliminated.
3.4 Analysis
Ones the loads are determined; different structural analyses can be carried out based on needs. The results
from these analyses are extracted and checked for yielding, buckling and ultimate strength automatically at
desired locations.
Different Analyses that are usually performed are 1) Stress Analysis of Ship Structures and components
2) Vibration Analysis of Ship Structures 3) Ultimate Strength Analysis 4) Transient Dynamic Analysis and
Strength Analysis under Impact loading 6) Thermal Analysis
Before these analyses could be actually performed on the full-scale model, they are always backed up with
a thorough in-house research on simpler structures like plate or plate with stiffeners etc. Suitable ANSYS
models are made and appropriately loaded and boundary conditions are applied. Various analyses are then
performed and the process is thoroughly checked and fine-tuned for mesh size, element type, time step (for
transient dynamic and Non- Linear analysis) and other FE properties. The results of these analyses are
checked with either published or experimental results. Ones these methods are established they are
performed on the complex ship structures and components.
This reduces the chances of errors due to wrong or un-established methods and as the FE parameters are
thoroughly checked, the results are close to the actual results. Further the analysis, which are done on
complex ships structures are also verified by measurements on actual ship structures. Once the results are
verified, the method is finalized for subsequent analyses.
Some of the analyses are briefly described below.
3.4.1 Stress Analysis of Ship Structures and components
Stress analysis is usually performed to find the overall strength of the ship and its components. As there are
always two or more loads acting on the structure at a time and as these loads change, based on the loading
condition, hence it becomes necessary to prepare different load cases and then to analyze all or some of the
load cases independently, as required. The number of load cases to be investigated depends on i) the
number of envisaged cargo and ballast loading conditions and ii) number of wave cases (snapshot load sets)
to be considered for each ship loading case. Sometimes it becomes necessary to carry out the structural
analyses for a large number of load cases, making it absolutely essential to develop automatic processes.
Thus several interfacing software programs, which connect with the FE analysis software, have been
specially developed e.g. for transfer of all load data into the FE model and apply automatically. ANSYS,
and the use of its macro and APDL programming helps a lot in making this process simple and error free. A
simple static analysis is usually performed taking Non-Linearity (if required) into consideration.
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Analysis of ship structures using ANSYS SeaTech Solutions International (S) Pte Ltd
The results are extracted and checked for yielding, buckling and ultimate strength automatically at desired
locations.
Few Stress analysis projects are briefly described below
3.4.1.1 Advanced Analyses of Complex Structural Systems (Bulk carrier) Using FE Codes- The
complex modeling of the complete Bulk Carrier, (Fig. 1) detailing each of its components requires
advanced modeling technique and a keen technical mind. We model Bulk Carriers and other large vessels
and perform direct strength Analysis with varied load cases to check the strength of the vessel.

Fig 1. Bulk Carrier Model (Stress Distribution) Fig 2. VLCC Tanker Model

3.4.1.2 Optimization Of Scantlings For VLCC Tanker By Finite Element Method - On a project for
unification of Ship Scantlings, a proposal of IACS working group, we had carried out calculations of
scantlings, based on IRS own rules and direct structural analysis by finite element method for Panamax
bulk carrier (Fig. 2) and a double hull tanker of 300000 DWT. The efficient use of FE (using ANSYS)
and the results were well appreciated at international forums.

3.4.1.3 Buckling Analysis of Mini Bulk Carrier - The Static (structural stress) analysis of Mini Bulk
Carrier (Fig 3) was carried out to check the strength adequacy due to local (Hydro static and hydro
dynamic pressure and cargo load) and global loads (Still water bending moment and wave bending
moment). For this various pressures, Forces and Moments were all applied simultaneously for the
analysis. The hatch coaming area has been specially investigated for buckling for various axial loads.
The complete project was done in a record time, which would have not been possible without the use
of advanced FE software, ANSYS.

Fig 3. Mini Bulk Carrier (Stress Distribution) Fig 4. FE Model of Fast patrol Vessel

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3.4.1.4 Static And Dynamic Analyses Of Fast Patrol Vessel - A static and free vibration analysis of
the vessel was done to simulate the loads resulting from lightweight, high speed and slamming
pressure. The vessel, made up of steel and aluminum (Fig 4), has to be checked against the water jets
and slamming loads. The static analysis was carried out under these loading conditions. The free
vibration analysis of the entire vessel and superstructure were performed to estimate the natural
frequency and the mode shapes for avoiding resonance with other machinery components.

