This paper discusses the key challenges associated with the design of a Dry Tree Semi (DTS) global configuration with robust performance and easy execution, as well as the integration of the topside facilities with hull at quayside. The authors will address the issues associated with a safe wellbay design in order to satisfy all functional requirements of simultaneous drilling and production operations, and compensation of riser stroke. More importantly, the authors will present technically robust and cost-effective solutions to overcome these challenges through extensive engineering assessment, computer simulation, and model test verification. The authors focus on improving functional and operational safety of the DTS system while minimizing project execution risks. Through innovative design, engineering analysis, and model test verification of the DTS system, important and valuable results will be presented in this paper, which include overcoming various conflicting issues associated with wellbay arrangement and functional performance under simultaneous drilling and production operations; compensating for the most challenging issue of riser performance on a DTS utilizing long stroke riser tensioner technology; and deriving a hull-mooring-riser global configuration that not only carries robust performance during operation, but also offers the benefits during project execution.

The use of semi-submersible platforms has become increasingly popular due to its ability to carry large topsides and the possibility for quayside integration. With recent exploration successes in ultra-deepwater fields of the Gulf of Mexico, major oil and engineering companies are keen to look for a safe, reliable and cost-effective dry-tree option to maximize the value of deepwater field developments. Dry-tree semi-submersible (DTS) emerges as such an option to overcome the water depth and size limits imposed by TLP and Spar, respectively, and enables the platform to carry a large well array and payloads in ultra-deep water. This paper presents the offshore industry’s multi-year efforts to mature two promising semi-submersible platform concepts that can accommodate long-stroke dry-tree risers and have large drilling and production capabilities. Results of technology development and qualification will be highlighted with details on hull performance and hull/riser interfaces. Key structural, mooring and riser analyses and scaled model test results including the long-stroke riser tensioning system will be presented. Remaining challenges that need to be overcome to advance the DTS concepts from “technology acceptance” to “project readiness” will also be discussed.

In continued pursuit of safe and economic field development solutions in deep and ultra-deep waters, the authors in this paper present a project oriented and technology robust dry tree semi-submersible floater concept. The proposed dry tree floating production and drilling system is based on the proven technology of a conventional four-column deep draft semi-submersible and comprises topside facilities, hull and mooring system, top-tensioned risers, and steel catenary risers.The paper discusses engineering analysis, computer simulation, and model test validation carried out in the investigation of the proposed concept. Sample design cases for ultra-deep water fields in the Gulf of Mexico are presented, where the authors discuss design philosophy, global configuration, and coupled responses of the dry tree semi-submersible hull with mooring and riser system. Safe design and layout of the wellbay hosting the top-tensioned riser system and its interfaces with hull and topsides are addressed as the core of the concept technology. In addition, constructability of the proposed dry tree semi concept is discussed by outlining project execution options under realistic infrastructure and facilities constraints during project construction, integration, and installation phases. Through engineering design, analysis and verification, the authors demonstrate robust performance and cost-effectiveness of the proposed concept, particularly the attractive quayside integration and commissioning. In summary, this paper presents a project oriented dry tree semi-submersible floating system concept with potential improved cost efficiency and reliability over current technology.

This paper discusses how to enhance air gap design and assessment for the deepwater floating systems. The authors first establish an analytical method to predict air gap considering global upwelling due to wave diffraction, as well as random wave effects. Then the authors apply the analytical method to simulate the air gap of typical deepwater floating systems through uncoupled analysis combining frequency domain and time domain global motion analysis. For comparison, a coupled analysis with hull structure, mooring and riser system is also carried out. The authors then look into model test verification and correlation analysis of air gap prediction, especially how to deal with random motion effects on deriving the extreme values of air gap. Furthermore, the authors illustrate how to improve deepwater floating system performance and cost-effectiveness by utilizing a practical and reliable air gap design approach.The air gap simulation results derived from the proposed analytical procedure correlate well with the wave basin model test results. This analytical procedure has also been applied to deepwater floating system design for field development projects, which can be considered as a practical and reliable air gap design solution for optimizing global configuration to improve deepwater floating system performance and cost effectiveness.