The SDS Skip Subsea Deployment Systems Ltd. Subsea Deployment - - PowerPoint PPT Presentation

Subsea Deployment Systems Ltd.

The SDS Skip

Subsea Deployment Systems Ltd.

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SUBSEA SKIP

• An alternative to enhance the recovery of structures, spool

pieces, mattresses etc. during decommissioning work

• Can be used to transport complex structures or spool pieces to

field for installation by another vessel

• Reduces working and transit time for more costly vessel
• Operations have decreased weather dependency
• Safer than lifting to deck
• Schedule flexibility

Subsea Deployment Systems Ltd.

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The SKIP consists of UPPER SEALED TUBULARS designed to required water depth (typ. 150m). The amount of buoyancy is sufficient to render the SKIP slightly positively buoyant when without payload and the lower tubular fully flooded

SKIP

Subsea Deployment Systems Ltd.

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The SKIP consists of LOW ER FLOODABLE TUBULARS which have ballasting

• facilities. The displacement of the lower tubulars corresponds to the submerged

carrying capacity of the SKIP . The empty SKIP will be fully ballasted during the submerged tow and all valves will remain open to the sea to avoid hydrostatic collapse

SKIP

Subsea Deployment Systems Ltd.

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The CASTLES are positioned above the majority of the upper tubulars and protrude above the waterline in the deep draught condition allowing fine tuning of the trim

SKIP

Subsea Deployment Systems Ltd.

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The SI DE AND DOUBLE BOTTOM TANKS will contain ‘permanent’ water ballast and will be left open to sea at points above their upper boundary to ensure they do not contribute to free surface effects when submerging or surfacing the SKIP . They will have ballasting facilities

SKIP

Subsea Deployment Systems Ltd.

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The HOLD is completely water tight and allows the SKIP to be brought to shallow

• draught. The depth of the SKIP hold ensures that payload is low and buoyancy

high and ensuring a positive separation between the Centre of Buoyancy (CoB) and the Centre of Gravity (CoG) at all times

SKIP

Subsea Deployment Systems Ltd.

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The CONTROL CHAI NS are lowered into chain CONTROL TOW ERS to control the SKIP during set-down and recovery. The weight of the chain supported by the SKIP at the base of the chain towers is used to control the height of the SKIP . The length (weight) of chain suspended within the chain towers provides lateral and rotational control of the SKIP

SKIP

Subsea Deployment Systems Ltd.

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The TOW CHAI N CLUMP W EI GHT is inserted into the tow rigging to provide the necessary weight to submerge the SKIP from the deep draught tow condition to the submerged tow condition. It also acts as an anchor for the SKIP when parked above the seabed

SKIP

Subsea Deployment Systems Ltd.

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HOW SDV WORKS

Subsea Deployment Systems Ltd.

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CONTROL CHAIN TOWERS & DYNAMICS

Control Chain / Towers

• Vertical Control
• Lateral Control
• Rotational Control
• On‐bottom Weight

Dynamics

• Natural Period > 120s
• Response Amplitude <

20% of Surface Vessel

• Soft‐Soft Landing

Subsea Deployment Systems Ltd.

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SKIP

• Skip hold can be de‐ballasted to bring it to shallow draught
• Skip can be used for both subsurface and surface transport
• Demonstrated skip
• Length

32m

• Width

16m

• Depth

8m

• Weight

300Te

• Capacity (sub)

300Te

• Depth rating* 150m

* May be deeper if upper tubular pressurised

Subsea Deployment Systems Ltd.

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SKIP BASICS

• Upper tubulars are fully sealed and buoyant and designed for the

maximum water depth

• Lower tubular compartments are designed with a ballasting facility
• Side and double bottom compartments are fully flooded and open

to the sea

• Skip fitted with remotely operated system (power and control

pods) to control the de‐ballasting function of the hold

• The hold is fitted with centrifugal pumps powered by the

pressurised air in the lower tubular compartments

Subsea Deployment Systems Ltd.

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SKIP SEABED OPERATIONS

• The skip will be placed on the seabed by use of the control chains
• The control chains will be left in the towers to provide stability
• The skip will then be loaded with spool pieces, mats, structures
• etc. (Note: All lifts will be coarse weighed by the crane and evenly distributed within

the skip)

• Smaller items, mattresses

and gabions may be placed in steel wire cargo nets before being landed into the skip

Subsea Deployment Systems Ltd.

