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Article: Advanced Double Hull Concept Reduces Costs, Increases ...

Special Issue:

Ships Navy Experts Explain

the Newest Material &

Structural Technologies

AMPTIAC is a DOD Information Analysis Center Administered by the

Defense Information Systems Agency, Defense Technical Information Center


The issue you hold in your hands has been 14 months in the

making. It began with a simple idea: turn the spotlight on the

age-old art of building ships. We wanted to show the exciting

new technologies that are offering novel materials for ship construction,

changing the way ships are built, and indeed creating

one of the most fundamental shifts in Navy combatants since

steel replaced wood.

This simple mission turned

out to be much more complex.

The project underwent a number

of different iterations, but

finally settled in and came

together. It has been a labor of

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and tanks often grab the

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and materials engineering systems fielded in today’s military.

Nothing has the complexity, impact, size, and sheer force of a

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So here it is, finally, and I am thankful that it is done. Not

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but we are proud because AMPTIAC has compiled something

that probably has not existed before: an overview of the

newest technologies being incorporated into structures and

materials for use aboard Navy combatants. And the people providing

the perspective are the experts at the Office of Naval

Research, NSWC-Carderock, and the Naval Research Lab. You

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Jeffrey E. Beach, Director

Survivability, Structures, and Materials Directorate

Daniel D. Bruchman

and

Jerome P. Sikora, Branch Head

Design Applications Branch

Carderock Division, Naval Surface Warfare Center

West Bethesda, MD

INTRODUCTION

The primary hull structure of a modern surface ship can provide

practical solutions for a multitude of obstacles facing a commercial

or combatant design. Acquisition cost, life cycle costs,

detectability, survivability, speed, range and payload are all, potentially,

important factors when selecting a platform to perform a

specific mission. For improved performance in many of these

areas, significant resources have been allocated for further development

of structural technologies used in both primary and secondary

structural applications. Often these technological options

increase the cost and complexity of the structure, have limited

impact on overall ship operational capabilities, and are, therefore,

only utilized in very weight-critical locations and designs.

In the late 1980’s, the Structures and Composites Department

at the Naval Surface Warfare Center, Carderock Division

(NSWCCD) began assessing the feasibility of the Advanced

Double Hull (ADH) concept with attributes that can significantly

impact all the areas in the design space. The ADH concept

consists of an inner and outer hull, connected by longitudinal

web girders. The resulting cellular structure

exhibits increased structural capacity

under typical seaway loads and the opportunity

for improved resistance to weapons

effects. The ADH concept is shown in

Figure 1.

Figure 1. Graphic Depiction of ADH Concept (Left) and Traditional

Ship Construction (Right).

GENERAL BENEFITS OF THE ADH

The ADH structure is unidirectional; all of the shell and deck

material acts in the longitudinal (parallel to the long axis of the

ship) direction. The transverse (perpendicular to the long axis of

the ship) frames (welded stiffeners) have been eliminated and the

transverse bulkheads provide the support for the longitudinal

structure. The elimination of transverse frames in an ADH configuration

has a number of positive effects on hull girder cost

and performance.

First of all, the elimination of the transverse framing leads to a

geometric simplicity that produces significant acquisition cost savings.

It contributes to an overall reduction in the number of piece

parts, substantially reducing labor costs associated with the cutting

and welding at longitudinal stiffener and transverse frame intersections.

Figure 2 and Table 1 illustrate the potential piece-part

reduction of a typical destroyer assembly. The geometric simplicity

also lends itself to automated fabrication processes.

Further, the elimination of transverse frames and the smooth

interior surfaces of the ADH structure greatly facilitate the

installation of distributive systems,

painting, and their associated costs.

A visual comparison is provided in

Figures 3 and 4 showing the relative

complexities associated with the installation

of these systems on a conventionally

framed combatant and an ADH combatant,

respectively.

The distributive systems added to an

ADH structure would not have to navigate through

and over transverse structure. In fact, a cost comparison was

conducted for electrical, HVAC, and piping systems in a

destroyer. The analysis estimated that the use of an ADH

configuration would result in a 17.5% labor hour savings in the

electrical craft, a 17.9% labor hour savings in the sheet metal

craft and a 23.6% labor hour savings in the piping craft.

