Marine riser system

01

Choose the coupling type and tension rating

NOV defines a marine drilling riser as a tubular conduit that extends the well bore from the wellhead equipment on the seafloor to a floating drilling rig, connecting the subsea BOP stack to the vessel. Its primary functions are fluid communication through the main bore during drilling, through the choke and kill lines when the BOP is controlling the well, and through auxiliary hydraulic and mud boost lines; guiding tools into the well; and serving as the running and retrieving string for the BOP stack. A riser joint is a section of main tube with box and pin ends, carrying choke, kill and optional auxiliary lines and their brackets. [1]

NOV's data sheet lists flange-type and dog-type couplings with tension load ratings of 1,125,000, 2,000,000, 3,000,000, 3,500,000 and 4,000,000 lb, on 21 in main tubes (and one 42 in design rated 3,000,000 lb), with flange diameters of about 35.6 to 50 in depending on type. [1]

Editorial calculation: 3,500,000 lb is about 15.6 MN; 21 in is about 533 mm. [1]

A Cameron patent, US 8,657,013 B2 (Riser system), sets out why tension and wall thickness escalate in deep water: the vessel applies tension at the top to carry the riser and the mud inside it, so a stronger riser needs a thicker wall, which adds weight and in turn needs more tension. It notes that drilling mud can be about twice as heavy as seawater, giving roughly 0.5 psi per foot of extra internal pressure, so at 10,000 ft depth there could be 5,000 psi inside the riser relative to the surrounding sea. Its answer is a riser section with tie rods between the end flanges that share the tensile load with the main tube. [3]

Editorial calculation: at the patent's 0.5 psi per foot, a 3,000 m (about 9,840 ft) water depth gives about 4,900 psi (34 MPa) of internal differential at the lower riser, and the full mud column also adds to the top tension the tensioner must hold. The mud density is the patent's illustrative figure; use your mud programme. [3]

Give the supplier the rig and spider/gimbal interface, water depth, mud weight, required tension rating, coupling type, main tube size and wall, auxiliary line set and pressures, joint and pup lengths, buoyancy depth rating, telescopic and termination joints, inspection and certification needs, and the number of joints.

02

Set auxiliary lines, joint lengths and buoyancy

NOV's configurations range from 2-line (choke and kill) to 6-line sets adding booster or mud return, seawater, two hydraulic lines and glycol. Choke and kill lines are rated 10K or 15K psi (for example 6.5 × 4.5 in or 5 × 3 in), booster lines 3K, 5K or 7.5K psi, hydraulic lines 3K or 5K psi in 2.5 in Schedule 80 of 316 or duplex stainless, and a glycol line 15K psi; lines are spaced 60° or 90° apart around the flange. [1]

Standard joints are 50, 75 or 90 ft long, with pup joints from 5 to 90 ft in set increments, and main tube walls from 5/8 in to 15/16 in. [1] A Huisman patent describes a riser joint whose buoyancy modules carry umbilical gutters with a resilient cover, so umbilicals can be loaded parallel to the riser pipe. [2]

Editorial calculation: a 75 ft joint is about 22.9 m, and 15K psi is about 103 MPa. [1]

03

Fit buoyancy through the rotary and rate it for depth

US patent 6,155,748 (Riser Systems Technology, priority 1999) sets out the size and depth constraints on drilling riser buoyancy. It describes riser joints "generally 50 feet in length" with mechanical connections at both ends, deepwater drill sites in 8,000 to 12,000 ft of water, and syntactic foam of hollow microspheres in an epoxy or plastic matrix as passive flotation whose buoyancy cannot be adjusted after installation. As water depth increases, so does the buoyancy required per length of riser joint and the density requirement on the foam. [4]

The patent's answer is a dual-phase system: passive syntactic foam modules plus active modules with pressure-containing bladders that can be charged with gas, so average buoyancy per foot can rise without enlarging the outside diameter beyond the 48 in rotary. [4]

Editorial calculation: a 48 in rotary around a 20 in riser leaves 14 in of radial space for foam, auxiliary lines and clamps. At the patent's 0.500 psi per foot, foam at 12,000 ft sees about 6,000 psi (41 MPa) of external pressure, and at 10,000 ft about 5,000 psi (34 MPa). These depth figures match the scale of the 5,000 psi internal differential in the Cameron patent above, but act from outside on the buoyancy, not inside the main tube. [4] [3]

Ask the buoyancy supplier for the module outside diameter against your rig's rotary, the rated depth and test pressure, the net uplift per joint at that depth, and the water absorption allowance over the service life. [4]

04

Before ordering riser joints: coupling, line, buoyancy and certification questions

Ask any supplier for coupling design and tension rating, main tube material and wall, auxiliary line sizes and pressure ratings, joint and pup lengths, buoyancy module depth rating and lift, weights in air and water, compatibility with your spider and BOP, certification to the applicable standard, and inspection and repair services.

05

Main tube, couplings, choke/kill and auxiliary lines, pup joints and buoyancy

NOV's riser sheet supplies the tube, coupling and line rows; the buoyancy row draws on the Huisman patent.

06

Riser joint data from the NOV sheet, and the six values it does not give

NOV's sheet gives the diameter, wall, joint length, coupling tension and line ratings; temperature, weight, bending radius, corrosion allowance, yield strength and fatigue life are not stated in the sources reviewed.

07

Send the rig, well and riser data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Rig: [type; spider; gimbal]
Well: [water depth; mud weight; BOP]
Coupling: [type; tension rating]
Main tube: [OD; wall]
Lines: [choke/kill psi; booster; hydraulic; glycol]
Joints: [lengths; pups]
Buoyancy: [depth rating]
Certification: [standard; inspection]

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