Using the time domain finite element analyses and considering 95% non-exceedance installation currents, the dropped PLET and flowline configurations as well as the resulting footprints on the seabed in the vicinity of two existing gas export pipelines were used as the guides in the development of the procedures for the laydown installation of the PLET and flowline. The procedures minimized the risk of damaging existing live gas export pipelines if the PLET and flowline were accidentally dropped during the attachment and laydown process. The PLET and flowline were both successfully installed in the target box using the procedures established through this detailed engineering study.
Pipelay termination in the vicinity of live pipelines is a relatively highrisk operation. Dropping the PLET (pipeline end termination) and flowline during the lay down stage may cause serious damages to the existing infrastructures as well as hydrocarbon release, project delay, and economic losses.
A 6″ MEG (mono-ethylene glycol) injection flowline is to be laid next to two 22″ live gas export pipelines for accessing the tie-in point. Due to a prior incident involving a dropped PLET and flowline which the Owner of the 22″ pipelines had previously experienced during the previous phase of the development, the Owner required the project team to perform a detailed dropped PLET and flowline study in order to understand and, thus, develop installation procedures that minimize the risk of damaging these live pipelines.
The flowline is to be S-laid, recovered to the vessel side hang-off platform, integrated to the PLET, and then laid down to the seabed using the PLET Handling System (PHS) from the side of the vessel, as shown in Fig. 1. Since the PLET structure is attached to the flowline in a catenary configuration, i.e., not a free-falling dropped object, DNVGL-RP-F107 (2017), the risk-based dropped object analysis guideline, is not well suited for this application.
