The nature of seabed interaction both in catenary risers and J-lay pipelines is the main boundary condition governing the performance in the touchdown zone. Design must ensure that the curvature remains well within elastic limits, and that fatigue damage remains acceptable during the life. Closed form solutions, although limited in their accuracy because of idealisations of the system response, offer a first step in assessing the system performance. The paper compares the results of different boundary layer models with those from finite element analysis to evaluate the accuracy and consistency of solutions for initial design assumptions and fatigue assessment.
In recent decades, increasing of the energy demand and development of Marine energy has created new challenges in design and installation of subsea pipelines. One of the most popular technics used for pipelines in deep and ultra-deep waters is the J-lay methodology. The most important characteristics of the J-lay method are (i) the reduction of the distance between the vessel and the touchdown point (TPD), thus facilitating the dynamic positioning, (ii) the drastic reduction of the horizontal force at the barge to be supplied by the vessel engines, and (iii) the elimination of the overbend curved part at the end of the stinger, which, among others, can produce dangerous high stresses and strains. Furthermore, it reduces the pull tension required at the vessel to lay the pipeline and eliminates the long and vulnerable stinger (Lenci and Callegari, 2005). Disadvantages of this method include: (i) that the processing operations are more difficult along the steep ramp, leading to slower laying rates, (ii) the capability to lay the pipeline within a narrow corridor with stringent lay tolerances, and (iii) bigger vessels with greater installed power are needed to grant the dynamic positioning under all possible operating conditions.