Applied Ocean Research 46 (2014) 131143
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Applied Ocean Research
journal homepage: www.elsevier.com/locate/apor
eliability centered modeling for development of deep water Humanccupied Vehicles
. Vedachalam , G.A. Ramadass, M.A. Atmanandational Institute of Ocean Technology, VelacheryTambaram Main Road, Pallikaranai, Chennai 600100, India
r t i c l e i n f o
rticle history:eceived 3 January 2014eceived in revised form 24 February 2014ccepted 3 March 2014vailable online 3 April 2014
a b s t r a c t
Human Occupied Vehicle operations are required for deep water activities such as high resolutionbathymetry, biological and geological surveys, search activities, salvage operations and engineering sup-port for underwater operations. As this involves direct human presence, the system has to be extremelyreliable. Based on applicable standards, reliability analysis is done on 5 key representative functions withthe assumption that the submersible is utilized for ten deep water missions per year. Analysis is done onthe results obtained to find the influence of the subsystems on the reliability of the overall submersible.Analysis include, influence of battery technologies and reliability centered battery and hydraulic systemconfigurations. Dependence of seal sizes and seal seat surface finish on the leak tight integrity of the per-
uman Occupied Vehicleersonnel sphererobability of failureeliability
sonnel sphere is also discussed. It is found that for submersible housing 75 kWh energy storage batteries,the probability of failure of the hard tank buoyancy ascent function with lead acid batteries configuredfor 300 V terminal voltages and non-redundant hydraulic configuration is 37.74%. The probability of fail-ure can be reduced to 5.24% with lead acid batteries with terminal voltage configured to 120 V and withredundant hydraulic configuration. The results presented shall serve as a model for designers to arriveat the required trade-off between the capital expenditure and the required reliability.
Human Occupied Vehicles (HOVs) are useful tools for theesearches investigating deep sea life and for exploring oceanesources. Even though unmanned vehicles have improved maneu-ering capabilities and excellent vision systems which resembleirect observation, HOVs provide a feel of direct physical presenceor the researches. The successful operation of the first generationOV, Trieste [1,2] at a water depth of 10,906 m in the Marianarench triggered initiatives for the development of more efficientOVs. Further technical developments have greatly expanded theperating range and improved the operational efficiency of theOVs used in scientific research. The second generation HOV which
s centered on the development of a lighter hull for the crew,mprovement of the power supply for propulsion, and establish-
ent of reliable systems, include Alvin  of USA, Nautile6000 ofrance, Shinkai6500 of Japan , MIR 6000 submersibles of Russiand Jiaolong 7000 of China, developed during the period since 1964
With the experience gained in the development and successfulualification of deep water unmanned Remotely Operable Vehicle
Corresponding author. Tel.: +91 9884869101.E-mail address: firstname.lastname@example.org (N. Vedachalam).
141-1187/$ see front matter 2014 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.apor.2014.03.001
2014 Elsevier Ltd. All rights reserved.
ROSUB 6000 , the National Institute of Ocean Technology withthe objective of augmenting Indias capability in deep sea research,is planning to develop a HOV capable of operating in deep watersand used for carrying out scientific exploratory activities [8,10].
As HOVs are not electrically powered from the surface, theyrequire self-contained power supply with high energy storage which increases the weight and volume of the submersible, whichin turn limits its operational endurance. When a human is in thesystem he/she must be protected from the hostile deep waterenvironment, and hence, the reliability of the system is of utmostimportance . This calls for man-rating certifications, and is doneby bodies including the American Bureau of Shipping (ABS) ,Germanischer Lloyd , Det Norske Veritas (DNV)  and oper-ational guidances by International Marine Contractors Association(IMCA) .
This paper reviews the recent technological developments in thesystems required for reliable operation of deep water HOV. Basedon the applicable DNV guidelines RP-203 for qualification proce-dures for new technology  reliability analysis are made on thefive identified key representative HOV functions, with the assump-tion that the HOV is utilized for ten deep water dives per year, each
deep water mission clocking 4 h of subsea operation. Studies werecarried on the results obtained to find the subsystems influenceon the overall submersible reliability and it is found that energystorage batteries and hydraulic systems are major contributors in
132 N. Vedachalam et al. / Applied Ocean Research 46 (2014) 131143
Table 1Standards followed for FIT estimation of subsystem components.
