CA1090200A

CA1090200A - Launching and recovering submersible craft

Info

Publication number: CA1090200A
Application number: CA287,917A
Authority: CA
Inventors: Chengi Kuo
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-10-01
Filing date: 1977-09-29
Publication date: 1980-11-25
Also published as: FR2366167A1; NO144600B; DE2744100A1; NO144600C; NL7710702A; NO773354L; GB1597477A

Abstract

A B S T R A C T

A method and apparatus for launching and recovering submersible craft or equipment through a moonpool in which an airtight or nearly airtight closure is placed over the moonpool so that surging in the moonpool is avoided or significantly reduced.

Description

)20~

This inven~ion relatesto a method of launching and recovering diving bells, diving chambers and submersibles, hereinafter referred to as "submersible craft", or underwater equlpment ~hereinafter referred to as "equipment") through a moonpool of a ship, semi-submersible or other floatin~
vessel (herelnafter referred to as a "vessel") or from a ri.g, platfor~, pie~ or other fixed structure~ hereinafter referred to as a "fixed structure" and to a vessel or fixed structure for launching and recovering such submersible craft or equipment.
Hereinafter and in the claims 3 the term "sea" should be interpreted as including areas of sal.t water and fresh water.
It is known to launch and recover submersible craft - and equipment in an area of sea surface that is located within a vessel or structure and communicates with the sea surroundi~g the vessel through an opening which lies entirely be].ow sea level. Such a structure is called a moonpool and usuall-y consists of.an upright shaft which extends up into tre vessel and is open at its lower end. The submersible craf~ and equipment are launched through the surface of the sea in the upri~ht shaft and can pass out through the opening at; the lower end of the shaft into the surrounding sea. In the usual arrangement the ~pper end of the shaft is open to the atMosphere.
The surface of the sea within the moonpool surges that is it rises and falls, by an amount dependellt on the state of the sea, and especially on the wave frequency. During a swell OZ~

of a particular frequency which does not necessarily correspond to the heaviest sea states possible, the surge amplitude within the moonpool may be considerably greaker than the wave amplitude in the seaways. This may result in danger to life or damage to or loss of craft or equipment and thus prohibits launching or recovery operations.
The object of the present invention is a method of launching or recovering submersible craft or equipment through a moonpool in which the problems of surging in the moonpool are reduced or avoided, and to provide a vessel or fixed-structure with a moonpool adapted to reduce or avoid the problems of surging.
Accordingly, the present invention in one aspect coinprises a method of launching and recovering submersible cr~ft or equipment through a moonpool of a vessel or fixed structure comprising the provision of an enclosed space over and bounded by the surface of the sea within the moonpool and launching said submersible craft or equipment from wi1;hin said space or recovering said submersible craft from the sea into said space, the enclosed space being airtight or nearly airtight so that surging in the moonpool is avoided or significantly reduced compared with a similar t~6 ~mO~6R4 ~rrangement in which ~he top of the moonpool is open?.
By this method an enclosed column of sea is provided which, due to the airtight or nearly airtight enclosed space above the column, will have a reduced surge motion - 3- i 109020~
relative to the ambient sea or no surge motion at all.
Therefore~ in adverse conditions launching and recovery of a submersible craft or equipment within the enclosed space will be possible in circumstances where it would be prohibitive i~
`5 the moonpool were exposed to atmosphereO
The present invention in another aspect provides a vessel adapted for use in launching and recovering a submersible craft or equipment, said vessel having a moonpool open at its lower end for ~assage of the craft or equipment out of or into the moonpool and which is closed at its upper end to form an enclosed space above and bounded by the surface of the sea contained within said moonpool when the vessel is afloat and the moonpool is filled with sea to the level of the ambient sea~ the enclosed space being airtight or nearly airtight so that surging in the moonpool is avoided or significantly reduced compared with a similar arrangement in which the top o~ the moonpool is open.
Preferably, the moon?ool is closed at its upper end by a housing on the deck or floor into which the upper end of the moonpool opens, the nousing containing means for launching and recovering th~ craft or equipment.
Preferably also, said housing has a door or hatch which is airtight when closed.
The lower end of the moonpool may be extendible downwards to a location below the normal leveL.
In a further aspect, the present invention provides - a fixed ~tructure adapted for use in launching and recovering ~0~020(~