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MATTRESSES

• Mattresses are loaded into steel wire cargo nets (Quicker than using

speed loaders)

• The nets are then course weighed and loaded into segregated

areas within the skip

• The rigging is disconnected and hung
• ff topside of Skip (no need to enter

hold) (same rigging / cargo nets would be used

for offloading onshore)

Subsea Deployment Systems Ltd.

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DE‐BALLASTING THE SKIP

• When skip fully loaded the surface vessel will connect air hoses
• Lower tubulars will be de‐ballasted (pressurised to water depth)
• If skip not fully loaded; only some compartments de‐ballasted or

ballast weight is used

(Note: The pressure in the lower tubulars will later be used to de‐ballast water in the main hold)

Subsea Deployment Systems Ltd.

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SKIP RECOVERY

• The following measures ensure safe recovery of the skip
• Coarse weighing of each load by the crane
• There is excess weight in the control chains to prevent early float‐off when de‐

ballasting

• There is excess weight in the tow chain clump weight
• As the control chain are recovered the loaded skip rises off the

seabed

• Once off the seabed, the load, trim & pitch are assessed
• If required, the skip would be re‐trimmed before starting the

submerged tow

Subsea Deployment Systems Ltd.

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SURFACING OF THE SKIP

• Surfacing of skip would normally be done inshore in calm waters;

however, it may be done offshore if weather permits

• The hold would be partitioned to ensure negligible shifting of

payload during the surface tow

• During the submerged tow the skip is unaffected by the surface

weather minimising roll and pitch

Subsea Deployment Systems Ltd.

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SURFACING INTEGRAL SKIP

Upper tubular

(fully sealed, buoyancy equals submerged weight

• f skip)

Lower tubular

(c/w ballasting facilities)

Side and Double Bottom Tanks

(high point open to the sea)

Hold

(water can be pumped out of the hold using pressurised air from lower tubular)

Remotely operated

(Valves will be operated remotely for de‐ballasting)

Subsea Deployment Systems Ltd.

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SURFACING OF THE SKIP

• Tow vessel will slow down

to dead slow ahead

• Tow vessel recovers the

chain clump weight to bring the skip to surface

• Tow vessel will break out

the chain clump weight

Subsea Deployment Systems Ltd.

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SURFACING OF THE SKIP

• The skip will float with

upper tubular members breaking the surface due to the excess buoyancy

• Water trapped inside

‘hatch coaming’ above tubular is allowed to escape to match the

• utside water level

Subsea Deployment Systems Ltd.

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SURFACING OF THE SKIP

• When the skip has settled

water pumps will be activated to expel the remaining water in the hold

• The water pumps will be

driven by the energy stored in the lower tubular

Subsea Deployment Systems Ltd.

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SURFACING OF THE SKIP

• The skip will be

completely emptied of water within the hold

• To ensure stability

throughout, ‘permanent’ water ballast will be kept in the side and double bottom tanks

Subsea Deployment Systems Ltd.

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SKIP STABILITY

• A positive separation

between CoG and CoB when submerged

• Flooded side tanks ensure

that hull free surface effects during ballasting

• perations do not cause

instability during surface

• perations

Subsea Deployment Systems Ltd.

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TOW FORCES

• 300Te SDV Skip:
• Tug BP 50Te @ 3.0knots
• Tug BP 120Te @ 5.0knots
• Tug BP 220Te @ 7.0knots

(Note: If conditions are benign then surface tow will be faster)

Subsea Deployment Systems Ltd.

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Lerwick 95nm Nigg 210nm Burntisland 270nm

SOME DEMOB LOCATIONS

Bergen 130nm Haugesund 110nm Stavanger 135nm

Subsea Deployment Systems Ltd.

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SCHEDULE

• All preparation works done in advance (not schedule critical)
• No surface operations (less weather sensitive)
• Typical turnaround central North Sea 4 days

Subsea Deployment Systems Ltd.

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CAPEX / OPEX

• CAPEX
• 300Te (wet) capacity approx. £1.5‐2.0m.
• OPEX
• AHT or Tug day rate vs. Construction Vessel time and weather

downtime (can be significant when avoiding to recover to deck)

Subsea Deployment Systems Ltd.

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SAFETY AND ENVIRONMENT

• Low tech / fail safe
• Operations can be suspended at any time
• The overall operation is less sensitive weather
• Structures , spool pieces, mattresses etc. are not recovered to

deck

• Reduces working and transit time for more costly vessel

Subsea Deployment Systems Ltd.