subsea structures

8/11/2019 Subsea Structures

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Structural design must be considered early in asubmersible vehicle development program soonafter general mission characteristics have beenoutlined, such as operating depth, speed,

manning, endurance, and payload. Size isobviously the predominant consideration in hulldesign, and is influenced by these variables. Thestructural design of the hull may be considered

in two broad divisions: the pressure hull and theexostructure, the latter commonly called thenonpressure hull or outer hull.


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The design process generally differs for thesebecause of their differing functions anddesign loads, but, of course, there is aninterrelationship, and the design of eachinfluences the other, the pressure hull beingparamount. Various failure criteria includingyielding, instability, fatigue, fracture, creep,and corrosion greatly affect the structuraldesign.


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The single unstiffened sphere frequently hasbeen used for deep operating, smallsubmersibles because of its attractively lowbuoyancy factor. For an ideal sphere with nostiffeners or reinforcements the buoyancyfactor is:

s w = densities of the shell and water,respectively,

p = hydrostatic pressure, and

max = maximum stress in an ideal sphere.

where


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By comparison, the buoyancy factor for anideal cylinder with no stiffeners orreinforcements is:


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Thus, in the ideal case, the single sphere has a33% advantage in structural efficiency. Theweight advantage of the single sphere is evengreater when compared with a cylinder withstiffeners and reinforcements. However, whentotal weights are compared, consideringhydrodynamic form and internal and externalarrangements, the net weight advantage of thesingle-sphere vessel declines. Another advantageof the single sphere is that it allows for a fairlyshort and acceptably light vehicle that can be

handled by a single shipboard crane on a tender.Thus, many of the shallower depth submersiblesare of single-sphere design, with a crew of justtwo or three.


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The stiffened cylinder generally permits superiorhydrodynamic form, better internalarrangements, lighter exostructures, and lowerfabrication costs. It is also less affected by initialgeometric imperfections than shells withcompound radii of curvature. The ring-stiffenedcylinder is used less frequently for deeperoperating depths, because of its less favorablebuoyancy factor. However, there are situations

when material selection and fabricationtechniques can justify the use of ring-stiffenedcylinders down to 6100 m (20 000 ft).


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Penetrations are a major design consideration.There are local stress variations around thepenetrations that affect the design depthlimitations, as well as the cyclic life of thepressure vessel. Small penetrations affect only

local stresses, but large penetrations could affectthe design collapse depth. An efficientpenetration design cannot be achieved simply bycompensating for the shell material removed, asis the case for penetrations in less efficient

pressure vessels. Detailed design and analysistechniques must be used to achieve a balanceddesign (if possible), avoiding both over-reinforcement and under-reinforcement.


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For deep-depth submersibles, major penetrations

are limited to access hatches, electricalconnections, and viewports. Hydraulicpenetrations, stuffing tubes, and shafts normallyare avoided in order to minimize any possibility ofleaking.

Access hatches can be categorized as being eitherof the seat type or plug type. The seat type ofhatch normally is used in relatively shallow depthsubmersibles, since it is more easily fabricatedand costs less; however, it is somewhat heavierthan a plug hatch. The plug hatch generally isused for deep-diving submersibles so as to saveweight. It is used normally in shells of uniform in-plane stresses, such as spheres, since thestiffening ring around the hatch in a sphere is ofuniform thickness.


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The pressure hull, because of its thick scantlings,high-performance material, and tight fabricationtolerances, is relatively costly and constitutes alarge part of the total vehicle weight. Largepressure-resistant structures become especiallyimpractical for deep-diving submarine craft. For

these reasons, the pressure hull size is minimized,sufficient only to accommodate personnel, anymission equipment that must be inside, systemcontrols, and perhaps certain emergency lifesupport equipment. All other components andsubsystems are placed outside the pressure hulland, for the most part within the supportive andprotective confines of an exostructure.


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Maximizing cost-effectiveness essentially

means arriving at one of many solutions to thedesign problem, posed by missionrequirements and design constraints, whichidentifies the mission system best able to

accomplish the mission task(s) at least cost.For a mission system including a mannedsubmersible as one of its systems, one way ofexpressing this statement as an equation is:


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The first term of the equation's right side isthe measure of the system's expeditiousnessin performing given underwater tasks. Thesecond term is the mission system's cost perdive day , which is "bottom line" economicdata for system operations. Optimizationresults from minimizing 1/ CE or maximizingCE itself. Figure 2 shows a breakdown ofconsiderations involved in mission systemcost-effectiveness studies.
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The relationship between mission andperformance requirements is illustrated by twoexamples involving the mission requirements for(1) task-site depth and (2) details of work objects.Task-site depth leads to the performance

requirements for maximum operating depth, D o ,and collapse or design depth, D c. The term D o is atleast equal to the maximum site depth while:

where F is known as the factor of safety whichaccounts for structural design uncertainties.