When combined with other material, labor, and yard costs,

construction expense for an ADH destroyer can be reduced by

$60 million when compared to a conventional ship.[1]

The AMPTIAC Quarterly, Volume 7, Number 3 5


Panel

Stiffeners

Compensation

Rings

Collar Plates

Transverse

Floors

Flange & Tangency

Chocks

Chafing

Rings

Shell & Inner

Bottom

Longitudinals

& Stringers

Longitudinal

Girders

Longitudinal

Girders

Keel

Brackets

Panel Stiffeners

Figure 2. Piece-parts Used In Framing a Conventional Destroyer Hull Section.

Table 1. Comparison of Piece-part Requirements in a

Conventional and ADH Destroyer Assembly.

Number of Basic Structural Pieces

Item ConventionalADH Variance

Design Design < - >

Collars 200 0

Chocks 120 0

Panel Stiffeners 40 0

Access 40 11

Opening Rings

Chafing Rings 100 20

Deep 11 13 +2

Longitudinal Girders

Intermediate 6 0

Shell Longitudinals

Intermediate Inner6 0

Bottom Longitudinals

Individual 50 0

Floor Panels

Docking 10 10 –

Keel Brackets

Deep Long'l Girder7 9 +2

Panel Stiffeners

Total Basic 584 63

Ultimately, cost is most often the metric that governs the

selection of a ship platform. Before a new technology is adopted

into a platform’s design, significant acquisition and/or life

cycle cost reductions as compared to existing processes and

designs must be demonstrated. Acquisition cost is easily quantified

and readily demonstrated with the ADH concept.

However, life cycle cost reduction potential can be difficult to

demonstrate – particularly for a combatant. Life cycle costs

encompass recurring expenses such as routine maintenance,

painting, corrosion management, structural failures resulting

from typical seaway or combatant operations, military and

political ramifications if a platform cannot complete a mission,

and replacement costs if a ship is lost at sea. The ADH concept

offers a unique solution that significantly reduces costs and

risks throughout the operational life of the ship.

The structural material comprising the transverse frames in

conventional, grillage construction (utilizing both transverse

and longitudinal welded stiffeners) is redistributed into the

inner and outer hulls in an ADH configuration - increasing the

hull girder section modulus. This lowers the primary, seawayinduced

longitudinal bending stresses and increases the fatigue

life of the structure. The elimination of the transverse frames

further increases the hull girder fatigue life by significantly

reducing the number of crack initiation sites at the intersections

of longitudinal stiffeners and transverse frames.

The increased section modulus also gives the ship designer

flexibility to isolate (both acoustically and from shock) internal

decks, because they no longer need to resist primary loadings.

Similarly, ADH technology can be utilized to incorporate modular

design capabilities into a ship system. The inherent

6

The AMPTIAC Quarterly, Volume 7, Number 3


Machinery

Platform

Plate & Beam

Web Frame

Longitudinals (Typical)

strength of an ADH hull form can also allow for large openings

in deck structures, simplifying the removal and replacement of

obsolete systems.

Pipe (C)

Figure 3. Typical Structure-System Interface on

Conventional Combatant.

Pipe (C)

Pipe (B)

Longitudinal BHD Plating

Plate & Beam

Smooth Cored Deck

Machinery Platform

Pipe (B)

Advanced

Double Hull

Pipe (A)

Inner

Plating

Figure 4. Typical Structure-System Interface on an ADH

Combatant.

Figure 5. Rigid Vinyl and Finite Element Analysis

of Stress Behavior in ADH Structures.

Smooth

Hull

Pipe (A)

STUDYING THE MECHANICS OF THE ADH

Over the last several years substantial research into the ADH

has documented the structural characteristics and performance

of the concept under a variety of loading conditions. This

research has primarily focused on the performance of a prismatic

ADH structure located in the mid-section of the ship

(where the sides are roughly parallel). Research has included

investigations into the stress behavior of prismatic ADH structures

[2] (Figure 5) and their ultimate strength (Figure 6).

