Full-Scale Physical Modeling of Pipeline Instability on a Sloping Seabed
Authors: Gao Fuping , Cao Jing , Han Xiting , Sha Yong , Zhang En-yong , Wu Yingxiang , Cui Jinsheng 
Published: May 2011

For the oil and gas exploitation in deep waters, e.g. the continental slopes at South China Sea, the seabed slope would have influence on the pipeline on-bottom stability. In this study, an actuator-driven pipe-soil interaction facility has been specially designed and constructed, for full-scale physical modeling of the current-induced pipeline instability on a sloping sand-bed, including upward instability and downward instability. Based on dimensionless analyses, an ultimate lateral-soil-resistance coefficient is proposed to describe the pipe-soil interaction, and a series of tests have been conducted to reveal the mechanism of pipe instability on a sloping sand-bed. Experimental results indicate that, sand-bed slope angle, pipe's submerged weight and end-constraints have much effect on the pipe stability on a sloping sand-bed.

Introduction

As more and more deepwater oil/gas fileds have been recently found at the continental slope of South China Sea (SCS), the stability of deepwater pipelines on a sloping seabed attracts much attention from the engineering designers and researchers. The seabed in SCS holds rich varieties of its topographic feature including the vast continental shelf, the continental slope and deep sea basin. At the continental slope of SCS, the water depth is generally between 150 to 3,500 meters, and the seabed slope angle changes much at various locations, e.g., the slope angle ranges from 2 to 4 degree at the lower continental slope of northern SCS, and ranges from 3.2 to 7.8 degree at the lower continental slope of southern SCS (Liu et al., 2002). Nevertheless, the influential factor of seabed sloping angle has not been involved in the existing design codes or recommended practices regarding the pipeline on-bottom stability (see, Det Norske Veritas, 2007)

To avoid the occurrence of pipeline on-bottom instability, i.e. the the pipe breakouts from its original site, the seabed must provide enough soil resistance to balance the hydrodynamic loads upon the untrenched pipeline. The pipeline on-bottom stability involves a complex pipe-soil interaction process. Since the 1980s, mechanical actuator experiments have been carried out and some pipe-soil interaction models were proposed to predict pipeline stability induced by the ocean wave in shallow waters (e.g., Wagner et al., 1987; Brenodden, et al., 1989; Zhang, et al., 2002; Teh et al., 2003).

As the oil and gas exploitation moving into deeper waters, ocean current becomes the prevailing hydrodynamic load upon submarine pipelines. For describing the current-induced pipeline lateral stability, Gao et al. (2007) conducted a series of flume tests and established an empirical relationship between the dimensionless submerged weight of pipeline and Froude number. Recently, pipe-soil interaction mechanism for steady flow-induced instability of a partially embedded pipeline was further investigated experimentally by employing mechanical actuator to simulate hydrodynamic loads on the pipe (Gao et al., 2011). Nevertheless, the aforementioned studies focused mainly on pipeline on-bottom stability on the horizontal seabed. The effect of seabed slope angle on the pipeline on-bottom stability is far from being well understood.

In this experimental study, the pipeline on-bottom stability on a sloping sandy seabed is investigated with a novel pipe-soil interaction facility, which is capable of modeling the prototype-size pipeline (diameter from 0.2 up to 0.5 m) on an inclined seabed with the slope from 0 to 30 degree. The effects of seabed slope, pipe submered weight and end-constraint on the pipeline on-bottom stability are investigated experimentally.