DNVGL-RU-UWT-Pt3Ch7 Stability and buoyancy

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DNV GL AS

RULES FOR CLASSIFICATION

Underwater technologyEdition December 2015

Part 3 Pressure hull and structures

Chapter 7 Stability and buoyancy

FOREWORD

DNV GL rules for classification contain procedural and technical requirements related to obtainingand retaining a class certificate. The rules represent all requirements adopted by the Society asbasis for classification.

© DNV GL AS December 2015

Any comments may be sent by e-mail to [email protected]

If any person suffers loss or damage which is proved to have been caused by any negligent act or omission of DNV GL, then DNV GL shallpay compensation to such person for his proved direct loss or damage. However, the compensation shall not exceed an amount equal to tentimes the fee charged for the service in question, provided that the maximum compensation shall never exceed USD 2 million.

In this provision "DNV GL" shall mean DNV GL AS, its direct and indirect owners as well as all its affiliates, subsidiaries, directors, officers,employees, agents and any other acting on behalf of DNV GL.

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Rules for classification: Underwater technology — DNVGL-RU-UWT-Pt3Ch7. Edition December 2015 Page 3Stability and buoyancy

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CURRENT – CHANGES

This is a new document.

The rules enter into force 1 July 2016.

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CONTENTS

Current – changes...................................................................................................... 3

Section 1 General....................................................................................................... 51 Classification...........................................................................................52 Documents for approval......................................................................... 53 Definitions.............................................................................................. 5

Section 2 Intact stability............................................................................................61 Buoyancy in surfaced condition.............................................................. 62 Load cases for stability.......................................................................... 63 Assumptions for the calculation............................................................. 94 Righting levers..................................................................................... 105 Heeling levers.......................................................................................106 Criteria for intact stability.................................................................... 11

Section 3 Stability in damaged condition................................................................. 161 Degree of damage................................................................................ 162 Surfaced submersible........................................................................... 163 Submerged submersible....................................................................... 16

Section 4 Diving, trimming and heeling tests...........................................................171 General................................................................................................. 172 Diving test............................................................................................ 173 Heeling test submerged........................................................................174 Heeling test surfaced............................................................................185 Trimming test....................................................................................... 18

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SECTION 1 GENERAL

1 Classification

1.1 Submersibles will be assigned Class only after it has been demonstrated that their buoyancy and their staticand dynamic stability in intact condition is adequate for the service intended. The level of intact stability forsubmersibles shall generally meet the standard defined in the following.

2 Documents for approvalThe documents to be submitted are summarized in Pt.5 Ch.6 Sec.2 [5].

3 DefinitionsObject

WatertightA component / piece of the construction is considered watertight if it is capable of preventingthe ingress of water at the hydrostatic head for which the component / piece and itssurroundings has been designed for.

Weathertight Weathertight in relation to an opening and its cover means, that water will not penetrate intothe surfaced submersible in any thinkable seaway condition.

Centre of buoyancy

The centre of buoyancy has the designation B↓ for the submerged submersible and is related tothe submerged displacement Δ↓.The centre of buoyancy with the designation B↑ for the surfaced submarine is related to thesurfaced displacement Δ↑.

Centre of gravity The centre of gravity for the relevant load condition and has the designation G.

Metacentre

The metacentre with the designation M is the intersection between the resultant force line ofthe centre of buoyancy and the center line valid for heeling angles 0° <φ ≤ 5°. The center ofbuoyancy is given by the center of gravity of the displaced water. For bigger heeling angles φthe metacentre shall be evaluated adequately.

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SECTION 2 INTACT STABILITY

1 Buoyancy in surfaced condition

1.1 Depending on the type of submersible and the operation area the distance between the waterline in fullysurfaced condition and the upper edge of entrance openings, air pipes, etc. which may be open for surfacedoperation, shall be approved by the Society.If there are bulwarks/hatchway coamings which are open at the upper side, adequate bilge systems shall beprovided.

1.2 All submersibles shall have a sufficient reserve of buoyancy in surfaced condition to meet the stabilityrequirements of this section. This buoyancy reserve shall be proven by calculation.

1.3 The buoyancy reserve shall be at least 10% of the pressure tight volume (overall displacement) of thesubmersible.

1.4 For open diving tanks it shall be proven, for all intended seaway conditions, that sufficient buoyancy isavailable at the heeling and trimming conditions to be expected.