Ford, et. al. were able to use the finite element method in

conjunction with rigid vinyl model tests to evaluate the stress

behavior of an ADH structure. It was shown that, for a given

ADH geometry, the stress state of the structure could be predicted

using classical strength of material approaches. Further

investigations were conducted to determine the collapse

strength of ADH cellular structures, and are pictured in Figure

7.[3]

Devine tested a series of large 3-cell and 5-cell ADH

columns to collapse under axial loading conditions. A significant

amount of scatter was observed in the behavior of the

columns, particularly in the columns with plates having a slenderness

ratio (length/thickness) typically used in ship construction.

Devine showed that classical plate-buckling equations

that did not account for welding-induced residual stresses

were somewhat unconservative. There was a significant

reduction in the column collapse strength resulting from the

welding-induced residual stresses, which must be accounted

for in the design process.

The Oil Pollution Act of 1990 was enacted following the

Exxon Valdez tragedy in 1989. This legislation requires newly

built tankers trading in US waters to be fitted with double hull structure.

It also set a schedule for phasing out single hull tankers currently

in service. Significant assets were, therefore, allocated in the

ADH program to evaluate and improve the grounding and stranding

behavior of double hull configurations (Figure 8).[4, 5, 6, 7]

Rodd was able to demonstrate through a series of large-scale

grounding tests, that inner hull rupture occurred at the transverse

frames of conventional grillage construction. Removal of

transverse frames in the ADH allowed the structure to

absorb substantially more energy before inner hull failure.

Additional crash resistant features were also

developed to further enhance the configuration’s

damage resistance.

In addition to the extensive research to determine

the structural characteristics and behavior of

ADH structures under conventional loading conditions, there

has also been substantial research to determine their behavior

under combatant loading conditions. This included analysis of

explosions above the waterline (called AIREX) and below (called

UNDEX) (Figure 9).

NSWC conducted an extensive numerical analysis of an

ADH cruiser subjected to an underwater explosion.[8] Results

indicated that an ADH configuration was better at attenuating

The AMPTIAC Quarterly, Volume 7, Number 3 7


Figure 6. Test Articles for Ultimate Strength Analysis of

ADH Structures.

Figure 7.

Measuring the

Collapse

Strength of

ADH

Structures.

shock on equipment by reducing accelerations and peak velocities

during the UNDEX event.

Research to date indicates that, compared to a conventional

grillage type of structure, the ADH is generally stronger and

more damage resistant under the tested loading conditions, with

little increase in weight. Both acquisition and life cycle costs are

reduced in the ADH concept due to geometric simplification,

piece-part reductions and the associated facilitation of painting

and the installation of distributive systems. Furthermore, if the

space between the hulls is properly utilized and the right materials

selected, the ADH concept offers the potential for significant

reductions in IR, acoustic and magnetic signature levels. The extensive

research to date has culminated in the development of guidelines

for ADH structure design.[9]

MANUFACTURING/MAINTENANCE ISSUES

WITH THE ADH CONCEPT

There are significant manufacturing, maintenance and inspection

issues unique to the ADH hull form. These issues

generally revolve around the ability to gain access to the

interior cell spaces and are magnified for smaller ship classes

as the separation between the inner and outer hulls is reduced

for weight and volumetric considerations. As the distance

between the inner and outer hulls increases beyond about one

meter, more conventional construction, maintenance and

inspection methods can be employed.

Conventional welding techniques can be used for all aspects of

ADH construction until the inner shell region is welded to the

longitudinal web girders in a design with a hull separation less

than one meter. Then, because of the limited access to the interior

space, other welding processes may be required. These

processes may include automated welding techniques and can be

accomplished from either inside or outside the cellular structure.

Laser welding, electron-beam welding, and friction-stir welding

(please see accompanying article in this issue by DeLoach, et. al.)

have all been investigated and are viable options. Most near-term

applications being considered could be effectively fabricated

using manual or mechanized welding techniques.

Maintenance and inspection issues of the small interior cellular

structure have been investigated. Robotic technologies have

been studied [10] and proven to be effective means of performing

inspections and routine maintenance, such as painting and

minor repairs when necessary (see Figure 10).

Corrosion maintenance and repair costs of Navy combatants

are estimated to be at least $1.2 billion a year for approximately

360 ships.[11] Material selection, coatings and manufacturing

techniques must be carefully evaluated to minimize corrosion.