3.4.1.5 Transverse Strength Analysis of LSTL Hull Structure - The transverse strength analysis is
done to simulate the loads due to oblique wave, rolling of the ship and unsymmetrical cargo loading.
The transverse strength computations of the Landing Ship Tank (Large) [LST (L)], (Fig 5) has been
examined for following loading conditions: a) Lightship b) Transport role ‘A’ & ‘B’ vehicles –
Departure c) Transport role ‘A’ & ‘B’ vehicles – Arrival d) Beaching role ‘A’ & ‘B’ vehicles – Arrival
e) Replenishment role – Departure f) Replenishment role – Arrival. The FE results helped, the
shipyard, a lot to modify the design to withstand these loads.
Fig 5. FE Model of LST (L) -Stress Distribution Fig 6. Effect of Torsional Loads on Acid Carrier

3.4.1.6 Effect Of Torsional Loads On An Acid Carrier Of Wide Hatch Opening - Torsional analysis
is important for vessels with wide hatch openings and single bottom structure (Fig 6). The main
objective of this study was to develop a method to improve the structural adequacy with respect to
warping displacement and stresses, by 3-D finite element method. The analysis was carried out using
ANSYS, for cargo loading, dynamic forces and hydrodynamic torque due to oblique wave with
appropriate boundary conditions. The results were verified with the one, available in literature and
were found satisfactorily matching. Hence suitable modifications were suggested to the client.

3.4.1.7 Flexure analysis of Advanced Offshore Patrol Vessel - The objective of this project was to
determine the axial stress on longitudinal member as well as on super structure due to total hull girder
bending moment. Full ship (Fig 7) has been modeled from frame no.32 to frame no.140 so that whole
superstructure is covered. Total bending moment was applied on both ends of model. The results
showed that the axial stress on one of the deck was more than the critical buckling strength. Hence,
modifications were suggested to the client.

Fig 7. Deflected Shape of AOPV Fig 8. Ship at Campus
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3.4.1.8 Ship at Campus - The Ship at Campus, designed to be used to train Seafarer in general and
engineering training in particular. This 23 m long (950 tones DWT), half ship (Fig 8) will be erected
on a concrete foundation. The main engine will be kept at a separate concrete foundation to avoid
transmission of vibrations to the main hull. ANSYS was used to help the client to design the scantlings
of both ship and the foundation. Finite Element analysis, using ANSYS, was carried out to calculate
the loads on the foundation and strengthen the moon-pool area. The scantlings were optimized by
detailed analysis to safely transmit the required dead weight and lightweight into foundations.

3.4.1.9 3-D static analysis of cold water box for OTEC barge - This structure is connected to the deck
of the barge of an Ocean Thermal Energy Conversion plant, (Fig 9) which generates electricity from
the seawater temperature gradient. The cold water pipes are attached with this structure from 2 km
depth of seabed. The self-weight of the box, hydrostatic, hydrodynamic (wave &current), buoyancy
and pipe weight were considered as the applied loads. The static analysis was carried out using a
simplified model to find out the strength adequacy of the structure under the applied loads.

Fig 9. Cold Water Box on Barge Fig 10. Gun Support Structure

3.4.1.10 Gun Support Structures - The natural frequencies and stresses for stated recoil loads and
loads due to wind, ship motions etc were analyzed using ANSYS for the Gun Support Structure (Fig
10) of a vessel for a foreign agency. Different loading cases were analyzed to ensure the adequacy of
the structure in all possible load combinations. Modifications were suggested based on thorough study
of the results.

3.4.1.11 Design Analysis of Forward part of Hull Structure for Air Defense Ship - The shell
structure forward of FR 16 of ADS (Fig 11) has been analyzed to study adequacy of structural strength
against Bottom Slamming pressure and Bow flare impact pressure. The slamming and bow flare
pressure was calculated as per IRS and LRS Naval rules. The detailed finite element analysis has been
done for various models at different locations. Thorough In-house research was done before finalizing
the method for analysis of such a real time condition.

Fig 11. Forward Part of hull Structure (ADS) Fig 12. Stress Distribution on Skeg
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3.4.1.12 Design Analysis of Skeg Structure - A detail 3-dimensional FE analysis of the skeg
structure (Fig 12) of Air Defense Ship has been carried out to check the structural adequacy of the
proposed design for two different loading conditions. Static Analysis (Linear) of skeg structure has
been done to find out the maximum deflections and combined stresses after docked condition. While
non-linear analysis, has been carried out to check for buckling in the process of docking of the ship.

