Subsea Tubing Unit: Geo – STU

Overview

While some subsea wells can be plugged and abandoned from a light well intervention vessel, many have to be plugged and abandoned from a jack-up or floating drilling vessel. Geoprober has been working on subsea equipment that will enable light well intervention vessels to abandon a significantly percentage of subsea production  wells.

Challenge

In order to comply with current regulation, the gauge cable present on tubing in many subsea production wells will have to be removed to ensure there is no leak path through permanent cement barriers. As a result many operators are currently using a jack-up or floating drilling vessel to pull all the tubing and then set a cement plug across the casing.

Solution

The Geo-STU enables many of such subsea production wells to be plugged and abandoned from a light well intervention vessel by cutting and recovering sections of tubing, providing a window over which a cement plug can be set.

The Geo-STU can be utilised for wells with subsea horizontal trees as well as subsea vertical trees and comprises of two packages, a well control package and a reciprocating gripper package. The well control package has a series of shear rams, an annular preventer, diamond wire saw and an orientating slip while the reciprocating gripper package has two dual gripping units that are mounted on cylinders that allow them to reciprocate independently.

The operation is performed from a light well intervention vessel with the two Geo-STU packages deployed onto the subsea tree. Following this a wireline lubricator is then run into the annular BOP (in the lower Geo-STU package) and held in the grippers (in the upper Geo-STU package) to enable necessary wireline operations to be conducted.

A tubing hanger revival tool mandrel is run in the upper grippers and then reciprocated into the tubing hanger. Once unlocked, the retrieval tool and tubing hanger and attached casing are reciprocated so that the tubing hanger is just below the upper package. The tubing is hung off below the hanger in specially designed orientating slips and cut enabling the retrieval of the tubing hanger.

Sections of tubing can then be reciprocated out of the well, cut using an in-built saw and then recovered to the vessel on coil tubing in lengths that can be managed on the vessel. The connection mandrel is re-run to allow the next section of tubing to be pulled. The process is repeated until a sufficient gap has been created for the cement plug.

At any point during the process, a subsea lubricator can be run into the upper grippers, enabling wireline operations such as perforating and logging. Once all of the desired cement plugs are set, the wellhead can be removed.