1.5 Reserve buoyancy can be created with diving cells and hard foam elements. The foam elements shall besafely protected against external damage. Their pressure resistance, water absorption and resistance againstultraviolet radiation shall be according to requirements of Pt.2 Ch.5 Sec.7. Heel compensation with hard foamelements shall be avoided.

Guidance note:For calculation and checking of the weight and moment balance of the submersible, loading instructions are recommended to beavailable to the operator.

---e-n-d---of---g-u-i-d-a-n-c-e---n-o-t-e---

1.6 In case of submersibles with floodable compartments, e.g. transfercompartments of lock-in lock-outsubmersibles, the buoyancy behaviour of the partly flooded submersible shall be investigated. The stabilitydocumentation shall state in which flooding condition buoyancy is still sufficient.

2 Load cases for stability

2.1 GeneralIn general the stability load cases in [2.2] – [2.11] shall be investigated. If these load cases shall not beapplied for the actual case, other or additional load cases shall be agreed with the Society.

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All load cases shall be investigated with the lowest as well as the highest defined water densities, comparePt.3 Ch.1 Sec.4 [2.1].The submersible shall be assumed free of horizontal inclination (even keel) in submerged condition.Tanks with great breadth resp. great length shall be protected against the influence of free liquid surfaces byseparating/ baffle plates.A summary of the different load cases is given in Table 1.

2.2 Load case 1a: Surfaced, start of the journey with 100% stocks, nopayload NLThe submersible is fully equipped and manned, diving tanks are empty, trimming tanks 50% full,compensating tanks empty, stocks and fuel tanks (if existing) 100% full, any working gears drawn out, nopayload NL (means no passengers and/or no picked up materials, additional equipment, etc.)

2.3 Load case 1b: Surfaced, end of the journey with 10% stocks, nopayload NLThe submersible is fully equipped and manned, diving tanks are empty, trimming tanks 50% full,compensating tanks empty, stocks and fuel tanks (if existing) 10% full, compressed air 50% and oxygen10% full, any working gears drawn out, no payload NL. For submersibles with sole electro drive and relativelylittle quantities of compressed air and oxygen it shall be investigated, if this load case is essentially differentfrom load case 1a and shall be checked separately.

2.4 Load case 1c: Surfaced, end of the journey with 10% stocks, nopayload NL, release of solid ballastThe submersible is fully equipped and manned, diving tanks are empty, trimming tanks empty, compensatingtanks empty, stocks and fuel tanks (if existing) 10% full, compressed air 50% and oxygen 10% full, anyworking gears drawn out, no payload NL. The solid ballast is released as aid for surfacing.

2.5 Load case 2a: Surfaced, start of the journey with 100% stocks, withpayload NLThe submersible is fully equipped and manned, diving tanks are empty, trimming tanks are 50 % full,compensating tanks empty, stocks and fuel tanks (if existing) 100% full, any working gears drawn out, withpayload NL (means with passengers and/or picked up materials, additional equipment, etc.)

2.6 Load case 2b: Surfaced, end of the journey with 10% stocks, withpayload NLThe submersible is fully equipped and manned, diving tanks are empty, trimming tanks 50% full,compensating tanks empty, stocks and fuel tanks (if existing) 10% full, compressed air 50% and oxygen10% full, any working gears drawn out, with payload NL. For submersibles with sole electro drive andrelatively little quantities of compressed air and oxygen it shall be investigated, if this load case is essentiallydifferent from load case 2a and shall be checked separately.

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2.7 Load case 2c: Surfaced, end of the journey with 10% stocks, withpayload NL, release of solid ballastThe submersible is fully equipped and manned, diving tanks and trimming tanks are empty, compensatingtanks empty, stocks and fuel tanks (if existing) 10% full, compressed air 50% and oxygen 10% full, anyworking gears drawn out, with payload NL. The solid ballast is released as aid for surfacing.

2.8 Load case 3a: Submerged, start of the journey with 100% stocks, nopayload NLThe submersible is fully equipped and manned, diving tanks are flooded, trimming tanks 50% full,compensating tanks 20% full, stocks and fuel tanks (if existing) 100% full, any working gears drawn in, nopayload NL.

