An Extendable Draft Semi-submersible (EDS) is a low-motion floating production system concept focused on flexibility and low cost. An Extendable Draft Semi-Submersible (EDS) can support full drilling and production facilities that can be prepared for deployment at a dockside facility with limited water depth. The EDS combines the favorable features of deep-draft Semi-submersibles, Truss Spars and other reduced-heave floaters, making its in-place motions superior to shallower-draft floaters for supporting dry-tree risers and SCRs.
EDS is a self-installing design, which delivers and deploys a fully operational system without the aid of crane vessels. The EDS structure consists of a floatable deck, stabilizing columns and heave plate/pontoon connected to the bottom of the columns. The barge-type deck is designed to float the entire structure, including topsides, to the near shore site. Upon arrival, the columns and heave plate/pontoon is lowered down using strand jacks either in a single or dual direction mode and locked into the deck to the locking draft. The use of strand jacks allows the lowering operation to be performed in a higher sea state than normally used for offshore installation thereby increasing the operability. The deck is then raised to the operating elevation by de-ballasting the columns after pawls are engaged between deck and columns for rigid connection between them. The pawl connection between columns and the deck is reversible and the columns can be raised above the deck to enable the platform to be towed to a new location for redeployment or brought to quayside.
An alternative to the self-installing design is installing the topsides by a floatover technique, similar to what has been done for several semi-submersibles in Brazil.
The EDS motions are suitable for Dry Tree application and SCRs in different areas around the world including areas with severe storm environment like Gulf of Mexico, Weastern Australia, etc. The concept provides flexibility in constructing the EDS that facilitates a high level of local content in most areas of the world.
This paper describes the features of the EDS platform along with the fabrication and deployment offshore.