ASNE International Conference on Launch & Recovery, November 14-15, 2012, Linthicum (MD).

Design of an Unconventional ASV for Underwater Vehicles Recovery:

Simulation of the motions for operations in rough seas.

Stefano Brizzolara and Chryssostomos Chryssostomidis

Massachusetts Institute of Technology

Department of Mechanical Engineering and Sea Grant College Program,

Cambridge (MA), 02139. stebriz@mit.edu / chrys@mit.edu

ABSTRACT

Paper introduces main characteristics and particulars of an

innovative design for an Unmanned Surface Vehicle to

autonomously launch and recover AUVs (Autonomous

Underwater Vehicles) in open sea. The USV has an

unconventional SWATH hull shape and in its smaller size

version, it has dedicated hangar at midship that can host one

medium size AUV, completely recovered onboard. The

focus of this paper is concentrated on the prediction of the

steady and unsteady hydrodynamic characteristics in terms

of hull resistance and motion in waves. The seakeeping

prediction is made by a fully viscous 3D Unsteady Navier-

Stokes solver. The predicted pitch and heave response of the

USV-SWATH in relatively high regular waves, in the non-

linear regime, are compared with those of an equivalent

catamaran vessel and dramatic reduction in vertical motions

and accelerations are found. On this good basis the paper

concludes presenting a first hypothesis of the L&R system

based on self-tensioning winches with hoists and belt

assemblies.

KEY WORDS

Unmanned Surface Vehicles (USV), Small Waterplane Area

Twin Hull (SWATH) vessel, AUV launch and recovery,

SWATH motions in waves.

1.0 INTRODUCTION

Persistent monitoring of large sea stretches, involving

cooperative networks of Autonomous Underwater Vehicles

(AUV) is a leading research topic in the field of underwater

robotics for scientific/civilian application (water quality,

oceanography) as well as for other military purposes, such

for instance in security/surveillance tasks. Interesting

technological advancements have been reached both in the

design of the underwater vehicles with increased energy

storage capabilities leading to larger endurances (Wilson

and Somlyody, 2009), as well as in the autonomous

intelligence. On this latter topic, it is worth mentioning the

work of Bahr et al. (2009) on distributed acoustic navigation

systems, with adaptive path planning in order to achieve

cooperative positioning for missions of extended duration

over large areas. With the same final aim, Lolla et al. (2012)

propose new numerical algorithms to determine time-

optimal paths of ocean vehicles in continuous dynamic

flows.

Huntsberger & Woodward (2011) transfer the experience

made by NASA JPL on space autonomous vehicles to

marine vehicles, supporting the idea of a cooperative

network of surface and underwater vehicles (see the sketch

of Fig. 1) to provide adaptive mission capabilities by means

of a proprietary control system, as opposed to the open

source software framework MOOS-IvP developed at MIT

(Balasuriya et al., 2009; Benjamin et al. 2011).

Fig. 1. The small USV-SWATH as the key element of an

integrated cooperative AUV network

The cited studies are necessary but not sufficient

technologies to effectively realize a persistence system of

underwater vehicles (AUVs network). The practical

realization is limited, in fact, by the relatively small

endurance between regular maintenance intervention to the

AUVs: first of all, for their batteries recharging. This task is

still delegated to manned ships equipped with special cranes

(for launching and recovering of the AUVs) that usually

take also care of bringing the vehicles in the specific theatre

of operation.

From all above considerations it is easy to derive that an

Unmanned Surface Vessel (USV) specifically dedicated to

support Autonomous Surface Vehicle (ASV) and capable of

launching, retrieving and recharging them in place, is the

real key to the realization of a true persistent sea monitoring

system of AUVs. In fact, following a practical operational

necessity of the NURC (NATO Undersea Research Center,

La Spezia, Italy), the design and development of an

innovative ASV based on an Unconventional SWATH hull

form was initiated. The basic layout of the vessel, presented

in Fig.2 and Fig.3, is based on an unconventional optimized

hull shape for the lowest powering requirement and reduced

motions in a sea state. A summary of the vessels main

design features is published in Brizzolara et al. (2011). The

prevalent mission profile of the USV is the transportation of

medium sized AUVs used at the centre (i.e., Folaga,

Hydroid, Blue-Fin 9, etc.) from an onshore base to the

operational area being; moreover the platform itself can be

equipped with different sensors to carry out measurement

campaigns on its own. The following two sections supply

more detail information of the numerical methods used to

optimizer the hull form and about the quite unique

seakeeping performances in waves that have been recently

predicted and never presented before.