2.9 Load case 3b: Submerged, end of journey with 10% stocks, no payloadNLThe submersible is fully equipped and manned, diving tanks are flooded, trimming tanks 50% full,compensating tanks 80% full, stocks and fuel tanks (if existing) 10% full, compressed air 50% and oxygen10% full, any working gears drawn in, no payload NL.

2.10 Load case 4a: Submerged, start of the journey with 100% stocks,with payload NLThe submersible is fully equipped and manned, diving tanks are flooded, trimming tanks 50% full,compensating tanks 10% full, stocks and fuel tanks (if existing) 100% full, any working gears drawn in, withpayload NL.

2.11 Load case 4b: Submerged, end of journey with 10% stocks, withpayload NLThe submersible is fully equipped and manned, diving tanks are flooded, trimming tanks 50% full,compensating tanks 70% full, stocks and fuel tanks (if existing) 10% full, compressed air 50% full, anyworking gears drawn in, with payload NL.

2.12 Further load casesIf required, further load cases shall be investigated in agreement between operator of the submersible andthe Society.

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Table 1 Summary of stability cases

3 Assumptions for the calculationFor unification and comparability between different projects the following assumptions shall be observed forthe calculations submitted to the Society:

— The displacement shall be computed in metric tons (1000 kg).— The range of the salt content for which the submersible shall be used shall be defined clearly.— The weight of the crew respectively passengers is normally assumed with 75 kg/person, depending on

area of operation and the duty of operation this assumption may be too low and shall be agreed with theowner/ operator.

— The density of all liquids used in tanks and bunkers shall be agreed with the owner/ operator.

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4 Righting levers

4.1 DefinitionA righting lever h is defined as follows:

h =

Δ↑ = surfacedΔ↓ = submergedB = centre of buoyancyG = centre of gravity

4.2 Surfaced submersibleThe righting levers hSW of the surfaced submersible in still water shall be evaluated for the load cases definedin [2] and for the following conditions:The levers in still water shall be evaluated for the heeling angles φ = 10°, 20°, 30°, 45°, 60° and 75° andshall be presented as lever arm curves. For operation in a seaway see [6.5].

4.3 Submerged submersibleThe righting levers of the submerged submersible are following from the fact that the centre of gravity shallbe situated below the centre of buoyancy:

hSub = · sin φ

5 Heeling levers

5.1 DefinitionA heeling lever k is defined as follows:

k =

The heeling levers shall be determined for heeling angles φ = 10°, 20°, 30°, 45°, 60° and 75°.

5.2 Surfaced submersibleThe following heeling influences shall be considered:

5.2.1 Free liquid surfacesIn partially filled tanks and bunkers the liquids have free surfaces which contribute to the heeling moment bya heeling lever kF as follows:

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pi = mass of liquids in tank/bunker i with free liquid surface [t]bφi = change of the centre of gravity in relation to the upright submersible, measured parallel to the

design water line [m]

5.2.2 Turning circleAs far as necessary the motion in the turning circle with heeling lever kD shall be considered. See also Pt.3Ch.2 Sec.2 [2.6].

5.2.3 WindThe calculation of the wind forces at the elements above the waterline shall be done according to Pt.3 Ch.2Sec.2 [2.1] This results in a heeling lever kW.According to the area of operation the values for the wind velocity to be used shall be agreed with theSociety.

5.2.4 Ice loadsIf the planned area of operation of the submersible requires it, the calculation of the ice loads at theelements above the waterline shall be done according to Pt.3 Ch.2 Sec.2 [2.4]. This results in heeling leverkE.

5.2.5 Load-handling loadsDuring the transfer of equipment, provisions and persons from land or the support ship to one side of thesubmersible a heeling lever kP may be considered.

5.2.6 Tow-rope pullDuring towing of the submersible a heeling lever kT may be created by the tow-rope pull.

5.2.7 Payload NLIf the payload respectively the working devices are able to change their position in transverse direction, aheeling lever kN shall be considered.

5.2.8 Supply lines/umbilicalsFor non-autonomous submersibles transverse pull forces may occur – depending on the arrangement -because of the supply lines from the support vessel. This shall be considered as heeling lever kV.

5.2.9 Jettisoning of ballastIf the ballast is released as surfacing aid, the centre of gravity is shifted upwards, thus reducing the distanceGM.