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The shallow- and deep-water columnsrequire brief explanations at this point; theother environmental segments are easilyvisualized. In this regard, the water column

can be viewed from three perspectives: (1)the effect of surface waves, (2) the generationof wavemaking resistance and (3) thevariability or constancy of environmentparameters, primarily water temperature. In(1), the depth D sw separating the shallow anddeep water columns is approximately:


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where is the significant surface -wavelength. Surface waves are felt above thisdepth, causing motion of the submersible. Inperspective (2), the depth, D WR , separating

these columns is about.

where F is a factor that increases with thespeed of the submersible to a maximumvalue of approximately 3.0 and d m is thevehicle's maximum diameter.


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In the designer's pursuit of these studies, thesubmersible may be decoupled from the restof the mission system if certain assumptionsare made. They are that (1) the characteristicsand operating costs per year of other (I)systems of the mission system are notchanged significantly by variations in size andweight of the submersible design alternativesconsidered and (2) all alternatives arecomposed of reliable systems.


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Then, the submersible's cost-effectivenessequation may be written as:

where, for the assumptions made, it may be

further assumed that the term dive days peryear is a constant, C , for all alternativesdeveloped.


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Dimensions estimated from volume whichpressure hull(s) must enclose. Relationshipsbetween volumes and dimensions for variousshapes are shown in Fig. 8. One expressionfor volume, ( V PH ), is

n = number of persons in pressure hull,

V m = cubic/man data from Fig. 9, and

C = fraction of V PH occupied by items inside pressure hullother than persons. Use similar design data.

where


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Envelope volumes, ( V E )enclosed envelopes volumes may beestimated from the expression:

where V E is the volume of displacements ofpressure hull and all external items within theenvelope plus volume of free flooding waterwithin envelope and C B is the block

coefficient relating principal dimensions andshape which may be estimated from similardesigns.


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Open-frame envelopes volumes may beestimated from the expression:

Envelope composed of individual shapes volume may be estimated from the simpleexpression:

Displacement of submersible A firstestimate of displacement may be obtainedusing the expression:


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where P is the packing factor, derived fromsimilar designs, and is the specific weightof seawater. P reflects the efficiency withwhich the pressure hull(s) and external itemscan be "packed" into an envelope of giventype and shape. 1.0-P is a measure of theamount of free-flooding water filling the voidspaces within the envelope.


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Energy storage capacity: The capacity of themain electrical-energy storage system equalsthe total energy requirement, the area of thepower-energy profile diagram of Fig. 10, plusreserve energy.


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Pneumatic energy storage requirements ofteninvolves these requirements for both highand low pressure systems; high pressuresystems are used in designs with maximumoperating depths down to about 610 m (2000ft). In both systems,

v 1 = total volume of air stored at a working pressure of p 1 in n high-pressure air flasks having a volume of v F per flask; n flasks oftenare called an air bank,

p 1 = working pressure; representative values for high-pressuresystems are 16.54 10 6 Pa (2400 psig) and 20.68 10 6 Pa (3000psig)and for low-pressure systems 9.8 10 6 Pa (1422 psig).

where


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P 2 = pressure at maximum ballast-blowingdepth for high-pressure systems and pressureat or near the surface for low-pressuresystems,V

2 = total volume of seawater ballast which

can be expelled by a fully charged air bankagainst a pressure of p 2 , andR = a factor allowing for a reserve of storage

volume and derived from similar design data.


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The pneumatic energy storage for the highpressure system to obtain v 1 necessitates abreakdown of v 2 . Since these systems areoften used to discharge variable as well asmain ballast, the following expression may beused:

N C = number of cycles of variable ballast per dive,

v VBC = average volume of variable ballast expelled percycle against a pressure of p 2 .

where


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v MB = total volume of main ballast seawater, andK = fraction of total volume of main ballastexpelled against a pressure of p 2 .For high-pressure systems, the value of K depends on design decisions regarding initiationof the normal and emergency ascents from themaximum operating depth. Normal ascent maybe initiated by dropping ascent weights or byinjecting "bubbles" of high-pressure air into themain ballast tanks the bubbles, of course,expanding with decreasing depth. If the bubblescheme is used, with or without dropping ascentweights, K may be 0.20 or less.


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Classification and pplicationsRemotely operated vehicles are generally classified(MTS 1984) into six types and the primary differenceis the means of propulsion. Some typicalcharacteristics and applications are also shown. TheROV is usually near neutral buoyant and thus canhover for inspection or observation purposes usingvideo, sonar, or still camera.

It can also push or pull, lift, and connect or disconnectsystems using only small forces on the order of 444.8N (100 lb) to 889.6 N (200 lb). Vehicle shapes aretypically block shape when forward speeds greaterthan 3.71 kph (2 kts) are not needed, and torpedoshaped when greater speeds are desirable. Theresponse of the vehicle is largely determined by thecomparison of the available thrust to the weight of thevehicle. This thrust to weight ratio defines thequickness of response or how fast the vehicle canaccelerate to its steady state speed.