a submersible craft or equipment, said fixed structure having a moonpool open at its lower end for passage of the craft or equipment out of or into the moonpool, and which has a closure at its upper end to f'orm an enclosed space above and bounded by the surface of the sea contained within the moonpool, the enclosed space being airtight or nearly airtight so that surging in the moonpool is avoided or significantly reduced compared with a similar arrangement in which the top of the moonpool is open.
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. . ..

_ _ Q~O

Embodiments of the present invention will now be describedg by way of example, with reference to the accompanying drawings, in which:-Figure 1 gives a diagrammatic view of a vessel according to the present invention, Figure 2 is an illustrative view of another vessel according to the present invention;
Figures 3 and 4 illustrate a semi-submersible in li~ht conditions and operating conditions respectively;
Figure 5 is an illustrative view of a fixed structure according to the present invention; and Figure 6 is a graph showing the damping effec-t of closures with less than perfect seals.
Referring to Figure 1, a vessel 10 for use in launching and recovering a submersible craft or equipment 12 has a moonpool 14, in the form of a shaft, preferably vertical, . passing downwards through the vessel from an open deck 16 to a hole in th.e bottom of the hull, and filled with sea water to the level 18 of the sea in which the vessel is floating.
A closure, in the form of.a housing 20~ is provided over the moonpool 14, on that part of the deck 16 into which the upper end of the moonpool opens, the housing 20 being closed crf from ambient atmosphere so that within the housing and within the part of the moonpool above the surface 18 of the sea therein, there is formed an airtight or nearly airtight enciosed space 22, the effect of which is to dampen or xo~

eliminate surge of sea water within the moonpool which would occur due to wave frequency if the moonpool were open to atmosphere.
The housing 20 has a door or hatch 2l~ which is airtight when closed and any ports or ventilators (not shown) in the housing are closable and are closed at least when the moonpool is to be used and when the waves around the'vessel are at the adverse frequencies.
The housine contains lifting tackle 26 by which,a craft or equipment 12 is to be launched or recovered through the moonpool.
The lower end of the moonpool may be bellmouthed.
Optionally, the lower end ol the moonpool shaft is extendible as shown at 28 to a depth lower than the keel of the vessel 10.
In the second embodiment shown in Figure 2, the upper end of the moonpool 14' is closed by a closure in the form of - a hatch, deck or lid 20' which is preferably of transparent material for observation purposes, and which has an aperture 30 having a sub.stantially airtight seai through which aperture can pass the cable 34 of the 'lifting tackle 26' for launching or recovering a cra~t or eqllipment and also ]ife support transmission cables. Such a hatch, deck or lid may, for example, be in t~o parts 30a, 30_, whic'n slide or pivot horizontally apart as shown in dotted lines to uncover the moonpool. In this embodiment, the launching and recovery of a craft or _ 7 --~9oz~

equipment 12' could take place in adverse conditions because the surge within the moonpool would be sufficiently dampened due to the airtight or nearly airtight enclosed space below ~he hatch when the hatch is closed, but the return of the craft or equipment to the deck may have to be delayed if at that time conditions are unsuitable for opening the hatch.
The lower end of the moonpool may be downwardly extendible as in the first embodiment, and~orbellmouthed.
In a third embodiment, shown in Figures 3 and 4, the vessel is a semi-submersible40`3, and has a moonpool 14" as described with reference to Figure l, i.e. opening into a housing 20". It will be understood that a moonpool, enclosed by a dec~, lid or hatch as described with reference to Figure 2, could be provided instead The cylindrical body forming the moonpool 14" extends below the deck 16" of the semi-submersible but only to a distance which, when the semi-submersible is in light condition, is above the water--line; such an arrangement reduces resistance to the semi-subrnersible's passage through the water en route to its working location.
~efore use, the semi-submersible is lowered to its operating condition and the lower end of the moonpool becomes s.ubmerged and partially filled with sea water so that an enclosed space is provided above the surface of the sea within the moonpool.

