This paper discusses the importance of foundation and soil response considerations in pipeline design and installation. The design and optimization of many of the latest high pressure and high temperature pipelines are now strongly influenced by the selection of appropriate soil parameters and foundation models. This paper describes some developments in conventional pipeline design for both static in-place conditions and dynamic installation conditions.
It is suggested that advances in soils investigation and survey techniques for pipeline routes are also needed and the paper reviews the current state-of-art. Some recent burial technologies are also discussed. Many of these, such as soil thermal and electrical resistivity response, are becoming increasingly significant as a result of the latest developments in deepwater sites. Also acknowledged in the paper are advances in non-trenching, or the engineered avoidance of trenching requirements based on improved knowledge of site specific conditions.
In common with any structure that rests on or in the soil, a pipeline has a foundation that should remain stable in a controlled way throughout its design life. However, a pipeline is a relatively lightweight structure that is usually founded in the shallowest seafloor soils. In this respect the engineering of an offshore pipeline foundation can raise some challenges which may not be familiar to a soil mechanics engineer who is used to designing the foundations for large heavy structures which are founded in soils deep below the seabed.
In fact the problems faced by a pipeline engineer can be rather daunting and complex. For example, the pipeline may require to be buried in a metre or so of seabed soil which must first be excavated, remoulded and then placed on top of the pipeline by one of range of specialist machines. This spoil may then be required to provide an uplift resistance against the pipeline for its design life. During this time it may be loaded cyclically as a hot product is transported through it. The spoil may be required to provided the pipeline with the thermal and electrical insulation as well.
Such challenges bring opportunities to develop new design techniques. While a considerable amount of work has been carried out recently in the optimization of the design of the pipeline themselves (for example the use of strain based design and thermal pipe-in-pipe systems), there has been somewhat less attention placed on the potential for optimisation of the foundation and geotechnical design.
In some way this is rather surprising. Particularly when it is considered that soils issues on pipeline design and construction often represent a significant proportion of the cost and often an even more significant proportion of the risk involved in such a development For example, budgets for protection of a pipeline system by sand, gravel or rock dumping as alternative to trenching can be up to order of US$1 million per kilometer of route length.
For a variety of reasons, pipelines may require to be trenched (lowered into an open trench excavated below seabed level)