In an ADH combatant, the increased horizontal surfaces

where water can accumulate and the generation of additional

moisture caused by variation in temperature between the inner

8

The AMPTIAC Quarterly, Volume 7, Number 3


Figure 8. Studying the Grounding Response of ADH

Structures.

and outer cell walls can aggravate corrosion problems.[9]

Several coatings were evaluated for ADH applications to help

minimize corrosion problems. In general, high build epoxies

were the most effective in both long- and short-term exposure

tests [12] and often outperformed the standard Navy F150/151

epoxy system. Pre-coating ADH structural components also

proved effective in reducing coating costs.

STAINLESS STEELS

Budget conditions mandate that ship construction costs be

reduced in future combatant construction. Many technologies

and systems that would improve combatant performance are not

considered because they increase acquisition costs, and the

implementation of stainless steels for hull construction is one

example of this. Although preferable to ordinary steel construction

for many reasons, austenitic stainless steels and other nonmagnetic,

corrosion resistant materials are not often considered

in combatant designs because of acquisition cost constraints.

The ADH concept provides a practical solution to the combatant

need for a more survivable hull form. It offers a unique

geometry and significant cost savings during construction and

the installation of distributive systems that enable the practical

implementation of many multi-spectral ship signature reduction

options. These options include the use of austenitic stainless

steels, not only for their ability to reduce the magnetic signature,

but also for their high resistance to Charpy impact and explosion

bulge loading. Their high elongation before rupture can also

lower hull girder bending stresses acting on the hull during an

UNDEX event, and significantly increase survivability in these

conditions.[13] In Table 2, the properties of commonly used

magnetic shipbuilding steels are compared with select non-magnetic

stainless steels.

There are a number of reasons why the ADH concept is an

ideal platform to introduce non-magnetic stainless steels. The

estimated $60M savings for a 9,000 long-ton ADH combatant

can more than offset the added expense in material and fabrication

costs of a comparable stainless steel combatant. Further,

because the ADH concept lends itself to automated welding

processes, fume controls mandated by the Occupational Safety

and Health Administration (OSHA) are less problematic. These

requirements were set forth to protect personnel from the fumes

generated during the welding of non-magnetic steels and nickelbased

alloys.[14] Finally, automatic and robotic welding operations

enabled by the ADH’s geometric simplifications can ensure

a more consistent, high quality weld than possible with manual

welding techniques. This is doubly important when welding

non-magnetic stainless steels, as the weld quality must be

extremely high in both weld surface condition and the weld

deposit itself.[15]

To identify the costs and benefits of a stainless steel ADH

combatant, a comprehensive research and development program

began in the late 1990’s. In this program, several stainless

steels were evaluated based on the following criteria: weldability,

welded properties, yield strength, elongation, residual stress

effects, fabrication distortions and crevice and stress corrosion

susceptibility. The materials also had to be non-magnetic. Based

on the selection criteria, the focus of the study centered on

austenitic stainless steels because, although the high nickel and

chromium content makes the alloys expensive, they exhibit

Figure 9. ADH Structures in UNDEX and AIREX Tests.

The AMPTIAC Quarterly, Volume 7, Number 3 9


Figure 10. Robotic Maintenance and Inspection of ADH

Structures.

excellent ductility, formability and superior corrosion resistance.

Austenitic stainless steels evaluated include AL-6XN, Nitronic

50, and the 300 series. The material costs for plating ranged from

$1.90/lb for the 300 series, $3.10/lb for the Nitronic 50, to

$4.00/lb for AL-6XN.

Based on the results from the stainless steel research and development

program, several performance and cost characteristics

were identified. Of the metals investigated, the 300 series steels

were particularly susceptible to crevice corrosion. In general, the

AL-6XN provided the best corrosion protection and is recommended

for the most corrosion-sensitive locations. Both the AL-

6XN and the Nitronic 50 exhibited yield stresses comparable to

AH 36. The 316L had a yield stress value around 30 ksi.

