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Summary

One of the challenges for the development of high-pressure oil and gas prospects in ultradeep water is the high capital-expenditure (CAPEX) cost of subsea flowlines and risers. Installing a high integrity pressure protection system (HIPPS) in the subsea system allows the flowline to be designed to just above well flowing pressure instead of well shut-in pressure. On the basis of flow simulations, pipeline design, and economic analysis, this paper demonstrates that installing subsea HIPPS results in the following:

•Lower installed cost of flowlines and risers.

•Early payback with incrementally higher flow of oil and gas.

•Secondary benefits including higher flowing wellhead and flowline temperatures.

Another important benefit from using subsea HIPPS is that the weight of the risers hanging off at the host facility can be reduced substantially. Otherwise, heavy-walled risers may restrict the tie-in of the deepwater prospect to host facilities.

With pressure decline over time, the thick-walled flowlines become overkill and tend to restrict the well flow rate. With subsea HIPPS, the larger flow area in the subsea flowlines will allow the field to be abandoned at a lower reservoir pressure. This means that that the total recoverable reserves from the field can be increased. The major finding of this paper is that for high-pressure reservoirs in ultradeep water, installing HIPPS subsea accelerates cash flow and increases the project net present value. These benefits will be illustrated on the basis of the analysis of a deepwater field-development design case.

Introduction

A major challenge for the development of a deepwater project is the ability to recover reserves at a reasonable CAPEX investment for the installation of flowlines and risers. One of the main purposes of this paper is to illustrate the use of HIPPS for providing a pressure break between the subsea systems rated to full shut-in pressure and the flowline and riser rated to a lower pressure.

The base case for developing a high-pressure reservoir tie-in to an existing facility is to have the topside HIPPS option. Flow-assurance analysis was performed using OLGA to evaluate the benefits of using the HIPPS system subsea. Flow-assurance analysis provides the critical input in establishing the length of “fortified” section required in the flowline. The length of fortified section is based on how far the pressure wave will travel before the HIPPS valves have enough time to sense and isolate the lower-rated flowline from overpressure.

The flow-analysis study has been divided into three objectives:

•Size the flowlines and risers for topside HIPPS and subsea HIPPS options.

•Define thermal insulation requirements, and study the impact of subsea HIPPS on thermal performance.

•Establish the length of the fortified zone required for subsea HIPPS on the basis of dynamic pressure response.

Design Example

A typical high-pressure field in 4,000 ft of water depth with multiple wells needs to be developed as a subsea tieback to an exiting host facility 10 miles away. This field has a well shut-in pressure of 11,500 psi. The high initial shut-in pressure at the wellhead requires very thick flowlines. Alternatively, subsea HIPPS can be installed and the flowline can be derated downstream of the subsea HIPPS. Comparative analysis was performed for the most common flowline sizes of 6 and 8-in. line size.

Multiple wells were considered for this development. These multiple wells will be daisy chained and produced back to the existing host facility through dual flowlines.

Fig. 1 shows the layout of the generic field. The key assumptions are also listed in Table 1.