Combination Strings in Casing Design

Combination Casing Strings - Load Distribution Engineering, Grade Selection, and Deep Well Design Optimization

A combination string is a casing design that uses two or more grade-weight combinations within a single casing string to match the mechanical loads at each depth without overdesigning the entire string to the most demanding load case. It is simultaneously a structural optimization problem and a cost-engineering decision: a 9-5/8" production casing string running from surface to 18,000 ft TVD designed uniformly in P-110 53.5 ppf would weigh 962,500 lbs in air, require a 1.6M lbs tensile rating at the top, and cost approximately $1.8M in tubular material alone. The same well with a three-section combination string - P-110 53.5 ppf at the top, P-110 47.0 ppf in the middle, and L-80 43.5 ppf at the bottom - reduces string weight by 140,000 lbs, lowers material cost by $280,000-400,000, and keeps every design safety factor above minimum. In deep wells, HPHT environments, and extended-reach wells where collapse governs at the bottom and tension governs at the top, combination strings are not a convenience but an engineering necessity - the alternative is either overdesigned (expensive) or underdesigned (unsafe) at every depth except one.

1. The Physical Basis for Combination Strings - Why Load Requirements Change with Depth

1.1 Tension Decreasing, Collapse Increasing with Depth

Governing load at each depth:

Tension is maximum at the top of the string (bears full buoyed weight below) and zero at the shoe.
Collapse is maximum at the bottom of the string (maximum external pressure differential) and lowest at the top.
Burst is maximum wherever the internal-external pressure differential peaks - often near the shoe in a gas kick scenario or near surface in a gas column well.

Worked example - 9-5/8" production casing at 18,000 ft TVD:
MW = 14.0 ppg OBM, Cement top at 10,000 ft MD

Tension at surface (buoyed string weight):
BF = 1 - 14.0/65.5 = 0.786
Buoyed weight (uniform 53.5 ppf) = 53.5 x 18,000 x 0.786 = 756,756 lbs

Collapse at shoe (maximum external pressure, gas-evacuated inside):
External pressure = 0.052 x 14.0 x 18,000 = 13,104 psi
Internal pressure (evacuated) = 0 psi
Net collapse pressure = 13,104 psi

P-110 53.5 ppf collapse rating = 10,780 psi → FAILS collapse at the shoe
L-80 72.0 ppf (heavier wall) collapse rating = 14,400 psi → passes, but heavy and expensive
P-110 58.4 ppf (heavier weight) collapse rating = 12,740 psi → still fails

Solution: use heavier/stronger casing only at the bottom where collapse is critical; maintain standard weight at the top where tension governs.

1.2 Safety Factor Profiles Along the String

Depth Position	Governing Load	Minimum Industry SF	Consequence of Underdesign
Top 20-30% of string	TENSION (primary)	1.6-1.8	Casing parting; wellhead overload; surface blowout
Middle section	BURST (primary) + Tension	1.10-1.15 (burst)	Burst failure during kick; external casing integrity loss
Bottom 30-40% of string	COLLAPSE (primary)	1.0-1.1 (collapse)	Collapse during production or cementing - permanent deformation, rig workover required

2. Combination String Design - Calculation Workflow

2.1 Section Length Determination - Setting the Crossover Point

Tension crossover depth - where the bottom section can safely bear the tension from below:
L_bottom (ft) = F_tensile_rating_bottom / (W_ppf x BF)

Worked example - 9-5/8" two-section combination string:
Bottom section: L-80 47.0 ppf, tensile rating = 1,095,000 lbs
Top section: P-110 53.5 ppf, tensile rating = 1,710,000 lbs
BF = 0.786, total string to 18,000 ft

Maximum safe length of bottom section L-80 47.0 ppf (SF = 1.6):
L_bottom = 1,095,000 / (1.6 x 47.0 x 0.786) = 1,095,000 / 59.12 = 18,523 ft

This means the L-80 47.0 ppf could be run all the way to surface from a tension standpoint. But does it pass collapse at 18,000 ft?

L-80 47.0 ppf collapse rating = 7,890 psi
Required collapse resistance at shoe = 13,104 psi
SF_collapse = 7,890 / 13,104 = 0.60 → FAILS collapse → L-80 47.0 ppf cannot be used at the shoe

Bottom section must use heavier wall: P-110 58.4 ppf
P-110 58.4 ppf collapse rating = 12,740 psi → SF = 12,740 / 13,104 = 0.97 → still marginal
Must use P-110 72.0 ppf or higher-grade V-150 47.0 ppf for the deepest section.

