Landing string design and strength check in ultra-deepwater condition

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Abstract

Landing string used in ultra-deepwater casing running operations sustains complicated loads, including axial load, slip crushing load, bending load and dynamic load. Landing string design methods based on tension load are presented in detail. In these methods, sufficient margin of over pull is taken into account for the extreme axial load during surface casing running operations and environment loads caused by wave, current and vessel offset and heave. Landing string strength check is discussed to evaluate the safety of landing string in harsh environment. According to the analysis, slip crushing capacity is one of the critical limitations for landing string; bending stress caused by currents is great at the top of landing string at the beginning of operations while it will decrease by adjusting the vessel position after setting the casing below mud line.

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

References (8)

  • R.J. Adams

    Proven landing string design for ultra-deepwater application

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  • American Petroleum Institute

    API RP 2A-LRFD: Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms – Load and Resistance Factor Design

    (1993)
  • American Petroleum Institute

    API RP 7G. Recommended Practice for Drill Stem Design and Operating Limits [S]

    (1998)
  • J.W. Breihan et al.

    Landing strings: design, manufacture, inspection and use issues

    Journal of Petroleum Technology

    (May 2002)
There are more references available in the full text version of this article.

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