OUTLINE METHOD

A typical installation will consist of the following steps:

  • Load-out

  • Shallow draught surface tow

  • Ballasting and trimming

  • Deep draught surface tow

  • Submerged tow

  • Positioning

  • Set-down

  • Ballasting

  • Float-off

 

LOAD-OUT
The structure may be loaded-out into the Subsea Deployment Vehicle (SDV) by a variety of methods depending on the available equipment and draught.  Load-out options include:-

  • Direct lift

  • Lift, submerge and float-over

  • Float-over in a dry dock

  • Submerge and float-over using a floating dock or submersible barge

  • Submerged docking


SHALLOW DRAUGHT SURFACE TOW
When there is limited water depth at the load-out location, the SDV and structure will be towed initially in Shallow Draught Surface Tow Mode until reaching a suitable location for flooding the hulls.


BALLASTING AND TRIMMING
When the water depth is suitable, the SDV will be ballasted down by flooding the hulls.  The attitude of the SDV will be checked and adjusted as necessary.


DEEP DRAUGHT SURFACE TOW
The SDV and structure will then be towed in Deep Draught Surface Tow Mode with only the castles and control chain towers breaking surface.

 

SUBMERGED TOW
When the water depth is suitable, the tow vessel will pay out the tow wire and tow chain clump weight.  The tow chain clump weight will cause the SDV to submerge.


The tow vessel can adjust its speed and the length of the tow wire to maintain the SDV at a suitable depth.  The SDV may be lowered as the water depth increases by paying out on the tow wire.  The position and inclination of the SDV will be monitored throughout by acoustic interrogation.


When approaching the field, the tow vessel will slow down and adjust the tow wire to keep the tow chain clump weight off the seabed until in a designated parking area.


The tow vessel will then pay out the tow wire until the clump weight  rests on the seabed.  The SDV and structure will now be “anchored” and float above seabed.


Ideally the length of tow wire between the clump weight and SDV will be marginally greater than the distance between the parking area and the final target location.  This allows final set-down without the need to lift and re-position the tow chain clump weight.

 

VIDEO - Orcaflex Simulation of Submerged Tow - Real Time (58s)

The first half of the animation shows the movement of the surface vessel during tow in 4m regular waves with the SDV at 500m depth. The second half of the animation shows that the response of the SDV is minimal due to the dampening effect of the tow chain clump weight.

 

POSITIONING
The SDV will be positioned by means of two control chains suspended from the installation vessel and lowered into the chain towers. The height of the SDV will be adjusted by raising or lowering the control chains. The position and orientation of the SDV will be adjusted by moving the installation vessel and / or the crane.


In deeper water the tow vessel may also be utilised to aid in positioning and heading control by lifting the tow wire and chain clump weight clear of seabed.

 

VIDEO - Orcaflex Simulation of Lowering SDV Onto Seabed - Real Time - Edited Version (167s)

The animation shows the control chains being lowered into the control towers and the subsequent lowering to the seabed. This is a slow operation and therefore this real time version of the video clip is edited to reduce its length. The initial screen shows the surface vessel deploying the control chains into the control tower. The second screen shows the lowering of the SDV to approximately 10m above seabed. Following screens show the surface vessel stepping 30m to port, then stepping 30m forward and finally changing the heading 45 degrees to starboard. The final screen shows the SDV being lowered onto the seabed and control chains lowered fully into the control towers.

 

VIDEO - Orcaflex Simulation of Lowering SDV Onto Seabed - 30 x time - Full Version (142s)

The animation shows the full lowering sequence described above but at 30x the frame speed.

 

SET-DOWN
Once the DSV is in the correct position and orientation the structure will be landed by lowering the control chains.  The control chains will then be fully lowered into the chain towers and temporarily disconnected.  The control chain connector will be located on the top of the chain tower ready for reconnection after ballasting.


The weight of the control chains will contribute to the initial on-bottom stability, i.e. prior to ballasting.


BALLASTING
Ballast weight will be added to the ballast chain lockers by the surface vessel crane to balance the weight of the structure.  The SDV will then be slightly negatively buoyant and will rest on the structure.  The SDV will now be disconnected from the structure.


FLOAT-OFF
The installation vessel will now re-connect to the two control chains and raise them until the SDV is neutrally buoyant.  Further raising of the control chains will lift the SDV clear of the structure.


The SDV will then be manoeuvred in a near reverse of the installation procedure until clear of any subsea assets.  The control chains will then be removed completely from the towers allowing the SDV to float above the seabed while still anchored by the tow chain clump weight.