E L S E V I E R 0 1 4 1 - 1 1 8 7 ( 9 4 ) 0 0 0 2 3 - 9
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Scale effects on a unique launch sequence of a
gravity-based structure
Subrata K. Chakrabarti
Chicago Bridge & Iron Teclviical Services Co., Plainfieid, IL, USA
(Received 25 May 1993; accepted 14 August 1994)
There are many different techniques that are used in launching an offshore structure. The suitability of these techniques depends on the type and size of the offshore structure built on the shore. Many of these techniques are routinely model tested to ensure feasibility of the sequence of launching. This paper describes one such model test of a gravity-based offshore structure. The launching method in this case is unique and required model testing to generate confidence in the method. The modehng problems encountered in the launching and a necessary solution are discussed here,
I N T R O D U C T I O N
Most offshore structures are built on land close to water and transported to their final destination. A t the site, they are submerged and installed at the prescribed location. Thus, a completed offshore structure goes through three distinct stages before i t is ready f o r production and operation at site. These are considered as launching, towing, and submergence.
I f an offshore structure is small i n size or i f the structure does not have a large buoyancy module, i t is carried on a barge. One o f these structures is a jacket type drilling and production structure. The jacket sUdes down into the water f r o m the barge rails before being submerged at the site. Bhattacharyya^ described the scaling technique applicable to the launching of a jacket structure. The installation procedure^ and the model testing^"^ have also been described i n the literature.
A graving dock is a popular method of building and launching large-volume structures offshore for towing to the site f o r installation. The Khazzan o i l storage structures operating i n the Persian G u l f were built this way. Some o f the large concrete structures i n Norway are built i n the deep water i n fjords by ballasting the structure as the construction progresses. Then the structures are unballasted to a shallow draft before towing out of the fjords.
A U N I Q U E L A U N C H
Submersible drilling rigs with large bases were built by
C B I at its Pascagoula Construction Y a r d i n Mississippi. I n the yard the launching dock was much higher than the existing water level at the launch site. Therefore, a unique, patented launching procedure was developed. Open-bottom buoyancy cans (up to 10-7 m or 35 f t i n diameter) were designed to support the structure on a cushion of compressed air during launch and load out i n the sheltered bay off the Gulf o f Mexico (Fig. 1). The buoyancy cans were ballasted and submerged beneath the rig i n deeper water i n the bay before final towing, allowing the rig to be towed under its own buoyancy.
Since this launching procedure had not been used in the past, a test was conducted to investigate the launching sequence and the stability of the rig on the flotation cans. A simple model of the rig was built at a scale o f 1:48. Nine buoyancy can models f o r supporting the r i g were also built. The procedure outlined i n the launching sequence was followed i n the test i n 15 discrete steps, and the pressures i n each can were monitored and adjusted as necessary. A i r supply reservoirs were connected to each flotation can to accurately model the soft volume needed i n the test with respect to the prototype can volume. (Note that soft volume refers to the air trapped i n an open-bottom enclosure as opposed to hard or buoyant volume.) During the launch, the mat was supported at the bow by soft volume buoyancy cans and at the stern by a 12-7 m m (2111) diameter aluminum r o d ,
which supported the model at the centroid of the launch beam reaction. The seakeeping characteristics o f the rig while supported by all nine cans i n wind and waves were studied. Finally, the cans were ballasted beneath
Fig. 1. Launch of CBI 85 submersible from dock. the structure, allowing the rig model to submerge to its
design towing draft.
M O D E L I N G OF S O F T - V O L U M E CANS
Under normal testing conditions, the atmospheric pressure in the laboratory is close to the atmospheric pressure prevaihng at the site. The hydrostatic head in a soft volume depends on the pressure difference between the inside and outside air pressure and, as such, poses no scaling problem. However, this head is directly related to the compressibility of the air inside the soft volume which depends on the absolute pressure. Assuming that air follows the perfect gas law,
P F = constant (1) where P is the absolute pressure (atmospheric -|-
hydro-static) of air in the soft volume, and V is the trapped air volume, this law must be met in the model as well as the prototype. I f the geometric similarity is maintained between the model and prototype then the volume, V, can be modeled properly. However the pressure, P, includes the atmospheric pressure which does not scale. The pressure in the model environment is higher than the scaled down value since the scale factor is greater than one. I n fact, the distortion is greater as the scale factor becomes larger. There are two ways to circumvent this problem. I n the model, the soft volume members may be placed under vacuum (of appropriately scaled negative or below atmospheric pressure) which is difficult to accomplish. The other method is to distort the soft voluiue so that the gas law is satisfied in the model scale. This is achieved by connecting additional
volume to the soft volume members. I t is clear that as the pressure changes, the amount o f this extra volume also changes. Thus, in a test, e.g. during deballasting, a variable reservoir volume is needed.
