The exploration of offshore oil & gas has been moving to deepwater fields as big reservoirs have been found and technologies have improved. Presently, the most active areas in deepwater oil & gas field development are Africa and the GOM (Gulf of Mexico) in North America.
There are many technological challenges in developing deepwater oil & gas fields such as: flow assurance, subsea systems, riser systems, surface production structures, transportation systems, etc. This paper gives an overview of each deepwater field development concern listed above. The design and installation issues of deepwater pipelines are discussed in detail.
Until 10 years ago, from a European perspective the answer was simple, 200 metres and deeper, essentially the edge of the continental shelf. When viewed globally the answer is not so simple. The Gulf of Mexico, Brazil and West Africa have seen deepwater records tumble as discoveries and production has come from depths greater than 1,000 metres. In April 1998 the record was pushed to 1,709 metres, the current deepest producing field is Marathon Oil's Camden Hills (Oct 2002) at a water depth of 2,198 metres and within the next five years the record is expected to exceed 2,500 metres. Unocal's Trident field could quickly increase the record to 2,953 metres should it come onstream in 2006.
The reasons deepwater developments outrun the onshore and shallow water field developments are:
Limited onshore oil & gas sources (reservoirs)
Relatively larger (~20 times (oil) and 8 times (gas)) offshore reservoirs than onshore
More investment cost (>~20 times) but more returns
Improved geology survey and E&P technologies
The simple graph below shows the worldwide trend in maximum water depths within each year band.
Source :http://www.deepwater.co.uk/info.htm
1.1 Flowline, Pipeline, & Riser System
Oil was transported by wooden barrels until 1870s. As the volume was increased, the product was transported by tank cars or trains and eventually by pipelines. Although oil is sometimes shipped in 55 (US) gallon drums, the measurement of oil in barrels is based on 42 (US) gallon wooden barrels of the 1870s.
Flowlines transport unprocessed fluid - crude oil or gas. The conveyed fluid can be a multi-phase fluid possibly with paraffin, asphaltene, and other solids like sand, etc. The flowline is sometimes called a "production line" or "import line". Most deepwater flowlines carry very high pressure and high temperature (HP/HT) fluid.
Pipelines transport processed oil or gas. The conveyed fluid is a single phase fluid after separation from oil, gas, water, and other solids. The pipeline is also called an "export line". The pipeline has moderately low (ambient) temperature and low pressure just enough to export the fluid to the destination. Generally, the size of the pipeline is greater than the flowline.
It is important to distinguish between flowlines and pipelines since the required design codes are different. In America, the flowline is called a "DOI line" since flowlines are regulated by the Department of Interior (DOI 30 CFR Part 250: Code of Federal Regulations). And the pipeline is called a "DOT line" since pipelines are regulated by the Department of Transportation (DOT 49 CFR Part 195 for oil and Part 192 for gas).
Figure 1.2.1 Flowline/Pipeline/Riser System
1.2 Pipelines
Subsea pipelines present very different engineering challenges as faced by onshore lines. They must deal with higher external pressures and colder temperatures, and unless buried or trenched in the seabed, subsea pipelines are subject to currents and tidal movements.
In addition to providing pipeline services through all project stages - from conception to operation to decommissioning - we add value to our clients' projects and assets by ensuring that client and regulatory requirements are met.
A total of 175,000 km (108,740 mi.) or 4.4 times of the earth's circumference of subsea pipelines have been installed by 2006. The deepest water depth that pipelines have been installed is 2,414 m (7,918 ft) in the Gulf of Mexico (GOM) by Anadarko for the Independence Hub project in 2007. The record is broken by Petrobras Cascade flowlines which are installed in 2,689 m (8,820 ft) of water in GOM in 2009 [10].
The longest oil subsea tieback flowline length is 43.4 miles (69.8 km) from the Shell's Penguin A-E and the longest gas subsea tieback flowline length is 74.6 miles (120 km) of Norsk Hydro's Ormen Lange, by 2006 [1]. The deepwater flowlines are getting high pressures and high temperatures (HP/HT). Currently, subsea systems of 15,000 psi and 350oF (177oC) have been developed. By the year 2005, Statoil's Kristin Field in Norway holds the HP/HT record of 13,212 psi (911 bar) and 333oF (167oC), in 1,066 ft of water.