Fig. 2. USV- SWATH: view from below, hangar doors for

AUV launch and recovery are colored in gray.

2.0 VESSEL DESIGN FEATURES

This section intends to give the main features and

characteristics of the USV (paragraph 2.1) and its peculiar

hydrodynamic performance characteristics that have been

optimized and verified on extensive studies (paragraph 2.2).

More details of the optimization method for the

hydrodynamic design of the hull are given in Brizzolara &

Vernengo (2011),

2.1 General Arrangements Plans and Main Operational Characteristics

Fig. 3 shows a 3D-view of the general arrangement of the

small USV-SWATH. The underwater hulls are 6m long and

the vessel full load displacement is about 4.2t; design speed

is 12 knots. The propulsion system is diesel-electric with

two gen-sets of 25kWe in the main body (brown boxes in

Fig.3), two fast and compact DC brushless electric motors

about 20 kW in the lower hull connected through

epicycloidal gears (red cylinders) to the slow turning fixed

pitch propellers. Four battery packs are fitted in the lower

part of the struts (gray boxes). Fuel and compensation

ballast tanks are fitted in the central portion of the lower

hulls (red and yellow colored portions). They are sized to

ensure a sufficient reserve of energy to cover a range of

about 120 miles (at 12 knots) and recharge a few AUVs in a

single sortie.

The upper structure (main body) is subdivided in three

sections by two watertight bulkheads. The central section

hosts the L&R and recharging system for AUVs: in the

example of Fig. 3, a 2m long vehicle is presented inside the

docking section, in its pulled-in position. Two pairs of

stabilizer fins are fitted on each underwater hull. They are

activated by an integrated ride control system to correct the

dynamic trim and sinkage of the SWATH at high speeds

and to dampen roll, pitch, heave motions in waves.

The connection between the main body and the struts is

dismountable and the three main pieces of the vehicle are

watertight. Electric and fluid transmission between the main

body and lower hulls are realized by watertight connections.

The whole AUV can be dismounted can fitted into a

standard 40’ container to be easily transported.

Fig. 3. General Arrangement of the Small USV-SWATH, main systems are schematically represented

Patent Pending

2.1 Hull Hydrodynamic Design and Optimization

The SWATH hull form shape has been optimized to achieve

the lowest drag at cruise speed, 12 knots, corresponding to a

length based Froude number Fn0.8. The automatic

parametric optimization procedure, whose high level flow

chart is given in Fig.4, is based on a viscous-inviscid-

interaction solver, namely a free surface panel method and

thin boundary layer solver, weakly coupled through

transpiration velocities. Details of the solver and a first

example of application in a case of a larger vessel are given

in Brizzolara (2004).

Fig. 4. Highlevel flow chart of he parametric optimization

method to find the best hull form

Design constraints, such as maximum weight and main

dimensions, were assigned to guarantee the manageability

and transportability of the vehicle on land and offshore. This

resulted in rather short hulls (L=6m). So, the sufficient initial

transverse and longitudinal metacentric heights were

obtained by means of a twin canted design of the struts and

by positioning them at the forward and aft ends of the vessel,

as presented in the exemplary shape of Fig. 5.

A new hull geometry generation module has been developed

with respect to Brizzolara (2004). The new formulation is

based on a fully 3D representation of the hull and twin

canted strut surfaces with B-Spline surfaces. A new

optimization algorithm based on multi-objective,

constrained, genetic algorithms was also selected to drive the

choice of free parameters toward convergence. The main

details of this new method are given in Brizzolara &

Vernengo (2011). The parametric geometry definition uses

the least number of free parameters that permit to generate

unconventional shapes of the type presented in Fig.5. The

free parameters are the coordinates of the control vertices of

the B-spline surface that regulate the contraction of the

transverse sections in between the two relative maximum

area sections and the two curvature radii at the aft and fore

end of the hulls. A fixed aspect ratio of the elliptic transverse

sections has been used for the whole underwater body. The

geometry of the struts has been also fixed, being designed

with regards to the transverse and longitudinal stability. An

inequality constraint was given on the design displacement

with a 3% allowance margin. The value of the wave

resistance coefficients predicted for the optimum hull shape

are listed in Table 1, together with those of a conventional

shape taken from series 58 (at equal prismatic coefficient)

and of an initial guess of unconventional shape taken from

previous studies. As reported in table 1, the optimized hull

form is able to nearly halve the wave resistance of the

conventional shape. These excellent improvements were

verified by a higher fidelity CFD solver, namely a finite

volume RANSE solver (CD-Adapco, 2008) already

validated with success in case of high speed SWATH vessels

(Brizzolara & Villa, 2009). Near field wave patterns

predicted with the RANSE solver at different full scale

speeds are presented in Fig.6. the positive wave cancellation

effect at 12 knots is evident by the wave contour plots.