2.2 Three-Section Combination String Design

Section	Depth Interval	Casing Grade / Weight	Governing Load	Tensile Rating
Top (Section 1)	0 to 6,000 ft	P-110 53.5 ppf	Tension	1,710,000 lbs
Middle (Section 2)	6,000 to 13,000 ft	P-110 47.0 ppf	Burst + Tension	1,500,000 lbs
Bottom (Section 3)	13,000 to 18,000 ft	Q-125 58.4 ppf	Collapse	2,205,000 lbs

2.3 Full Safety Factor Verification at Each Section

Verification of the three-section combination string:

Section 3 collapse check (0 to 5,000 ft of Section 3, from 13,000-18,000 ft):
Q-125 58.4 ppf collapse rating = 16,020 psi
Max collapse pressure at 18,000 ft = 13,104 psi
SF_collapse = 16,020 / 13,104 = 1.22 → acceptable (min 1.0-1.1)

Section 2 tension check (tension at top of Section 2 = weight of all below):
Weight Section 3 (buoyed) = 5,000 x 58.4 x 0.786 = 229,548 lbs
Weight Section 2 (buoyed) = 7,000 x 47.0 x 0.786 = 258,426 lbs
Total tension at top of Section 2 = 229,548 + 258,426 = 487,974 lbs
P-110 47.0 ppf tensile rating = 1,500,000 lbs
SF_tension at crossover = 1,500,000 / 487,974 = 3.07 → comfortable margin

Section 1 tension check (bears full string below):
Weight Section 1 (buoyed) = 6,000 x 53.5 x 0.786 = 252,396 lbs
Total at surface = 252,396 + 258,426 + 229,548 = 740,370 lbs
P-110 53.5 ppf tensile rating = 1,710,000 lbs
SF_tension at surface = 1,710,000 / 740,370 = 2.31 → acceptable

Material cost comparison:
Uniform P-110 53.5 ppf to 18,000 ft: 18,000 x 53.5 x $1.20/lb = $1,155,600
Combination string as designed: (6,000 x 53.5 + 7,000 x 47.0 + 5,000 x 58.4) x $1.20/lb
= (321,000 + 329,000 + 292,000) x $1.20 = $1,129,200
Saving = $26,400 in pipe weight alone + $80,000-120,000 on grade upgrade for collapse section vs upgrading the full string

3. Application in Challenging Well Environments

3.1 Deepwater Wells - Multiple Stress Drivers

Deepwater Challenge	Effect on Casing Load	Combination String Response
Long air gap (rig to mudline)	High tension in riser; no buoyancy above mudline	Premium grade / heavier weight in top section for tension
High external hydrostatic (seawater)	Collapse load from 0.444 psi/ft x water depth at mudline	Heavy wall or high-grade section at and below mudline
Annular pressure buildup (APB)	Trapped annular fluid expands during production; high internal pressure on outer string	High burst grade in section exposed to APB zone
High HPHT formation pressures	High burst requirement from formation kick scenarios	V-150 or special alloy grade at HPHT section; standard grade elsewhere

3.2 HPHT Wells - Burst-Collapse Reversal with Temperature

Temperature derating of collapse resistance:
Collapse resistance decreases at elevated temperature (yield strength reduction).

Yield strength derating factor (approximate, carbon steel above 250°F):
Yield_T = Yield_20°C x [1 - (T - 250°F) x 0.0003] for T in °F

Worked example - HPHT well at 350°F BHT:
P-110 nominal yield = 110,000 psi
Yield at 350°F = 110,000 x [1 - (350 - 250) x 0.0003]
= 110,000 x [1 - 0.030] = 110,000 x 0.970 = 106,700 psi at temperature

Collapse resistance scales approximately with yield ratio:
Collapse_T = Collapse_20°C x (Yield_T / Yield_20°C) = 10,780 x 0.970 = 10,456 psi at 350°F

This 3% derating can convert an acceptable SF to below minimum at high temperature.
HPHT combination strings must include temperature-derated collapse ratings in the design verification - API tables are for standard temperature conditions.