5.3 Submerged submersibleFor the submerged submersible the following influences which lead to heeling levers shall be considered:

— free liquid surfaces in tanks and bunkers, compare [5.2.1] with heeling lever kF. Δ↓ shall be insertedinstead of Δ↑.

— motions of the submersible in the three dimensional space with heeling lever kD , compare [5.2.2] andPt.3 Ch.2 Sec.2 [2]

— influence of payload with heeling lever kN, compare [5.2.7]— tension forces from the supply line for non-autonomous levers with heeling lever kV, compare [5.2.8]— jettisoning of ballast

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6 Criteria for intact stability

6.1 Summary of influencesThe righting and the heeling levers are summarized in Table 2.

6.2 Proof of stability6.2.1 Lever arm curvesThe proof of sufficient stability shall be done by comparison of the curves of the righting levers h and theheeling levers k. Combination of heeling moments according to Table 2 shall be considered. Decisive for theevaluation is the size of the remaining righting lever and of the static angle of heel. As remaining lever hremthe remaining lever above the curve of the heeling levers is designated, see Figure 1.

6.2.2 Minimum values of stabilityBetween the angle of the static balance φstat and the angle of immersion of the first unprotected opening φrefor the angle φ’ of the second intersection of the curves of the heeling and righting levers or the intersectionof the righting lever with the abscissa, whereas the smallest angle shall be considered, positive remaininglevers hrem shall exist. Generally the remaining lever hrem should be at least 0.05 m in submerged conditionand at least 0.1 m in surfaced condition. in an actual case this value shall be agreed with the Society.

Decisive for the proof of initial stability is the size of the value (= vertical distance between centre of

gravity and metacentre) for the surfaced submersible respectively the distance (= vertical distance

between centre of buoyancy and centre of gravity) for the submerged submersible. The minimum values aredefined in Table 4.

Figure 1 Lever arm curves

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Table 2 Summary of the righting and heeling lever arms

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Table 3 Summary of the righting and heeling lever arms (continued)

Table 4 Minimum values for sufficient stability

Operating conditions Stability criteria [m] Distance [m]

Surfaced Distance 0.101

Submerged Distance 0.051

1) This value shall be agreed with the Society for an actual case

6.3 DraughtThe draught of the surfaced submersible is the permissible draught which is possible because of buoyancyand stability. In general it will be achieved by load case 2a.The draught shall be clearly marked in an unchangeable way on both sides of the submersible admidships asusual for vehicles classified by the Society.

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6.4 Trimming diagramThe trimming diagram serves as graphic presentation of the range of mass and trimming moments inlongitudinal direction of the submersible, which can be reached by the control of the water in compensatingand trimming tanks as well as in special spaces (e.g. diver’s lock-out).These possibilities shall be shown in the trimming diagram in form of a polygon line. It shall be proventhat the displacement due to additional loads (e.g. because of taking up of payloads) can be compensatedby changing the filling of the compensating tanks as well as the trim by filling the trimming tanks. If thepoints representing the different load cases are lying within the polygon line, compensation is possible withoperational measures on board.

6.5 Dynamic intact stability

6.5.1 This possible stability case for the surfaced submersible in a seaway shall be investigated in addition to[4.2] if longer journeys in a certain sea area with predominated wind direction and certain required coursesof the submersible are planned, compare CONOPS according to Pt.3 Ch.1 Sec.1 [4].In this case it is advisable to perform such investigations already in the design stage:

— for the submersible in the seaway in wave crest condition: hC— for the submersible in the seaway in wave trough condition: hT— for the submersible in the seaway, average value of wave crest and wave trough conditions: hWV.

In the seaway especially the length of the submersible in relation to the critical wave lengths and heights inthe operation area shall be considered. Thus a reduction of the above conditions may occur.

6.5.2 For independent submersibles the dynamic stability shall be investigated in any case.The Society is able to offer for such a case special advice and relevant computation procedures.

6.6 Special construction typesFor special types of construction of submersibles the criteria defined above may not be directly applicable. Inthis case special agreements shall be made with the Society.

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SECTION 3 STABILITY IN DAMAGED CONDITION

1 Degree of damageAccording to Sec.1 [1.2] for the stability in damaged condition will be assumed that:

— the pressure hull is undamaged— the exostructure is distorted— one important tank (e.g. a diving tank) within the exostructure fails because of damage— an emergency operation of the submersible is possible without endangering the crew and passengers.