The stainless steel ADH hull form exhibited excellent multispectral

signature control characteristics. The magnetic signature

alone was an order of magnitude less than that of an ordinary

steel hull. However, this did require special handling and

cutting techniques akin to those used for other non-magnetic

metals such as aluminum. The space between the hulls could

also be designed to significantly reduce the IR signature as well

by the inclusion of active or passive insulators.

CONCLUSIONS AND REMAINING CHALLENGES

The Advanced Double Hull concept can significantly

reduce both acquisition and life cycle costs. The cellular

structure exhibits increased structural capacity under

typical seaway loads, superior fatigue performance, and

the opportunity for improved resistance to weapons

effects. The inherent strength of an ADH hull form also

offers the opportunity to isolate internal decks acoustically

and from shock, providing a less detectable, more

survivable hull. Further, the ADH concept is an ideal

platform to construct out of stainless steels, thus providing

the opportunity for multi-spectral signature control.

Several issues however, remain to be investigated. ADH technology

can’t efficiently be implemented approaching the bow

where the hull form rapidly changes and exhibits flare. Therefore

optimum details need to be identified in the regions where the

ADH structure transitions to a conventional structure. Work has

begun to evaluate the performance of non-prismatic ADH structures,

but further research is necessary. When constructing a nonmagnetic,

stainless steel ADH, a dedicated yard with special tooling

may be necessary. Additionally, there are higher levels of distortion

resulting from the welding process of the austenitic stainless

steels and flame straightening may be required.

REFERENCES

[1] J. Beach. “Evaluation of Advanced Double Hull (ADH)

Including the Use of Stainless Steels”, Presentation to SC-21

Program Office, April 1997

[2] H. Ford, J.M. Grassman, and R. Michaelson. “Stress

Behavior of Advanced Double Hull Structures”, Paper No. 10.

Proceedings of the Advanced Double Hull Technical

Symposium, 25-26 Oct, 1994, at Gaithersburg, MD

[3] E. Devine, J. Ricles and R. Dexter. “Instability and Collapse

Strength of Advanced Double Hull Structures”, Proceedings of the

Advanced Double Hull Technical Symposium, Paper No. 6,

Gaithersburg, MD, 25-26 Oct, 1997

[4] J. Rodd, with M. Phillips and E. Anderson. “Stranding

Experiments on Double Hull Tanker Structures”, Proceedings of

the Advanced Double Hull Technical Symposium, Paper No. 2,

Gaithersburg, MD, 25-26 Oct, 1994

[5] J. Rodd. “Frame Design Effects in the Rupture of Oil Tanks

During Grounding Accidents”, DERA, Proceedings of the

International Conference - Advances in Marine Structures III,

Table 2. Material Property Comparison of Commonly Used Shipbuilding Steel and Select Non-magnetic Stainless Steels.

Ultimate Yield Elongation Charpy Modulus Density

Material Alloy Strength (ksi) Stress (ksi) (% in 2-in.) Toughness (ft-lb) (x10 6 psi) (lb/in 3 )

Shipbuilding AH36/ 71-90 51 22 17 transverse 30.0 0.283

Magnetic DH36 25 longitudinal

Steel

(20 °F DH36)

(0 °F AH36)

Non- Nitronic 50 104 57 60 103-110 (75 °F)

Magnetic 83-88 (-100 °F) 26.2 0.285

Steel AL-6XN 108 53 47 140 (70 °F)

100 (-200 °F) 28.3 0.291

10

The AMPTIAC Quarterly, Volume 7, Number 3


Dunfermline UK, 20-23 May, 1997

[6] T. Wierzbicki, E. Rady, D. Peer and J.G. Shin. Damage

Estimates in High Energy Grounding of Ships, Massachusetts

Institute of Technology, Department of Ocean Engineering,

Tanker Safety Program, Report No. 1, June 1990

[7] T. Wierzbicki, D. Peer and E. Rady. The Anatomy of Tanker

Grounding, Massachusetts Institute of Technology, Department

of Ocean Engineering, Tanker Safety Program, Report No. 2,

May 1991

[8] H. Gray. “Survivability Implications of Advanced Double

Hull Combatants”, Proceedings of the Advanced Double Hull

Technical Symposium, Paper No. 18, Gaithersburg, MD, 25-26

Oct, 1994

[9] J. Sikora, and E. Devine. Guidelines for the Design of

Advanced Double Hull Vessels, NSWCCD-65-TR-2001/04,

West Bethesda, MD: Naval Surface Warfare Center, Carderock

Division, 2001

[10] M. Gallagher, T. Barbera, and M. Bankard. “Inspection and

Maintenance of Advanced Double Hull Cells”, Paper No. 17.

