Is Scour Important for Pile Foundation Design in Deepwater?
Authors: Li Ya , Chen Xingnian , Fan Shejun , Briaud Jean-Louis , Chen Hamn-Ching 
Published: July 2009

Since strong bottom currents with long durations have been observed in many deepwater developments, scour hazards may become a serious concern which can have major consequences for pile foundations and associated equipment. While bridge scour has been studied and well documented, offshore scour prediction remains an uncertain art. In most cases, local experience is relied on as the best guide for scour prediction, and an operating strategy of monitoring and remediation is considered necessary. This can result in excessive contingencies and design allowances and the need for expensive and time consuming monitoring. In this paper, selected bridge scour concepts are extended to apply to deepwater pile scour prediction after necessary modifications. Pile behaviors with the presence of scour are studied, to identify the sensitive scour range for various types and sizes of offshore piles.

Introduction

There has been a rapid growth in the number of structures installed in deepwater. Considering the tremendous investment and the extremely harsh environment that are associated with offshore projects, foundations of offshore structures have to meet very stringent requirements. In current offshore foundation design practice, one of the common and plaguing issues is how to select the appropriate scour parameters and evaluate the scour effect. The presence of seabed scour " affects both lateral and axial pile performance and capacity?? as stated in API RP 2A-WSD (2005). However, " scour prediction remains an uncertain art. The sediment transport studies may assist in defining scour design criteria but local experience is the best guide. The uncertainty on design criteria should be handled by robust design or by an operating strategy of monitoring and remediation as needed.??

Scour is the erosive action of flowing water, which excavates and carries away materials from streambeds and banks. It is a very complex process involving soil, flow and the geometry characteristics of structures placed in the flow path. These three elements by themselves are also defined by many variables. Scour is initiated when the flow generated bed shear stress is larger than a threshold value of the soil erosion resistance, which is known as the soil critical shear stress (tc). A chart showing the typical value of critical shear stress for various soils was developed by Briaud el al (1999) and is presented in Fig 1. Around a foundation, the total scour usually is comprised of general scour and local scour. The general scour occurs on an open seabed where the flow itself is strong enough to erode away the seabed materials and is expressed by seabed scooping in a relatively large area. Local scour involves removal of material from around structures placed in the flow path and is caused by the acceleration of flow and resulting vortices induced by the flow obstruction. Development of local scour is presented as a scour hole around the foundation. Fig 2 shows a local scour developed around a cylindrical pile in cohesive soils studied in Texas A&M University by Briaud et al (2003). It is known that scour in cohesive soils (clays) differs from that in loose granular materials (sand/silt) mostly by the slow erosion rate. Under a constant flow, while it may take hours to reach the maximum scour depth in sands, weeks and months of time is required to complete the same process in clays.