MONACO -- Attendees of the DOT conference today were presented with an overview of the “E-Semi”, an extendable draft semisubmersible. Conference presenters Anis Hussain and Edwin Nah of the Deepwater Technology Group at Keppel FELS and John Murray and Christopher Barton of FloTEC demonstrated that the E-Semi has motion characteristics close to that of a SPAR when the second tier pontoon (STP) is extended, and is able to maintain the draft of a conventional semi-submersible when the STP is retracted.
Such flexibility allows deck integration/installation to be done at the quay side or shallow water near shore. Unlike a Spar, E-Semi is re-deployable once it has completed its mission at a particular field, since the reduced draft allows it to be dry transported to an alternative site. The configuration of the hull and topside is such that it allows independent parallel construction of the hull and the topside at potentially different locations leading to ease in construction planning and execution. Integration of the topside with the lower hull at the quayside also removes the offshore installation operation and eliminates the requirement of a favorable weather window for topside installation thereby reducing the risk of waiting on weather which results in reduced overall installation costs.
This paper provides an overview of the complete design and development work inclusive of model tests that have been done in the evolution of the ultra deepwater dry tree production unit, E-Semi. Providing new details on previous presentations about the technology, the presenters gave an update on the status of the on-going work. Results on structure strength, such as global strength analysis, fatigues screening, local strength analysis of critical connections were discussed; results from stability analysis, such as Intact, damaged stability and stability during deployment of STP were discussed and the plan for ultimate strength and fatigue test of the grouted critical connection was presented.
The extendable semisubmersible is a hybrid, conventional and truss structured semisubmersible. It is aimed at providing a solution for development of small marginal fields that are located in deep waters and harsh environment. The E-Semi consists of an upper hull similar to a typical ring pontoon semisubmersible hull and has square stabilizing columns at the corners of the ring pontoon. It has retractable extension legs that connect the upper hull to a Second Tier Pontoon (STP). In operational draft, the STP will be extended below the upper hull baseline. During transit, the STP would be fully retracted.
The configuration of the hull and topside is such that it allows independent construction leading to ease in construction planning and execution. The entire unit consisting of the topside, upper hull and STP can be constructed, integrated and commissioned at Keppel FELS yard. This significantly shortens the time line from project start to first oil. One main reason for this would be the fact that being able to integrate the topside to the upper hull and Second Tier Platform (STP) at KFELS quayside effectively removes the weather window that in many instances constrains the integration time on location. If compared to other similar concepts, there is a saving of up to 12 months in terms of project execution time. Fabrication time for the hull of other concepts would take approximately 18-20 months and about three years from project start to installation of hull on site.
The attractive advantages of the E-Semi are as follows, according to the paper’s presenters:
•Hull and Topside integration, commissioning inshore: much time and cost is saved by the fact that the structural configuration of E-Semi allows hull and topside integration possible inshore. This significantly reduces time and cost as compared to offshore integration
• Lower Motions Characteristics: The motions of the E-Semi are much lower than that of ships and semi type hulls. This translates to less “down-time” and riser fatigue, the authors say
• Dry Trees: With better motion response, the E-Semi permits top-tensioned risers with dry trees, which lowers intervention costs and work-over time.