High-density polyethylene pipe manufactured as PE100 to ISO 4427-2 and classified under ASTM D3350 with a cell class such as 345464C is routinely specified for dredge discharge and marine outfall service. The governing design problem—HDPE Pipe for Dredging and Outfall: Diameter, Buoyancy and Sinking—requires simultaneous evaluation of outside diameter, wall thickness, flooded or empty internal condition, and installed ballast mass. Because solid-wall HDPE has a density of 0.941–0.965 g/cm³ when measured by ISO 1183-1 Method A, and seawater at 20 °C and 35 PSU has a density of approximately 1,025 kg/m³, an empty HDPE string placed in seawater generates positive buoyancy proportional to the square of the outside diameter. For outside diameters from 200 mm to 1,200 mm, the calculated empty-pipe uplift for SDR 26 ranges from roughly 28 kg/m to more than 990 kg/m. A 315 mm line already exceeds 68 kg/m of uplift, and a 630 mm line reaches approximately 275 kg/m. The same pipeline can become negatively buoyant during dredging when filled with sand-water slurry, but reverts toward positive buoyancy if the pump trips and the line is backfilled with seawater. The following sections examine diameter-dependent hydraulic capacity, ballast hardware, and sinking procedures, with all force values stated as calculated from nominal dimensions and standard material densities.

How Does Outer Diameter Change Empty-Pipe Uplift and Ballast Demand?

Design calculations start from the displaced seawater volume over the outside diameter D_o. The buoyant force per metre is F_b = ρ_sw · g · π D_o²/4, where g = 9.81 m/s². The in-air pipe mass per metre is m_p = ρ_PE · π (D_o² − D_i²)/4, with D_i = D_o − 2e and e = D_o/SDR. The empty-pipe uplift is U = F_b/g − m_p. For a 630 mm outside diameter SDR 26 line with nominal wall thickness 24.2 mm and inside diameter 581.6 mm, the annulus mass is approximately 44.0 kg/m at 955 kg/m³ and the displaced seawater mass is approximately 319.5 kg/m at 1,025 kg/m³, giving an empty-pipe uplift of 275.5 kg/m. At 1,000 mm outside diameter SDR 26 with a 38.5 mm wall thickness, the annulus mass is approximately 111 kg/m and the displaced seawater mass is approximately 805 kg/m, giving an uplift of 694 kg/m. Because the PE density is below the seawater density, water-filled pipe remains slightly buoyant; the 1,000 mm SDR 26 line flooded with seawater at 1,025 kg/m³ still has upward force of approximately 8 kg/m due to the immersed volume of the polymer annulus. This residual buoyancy is the reason that sinking cannot be achieved by flooding alone, and external ballast is required for a stable subsea string.

During cutter-suction dredging, the flow condition inside the line changes the buoyancy state independent of the installed ballast. A 1,000 mm outside diameter SDR 26 line with inside diameter 923 mm carries approximately 836 kg/m of internal fluid at a slurry density of 1,250 kg/m³. The same pipe annulus has a mass of 111 kg/m, and the displaced seawater mass is 805 kg/m; the submerged line mass is therefore approximately 142 kg/m downward, meaning the pipeline is negatively buoyant without added ballast while the slurry is flowing. If the dredge pump trips and the slurry drains or is replaced by seawater, the internal mass drops to approximately 686 kg/m, and the line returns to a near-neutral upward force of approximately 8 kg/m. If the line drains completely to air, the original 694 kg/m uplift reappears. This is why dredge crews monitor pump discharge pressure and valve line position before releasing any float or pontoon. A pressure drop on the commissioning gauge is not sufficient; the line must be independently verified to be filled with slurry or externally ballasted before sinking. Sand-water mixtures in HDPE lines are typically transported above the critical deposition velocity, which for 500–1,000 mm lines and dredged sand with median grain sizes from 0.2 mm to 0.8 mm is commonly held between 3.0 m/s and 5.0 m/s. A 600 mm outside diameter SDR 17 line at 4.5 m/s has a nominal internal flow area of approximately 0.220 m² and passes about 0.990 m³/s, or 3,560 m³/h; the density and solids concentration are determined by the dredger’s mixture pump and production rate, not by the pipe itself. Published measured values for transient buoyancy during pump trip on production dredges are limited, so the design must conservatively assume that the line can return to empty-pipe positive buoyancy.

