Assessment of Free Spanning Pipelines Using the DNV Guideline

摘要

This paper addresses free span assessment of pipelines both in the design phase and during operation. The assessment is based on the new DNV Guideline for free spanning pipelines. Free spans often become a problem both in the design phase and during operation of pipelines. The spans may develop due to several reasons such as installation on uneven seabed or due to scouring effects. Such spans may be subjected to fatigue damage caused by direct wave and vortex induced vibrations (VIV). Despite of the significant costs involved with free spans, they are normally designed according to unduly conservative concepts and very simple analytical methods. An official DNV Guideline (draft version) for free spanning pipelines has been issued in December 1997. The objective of this guideline is to provide rational state-of-art design criteria and guidance on fatigue design, methods for free spans subjected to combined wave and current loading. It is based on the technical development within free span technology in recent R&D project as well as design experience in recent and ongoing projects. This paper gives a brief introduction to the DNV Guideline. Then strategies for different phases with respect to free span design and span intervention are discussed. Some examples are presented to demonstrate the use of these strategies in connection with the calculation methods and acceptance criteria given in the guideline.