The SDS Skip. Subsea Deployment Systems Ltd.

Transcription

1 The SDS Skip

2 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 2

3 SKIP 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 3

4 SKIP The SKIP consists of LOWER 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 4

5 SKIP 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 5

6 SKIP The SIDE 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 6

7 SKIP 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 7

8 SKIP The CONTROL CHAINS are lowered into chain CONTROL TOWERS 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 8

9 SKIP The TOW CHAIN CLUMP WEIGHT 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 9

10 HOW SDV WORKS 10

11 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 11

12 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 Width Depth Weight Capacity (sub) 32m 16m 8m 300Te 300Te Depth rating* 150m * May be deeper if upper tubular pressurised 12

13 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 13

14 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 14

15 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 off topside of Skip (no need to enter hold) (same rigging / cargo nets would be used for offloading onshore) 15

16 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) 16

17 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 deballasting 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 17

18 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 18

19 SURFACING INTEGRAL SKIP Remotely operated (Valves will be operated remotely for de ballasting) Hold (water can be pumped out of the hold using pressurised air from lower tubular) Side and Double Bottom Tanks (high point open to the sea) Upper tubular (fully sealed, buoyancy equals submerged weight of skip) Lower tubular (c/w ballasting facilities) 19

20 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 SURFACING OF THE SKIP 20

21 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 outside water level SURFACING OF THE SKIP 21

22 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 SURFACING OF THE SKIP 22

23 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 SURFACING OF THE SKIP 23

24 A positive separation between CoG and CoB when submerged Flooded side tanks ensure that hull free surface effects during ballasting operations do not cause instability during surface operations SKIP STABILITY 24

25 TOW FORCES 300Te SDV Skip: Tug BP 3.0knots Tug BP 5.0knots Tug BP 7.0knots (Note: If conditions are benign then surface tow will be faster) 25

26 SOME DEMOB LOCATIONS Lerwick 95nm Bergen 130nm Nigg 210nm Burntisland 270nm Haugesund 110nm Stavanger 135nm 26

27 SCHEDULE All preparation works done in advance (not schedule critical) No surface operations (less weather sensitive) Typical turnaround central North Sea 4 days 27

28 CAPEX CAPEX / OPEX 300Te (wet) capacity approx m. OPEX AHT or Tug day rate vs. Construction Vessel time and weather downtime (can be significant when avoiding to recover to deck) 28

29 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 29

30