Optimising FPSOs in extreme environments

Paul Boughton

Wojciech Misiag explains the importance of understanding the environmental conditions at a site in order to determine when offloading operations need to be suspended or when an offloading tanker needs to be disconnected.

Oil and gas companies continue to move into new and increasingly harsh and remote environments to meet the world's growing demand for energy. For example, the potential wealth of natural resources in the Arctic is leading organisations to show an increased interest in this region. However, exploring new frontiers, as well as constructing new facilities carries risks and underlines the importance of understanding, mitigating and managing the associated operational risks.

Optimising the design, operability and safety of FPSOs (Floating Production Storage Offloading vessels) is one such area that must be duly considered.

Deepwater oil and gas resources are an important part of the energy mix, especially in the US, where offshore production in the Gulf of Mexico has reversed a 23-year trend of declining domestic oil production. According to a new assessment by the US Geographical Survey released earlier this year, 75% of the world's undiscovered, technically recoverable conventional oil, (exclusive of the US) is in four regions: South America and the Caribbean; sub-Saharan Africa; the Middle East and North Africa and the Arctic provinces portion of North America.

Although these regions represent the richest areas of energy resource, they also signify the most challenging operational environments for oil and gas companies. As well as tropical storms and hurricanes in areas such as the Caribbean for example, the harsh winter weather in the Arctic continues to be the most dangerous and expensive place for oil and gas exploration. Equipment must be specially designed to withstand the frigid temperatures and limited transportation access and long supply lines can significantly reduce the available options, whilst increasing costs. Higher salaries are also required in order to attract personnel to work in such an isolated and inhospitable area.

Despite these difficulties, interest in the Arctic is on the up - especially at a time when oil and natural gas fields in other areas are depleting. The combination of environmental conditions experienced in this region is unlike those in existing fields therefore, companies must ensure they have a deep understanding of their influence on oil and gas exploration. This will allow for the design of their marine operations to be optimised and the associated risks when operating within such harsh and remote environments, to be mitigated.

Planning marine operations in these demanding environments is no mean feat. Each stage has its own associated risks which must be managed effectively. The number of FPSOs being put into service is continuing to increase. FPSOs are known to be a particularly attractive and flexible option in remote or deepwater locations as they eliminate the need to lay expensive long-distance pipelines from the processing facility to an onshore terminal.This can provide an economically attractive solution for smaller oil fields which can be exhausted in a few years and do not justify the expense of major investments in fixed infrastructure including pipelines. Furthermore, once the field is depleted, the FPSO can be moved to a new location. However environmental conditions can significantly impact the operation of FPSOs and in turn production, therefore the risks must be managed.

The Arctic for example, presents challenges in the prediction of weather and ice flow patterns for the design of FPSOs and transport systems, as well as the evaluation of ice-induced loads on vessels during exploration operations. By overcoming these challenges, companies can minimise downtime.

In order to stay in place, FPSOs are either moored to the seabed, or utilise Dynamic Positioning (DP), a computer-controlled system which automatically maintains the FPSOs position by using its own propellers and thrusters. Regardless of the option used, severe metocean (wind, wave and currents) conditions cause motion of the FPSO and loads in the mooring system, if used. Therefore, companies must analyse these conditions and responses early in the design stage so that the optimal choices can be made between DP and mooring - and the type of mooring, if this is the preferred option.

Further, the system for mooring shuttle tankers to the FPSO has to be considered carefully. This can be done either with tandem mooring (generally easier both to remain attached and for arrival and departure) or using side by side mooring (generally required because of restrictions in the ability to transfer the product through long or flexible hoses). An unsuitable mooring solution can result in unnecessarily frequent disconnection of the shuttle tanker from the FPSO and subsequently a suspension in production. Badly designed mooring systems are also a safety issue and could result in a fatal accident, so this aspect obviously deserves attention early in the design process.

Side by side mooring requires motion analysis of both ships. For both systems, the effect of unexpectedly deteriorating weather conditions such as sudden squalls needs to be accounted for when designing the ship to ship connections and developing procedures. Also, tug assistance may be required to support safe arrival and departure of shuttle tankers for both systems. Analysis and simulation of the moored shuttle tankers and of their manoeuvring at arrival and departure allow for risk mitigation to be put in place so that oil and gas companies feel comfortable, while minimising the risk of an unnecessary suspension of production.

It's important to note that the required marine transportation doesn't stop at the FPSO. Organisations must also consider the shuttle tanker's journey between the FPSO and the terminal or port where the discharging of oil and gas occurs. Operators want to know how many tankers they will need to guarantee production isn't interrupted because of insufficient offloading capacity. This depends on the metocean conditions at the site of the FPSO, the terminal and the route between the two. This means that expected metocean conditions in a severe month have to be known. The resulting delays can then be accounted for in the required fleet size estimations.

Failure to account for weather conditions may result in underequipped shuttle tankers having too weak thrusters or insufficiently powered tugs at FPSO sites or at terminals. This in turn, could lead to the tankers having to abort berthing manoeuvres too frequently, thus increasing the downtime of operations and halting production at the FPSO site.

Utilising an effective simulation tool such as BMT PC Rembrandt at the feasibility stage will allow the safety of arrivals and departures of manoeuvring shuttle tankers to be assessed depending on the environmental conditions and provide oil and gas majors with essential information on the operability of FPSOs and oil terminals.

Supporting companies in the assessment of terminal navigational safety is key. By providing simulation technology the limiting conditions for manoeuvring at berths and the required tug power can be determined. The design of port and terminal access channels also has an influence on vessel safety, terminal construction and maintenance costs and must therefore be, appropriately assessed.

Optimising the design, operability and safety of the marine transportation within the feasibility stage can provide companies with a larger operational window, increasing production and throughput. Furthermore, understanding the motions and loads can lead to a reduction in maintenance costs. Such analyses can also provide reassurances for the wider community, critically the regulatory bodies and the Governments who will have the final say on whether or not companies can begin exploration in a particular region.

BMT ARGOSS offers metocean consultancy, weather forecasting, and state-of-the-art systems for emergency response, vessel simulation and performance monitoring.

Wojciech Misiag is a Naval architect at BMT ARGOSS, a subsidiary of BMT Group, Marknesse, the Netherlands.

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