After the submersible rig has been launched f r o m the construction yard bulkhead, it will be supported by freely floating soft volume cans. These support cans are modeled as shown in Fig. 2. Consider a prototype can of unit cross-sectional area initially floating i n Position 1 with its freeboard equal to H^^. The internal pressure, P i p , corresponds to a differential head, H^^, between the water levels inside and outside. I f P^ represents atmo-spheric pressure
P i p = P o + / / l p (2) The air volume in the can is F j p ,
Vxv = Hop + ^ i p (3) Note that the volume has been normalized by the
cross-sectional area o f the cans.
I f the floating can is displaced by an amount, A / / p , to Position 2, its internal pressure will now be P2p.
P2p = P o + (4)
where is the differential head between the water level inside and outside of the can. The new volume is F , , (5) 2p' V2„ = / / o p + A / / p + //2p P. A Hp P. POSITION 1 . 2
Assuming the product PV to be constant between Positions 1 and 2 ,
(Po + i ï l p ) ( / / o p + i / l p ) = (Po + / / 2 p )
x ( / / o p + A / / p + / / 2 p ) ( 6 ) Solving this equation for the differential pressure head
at Position 2 , one gets
H 2p = ((Po + / / o p + A i / p ) 2 + 4 { P o ( / / i p A i / „
+ / / l p ( i / o p + / / l p ) } ) 1/2
| ( P o + / / o p + A / / p ) ( 7 ) When the can is modeled with a scale factor of A, the gas law gives H lp A //nr. H A lp A 2p A //, op A A / f „ H-A 2p A ( 8 )
where fJ is a volume of air that must be added to the can volume to compensate f o r the fact that Pq will not be scaled in the model. Solving for this added volume,
P o ( A - l ) ( i / i p - i Ï 2 p - A / / p ) A ( / / 2 p - H
(9) Since the above relation assumes the density of the supporting liquid to be equal f o r the prototype and model and we are representing a prototype that floats i n sea water by a model floating in fresh water, the equation f o r fJ must be modified to account f o r this difference.
I n the following, the subscript 'sw' refers to sea water and the subscript ' f w ' refers to fresh water. Consider the same prototype can floating i n sea water as well as in fresh water with an identical mass of air in the can f o r both cases then
/ p s w ^ s \ v / p f w ^ f w ( 1 0 )
For the can to be floating i n equilibrium in both cases, the upward force on the can must be the same whether the can is floating in sea water or fresh water. I f the upward forces are equal then the following must be true assuming that the buoyancy force f r o m the submerged can waU is nearly the same for both cases:
( 1 1 ) P psw • p f w
and
^ s w ^"pfw
The can pressures are taken to be
- psw - ^ I p s w T s w
( 1 2 )
( 1 3 ) and
P o + / / i p f w 7 f w ( 1 4 ) where 7 is the water density. Solving f o r the fresh water
differential head one gets 7sw
H I p f w 'H
7 f u
Ipsw ( 1 5 )
which shows the fresh water differential head to be equal to its corresponding sea water head multipUed by the ratio of sea water density to fresh water density. Multiplying the differential pressure head terms in eqn (9) by the ratio 7 s w / 7 f w one gets
P o ( A - l ) Tsw 7f\v ( / / Ipsw //2psw) - A i / p A ( (Hjpsw -~ / / i p s w ) v 7 f w / ( 1 6 )
which gives the reservoir volume that should be added to the model can floating in fresh water in terms of the differential head of the prototype can floating i n salt water.
