In ultra deepwaters, helical tensile armour wires under certain loading conditions may exhibit localized deflection in either the radial (out-of-plane) or lateral (in-plane) direction when subjected to significant high axial compression and bending. This phenomenon is often referred to as birdcage buckling. This paper presents the development of a total strain energy approach for modeling the buckling and post-buckling behaviour of these wires and for illustrating the characteristics of such behaviors. The paper presents the summary of the full scale offshore (Deepwater Immersion) DIP tests performed by Wellstream to date, all of which have been successful in resisting the failure modes. In addition, the results of pressure chamber tests are also presented and discussed, especially some of the key test constraints which can have a significant influence on the final result as observed on a recent 10-inch structure chamber test. The presented model provides a tool to define and implement the design criterions against such failure modes. The further validation of the model through comprehensive tests is planned, which is critical to ensure the ability to economically design and optimize flexible pipe structures to prevent this phenomenon, and to facilitate more cost effective products that meet the ultra deepwater design challenges.
Unbonded flexible pipe has been a proven technology for riser solutions in offshore oil and gas production since the 1970s with over 2000 risers installed. The operating envelope for flexible riser configurations has continually expanded to meet the challenges of both shallow and deep water applications [1]. This paper presents recent innovations in technology for flexible riser solutions to enable oil and gas development in water depths as low as 20m with required system reliability as well as cost effectiveness. For shallow water applications, the traditional technology is the wave or S configuration. S configurations require a structure such as a mid-water buoyancy arch (MWA) to support the riser configuration, which increases the cost of both fabrication and installation. The wave configuration with distributed buoyancy is a more cost-effective approach in terms of installation. The disadvantage of this solution is that the riser could either float to the water surface or sink to the seabed when its content density varies or the floating production, storage and offloading vessel (FPSO) deviates from its nominal mooring position during field production. A new modified wave configuration, referred to as the Weight Added Wave (WAW) configuration (Patent pending) is presented, which enjoys the low installation cost of the wave configuration and performance reliability of the S riser configuration. The WAW configuration has been applied to two FPSO shallow water field developments and the results are presented herein to confirm the solution for real life applications.
Offshore exploration for oil and gas is being performed in even more challenging waters, with fields being developed in water depths of 2000 meters and greater. To recover hydrocarbons from these depths, a number of technical challenges are presented to the designers of riser and offloading systems. Metocean characteristics and relatively low reservoir temperatures compound the challenges. This paper discusses how unbonded flexible pipe technology has overcome the challenges of floating production systems in 2000 meters water depth and how its continued development enables solutions for ultra deep water riser systems. During the discussion of the status of flexible pipe technology, an overview is given to the advancements in flow assurance technology and integrity monitoring, both of which can evolve as integral parts of the flexible pipe. These achievements in technology are applied to the design of large diameter offloading and export systems that are being considered for large field developments where crude oil is being offloaded to other floating structures and tankers. The paper details the design and detailed analysis of the unbonded flexible pipe solutions developed by Wellstream to overcome the deepwater challenges associated with large diameter export riser systems. Design comparisons are developed with the alternative products currently proposed. Finally, the paper concludes with an economic appraisal of why flexible pipe systems should be considered a cost effective solution in the overall CAPEX and OPEX of deepwater fielddevelopments. Introduction The exploration of oil and gas offshore is accelerating to even greater depths with each new discovery. Deepwater drilling in the US GOM reached a record high[1] in the final week of 2000. A total of 40 rigs were drilling in the region compared to 26 a year previous. Activity in the ultra-deep is also growing with seven rigs working at the end of the year. The deepest well was drilled by BHP Petroleum at a water depth of 2,545 meters. The recovery of hydrocarbons at such depths presents challenges to the designers and installers of riser and off-loading systems. Since unbonded flexible risers are an enabling solution for floating production and storage systems, being a proven technology spanning three decades, these challenges have already been overcome for the smaller diameter pipe structures in water depths up to 2000 meters. Continued developments will have much larger internal diameters qualified to these water depths in the near future. Unbonded flexible pipe is a multi-layer structure of helically wound metallic wires, tapes and extruded thermoplastics. Each layer has a unique role in the flexible pipe. Refer to Figure 1. The inner metallic layer, which is interlocked, provides the collapse resistance. An extruded thermoplastic layer provides the seal for the fluid to be transmitted through the pipe. An interlocked steel layer wrapped over this thermoplastic layer provides resistance to internal pressure and radial compression. A dual layer of counter-wound tensile armors provides the tensile strength to the pipe structure.
Unbonded flexible pipe has been successfully operating in oil/gas production applications on the North Slope of Alaska since 1995 as documented in OMAE 1995 paper “Arctic Applications of Flexible Pipe for Production Pipelines and Infield Fluid Transport Systems”, Reference [1]. The pipe and end terminations were subjected to extensive material and full scale testing to confirm suitability for arctic applications and the results are documented in the paper. The products deployed on the tundra were designed to ensure successful, safe operation and mitigate risk for the initial application. The robust design included pipe layers and end fitting materials that exceeded fit for purpose needs and resulted in a heavier, more costly design than was actually needed. To ensure cost effective solutions are available for future projects in the regions, Wellstream in conjunction with the Prudhoe Bay Operating Unit developed an optimized flexible pipe that met the onerous requirements of arctic applications. Pipe specifications included 5.5-inch ID, 3600psi design pressure, thermal loss and cooldown limitations, maximum operating temperature of 140°F and minimum operating temperature of −50°F. This paper will present the results of the design, engineering and analytical process, full scale testing and installation activities. A detailed comparison between the initial products and the current optimized design is presented to illustrate the substantial weight and cost savings achieved.
Offshore developments must evolve if the industry is to unlock subsea reserves and increase recovery. A novel method of achieving these goals is through use of a systematic approach to subsea tiebacks that combines new technologies with minor modifications to the existing topsides equipment. The tieback strategy is a system-based approach with a combination of qualified disruptive technologies and field-proven solutions that will improve costs, reduce the number of interfaces, minimize the modifications needed on the topsides, critical for platforms with space and weight limitations, and maximize the use of existing assets. Significant investment was made to qualify disruptive and groundbreaking technologies to make this possible. The following summarizes the main components of the subsea system: –Seabox subsea water treatment and injection provides higher quality water for reservoir injection and increased recovery–Subsea chemical storage that allows longer subsea tiebacks and mitigates weight and space limitations on topside structures; only power and communication are needed from the topside–Subsea treatment of produced water subsea for either discharge directly to sea or to re-inject–Subsea automatic pig launcher combined with a single pipeline that ensures continuous flow at a lower capex–All-electric controls and valves eliminate the need for utility pipelines and expensive umbilicals–Applying field-proven tie-in systems integrated with flexible pipe solutions and customized subsea structures In addition to exploring the above components, this paper also explores combining the components into a comprehensive system. The system-based approach will unlock previously uneconomical reservoirs.
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