FLOATING TECHNOLOGY: Software improves modeling of drilling and intervention in field life economics

Operators are turning increasingly to simulation software to address deep-water development uncertainties. One of the newest tools, called Sloop-2, has just been completed following a two-year program sponsored by nine oil majors. It has already been used to evaluate project scenarios in the Gulf of Mexico and West of Shetland. Sloop (Simulation of Long-Term Offshore Oil & Gas Production) has been developed by London-based BMT Fluid Mechanics.

Work on the first version started in 1995, at the time specifically at Conoco's request, to aid evaluation of potential production systems in mid-Norway's deepwater Voering Plateau. According to BMT's Stephen Rowe, "we were comparing scenarios using dry trees versus others using wet trees in this harsh environment, and the pros and cons of a lower capex (capital expenditures) FPSO (floating production, storage, and offloading vessel) against a higher cost TLP (tension leg platform). The direct well entry afforded by the dry trees for workovers/intervention can be a big benefit in this type of location, and perhaps worth paying for."

In 1998, BMT gathered further support from Arco, BP (Amoco), Elf, Marathon, Mobil, Saga, Statoil, and Total for a second version of Sloop, which could model the complicated behavior of floating systems more realistically. One of the key aims, which has since been achieved, was to better model intervention and drilling tasks. The key aims account for a high proportion of deepwater project costs as well as impacting revenue through delaying production.

Learning from experience

"There had been some disaster fields in the Gulf of Mexico where intervention hadn't worked," Rowe points out, "leading to some wells clogging up. Some fields may also have a higher-than-normal risk that wells will not perform as hoped, and will require excessive intervention to keep them flowing. Simulations can be influential in quantifying the extent of this financial risk."

Computer simulators are derived from mathematical representations of the various oilfield processes (physical, logistical, meteorological, and commercial), which shape production performance. Sloop-2 and other simulations, run in the "event domain," predict the system's status as impacted by successive events. Sloop-2 also factors in the effects of incidents, such as equipment failure and subsequent repair times.

As the simulation progresses, data is compiled on production and operational performance, including resource utilization (percentage of time during which the contracted rig was employed). Important parameters can be varied, allowing a plot of the overall performance measures against that parameter.

"This is one of the greatest benefits of running simulations of this type," Rowe says, "because it helps the user understand how things interact. Fields are very complicated systems, often behavimg in unexpected ways. Simulations help us to understand this sometimes counter-intuitive behavior. To date, 800-plus simulations have been generated by Sloop-2 users and by BMT. In many cases, the outcome has been that production efficiency of a chosen concept has differed from initial assumptions."

Shifting from assumptions

In most deepwater regions, aside from the normally tranquil Gulf of Guinea, the metocean environment can affect performance of the production installation and its various sub-systems (risers, process plant, and others), and repair and maintenance operations can be severely restricted. In the Atlantic Margin in particular, these operations rely on extended periods of weather suited to the systems' operating limits, and also availability of a suitable rig for interventions.

If neither applies, production will be dragged down, and meantime other problems may arise, field life economics increasing the competition for resources and perhaps triggering a need to bring in further resources. Sloop is designed to reduce these complex, inter-dependent factors to a simple value of overall production efficiency. Data required for a typical SLOOP simulation include:

• Metocean parameters over time, such as significant wave height, wind and current speed and direction
• Anticipated hardware, well failure and maintenance frequencies, and the resources and tasks required to perform the repair
• Vessel motion characteristics, including heave, and, possibly, thruster capability to allow modeling of DP-assisted performance
• Process plant operating limits, as this must be shut down when vessel motions are excessive
• Risers and/or mooring system performance
• Export systems, storage, offloading rate and distance to port for visiting shuttle tankers.

However, sometimes early on in the project planning phase, very simple simulations containing little detailed information can also be informative, according to Rowe. Applications of Sloop-2 for deepwater include a drilling program West of Shetland for Conoco and Arco (prior to its takeover by BP). Here, simulation was performed to help plan the exploration program, assess alternative field development options, and also the cost implications of drilling at different times of year.

More recently, Vastar Resources used the simulator to compare alternative production schemes for its Horn Mountain project, where a Spar was eventually chosen.

Most of the study scenarios to date have related to production schemes for FPSOs and Spars in the Gulf of Mexico and West of Shetland. Most concern oil production, although one West of Shetland gas field project has been modeled. Both "snapshot" and field life simulations have been run.

Snapshots represent a year in the life of a field, but are run for many years with differing weather and random failure events to obtain statistically reliable data on the field's performance at that stage of its life.

Field life simulations follow the project from development drilling through to final shutdown, with the entire "life" being re-modeled many times for a statistically reliable outcome. A key aim of Sloop-2 users to date has been to generate a library of field situations for future use. Earlier this year, program users also attended a two-day workshop to develop simulations for alternative production schemes for a field development study set by host BP.

Comparison study

A paper presented jointly at this year's OTC by BMT and Exxon-Mobil summarized four bases cases simulated using Sloop-2 for FPSOs and Spars in the Gulf of Mexico and West of Shetland. All were oil fields with projected daily throughput of 150,000 bbl, with water depths of 2,000 meters in the Gulf of Mexico and 1,000 meters West of Shetland. Onboard storage was 1 million bbl in all cases, except the Gulf of Mexico Spar, which exports to a pipeline. Among the conclusions were:

• The Spar systems with dry trees achieves higher efficiencies than the FPSOs with their subsea trees, the former being easier to maintain and repair, and less sensitive to weather and vessel mobilization delays.
• The free weathervaning solution for the West of Shetland FPSO could be replaced by one with sufficient thrusters and control to orient itself into the waves to reduce roll, thereby limiting process shutdowns caused by excessive roll.
• Production efficiency of Spars, due to their dry trees and the minimal time needed to mobilize intervention equipment, is generally insensitive to well quality. However, the West of Shetland metocean climate can make well management very difficult in a "Bad Well" case, causing efficiency to plummet to 57% against 87% for the base case. The problem is compounded as resources struggle to keep pace with intervention demands; the only quick fix is to charter another rig.

According to Rowe, Sloop-2 can also be used to predict the cost of wells and the attendant rate of return. For one West of Shetland project, the model "drilled" a cluster of six wells 100 times, to examine the value of ensuing returns over a three-year period. "We looked at batch drilling and sequential drilling. Everyone had thought that the batch technique would be better, but our simulation showed a $100 million improvement using sequentially drilled wells.

"When we examined the simulation in detail, it became clear why the batch drilling was not giving us the expected benefit. This is a good example of how simulations can promote understanding.

"We are considering numerous further enhancements to the software, although it's up to the client companies to decide if and when they're needed. There could be a follow-up joint industry project, or it could be that a client that runs into unexpected intervention problems will pay us to enhance the software." The software is also available for non-sponsors and can be run as a consultancy tool on their behalf by BMT.