Fig. 5. Unconventional SWATH shapes with twin canted struts

Table 1. Wave Resistance Coefficients predicted for the

conventional, optimized and a first tentative hull shape

Optim. Original Conv. CW*103 1.915 3.336 3.533

CW % - 45.8 - 5.6 0.0

Fig. 6. Wear Field Wave Patterns generated by the best hull

form at different speeds (indicated in knots)

The total resistance evaluated by means of RANSE model in

the complete speed range of Fig. 6 has been used for the

design of the optimum propeller with a procedure similar to

that successfully used for underwater vehicles (Brizzolara &

Gaggero, 2010). The total brake power of 40kW (selected

with a 20% allowance margin) was split on two propellers of

0.7m diameter.

3.0 MOTION IN WAVES

The same finite volume RANSE solver with VoF method for

free surface calculations has been recently used to predict

the heave and pitch transfer functions of the vessel in regular

waves. In parallel, a new hybrid numerical method for

seakeeping predictions, based on 2D fully viscous

calculation of added mass and damping coefficients. The

main module successfully validated in case of conventional

monohull sections (Bonfiglio et al., 2012) is currently under

validation for catamaran and SWATHs sections. When

finished this new numerical method is expected to reduce the

calculation times, without any significant loss of accuracy

(apart from 3D effects), with respect to the 3D method used

in this study that required about 15,000 CPU-hours to

simulate all the cases needed for just a single hull type.

3.1 U-RANSE Solver and 2-DOF Model

A state of art of URANSE finite volume solver with mixture

of fluid method for free surface flows has been adapted to

the problem of the prediction of non-linear ship motion in

waves, as already tested by the author in other cases (Grasso

et al., 2010). The generic CFD solver (CD-Adapco, 2008)

can use a variety of turbulent models and solves for the

dynamic equilibrium of a floating body, whose motion is

predicted integrating the dynamic ODE of the rigid body

motion in 6 degree of freedom.

The RANS equations can be expressed, in the case of an

incompressible fluid mixture, as following:

Mmixmix STUPU Re

0

U (2)

where U is the average velocity vector field, P is the average

pressure field, μ is the dynamic viscosity, TRe is the tensor of

Reynolds stresses and SM is the vector of momentum

sources. The component of TRe is computed by means of a

realizable k-ε turbulence model that, according Boussinesq

hypothesis, implies:

ijijt

ij

i

j

j

itij

k

kxx

3

22

3

2Re

D

UU

(3)

Where μt is the turbulent viscosity, k is the turbulent kinetic

energy. To save cells close to the hull surface, a two layer

analytical wall functions suitable for ship full scale Reynolds

numbers have been adopted to extrapolate the velocity in the

prismatic layer of cells closest to the wall. According the

model a first thin linear (laminar) sub-layer close to the wall

is considered and then the extrapolation continues in the

outer region by a usual universal logarithmic law valid for

thin boundary layer. For this model to be valid, the centroid

of the first cell near needs to lie within the log-law region

where y+<100.

The RANS solver is based on a Finite-Volume method

allowing for non-structured polyhedral elements to discretize

the physical domain. The momentum and mass conservation

equations for an incompressible single virtual fluid, whose

density and viscosity properties considered as a mixture of

two uncompressible phases, given in eq. (2) are solved with

one more transport equation for a scalar function that defines

the fraction of hew liquid phase (water) present in each cell:

waterairmixwaterairmix

divt

)1(,)1(

0U (4)

This very easy model is as simple as powerful and

demonstrated its ability to find the correct shape of the free

surface between water and air in many complex non-linear

problems, including wave breaking and spray as for instance

in case of planing hulls (Brizzolara & Federici, 2010). To

solve the time-marching equations, a second order implicit

solver is used. SIMPLE method is used to conjugate

pressure field and velocity field, and the AMG (Algebraic

Multi-Grid) solver to accelerate the convergence of the

solution of (2) at each time step. In our particular case, many

iterative steps are needed to minimize the residuals in order,

in turn, to obtain an accurate prediction of ship motions in

waves. In fact, each time step, the forces acting on the hull

are calculated by integration on the wall boundary

corresponding to the hull of the tangential and normal

stresses that needs to be accurately estimated.

