Landing Strings and Slip Crushing (A Hand’s View)

Drilling engineering deals with planning all aspects of drilling an oil well. While this may

require some specialized training, it wouldn’t hurt to talk about some fundamental

concepts. It might help us hands out in the field get a better perspective of the big picture.

Some of this planning involves selection of the tubulars required. Some variables to

consider are:

• Outside diameters (OD)

• Inside diameters (ID)

• Wall thicknesses

• Mechanical properties of tube section, connection and upset

• Upset type

• Connection type

• Maximum weight of string

• Tools needed to handle string (rotary slips, elevators, safety clamps etc.)

• Interaction between the tool gripping surfaces and pipe surfaces

Landing String vs. Drill String

The IADC Lexicon describes a landing string as, “Jointed pipe used to run casing strings, liners,

or tubing. NOTE: A landing string can be designed to have a higher load capacity and is often

inspected to a higher acceptance criterion than a string used for drilling.”

A drill string on the other hand, is the drill pipe used for applying torque and force to the

drill bit.

Back when I was still just a young finger way back when, I had no idea what the difference

between the two was, I just did what I was told by them old-timers. Time has a way of

changing our perspectives of things, and there’s nothing wrong with a little perspective.

Like the old saying goes, “If you can’t explain it simply, you don’t understand it well

enough.”

If my understanding is correct, a landing string carries more weight (tensile load), but it

isn’t subjected to rotary torque, only make-up torque. Landing string components may also

have larger wall thicknesses and have higher yield strengths to increase their tensile

capacity.

Drill strings on the other hand, will carry more complex loads like axial loads, torsional

loads and sometimes even bending loads at the same time. The drilling engineers call this

combined loading. Us hands call it, “A lil bit of pull, a lil bit of twist, and a lil bit of bend to

it.”

Minimum Yield Strength (MYS) – Stress at which a material will start to permanently

deform. Steel is very elastic, so when the driller pulls past Martin-Decker on stuck steel

pipe that thing is going to stretch, but it will always spring back after the load is released,

so long as the yield stress hasn’t been exceeded. If you pull on it too much, it will spring

back most of the way but stay stretched just a little bit. This is the definition of yielding.

Tensile stress – Tensile force divided by cross-sectional area (𝐹 𝐴⁄ ). A tensile force must be

perpendicular to the cross-sectional area.

Cross-sectional area – Area of the pipe if you were to cut it with a saw and look straight

down on it.

Basis for calculating cross-sectional area:

Most pipes are round inside and out. We know the area of a circle is A = π𝑟2. Another way

to express the area of a circle in terms of diameter is 𝜋

4𝐷2. The reasoning behind this is that

the diameter is twice the radius, so D = 2r and conversely, r = 1

2D. If you take the original

equation

A = π𝑟2 and replace r with its equivalent 1

2D, you have, A = π(

1

2D)2.

Applying laws of exponents, rearrange it to A = π(12

22 𝐷2). This simplifies to

A = 𝜋

4𝐷2.

When you cut a pipe and look down on it, the cross-section looks like a ring. The area of the

ring is basically just the area of the hollow inner part subtracted from the whole area. In

symbolic form this looks like:

𝐴𝑟𝑖𝑛𝑔 = 𝜋

4𝑂𝐷2 -

𝜋

4𝐼𝐷2

Factor out the common factor to get the commonly known equation:

𝐴𝑟𝑖𝑛𝑔 = 𝜋

4(𝑂𝐷2 - 𝐼𝐷2)

A Hand’s Look at the Landing String

I was looking at this paper1 put out by the AADE (American Association of Drilling

Engineers). It had some interesting information on landing strings. I was doing my best to

try and understand what the paper was trying to say. According to the paper, landing

strings have four failure modes to consider:

• Tube body yield

• Connection yield and shoulder separation

• Slip and upset area damage

• Pipe crush by rotary slip

Industry best practices require these to be addressed when designing a well. Let’s take a

quick “Hand’s View” at each of them.

Tube Body:

As we all know, this is the main part of the pipe, the middle part. The available tensile

capacity of the tube body is defined below.