3.4.1.13 Mast Static and Dynamic analysis for various vessels - The strength of the mast structure
(Fig 13) was estimated due to the wind and ship motion loads. This mast accommodates radar antenna
bases and for the proper functioning of the equipment, the deflection of the mast for operating
frequencies of the radar should be limited, hence a transient dynamic analysis was also done to
determine the deflection due to the ship motion. The structure was modified to restrict the deflection
and resulting stresses within allowable limit. The adequacy of the mast structure subjected to wind
load and ship motion load are checked on the modified structure and the stresses are kept within
allowable limit.

Fig 13. Deflection of Mast Fig 14. Deflection Plot of Helideck

3.4.1.14 Structural Analysis Of Heli-Deck Of Advanced Offshore Patrol Vessel - The main objective
of the structural analysis of Heli-Deck for Advanced Offshore Patrol Vessel (Fig 14) was to determine
the strength of heli-deck for normal landing and emergency crash landing of the helicopter. The
analysis was also carried out based on NES154 standards. The elasto-plastic analysis was carried out,
using ANSYS, based on allowable permanent deflection criteria.

3.4.1.15 Ship Lift System - The Ship Lift System,
shown in Fig 15, is used to transport the naval vessel
from the sea to the repair shed. These whole systems
have upward, longitudinal and transverse movement
during the full operation. The transverse movement at
a junction was found to have problems during shifting
from longitudinal movement at the rail and crossing
piece.
Hence, a FE analysis was done to analyze and re-
design the transfer system so that it could effectively
transfer the ship from sea to desired repair location.
The recommendation suggested was rectified in the
system and tested successfully.

Fig 15. Ship Lift System
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3.4.2 Vibration Analysis of Ship Structures
With the progress made in the field of FE analysis and methods developed to cut down time, it is now
possible to use FE analysis to predict vibration characteristics of ship. It is desirable to perform a Free
vibration analysis during design stage of a vessel to estimate the natural frequencies in various mode
shapes, as any changes later are difficult, if not impossible. This analysis is normally carried out for
important and representative ship loading conditions e.g. Light Ship, Fully loaded departure/arrival and
Ballast departure/arrival conditions, which cover the major variation of loading during voyage. Added mass
is suitably incorporated to simulate the vibration of elements in contact with water.
In respect of the global i.e. hull girder vibration modes; it is customary to investigate the 1,2,3 and 4 noded
vertical, horizontal, torsional modes and the respective natural frequencies of vibration. These are
compared with the major excitation frequencies e.g. propeller blade frequency, engine/ shaft frequency,
main generators frequency etc. to avoid resonance. Similarly it is useful to model local structures for e.g.
engine room, funnel and the super structure/deck houses to avoid potential area of resonance. Necessary
corrective action can be taken at the design stage by rearrangement of structural elements and/or by
increasing the scantlings.
The forced vibration analysis wherein the time varying forces from machinery (propeller, engine etc.) are
also taken into account can provide useful information such as the displacement, velocity and acceleration
at critical locations. These responses can be compared to ensure compliance with acceptable standards.
Few important vibration analysis projects are briefly described below
3.4.2.1 Determination of natural frequencies of a cement carrier - During the sea trail of this cement
carrier, shown in Fig 16, excessive vibration was sensed and also measured by our surveyor. In view of
that, finite element analysis of superstructure, including engine room and steering gear compartment
was carried out to find out the natural frequencies and mode shapes. From the analysis result,
excessive vibration was found at the same locations, which the surveyors had detected during the trial.
The remedial measures to reduce excessive vibration were identified and made. After the modification
the vibration and noise level came down to the acceptable limits
Fig 16. Mode Shape of Cement Carrier Fig 17. Mode Shape of 500 Passenger Vessel

3.4.2.2 Vibration Analysis of 500 Passenger Vessel - Vibration analysis for a passenger vessel
designed for well-known Indian Shipyard, Fig 17, was carried out to estimate the natural frequencies
of the first 40 modes of vibration in various loading conditions. A 3D shell-beam model of the
complete ship in all details was created using the pre-processor of ANSYS 8.0. The vertical, horizontal
and torsional modes of hull girder frequencies for various loading conditions were compared with the
frequencies of engine, shaft and propeller. Suitable recommendations were made to avoid resonance.

