Subsea Casing-While-Drilling System with Inner Drill String
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Solution Overview
Problem
Current casing-while-drilling equipment designed for land wells is not suitable for subsea well drilling due to size compatibility issues between the outer string of casing and the high pressure wellhead housing, leading to challenges in installing larger diameter casings and preventing borehole collapse in unconsolidated formations.
Innovation Solution
A downhole drilling assembly is attached to an inner string of casing, which is lowered through an outer casing string, allowing for drilling and cementing without retrieving the drill bit, and a tieback member is used to secure the inner wellhead housing, enabling deeper drilling and reducing the risk of borehole collapse.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If casing-while-drilling equipment designed for land wells is used in subsea wells, then the drilling process can be simplified, but the equipment is not compatible with subsea wellhead housing dimensions
Solution Approach 1:
The patent modifies the dimensions and structural parameters of the drill bit and casing assembly to match subsea wellhead housing specifications. The drill bit is designed with specific outer diameter constraints and the casing string is configured with appropriate wall thickness and connection types to be compatible with subsea wellhead landing shoulders and sealing mechanisms.
Solution Approach 2:
The drilling system is divided into separate functional segments: the outer casing string that remains in the wellbore, the inner drill string with drill bit for drilling operations, and the tieback member for securing the assembly. This segmentation allows each component to be optimized independently for its specific function while ensuring overall system compatibility.
2Ease of repair
If a retrievable drill bit is used for drilling the hole for the inner casing, then the bit can be recovered, but it cannot be pulled through the high pressure wellhead housing due to size constraints
Solution Approach 1:
The drill bit is designed as a disposable component that remains in the wellbore after drilling operations are complete. Rather than attempting to retrieve the bit through the wellhead housing, the bit is left in place and the casing is cemented around it. This eliminates the retrieval operation entirely while maintaining cost-effectiveness.
Solution Approach 2:
The casing is lowered into the wellbore before the drill bit is removed or abandoned. The casing string is positioned and secured in advance, creating a protective barrier around the drill bit before any retrieval or abandonment operations occur. This preliminary casing installation prevents wellbore collapse during the transition from drilling to completion operations.
3Reliability
If the outer string of casing is used with standard subsea wellhead housing, then the wellhead can be installed, but the inner diameter of the casing is larger than the wellhead housing allowing proper sealing
Solution Approach 1:
The outer casing string is manufactured with precise dimensional parameters including outer diameter, wall thickness, and connection dimensions that match the subsea wellhead housing specifications. The casing hanger landing shoulder is positioned at a specific depth and configured with exact dimensional tolerances to ensure proper engagement and sealing with the wellhead housing.
Solution Approach 2:
A casing hanger is introduced as an intermediary component between the outer casing string and the subsea wellhead housing. The casing hanger provides the sealing interface and mechanical connection, allowing the casing string to be securely installed in the wellhead housing while accommodating slight dimensional variations through its flexible sealing elements.
4Strength
If drill pipe is used to drill each section and then casing is run and cemented, then the wellbore can be strengthened, but the open borehole may collapse in unconsolidated formations due to formation pressure
Solution Approach 1:
The outer casing string is installed and secured in the wellbore before drilling operations begin. The casing acts as a preliminary structural support that prevents wellbore collapse in unconsolidated formations. The casing is cemented in place to create a permanent barrier that maintains wellbore stability throughout subsequent drilling and completion operations.
Solution Approach 2:
The casing string serves as a protective barrier that cushions and protects the wellbore from collapse pressures in unconsolidated formations. By installing the casing beforehand, the system creates a pressure barrier that prevents formation water from entering the wellbore and causing borehole instability before drilling operations commence.
Data Source
AI summary
At least a portion of a subsea well is drilled casing-while-drilling techniques. A landing shoulder is formed within an outer string of a casing. The outer string of casing is secured to an outer wellhead housing above the landing shoulder and lowered into the wellbore. A casing hanger is attached to an inner string of casing and lowered through the outer string of casing into the wellbore. A drill bit at the at the lower end of the inner string of casing is rotated to deepen the well. The string of inner casing moves downward as the well is being deepened until landing the casing hanger on the load shoulder. A tieback member is attached to an inner wellhead housing and lowered into sealing engagement with the casing hanger. The inner wellhead housing lands in the outer wellhead housing.