Application of Lessons Learned From Field Experience to Design, Installation and Maintenance of FPS Moorings
Authors: D&apos;Souza Richard , Majhi Sai 
Published: May 2013

The integrity of Floating Production System (FPS) mooring systems is critical to safety and availability of a deepwater field development. While current design codes and methods result in robust moorings, there are documented cases of FPSs having experienced mooring integrity issues, that have resulted in mooring line replacement and production shut-in. There have been several industry initiatives over the past decade to analyze these mooring failures and determine their root causes. This paper presents lessons learned from these failures and provides specific recommendations to improve design and performance of future FPS mooring systems over their life cycles.

Introduction

An analysis of mooring failures and integrity issues in the last decade has shown that a significant number of Floating Production Systems have had mooring failure incidents many catastrophic, requiring partial or complete mooring replacement. This indicates a trend in which the intended design performance of moorings does not meet operational performance. To ensure reliability of a mooring system throughout its life cycle, it is essential to capture and apply lessons from industry failures and operational feedback. This paper presents a summary of mooring lessons learnt encompassing design, fabrication, installation and operations from failure analysis and field experience.

Current industry codes are based primarily on North Sea experience in shallow to medium water depths and are not necessarily applicable to other geographical areas. Though clear on required safety factors, they do not provide guidance on complex issues such as W. Africa squall transient loads, statistical basis for estimating design loads and Out-of-Plane Bending (OPB) design. Lessons learnt and operational feedback are either slow to be adopted or do not make their way into the codes due to confidentiality and liability concerns.

Many engineering companies undertake mooring design using ‘analysts’ with no field experience resulting in a lack of installation and inspection friendliness of these systems. Uncertainties in deterioration mechanisms and load case definition are not always properly accounted for in design. Interface gaps among disciplines result in incorrect or dated design assumptions unaccounted for in the final design.

One of the major pitfalls of hardware procurement is that projects rely primarily on ‘Class’ designation for quality assurance and control (QA/QC) of hardware manufacture which are often inadequate. There are significant variations in compliance between project specifications and delivered quality among manufacturers. Material traceability is generally inadequate and test results on manufactured hardware vary depending on the competency of the testing facility employed.

Installation engagement occurs late in the design stage and proper interface between the installation contractor and design disciplines is lacking. Though detailed procedures are developed they are not always adhered to because of schedule constraints, cost (vessel day rate) and inadequate supervision resulting in installation damage. As-built reports are not always mandated and the performance of the as-installed mooring system is seldom assessed.