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An estimate of the speed the vehicle can movethrough the water or the amount of current inwhich the vehicle can hold its position is made byequating the available thrust in a particular

direction to the drag force given by

where is the mass density of water, Cd is thedrag coefficient that is a function of the Reynolds

number and may vary from 0.2 to 2.0, A d is theprojected area normal to the flow direction, andVc is the current velocity or speed of the vehicle.


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The available thrust for a propeller is usuallydetermined from the standard bollard pull tests andthis information is sometimes available frommanufacturers. It can also be estimated from theavailable power by assuming that a thruster developsabout 111.2 N (25 lb) to 133.4 N (30 lb) of thrust perhorsepower (Shatto 1991). Also, the available thrustcan be theoretically determined by computing thechange in momentum of the water that is acceleratedthrough the thruster using

where F T is thruster force, is the mass density ofwater and A T is the cross-sectional area of the

thruster's stream of water being accelerated from zerovelocity to the average velocity (V T) through thethruster.


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The velocity of the water through the thrusteris determined from

where N is the revolutions per minute (RPM)of the propeller, E is the propeller efficiency,and P is the propeller pitch. Thrusters usually

have a large hub, and its cross sectional areais subtracted when calculating A T.


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Thus,

where d p and d h are the propeller and hubdiameters. The thruster intake is usually notin still water because the vehicle is movingthrough the water or holding against acurrent. When the thruster has a fixed pitchand a fixed maximum speed, the thrusterforce (F T) is reduced approximately by theratio of V C/V T and is expressed as


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Calculation of vehicle performance requires

knowledge of the vehicle drag coefficient C d andthe propeller efficiency E. These parameters areusually determined from empirical results. Thedrag force of the tether should also beconsidered, and the drag on a long tether can

exceed that of the vehicle.The vertical thrust developed by the vehicleaffects the response in the vertical direction, andit also determines the amount of weight that canbe lifted or carried by the vehicle. The amount ofweight the vehicle can lift is typically called thedead lift weight. The amount of thrust availablefor vertical acceleration is determined bysubtracting the dead lift weight.


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Vertical thrust is also used to control thevehicle attitude and to adjust the pitch angleof the vehicle when the manipulator lifts ordrops a weight. This is only needed when the

loads to be lifted cause excessive pitch orroll.The ROV usually has a relatively large GB thatmakes them very stable. For a typical workvehicle, it might have a GB of 27.9 cm (11in), a weight and buoyancy of 3500 lb and astiffness of 56 ft-lb/degree at zero pitch androll.


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The angle of pitch when picking up a load can bedetermined by

where H is the horizontal moment arm from theload centerline to the ROV's vertical center ofthrust, W L is in-water weight of load, W a is thein-air weight or buoyancy of the ROV, and BG isthe distance between center of buoyancy andcenter of gravity. For the vehicle described above,the ROV could lift a weight of 100 lb at a radiusof 2.1 m (7 ft) resulting in a pitch angle of 12.6.


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The effect of the location of the center of gravityin floating and submerged vessels is illustrated inFigure 6-4. Lowering the center of gravityincreases the stability by increasing theseparation of the forces of weight and buoyancy.

Lowering the center of gravity can also change aheeling moment to a righting moment.The longitudinal separation of B and G effects thedraft and trim of the vessel. For the submergedbody, the center of buoyancy does not move, andpositive stability requires that G remain below B.An unstable condition occurs when G movesabove B.


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OverviewAll submersible vehicle designs must embody operational

safety considerations. From the selection of the hullstructural material to the publication of an operationalmanual, design process decisions must consider theimpact of all choices on vehicle reliability and particularlyon the safety of personnel who will operate and occupy thesubmersibles.

Safety considerations touch all elements of a submersible'sdesign. A submersible meeting key mission requirementsrelating to speed and diving and surfacing times mustconsider stability while on the surface, while submerged,and while passing through the sea-air interface at varioussea states. A particular mission may demand minimizingtime on station, thus requiring rapid handling of thesubmersible by the mothership. Submersibles which arelaunched from support platforms must be able to belowered onto and lifted from the sea surface safely as wellas expeditiously.


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For certification classification of a submersible,

its design, fabrication, and testing must be incompliance with one of the sets of classificationagency rules, such as Lloyd's Register ofShipping, Det Norske Veritas, or the AmericanBureau of Shipping.

In addition, governmental agencies such as theU.S. Coast Guard have rules which govern theoperation of equipment the requirements forinstalled equipment, and the qualifications foroperating personnel. Submersible characteristicsrelating to safety imposed by classificationagencies and a detailed discussion of the U.S.Navy's role in establishing submersible safetyprograms.


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Also, the diver-occupied spaces must haveair purification and heating facilities toensure divers' safety and well-being.The entire design must have a common goal

of safety of personnel and equipment. Withthe constraint of size, weight, and costs, thechallenge is great for the designer to create asubmersible meeting the stated mission

requirements and at the same time ensurethat the end product will be a safe,operational vehicle.


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