Negative Planning LL with Wellesley

Hemispheres (36/1-4) – P&A

What happened:

Cement plug was pumped without incident, POOH through cement was without incident. Circulated clean at 2500 lpm / 80 bar.

Observed some black spots in the sea water returns on bottoms up. This was clumps of cement and 5 m3 was diverted to slops.

A standard 7″ OD bull nose / jet sub was run after the EZSV RT was recovered. This was ineffectual. The jets point downward at a 45° angle and not straight out at 90° against the BOP.

BOP was unlatched and recovered after the jetting run.

When BOP was inspected at surface it was observed that the ram cavities were filled with hard cement.

What was planned:

Set a 200 m cement plug on top of a 13 3/8″ EZSV at 686 m. This was the final abandonment prior to recovering BOP and leaving location.

Plug was pumped through the 13 3/8″ EZSV running tool on the end of a 411 m long 3 /2″ DP stinger.

TTOC was 486 m. Stinger was pulled through the cement plug and string / well circulated clean at 455 m ( 1 x stand above TTOC ) with 2500 lpm flow.

When circulating clean 3 1/2″ DP was through the BOP.

Cement plug was set in seawater, no spacer used.

Run a jet sub and clean up the BOP prior to recovering BOP.

What was learned:

The likely cause of the cement in the BOP cavities was lack of annular velocity with the 3 1/2″ stinger through the BOP.

Any cement being cleaned off / circulated out would not have had enough annular velocity to be cleaned out of the hole and settled inside the BOP.

Flow rate of 2500 lpm was max flow rate through the 13 3/8″ EZSV running tool.

What actions are required:

There are several steps that can be taken to prevent this from happening again.

1) Do not use 3 1/2″ DP for the stinger. Use standard 5 7/8″ DP to run the 13 3/8″ EZSV on.

2) If 3 1/2″ is preferred as a stinger: Modify the following space out variables to ensure that when circulating clean above the TTOC you have 5 7/8″ DP through the BOP:

1) Shorten stinger length to match plug length,

2) depth of EZSV – make it deeper if required.

General thought is that 200 m plug is sufficient in this casing size so should not be shortened.

3) Dolphin should function the BOP rams immediately following this cementing operation to help clear the ram cavities. The choke, kill and booster lines should be flushed as part of this “after cementing” routine.

4) 4000-4500 lpm flow rate would help lift any excess cement. To achieve these higher flow rates the 13 3/8″ EZSV running tools needs to be bypassed. This could be handled by the addition of a drop ball circulating sub above the EZSV running tool. The other way of handling this is to POOH with the EZSV RT, lay it out and and make a dedicated trip with 5 7/8″ DP to circulate at full rate.

5) The standard 7″ jet sub equipment that Dolphin uses is ineffectual and needs a re-think. There are plenty of dedicated larger OD tools with 90° nozzles designed for cleaning BOP cavities, Coretrax BOP jet sub, Sub-drill Vort-X tool. SLB dual action bypass sub. These are just three of many available large 16″ OD tools designed to clean the BOP cavities effectively.

What Was Done

Balanced cmt plug was designed, planned and executed in the normal way. Plug was displaced with 1 m3 under-displacement, and pipe was pulled dry. Theoretical top of cmt plug was 250 m below seabed, and zero excess was used on the calculated cmt volume. In addition the cmt stinger was pulled up 30 m above theoretical top of cement to avoid getting cmt slurry back to surface.