Polyester is the most widely used synthetic fiber in modern mooring systems. Though its use is widespread, very simplistic models of its nonlinear load-elongation behaviors are typically utilized in mooring analyses for predicting maximum line tensions and facility offsets. This oversimplified approach can lead to under/over conservative results affecting both mooring system and riser design. A more accurate approach is to separate the permanent elongation of the rope from its non-linear rope stiffness properties. Handling these variables (stiffness and permanent elongation) independently is key in accurately representing the mooring system's response. Different mooring scenarios (e.g. MODUs, FPSOs, SPARs) are affected in different ways by the use of an oversimplified load-elongation model of polyester behavior. This study will address these affects for both a Semi-submersible and an FPSO under metocean conditions typical of the Brazilian basins.

Through a detailed rope testing plan, rope non-linearities and length effects as a function of mean load and frequency of loading oscillations can be fully captured for use in mooring analyses, better predicting actual mooring system responses. One such rope testing plan will be discussed and shown how the data is analyzed and prepared for use in the mooring analysis.

By modeling the true non-linear stiffness and being able to capture the permanent non-recoverable elongation of fiber mooring ropes in a mooring analysis, the designer can more accurately assess the vessel motions during a storm event and more effectively plan component lengths such that re-tensioning operations will not significantly impact the facility's ability to operate. Such accurate vessel motions and offsets from this analysis method also aid greatly in the riser and other system designs that are integral parts of permanently moored facilities.

Hydrodynamic Model Overview

For the semi-submersible examined in this case study, a generic Mobile Offshore Drilling Unit (MODU) model was created with symmetry along each horizontal axis. This symmetry, in conjunction with an assumed flat seafloor, allowed for metocean conditions to be applied only on one quadrant of the model to determine the controlling environment direction. The chosen base model was a modified Aker H-3 Series semi-submersible. The approximate vessel dimensions are ~74m×63m with a operating draft displacement of ~28,000 mt. The following figure shows the hydrodynamic model of the semi-submersible that was created in AQWA and used to generate the RAOs and wave drift forces.

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