Proceedings of the Advanced Double Hull Technical

Symposium, 25-26 Oct. 1994, at Gaithersburg, MD. 1994

[11] D.D. Thurston. Recurring Fleet Corrosion Issues. Paper

read at CORROSION 97 (NACE International) Group

Committee T-9B (CINCLANTFLT maintenance Technical

Advisor), at New Orleans, LA, March 1997

[12] H. Hack, Holder, A., Clayton, N., Dobias, P., Arazy, C.,

Bowles, C., Brush, N., Vasanth, K., Katilaus, J., and Jean

Montemarano. “Corrosion Control of Advanced Double Hull

Combatants”, Proceedings of the Advanced Double Hull

Technical Symposium, Paper No. 16, Gaithersburg, MD,

25-26 Oct, 1994

[13] E. Moyer, M. Pakstys, and C. Milam. “The Effects of

Plasticity on the Whipping Response of Surface Ships and

Submarines”, Proceedings of the 62nd Shock and Vibration

Symposium, Vol. II, Springfield, Virginia, 29-31 Oct, 1991

[14] National Shipbuilding Research Program. “Impact of

Recent and Anticipated Changes in Airborne Emission Exposure

Limits on Shipyard Workers”, Report NSRP0463 (March 1996)

[15] Marine Engineers Review. “Corrosion Resistance of

Stainless Steels for Chemical Tankers”, March 1972

Dr. Jeffrey E. Beach is Director of the Survivability, Structures, and Materials Directorate at the Carderock Division,

Naval Surface Warfare Center where he has been employed since 1969. He earned B.S. and M.S. degrees in aerospace

engineering from the University of Maryland and has had additional graduate training in naval architecture and

structural engineering. He earned his doctorate in engineering management at The George Washington University.

Dr. Beach manages a diverse program of exploratory, advanced, and engineering development research aimed at

the advancement and application of materials and structures to marine vehicle systems. He is active nationally and

internationally in professional societies, organizations, and exchanges, and has published and presented numerous

technical papers in the international community. Among his awards are the 1984 American Society of Naval

Engineers Solberg Award for research and the 1998 ASNE Gold Medal Award.

Daniel D. Bruchman has been employed at the Naval Surface Warfare Center, Carderock Division since 1986 when

he received a BS degree in Civil Engineering from the University of Mississippi. Currently, he is a Naval Architect in

the Structures and Composites Department at Carderock, responsible for the technical analysis and evaluation of

advanced structural systems under consideration for Navy surface ships and submarines. Mr. Bruchman began his

career specializing in the use of numerical methods to analyze conventional and unconventional hull forms. His design

of a ship collapse facility, capable of testing large scale ship structures to collapse under pure moment or momentshear

combinations, has contributed to a much greater understanding of the global collapse of vessels under these

conditions. Recently, he has been involved in the development of high strength/lightweight materials and structural systems

for high speed ships while also serving as the Advanced Double Hull program coordinator.

Jerome P. Sikora has been employed at the Naval Surface Warfare Center, Carderock Division since 1969 when he

received a BS degree in Physics from the University of Detroit. Currently, he is the head of the twenty-member Design

Applications Branch within the Structures and Composites Department, a position he has held since 1986. The Design

Applications Branch is responsible for developing structural design criteria and design tools for Navy surface ships

and submarines. Mr. Sikora began his career specializing in the field of static and dynamic optical measurements of

such structural parameters as stress, deformations, and vibration modes. He has been instrumental in the development

of unconventional vessels and offshore structures such as SWATHS, tri-hulled ships, and the mobile offshore base. Over

the past two decades, he has developed probability-based loads criteria for conventional and SWATH ships.

Recently, he has been involved in developing structural design criteria and design methods for the cellular structure of

the Advanced Double Hull.

The AMPTIAC Quarterly, Volume 7, Number 3 11

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