3.4.2.3 Vibration Analysis of OPV’s, FAC’s and AOPV’s - Vibration Analysis of Naval Vessels is of
prime importance due to the kind of conditions and the type of service they operate on. We model the
entire vessel, shown in Fig 18, carefully and perform Vibration analysis to find the natural frequency
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of the same and also to check if forcing frequency matches with the natural frequency to create
resonance. Suitable modifications, if any, are then recommended to the yard.
Fig 18. Mode Shape of a Naval Vessel Fig 19. Mode Shape of a Mast

3.4.2.4 Vibration Analysis of Masts fitted in Various Vessels- Masts fitted on ships carry important
navigation and communication components, which are very sensitive to vibrations. And, it is
constantly acted upon by wind and acceleration loads. Hence it becomes very important to check the
vibration and strength of masts for the specified operation. The mast is modeled in complete detail and
free vibration analysis is performed, Fig 19, fitted to check if it could operate safely with the level of
vibrations, which arise due to the running of the main engine, Pumps etc. Modifications if any, are
suggested to the client.

3.4.3 Ultimate Strength Analysis
Collapse of hull girder is an important failure mechanism for ships as it can cause huge losses of life &
property and marine pollution, particularly in case of large ships. Therefore it becomes essential to check
the ultimate hull girder strength against the combined vertical wave bending moments and still water
bending moments, including those in the flooded conditions. The ultimate hull girder strength against
bending can be calculated using either the simplified methods such as that based on incremental-iterative
approach or by nonlinear FE analysis.
In the incremental-iterative approach the stress-strain relationship for all stiffened plate elements are
established separately for when under tension and when under compression. Finite Element Method, which
not only takes into consideration complex geometries associated due to initial imperfections but also the
geometrical non-linearity and the material non-linearity. It is possible to include the effects of internal loads
due to ballast water and cargo. The midship section is modeled including all longitudinal members between
web frames and moments are applied incrementally on the model end. The analysis results in the curvature
vs the ultimate moment carrying capacity curve to provide the ultimate hull girder bending moment
capacity

3.4.3.1 Ultimate Strength of Tankers, bulk Carriers and Container Ships - The aim of this project
was to develop a procedure to determine the ultimate strength of the hull girder of ships considering all
possible modes of progressive failure. The work was carried out as a part of combined research
conducted by the Asian Class Societies. All longitudinal strength members between two web frames
were modeled in detail for the full transverse sections, using shell elements and fine mesh. Static non-
linear elasto-plastic analysis was carried out considering the global vertical bending moments (Sagging
and Hogging) and horizontal bending moments applied at the ends of the structure. The Ultimate
collapse moment was determined for different ships and was compared with Rule allowable bending
moments to provide an indication of the reserve bending strength at the very extreme loads.

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35000
30000
25000 m)
m
20000 Base (
m
15000 n fro
10000 ocatio
L
5000
0
-350 -250 -150 -50 50 150 250 350
Compression Axial Stress (Mpa) Tension

Fig 20. Stress Distribution (Mid-Ship Section) Fig 21. Variation of Axial Stress along Depth of Ship

3.4.3.2 Determination of ultimate collapse pressure of hatch cover - In certain cases of bulk carrier
total losses, failure of the foreside hatch covers due
to abnormal waves has been sited as initial cause
leading to flooding and subsequent loss. In this
project a method was developed, for determining the
ultimate strength of hatch covers by non-linear
FEM. The load is increased in step increments and
both the region of material plasticity and deflection
are monitored. The practical ultimate strength is
determined up to a point when the hatch cover fails
to perform its assigned function of keeping sea
outside of the ship.

Fig 22. Stress Plot of Hatch Cover
3.4.4 Transient Dynamic and Strength Analyses under Impact Loading
The wave loads discussed above are dynamic in the sense that they are time varying, the frequency of the
cyclic variation being similar to the frequency of encountering waves. This being comparatively low, it is
possible to consider them as quasi-static loads and add to the still water loads. However there are other
types of highly dynamic loads, which are characterized by very high amplitude but short durations usually
lasting up to few milli-seconds e.g. slamming loads, under water shocks, contact/ collision etc. To fully
understand the effects of such loads the use of a non-linear transient dynamic analysis is necessary.
Nonlinear transient dynamic analysis is carried out in time domain, wherein the loads are varied at each
time step. Material and geometrical non-linearities are accounted, thus it is possible to investigate post-
elastic stage and estimate any permanent deformation left after the load history.
This kind of analysis usually requires extensive research due to the extreme sensitivity of the results on the
Finite Element Parameters.
3.4.4.1 Analysis of FSP subjected to Underwater Explosion - Now a day, all naval vessels in addition
to analysis to Normal Operational Loads, needs to checked for Explosive loads as well. We have
already started developing methods to study this complex phenomenon of structures subjected to
Explosive Loads. In a preliminary study we have analyzed a Floating Shock Platform (Fig 23) under
shock loads. The results matched closely with a similar study made by a Naval Department.

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