Batteries, Power electronicconverters, Transformers,Isolators, Motors, Pumps, Lamps,Cameras
MIL and IEEE
CPU, DC-DC Converters, Fuses,Connectors, Ethernet Converters,Input and Output modules forData acquisition cards
Subsea sensors and transmitters,Solenoid Valves, O-rings, Gaskets
OREDA, NSWC, ABS
Electronic Pressure cases, ABS, DNV, Germanischer Llyods
comprising of spheres, pumps and compressed air bottles fordescend, traverse, and ascend operations, navigation systems forposition determination and safe vehicle maneuvering, Ship-HOV
Table 2Failure-in-time data used for configuration.
Component FIT (in 109 h)
BatteriesLead acid battery 12 V, 100 Ah 3140Lithium ion battery 12 V, 3.8 Ah 237
Control and network electronicsReal time controller 41Analog input/Output module 12Ethernet switch 70Industrial Computer 527
Vision support systemsCamera 355Light 350
Navigational sensorsPhotonic Inertial Navigation Sensor (INS)with Micro-Electro-Mechanical Systems(MEMS) based INS in redundancy
Doppler Velocity Log (DVL) 300Power conditional systems
DCDC converter 75DCAC converter 245
Propulsion systemsThruster motor electronic controller 477Thruster Brush-Less Direct Current (BLDC)motor of 8 kW capacity
Hydraulic systemsPump 5400Pressure sensor 60Control valves 341
Fig. 1. Reliability bath tub curve indicating the life cycle .
eciding the HOV operational reliability. A detailed analysis on thenfluence of battery chemistry, battery architecture, and hydraulicrchitecture and personnel sphere seals is presented.
. Major standards followed for reliability modeling
The following is the major list of standards followed:
1) FIDES Guide for the estimation of Reliability  for electroniccomponents and systems, considering mission and environ-ment specific analysis.
2) MIL HDBK 217F, Military handbook for Reliability Estimation of Electronics Equipment.
3) OREDA Handbook  for Offshore Reliability Data.4) IEEE 493 IEEE Recommended practice  for Design of reliable
Industrial and Commercial Power Systems.5) DNV RP-A-203-Qualification procedures for new technology
.6) Handbook for reliability prediction procedures for mechanical
equipment by Naval Surface Warfare Center (NSWC) .
The failure rate determination for the system components wasone by the following methods:
(a) Based on the manufacturers data and interpretation accordingto the mission profile.
b) For systems where Failure-in-Time (FIT) data are not avail-able, failure rates are calculated from component failure datafrom the respective standards (FIDES, IEEE, and MIL), takinginto consideration the mission profile, operating conditions andoperating stresses.
FIDES  approach is based on physics and failures, sup-orted by the analysis of test data, feedback from operations andxisting models along with statistical interpretations over theormal operating life period of the system and is indicated inig. 1.
The provision in the FIDES standard considering the influence ofhe operating temperature, amplitude and frequency of the tem-erature changes, vibration amplitude, humidity and operatingtresses based on Arrhenius, Norris and Basquin laws  waspplied for calculations. The provision given for accounting man-facturing and integrating quality factors are also applied. Thetandards also provide Commercially-Off-The-Shelf (COTS) boardspproach for calculating the failure rates of the systems for theefined mission profile with the functional requirements and mis-ion profile as inputs from the user. The failure rate determinationor the components is done at the circuit component level, and is
ncorporated in the reliability trees . Table 1 gives the standardsollowed for the major systems and components.
Table 2 shows the FIT values of major systems considered foromputations. The values are obtained from indicated standards
Personnel Sphere, Entry Hatches,View Ports
and the values arere-computed based on the actual mission profileand operating conditions adopted.
The TOTAL-SATODEV GRIF tool  is used for realizing failuretrees.
3. Overview of HOV systems
Fig. 2 shows the overview of typical deep water HOV . Themajor systems of the HOV include the pressure rated personnelsphere (PS) for human occupancy, equipped with hatch and viewports. They house the systems required for the control of the HOV,communication aids and life safety systems required for the crew.Electro-optical penetrators enable electro optical communicationfrom the external systems. The propulsion system comprises ofbatteries and thrusters to enable the vehicle maneuverability inmultiple degrees of freedom, variable buoyancy enabling systems
OthersShape memory alloy element 92Subsea cables 244Subsea connectors 20
N. Vedachalam et al. / Applied Ocean Research 46 (2014) 131143 133
of a h
Fig. 2. View of the major systems
ommunication systems, Emergency Surfacing Systems andntanglement release systems. The details of the subsystemsonsidered for analysis are shown in Table 3.
