Feasibility studies for development of offshore fields must satisfy both technical capability and economic success. To accomplish these objectives, feasibility studies are subdivided into a number of specific tasks to provide a systematic, sequential approach to developing an optimum design. The primary tasks to be performed are selectionof a project team, selection of criteria, description and evaluation of alternative concepts, preliminary design of the selected conceptual costingand economic evaluations. The method of approach to such studies, the requirements for each task and its objective are discussed.

The objective of a feasibility study for development of any field, onshore or offshore, is to select an optimum production system for the operator. Feasible by definition is that which is capable of being done, carried out, used or dealt withsuccessfully. To be feasible, the plan must be suitable, reasonable and practical within the limits of ability or capacity. Thus, any feasibility study must satisfy both technical capability and economic success. Onshore development does not generally require indepth feasibility assessment, because the fields are usually more easily accessible and options are numerous. Exceptions would be locations in harsh environment areas such as arctic or very remote areas with nonexistent infrastructure. However, both technical and economic feasibility must be carefully determined for offshoredevelopment, because the locations are not easilyaccessible and development costs are exceptionally high in comparison to onshore development.

To accomplish its objectives, such a study is divided into a number of specific tasks. Initially, the criteria and field development concepts are established. The various concepts are then studied both qualitatively and quantitatively to assess technical feasibility and arrive at cost and schedule estimates. The various concepts can then be ranked for selection of a preferred case whichis studied in more depth. The preferred case design focuses on preliminary design and inplace analysis of operational requieraents. Installation, hookup and commissioning are included in this phaseof the study. Definitive costs and schedules can then be developed for budget purposes and, more importantly, for economic projections and a final decision.

This paper will describe the method of approach to feasibility studies for offshore field development, discuss the requirements for each task and its objectives.

Feasibility studies can be subdivided into a number of discrete tasks to provide a systematic, sequential approach to development of an optimum system. The level of effort in these studies is less than a detailed design, capable of being implemented in a shop or fabrication yard, but shouldprovide a technically superior system defined to a level where an accurate cost estimate can be produced and a detailed design can proceed. Evaluation of the various concept alternatives should beflexible and when any concept is clearly not technically feasible or cost-effective, further effort on that concept should be discontinued. If no concept is feasible, the study should identify those aspects which require either modification to existing or development of new technology.

The general tasks to be performed in a feasibility study are given in Table 1. Each task is usually subdivided into further tasks for a study plan similar to the one shown in Figure 1.

TABLE 1

FEASIBILITY STUDY TASKS

1. Organization of Project Team. 
2. Selection of Criteria. 
3. Description of Alternative Concepts. 
4. Evaluate Technical Feasibility and Costs. 
5. Rank Alternatives and Select Concept. 
6. Preliminary Design of Selected Concept and Evaluation. 
7. Development of Costs, Schedules, and Economic Projections. 
8. Sensitivity Analysis. 
9. Recommendation. 
1. Organization of Project Team. 
2. Selection of Criteria. 
3. Description of Alternative Concepts. 
4. Evaluate Technical Feasibility and Costs. 
5. Rank Alternatives and Select Concept. 
6. Preliminary Design of Selected Concept and Evaluation. 
7. Development of Costs, Schedules, and Economic Projections. 
8. Sensitivity Analysis. 
9. Recommendation. 
Fig. 1

Study plan.

Project Team

Feasibility studies require input from a wide range of professional disciplines and experience. The project team should preferably be headed by aproject manager with a broad experience in developing and operating offshore fields. Figure 2 shows a typical project team organization. Engineering skills include structural, geotechnical, marine, facilities and petroleum. Input from individuals having construction, installation and operatingexperience is essential. Support from drafting, secretarial and administrative functions are always required and should be designated on the project team. Actually, project management and support services represent a significant amount of effortto a feasibility study, as illustrated in Figure 3, and is often a badly underestimated aspect of the project. It is important to note that engineering time and cost represent only 45% of the total study effort but depending upon the specific study, this percentage could be as high as 60%. The important fact to always consider is the great amount of time and cost required by drafting, documentation, and miscellaneous items such as meetings and printing. The contribution required by these tasks is often unappreciated in planning a study and frequently overrun their budgets.

Fig. 2

Typical project organization.

Fig. 2

Typical project organization.

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Fig. 3

Allocation of effort.

Fig. 3

Allocation of effort.

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Criteria Selection

Establishing sufficient and reliable criteria for the study is the most important task to be accomplished, yet is often treated with nonchalanceor as a lesser important task in order to get to the more interesting tasks. Indifference to the selection of the basic criteria can result in the study yielding seriously incorrect choices being made with disastrous economic impact when that choice is implemented.

General criteria requirements for an offshore field development study are given in Table 2. Although all of these criteria are important, some have a much greater impact upon the study than is realized in many cases and create considerable problems to the project team in developing an optimum concept.

TABLE 2

CRITERIA REQUIREMENTS

  1. RESERVOIR DATA

    • Bottom Hole Pressures

    • Physical Properties of Hydrocarbons

      1. Compositional Analysis

      2. GOR

      3. Gravity

      4. Viscosity

    • Reservoir Temperatures

  2. PRODUCTION CHARACTERISTICS

    • Per Well Producing Rate

    • Flowing BHP and Wellhead Pressures

    • Flowing Wellhead Temperature

    • Reservoir Drive Mechanism

      1. Water Drive, Anticipated Water Cut

      2. Depletion, Gas Lift

      3. Pressure Maintenance or Secondary Recovery

  3. GEOLOGIC INTERPRETATIONS

    • Areal Extent, Thickness of Reservoir

    • Fault Complexity

    • Oil/Water, Gas/Oil Contacts

  4. Water Depth

  5. Distance From Shore Base and/or Terminal

  6. ENVIRONMENTAL CONDITIONS

    • Wind, Wave, Current

    • Ambient Temperatures

    • Ice Accretion

    • Seismic Activity

    • Visibility Conditions

  7. Soil Criteria

  8. Functional and Operational Requirements

  9. Governing Codes of Practice

  10. Special or Unusual Design Codes

 
  1. RESERVOIR DATA

    • Bottom Hole Pressures

    • Physical Properties of Hydrocarbons

      1. Compositional Analysis

      2. GOR

      3. Gravity

      4. Viscosity

    • Reservoir Temperatures

  2. PRODUCTION CHARACTERISTICS

    • Per Well Producing Rate

    • Flowing BHP and Wellhead Pressures

    • Flowing Wellhead Temperature

    • Reservoir Drive Mechanism

      1. Water Drive, Anticipated Water Cut

      2. Depletion, Gas Lift

      3. Pressure Maintenance or Secondary Recovery

  3. GEOLOGIC INTERPRETATIONS

    • Areal Extent, Thickness of Reservoir

    • Fault Complexity

    • Oil/Water, Gas/Oil Contacts

  4. Water Depth

  5. Distance From Shore Base and/or Terminal

  6. ENVIRONMENTAL CONDITIONS

    • Wind, Wave, Current

    • Ambient Temperatures

    • Ice Accretion

    • Seismic Activity

    • Visibility Conditions

  7. Soil Criteria

  8. Functional and Operational Requirements

  9. Governing Codes of Practice

  10. Special or Unusual Design Codes

 

The data which are most frequently treated in an offhand manner are production characteristics. Individual well producing characteristics such asrate, flowing pressures, GOR and temperature, have enormous impact on design and ultimately cost. The results of unrealistic selection of these data can produce overdesigned or seriously underdesigned facilities. The choice of development concepts will be affected by the number of wells to be drilled, spacing of wells and topside facilities required, as much as other criteria such as weather, environmental and regulatory requirements. The need for multiple rigs on a platform, multiple platforms, water flood, gas lift, gas disposal and water disposal add complexity to design and have tremendous impact on cost. Designing a development program to include capacity not required could jeopardize a decision to develop.

Reservoir drive mechanisms also have significant influence on the development concept selection. The need for water flood equipment or gas injection equipment should be realistically designated.

Establishing criteria based on prevalent data used in one area, such as the North Sea, for potential development in much different environmentaland climatic conditions is common and should be avoided. Governmental regulations in some areas have significant cost impact on design and criteriaselection must recognize that impact. Designing to regulatory standards required in one area whichare not required in the area of study should be avoided. This does not suggest one should ignore prudent environmental and safety criteria but should recognize that such standards are set for those particular needs and may not be the same for the area of interest.

Another common mistake in criteria selection is setting unrealistic operating requirements. An example is the result of overestimating the number of persons to be accomodated on the production facility. Space is always limited in offshore structures, thus an overestimate of personnel requirements affects the design of the structure, in addition to living quarters, fresh water supply, power, fuel and storage requirements.

Alternative Concept Descriptions

This task establishes the various alternative concepts for initial evaluation. In the vast majority of cases, conventional piled steel jackets are used for offshore development but even this basic concept has numerous options. Also, these cases are usually in locations where water depths, environmental conditions and distance from adequate shore-based facilities are such that technical feasibility is not a question. The alternate concepts then primarily focus on economic conditions. However, as potential fields are located in deeper waters, in harsh climate conditions or in very remote areas, the technical feasibility of alternate concepts can be of primary importance.

There are four basic alternatives available for offshore development, each with multiple options. By far, the most common development scheme is the use of fixed structures with pipelines to a shore-based terminal. Movement of hydrocarbons canbe via onshore pipelines, by tanker from a shoreline terminal or by tanker loaded at an offshore terminal (usually an SPM buoy). Structural optionsinclude:

  • o

    Conventionalpiled platforms.