Modeling the prototype sea water with fresh water also affects the model's freeboard. Again considering the same prototype can floating i n both sea water and fresh water then ' p s w //, opsw H Ipsw and ^ f w / / o p f w + / / ipfw ( 1 7 ) ( 1 8 ) Using eqn ( 1 4 ) for i / i p f w and the equahty i n eqn ( 1 1 ) and solving f o r i / o p f w gives
/ / , Opfw opsw + H Ipsw
7sw 7fw
(19) The fresh water freeboard of the model, i?omfw> will, therefore, be / / o m f w / / , o p f w opsw omfw 7sw • / ' i p s w i - — 7 f w A
Since the model should float at //opsw ^ omfw A ( 2 0 ) ( 2 1 ) ( 2 2 )
to dupUcate the prototype, the model must be raised i n the water by the amount A i ï o m f w where
7 f u AH, o m f w
/ / , opsw H, opsw H ipsw
A or / / i p s w I 1 A / / o m f w = - • V 7 f v
J
( 2 3 ) ( 2 4 )Table 1. Extra volumes required with launch cans floating at initial freeboard and pressure
Can No. Prototype Model Prototype pressure Model pressure Reservoir volume
freeboard freeboard (in) (ft of sea water) (in of water) (ft')
(ft) 1 8-50 2-125 5-85 1-50 8-55 2P and 2S 8-50 2-125 16-00 4-10 8-23 3P and 3S 8-50 2-125 16-39 4-20 8-30 4P and 4S 8-50 2-125 16-00 4-10 6-67 5P and 5S 8-50 2-125 15-22 3-90 10-24 l i n = 25-4mm; l f t = 0-3m.
To raise the model to the correct elevation one can subtract the amount A i ï o n , f w f r o m the reservoir volume. This will i n effect move the volume A 7 / o ^ f „ f r o m the reservoir into the can, causing it to float at the correct freeboard. The compressibiUty of air is still correctly modeled because the can-reservoir system remains constant. The equation for is now
/ ' o ( A - l ) K 7s, L7fw psw A — j (//2psw ~ / / i p s w ) \ 7 f w / / / i p s w l 1 + • 7sw 7fw A (25)
which gives the reservoir volume that will allow one to correctly model the compressibility of air and cause the model to float with the correct freeboard. The volume calculated by eqn (25) will have dimensions of cubic units per square unit of model cross-sectional area. A n example of the extra volume needed f o r proper modeling is given as follows. Assume a scale factor of 1:48 f o r the model. Also, consider a prototype freeboard for a can to be 2-6 m (8-5 ft) and the prototype pressure to be l-78m (5-85 ft) of sea water. Then, the calculation based on the above formulae gives an extra reservoir volume i n the model (with fresh water) o f 0-24 m^ (8-55 ft^). This is a substantially large volume compared to the model volume. The reservoir volumes needed f o r the floating condition of the rig on cans are shown i n Table 1.
S U B M E R S I B L E R I G M O D E L Mat
A 1:48 Froude scale model o f the mat portion o f the drill rig was constructed. The side walls of the mat were made f r o m 19 m m ( | i n ) thick polyethylene glycol treated sugar pine and corner braced with wood blocks and 1-6mm ( j ^ in) sheet aluminum splicing plates. A l l joints were glued with waterproof glue. The top and
bottom of the mat structure were fabricated f r o m single pieces of 3-2 m m ( | in) i n thick plexiglass and secured to
the side walls with a clear silicone adhesive and brass flat head screws on a 102 m m (4 in) spacing. A three legged adjustable platform was then constructed o f 38 m m (12 in) diameter aluminum pipe and socket fittings and secured to the mat structure. A 22-7 kg (50 lbs) weight was bolted to the platform and positioned so as to place the center of gravity of the completed structure at the point shown i n Fig. 3. Thus, the geometric similarity was maintained only where it was required for this test.
Since the top and bottom of the mat were transparent, positioning of the launch cans was accomphshed by locating circles of various colors in the appropriate places and matching and centralizing larger colored circles placed on the top surface of the launch cans. Cans and reservoir volumes
The launch cans were also Froude scaled with the exception that the displacement of steel thicknesses was not scaled. The vertical shell walls of the cans were constructed of 0-76 m m (0 03 in) thick clear lexan. The single vertical joint i n the shell was a butt joint supported by a 6-4mm ( | i n ) wide back-up strip and solvent glued. The top of the can was held concentric by a 6-4mm ( | in) thick by 15-9 mm ( | i n ) wide ring o f plexiglass. A t the hexagonal points the ring was enlarged to support hard buoyancy cans. A n internal buoyancy skirt was made f r o m 0-76 m m (0-03 in) thick lexan with a 3-2mm ( | i n ) thick plexiglass plug. This unit was then solvent glued to the center o f the launch can. A 127 m m (5 in) length o f 6-4 m m ( j i n ) O D plastic tubing, fish mouthed at the top end, was glued to the outside of the inner skirt as a tubing connection and air entry into the launch can. A 10 f t length of soft PVC tubing hnked to a smaller tubing, was used to connect the launch can to the reservoir volume (see Fig. 4).
The compensating reservoir volumes were fabricated f r o m purchased, polypropylene containers 0-71 m (28 in)
diameter X1-07 m (42 in) high. These containers were
sealed with a 19 m m ( | i n ) plywood cover fined w i t h 0-76 m m (0-03 in) sheet lexan and held in place with lead weights and C-clamps. Each container was provided with a valved 19 m m ( | in) diameter bulkhead fitting at the bottom of the container. Water was then added to the container via this fitting so as to provide a liquid piston to vary the volume o f the reservoir.
W/L
Fig. 3. Platform model supported on flotation cans.
Launch platform
The launch platform was fabricated by screwing two 19-1 m m ( I in) thick, 12 x 2-4m ( 4 x 8 f t ) sheets of plywood together on three unistrut beams. This flat surface was then placed and secured to two 1-3 m (50 in) long, 51mm (2 in) diameter schedule 40 pipes socketed in the sockets of the wave tank wall. These cantilevered pipes were then backstayed to the wave tank rail by a turnbuckle-strap arrangement. The platform balancing device consisted of a l-52in x 12-7mm (60 in X j in) diameter sohd aluminum rod held in place by 12-7 m m ( j i n ) thick slotted plywood cleats fastened to the platform. The aluminum bar was moved to the various slots as corresponded to the foot print e.g. of the rig
as it was moved out f r o m the bulkhead. See Fig. 5 f o r detaüs.
The water level of the wave tank was adjusted to 54mm or 2^ in (2-6m or 8-5ft prototype) below the top of the balance rod of the leveled launching platform to model the prototype launch site platform.
The righting moment of the rig by itself was estabhshed i n the longitudinal and transverse directions as follows. The rig was lowered i n the water to float by itself. Weights were placed at the bow and stern ends at known distances and the angle of tilt of the rig i n the longitudinal direction was recorded. Similarly, weights were placed on the transverse line through the e.g. at the port side edge of rig and the transverse angle of tilt recorded.
Fig. 4. Layout of tubings between launch can and reservoir volume. Instrument panel
Each launch-can pressure was measured by a 203 mm (8 in) well type, Dwyer Manometer (Model 308). These manometer gauges have a minor scale division of 2-5 m m (OTin) and are cahbrated to read in inches of water. Each launch can was provided with a separate manometer, 6-4 m m ( | i n ) diameter pressurizing valve and 6-4 m m (4 in) diameter vent valve.
L A U N C H I N G O F M A T O N C A N S
During the launching, the mat was supported at the bow
by the floating soft volume cans and at the stern by the 12'7mm (0-5 in) diameter aluminum rod. The aluminum rod at the stern was positioned to support the model at the centroid o f the launch beam reaction. The launch procedure was simulated i n 15 discrete steps (Table 2). The structure's overhang past the bulkhead face, the launch beam reaction centroid location, and the pressure in each can were pre-calculated (Table 3).
