CORRODED RISER LINING
The repair and maintenance required for the corrosion of carbon steel water injection pipelines in offshore oil platforms can significantly impact operational expenditure.
In the central North Sea, Apache’s Beryl Bravo platform had begun to experience an increase in such expenditure due to the contamination of seawater within two highly corroded 130m risers.
At a cost of approximately £1,500,000, the replacement of the risers was infeasible and operators Mobile North Sea Ltd engaged Radius Subterra to retrofit the risers with a polyethylene pipe liner in order to extend their life of field.
Possessing two opposed 45 degree 5-D bends in close proximity to the sea bed, there were concerns that the use of thick walled polyethylene pipe may not bend sufficiently and become damaged.
Furthermore, at a low sea temperature of 5 degrees, the polyethylene pipe would become stiffer and thus require an increase in force for it to negotiate the bends.
Added to the potential variations in the bore diameter of the risers between 122mm to 130mm, it was essential to ensure that seawater did not remain trapped within the pipe and continue to corrode it.
Whilst the sliplining of the risers with smaller diameter pipe would require less pull force, this was discounted due to an unacceptable loss of riser capacity and an unacceptable retention of seawater.
In order to ensure that the force required to pull the liner through the risers and negotiate the tight bends did not damage the pipe, and a close fit against the host riser was achieved, Radius Subterra chose to combine both their Subline DR and Subline PF techniques.
The Subline DR system concentrically reduces the diameter of the pipe by pushing it through rollers and allows it to be installed using traditional sliplining techniques before reverting its diameter to form a close fit with the host pipe. Whilst the Subline PF system folds and binds the pipe into a “U” shaped cross section that enable it to more easily negotiate tight bends.
By combing these solutions, Radius Subterra would be able to use a low pull force to preserve the liner integrity and ensure a close fit with the host riser, despite variations in diameter.
Using CCTV to ensure the riser bores were clear of obstructions, the liner was installed from the top down using a pulling arrangement that comprised of a winch on the top platform and pulling wire weighted to the sea bed and returned topside. This enabled it to be controlled from the platform without the assistance of divers.
Each liner was installed at a pull through rate of approximately 1.25m per minute and with winching forces lower than expected; an effect ascribed to the lubricative effect of the water and additives.
Once in position the liner was then filled with water to revert it to a circular shape by gradually increasing the pressure to break the restraining bands. The pressure was then further increased to expand the liner fully to the 130mm diameter of the riser bore, expelling seawater from the space and providing a close annular fit.
Post installation CCTV of the liner showed an excellent fit to the host riser pipe had been achieved and the risers were connected to the pipeline system to return them to service.
Radius Subterra adapted their already existing technologies to devise a method that succeeded in rehabilitating a riser with varying diameter and tight bends, whilst ensuring the expulsion of seawater from the riser bore.
As a result, the cost of rehabilitation to Mobile North Sea Ltd was a fraction of the cost to replace the risers and life of field of the platform has been significantly extended.Download: