Driver Options for Offshore Applications
Rainer Kurz, Manager - Systems Analysis
Cynthia Sheya,Principal Project Manager
Praveen Singhal,Sales Manager, O & G
Solar Turbines Incorporated

Offshore oil and gas production requires both electric and mechanical power for various applications. In general, the electricity is generated on the platform by gas turbine driven generators. Traditionally, gas turbines have also been the driver of choice for, compression and larger pump services. An alternate approach is, to use electric motor drivers for these applications. In this case, the power for the electric motor and the utility power is supplied by larger gas turbines used as a central power generation plant. This article discusses evaluation considerations to aid in decisions to go with either solution.

Offshore oil and gas production requires both electric and mechanical power for various applications. In general, the electricity is generated on the platform by gas turbine driven generators. Traditionally, gas turbines have also been the driver of choice for, compression and larger pump services. An alternate approach is, to use electric motor drivers for these applications. In this case, the power for the electric motor and the utility power is supplied by larger gas turbines used as a central power generation plant. This is sometimes referred to as the "All Electric" solution. There are several important factors to be evaluated when considering options and selecting the optimum solution for this type of application.

Offshore oil and gas production use a variety of production platform and FPSO designs, depending generally on water depth. In many instances, the necessary electrical power is generated on the platform, and this is one of the assumptions for this study. There are other scenarios, where power is generated onshore, or surplus power from other platforms is used. We also will not consider situations where an existing electric power generation system can be used, for example for conversions.

Since the topsides of any of these systems have to be planned and designed based on data with a high uncertainty, and further, since the design parameters (such as flow, vapor and liquid composition, and pressures) can and will change significantly over the life of the project, flexibility is one of the paramount requirements. Typical offshore or gas field applications show a fast ramp up of power requirements (and oil production), with a slow decline in later years.

While not a paramount requirement, but yet important in any technical design , is the requirement of simplicity. The best engineering solution is generally the one that is the least complex, least costly, most reliable, most durable, and most maintainable. This is especially true for remote locations such as offshore platforms.

The production facilities on a platform may include gas turbine driven generators, compressors for gas gathering, flash gas compression, gas boost or gas export. Enhancing the oil yield may require water injection (using high pressure water pumps), compressors for gas lift or compressors for gas re-injection. Compression for CO2 injection might also be used. Other topside equipment, such as crude oil pumps, heaters and others, use electric power [1].

Any possible solution has to be evaluated based on first cost, cost of operation (this includes maintenance, and fuel cost if applicable), emissions (this also depends on pertinent regulations as well as operating company guidelines and can include one or more of NOx,CO,CO2,UHC,VOC and particles), reliability, availability, and flexibility. Depending on user philosophy, Total Cost of Ownership, which includes all the above factors at a discounted level, may be used to compare different alternatives.

The fundamental choice for the compression equipment is, to either drive the compressors with gas turbines, or to use constant speed electric motors or variable speed electric motor configurations. The former choice will lead to smaller gas turbines for power generation. There are choices for different electric motor drives. Some use constant speed motors, while others allow to vary the speed of the driven compressor, either by feeding the motor through a variable frequency drive (VFD), or by using a variable speed gearbox. Electric motor drive systems have to be carefully evaluated to be able to start, in particular if the intent is to start a compressor from pressurized hold. Constant speed drives require the electrical system to supply the large inrush current necessary during start, unless softstarters or reduced voltage starters are used.

First cost and cost of operation depend largely on the amount of installed power. Kurz and Sheya [1] have shown, that in general, any electric motor driven solution will increase the amount of gas turbine power. This sounds paradoxical, but it is due to fact that there are significant system losses to generate electricity, bring it from the generator to the compressor drives, and feed the variable frequency drives and electric motors. If constant speed motors are used, the power supply has to be oversized to be able to start the motors. Constant speed compressors that can only be controlled by throttling or recycling have to be significantly oversized. Otherwise changing operating conditions (for example, changes in gas composition) will require costly and time consuming modifications to the compressor or its gearbox [2].

