Integrity Management of Mooring Systems

Abstract

Mooring systems remain an important component of an overall field development. There are numerous sources in literature describing challenges with mooring systems as well as potential solutions. The goal of this paper is to present a number of new technologies which can be applied jointly in an integrated framework to monitor integrity of subsea mooring components. The paper will cover (1) direct tension measurements using Vibrating Wire Gauges, (2) warning systems based on anomaly detections from GPS measurements and (3) mooring line force estimation method and fatigue using time-domain simulation methods. Vibrating Wire Gauges are a potential alternative to the use of in-line load cells or inclinometers for measuring mooring line tensions. These sensors have a proven track record in the Geotechnical Industry. A sensor dedicated for the offshore industry has been developed. The main benefit of this type of sensor is the ease of retrofitting in an offshore environment. Long-term stability tests and dynamic tests were executed and will be presented. Procedures for installation and maintenance were developed. GPS measurements have widely been used in the industry to identify mooring failures. Post-processing methods of these measurements range from application of watch circles to Artificial Neural Networks. However, there remain important challenges with performance of these warning systems under the influence of environmental loads. The authors will show a basic concept which can overcome these limitations. High accuracy floater position and motion measurements can be used in combination with a numerical model to determine mooring line forces in the field. This approach combines the numerical models used in the design with onboard integrity and maintenance procedures. The technical and organisational challenges of such approach are discussed. An onboard system able to capture system drift is used to update the numerical model and correct for deterioration of the mooring system over time. In-service measurements have been used to demonstrate and validate the concept. The methodology has been implemented and installed on an offshore asset which will become active in Q1 2019. A number of industry solutions for mooring line integrity methods have been compared. All presented solutions have a role in the total mooring subsea integrity management program. Integration of these components together or supplemented with numerical analysis can be used to develop an overall mooring subsea integrity management plan and philosophy.