Drilling Riser Stack-Up Model

The weight in air and seawater for the telescopic joint, flexjoints, LMRP, and BOP need to be defined for riser analysis. The submerged weight and dimensions (length width height) for the production trees, manifolds, and jumpers are required for the dual activity interference analysis. The properties of the auxiliary rig drill pipe and wire rope will also be used in the interference analysis. The recommended tensioner forces used in the analysis are calculated based on the mud weight. In the analysis, the drilling mud densities are typically assumed to be 8.0, 12.0, 16.0 ppg, etc. The maximum allowable bending moment in the casing may be determined assuming the allowable stress is 80% of yield strength. The hydrodynamic coefficients to be used in the analysis include the normal drag coefficient and the associated drag diameters for the bare and buoyancy joints. The tangential drag coefficient may be taken from API RP 2RD , Section, Equation 31. For the LMRP and the BOP, the vertical and horizontal drag areas and coefficients may be provided by suppliers. The red alarm is typically 60 sec before disconnect point, and the yellow alarm is roughly 90 sec before the red alarm.

Vessel Motion Data

The required vessel motion data include the following:

  • The principal dimensions of the vessel;
  • The mass and inertia properties at maximum operation draft;
  • Reference point locations for RAOs (Response Amplitude Operator);
  • The survival draft RAOs for various wave directions;
  • The maximum operating draft RAOs for various wave directions;
  • The transit draft RAOs for various wave directions.

In addition, wave drift force quadratic transfer functions for surge, sway, and yaw are required to conduct irregular wave force calculations in a drift-off analysis. Wind and current drag coefficients for the vessel are also required.

Environmental Conditions

Generally angles denote the direction “from which” the element is coming, and they are specified as clockwise from true north. Tidal variations will have a negligible effect on the loads acting on deepwater risers and may be negligible in the design. Environmental conditions include:

  • Omnidirectional hurricane criteria for the 10-year significant wave height and associated parameters;
  • Omnidirectional winter storm criteria for 10- and 1-year return periods;
  • The condensed wave scatter diagram for the full population of waves (operational, winter storm, and hurricane);
  • Loop/eddy normalized profiles;
  • The 10- and 1-year loop/eddy current profiles along with the associated wind and wave parameters;
  • The bottom current percent exceedance and the normalized bottom current profile;
  • The combined loop/eddy and bottom current normalized current profile as a fraction of the maximum;
  • The combined loop/eddy and bottom current profiles for a 10-year eddy þ 1-year bottom, and 1-year eddy þ 1-year bottom;
  • A 100-year submerged current probability of exceedance and profile duration.

Background current is the current that exists in the upper portion of the water column when there is no eddy present. Mean values of the soil undrained shear strength data, submerged unit weight profile, and 350 profiles are used along the soil column to calculate the equivalent stiffness of the soil springs, for analysis of the connected riser.

Cyclic p-y Curves for Soil

The methodology for deriving a p-y curve for soft clay for cyclic loading was developed by Matlock [7]. A family of p-y curves will be required to model the conductor casing/soil interaction at various depths below the mudline.


[1] World Oil, Composite Catalog of Oilfield Equipment & Services, forty fifth ed., Gulf Publishing Company, Houston, 2002/03.

[2] International Standards Organization, Petroleum and Natural Gas Industries – Design and Operation of Subsea Production Systems – Part 7: ompletion/Workover/Riser System, ISO 13628-7, 2005.

[3] T. Clausen, R. D’Souza, Dynamic Risers Key Component for Deepwater Drilling, Floating Production, Offshore Magazine, vol. 61, (2001), May.

[4] P.R. Geiger, C.V. Norton, Offshore Vessels, Their Unique and Applications for the Systems Designer, Marine Technology, vol. 32 (No. 1) (1995) 43–76.

[5] American Petroleum Institute, Recommended Practice for Design, Selection, Operation and Maintenance of Marine Drilling Riser Systems, API-RP- 16Q (1993).

[6] American Petroleum Institute, Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platform (TLPs), API-RP- 2RD (1998).

[7] H. Matlock, Correlations for Design of Laterally Load Piles in Soft Clay, OTC, 2312, Offshore Technology Conference, Houston, Texas, 1975.

[8] K. Vandiver, L. Lee, User Guide for Shear7 Version 4.1, Massachusetts Institute of Technology, Cambridge, 2001, March 25.

[9] J.F. Archard, Contact and Rubbing of Flat Surfaces, Journal of Applied Physics, vol. 24 (No. 8) (1953) 981.