Offshore locations, which use large diameter coiled tubing for wellbore intervention, are currently limited by the lifting capacity of the cranes on the platforms. Often intervention projects require a longer length of coiled tubing on a spool than can be lifted onto the platform. Historically this problem has been resolved by connecting two or more spools of coiled tubing on the platform with a butt weld. Safety considerations, logistical issues and repeatable low-cycle fatigue performance have made this practice undesirable.
Spoolable connectors have addressed concerns associated with butt welds, however limitations on connector fatigue life, elastomer durability and post connection integrity verification exist.
A mechanical spoolable connector and an assembly skid have been introduced in the North Sea. An integrity test verifies the tensile strength of the newly made connection. Testing has shown that the connector does not affect ballooning of the coiled tubing due to bending under pressure.
The assembly skid provides improved safety and efficiency when handling, preparing and installing a spoolable connector. Remote operation of the pipe handing equipment has removed the operator from the work area when pipe is moving, which reduces the exposure to this hazardous task. This equipment provides a safe work environment for efficiently handling and preparing the coiled tubing during the assembly and disassembly process.
The connector together with the assembly skid was used successfully on the Gullfaks B platform in the Norwegian sector of the North Sea for sand cleanout, scale milling and plug setting operations.
Even though coiled tubing offers operational flexibility, it comes with its unique challenges when compared with other conveyance methods. As the wells become deeper and CT applications require higher flow rate to achieve operational success and increased stiffness to reach target depth, the strings are also getting heavier. With the capacity of offshore platform cranes lagging behind the coiled tubing string lift requirements in most offshore environments, operators have tried to overcome the weight challenge by using one or more techniques that is available today in order to utilize optimally sized coiled tubing in the harsh offshore environments.
The most common techniques used to overcome the weight challenge today are: field butt welding of multiple CT strings, boat spooling, use of smaller CT string and lately joining of multiple strings with spoolable connectors is becoming accepted.
Each of the techniques has their challenges and limitations. For example butt welding coiled tubing requires special skills - welder, manual handling of the pipe, complex procedures and rigorous inspection of the butt weld and could be very time consuming. The resulting weld only has ~ 50% low cycle fatigue life compared to the parent pipe material and its performance can be difficult to predict or manage.
This paper describes a spoolable connector/assembly skid testing and deployment in the Norweigian sector of the North Sea to alleviate the challenges of joining coiled tubing to obtain a longer work string under field conditions.
The Schlumberger Coiled Tubing Services engineering team and the operational teams in Norway and Denmark embarked on finding a solution to the logistical problem associated with handling and lifting of larger OD CT strings on offshore installations.