Wellhead protection concepts for subsea marginal developments - Grand Banks of Newfoundland

Fowlow, Dennis Chad (2007) Wellhead protection concepts for subsea marginal developments - Grand Banks of Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

As the offshore industry matures on the Grand Banks, the desire to tap into more marginal oil and gas reserves will be realized and efficient exploitation will be required. A study has been undertaken to enable evaluation of wellhead protection concepts for subsea marginal developments located on the Grand Banks of Newfoundland. The study has focused mainly on concepts that maintain probability of well blowout as a result of freely floating and scouring icebergs below accepted levels of risk. The investigation provides a framework from which intelligent decisions can be made regarding the relative benefits and costs of different protection concepts. -- In order to represent both ends of the spectrum in terms of size and architecture, two subsea marginal field development scenarios, considered typical for the area, were selected. These include (1) single well and (2) clustered multi-well developments tied back to an existing production facility. -- A thorough overview of existing wellhead protection technical solutions such as Open Glory Holes, Cased Glory Holes, Caisson Wellhead Systems and Protective External Barriers were presented. Existing failsafe systems such as surface controlled-subsurface safety valves (SCSSV's) were also investigated in detail to determine their reliability and potential effectiveness for protection against uncontrolled well blowout in the case of catastrophic wellhead damage due to an iceberg encounter. -- A minimum risk acceptance criterion associated with wellhead protection in the region was established to be less than 1 x 10⁻⁵ per annum. Utilizing existing methodologies developed from simple geometric models along with the appropriate iceberg data, an analysis was performed in order to determine the encounter and contact probabilities to wellhead facilities as they relate to the various protection concepts. -- To support the selection and decision making process, a cost analysis was performed. The methodology used in the analysis involved a full comparison of capital expenditure (CAPEX) incorporating the risks associated with iceberg contact. Consequences resulting from an iceberg contact such as lost production, environmental cleanup and replacement I repair costs are factored by the probability of that event occurring. -- Results of the study indicate that SCSSV's and other fail-safe systems offer an obvious solution for reduction in overall risk and up-front development costs. The cost analysis indicates the "Modified Cased Hole" protection concept to be most attractive protection solution from a combined cost & risk approach. A conventional "unprotected" subsea well installation for the Grand Banks may prove to be a feasible development scenario given further research. -- Additional work is recommended addressing issues such as well downhole response mechanisms, SCSSV reliability and refinement to the inherent conservatisms & limitations in well blowout probability calculations.

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