Disputas: Arash Zakeri

Prøveforelesning

Bedømmelseskomité

Professor/Director Mark Cassidy, Centre for Offshore Foundation Systems, The University of Western Australia, Perth, Australia
Professor Emeritus Fridtjov Irgens, NTNU, Trondheim
Professor Henning Dypvik, Institutt for geofag, Universitetet i Oslo

Leder av disputas:  Professor Nils Roar Sælthun

Veileder:  Prof. Kaare Höeg, Prof. Farrokh Nadim

Sammendrag

Estimating the impact forces exerted by a submarine debris flow on a pipeline installed on the seafloor is a challenge. To that extent, the Geoscience Department of the University of Oslo in affiliation with the International Centre for Geohazards (ICG), both located in Oslo, investigated the drag forces on submarine pipelines caused by debris flow impact. To start, an extensive literature review was carried out assessing the state of the art both for subaerial and submarine debris flow impact situations. For the latter, the early investigations were inspired in the wake of the 1969 Hurricane Camille in the Gulf of Mexico which inflicted significant damages to three offshore platforms and the associated pipeline network, and continued into the mid 1980’s. It was concluded that the methods available are limited in terms of practical application and provide a wide range of estimates - hence, considerably more work is required to advance the state of the art.

Later, an experimental program was performed to investigate the impact of clay-rich debris flows on two pipeline installation scenarios: 1) suspended pipeline and 2) laid-on-seafloor pipeline. The program consisted of a series of flume experiments accompanied by Computational Fluid Dynamics (CFD) numerical analyses through which, impact of a subaqueous gravity flow of clay-rich slurries on a pipeline was modelled. A fluid dynamics approach was adopted to analyze the data. The experimental program performed very satisfactorily.

The outcome is a method that makes it possible to estimate the drag force exerted on a suspended pipeline or a laid-on-seafloor pipeline upon impact with a clay-rich submarine debris flow. For suspended pipelines, the method covers all angles of incidence to the pipe axis. The method, which covers a wide range of impact velocities, is considered directly applicable to prototype situations for design purposes. Further, two conceptual mitigation and control measures are introduced for protecting pipelines against submarine debris flow impacts.

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