The paper presents a new subsea structure foundation concept and installation method, called the post-installed mudmat. With this new method, the thermal expansions and displacements of HP/HT flowlines are no longer limiting criteria for the subsea structures. It is applicable to the pipeline end termination (PLET), either first end or second end, and In-Line Tee structure to be laid in any mode (S, J or Reel lay). Combined with Subsea 7 experience of installing structure in all modes, the study is based on computer works to verify the installation sequence, the pipeline integrity and the geotechnical strength.
The post-installed mudmat concept is applicable to the projects where the pipe laying method includes structures. In the present application, the pipeline is laid in standard S (also applicable to J or Reel lay) mode with a piping module, including valve and connectors, going through the vessel tensioners and stinger also called firing line. Generally two mudmat wings are added before reaching the stinger and then unfolded subsea. In the new method, a purposely designed mudmat and associated devices is post-installed at sea-bed level on top of a previously laid simple pipeline and module. The specific mudmat guiding and securing device allows the pipe module leveling and weight transfer before the mudmat fulfills its foundation functionality.
Comprehensive computer simulations have been performed to verify the laying of pipe module and the new post-installed mudmat on the sea-bed. Calculations confirm the integrity of the pipeline at anchor point to the mudmat after installation. Geotechnical simulations also verify the piping module temporary bearing and the final foundation capacity and settlement. The post-installed mudmat design brings a significant contribution to the pipeline laying method. The mudmat size is no longer a limiting parameter for its integration in the main line. Compared to its opposite design, the pre-installed mudmat, it is not impacted by the installation tolerances which would make it larger and longer. It can also be fixed, sliding or fitted with rails to accommodate the pipeline thermal expansion without significant impact on the piping module design going through the main line. It presents an alternate to other method with separate deployment table generally retained for PLETs or In-Line Tees too large for the standard main line. This post installation process is patent pending.