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:
•
•
•
•
•
•
•
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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 = π𝑟 2 . Another way
𝜋
to express the area of a circle in terms of diameter is 𝐷 2 . The reasoning behind this is that
4
1
the diameter is twice the radius, so D = 2r and conversely, r = D. If you take the original
2
equation
1
1
2
2
A = π𝑟 2 and replace r with its equivalent D, you have, A = π( D)2.
Applying laws of exponents, rearrange it to A = π(
12
22
𝐷2 ). This simplifies to
𝜋
A = 𝐷2 .
4
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:
𝜋
𝜋
𝐴𝑟𝑖𝑛𝑔 = 4 𝑂𝐷2 - 4 𝐼𝐷2
Factor out the common factor to get the commonly known equation:
𝜋
𝐴𝑟𝑖𝑛𝑔 = 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:
•
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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,
𝜋
Tensile Capacity = (𝑂𝐷 2 - 𝐼𝐷 2 ) x MYS
4
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:
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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 𝐹𝑜𝑟𝑐𝑒⁄𝐴𝑥𝑖𝑎𝑙 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:
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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)
𝑙𝑏
SH: hoop stress (𝑖𝑛2 )
𝑙𝑏
ST: tensile stress (𝑖𝑛2 )
Equations:
SH
ST
= √1 + [
K=
𝑃𝑊
𝑃𝐴
𝐷𝐾
2𝐿𝑠
]+ [
1
tan(𝑦+𝑧)
=
𝑆𝐻
𝑆𝑇
implies that,
𝐷𝐾 2
2𝐿𝑠
]
(hoop stress to axial stress ratio)
(transverse load factor)
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
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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