Structural casing or conductor pipe provides a foundation for deepwater well and is designed to withstand the expected axial and bending loads. A very conservative design approach with worst case scenario is usually adopted for structural casing in exploration wells because the lack of information and high risk associated to deepwater operations cannot be underrated.

This paper will present the evolution from a conservative to a fit-for-purpose engineering design approach. A design review with the latest interpretation of regional geological understanding derived from actual drilling experience was performed to quantify the associated uncertainties in the design phase. The result was compared with an independent analysis and validated during the actual installation performed by rig contractor. The project team supplemented this paper with a case study by sharing the conductor installation results from the jetting operation.

In the early stage of deepwater exploration wells in Southeast Asia (SEA) region, many operators set their first string or structural /conductor pipe at around «80 m below mudline. This is a normal industry practice adopted from experiences in the GOM deepwater operation or operators' own experience in their deepwater field. This structural casing is generally jetted down with internal jetting BHA and drilled in riserless mode to the predetermined depth. This first string will not be cemented, and a subsequent surface casing will be installed and cemented to seabed. These two (2) strings will provide the foundation for a deepwater well to withstand the expected load, which are axial loads and bending loads. The conservative design generally uses a large size structural pipe like 36 in. OD with wall thickness from 1.5 in. to 2.0 in. WT. The second surface casing typically uses a large size pipe like 20 in. OD with moderate wall thickness. A weld-on connector are generally used for both structural and surface casings to accommodate the expected axial and bending load resulting from the installation of BOP and marine riser. The paper will describe the progression and transformation from the conservative to a fit-for-purpose design by optimizing the structural casing setting depth, its wall thickness and employing slim well architecture specifically for shallow well objectives below mudline. The most significant finding is that the slim design has proven to be suitable and was successfully installed in three (3) ultra- deepwater exploration wells in Brunei. This paper also will share the Company's experience and observation during the structural conductor installations to achieve efficient execution during jetting operations.

The novelty of the fit-for-purpose design is in the ability to change the design norm resulting in substantial cost savings to the company and meeting the well objectives.

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