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...
Pipeline Vibration Dampers (PVDs) have been utilized to mitigate wind induced vibrations for above-ground pipelines in the Arctic Circle for decades. Although such pipeline vibrations are relatively small, the accumulation of vibration cycles can cause excessive fatigue at pipeline joints. This paper considers the application of PVDs to help suppress vortex induced vibrations (VIV) of subsea pipelines, and presents an analytical method for estimating PVD weight, damping, and tuning frequency to provide optimal vibration reduction. The paper also reviews some of the fundamental VIV concepts and provides an overview of the PVD behavior and equations of motion. Akin to the above ground implementation, each PVD may have to be self-contained in a small pressure vessel for the subsea application in order to avoid hydrodynamic interactions with the surrounding sea water and ensure the PVD mechanical performance. The PVD devices may be used exclusively for VIV mitigation, or they may be used in combination with helical strakes to help suppress VIV.
A study is presented of the potential capacity of pipeline vibration dampers (PVDs) to mitigate vortex induced vibration (VIV) for subsea pipeline spans. The study also makes evaluations for VIV fatigue damage reductions that could be accomplished by PVDs for the spans. While it is recognized that helical strakes and other similar VIV suppression devices are widely used, PVDs are studied herein as a potential standalone option for VIV mitigation or a tool that can be used in combination with strakes. Solutions are presented which indicate that PVDs can substantially reduce VIV of subsea pipeline spans, and there is potential to develop them into practical design tools for such spans. The study is based on the use of representative subsea pipeline span conditions.
Suspended from the Atlantis Production Quarters Semi-submersible in over 7000 ft water depth, the 24-inch diameter Mardi Gras Oil Export Steel Catenary Riser (SCR) presented many challenges for design and installation. In particular the touchdown region of the riser was subject to severe fatigue and extreme loadings. The Engineering Critical Assessment (ECA) at the weld OD generated defect acceptance criteria that would be impractical to apply with the available inspection technology. To mitigate this, the decision was made to remove the weld caps in the most critical portion of the SCR. This paper describes the design drivers that led to this decision and the steps taken to ensure successful implementation.
Metocean conditions and connections to dynamic floating facilities make riser systems for deepwater projects among the most challenging equipment to design and fabricate. The ends of the risers experience substantial complicated fatigue and tensile loads, and when production fluid includes hydrogen sulfide (H2S) the riser materials' resistance to fatigue and fracture can be diminished. The riser girth welds, in particular, must be specially designed and qualified on a project-toproject basis.Forged high-strength steel tapered stress joints at the ends of free-standing risers address the high tensile loads and manage the fatigue demand. However, the girth welds connecting the tapered stress joint to seamless pipe must be specially qualified for fatigue. This paper will discuss the design, qualification and fabrication of the tapered stress joint girth welds for superior fatigue and fracture performance in mildly sour conditions for service in deepwater regions, like the Gulf of Mexico. The tapered stress joints were of 80ksi A182-F22 material, and the pipe was API 5L X70. Qualification included an elaborate testing program of the sensitivity of welding parameters, such as post-weld heat treatment, on weld hardness required for mildly sour service and pipe yield and tensile strength. Full-scale resonant fatigue testing demonstrated reliable C-class fatigue performance. In addition, a thorough engineering critical assessment (ECA) provided a basis for NDE system design and flaw acceptance criteria.
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