An Analysis of Subsea Crown Plug Operations in Horizontal X-mas trees

Abstract

In this thesis, the operations regarding the tubing hanger crown plugs and the internal tree cap are studied and analyzed based on experience data collected from the Statoilo perated fields of Norne, Gullfaks, Åsgard and Heidrun.

The Internal Tree Cap (ITC) has been analyzed considering operations involving puling of the lower Tubing Hanger Crown Plug (THCP) through the ball valve. Due to the extent of the problems seen with operating the ball valve, the cap is recommended pulled before pulling the lower crown plug. A contingency procedure is described in form of a decision tree, and should be use if the cap for some reason is not pulled.

For the lower THCP problems have been identified for both pulling and setting operations. Problems with the Upper Wireline Plug (UWLP) are mostly during setting operations.

For the lower THCP premature setting of the plug is one of the main problems. The premature setting is seen in connection with use of a brass pin as the upper shear pin in the CPHRT. A steel-steel combination for the two shear pins in the hydraulic running tool is therefore recommended. The CPHRT should also always be checked for water intrusion in order to ensure that the pistons in the RT are free to move all theway down when trying to lock the dogs into the tubing hanger nipple profile. It is recommended that the number of pistons in the running tool and the setting pressure should follow the table given in the Halliburton manual.

Different methods have been used for pulling the THCP. Equalizing across the plug isseen as a necessity for a successful pulling operation. The pulling method with the highest success rate is the”Pressure up above” method. This method is currently just in use for the RLWI system but it is recommended that it be adapted and used for pulling through the WOR. Braided wire should be used when pulling the plug.

Displacing fluid through the ¼” TCI-line between the upper and lower plug is seen asone of the challenges when trying to set the UWLP. Calculations done for setting inthe WOR and MR show that the pressure needed in the riser is dependent on the displacement velocity. The higher the pressure in the riser the faster the fluid willdisplace through the TCI-line. In the MR only hydrostatic pressure is available so alonger displacement time should be accounted for.

A design failure with the UWLP setting tool has been investigated. When the runningtool is set in compression, a premature energizing of the plug locking dogs can occurand the plug is not able to set. It is recommended that the running tool should be held in tension until the plug has landed and it should be evaluated if a re-design of the tool is needed.

New technology regarding the crown plugs is presented. This new technology includes an equalizing crown plug, a motorized setting tool and a pulling/jetting toolall under development for FMC Technologies.

Best practice guidelines are developing for the installation and retrieval of the crown plugs based on the analyses performed. Best practice guidelines and recommendations are also made for operating the ITC.

The analysis and recommendations given in this thesis should be used as a basis for developing a finalized best practice for crown plug operations.