As the exploration and development of offshore energy reserves moves into ever deeper waters, the use of Dynamically Positioned (DP) vessels to undertake drilling and/or completion work is being considered as an alternative to the more traditional methods. The use of DP vessels is perceived as introducing the additional risk associated with the potential loss of station keeping during drilling or completion operations. At least one major operator is now assessing this risk and the corresponding consequences as part of thedecision making process.

This paper illustrates a recent study that considered a variety of deepwater completion operations by examining the following key features of the project.

  • ?The basis for the risk analysis.

  • The collection of input data for the model, including the contribution of expert opinion provided by Completion Engineers and DP experts.

  • The methodology adopted to evaluate the risk costs associated with individual operations.

  • The interpretation of the model outputs.

Finally, examples of points at which the model may be of use within the decision making process throughout the deepwater development cycle are provided.


In recent years several new generation DP vessels have been contracted to undertake deepwater completion operations rather than use the conventional moored vessel option. The decision to use the DP vessel is based on the operational benefits afforded by such vessels for deepwater operations. However, concern has been expressed that undertaking completion operations using a DP vessel could expose the operator to increased risks should a catastrophic loss of station keeping occur.

As part of the many risk assessment and mitigation exercises undertaken prior to the initiation of the completion operation that forms the basis of the studies reported in this paper, the operator's management wished to establish the expected ?risk cost' of using a DP vessel compared to alternative solutions. Risk cost may be defined as a financial measure of the costs associated with the risk of undertaking the proposed operations. It may be used alongside more familiar factors such as Capital Expenditure (CAPEX) and Operating Expenditure (OPEX) when comparing the options available in the overall decision making process.

It should be noted that the results of the current exercise only attempt to quantify the risk cost of the completion operation that arises as a result of a ?loss of station keeping'. This represents only a part of the risk picture. Other risk factors, outside the scope of the current study, that could be considered include, but are not limited to, the following.

  • General hazards of deepwater vessel operations, e.g. those associated with anchor handling (not relevant to DP vessel operations).

  • Operational hazards arising from working over sub-sea equipment, e.g. dropped object and anchor line damage to flowlines, umbilicals and wells.

  • Logistical support requirements, e.g. crew change out or hurricane evacuation.

The specific background to this study concerns the station keeping characteristics of DP vessels compared to those of moored vessels.

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