Abstract

The increase in water depth of offshore drilling combined with heavier casing loads has led to dramatic increases in the load requirement of landing tubulars. Landing tubulars to handle these high loads are relatively new, non-standard and generally application specific. As a result, there are no industry specifications or guidelines relating specifically to these products. This paper explores these issues related to landing tubulars and provides a methodology for design, specifying, and review of ultra-critical landing tubular strings.

Introduction

Landing strings are typically defined as tubulars with rotary shoulder connections with wall thickness in excess of those offered by API specifications for drill pipe¹. The primary function of a landing string is to support heavy casing loads. The advantage of a landing string over using longer casing strings and then pulling the unneeded casing is the speed of use and robust nature of the rotary shoulder connections used on landing tubulars. The conventional configuration of the landing string compared to drill pipe also allows the use of conventional rig-handling equipment (Figure 1). It was originally thought that landing strings would serve dual purposes, running casing and drilling. The trend is to use landing strings specifically for lifting capacity with limited if any drilling service. The demand and number of landing strings will likely be larger than originally expected. Originally landing strings were custom designed for specific applications. The lack of common terminology and clear understanding has lead to miscommunication and could lead to potential problems in the future. To date over 30 landing strings in excess of 200,000 feet have been produced. A list of the sizes that have been manufactured are shown in Table 1.

Landing String Designation

There is no standard method of relating an upset pipe weight designation to the wall thickness for these strings, as there is no established standards for the upset. In order to prevent confusion between the manufacturer and the end user, it is recommended that the material be specified by nominal wall thickness. This is especially critical when specifying a particular Minimum Remaining Body Wall (MRBW). API standards require a minimum of 87-½% of nominal wall¹. The growing trend is to order by agreement with the manufacturer either 90% or 95% MRBW. However, in many cases the wall thickness may preclude the rolling mills from offering the 90% and 95% options.

Design Considerations

There are three areas that must be examined when designing a landing tubular. First is the tube body. The tube cross-section area and material yield strength determine the lifting capacity of the assembly:

• Equation (1)

Typically the designer backs into a required cross-section by dividing the required lifting capacity by the Minimum Specified Yield Strength (Ys) of the Grade selected. The rotary shoulder connection must be designed to exceed the lifting capacity of the tube body. The weld or attachment of the tool joint must also exceed the lifting capacity of the string.