Design Aspects and Benefits of Sandwich Pipes for Ultra Deepwaters

Oil&gas production on deep and ultra deepwater scenarios require very thick walled steel pipelines or heavy Pipe-in-Pipe systems, which are expensive and difficult to install. Sandwich Pipe is a new pipeline concept composed of two concentric steel pipes separated by and bonded to a polymeric annulus that provide the adequate combination of structural strength and thermal insulation. In later works, Sandwich Pipes have been analyzed regarding to the structural strength and results have indicated good relation between steel weight and external pressure resistance, mainly when compared to the Pipe-in-Pipe system. To fulfill the thermal insulation requirements, an adequate polymer for the annular layer should be selected, combining both insulation requirements and good bonding strength to steel, which are determinant for the adequate performance. Sandwich Pipes with typical inner diameters of those employed in the offshore production are analyzed numerically to evaluate the ultimate strength under external pressure. Polyurethane based materials with different mechanical and thermal properties are selected. Experimental tests are performed to evaluate adhesion strength to steel, which are used as input for the numerical models. Non-linear geometry, material and contact properties are included, as well as temperature effect on polymer stiffness. To estimate the insulation capacity for each option the global heat transfer coefficient is calculated and a maximum “U value” is considered for all systems. Also, both numerical and analytical analyses are employed to design a PIP system for a hypothetical offshore field. Similar conditions to the newly discovered sub-salt fields offshore Brazil are adopted, i.e. 2500 m water depth and 80°C produced fluid temperature. API 5L specifications table are used for thickness and diameter selection and API RP1111 is employed to design PIP inner pipe. Results indicate that Sandwich Pipes with adequate strength and insulating annular material can generate significant advantages in relation to the PIP system. In addition to the sandwich structure benefit, the initial out-of-roundness maximum diameter directions between inner and outer pipes have a secondary role that produces additional strength for the collapse pressure resistance. Lower steel weight is calculated in all cases, even when the outer pipe is larger. All cases yielded lower submerged weight, which is an important parameter for installation purposes, because less expensive lay vessels may be required.