Sandwich Pipes for Ultra Deepwater Applications
Authors: Castello Xavier , Estefen Segen Farid 
Published: October 2008

Abstract
Ultra deepwater scenarios, related to water depths beyond 1,500 m, require very thick walled steel pipelines or pipe-in-pipe systems, which are expensive and difficult to install due to excessive weight. Sandwich pipe is a new concept composed of two concentric steel pipes separated by and bonded to a polymeric annulus that provide the combination of high structural strength with thermal insulation.
Previous results indicate that collapse pressure is strongly dependent on the polymer stiffness. The adhesion property is also important and can affect significantly the pipeline external pressure resistance due to the relative displacement between layers. Sandwich pipes can minimize steel costs and facilitate the ultra deepwater installation, with thermal and structural performance close to pipe-in-pipe systems.
In this work, experimental tests and numerical models are employed to verify the influence of the inter-layer adhesion on the ultimate strength under external pressure and longitudinal bending of a sandwich pipe prototype. The maximum shear stress obtained from sandwich pipe specimens bonded by a particular adhesive indicated the adhesion levels to be adopted in the numerical simulations. Contact model combined with non-linear springs that connect the steel pipes to the polymer layer was employed to analyze both bonding and slipping conditions. As expected for a sandwich structure, the strength is strongly dependent on the interface stickiness. The analyzed geometry is able to withstand a water depth up to 3,000 meters with a bonding strength corresponding to only 10% of the idealized perfect adhesion condition.
Finally, sandwich pipes with typical inner diameters of those employed in the offshore production are analyzed numerically to evaluate the ultimate strength under external pressure. The annular material must have both adequate mechanical strength and low thermal conductivity properties to satisfy the operational requirements. Some polymeric materials with different properties are selected. The global heat transfer coefficient is determined in each case to attend the thermal insulation requirements of an oil field.

Introduction
New concepts for submarine pipelines and risers have been proposed recently in order to achieve flow assurance in deepwater environment. It is the case of both pipe-in-pipe (PIP) and sandwich pipe (SP). PIP is composed of two concentrically mounted steel pipes with the annular space filled with either circulating hot water or materials with known thermal insulation properties. The objective of this type of pipe is to increase the thermal insulation capacity to prevent blockage of the line caused by dropping fluid temperature below that required to form paraffin or hydrate. One of the advantages of PIP system is the possibility of using materials with excellent thermal properties, considering that the structural integrity is provided independently by the outer and inner steel layers, Grealish and Roddy (2002).
In the case of SP, object of this study, the annular layer characteristics differ from PIP by satisfying simultaneously mechanical and thermal requirements. Therefore, greater structural strength combined with adequate flow assurance can be obtained. Sandwich structures are a particular kind of composite characterized by the combination of different materials bonded together, contributing with their single properties to the global structural performance. Usually, the sandwich structure is divided in three layers: two external thin and stiff and a central thick and flexible core. The external layers are bonded to the core to allow the load transfer between the components. Numerical and experimental studies have been carried out to obtain data about the mechanical behavior of this kind of structure not very well understood so far, as done by Borselino et.al. (2004) and Sokolinsky et.al. (2002). Sandwich structures, i.e. light and stiff panels, have been employed in the naval industry mainly, searching the advantages associated with weight reduction, fuel economy, stability during navigation and corrosion resistance, as mentioned by Mouring (1999). Several multilayered applications are found with thermal insulation purpose for submarine pipelines and equipment in the offshore industry, but the benefit of the structural performance of this kind of structure has not been yet pursued for deepwater pipelines and risers, as it is the case of the present work.