The Liza risers comprise production risers, water injection risers and gas injection risers, and a lazy wave configuration is selected considering FPSO motion, reservoir fluid and overall project execution requirements. During operation, the risers are expected to move cyclically with small vertical displacement amplitudes (e.g. 0.1% to 1% of the riser diameter), and a key design issue is the fatigue life of these risers at critical locations including the touch-down zone which will be governed by the seabed stiffness. The role of soil response on fatigue life of riser with buoyancy has been investigated through nonlinear finite element and comprehensive lab and field testing program. Published methodologies for determining seabed stiffness values for risers concentrate more on larger amplitude motions based on the design requirements of steel catenary risers.
The paper presents the sensitivity of the fatigue life at TDP to various soil model and provides insight in the results. Also included is the importance of site specific soil investigation in the context of design of riser.