ABSTRACT

Polyester ropes change length after experiencing high loads in operation. In benign waters, mooring systems with polyester ropes have been used to a large extent, where the length change has not been reported to be an issue or has been handled by mooring line length adjustment. When moving to harsh environment installations, length change becomes a potential issue, both for offset and mooring system performance. The length changes in a polyester rope taut mooring system in operation in harsh environment has been studied, and the findings with respect to both elongation, contraction and stiffness are presented in this paper.

INTRODUCTION

Polyester ropes have been used for many years in long term mooring of offshore installations, typically in deep waters with taut mooring lines. A particular challenge has been the increase in line length, referred to as permanent stretch, following higher tensions than previously experienced by the polyester ropes. As these mooring lines tend to be very long (typically 2000-3000 m), even the smallest change in length (in percent) can make a significant impact on the total line length. If not actively adjusted during operation, this will affect the mean tension in the mooring system, which is an important parameter for both extreme line tension and offset during storm events.

For long term mooring with taut mooring lines in deep waters, the permanent stretch has typically been handled by either (i) tensioning the ropes to the highest possible load during or just after installation, (ii) designing the mooring system to allow for increased line length, (iii) use winches to pull in the ropes after elongation, or (iv) a combination of these strategies. This has worked well, especially as many of the polyester rope mooring systems have not been in very harsh environment areas, and/or have been equipped with winches enabling line length adjustments after peak loads leading to an elongation of the ropes.

For mooring systems on installations not able to adjust length in operation, predicting the final length of the ropes can be challenging. This now becomes an issue for turret moored FPSOs in deep water (taut mooring) and harsh environment, and for floating wind turbines with hybrid semi-taut mooring systems in shallow water. Understanding the change-in-length becomes vital for design of the mooring systems, and experience from taut systems in benign environment may not be covering the upcoming applications.

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