Formation and Development of Seabed Trenching from Subsea Inspection Data of Deepwater Gulf of Guinea Moorings

Abstract

It is now recognized that seabed trenching in taut deep water moorings can reach the suction anchor and the chain padeye depth, with the safety of the mooring system becoming a concern. However, the actual formation of such trenches and their development with time are still largely unknown, as well as the relation with the characteristics and dynamics of the mooring systems. The analysis of a database of subsea inspection data from deep water Gulf of Guinea mooring systems is presented. The trench data, from in-service subsea inspections of mooring systems for five FPSO vessels and off-loading terminal buoys, comprise the measurement of trenches width and depth and of anchor-to-trench distances. Because similar metocean and soil conditions apply to the ten mooring systems, the study offers a unique database for better understanding the formation and evolution of this seabed trenching. The subsea inspections were carried out between 2014 and 2017 but the study covers a much larger time span of moorings performance as the FPSOs and off-loading buoys were installed between 17 years and 5 years ago. The trench development is studied in relation with the moorings characteristics and dynamics and is illustrated with typical features and trench geometries for the two types of floaters. The results obtained represent a key improvement for understanding this seabed trenching phenomenon and are most probably applicable to all deep water moorings of the Gulf of Guinea area. Preliminary recommendations about design aspects and possible mitigations that can be considered for limiting the seabed trenching are also addressed in the paper.