Located in 8,200 feet water depth, the Cascade & Chinook subsea development risers are the deepest production risers in the world. These are also the first offset free-standing hybrid risers to be installed in the Gulf of Mexico, and the world's first use of hybrid risers in combination with a disconnectable turret-moored FPSO.This paper presents the unique benefits and challenges associated with using hybrid riser tower technology in the deepwater Gulf of Mexico, based on the experience gained by both client and contractor in executing the Cascade & Chinook riser project.The authors review the history of the Cascade & Chinook riser system, from concept selection through engineering, procurement, construction, installation and pre-commissioning. The most significant challenges and the key project management decisions are highlighted, covering: field layout considerations; strength and fatigue performance; design verification; interfaces with the production facility and subsea infrastructure; procurement and fabrication logistics, and spare materials philosophy; installation engineering and execution; safety and quality management; and regulatory compliance. The project management strategy, and the relationship between client and contractor through the project life cycle, are presented and critically assessed.The paper concludes with an evaluation of the delivery time, cost and project execution risks associated with the hybrid riser concept, and compares this with more traditional deepwater riser solutions, in the context of a deepwater Gulf of Mexico development. OTC 21857"bottom current", which actually extends over several thousand feet up from the seabed, is important with respect to vortexinduced vibration (VIV) and umbilical-to-riser clashing. Figure 1: Cascade & Chinook Phase I Concept SelectionThe Cascade & Chinook development will utilize an FPSO that can be disconnected from the mooring system 6 . This allows the FPSO to sail away from the danger of approaching tropical storms or hurricanes. When released, the disconnectable turret buoy descends to a neutral depth of more than 120 feet below sea level where it, together with the connected mooring lines, risers and umbilicals, are sheltered from the extreme wave forces of such storms.Production facility type and water depth are generally two key parameters for riser concept selection and this was the case for Cascade & Chinook, i.e., the deepest risers in the world combined with a disconnectable turret-moored FPSO. At the time of concept selection, flexible pipe was not qualified for the water depth. While steel catenary risers have been successfully used with semi-submersible and spar hulls in the Gulf of Mexico, and with spread-moored FPSOs in West Africa, they were considered to present too much risk for the Cascade & Chinook project for the following reasons.• Early studies of fatigue performance, and review of literature investigating use of SCRs with an FPSO in the Gulf of Mexico 7 , indicated that it might not be possible to achieve the required s...

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This paper was selected for presentation by the OTC Program Committee following review of informa!lon contained In an abstract submined by the author(s). Contente of the papar, as presented, have not been reviewed by the Offshore Technology Con farenca and are subject to corraotlon by the author(s). The material, as praaanted, does not necessarily reflect any position of the Offshore Technology Conference or ite officers. Permission to COPYis restricted to an ebstrsct of not more than 3C0 words. Illustrations may not be copied. The abstract should centakr censplcuoua acknowledgment of where and by whom the papar is prasenled. ABSTRACT This paper discusses the factors that operators considering FPSO developments in harsh environments should evaluate when selecting between a permanent or disconnectable Single Point Mooring system. The paper first reviews the state of technology for both mooring types, focusing on turret-type systems. A global approach is presented by defining the quantitative and qualitative factors that must be considered in the selection and how each one can be addressed.These factors include mooring system size, green water, tanker hull longitudinal strength, system cost and schedule, reliability, safety, risks to project schedule and cost, and risk of operating downtime.The sensitivity of mooring system feasibility, size and cost to water depth is quantified.

This oaoer was resented at the 19th Annual OTC in Houston. Texas. Aoril27-30. 1987. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. ABSTRACTI n mid 1987 a 250 000 DWT converted t a n k e r w i l l be connected, through a r i g i d moorimg yoke, t o an a r t i c u l a t e d column which was i n s t a l l e d d u r i n g October 1986 i n t h e VEGA f i e l d o f f s h o r e from S i c i l y . T h i s ARC-YOKE Mooring System i s equipped t o handle an estimated p r o d u c t i o n o f 60 000 b a r r e l s per day w i t h a system a l l o w i n g a d d i t i o n o f d i l u e n t t o t h e crude c i r c u l a t i o n i n o r d e r t o a v o i d clogging. The Paper g i v e s a complete d e s c r i p t i o n o f t h e system and some d e t a i l s o f t h e f a b r i c a t i o n o f t h e most noteworthy components, such as t h e l a r g e -s i z e u n i v e r s a l and t r i -a x i a l j o i n t s . A l a r g e p a r t o f t h e paper i s a l s o devoted t o column i n s t a l l a t i o n , which r e s o r t e d t o o r i g procedures i n order n o t t o r e q u i r e t h e need underwater assistance. t h e i n a l f o r