Flexible riser configurations in harsh environments require riser buoyancy in order to decouple vessel induced motions from the seabed interface. This is achieved through either wave (distributed buoyancy) or S (subsea buoy) solutions.
In the UK sector of the North Sea circa 30% of all floating production system field developments utilise subsea buoys in Lazy-S configurations. The majority of these fields have been in service for many years and designed prior to the widespread adoption of current industry standards, the analytical rigour available today, and better characterisation of the metocean environment. In many cases original system design has also been for 50yr return period conditions, compared to the specified 100yr return period events required in todays codes and standards, e.g. ISO 13628-2 .
Therefore, when replacement riser or life extension work is performed on existing structures or new developments are being designed there can be significant challenges in confirming the applicability of traditional system configuration designs.
The principal challenges with these configurations is minimising sag bend compressions driven by differential buoy vs. vessel motions and maintaining the riser or umbilical minimum curvature and compression criteria at the seabed touch down. The latter point is a particular problem for umbilicals routed via Lazy-S configurations owing to their relatively low weight and stiffness, and constraining MBR criteria.
This paper considers the applicability of Lazy-S configurations as a solution to modern harsh environment field developments and the evolution in Lazy-S system design to address the design challenges with particular emphasis on the TDP response.