Unconventional Method of Subsea Umbilical Retrieval Using Anchor Handling Vessel
Authors: Dharma Dian P. D. S. , Rashid M. F. , Berwanto A. , Abdullah A. Z. , Obaidelah A. T. , Ghazali I. 
Published: October 2023

A deepwater field in West Africa was decommissioned and subsea facilities retrieval operation was carried out as part of the Abandonment and Decommissioning (A&D) plan by the Operator. Subsea umbilical and Umbilical Termination Unit (UTU) were among the subsea equipment that had to be recovered from the seabed for abandonment. Subsea umbilical retrieval operation was executed in two phases, Phase 1 for dynamic umbilical retrieval and Phase 2 for static umbilical and UTU retrieval. While in phase 1 the dynamic umbilical was retrieved conventionally via reversed installation method using vessel with Vertical Lay System (VLS), the retrieval of static umbilical in Phase 2 adopted an unconventional approach using Anchor Handling Vessel without the VLS. This paper will discuss the simpler methodology for umbilical retrieval using Anchor Handling Vessel together with the engineering process and operational challenges faced during the offshore execution.

Retrieval of subsea facilities which normally being regarded as the reversed of installation requires extensive effort and precise engineering to ensure the operation can be executed according to time, budget, and scope. The engineering scope of works conducted were structural analysis to verify the structural integrity of UTU as well as umbilical recovery analysis to verify that the equipment would be able to withstand the load exerted during lifting and retrieval activities. Along with the main procedures, contingency retrieval methods and installation aids were prepared to guarantee a wholesome operation and minimize the risk of downtime.

Retrieval of umbilical and UTU using Anchor Handling Vessel is a field proven methodology that can offer significant benefit to Operators and Service Providers in terms of flexibility in usage of vessel (i.e. vessel type) and its availability for the operation, by means of wider vessel selection. By utilizing a simpler retrieval methodology, Anchor Handling Vessel is a viable solution for field decommissioning and umbilical retrieval operation is no longer limited by the capability of vertical lay vessel.

The key to successful execution of subsea facilities retrieval was identifying the critical operations and addressing its associated risks. Adopting lessons learnt and/or best practices coupled with fast, decisive troubleshooting in managing technical challenges that were encountered during the execution were the essential factors for successful offshore campaign. Effective interfaces and integrations between operator and contractors of various work scopes was crucial in achieving a win-win situation for all parties to meet the project goal at minimum operational cost.

While it was known that reversed installation methodology could be adopted for retrieval of subsea facilities, implementing the simpler retrieval methodology that was unique to the case due to abandonment and decommissioning nature, was proven to be more attractive and beneficial to be replicated in the future subsea abandonment and decommissioning projects.