For each launch step, the rig and support rod were positioned at the desired locations and the mat was held level by hand as the cans were pressurized to the speci-fied values. The mat was then released and the angle f r o m horizontal was noted. The pressures in the cans were then adjusted as necessary for the cans to hold the model level
r T ~ L
step
Table 2. Location of rod and rig on launch platform (lin = 25 4mm)
j
" s • " 8 " " 0 "
'O' overhang (in) 'S' e.g. from stern (in) 'B' e.g. from bullchead (in)
1 14-75 19-25 15-50 2 14-75 16-83 17-93 3 19-25 16-00 14-25 4 23-75 14-48 11-28 5 23-75 13-70 12-05 6 28-25 12-53 8-73 7 33-00 10-93 5-58 8 33-00 8-93 7-58 9 37-50 7-40 4-60 10 42-00 3-75 3-75 11 42-00 3-73 3-78 12 43-75 2-94 2-81 13 45-50 2-00 2-00 14 45-50 2-18 1-83 15 Floating on launch cans
and the new pressures were noted. On the final launch step the model was allowed to float free of the bulkhead, sup-ported entirely by the nine soft volume cans. The submers-ible model was successfully launched several times following the procedure outlined through computation. The pressures in each can during the launching step were noted (Table 4). Observing the can pressures during the launch serves basi-cally to check the statics calculations made earlier. Through-out the launching, the model appeared to be quite stable.
S E A K E E P I N G O F R I G O N C A N S
The seakeeping of the rig when supported by the cans was tested by floating the model i n waves (Fig. 6). The model was oriented so that the waves approached i t f r o m several different directions. The rig exhibited good seakeeping characteristics when exposed to wind and waves. The model experienced little motion i n rofl or pitch.
Table 3. Launch steps (model scale) Step Overhang
(in)
Distance from stern to reaction centroid (in)
Can pressures (in of water)
No. 1 Nos 2P and 28 Nos 3P and 3S Nos 4P and 4S Nos 5P and 58
1 14-75 19-25 0 — — — — 2 14-75 16-83 2-47 — — — — 3 19-25 16-00 3-13 — — — — 4 23-75 14-48 4-24 0 — — — 5 23-75 13-70 3-35 1-89 — — — 6 28-25 12-53 2-47 3-48 — — — 7 33-00 10-93 2-47 4-52 0 — — 8 33-00 8-93 2-47 4-52 1-89 — — 9 37-50 7-40 2-24 4-52 3-16 — — 10 42-00 3-75 2-11 4-65 4-65 0 — 11 42-00 3-73 1-71 4-60 4-60 2-29 — 12 43-75 2-94 1-58 4-60 4-60 3-27 — 13 45-50 2-00 1-34 4-60 4-60 4-55 0 14 45-50 2-18 0-83 4-60 4-60 4-55 1-71 15 Floating — 0-33 4-60 4-60 4-55 3-18 level on cans
Table 4 . Can pressures during launch (in of water)
Step No. 1 Nos 2P and 2S Nos 3P and 3S Nos 4P and 4S Nos 5P and 5S
Theo. Act. Theo. Act. Theo. Act. Theo. Act. Theo. Act.
2 2-5 2-75 — — — — — — — — 3 3-1 3-4 — — — — — — — — 4 4-2 4-45 0 0 — — — — — — 5 3-4 3-35 1-9 2-15 — — — — — — 6 2-5 2-45 3-5 3'9 — — — — — — 7 2-5 2-6 4-5 4-7 0 0 — — — — 8 2-5 2-6 4-5 4-6 1-9 2-2 — — — — 9 2-2 1-9 4-5 4-6 3-2 4-15 — — — — 10 2-1 2-0 4-7 4-9 4-7 4-9 0 0 — — 11 1-7 1-9 4-6 4-8 4-6 4-5 2-3 2'0 — — 12 1-6 1-9 4-6 4-8 4-6 4-5 3'3 3-0 — — 14 0-9 1-5 4-6 4-9 4-6 4-5 4-6 3-0 1-7 1-5 15 03 1-5 4-6 4-1 4-6 4-2 4-6 4-1 3-2 3-9
Theo., theoretical pressures; Act., actual pressure required to hold model level D E B A L L A S T I N G O F CANS
Lowering the rig f r o m its position on top of the cans to the point where i t floats on its own requires that air be removed f r o m the cans. The model was lowered . into the water f r o m the position on top of the cans by venting air f r o m several different combinations of cans. During each debaUasting test, the rig began to level itself as soon as the mat entered the water and picked up buoyancy.