2 Surfaced submersible

2.1 Righting leversBuoyancy and righting levers may be reduced in relation to Sec.2 [4.2] by the changed centre of gravity ofthe exostructure and the loss of buoyancy of the damaged tank.

2.2 Heeling leversThe lever kF for free surfaces according to Sec.2 [5.2.1] shall be checked. Concerning the lever kW for thewind load according to Sec.2 [5.2.3] it has especially to be proven as far as the area of wind pressure and itscentre of gravity are changed because of the new floating conditions.

2.3 Criteria for buoyancy and stabilityThe following assumptions shall be met after damage:

— Openings to the pressure hull shall have sufficient freeboard to avoid ingress of water into the pressurehull with hatches open.

— The heel of the submersible shall not exceed φ = 22.5°. At the same time the trim forward and sternwardshall remain below 10°.

— A positive remaining lever hrem shall be guaranteed. Its value shall be agreed with the Society accordingto the type of construction.

3 Submerged submersible

3.1 It shall be checked how the centre of buoyancy is changed and if therefore more critical conditions arise.

3.2 Criteria for diving capability and stabilityThe following assumptions shall be met also after damage:

— The submersible has still to be able to surface in a safe way.— The centre of weight has still to be below the centre of buoyancy.

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SECTION 4 DIVING, TRIMMING AND HEELING TESTS

1 General

1.1 Practical tests with the fully equipped submersible intended for normal operation shall be performed for anewbuilding or after essential conversions.

1.2 If an identical series of submersibles is built on a yard, the following tests shall be performed for the firstsubmersible only. If identical submersibles are built in different yards, the tests shall be performed with thefirst submersible of each yard.

1.3 The tests shall be performed in presence of a DNV GL surveyor.

1.4 The tests defined in [2] to [5] shall be performed in the given sequence.

2 Diving testThe following steps shall be performed:

— An adequate depth of the water shall be chosen.— The density of the water shall be evaluated.— The diving tanks shall be filled completely with water, respectively completely deflated (in case of

compressible tanks). Entrapped air shall be avoided.— The weight distribution of the submersible in the hovering condition shall be evaluated by adequate filling

of the compensating tank(s).— By adequate distribution of the water between the trimming tanks the submersible shall be neutrally

trimmed and the required filling of the compensating and trimming tanks shall be evaluated.

3 Heeling test submergedThe heeling test with the submerged submersible serves to evaluate the centre of gravity under water asbasis for the stability considerations described in Sec.2 [2].The condition of the submersible is according to the diving test in [2].For the preparation of the test the following shall be considered:

— The influence of free surfaces in tanks, pipes, etc. shall be kept to a minimum.— Starting angles of heel of more than 1° shall be avoided using weights/ballast.

The following steps shall be performed:

— The heeling/trimming of the submersible shall be started by a displacement of delivered test weights tothe sides resp. forward and astern.

— These weights shall be chosen in such a way, that a heeling of 1.5 to 3.0 degrees to each direction occurs.— The heeling angle shall be measured with two suitable devices, one of which shall be at least a damped

pendulum with a reticule plate.

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— The test shall be repeated at least 2 times to each side resp. forward and astern with different heelingmoments, the average of the measurements shall be determined.

— At the end of the test the starting condition of the loads shall be reestablished and the floating conditionshall be checked for conformity with the starting position.

— The displacement shall be constant during the test.

4 Heeling test surfacedThe heeling test with the surfaced submersible with 100% empty diving tanks serves to evaluate the centreof gravity surfaced as basis for the stability considerations described in Sec.2 [2].For the preparation of the test the following shall be considered:

— The test shall be performed in calm water with only slight wind.— The draught forward, midships and astern shall be evaluated at port and starboard.— All tanks shall be completely empty to avoid for certain the influence of free surfaces, the valves of the

piping systems shall be closed.— Starting angles of heel resp. starting trimming angles of more than 1° shall be prevented using additional

ballast.— The displacement shall be constant during the test.

The same steps shall be performed as for the heeling test submerged [3].

5 Trimming testFor the fully surfaced submersible with 100% empty diving tanks it shall be investigated by variation of thefilling of the forward and aft trimming tanks if a floating condition on even keel can be reached and whichtrimming conditions can be reached forward and aft.For the evaluation of the trimming status suitable measuring devices shall be used or the draught marksforward and astern shall be read.

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