Riserless Top Hole Drilling Deploying a Low Cost Vessel

Abstract

The main objective of this master thesis has been to investigate the possibility of utilizing the concept; riserless top hole drilling deploying a low cost vessel with consideration to formation pressures, regulations, risks and safety requirements. The economical aspect has also been included in the thesis to evaluate the value of the method.

On a general basis one can say that the petroleum industry has focused on developing the large fields which will profit from a single-stand solution for years. The marginal fields have been downgraded due to being less economically attractive. The industry has started to realize the potential of tie-back solutions but still need innovative technology to drill and complete the production wells at a low cost. The advantages of standardizing the subsea equipment for this type of developments have not yet made use of its full potential.

The concept of drilling the well fundament from a low cost vessel, and further installing the XMT on the Jetta main production well has been investigated in this thesis. This thesis has emphasized drilling of production wells due to the importance of developing marginal fields while the infrastructure still exists. The riserless top hole drilling from a vessel concept is based on a cost efficient development of marginal fields. One of the main advantages is the elimination of twice running and installation of the BOP and riser. The vessel will install the XMT before leaving the location and the rig will install the BOP and riser on top, the remaining hole sections will then be drilled through the XMT.

The integrity of the well when drilling the top hole section riserless from a vessel has carefully been evaluated with consideration to loads and hydraulic analyses. The results show that a low cost vessel is capable of drilling and cementing the surface casing with a sufficient safety margin. In addition the vessel is shown to be capable of handling events as shallow gas and SWF. The concept has also been evaluated with consideration to regulations applying to all petroleum activities on the NCS. The proposed method is not yet defined in the regulations and thereby not allowed for, but the NPD is currently investigating the concept.

The economical assessment show that the potential cost savings for the proposed concept is in the order of 107 MNOK. The lower developments costs is mainly due to the lower spread rate of the vessel, but also due to time savings for installation of the XMT.

This thesis has concluded with that there is a large potential in developing marginal fields with standardized equipment utilizing the combination of a low cost vessel and a drilling rig. For the Jetta field development this concept could increase the profit and thereby make the concept more attractive for similar field developments.