To observe the behavior of the rig i n a situation where air is lost f r o m only one can while the rig is floating on the cans, the rig was floated level using the can pressures determined f r o m the launching tests and then unbalanced by venting air f r o m one can at a time. A stability problem was noted when the N o . 5S can (Fig. 3) was lost. After the rig tilted approximately 4 - 6 ° , the N o . 2P and N o . 3P cans were vented to bring the rig back to a nearly level position. After comparing the behavior of the model during the stability test with and without the
Fig. 6. Model test of the launch procedure.
auxihary volumes containing air, i t was apparent that the extra air volumes did influence the model behavior to a considerable extent.
O B S E R V A T I O N S AND D I S C U S S I O N
The launching of the submersible drilling rig on open-bottom cans has been model tested i n the wave tank. The areas covered under this investigation are: (1) the jacking o f the rig f r o m the launch beams onto the cans, (2) the seakeeping of the rig floating on cans under environmental conditions expected in the launch site bay, (3) the can deballasting sequence required to lower the rig into the water f r o m its position on top of the cans and (4) the stability characteristics o f the rig with the complete loss of air f r o m any one of the support cans. The following conclusions may be drawn f r o m this model test:
(1) The model is quite stable during the launching of the rig onto the nine support cans. The final phase of launching, lifting the mat off the bulkhead, is the most difficult part of the launching control. The difficulties in maintaining balanced pressures and air volumes i n the support cans during the l i f t - o f f are miiumized i f the bow is kept high enough to cause the rig to tilt back and maintain steady contact with the bulkhead until just before the stern lifts clear. The air volumes and pressures are much easier to control when the rig is held level side to side by the bulkhead. Tilting the rig towards the stern does cause higher loads on the bulkhead and these loads must be kept within allowable limits.
(2) The rig on soft volume cans behaves nicely under moderate (as expected in the bay) wind and wave loads. The natural periods o f the system in heave, roll and pitch are long and are no major threat to the rig.
(3) The structure was quite stable during the deballasting procedures. The mat did not remain level as it lowered to the water, but tipped towards the bow when the N o . 2 and N o . 4 pairs o f cans were vented simultaneously or the N o . 2, N o . 3 and N o . 4 pairs of cans were vented at the same time. When all nine cans were vented simul-taneously, the rig's stern entered the water before its bow did, by opening the vents on the N o . 2, N o . 3 and N o . 4 pairs of cans and as it was lowered to the water. I n all cases, the rig tended to level itself as the mat entered the water.
( 4 ) The rig was found to be reasonably stable (statically) with the loss of any one of the N o . 1, N o . 2, No. 3 or N o . 4 cans. However, when one of the No. 5 cans was lost, the stabihty of the rig was marginal. By interchanging the pairs of No. 2 and N o . 4 cans, the stability under the loss o f one of the N o . 5 cans could be improved slightly. I f the loss of air f r o m the N o . 5 can is slow enough that there is time to respond, corrective measures can be taken to level the rig by venting the N o . 2 and N o . 3 cans on the opposite side o f the rig.
C O N C L U D I N G R E M A R K S
The difficulty of launching a large-based submersible rig f r o m a quay much higher than the water level at the site
has been addressed with the help of an innovative launching scheme of open-bottom cans. This launching was model tested in a wave tank to study the feasibility of the scheme and to uncover any possible problem areas. The scafing problem with the open-bottom cans and a solution to correct for this scaling difficulty has been discussed.
The full-scale structures were successfully launched f r o m the site following the procedure outlined i n the model test.
REFERENCES
1. Bhattacharyya, S. K., On the apphcation of similitude to installation operations of offshore steel jackets. Appi.
Ocean Res., 6 (4) (1984) 221-6.
2. Graff, W. J., Introduction of Offslwre Structures, Design,
Fabrication, Installation. Gulf Publishing Co., Houston,
TX, 1981.
3. Sekita, K., Sakai, M . & Kimura, T,, Model tests on various launching methods for large offshore structures.
Proceedings of the Twelfth Annual Offshore Technology Conference, Houston, Texas, OTC 3836, May 1980, pp.
369-78.
4. Rowe, S. J., Model testing of launching and installation of steel production platforms. In Offshore Structures: The Use
of Physical Models in their Design. Construction Press,
Lancaster, 1981.
5. Bhattacharyya, S. K., Idichandy, V. G. & Joglekar, N . R., On experimental investigation of load-out, launching and upending of offshore steel jackets. Appl. Ocean Res., 1 (1) (1985) 24-34.