Fig. 7. Domain for seakeeping simulation of USV-SWATH

Steadily-progressing free surface waves can be generated

imposing an initial flow in the domain and time variant inlet

and outlet velocity fields of non-linear (up to the fifth order)

Stokes waves, according to the analytical formulation of

Fenton (1985).

A finite volume model with about 6.5M cells have been

created for the USV-SWATH, having the overall dimensions

quoted in Fig.7, in order to keep the wave reflection from

boundaries after a sufficient number of incoming waves. An

unstructured Cartesian/trimmed type mesh with progressing

refinements has been used to minimize the number of cells.

Close to the body 10 prismatic layers of cells have been used

to resolve the flow in the boundary layer and capture

eventual separation vortexes. The cell size far from body is

0.2L (L is the underwater hull length), while on the hull

surface is 0.7%L with a minimum of 0.06%L around the

area with higher curvature, automatically refined by the

surface meshing algorithm, as per Fig.8. Two anisotropic

nested cell refinements around the free surface have been

used, spanning 0.1L and 0.5L respectively. In the inner

region around the mean free surface, the mesh is composed

by prismatic cells measuring [3; 6; 0.5]% L in the

longitudinal, transversal and vertical directions, respectively.

The outer refinement region was used to adequately resolve

the wave orbital motion and it spans about twice the size of

the inner previous region.

Fig. 8. Cell refinement close to the hull and at the free surface

used for the seakeeping simulations

For the integration time step, a value smaller than T/200 was

used in all the simulation presented (T is the period of the

incoming regular wave).

3.1 Comparative Motion Prediction vs. Conventional Catamaran

The unsteady RANSE calculations in incoming bow waves

with the SWATH free to heave and pitch but stationary in

the surge motion direction have been systematically repeated

for different incoming wave lengths, having values ranging

from the same SWATH hull length L, up to three times the

SWATH length. All the regular waves had the same height,

nominally 3.5% of the SWATH hull length L, in order to be

representative of a rather extreme and non-linear condition.

To be able to compare the performance of the optimized

USV-SWATH with a possible conventional alternative, the

seakeeping CFD simulations have been repreated for a

catamaran vessel having a hard chine, deep-V hull form. The

catamaran hull has been obtained by trnaformation in

affinity from the well known Marintek catamaran

(Faltinsen, 2008, p.279). The original hull has been non

uniformly scaled with two different scale factors, one for

length and the second for beam and draft, to reach a length

of 7m and the same displacement of the SWATH. The

catamarn so obtained is presented in Fig. 9, in a snapshot of

the case with incoming wave length double the hull length

L. Fig. 10 presents a similar snapshot for the USV-SWATH

hull with an incoming wave length 2.5 times the hull length.

Fig. 9. Snapshot of the wave pattern around the Catamaran

predicted by URANSE solver (/L=2.0)

Fig. 10. Snapshot of the wave pattern around the SWATH

predicted by URANSE solver (/L=2.5)

Fig. 11. Comparison of the Heave RAO calculated for the two

equivalent vessels

A typical time hisotory of the pitch angle calculated during

the simulation for the two vessels is given in Fig.13, for the

case with /L=2.5. From the comparison of the maximum

amplitude during the simulations, it is evident that the

SWATH pitch motion is noticeably smaller than that of the

equivalent catamaran vessel, about one third. This holds true

over almonst all the investigated range of incoming wave

lengths as confirmed by the comparative graphs of Fig. 11

and 12. These graphs present the heave (3) and pitch (5)

Response Amplitude Operators as derived from the time

histories of the simulated motions. The pitch RAO is

calculated as the ratio of the pitch max amplitude in radians,

to the product of the wave number k=2 and the wave

amplitude a. The heave RAO is simply the max heave

amplitude divided by the wave amplitude. In addition to

pitch, also the amplitude of heave motion of the SWATH is

substatially smaller than that of the catamaran, especially for

higher wave lenghts (>2.5L), where the catamaran vessel

substantially follows the wave. The SWATH vessel, instead,

has a resonance peak of the heave RAO for wave lengths

between 2 and 2.5 the hull length, where in any case

registers a 2/3 heave amplitude with respect to the

catamaran. For longer and shorter waves the SWATH heave

motion amplitude is less than 1/3 the wave amplitude.

No distinct peak is noted in the pitch RAO for the SWATH:

this is due to the twin canted struts design that smooths out

and shifts the resonance peak towards very high frequencies.

Fig. 12. Comparison of the Heave RAO calculated for the two

equivalent vessels

4.0 LAUNCH AND RECOVERY SYSTEM

The excellent performance in terms of vertical plane motions

in bow waves confirms the USV-SWATH as the best

candidate vessel to perform launching and recovering of

AUVs at sea. In fact, the research activities are intended to

continue with the design studies for the system to lift up and

down the AUV from the midship hangar.

