There are numerous modifications to offshore oil & gas installations due to e.g. tie-in of new fields and wells. When an existing production system is considered host for a tie-in there are a number of issues that need to be considered; the composition is different, and there is probably also different pressure, temperature and flow rates.
A design that once was deemed fit may be severely challenged. Fire loads may also have changed, and corrosion and wear & tear may have taken away capacity.
So what do you do if the flare capacity is too low?
Doing modifications to the flare system is normally not an option. More or less feasible alternatives include changing the size of the segments by i.e. introducing another segment, replacing weaker piping and flowlines with better materials or larger wall thickness, or adding passive fire protection (PFP).
Example of planned modification
A modification to a platform located in the North Sea was planned. The modification had impact on several process segments, including 53 flowlines and pipe spools, whereof 19 were exposed to fire. These belonged to four process segments, with operating pressures ranging from 3 to 33 barg. The inventory was a multiphase fluid.
Cost saving alternatives for PFP
The peak fire load varied from 250 to 350 kW/m2 with a background fire load of 100 kW/m2. The initial analysis, using VessFire, revealed that 9 flowlines and piping spools with Nominal Pipe Size (NPS) (as shown in the table) would rupture. In order to possibly avoid costly PFP, we applied different pipe schedules and wall thicknesses; the heat-stress curves for the steel qualities were already available in VessFire. The results are shown below. With the revised schedule and wall thicknesses there were no unacceptable ruptures and hence no need for PFP. The fire insulation in this example is 40-50 mm cellular glass with a 15 mm alkaline earth silicate (AES) wool and 0.7 mm steel mantling.
Huge cost saving potential
With an estimated effort of 10 man-hours/meter pipe (including bends and support) for offshore application at 180 USD/mhr, the saving in PFP work was about 475.000 USD. Painting of pipes (if necessary) and scaffolding are not included. Then there is the saving in insulation costs; the results showed an estimated saving in PFP of 4.65 tons at the expense of an additional net weight increase of 3.52 tons of steel due to increased wall thickness. The piping and flowline surface area that would otherwise have been provided with fire insulation was about 263 m2.
The additional 8.2 tons of steel represents a minor cost increase. In addition to savings in installation cost and reduced offshore work, benefits include reduced risk of corrosion under the PFP on piping and flowline, reduced cost of inspection and re-application, as well as reduced cost of repair of PFP on piping and flowlines.
In addition to documenting blowdown capacity and fire integrity, the analysis also contributed to reduced installation cost and reduced inspection, maintenance and repair costs. This is actually the beauty of VessFire: It is truly a multi-discipline tool where Piping & Layout, Process, Safety and Material disciplines can meet, test and explore consequences of options before they conclude on mitigations.