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In the fourth embodiment shown in Figure 5, a fixed structure in the form of an offshore rig 50 has a moonpool 14"' opening into a housing 20"' as described with reference to Figure 1. It will be understood that a moonpool i.e. enclosed by a. deck, lid or hatch as described in Figure

2 could be provided instead.
In all embodiments, an airtighk or nearl~ airtight enclosed space is provided at least within the moonpool, which space is located above and bounded by the surf'ace of the sea.
The effect of the airtight or nearly airtight enclosed space is to reduce or eliminate surge motion of the sea within the moonpool relative to the waves of the open seag the effect being caused, at least in part, by an increase of air pressure within the enclosed space, no matter how slight, over atn.osphere pressure. Thus, launching or recovering of the submersible craft or equipment from or into the enclosed space car: be effected in sea conditions when such operation would be dangerous or impossible from an open moonpool because of - ad~erse swel .
The enclosed space may be pressurised by introducing - compressed air or other gas thereinto.
In the embodiment shown in Figures 2 to 5 parts similar to those shown in the Figure 1 embodiment are indicated by like numerals with the addition of indices.
Laboratory tests have been carried out in a wave tank, wherein an open tube was located upright with its lower portior immersed in water in the tank. A series of wave frequencies ~o~ozao were generated and readings taken of the surge in the open cylinder.
The top of the tube was then covered by a plate and the surge ceased3 the water level remaining steady for each generated wave frequency. The air pressure within the enclosed space was noted on a manometer. Readings were taken at different depths of immersion and using tubes of different dimensions.
The results are as noted hereunder in Tables 1 to 4.

WAVE_DATA
.
Dial Reading Freqllency Height Length (Hert~) cm m _ 2 .40 1.5 4.60 4 .5 2.1 4.00 6 .62 2.5 3.113 8 .77 2.7 2.40 .95 4.8 1.50 - 12 1.16 5.5 1.20 14 1.38 7.25 0.75 16 1.67 5.75 0.50 _ 10 Q2~0 EXPERIMENT A

Circular tube with diameter = 5.08 cms.
(1) Pressure readings in cms of water when top is closed.
. 1-- -Wave Surging Depth of Immersion (cm) Frequency Amplitude __ _ cms 10.16 17.78 24 .13 25.4 30.48 . . .
.40 0.0 1.l~ 1.2 1~4 1.2 .~
.50 o,o 2.3 2.1~ 2.4 2.4 2.0 .62 0.0 1.3 1. 2 1.0 1.1 l.o .77 0.0 0.6 o.4 0.5 o.5 o.5 .95 o.o 0.5 0.4 0.3 0.3 o.3 1.16 o.o 0.2 0.2 o.l 0.15 0.1 1.38 0.0 0.1 0.1 0.1 0.1 0.05 1.67 o.o _ _ _ _ _ (2) Amplitudes of surge at immersion depth = 24.13 cms . . I
Wave Frequency Top OpenTop t'losed Hertz cms c.ms . _ _ _ _ _ .40 1.6 t).0 .50 2.54 t).0 .62 2.54 t).0 .77 3.59 o.o .95 9.16 ~.0 1.16 4.44 o.o 1038 1.27- 2.54 o.o 1.67 0.64 - 1.27 0.0 ;

,.

'rA~3LE 3 ~ 2 EXPF,RIMENT B
circulaI~ ~ube with diameter = 10.16 cms.
tl) Pressure readings inlcms of water when top is ciosed.
. Depth of Immersicn (cms) Wave Surglng Frequency Amplitude _ Hertz 10.16 17.78 24.1325.4 _ _ _ .40 0.0 o.6 1.8 2.7 2.3 .50 0.0 3.3 3.1 3.6 2.8 . .62 0.0 1.ll 1.4 1.2 1.3 .77 0.0 0.7 o.6 o.6 o.6 .95 0.0 0.4 . 0.4 0.4 0.3 1.16 0.0 0.3 0~2 0.2 0.1 1 3~ U O U I 0.1 0.10.0~

_ _ (2) Amplitudes of surge at draft = 24.13 ., _ - .

Wave .F~equency Top Open Top Closed Hertz cms .cms .
_ .

.40 1.6 0.0 .50 2.54 0.0 .62 2.54 0.0 .77 3.81 0.0 .95 12.70 0.0 1.16 3.81 0.~
1.38 . 1.27 0.0 1.67 0.93 0.0 _ _ _ .

_ 12 -EXPERIMENT C
-Square tube with sides equal to 25. Ll cms.

(1) Pressure readings in cms of water when top is closed.

. _ Wave Surge Frequency Amplitude Depth of Immersion (Hertz) cms ?4 ~ 13 _ _. l .40 0.0 1.8 .50 . 0.0 3.0 . .62 0.0 1~2 .77 l~.0 0.7 .95 ~.0 0.3 1.16 0.0 0.2 1038 ~).0 0.1 ~.67 l~.0 _ zo~

The experiments indicated that once the closure is placed over khe moonpool, the space between the water and the closure will have a pressure around atmospheric pressure. The pressure will show slight fluctuations because the water level inside the moonpool will ta}ce the mean level of the external seaways and the variation of seaways will in ~urn change the water level. .It is estimated that the pressure variation will be of the order of 1.0 metre of water. Such pressure variations would be tolerable ko persons inside the space.
In normal operating conditions there would be only a few persons in the enclosed space and for prolonged working it is necessary to ensure continuing supplies of oxygen in t~e space. By having double doors in the hatch or housing with an airlock between entry and exit can be ef~ected without affecting the seal of the enclosed space.
Figure 6 shows the results of tests performed to exlmine the effect of a closure above the moonpool which is not a ?erfect seal. In the test a circular tube with a diameker of 10.16 cm. (4 inches) is immersed to a depth of 24.1 cmO
(9.06 inches). Waves of a frequency of 0.93 cycles per second were generated around the tube. This frequency was chosen because it was close to the resonance condition when the amplitude of surging in the tube approached a maximum when the tube was open. In the graph the vertical axis represents the ratio between the observed amplitude and the maximum amplitude of surge (that is when the tube is open to the atmosphere). The horizontal axis represents the ratio of the area exposed to the atmosphere to the cross-sectional area of the tube, that is effectively the area of the water surface. In the experiment the tube was first sealed completely before reducing the amount of sealing across the end of the tube. From the results it is concluded that the effectiveness of the seal in reducing the surging motion of the water decreases rapidly with an increase in the lea~ area ratio and that as the exposed area reaches seven percent of the cross-sectional - area the effect is substantially the same as if no seal was employed. The results are considered to be typlcal and away from the resonance condition when the absolute - amplitude of surge with the tube end open is lower, the effect is similar.
It is thus concluded that for the maximum damping effect the enclosed space a`~o~e the moonpool should be airtight but some significant damping of surge can be 20. achieved with an enclosed space that is nearly airtight.
In practice a leak of about one percent of the surface area of the sea in the moonpool will reduce the amplitude of surge to about one third the amplitude with the moonpool open.