Tensile Capacity – How much vertical load the tube can carry before yielding or,

Tensile Capacity = Cross-sectional area x MYS or,

Tensile Capacity = 𝜋

4(𝑂𝐷2 - 𝐼𝐷2) x MYS

If we want to increase available tensile capacity of the tube, we should increase wall

thickness and/or minimum yield strength.

1 Adams, Richard, et al. “Deepwater Landing String Design.” American Association of Drilling Engineers National Drilling Technical Conference, Omni, Houston, TX, March 27-29, 2001

Connections:

The connection is all about the threads and tool joint. The connection and tool joint are

usually made to be stronger and harder than the tube body.

Pipe spec sheets have all the important information about the different connection types. In

general, the relevant information about each connection is usually stuff like:

• Make-up Torque Min & Max (MUT)

• Optimum Make-up Torque

• Maximum tensile load before shoulder separation (for min and max MUT)

• Maximum tensile load before connection yielding (for min and max MUT)

• Tool joint OD & ID

• MYS of the connection/tool joint

• Tool joint torsional strength

• Tool joint tensile strength

• Upset type (internal, external or both)

It’s interesting to note that the max tensile loads are dependent on make-up torque. Also as

mentioned before, the connection torsional capacity for landing strings isn’t normally a

design consideration.

Slip and Upset Area:

The slip and upset area will always be equal to or larger than the tube body, never smaller.

Since the geometry of the joint sometimes changes abruptly at these areas, and because the

hard slip inserts engage the pipe surface at these locations, they are more susceptible to

cuts and scarring. Accordingly, these locations are inspected very carefully for any flaws

that could negatively impact the integrity of the landing string. There are many different

inspection methods for this, but all that is beyond the scope of this article.

Pipe Crush by Rotary Slip:

A diligent well designer won’t forget about the possibility of slip crushing. Of course, most

old hands know that slip crushing is more likely to occur with large OD, thin-walled, lower

grade tubulars, but this isn’t a guessing game, we need to be 100% certain. Just like the old

toolpusher once told me, “Some things, once they done, can never be taken back.” Boy, how I

wish I would have listened when I had that chance.

Most of us also know that the slip segments are a variation of one of the six classical simple

machines, a wedge. The mechanical advantage of a wedge is the length of the sloped part

divided by its width, and the definition of mechanical advantage is the amount of force

amplification. From this we can conclude that the amount of force amplification of a simple

wedge is the ratio of the length of its slope to its width or,

Mechanical Advantage = 𝑇𝑟𝑎𝑠𝑛𝑣𝑒𝑟𝑠𝑒 𝐹𝑜𝑟𝑐𝑒𝐴𝑥𝑖𝑎𝑙 𝐹𝑜𝑟𝑐𝑒⁄ = 1 tan 𝛼⁄ .

Sometimes they call the mechanical advantage the force factor or the transverse load factor.

The important thing to know is that when the tip angle of the slip (wedge) is high, the

mechanical advantage is low. When the tip angle is low, the mechanical advantage is high,

which means the amount of force amplification is high. With slips, a high mechanical

advantage will generate more crushing force. We can conclude that a smaller tip angle will

generate more pipe crushing force than a higher tip angle. A sketch of the wedge tip angle is

shown below for reference.

Figure 1. Mechanical advantage of a wedge depends on tip angle, α

Slip Crushing Factors:

The AADE paper says that with respect to slip crushing, there are three deterministic

factors and one variable factor. The deterministic factors are well known and predictable,

the variable factor has more uncertainty.

The three deterministic factors are:

• Pipe dimensions

• Slip dimensions

• Hook load

The variable factor is the coefficient of static friction (µ) between the slip backs and bowl

taper. This value is difficult to determine exactly because it depends on several factors like

amount of lubrication, type of grease, temperature etc. The coefficient of friction represents

how much the two objects stick together. A high value means high friction and vice versa.

There is a range of generally accepted values of the coefficient of static friction for slip

crushing, usually about 0.08 to 0.25. API 7K mandates this value be no higher than 0.08 for

load rating purposes, so we’ll just go with that for now.