  • o

    Concrete or bottom-founded platforms.

  • o

    Compliant structures.

  • o

    Tension leg platforms.

Alternative concepts include single integratedstructures containing drilling, production and accommodations, separate drilling structures with acentral production facility or separate drilling, production and accommodation structures.

Fixed structures with offshore storage and export terminals is being used more frequently. Structural options are the same as listed above but offshore storage options include:

  • o

    A permanently moored storage tanker with rigid yoke mooring.

  • o

    Subsea storage with a tanker loading structure or soft mooring buoy such as SPM, SALS, etc.

  • o

    Direct loading through a mooring arrangement (buoy or structure) to alternating shuttle tankers.

Floating production systems have been a proven development scheme for years but have not beenwidely used. Options for this concept include:

  • o

    Semisubraersible platforms with transport of hydrocarbons through a pipeline.

  • o

    Semisubmersible platforms with storage/off-loading alternatives discussed above.

  • o

    Tanker-based floating, production, storage, offloading terminals (FPSO) transport of oil by direct loaded shuttle tankers or through a remote mooring arrangement.

The two semisubmersible options use subsea completions but provide an opportunity to perform well workover operations direct from the production facility. The tanker-based system can be used with subsea completions or a well protector jacket but has the disadvantage of requiring a separate rig to perform well workovers.

The development scheme most recently being used is to produce full well stream production direct from subsea completions through long flowlines to existing, or shallow water, production facilities. This alternative may be the most promising scheme for very deep waters or low reserve (marginal) fields.

As offshore field development moves into deeper waters, present technology is being extended to its limits. Figure 4 shows the industry's present capability to develop deep water fields. Fixed platform capability obviously has limited technical capability, but more importantly, their costs become so high their economic feasibility becomes questionable. Figure 5 shows the general increase in cost of platforms with increasing water depth, as well as the relationship ofcost between the various alternatives. Although Figure 5 indicates certain alternatives would bemuch less costly in certain situations, each of these alternatives have disadvantages which mustbe thoroughly considered before arriving at a decision. In other words, a decision on a simple determination of cost alone is not necessarily appropriate. When contemplating offshore development in arctic environments, similar costs can be developed for the different structures depicted in Figure 6.

Fig. 4

Offshore development capability.

Fig. 4

Offshore development capability.

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Fig. 5

Offshore production platform cost relationships.

Fig. 5

Offshore production platform cost relationships.

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Fig. 6

Typical arctic development structures.

Fig. 6

Typical arctic development structures.

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The emphasis in evolving the various conceptsshould be on innovative applications of existing, state-of-the-art, technology with an objectiveof optimizing the configuration of each concept. The procedures used in evaluating the different concepts should be flexible and all concepts or applications do not necessarily receive equal attention. When it becomes clear that a concept istechnically inferior or unfeasible and/or is notcost-effective, it should not be pursued further.

The analysis of the concepts should establishthe major components of a system to a degree that reliable cost estimates can be made. The concepts are compared in a relative rather than an absolute sense. In analyzing the concepts, optimumcombinations of structures, transportation, installation, pipelines or storage facilities are sought. When the technical analysis and cost estimates are completed, the alternate concepts canberanked on the basis of technical merit, cost andschedule. From this ranking of the alternatives, one specific concept is selected for more indepth study.

Preliminary Design of Selected Concept

The objective of this task is to perform the design of the selected concept in sufficient detail to assure technical feasibility and develop accurate estimates of cost and schedules so thatthe detail design can proceed rapidly. The majorsubtasks in this part of the study are:

  • o

    Preliminarydesign of the chosen concept under expected environmental and operational conditions.

  • o

    Descriptionof the required facilities consistent with the drilling and production criteria specified and produce deck layouts, weight distributions and deck areas.

  • o

    Descriptionof the cost and schedule of the proposed development plan.

  • o

    A simulation of efficiency of the proposed concept where applicable. Efficiency is evaluated in terms of mechanical downtime, visibility reduction, wave conditions, storage requirements, etc.

Economic Projections

With the above design, costs and schedules, economic projections based on expected programs and production rates can be made to assure the economic feasibility of the selected concept. Sensitivity of economics to variations in cost, schedules and production rates should also be made.

The sequence and emphasis of the tasks discussed are not rigid and will vary for each particular case. Studies for development of fields in very deep water or harsh arctic environments may not reveal a clear selection which meets economic requirements and new concepts must be developed and evaluated. Regardless of the specific case, feasibility studies will be conducted generally as described in this paper. As conditions extend beyond existing technology, feasibility studies identify those needs and should provide conceptual solutions.

The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. Write SPE, P.O. Box 833836, Richardson, Texas 75083-3836. Telex: 730989 SPE DAL.

The author wishes to thank Brian Watt Associates, Inc. for permission to publish this paper and his associates for their assistance in its preparation.