A first idea is to adapt onto the vessel a system like those

implemented in so called “Travel Lift”, commonly used to

lift small/medium size boat and yachts (up to 500 tons) in

many shipyards around the world. The system designed in

the USV-SWATH needs, of course, some adaptation to

compensate for the yet-reduced motions of the surface

vehicles and consists, as per Fig.3 and Fig.14 of two couples

of electric driven self-tensioning winches one forward and

one aft, each one of them controlled on the effective tension

of the hoist and belt assembly. The longitudinal position of

the forward and aft couple of winches can be adapted to the

length of the AUV vessel to be lifted up, as for instance in

Fig. 14, which shows a 2m long AUVs ready to be lifted up.

The lifting operation starts with the lowering of the hoists

and hoist/belt assemblies into the water, at a sufficient depth

to be less sensitive to the wave drift forces (this is relevant

also for the AUV). Only a couple of thin stainless steel

cables of the hoists are piercing the water in this lower

configuration, so the wave drifting effect is reduced to a

minimum. The belts are semi-rigid and ballasted, so they

maintain their shape and position when lowered into water.

The belts contact surfaces are finished with a thick and dense

silicon rubber coating, in order to have the best grip on the

AUV surface.

The AUV is guided in between the two lower hulls of the

SWATH by a sonar beacon positioned in the inside of the

two hulls and when in place and correctly aligned gives the

confirmation signal to start the lifting manoeuver. During the

lift manoeuver hoists are controlled either on the basis of the

tension force acting on the hoists cable or on the acceleration

measured at the winch location. When lifted up the AUV is

secured against a soft stopper by calibrated tensioning of the

hoists.

The operational limits of this type of system will be verified

by means of multi-body seakeeping simulations, possibly

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Pitc

h A

mpl

itud

e [d

eg]

t/T

SWATH

Catamaran

Fig. 13. Pitch angle time histories for the catamaran and the SWATH vessels subject to the same incoming bow wave (/L=2.5)

with the same types of solvers used for the presented study

coupled to a multi-body time domain method for the

dynamic of the hoists cables when they are subject to the

AUV weight and wave loads.

Other types of systems, still based on winches or on rigid

articulated structures are also under evaluation and will be

probably topics of future research.

5.0 CONCLUSIONS

The resistance and seakeeping characteristics and main

design features of a new type of USV-SWATH, having

unconventional underwater hull shape and twin canted struts,

have been described in the paper along with the type of CFD

models specifically devised to optimize and verify the hull

hydrodynamic performance. The innovative hull form

permits to reach relatively high Froude numbers (Fn=0.8)

with a quite limited powering requirement, overcoming the

usual drawback of conventional SWATH hull forms that has

always prevented their use in many types of application on

small crafts. In fact, such an optimum hull shape permits to

decrease the total resistance of more than 25% at high speeds

with respect to a conventional design.

The seakeeping characteristics of the SWATH surface vessel

are, as expected, exceptionally good in comparison with

alternative types of hulls carrying the same payload. The

paper, in fact, presents a comparison of the heave and pitch

RAO calculated with a fully viscous 3D finite volume

solver. Main details of the CFD method and setup of the

numerical model have also been described. The comparison

has evidenced that the pitch motion of the USV-SWATH

reduces down to 1/3 of the value predicted for the

catamaran, while an average reduction of heave amplitude of

about 1/3 over the catamaran, at the largest relative wave

lengths.

These two main advantages make the USV-SWATH the best

candidate to perform autonomous launching and retrieving

of AUVs in open sea. Some idea for an automatic L&R

system based on an adaptation of a well-known and widely

used terrestrial system has been outlined in the paper and

will be better studied in a future extension of this research

effort.

Future studies in fact are expected to continue on the

evaluation of relative motions of the USV and the AUV

when in between the two SWATH lower hulls ready to be

lifted up, the motion damping due to active stabilizers, such

as fins or gyroscopic devices, as well as on the design of an

innovative wireless underwater recharging system from the

USV to the AUVs.

Seakeeping tests at the MIT towing tank are also scheduled

in the incoming year as well as a very possible construction

of the first prototype of the vessel according the patented

design presented in this paper.

AKNOWLEDGEMENTS

This work has been partly funded by ONR grant N00014-11-

1-0598. A special thanks to Kelly Cooper for her important

support of the research on design and optimization of

unconventional SWATH autonomous surface vehicles.

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