Shallow Water Pipeline Installation Analysis

Methodology

This analysis screens shallow water S-lay pipeline installations across a parametric matrix of vessels, pipe sizes, and water depths. Each case solves the catenary mechanics to find the minimum horizontal tension and evaluates five installation checks per DNV-ST-F101 and API RP 1111 methodology.

S-lay Catenary Theory

During S-lay, the pipe forms a catenary curve from the vessel stinger tip down to the seabed touchdown point (TDP). The catenary is characterised by parameter a = H / w_sub, where H is horizontal tension and w_sub is submerged weight per unit length.

• Arc length: s = a × sinh(x/a)
• Height above TDP: y = a × (cosh(x/a) - 1)
• Pipe angle: θ = arctan(sinh(x/a))

Check 1: Stinger Departure Angle

The required departure angle at the stinger tip must fall within the vessel's adjustable range. Large PLV: 2°-8°, Barge: 3°-12°.

Check 2: Top Tension

Tension at stinger departure: T_top = H × cosh(x_dep/a). Must not exceed vessel tensioner capacity (600 te or 250 te).

Check 3: Sagbend Stress (Critical for Shallow Water)

At the TDP, curvature κ = w_sub / H gives the tightest bend radius. Combined bending + axial stress: σ = E × OD / (2R) + H / A_steel. Must satisfy σ ≤ 0.72 × SMYS per DNV-ST-F101 installation allowable.

Check 4: Overbend Stress

Pipe bending over the stinger: R_ob = L_stinger / sin(θ_dep). Overbend stress must also satisfy the 0.72 × SMYS limit.

Check 5: Vessel Capability

Pipe diameter must be within vessel's handling range and water depth within operational envelope.

Go/No-Go Criteria

• GO: All checks pass AND max utilisation < 0.85
• MARGINAL: All checks pass AND max utilisation in [0.85, 1.00]
• NO-GO: Any check fails (utilisation > 1.00)

Applicable Codes & Standards

• DNV-ST-F101 (2021) - Submarine Pipeline Systems
• API RP 1111 (2015) - Design, Construction, Operation, and Maintenance of Offshore Hydrocarbon Pipelines
• DNV-OS-F101 (2013) - Submarine Pipeline Systems

Installation Screening Summary

All 60 parametric cases — 2 vessels x 5 pipe sizes x 6 water depths

Assumptions & Limitations

• All pipes are X65 grade (SMYS = 448 MPa, E = 207 GPa)
• Catenary solution assumes no current loading on the suspended span
• Sagbend and overbend stresses use simplified elastic beam theory
• Combined stress is conservative additive (bending + axial) per DNV-ST-F101 Sec 5
• Stinger radius approximated as R_ob = L_stinger / sin(theta_dep)
• No dynamic amplification applied — quasi-static installation assumed
• Pipe submerged weight includes anti-corrosion coating and CWC where applicable
• Seabed assumed flat at each water depth — no slope corrections
• Seawater density = 1025 kg/m3 throughout water column
• Installation stress limit factor = 0.72 per DNV-ST-F101 (2021)