Nearly 100% of the shallow water gas/oil product is transported to onshore processing facilities by pipelines. Approximately 46,350 km (28,800 miles) of offshore pipelines exist in the GOM [8]. In deepwater, the pipeline is still the most cost effective choice, regardless of design and installation difficulties. But transportation system using shuttle tanker with FPSO will be more attractive as the water depths become deeper and the fields are located further from the shore. In the following sections, deepwater pipeline system design and installation concerns are discussed.
High external hydrostatic pressure, irregular sea bottom profile, and corrosive crude product make the deepwater pipeline design more complicated. The challenging areas in deepwater pipeline designs are (but not limited to):
Material selection
Insulation
Free span mitigation
Installation
Repair
1.3 Risers System
Following the tragic and catastrophic accident in the Gulf of Mexico, all attentions are focused on offshore design of installations of oil platforms. For the safe production of oil, control of well pressures during drilling is a fundamental requirement. One of the pieces of equipment for offshore oil platforms is the riser which is a key component.
The structure systems that connect the platforms at the surface of the sea and the structures found on the seabed are called risers. Normally risers are made of steel or titanium pipe having a wall thickness of less than one inch with an outer diameter of less than thirty inches. The length of the risers depends on how deep the oil production is moving into the water. The deeper the water, the risers become longer and longer, some attaining up to 10,000 feet, but most of them are usually around 3,000 to 6,000 feet in length.
The main function of the riser system is to convey oil and appropriate fluids between the wells and the platforms. It is also designed to accommodate the drill bit and the drill string and in order to deal with the movement of the drilling platform, flexibility in the riser is important. To avoid the subsea wells to be affected by the rolling and pitching of the drilling platforms, slip and ball joints are placed in strategic areas in the risers.
Several types of risers are available ranging from flexible riser, steel catenary riser (SCR), free standing riser to top tensioned rigid riser. However, the most popular riser used in deepwater is the Steel Catenary Riser which offers major advantages over the 'conventional' flexible or 'hybrid' freestanding risers.
The offshore oil riser system normally consists of one or several tubular conduits which are suspended in a vertical position from the floating platform towards the sea bed. At the bottom end of each conduit, a jumper connection and tensioning assembly is available for connecting the bottom ends of the conduit to a subsea oil well. An apparatus is also installed for stopping any horizontal movement of the bottom end of the conduits while on the other hand a vertical tension is applied in the conduits by the means of a weight.
Riser is therefore considered as a vital element for offshore oil platforms, as a failure in the riser will result in stoppage of oil production and can also lead to pollution and spillage. Eventually, risers which are normally slender,experience major fatigue damage like cracks along the pipes, under cyclic load when in deepwater due to the fact that they are prone to vibration under shifting winds, waves and water currents.
Consequently, the impact of a riser failure involves a high risk of human injury leading to death, a considerable amount of pollution in the environment as well as very high economic and political consequences.
Source: http://www.helium.com/items/1829094-what-is-an-oil-riser
2.0 Literature Review
Table 1. Number of Deepwater Gas/Oil Discoveries in Gulf of Mexico (from MMS Website, March 2002)
Table 2.Deepwater Riser Systems
4.0 Discussion
4.1 Pipelines System
4.1.1 Pipeline insulation
Insulation of the pipeline externally is a means employed to keep the heat of the production above the cloud point preventing the formation of hydrates, waxes, and asphaltenes, which would diminish the effective flow through the pipeline or plug the pipe entirely. Traditional insulation systems have used a "wet' insulation material, which is typically polyurethane, polypropylene, rubber, or glass reinforced plastic. These materials' U value is limited to approximately 2 W/m2-oK (0.35 Btu/hr-ft2-oF).
Dry insulation, such as polyurethane foam or rockwool, can achieve better U values of approximately 1 W/m2-oK (0.18 Btu/hr-ft2-oF). The presence of water severely degrades the performance of dry insulation, so a pipe-in-pipe (PIP) system is required to ensure the low U value. By creation of a partial vacuum in the PIP system, U values can be reduced to 0.5 W/m2-oK (0.09 Btu/hr-ft2-oF) [9].
However, for many deepwater and long distance tie-back applications, lowering the U value may not be adequate to keep the high wax and hydrate formation temperatures. To overcome the limits of the above passive insulation system, an active insulation system, such as hot water circulation and electrical heating systems, has been introduced in deepwater field development. Burial of the pipeline to a certain depth or gravel dumping over the pipeline can also provide an insulating effect. Recent research shows that a combination of insulation coating and pipeline burial is more cost effective than the thick insulation coating.
