The offshore industry anticipates the need for production riser systems in ultra-deepwater fields where water depths are between 3,000m to 4,500m. The development of ultra-deepwater fields leads to many challenges on the selection of the riser concept and in some instances such applications may require extending riser technology beyond its current limits. Consequently, there is a need to understand the feasibility of riser systems in such ultra-deepwater applications and the technology gaps that exist. In ultra-deepwater, long suspended riser lengths will significantly increase the riser weight potentially leading to challenges with offshore installation related to laying vessel capability. High external and internal pressure on the riser will lead to the need for heavy wall pipes. Thick-wall riser pipe will bring about riser design challenges in fabrication of pipes, riser pipe welding, riser hang-off system selection, long-term fatigue design, and fabrication of specialty riser joints. By looking into these challenges, it is very important to select the most appropriate riser concept for the ultra-deepwater fields and understand the current pipe manufacturing limits, enabling technology needed for such systems, and technology gaps considering the critical points mentioned above. This paper addresses the key riser design issues considering wall thickness sizing, top tension, axial dynamics, selection of pipe material, design of key components, and installation issues. This paper evaluates feasibility of a number of production and export riser configurations for ultra-deepwater applications based on existing technology, identifies current technology limits, and determines technology gaps that exist. Methods of advancing the current capability to meet the requirements of frontier deepwater applications are also proposed in the paper.
For ultra-deepwater subsea wells, a riser system is required to conduct completion, intervention/workover and end of life activities. For ultra-deepwater riser systems with high temperature and pressure requirements, the intervention riser system often requires vessel interface optimization to achieve acceptable design response. The upper riser can be configured in several different ways, each with its own benefit from a safety, risk and performance perspective. This paper compares the riser response for various vessel interfaces for ultra-deepwater applications. As discussed above, intervention riser structural response is sensitive to the riser configuration at the vessel interface. For a typical intervention riser, due to ultra-deepwater and high tension requirements, the functional tension load may utilize up to 40% of yield strength thus decreasing the capacity available to accommodate bending and pressure loads. Vessel operators have options to modify the system configuration to improve the strength and fatigue response of the riser. The different vessel interface options include the tension lift frame (TLF) to vessel interface, the top tension application method and the use or otherwise of a surface tree dolly. Upper riser assembly (URA) loads may be optimized by use of rotary wear bushings, a cased wear joint assembly or flexjoints as a part of the stack-up. The various riser-vessel interface options are evaluated and compared in this paper. This paper highlights the riser design challenges for ultra-deepwater applications.
Coiled tubing (CT) is being increasingly used in open water mode for offshore light well intervention such as subsea hydraulic pumping applications. Traditionally coiled tubing has been popular in land based intervention applications; whereas for offshore applications using a CT deployed through a riser (in-riser mode) is very common. However more recently, light well intervention (LWI) operations with CT deployed in open water mode are gaining traction due to improved efficiencies compared to traditional intervention methods. Coiled tubing systems are an integral part of a LWI system and are used for injection and hydraulic pumping operations. In open water mode coiled tubing pipe is susceptible to direct hydrodynamic loading from waves and currents and vessel motions. The strength response and fatigue performance of the coiled tubing pipe can severely limit operability and increase down time for these operations when compared to riser based operations. In this paper we will present a case study where coiled tubing has been used for LWI and subsea pumping operations. The paper will highlight some of the key challenges in design and operation of open water mode CT systems for offshore applications, from a loading standpoint and will also discuss challenges arising from lack of industry standards and codes. Analysis methodology and outcomes from this study will be presented to demonstrate how the CT strength response limits operations. Multiple mitigation options that were used to enhance operability will be discussed: these include judicious use of operational parameters, field measurement based environmental data and pipe depressurization to attain feasibility in harsh environments. In addition, modeling refinements based on 3 Dimensional (3D) Finite Element Analysis (FEA) of the CT injector guides and strain based design criteria will be discussed. The paper will include recommendations based on experience from these case studies and highlight the need for a common industry standard to better assist Operators and OEMs with future designs.
Subsea rigid jumpers are designed to meet numerous criteria including thermal and pressure effects, environmental and riser / pipeline interaction loads, slugging, and other field specific requirements. Jumper VIV can be a concern in fields with strong bottom currents. Without the benefit of detailed VIV fatigue analysis, designers must rely on experience and engineering judgement on placement of strakes if VIV is identified as a concern. VIV mitigation is even more challenging because the jumpers can contain numerous long and short design options to accommodate tolerances for subsea well locations and installation tolerances of subsea PLETS and manifolds. This paper will discuss a case study on optimization of 12 M-shaped jumpers designed for a sour service application in Gulf of Mexico. VIV fatigue assessment of the preliminary jumper design and the methodology adopted to optimize the jumper design and placement of VIV suppression will be discussed. Challenges in meeting high target fatigue life due to sour service application will be discussed. The key challenges whilst optimizing an acceptable VIV suppression solution are the assumed effectiveness of strakes, cost / available inventory of strakes, and physical limitations for placement of strakes. This paper will highlight the trade-offs that are required to strike a balance between strength and fatigue design requirements when using straked buoyancy modules vs. regular strakes. The paper will also highlight the current limitations in design code that relies on standards developed for pipeline application. An alternative method / modification to the DNV F105 approach used to calculate the cross-flow induced in-line VIV fatigue damage is also discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.