Crushing Equations:

John Casner came up with these crushing equations back in 1972. He expressed the main

equation as a ratio of hoop stress to tensile stress.

What is the difference between hoop stress and tensile stress?

Hoop stress is the stress on a cylinder’s surface that acts circumferentially. Tensile stress

acts along its length, also known as axial stress. See Figure 2 below. Common engineering

practice uses a square stress element on the free surface of the part being analyzed. Arrows

on the element show the directions of the hoop stress and the axial stress. This stress

element is the starting point for later stress calculations.

Figure 2. Stress element showing hoop and axial stresses

Symbols used in crushing equation:

K: transverse load factor

D: outer diameter of pipe (inches)

Ls: length of slip insert engagement (inches)

y: tip angle of slip taper (degrees)

z: tan-1 (µ)

µ: coefficient of static friction

Pw: tensile capacity (lb)

Pa: slip crushing capacity (lb)

SH: hoop stress (𝑙𝑏

𝑖𝑛2)

ST: tensile stress (𝑙𝑏

𝑖𝑛2)

Equations:

SH

ST = √1 + [

𝐷𝐾

2𝐿𝑠] + [

𝐷𝐾

2𝐿𝑠]

2 (hoop stress to axial stress ratio)

K = 1

tan(𝑦+𝑧) (transverse load factor)

𝑃𝑊

𝑃𝐴 =

𝑆𝐻

𝑆𝑇 implies that, 𝑃𝐴 = 𝑃𝑊

𝑆𝑇

𝑆𝐻

Slip Crushing Capacity: The string weight at which the slips will start to damage the pipe by

crushing.

Tensile Capacity: The string weight at which the tube body will be damaged by too much

weight pulling down.

Conclusions:

If the desired weight of the total landing string is less than the slip crushing capacity (PA),

we can be confident the pipe won’t crush. (Maybe add in a little safety factor, 1.1-1.3 should

be good)

If the desired weight of the landing string is more than the slip crushing capacity (including

the safety factor), then there might be a danger of the pipe being damaged by crushing and

we’ll have to make some adjustments. Some of the options are:

• Use a slip with longer insert engagement length

• Redo the calculation using different grease for the slip backs. Different lubricants

have different friction coefficients. As the friction coefficient goes up, the transverse

load factor goes down. As the transverse load factor goes down, the slip crushing

capacity goes down. In other words, if the bowl is greased real good, the slip will

squeeze the pipe more. If the bowl is not greased real good, it won’t squeeze it as

much, but then the slip might get stuck in the bowl easier. Just like many things in

life, it’s about knowing what’s important and what ain’t, its all about balance.

• Increase wall thickness

• Switch to a different string altogether

Also be sure to check the slips and bowl real good for abnormal wear. Worn slips and bowls

increase likelihood of crushing. Make sure inserts are in tip-top shape too.

Since Ramey Martin Energy Tools is an API Q1 manufacturer of rotary slips to 7K spec, we

know all about inspection criteria of slips pretty good, so just contact us if you have any

questions. After all, one of our main objectives is helping our customers solve problems and

achieve their goals.

Bibliography

(These papers were used as reference while researching this blog article)

1. Hui, Zhang, et al. “Landing String Design and Strength Check in Ultra-Deepwater

Condition.” Journal of Natural Gas Science and Engineering, Elsevier, 12 June 2010,

www.sciencedirect.com/science/article/abs/pii/S1875510010000375.

2. “Definition of Landing String.” IADC Lexicon, 30 Oct. 2013,

www.iadclexicon.org/landing-string/.

3. Cantrell, et al. “Design and Qualification of Critical Landing String Assemblies for

Deepwater Wells.” OnePetro, Society of Petroleum Engineers, 1 Jan. 2008,

www.onepetro.org/conference-paper/SPE-112787-MS.

4. Adams, Richard, et al. “Deepwater Landing String Design.” American Association of

Drilling Engineers National Drilling Technical Conference, Omni, Houston, TX, March

27-29, 2001