The control architecture considered for analysis is shown inig. 3 where the real time controller is located inside the pressureated enclosure communicating with the computers inside the per-
onnel sphere with hardware supporting Ethernet communicationrotocol.
able 3etails on the considered typical HOV subsystems.
Propulsion systemPropulsion batteries Pressure-compensated lead acid type,
75 kWh energy capacity, 300 V terminalvoltage, 4 pods of each 26 kWh, and eachpod with 25 numbers of 12 V 100 Ahbatteries
Auxiliary batteries 24 V15 Ah Communication battery,24 V50 Ah emergency batteries
Energy budget 54 kWh for a 4 h subsea mission, 3.5 kW forascent buoyancy engine, 8 kW for PS loadsand control system, 10 kW for contingency
Thrusters/pumps 300 V 8 kW BLDC motor operatedControl systems Real time controllers with Ethernet based
network connectivity with personnelsphere, multi-application industrial gradecomputersand control system componentsdistributed inside exostructure mountedpressure rated enclosures and personnelsphere.
Buoyancy tanks Pressure rated tanks with sea water pumpin/out using pumps of 5 kW capacity. Tankinternal pressure feedback system
Manipulators Propulsion battery powered andhydraulically operated pumps and valves
Navigation sensors DVL guided fiber optic and MEMS basedgyroscope for navigation guidance.Redundant camera and lights for visualnavigation
Personnel sphere seals Pressure rated redundant gaskets for hatch0.5 m diameter and three view ports of0.25 m diameter
uman occupied submersible .
4. Functions considered for reliability analysis
Based on the recommendations of the DNV RP-A-203 standardsand procedures, Failure Mode Effective Criticality Analysis (FMECA)studies  are carried out on HOV operations, and it is identifiedthat the following key representative operations are studied fromreliability point of view.
(1) Thruster enabled visual survey operations with navigationalaid.
(2) Thruster enabled ascent operation.(3) Drop weight enabled ascent operation.(4) Buoyancy enabled ascent operation.(5) Manipulator operations.
4.1. Function 1: thruster enabled visual survey with navigationalaid
This function involves subsea maneuvering of the HOV withvisual and navigational aids. The propulsion function is realizedusing top and lateral thrusters when the HOV is in a near neutrallybuoyant condition  with the operating power provided by the on-board propulsion batteries. Visual support function constitutes a setof cameras and lamps located on the HOV exostructure. The posi-tion information required for the HOV is provided by an InternalNavigation Sensor (INS)  aided by Doppler Velocity Log (DVL)and acoustic based Long Base Line (LBL) systems [34,35]. The failurecontributions from the major subsystems toward this function areas tabulated below in Table 4.
Fig. 4 indicates the probability of a 4 h mission failure when 25%battery bank capacity fails during the start of the deep sea opera-tions. Fig. 5 shows portion of the tree forming part of propulsionsystem batteries in Fig. 4. It indicates that the major failure con-tribution (93.36%) is from the battery. As per the energy budgetdesign, four pods are required for a successful 4 h subsea mis-sion. Fig. 5 shows a condition when 1 of 4 pods fail during the
start (T0) of a 4 h deep sea mission. Present day battery banksare equipped with battery management systems  which pro-vides in situ battery information such as state of charge and stateof health which helps in advancing the decisions for surfacing
134 N. Vedachalam et al. / Applied Ocean Research 46 (2014) 131143
Fig. 3. Control architecture consid
hereby reducing the probability of failure during the rest of theission.
.2. Function 2: thruster enabled ascent
Vehicle stability, buoyancy, depth keeping and emergencyurfacing requirements of deep water HOV are defined by thetandards from classification societies . Present day ascentystems [2,3,31] fall under one or a combination of systems involv-ng soft tank variable buoyancy, hard tank buoyancy or usinghrusters. Alvin and Shinkai6500 use buoyancy engine or thrustersr a combination of both for an ascent a...