# 非開削HDD水平方向削孔および推進ケーシング用ステンレス鋼管選定：引戻し耐力と外圧照査

Canonical: https://jundapipe.com/ja/blog/stainless-steel-pipe-trenchless-crossings

河川や交通要衝を横断する非開削水平方向削孔において、ASTM A312およびA790ステンレス・二相鋼管を選定する技術指針。引戻し引張力、外圧座屈強度、および溶接検査要件を解説。

Trenchless Horizontal Directional Drilling (HDD) and microtunneling installations under navigable rivers, major highway corridors, and environmentally protected wetlands require pipeline materials capable of enduring high mechanical installation stresses without external protective coating breakdown. Unlike traditional open-cut trenching where external polyethylene wraps or epoxy coatings can be visually inspected prior to trench backfilling, trenchless methods pull continuous welded pipe strings through borehole drilling mud and abrasive gravel formations. Where aggressive soils, brackish groundwater, or corrosive transported media make carbon steel risky, specifying solid austenitic stainless steel to [ASTM A312](https://store.astm.org/a0312_a0312m-25.html) (TP304L / TP316L) or duplex stainless steel to [ASTM A790](https://store.astm.org/a0790_a0790m-24.html) (UNS S31803 / S32205) provides lifetime corrosion protection, eliminates external cathodic protection requirements, and delivers superior tensile strength.

Selecting between stainless steel grades and calculating required wall thickness for trenchless crossings involves evaluating three interdependent structural load cases: axial tensile pullback load during installation, external hydrostatic buckling collapse under groundwater and drilling fluid head, and long-term internal operating pressure. Citing a standard piping specification without addressing trenchless pullback stresses risks bore yielding or annular collapse.

## 非開削引戻し工程における許容引張力の算出方法

During trenchless pullback, the pipe string experiences combined axial tension from borehole friction, mud viscous drag, and bending stresses induced by drill path curvature. The maximum allowable installation tensile force ($T_{allow}$) is governed by the pipe steel cross-sectional area and the specified minimum yield strength (SMYS):

$$T_{allow} = 0.60 \times F_y \times A_{pipe}$$

Where $F_y$ is the material yield strength and $A_{pipe}$ is the steel wall area. Standard austenitic grades such as ASTM A312 TP316L provide a minimum yield strength of 170 MPa (25 ksi). In comparison, ASTM A790 UNS S32205 duplex stainless steel provides a minimum yield strength of 450 MPa (65 ksi)—over 2.6 times the load capacity of TP316L. Specifying duplex stainless steel allows project engineers to significantly reduce pipe wall thickness while maintaining high installation safety margins on long river crossings exceeding 1,000 meters.

Bending stresses ($f_b$) induced by the borehole trajectory radius ($R$) must be accounted for in the combined stress state: $f_b = E \cdot D_o / (2 R)$, where $E$ is Young's modulus and $D_o$ is outside diameter. Maintaining a minimum drill radius of $R \ge 1200 \times D_o$ ensures bending stresses remain within elastic limits.

## 外部静水頭圧と耐外圧座屈強度の照査

While empty during the pullback process, the casing pipe experiences maximum external hydrostatic head from drilling bentonite slurry and high groundwater tables. External pressure resistance must be verified against Timoshenko elastic buckling equations to prevent ovalization and collapse:

| Alloy Grade       | Standard  | Yield Strength (MPa) | Tensile Strength (MPa) | Elastic Modulus E (GPa) | Typical Application                   |
| ----------------- | --------- | -------------------- | ---------------------- | ----------------------- | ------------------------------------- |
| 304L (UNS S30403) | ASTM A312 | 170                  | 485                    | 193                     | Urban utility casing, low chloride    |
| 316L (UNS S31603) | ASTM A312 | 170                  | 485                    | 193                     | River crossings, industrial effluent  |
| 2205 (UNS S31803) | ASTM A790 | 450                  | 620                    | 200                     | Deep crossings, high tensile pullback |
| 2507 (UNS S32750) | ASTM A790 | 550                  | 750                    | 200                     | Marine subsea outfalls, sour service  |

Where external groundwater pressure is high, selecting a heavier wall schedule (such as Schedule 40S or Schedule 80S to ASME B36.19M) prevents buckling before internal line commissioning.

## 円周溶接部の品質基準と非破壊検査 (NDT) 仕様

Because trenchless crossings pull pipe strings through tight subterranean bores where post-installation defect repair is impossible, circumferential butt weld joints must satisfy the highest inspection standards. Purchase requisitions should mandate:

- Root pass uniformity: Full penetration GTAW (TIG) root passes with internal argon gas purging, ensuring a smooth, flush internal root bead that will not obstruct secondary cable pulling or product transport.
- 100% Volumetric examination: Radiographic testing (RT) or phased-array ultrasonic testing (PAUT) across all girth welds to ASME B31.3 Severe Cyclic conditions or API 1104.
- Chemical passivation: Chemical pickling and passivation of all shop-welded spools to ASTM A380, restoring the protective passive chromium oxide layer prior to field shipment.

Junda verifies material properties through hydrostatic testing, ultrasonic flaw detection, and mechanical testing at its certified [testing capabilities](https://jundapipe.com/production-environment#testing-capabilities) center. Full material traceability is certified under our ISO-accredited [quality assurance](https://jundapipe.com/quality) system.

## 非開削ケーシング・本管の見積依頼および仕様書記載要領

To obtain an accurate quotation for trenchless crossing projects, your engineering request should define the complete fabrication and dimensional envelope:

| Requisition parameter       | Incomplete description | Recommended specification format                       |
| --------------------------- | ---------------------- | ------------------------------------------------------ |
| Material standard           | Stainless casing pipe  | ASTM A312 TP316L or ASTM A790 UNS S32205 seamless      |
| Manufacturing route         | Standard pipe          | Cold drawn seamless, solution annealed and pickled     |
| Diameter and wall thickness | 8 inch pipe            | 8" NPS (219.1 mm OD) × Schedule 40S (8.18 mm nom wall) |
| Length specification        | Random length          | 11.8 to 12.0 meter fixed cut lengths with end caps     |
| Inspection and NDT          | Mill standard          | 100% Ultrasonic test to ASTM E213 + EN 10204 3.1 MTC   |

For detailed grade inventories and wall schedule calculations, explore Junda's [austenitic stainless steel pipes](https://jundapipe.com/products/austenitic-stainless-steel-pipes) or submit your bore profile and crossing drawings through our [contact page](https://jundapipe.com/contact) for a technical review.