Pipe-in-pipe top tensioned risers, in the forms of TLP/Spar dry tree risers and freestanding hybrid risers, are used in numerous deepwater developments worldwide. During the detail design of pipe-in-pipe systems, the complex interactions between the pipes are often misunderstood, leading to conservatism or possible errors in the estimation of the system fatigue life. This paper aims to provide an explanatory note on the interaction response of pipe-in-pipe riser systems and the considerations that are required to assess the stress, fatigue and VIV response of such systems. The methods considered in this paper could also be applied to multi tube risers.
The industry has seen an increasing number of application of multiple pipe risers, ranging from TLP/Spar dry tree riser and freestanding single line hybrid risers, in deepwater. All these designs have a fundamental design challenge of determining the response of two or multiple pipes within the system, in order to determine the extreme stress and fatigue life over the length of the riser. In all pipe-in-pipe risers centraliser are included in the design between the pipes and are considered essential to reduce differential bending in the system. Also centralisers can stop propagation buckling, when one string is in compression. Analysis of pipe-in-pipe risers is often performed considering an equivalent composite model forming a unique pipe, as analysis of a detailed dual pipe model is considered time consuming. This assumes that the equivalent pipe bending and tension are shared equally based on the bending and tension capacities of the pipes. Using a composite model is justified if the pipes have equivalent differential bending or that the bending moment of the system is small, therefore little fatigue or minimal stresses in pipes is expected. However, for certain riser systems, where the bending moment is high near the top and bottom assembly of the structure, it is important that the interaction between the pipes is clearly understood.