Because dredge pumps can trip against closed valves or a blocked line, surge pressure is assessed alongside diameter. The instantaneous pressure rise p_s = ρ · c · ΔV scales with wave speed c, mixture density, and velocity change. HDPE wave speed for water service is approximately 200–300 m/s, lower than steel. A sudden velocity change from 4 m/s to 0 m/s in a water-filled HDPE line with wave speed 250 m/s produces a theoretical pressure rise of about 10 bar, which is below the PN 16 rating of PE100 SDR 11 but above the PN 6.3 rating of SDR 26. For sand slurry at 1,250 kg/m³, the same transient exceeds 12.5 bar. Because dredge operators may run the pump against a momentarily closed valve, SDR 17 or SDR 11 is often selected for the pump discharge section irrespective of steady-state pressure. The surge criterion links back to diameter because larger internal flow area at fixed velocity produces larger pump-stop momentum, but the pressure rise is primarily velocity-dependent, not diameter-dependent.

Negative Buoyancy Hardware, Collar Spacing, and Thermal Expansion Limits

Empty-pipe uplift is counteracted with discrete ballast collars, bolt-on steel weights, or a combination of both. A concrete collar with dry density of 2,400 kg/m³ has a submerged weight in seawater equal to dry mass multiplied by 1 − 1,025/2,400, or 57.3% of its air weight. Thus a 500 kg dry collar contributes only 287 kg submerged weight. For a 630 mm SDR 26 line with an empty-pipe uplift of 275.5 kg/m and a specified negative buoyancy factor of 1.20, the required submerged ballast is 330.6 kg/m. If the selected collar has a submerged weight of 287 kg, the maximum spacing is 0.87 m. If a heavier 800 kg dry collar is used, submerged weight is 458 kg and spacing increases to approximately 1.39 m. For a 1,000 mm line with 694 kg/m uplift and the same safety factor of 1.20, required submerged ballast is 833 kg/m; a 1,500 kg dry concrete collar contributes 860 kg submerged and therefore must be placed at 1.03 m centres. These spacings are calculated from nominal dimensions and not from geotechnical or metocean load conditions. Bolt-on steel weights are more compact and have a submerged weight of 86.9% of dry mass in seawater, but they require cathodic protection and crevice-corrosion monitoring, particularly near the fusion beads. Continuous rigid concrete coating is generally unsuitable for HDPE because the linear thermal expansion coefficient of HDPE is approximately 1.5–2.0 × 10⁻⁴ m/m/°C, and discrete collars prevent accumulated strain transfer to the pipe wall. Elastomeric bearing strips are required between the collar and pipe to reduce point loading and allow small axial movement during pressure fluctuations and temperature changes.

Once the line is on the seabed, diameter also controls hydrodynamic drag and lift. A larger outside diameter presents a broader profile to wave-induced near-bottom orbital velocities; for temporary dredge lines and permanent outfalls, on-bottom stability is commonly checked under DNV-RP-F109 using pipe submerged weight, seabed friction coefficient, and hydrodynamic coefficients. For a sand seabed, a friction coefficient of 0.5–0.7 is often used; for soft clay, the coefficient may fall to 0.3 or below, and ballast demand increases accordingly. The objective is not merely negative buoyancy but the prevention of lateral displacement. For an HDPE outfall line, the dry weight of the pipe itself is low, so ballast spacing may be determined by wave-induced lift rather than by simple uplift. In shallow water with significant wave height above 1.5 m, an 800 mm empty line may require additional negative buoyancy or lowering to a buried trench because the hydrodynamic lift force can exceed the residual submerged weight. Operators frequently limit controlled sinking operations to Hs ≤ 1.0 m for outside diameters above 800 mm because published dynamic sinking data in higher sea states is limited.

If the Outfall Diffuser Is Submerged by Flooding Without Towing Tension

For long outfall strings, the pipe is often assembled onshore or in a sheltered anchorage, floated into position with temporary pontoons, and then sunk by controlled flooding. Diameter affects the sinking sequence because larger-diameter empty pipe displaces more seawater per metre and requires larger flood valves to admit water fast enough to avoid air pockets. The minimum bending radius during towing and sinking is typically limited to 20–25 times the outside diameter unless a detailed strain analysis is provided; for a 1,200 mm outside diameter line this limit is 24–30 m. As the line fills, the tension at the shore winch changes from the weight of the floating string to the submerged weight of the flooded string, and the winch must be sized for the maximum submerged ballasted load. HDPE lines joined by butt fusion welding according to ISO 21307-2017 should not be subjected to point loads at the fusion bead root during ballast placement; saddles and lifting straps must be wide and padded. Outfall diffuser sections are particularly sensitive because each riser and duckbill valve creates a local mass and drag anomaly; ballast collars are moved clear of diffuser boxes and placed on the main barrel only. If the line is sunk by flooding alone without external ballast, residual buoyancy remains because the polymer annulus is less dense than seawater; the outfall will not remain on the bottom unless ballast mass or trench backfill is added. This condition is sometimes mistaken in specification reviews when a water-filled pipe is assumed to be negatively buoyant.