Leman Riser Cutting Project

Case Study

Project Description and Objectives

The Leman ‘A' platform is currently being modified in order to extend it's working lifespan. One modification was the removal of a 30" OD riser (attached to the outside of one of the platform legs) and reconnecting it with a pipeline to a new riser that is positioned in a more protected area of the platform. Bibby Offshore therefore tasked Proserv Offshore to provide the cutting services and personnel so that the riser could be safely removed from the platform. Also included in the workscope was the removal of a 4" OD MEG line attached to the riser.

Riser details

The riser and Meg line were connected to the outside of one of the Platform's Western legs. Two bolted clamps connected the riser to the platform these clamps had to be unbolted by the divers in order to remove the riser. The maximum water depth was approximately -35m. The 30" OD riser had a wall thickness of 18 mm wall and the 4" Meg line had a wall thickness of 5 mm.

Cutting Sequence

The first section to be removed was the 4" Meg line. The Meg line was severed at levels -18m, -34m and one cut on the horizontal section on the sea bed using one of Proserv's reciprocating pipe saws. The saw was installed by the diver using a chain clamp. Once secured in position the hydraulically driven saw is fed into the Meg line by the diver using a hand crank mechanism mounted on the saw. Each cut took approximately 5 minutes to complete.

The next stage involved the removal of the 30" riser. In order to complete this task three cuts were required on the riser. In addition to this, six 4" diameter lifting holes had to be cut into the riser in order to safely remove each section of riser. All of the cuts were completed using Proserv Offshore's 10,000psi JetCut water abrasive cutting system and Proserv manipulators.

All of the holes were cut using Proserv's hole cutting manipulator. The manipulator is secured to the riser by two cargo straps wrapped around the riser and the manipulator. Once secured in position the manipulator is hydraulically driven and controlled by the Proserv technicians on the dive vessel. In order to stop the cut section falling into the riser the cutting nozzle was angled so that the cut piece would fall away from the riser and not into the riser.

Two angled cuts were required to be produced at the -20m and -23m level so that a wedge shaped section could be removed from the riser. The final cut would be completed at the -34m level so that the bottom 10m section of the riser could be removed. A circumferential chain manipulator (CCM) which drives itself around the circumference of the riser via a hydraulic motor was used to produce these three cuts. In order to produce two 10º cuts for the top wedge section a new guide band had to be designed and manufactured. The self supporting guide is clamped around the riser and then the chain manipulator mounted onto the guide band. Once secured to the guide clamp and riser the nozzle tracks around the riser producing an angled cut, with the speed of cut being controlled by Proserv Offshore technicians on the Dive vessel.

The circumferential cuts took 40 minutes to complete with the 4" OD lifting holes taking 15 minutes.

Results

The project was successfully completed during the summer campaign of 2007 with no lost time incidents or damage to the surrounding platform structures. All cuts were successfully completed with minimum downtime on equipment. The project proved that Proserv Offshore can produce a range of cutting options, which are flexible enough to allow them to be adapted to suit the clients requirements i.e. the angled cuts on the riser and bevelled hole cuts.

As well as providing the cutting equipment and personnel on this project Proserv Offshore also provided marine growth removal and dredging equipment to unbury and clean the riser and other structures.