1.1 Background
Oil and gas industries are one of the most modern and high technological industries among the others. Their essential existences in global world activities are truly powerful and un-displaceable. Even though they are not renewable energy, but the energy supply is still highly demanded.
Offshore oil and gas development is relatively recent historically. The first well located offshore in the Gulf of Mexico was drilled in 1947 at Kerr-McGee’s Ship Shoal block 32. It was 17 km from shore and in 6 m of water depth. (Palmer & King, 2004). Since then, the emerging of offshore oil and gas industries is growing drastically with high sophisticated technology. Not to mention, to attract and explore in deeper water.
In recent years, there has been an increasing trend towards ultra deepwater exploration. To date, Perdido platform is the world’s deepest offshore drilling and production activity at 2450 m (8,000 feet) water depth (Shell, 2011). Located 320 kilometers from the Texas coast in Alaminos Canyon Block 857, this spar platform can handle 100,000 barrels of oil per day and 200 million standard cubic feet gas per day.
Figure 1.1 – Deepwater Milestones (Shell, 2011)
The advancement of technology in ultra deepwater has been leading Brazil into one of promising offshore market. According to GBI Researh (2010), Brazil’s offshore crude oil reserves were 11,744.3 million barrels in 2008. Recent sub-salt discoveries (e.g. Tupi Field) have transformed Brazil into a country with one of the highest potential investment acreages globally. According to Saliés (2005), 33% from the total exploration area operated by Petrobras, a Brazilian state-owned oil operator, are at water depth below 1,500 m. In late 2011, the company confirms the discovery of oil and natural gas located in 2,313 m water depth of the Sergipe-Alagoas Basin offshore north east Brazil (MercoPress, 2011). This can be concluded that the future lies in ultra deepwater.
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Figure 1.2 – Petrobras Brazilian Exploration Leases per Water Depth (Saliés, 2005)
Though ultra deepwater developments are being promising, there are a lot of challenges that is always become our interest, in particular, the selection of riser concept. Ultra deepwater riser selection is one of the major drivers in the evaluation of technical and feasibilities of a project. As the preliminary field layout and floating platform type are selected based on reservoir, drilling, and environmental conditions, the riser selection is interdependent compare to them. A proper floating platform motion will offer reliable riser behavior, while a robust riser configuration will have impact on less design constraints on the platform and eventually on the project execution (Shu et all, 2011).
In this thesis, many riser concepts will be discussed. Among of these, the newly developed Catenary Offset Buoyant Riser Assembly (COBRA) concept is selected as the main topic of this thesis, in particular for offshore Brazil ultra deepwater environment. In general, COBRA presents a combination between Steel Catenary Riser (SCR) at bottom section and flexible jumpers at top section, with a long and slender subsurface buoyancy module on top of SCR section on which it is tethered down to sea bed. The flexible jumper is connected to the host platform and can effectively absorb the platform motions. According to Karunakaran et al (2011), with this concept, it can improves both strength and fatigue performance of the riser system.
1.2 Purpose and Scope
The emerging ultra deepwater market in offshore Brazil and development study on the new riser concept are the key points on this thesis. This thesis looks into further COBRA riser concept optimizations with regards to offshore Brazil ultra deepwater conditions. This thesis will capture a base case study of COBRA and sensitivity study of the base case. Among of these are the sub-surface buoyancy position with regards to the water depth, the flexible jumper end-connection configurations, and the buoyancy tethers configurations on the sea bed.
A static and dynamic analysis will be performed in conjunction with the above mentioned cases. OrcaFlex software will mainly be used to study the topics. In addition, VIVANA software will be used for fatigue due Vortex Induced Vibration (VIV).
The scope of thesis will consist of:
Chapter 2 gives review of general type of riser systems, challenges in ultra deepwater condition, and focus on the uncoupled riser system for ultra deepwater environment, including the COBRA riser concept.
Chapter 3 provides the code checks that are used in riser design. The LRFD code based on DNV code is the main focus on this chapter.
Chapter 4 gives the analysis methodology of the riser analysis, including some theoretical backgrounds that relevant on this thesis.
Onshore
Shallow w ater up to 300 m Deep w ater from 300 to 1500 m Ultra deep w ater (deeper than 1500 m)
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Chapter 5 gives the design basis of the COBRA concept study. These include general overview of the riser system, design parameter, model overview, and also the design acceptance criteria
Chapter 6 provides detail information of the COBRA concept base case study and correspondence response from the case. This includes static, dynamic, and fatigue responses.
Chapter 7 demonstrates the sensitivity study from the base case configuration from Chapter 6. The sensitivity methods are mainly focused on the riser system configurations. At the end of this chapter, a discussion on comparison summary is presented.
Chapter 8 gives the conclusion and recommendation from the study.
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