~o9oz~o It will be appreciated that other embodiment~ of the invention are possible besides those described above. For example a tank may be provided in a hold in a vessel the tank opening into the sea so that it i~ filled to the leve].
of the ambient sea. A cover or lid is provided to enclose the upper end of the tank to provide the alrtight or nearly airtight enclosed space in the tank above and bounded by the surface of the sea therein. The terrn "moonpool" used in this specification is intended to cover such an arrangemen~.
It will be noted that the embodiments described above are suitable only for craft or equipment which are tethered and raised and lowered from the moonpool into the sea.
The vessel or structure will have some mechanism of rails, pulleys or other mech~mical devices which are used for lowering and lifting submers:ble craft or equipment.

-t~ ~

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of launching and recovering submersible craft or equipment from the sea through a moonpool of a vessel or fixed structure in the sea, said method comprising the steps of filling the moonpool with sea to substantially the level of the ambient sea, providing an enclosed space over and bounded by the surface of the sea within the moonpool and launching said submersible craft or equipment from within said space or recovering said submersible craft from the sea into said space, said enclosed space being airtight or nearly airtight so that surging in the moonpool is avoided or significantly reduced com-pared with a similar arrangement in which the top of the moon-pool is open to the atmosphere.

2. A method according to Claim 1, in which the enclo-sed space has an area of leakage and limiting the area of leakage so as not to exceed one percent of the area of the surface of the water in the moonpool when the water is at its normal level in the moonpool.

3. A method according to Claim 1, in which the moon-pool has a constant cross-section throughout its height.

4. A method according to Claim 1, in which the level of water in the moonpool is substantially the same as the mean level of the seaway around the vessel or fixed structure.

5. In a vessel for use in the sea for launching and recovering a submersible craft or equipment, a moonpool carried by the vessel, said moonpool being open at its lower end for filling the moonpool to substantially the level of the ambient sea for passage of the craft or equipment out of or into the moonpool and being closed at its upper end to form an enclosed space above and bounded by the surface of the sea contained within said moonpool when the vessel is afloat and the moonpool is filled with sea to the level of the ambient sea, said enclosed space being airtight or nearly airtight so that surging in the moonpool is avoided or significantly reduced compared with a similar arrangement in which the top of the moonpool is open.

6. A vessel as claimed in Claim 5, in which the clo-sure forming the enclosed space includes a housing on the deck or floor into which the upper end of the moonpool opens, said housing containing means for launching and recovering the craft or equipment.

7. A vessel as claimed in Claim 6, in which said hou-sing has a door or hatch which is airtight when closed.

8. A vessel as claimed in Claim 5, in which said moon-pool has a lower end portion which is extendible downwards to a location below the normal level.

9. A vessel according to Claim 5, in which the enclo-sed space has an area of leakage not exceeding one percent of the area of the surface of the water in the moonpool when the water is at its normal level in the moonpool.

10. A vessel according to Claim 5, in which the moon-pool has a constant cross-section throughout its height.

11. A fixed structure adapted for use in the sea for launching and recovering a submersible craft or equipment, said fixed structure having a moonpool open at its lower end for fil-ling the moonpool to substantially the level of the ambient sea and for passage of the craft or equipment out of or into the moonpool, a closure at the upper end of said moonpool to form an enclosed space above and bounded by the surface of the sea con-tained within the moonpool, the enclosed space being airtight or nearly airtight so that surging in the moonpool is avoided or significantly reduced compared with a similar arrangement in which the top of the moonpool is open.

12. A fixed structure as claimed in Claim 11, in which the closure forming the enclosed space includes a housing on the deck or floor into which the upper end of the moonpool opens, said housing containing means for launching and recovering the craft or equipment.

13. A fixed structure as claimed in Claim 12 in which said housing has a door or hatch which is airtight when closed.

14. A fixed structure as claimed in Claim 11, in which the lower end of the moonpool is extendible downwards to a location below the normal level.

15. A fixed structure as claimed in Claim 11, in which the enclosed space has an area of leakage not exceeding one percent of the area of the surface of the water in the moon-pool when the water is at its normal level in the moonpool.

16. A fixed structure as claimed in Claim 11, in which the moonpool has a constant cross-section throughout its height.