OIL RIG RISER PIPE LINING
As North Sea oil production platforms age, remedial maintenance is required to sustain rig operations. In the Forties field, the Alpha platform had begun to experience a significant increase in the expenditure associated with the consumption of filters that are used to reduc the levels of seawater contamination in the diesel fuel to an acceptable level. Wood Group Engineering (North Sea) Ltd (WGE) were asked to identify the issue and engaged Radius Subterra to assist in the implementation of an economical solution to improve the quality of the fuel.
By 2000, an increasing consumption rate of filters used to reduce the inevitable seawater contamination in the diesel fuel was costing approximately £300,000pa across Apache North Sea Ltd’s Forties field.
On the Forties Alpha oil production platform, diesel fuel for the powering plant is stored in the hollow steel platform legs. The fuel floats on a column of seawater in the base of the leg. To decant fuel for use, seawater is pumped into the base of the leg through a 6” NB steel tube. The fuel is discharged from a second port elsewhere on the leg. Bunkering of fuel is the carried out by reversing this procedure.
A CCTV survey revealed substantial corrosion and perforation of the riser bore, particularly at the upper levels, which was significantly adding to the level of fuel contamination. A sludge/oil mixture adhering to the riser bore was also reducing its flow efficiency.
Enhanced diesel filtering was rejected in view of the already unacceptable fuel decontamination costs, whilst conventional pipe relining technologies would fatally compromise the platform’s stability, due to the need for access to both ends of the liner pipe and the removal of its contents. Single-end launched cured-in-place pipe lining technologies were also infeasible, as contact with the leg contents would have either contaminated the resin, or impaired the curing process. Strapping a new steel riser to the outside of the platform leg was a possibility but was expensive at approximately £3.5M.
It was essential that the chosen solution did not reduce bunkering rates to unacceptably low levels, to minimise any additional time spent by the supply vessels conducting the hazardous refueling task. Thus, Subterra suggested that sliplining the riser with an undersized PE pipe from a single access point might be feasible.
Full-scale yard trials confirmed it was possible to push a 110mm OD SDR 11 MDPE/PE-80 pipe around the 90o bend and on down the riser past the two 45 degree bends to the bottom along approximately 140m without collapsing.
The material’s suitability for use in contact with diesel fuel was confirmed and calculations supported that the PE pipe would not collapse under the predicted subatmospheric pressure excursions, despite the slight transverse indentations induced in the liner within the 90o bend.
To assure the stability of the inserted liner, Subterra also devised a shroud that could negotiate the bend with the liner pipe, and then be inflated from the liner insertion point to form a closed-end annular space around the PE liner in the bore of the bend.
Finally, Subterra used its in-house resin technology expertise to devise a bespoke grout that would have sufficient pot life to ensure that the closed annular space in the bend could be fully grouted, but which would cure within 2 hours, so as to expedite the completion of the works.
The riser was decommissioned and drained before the PE liner was inserted and the fast-setting annulus grout applied, with the full process taking approximately five hours. The lined riser was then reconnected and successfully recommissioned.
Radius Subterra developed a number of innovative adaptations of its existing technologies to devise a method, which succeeded in sliplining the riser from a single access point.
As a result, the revised target bunkering rate of 80m3/hour was achieved and the cost to complete works was less than one tenth of other proposed solutions at £250,000.Download: