Landing string design and strength check in ultra-deepwater condition
Introduction
During ultra-deepwater oil and gas well drilling operations, landing string is used to run casing into the hole below mud line. The axial load of landing string is great during casing running operations (Adams, 2001, Breihan et al., 2002). Especially in surface casing running operations, after the surface wellhead housing lands in conductor, it may sink into seabed once the friction strength between conductor and formation is not sufficient to support the weight of whole strings. In this case, conductor must be pulled out of hole with surface casing. The axial loads will include the friction resistance due to pulling out conductor. Landing string may reach its tensile capacity as axial load increasing. Surface casing running is operated in non-riser conditions. Tubular string is affected by wave, current and vessel movements which cause bending load and dynamic load on landing string (DeWayne Everage et al., 2005).
Design methods based on tension loads which are used in drill string design can also be used to design landing string (API RP 7G, 1998, Chen and Guan, 2000). However, those methods need to be modified because of the axial loads consideration in surface casing running operations and environment loads in offshore conditions. This paper focuses on design methods and strength check of landing string and analyzes the effect of environment loads on landing string safety.
Section snippets
Landing string design methods
Three methods are used in landing string design including safety coefficient method, slip crushing constant method and margin of over pull method. The following two points are deserved to be paid attention.
- 1)
Bending load and dynamic load caused by environments and vessel movements are always existing throughout the casing running operations, no matter tubular string is tripping or in slip. It must be sure that landing string has sufficient tensile capacity. Therefore, a margin of over pull MOP1
Landing string strength check
Landing string works under composite loads conditions which include axial loads, bending loads, dynamic loads and slip crushing loads. Bending loads and dynamic loads are related to environment and vessel performance. For safety consideration, total stress in landing string should not exceed the yield strength of material in the tolerance environment conditions.
Bending loads are caused by currents and vessel offset. The model of landing string can be simplified as vertical and horizontal
Example
Liwan is a natural gas reservoir located in ultra-deepwater area of South China Sea. An exploration well LW21-1-1 will be drilled in October 2010 which is going to be the deepest water depth well in this area. The water depth is 2461 m and bushing altitude is 31 m. According to the casing programs, 508 mm surface casing string will be run to 558 m below mud line.
Conclusions
- 1)
Adequate margin of over pull should be reserved in landing string design due to bending load and dynamic load caused by environments in ultra-deepwater conditions.
- 2)
Slip crushing capacity is one of the critical limitations for landing string to run longer and heavier casing to deeper depth. It is better to use a slip with longer contact area in casing running operations.
- 3)
Since bending stress caused by current is great at the top of landing string when casing stays in the surface layer with high
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Proven landing string design for ultra-deepwater application
World Oil
(July 2001)API RP 2A-LRFD: Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms – Load and Resistance Factor Design
(1993)API RP 7G. Recommended Practice for Drill Stem Design and Operating Limits [S]
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Landing strings: design, manufacture, inspection and use issues
Journal of Petroleum Technology
(May 2002)
Cited by (7)
Drilling and borehole techniques relevant to natural gas exploration and development: A collection of published research (2009-2015)
2015, Journal of Natural Gas Science and EngineeringCitation Excerpt :It is concluded that smaller wave height and longer wave period are good marine riser installation conditions that minimize transverse resonance. Zhang et al. (2010) reviewed the complex loads sustained by ultra-deepwater casing during its installation, and evaluated design methods based on tension load. The methods take into account the extreme axial load during surface casing runs, and the environmental loads associated with vessel and water-column movements, to leave a sufficient margin of over-pull.
Some research progress in deepwater drilling mechanics and control technology
2019, Shiyou Xuebao/Acta Petrolei SinicaNonlinear Dynamic Characteristic Analysis of a Landing String in Deepwater Riserless Drilling
2018, Shock and Vibration