Unconventional Approach Simplifies Steel Catenary Riser Decommissioning

Abstract

A major pipeline company was tasked with decommissioning the Morpeth tension leg platform (TLP) in the Gulf of Mexico (GoM) and needed to abandon a 22-year-old, 8-in gas export steel catenary riser (SCR). The conventional approach to decommissioning an SCR is to mobilize a topside winch package, which sometimes requires the removal of platform equipment. The next step is to carry out structural engineering checks on the platform, determine the wire's various drop point routes through the platform, and provide the engineering analysis necessary to handshake the load from the winch to a heavy lift vessel for abandonment. This was the original plan for the Morpeth TLP SCR decommissioning program. Instead of employing this traditional approach to remove the riser using a topside winch package and heavy lift vessel, the pipeline owner requested an alternative method that had been used previously for an international operator. This novel method employed ROVs and divers to install a series of clamps to grip and secure the riser prior to cutting and used a Multiservice Vessel (MSV) to manage the disconnected riser for abandonment on the seafloor. The appeal of this integrated solution is that it does not interfere with the platform's topside operations or equipment layout and can provide potential cost savings. This methodology was used for the first time in the GoM to remove a riser to make space for a new one to be installed. For this installation, a clamp was designed by the operator to sit on the J-lay collar and to support the weight of the riser being removed. The project was modeled in OrcaFlex to determine safety requirements and the service company carried out the riser removal. Following adaptations to accommodate the differences presented by the Morpeth TLP SCR, a similar approach was used in the SCR decommissioning project, with the service company's engineering team designing a combination of a friction and through-pin box clamp, determining their placement, mapping out the role of divers, and defining the necessary vessel movements for each phase of the operation. Following an initial survey to determine the profile of the SCR, the service company created an OrcaFlex model and decommissioning plan that would allow the vessel to safely execute the program. The model allowed the team to incorporate safety factors to ensure that the load of the riser could be managed and that the riser could be handled without jeopardizing the safety of the vessel or creating riser clashing during the process of cutting and swinging the riser away from the TLP. It was determined that a winch wire could be cantilevered off the stern of the vessel to provide overhead access to the riser and clamp connection point. With the clamps properly connected, the riser was severed in a controlled manner by ROVs and then abandoned within the right-of-way (along the pre-approved area) of the pipeline prior to the ends being plugged and covered. In comparing the actual dynamic loads monitored during the operation with a line rider, it was confirmed that the model was within 5% of the expected loads, estimated at 54 metric tons. This paper explains how an unconventional method of SCR decommissioning allowed execution entirely from the MSV without impeding the operator's decommissioning schedule or conflicting with the work of other contractors on the platform to provide substantial cost savings. The solution also can be used to replace existing risers, whether the useful life of the platform exceeds the useful life of the riser and/or in the case where new field development requires a new tie-in. Other deepwater, vessel-based decommissioning solutions include the abandonment and/or recovery of flushed umbilicals and potentially the abandonment of flexible catenary risers. Following successful execution of the SCR decommissioning project, the pipeline company requested a FEED for using this integrated decommissioning solution for additional offshore assets. It was also requested that the clamp design be ROV installable. The study is for a three-year program to abandon five export SCRs on multiple TLPs in the GoM with loads upwards of 350 metric tons. The abandonment method to be employed for the upcoming SCR decommissioning program is the same as the one described in this paper, with execution from a single, standalone MSV.