Fiber Moorings, Recent Experiences and Research: Updating API RP 2SM on Synthetic Fiber Rope for Offshore Moorings

Abstract

The 1 st Edition of API RP 2SM -Recommended Practice for Design, Manufacture, Installation and Maintenance of Synthetic Fiber Ropes for Offshore Mooring -was released in March 2001. Prior to then, most of the actual synthetic fiber rope mooring applications were installed in Brazil by Petrobras. Since the publication of RP 2SM, polyester moorings have been used in other deepwater basins, including the Gulf of Mexico, for both temporary drilling MODUs and permanent FPSs. Much has been learned from the actual design, manufacture, installation and operation of these systems by other operators and contractors throughout the past decade. This work has created an extensive knowledge base in the areas of both synthetic fiber rope behavior and mooring system design. To best capture these new learnings, an API Task Group assembled to perform a major update in developing a 2 nd Edition.API RP 2SM is the recognized standard for synthetic fiber offshore moorings in the Gulf of Mexico as well as other deepwater basins of the world. It is used in conjunction with API RP 2SK (Design and Analysis of Stationkeeping Systems for Floating Structures, 2005) and API RP 2I (In-Service Inspection of Mooring Hardware for Floating Structures, 2008) for the design, manufacture, installation and maintenance of both temporary and permanent synthetic fiber mooring systems. This paper will present the key changes in the update of this API RP. Reasons for the changes and significance on a synthetic fiber offshore mooring project will be discussed. Major changes in the RP include sections on elongation and stiffness testing, contact with the seafloor, creep rupture and axial tension compression fatigue. The new guidance in the RP will allow for improved synthetic fiber mooring systems design, installation and operation while also potentially reducing cost. IntroductionIn 1997, Petrobras installed a 12-point taut leg polyester mooring system on its P-27 semisubmersible floating production system in the Campos Basin, offshore Brazil. This installation was a first in the offshore industry, and since then Petrobras has installed more than 20 polyester mooring systems on semis, FSOs and FPSOs. In 2004, BP was the first to install a polyester mooring system in the Gulf of Mexico (GoM) on its Mad Dog spar. Anadarko followed shortly by installing a polyester system on its Red Hawk cell spar. Since then several other projects (Gomez, Tahiti, Blind Faith, Independence Hub, Thunder Hawk, Mirage and Perdido) have used polyester mooring systems in the GoM. Polyster moorings are planned for future GoM projects, including Petrobras's Chinook and Cascade development. Additionally, polyester has been used for the Kikeh spar moorings in Malaysia as well as several CALM buoy moorings and turret moorings throughout the world.

A creep–rupture model of synthetic fiber ropes for deepwater moorings based on thermodynamics

“…The experimental results including dynamic stiffness, creep and fatigue of polyester ropes made great contribution to the application of polyester ropes for deep water moorings. In 1997, Petrobras installed the first polyester mooring system in the Campos Basin, offshore Brazil [7]. After that polyester ropes have become the preferred option for depths down to 1500 m. However, with numerous discoveries of natural resources in deeper waters, the question of whether polyester ropes can be utilized and provide enough stiffness to maintain acceptable platform offsets at all depths has been raised.…”

“…For polyester and aramid mooring systems Nomenclature t time t 1 creep time t r total time of the creep rupture ε(t) total axial strain at time t ε ve (t) recoverable viscoelastic strain at time t (t), ij actual or nominal stress (t), ij effective (undamage) stress mass density Á specific entropy ϕ Helmholtz free energy q heat flux vector T temperaturē (m) (m = 1, · · ·, M ve int ; M ve int ≥ 1) viscoelastic internal state variables Q (m) thermodynamic conjugate forces associated with the viscoelastic internal state variables Y damage force c constant stress g 0 , g 1 designed to API 2SK safety factors, creep failure is not a typical design issue in the intact mooring condition [7]. However, for conditions which result in high fiber loads (greater than 70% of the fiber break strength) for a long duration, creep failure can be a concern [13].…”

Effects of Damaged Fiber Ropes on the Performance of a Hybrid Taut-Wire Mooring System

“…Therefore, the use of synthetic fiber ropes as main sections of the mooring lines was proposed in the 1960s [5]. In 1997, the first polyester taut-wire mooring system was successfully installed in the Campos Basin, offshore Brazil [6]. Since then, the polyester ropes used in taut-wire mooring systems have become the preferred option for depths up to 1500 m. However, with numerous discoveries of natural resources in deeper waters, the question of whether the polyester ropes can provide sufficient stiffness to maintain acceptable platform offsets at various depths has been raised [7][8][9].…”