The difference in installed power between platforms with gas turbine driven compressors, compared to electric motor driven installation, increases as the total power consumed by gas compression increases. In other words, platforms where a large amount of the power is consumed by the compressors tend to favor gas turbine driven compressors on the basis of first cost and maintenance cost. While the maintenance for the electric motors is relatively low, the power generating gas turbines have to be significantly larger for all electric solutions.

It is a valid assumption that the gas turbines for either solution can operate using a common fuel conditioning system. The fuel is typically associated gas from the oil production. The high voltage electric motors require a relatively complicated power distribution system with multi-tiered voltage power distribution and motorized, non-arcing switchgear. If the electric power demand exceeds about 10 to 15 MW, systems with multiple voltage levels become necessary; a high voltage system (11 or 13kV) will have to be installed in addition to the 4-6.6 kV medium voltage system on the platform.

Speed control is achieved through Variable Frequency Drives (VFDs), requiring isolation transformers, and either air cooling systems or above 6MW, water cooling systems (coolers, pumps, and piping). Ubiquitous harmonics will define the use of harmonics filters. Enough space needs to be planned for the motor control center and switchgear. This will later become important in the evaluation, because deck space is at a premium for any platform. Weight is of importance as well, since it defines the structural and buoyancy requirements of the platform.

Included in the considerations are the first cost of the required equipment, the installation costs, the total system weight and space requirements and its impact on platform design and fabrication cost. For example, when taking the entire electric system into consideration, the footprint and weight is likely higher than a turbine mechanical drive.

A typical, 10MW electric motor with the VFD system weighs about 30,000kg for the Motor and 30,000kg for the VFD, while a 10MW gas turbine weighs about 8-10,000 kg. The weights mentioned are understood without skid and ancillary systems. The space requirements for a 10 MW VFD are about 1.5m x15m, with a height of 2.5m in addition to the motor-compressor train. The footprint of a typical electric motor driven skid is typically 12m by 3m, while a gas turbine skid would have a very similar, footprint of 15m by 3m. Figure 1 outlines the weight and size comparison. The weights and footprints in this comparison include the skid and ancillary system for the gas turbine.

The electric drives, depending on the configuration, include the VFD, transformers, gearboxes, and the motors, respectively. It should be noted that, in addition to the weight and size of the compressor drives, electric drives also will increase the size and weight of the platform power generation.

Weight and size of the installation have a significant impact on the overall platform cost.

Even routine, planned maintenance can be disruptive for the platform operation. Maintenance requirements for the electric motor by itself are lower than the requirements for a gas turbine. However, the entire system (power distribution, VFDs) adds to the maintenance requirements for electric motors and should be included in any evaluation. These systems are usually custom designed, and therefore may cause significant downtime due to parts availability.

It has been argued, that the maintenance for a larger number of smaller gas turbines can be more disruptive than for a smaller number of larger engines, because the maintenance events occur more frequently. However maintenance costs are roughly proportional to installed power, and fired hours, although there is a slight advantage on a cost per MW basis for larger turbines. This is often outweighed by the fact that electric drives require more installed power.

The availability and reliability of electric drives is often assumed to be better than for gas turbine drives. Taking into account comparisons for entire systems including availability and reliability of the entire electrical and power generation system reveals that this is not true for many installations. Rather, the largest impact on reliability and availability of the entire system comes from the amount of spare units available. This applies to compressor drives and generator sets alike. The available data does not indicate an advantage for electric drive systems, under comparable conditions. Also, availability is driven not only by the frequency of machinery failure but more so by the time it requires to repair the problem. There are large differences between different manufacturers, and different service arrangements. Published availability data, including time to repair the problem is difficult to find.

The decision for any one of the described architectures is a difficult one. We have described a number of criteria that relate to first cost and life cycle cost of a project, including considerations such as the changing operating conditions platforms and FPSOs see over their design life. One of the key findings is that an electrified solution will always require more installed gas turbine power than a solution where the mechanical equipment is driven by gas turbines.

The main drivers for life-cycle costs for the equipment are first cost, fuel cost and maintenance cost. Any comparisons regarding economics have to be made on an overall system level, where weight, footprint, complexity, as well as reliability and availability have to be considered. Comparisons that stay at the component level will lead to incomplete results and incorrect conclusions. Sparing of equipment will improve equipment uptime, but again this has to be seen in the context of the entire platform.