3.3 High-Pressure Gas Wells - Burst Governs at Multiple Depths

In a high-pressure gas producer, a full gas column from reservoir to surface during a worst-case shut-in creates a significant internal pressure near the surface even when the formation pressure at depth is balanced by completion fluid. The burst design must account for this column effect:

Burst pressure at surface from gas column (full evacuation scenario):
P_burst_surface = P_reservoir - G_gas x TVD

Where:
G_gas = gas gradient (psi/ft) = 0.1 x SG_gas (approximately 0.05-0.12 psi/ft for typical gas)

Worked example - gas well at 14,000 ft TVD, P_reservoir = 8,500 psi:
G_gas = 0.08 psi/ft (mid-range dry gas)
P_burst_surface = 8,500 - 0.08 x 14,000 = 8,500 - 1,120 = 7,380 psi surface burst load

External (backup) pressure at surface = 0 psi (gas in annulus assumed worst case)
Net burst = 7,380 psi

P-110 47.0 ppf burst rating = 9,450 psi → SF = 9,450 / 7,380 = 1.28
P-110 53.5 ppf burst rating = 10,780 psi → SF = 10,780 / 7,380 = 1.46 → use at top section

4. Combination String Selection Matrix

4.1 Grade and Weight Selection by Well Type

Well Type	Top Section (Tension)	Middle Section (Burst)	Bottom Section (Collapse)
Moderate depth (<10,000 ft), onshore	N-80 or L-80, 47-53.5 ppf	J-55 or N-80, lighter weight	N-80, heavier weight
Deep well (10,000-18,000 ft)	P-110 53.5-58.4 ppf	P-110 47.0 ppf	Q-125 or P-110 58.4+ ppf
HPHT (>10,000 psi, >300°F)	Q-125 or V-150, heavy weight	P-110 or Q-125, standard weight	V-150 or special alloy (Cr13, 28Cr)
Deepwater (>3,000 ft water depth)	P-110 53.5 ppf + premium connection	P-110 47.0 ppf + APB mitigation design	Q-125 or V-150 at mudline interval
ERD (>20,000 ft MD)	Q-125 or V-150 (drag + tension)	P-110, lighter weight to reduce drag	P-110 47.0 ppf (collapse lower in deviated section)
Sour service (H2S present)	L-80 or C-90 (SSC resistant)	C-90 or T-95 (max 90-95k yield for NACE MR0175)	13Cr or Duplex SS if corrosion critical

5. Practical Design Verification Checklist

5.1 Design Check Sequence for Each Section

1. Define load profile at 5-10 representative depths: Calculate tension, collapse, and burst at each depth for each load case (running, cementing, pressure test, production, kick). Plot the result to identify where each load type governs.
2. Identify crossover depths: Determine where collapse takes over from burst, and where burst takes over from tension moving downward. These are the candidate locations for grade/weight changes in the combination string.
3. Select casing for each section: Choose the lightest, lowest-grade casing that satisfies all three safety factors at every depth within that section. The governing check changes by depth - verify all three at every boundary.
4. Check connection efficiency: Premium connections may be required at the upper section where pipe body strength is fully utilized. API connections at the lower section are often acceptable.
5. Verify triaxial loading at critical depths: At each grade crossover point, run a von Mises check to confirm the combined stress does not exceed yield despite passing the individual uniaxial checks.
6. Temperature derating in HPHT: Apply yield and collapse derating factors before accepting any section that operates above 250°F. API table ratings are issued at standard temperature.

Conclusion

The combination string calculation in this article - a three-section 9-5/8" design showing SF_tension = 2.31 at surface, SF_collapse = 1.22 at the shoe, with tension crossover depths calculated from tensile ratings - demonstrates why the design workflow must be performed at multiple depths rather than at a single worst-case point. The cost comparison, $1,155,600 for a uniform P-110 53.5 ppf versus $1,129,200 for the optimized combination string, understates the true saving: the combination string also avoids upgrading the entire string to the heavy-wall grade needed at the shoe, which would have cost an additional $80,000-120,000 in tubular material and increased total string weight by 4-6% - directly impacting hookload, wellhead, and rig capacity requirements. The temperature derating calculation reducing P-110 yield from 110,000 to 106,700 psi at 350°F, and collapse from 10,780 to 10,456 psi, illustrates why HPHT combination strings cannot rely on API table values alone.

Combination string design is a forward-looking engineering activity. The grade-weight crossover depth selected during well planning must remain valid throughout a 20-year well life that includes pressure tests, multiple workover interventions, production decline leading to artificial lift installation, and potential re-perforation of lower zones at higher pressures than originally planned. A combination string that passes all checks on day one but is designed with no margin for operational envelope changes will fail when production conditions evolve. The cost of selecting the correct combination string at the design stage is 30-60 hours of engineering time. The cost of a collapsed casing section at 15,000 ft discovered during a workover is $3M-20M in remedial work, sidetrack, and lost production.

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