As the offshore industry moves into deeper water, the effective reach of pipelines for crude oil transport to transfer facilities or shore is limited by the insulating power of the exterior coatings. It is that insulation which keeps the heat of the produced oil above the cloud point preventing the formation of hydrates, waxes, and asphaltenes, which would diminish the effective flow through the pipeline or stop the flow entirely by plugging the line.
Input parameters :U=3 W/m2K
Example of multilayer PP coating with FBE layer inside.
4.1.2 Pipeline Integrity Management
Pipeline integrity management services help preserve and improve their performance in the most economical manner, whilst complying with safety and environmental regulations.
Pipeline integrity management services include:
External corrosion management
Fitness for service assessments
Geotechnics and ground movement
In-line inspection services
Integrity management systems audits
Investigation of pipeline incidents
Survey of the pipeline route (onshore)
Pipeline upgrading
Welding technology services
Source http://www.gl-nobledenton.com
4.2 Riser System
4.2.1 Dry or Wet Trees in Deepwater Developments from a Riser System Perspective
Deepwater oil and gas fields are currently developed using wet (subsea) trees or dry (surface) trees, or a combination of both. Once the reservoir characteristics have been determined, the evaluation of development options for a new field is usually focussed around the type of floating production vessel required to develop the field, whilst the well and riser systems are often ignored until the development scenario has been selected.
Dry tree units provide direct access to the wells for workover and improved recovery but require motion optimised hulls to accommodate the riser systems and are considered limiting with respect to water depth and development flexibility. Although widely used for developments in shallow to medium water depth, dry tree units are not considered the optimum way to develop the deep and ultra deep opportunities, despite the industry's preference to extrapolate field proven solutions.
Subsea developments are suitable for widespread reservoir structures. They provide a degree of vessel and field expansion flexibility with simplified riser interfaces, but at the expense of high drilling and workover costs.
Due to the increasing costs of developing fields in deeper waters, the focus of development solutions should be placed on key 'enabling' technologies such as the well and riser systems, and in developing floating production systems which are safe, cost effective, flexible enough to accommodate changes, and capable of being built locally. This is particularly applicable in the upcoming oil and gas plays of the world such as South East Asia where, being remote to the current deepwater activities elsewhere in the world, construction vessel availability is limited and mobilisation costly. The ability to construct or convert the vessel locally will boost employment and the economy of the developing country.
Dry Tree with vertical well access
Wet Tree
Top Tensioned (TTR) riser
Catenary systems (Flexible /SCR)
Motion optimised vessels (Spar/TLP)
Free standing with catenary jumpers
High payload and wellbay design issues
Catenary Moored vessels (Semi / FPSO)
Drilling technologies (threaded construction)
Fatigue sensitivity
Flowline technologies (welded construction)
4.2.2 Types of Risers
4.2.2.1 Steel Catenary Risers
Characteristic
Advantages
Many installations to date
Cost effective solution for subsea tie backs
Deepest - 6,300ft (Semi)
Extension of the flowline
Large Diameter Exports:
20inch Gas (Na Kika)
24inch Oil (Mardi Gras)
Suitable for wide range of diameters and water depths
First Pipe in Pipe Production
Welded construction
Example of Steel Catenary Risers
4.2.2.2 Hybrid Risers
Characteristic
Advantages
Girassol Bundle Riser
4500ft (Prod., GL, WI)
Low fatigue sensitivity (Quasi-Static)
Single Line Offset Riser
Kizomba A - 3300ft (WAG)
Kizomba B - 4000ft (WI)
Pre-installation feasible
Concentric Offset Riser
Kizomba B - Production
Accommodates stringent thermal requirements
Flexible field layout
Riser base gas lift
Low vessel loads
Vessel interface criticality low
Example of Hybrid Risers
Conclusions
Deepwater pipeline installation record depth has been extended to 2,012 m (6,600 ft). As the discovery of large reservoirs and new technologies reduce costs and risks, operators can develop fields in deeper and deeper waters. The current deepwater field development status and available technologies are introduced in this paper. Challenges and emerging technologies in deepwater field development are also identified.
The available technologies discussed here will provide substantial support for deepwater development, but continuing progress in these areas will undoubtedly be required.
