Design and installation aspects in the use of high strength steels for deep and ultra-deepwater pipelines

The economics of large diameter, deepwater pipeline systems is governed mainly by installation considerations. Major technical difficulties need to be overcome in order to make laying of pipelines in deep and ultra-deepwater an economically viable proposition. This paper examines cost-effective methods for designing and installing pipelines in water depths down to 3500m. Design issues associated with hydrostatic collapse, local buckling, propagation buckling and on-bottom stability are discussed. The effects of fabrication tolerances such as ovalisation on hydrostatic collapse and the design of buckle arrestors are also considered. Additional benefits that may be gained from the use of high strength steels, such as X100 and X120 that are currently being developed by industry are investigated. The mechanics of propagation buckling are reviewed along with the types of buckle arrestors available for different installation methods. The need to satisfy on-bottom stability requirements and the effects of specific gravity (SG) on buoyancy are examined in conjunction with higher-grade materials. Installation of large diameter pipelines are reviewed in terms of S-lay and J-lay methods. The driving forces and limitations are assessed in terms of vessel capabilities. In particular, tensioner capacities are examined in light of the considerable weights having to be supported by vessels operating in deepwater. The possibility of designing and installing deepwater pipelines using higher-grade materials such as API 5L X100 and X120 are addressed in terms of increased strength and fabrication tolerances. The impact of major design aspects, such as hydrostatic collapse, propagation buckling, and on-bottom stability are analysed with respect to both S-Lay and J-Lay installation. A parametric study is included to assess the layability of pipelines in deep and ultra-deepwater by both S-lay and J-lay methods. It is concluded that the availability of higher-grade materials and the upgrading of existing vessels in terms of tensioners and mooring systems together with the use of, buoyancy aids and advanced NDT and welding techniques and improved manufacturing procedures to control fabrication tolerances can meet the future demands posed by the ultra-deepwater pipelines. However, these aspects need to be developed further.