Pipeline / Riser Isolation

[bosse1950]

I am working on a project where double block & bleed isolation is required to change out a 30" leaking & passing ESD Valve. We are investigating 30" x 30" mechanical hot tap fitting (s) hot tapping, & insertion of line stop(s). We are looking to achieve isolation by means of either two traditional stopples or one BISEP double block & bleed isolation tool.Also there are 2 possible locations - one is in the splash zone directly below the +15ft clamp and the other is sub sea ar -200ft in the horizontal. Are there any of you who could provide me with feedback on these types of installations based on past experiences? Your input will be greatly appreciated.

 

[BigInch]

I was saving this, but in case I don't make it... You seem to be concentrating on the tools to be used. That's good, but... here's some other ideas,

Check for high existing axial stress in the subsea section. If there is a long straight run of (inflexible geometry) pipe at the bottom of the riser and the first clamp off the bottom between pipe and jacket is close to the mud line, axial stress may be high. See if there is any indication of movement there. Is the riser out of alignment? That's an indication there has been some axial stress relief. If there has been no movement, stress may be highly compressive in the subsea run, so watch it if you remove a substantial amount of pipe wall. Resulting eccentric loads may decide to change the geometry of the wall in the vicinity. If the pipe runs straight out from the plfm for 200 feet or so and turns 30º or more, its a more flexible geometry and the pipe is less likely to be at high axial stress, but it could mean more bending.

Is it not possible to get far enough away from the plfm, outside the hazard radius of the possible gas cloud, to place the underwater stops? One less thing to worry about.

Has there been a side scan or visual to determine the extent if any of scour near the platform? Are there any spans in the vicinity of the proposed subsea stopple area?

I believe the storm tracks of recent years have been farther to the south than in previous history. Has there been any thought to possible increased scour activity from higher bottom currents and have higher significant wave loads on pipe in the splash zone been considered?

What marine fouling factor has been used to determine wave drag force and impact loads for stress checks in the splash zone. Many times the amount of fouling in warm waters is grossly underestimated at 6 to 8" thickness, whereas it often amounts to as much as 12" thick. The drag factor of marine fouling is very high. Make sure they did not use 1.4

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain