Click on the illustration above for descriptions of the key SDV components.



Lengths of chain suspended from the surface vessel are lowered into the chain towers to perform two main functions. Primarily the weight of the chain supported at the base of the chain towers is used to make the SDV neutrally buoyant. Secondly the length (weight) of chain suspended within the chain towers provides lateral control of the SDV. The position and orientation of the SDV can be adjusted by moving the surface vessel.



The SDV is designed to be inherently stable throughout all stages of submergence. During shallow draught tow the hull spacing will provide good stability with the metacentre well above the CoG. When the SDV is fully submerged the CoB is well above the CoG.



The interface connection between the subsea deployment vessel will vary depending on the structure to be deployed and the load-out method. It may be in the form of conventional slings, hydraulically operated pins or by means of a propriety connector such as Ballgrab.


In most cases this will involve the use of project specific interface beams which are attached to the structure at the design lift points and to standard connection points on the SDV. The interface beams may also serve as lifting beams if required by the load-out method.



The relationship between the forces acting on the SDV and structure during various stages of operation is demonstrated by the following example.

Assume a SDV with 520t net buoyancy and a 500t structure.


During submerged tow the SDV provides 20t excess buoyancy which is balanced by the weight of the tow chain.
During installation the tow chain rests on the seabed and the excess buoyancy of 20t is balanced by the control chains.


During the initial set-down an additional 40t of control chain is lowered into the chain towers.  This is balanced by a 40t reaction from the seabed.  This reaction also reduces the load on the connectors between the SDV and structure from 500t to 460t.


Ballast matching the weight of the structure, i.e. 500t, is added to the SDV ballast lockers.  At this point the SDV with ballast is 40t heavy and will rest on the structure.  The full weight of the structure plus the excess ballast is resisted by the seabed.  With the SDV resting on the structure the connectors between SDV and structure can be released.


The surface vessel then reconnects to the SDV control chains and raises until only 20t of control chain is supported by SDV.  Further raising of the control chain will lift the SDV clear of the structure.



The two main SDV configurations are square and U-shaped which facilitate different load-out methods. Both designs are constructed from modular units which can be varied to suit the required capacity.


U-shaped (left) and Square-shaped (right) SDV

The U-shaped SDV is a more compact design and typically has greater carrying capacity than the square shaped SDV.  However, this configuration requires a greater water depth since the structure is located below the SDV.


The square shaped SDV can accommodate the full foot-print of the structure between the hulls and cross / interface beams. This results in a shallower draught than for the U-shape and would typically be used where there is a limited water depth at the load-out location or along the tow route. However, this design has less carrying capacity due to the relative heavy cross / interface beams.