Technical Advances in the Design and Installation of Large Diameter Offshore Gas Trunklines

摘要

The world's sources of natural gas tend to be remote from the end users. Large diameter gas trunklines offer an efficient means of transporting natural gas from source to end user. This paper looks at technical advances in the design and installation of large diameter offshore gas trunklines, installed in both shallow and deep water. The importance of route selection is weighed up in terms of economics and its effect on the environment. The selection of material grade and the consequences on trunkline design are assessed, together with the wall thickness and mechanical design. On-bottom stability requirements are addressed in terms of analysis and concrete coating requirements for stability. The static and dynamic assessment of pipeline freespans is covered together with possible lateral and upheaval buckling and the need or otherwise for seabed intervention works. Fracture and fatigue is examined with regard to allowing movement of a trunkline. The principal approaches to trunkline installation are covered in terms of S-lay and J-lay, as well as vessels available in the industry. Key issues concerning installation of large diameter pipelines are addressed, together with tie-ins an emergency pipeline repair systems. Approaches to risk, safety and protection are addressed as well as testing and commissioning. Over the past three decades, the offshore pipeline industry has made significant advances in the design and installation of large diameter trunklines. The technology has been developed using advanced analytical tools to lay trunklines of 48" diameter and larger in shallow to medium water depths. In deep waters, the technology exists to lay pipelines in water depths down to 2,500m and beyond. Automated design tools have enabled more effective pipeline routing to be achieved, and attention to environmental impact issues have meant that pipeline projects now meet current demands for zero emissions. Mechanical design of large diameter trunklines has progressed through the adoption of limit-state design methods and application of advanced fatigue and fracture assessment to make full utilisation of linepipe material. On-bottom stability under extreme storm conditions can be achieved by using fit-for-purpose designs while whilst maintaining safety levels by use of reliability methods. Pipeline free spanning may be assessed via dynamic fatigue analysis using advanced analytical tools and methods developed to minimise seabed intervention works. There is still, however, scope for further advancement in the understanding of pipeline behaviour under vortex induced vibration. There is potential for upgrading existing vessels to improve layability in terms of both pipeline diameter and water depth. However, there is still work to be done to increase the size of gas trunklines to beyond 30" diameter that can be laid in deep waters. Production of higher strength steels and welding procedures, testing and pipeline installation, for example, still require further advancement. The oil and gas industry has responded to the demands for gas transportation through advances in technology and will continue to do so as trunklines move into more hostile environments.