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Home > User Interface > Analyses > Calculation Parameters > Types of Analyses > Drilling Riser Preliminary Calculation

Drilling riser preliminary calculation

Preliminary calculations may be carried out for drilling risers:

  • Recommended limits for design and operation: According to the API RP 16 Q, a table of inputs and outputs is provided to check the riser stability. For the selected drilling riser model, the minimum slip ring tension and top tension are calculated to ensure the riser stability of the riser, it means the effective tension is always positive all along the riser. Furthermore, the minimum tension to disconnect the riser with the LMRP is provided. The model may include or not one tensioner object and one slip joint. The results are displayed in the post-processing window, sheet labelled API RP-16Q Check, tab Minimum tension.

  • Iso-angle curves: According to API RP 16 Q, the top and foot angle must remain lower than maximum angle values. A DeeplinesGUI facility consists in providing pencils of static iso-angle curves. These curves are displayed in the post-processing window, sheet API RP-16Q Check, tab Iso-angle curves.

This facility is available when the analysis includes a drilling riser.

Warning : your analysis must not include more than one drilling riser and one tensioning system.

Otherwise, the select box Drilling Riser Preliminary Calculations is not accessible (shadowed) in the Analysis area of the Editing calculation parameters panel.

For a drilling riser analysis, users may select or not the option. If selected, the calculation parameters are accessible through the Advanced button. Six parameters are defined that are:

  1. The Submerged Riser Weight Tolerance Factor fwt

  2. The Buoyancy Loss and Tolerance Factor fbt

  3. The number n of tensioners subject to sudden failure (among the total number of tensioner units defined in the tensioner object).

  4. The Reduction factor Rf: Reduction Factor Relating Vertical Tension at the Slip Ring to Tensioner Setting to account for fleet angle and mechanical efficiency (friction and fleet losses)

  5. The LMRP minimum disconnect overpull.

  6. The maximum percentage of Dynamic Tension Limit (DTL)

These coefficients are used in the following equations :

\[ T_{min} = T_{SRmin}\frac{N}{R_f(N-n)} \]

where Tmin is the minimum top tension setting,

with

\[ T_{SRmin} = W_s f_{wt} - B_n f_{bt} + A_i [d_m H_m - d_w H_w] + (LMRP_{weight} + Minimum~~overpull) \]

and

  • \(T_{SRmin}\) = Minimum Slip Ring Tension

  • \(W_s\) = Submerged Riser (steel) Weight above the point of consideration

  • \(f_{wt}\) = Submerged Weight Tolerance Factor (minimum value= 1.05 unless accurately weighted)

  • \(B_n\) = Net Lift of Buoyancy Material above the point of consideration

  • \(f_{bt}\) = Buoyancy Loss and Tolerance Factor

  • \(A_i\) = Internal Cross Sectional Area of Riser, including Kill&Choke and auxiliary lines

  • \(d_m\) = Drilling Fluid Weight Density

  • \(H_m\) = Drilling Fluid Column to point of consideration

  • \(d_w\) = Sea Water Weight Density

  • \(H_w\) = Sea Water Column to point of consideration

  • \(N\) = Number of Tensioners (units) Supporting the Riser

  • \(n\) = Number of Tensioners (units) Subject to Sudden Failure

  • \(R_f\) = Reduction Factor Relating Vertical Tension at the Slip Ring to Tensioner Setting to

  • account for fleet angle and mechanical efficiency (usually 0.9 0.95)

  • LMRP_weight = Wet weight of the LMRP

  • Minimum overpull = The minimum overpull is the minimum tension required at the riser bottom to disconnect the LMRP.

Note

1) If there is no tensioner object in the analysis, Tmin values are not provided.

2) We turn the readers attention to the following points :

Ws is the steel apparent weight of the riser full of water, including the weight of the joints but also peripheral lines and connectors.

With the expression above, the weights and tensions are in fact homogeneous with masses (kg or tons or pounds). Indeed in the term \(A_i[d_m H_m - d_w H_w]\), Ai is an area in m, dm and dw of the densities in kg/m3 and Hm and Hw heights in m. Thus, one obtains values in kg.

In the above equation for TSRmin, the term related to external pressure (dwHw) is multiplied by the internal cross section area Ai of the riser (including auxiliary lines) rather than the external cross section area A0. This is because the buoyancy of the riser pipe walls dwHw(A0-Ai) has been included in the submerged riser weight Ws.

Tmin is the tensioner system tension that takes into account the losses due to friction and fleet angle between the tensioner units and the tensioner ring. The calculation of Tmin also takes into account the failure of one or several tensioners units.

TSRmin is the tension applied at the tension ring.

In practice, in DeepLines, the tension is applied at the extremity of one joint (drill joint or outer barrel for example) at the tensioner ring location. Hence, this load corresponds to TSRmin.

3) The Preliminary calculations do not require static nor dynamic calculations. Nevertheless, computation must be launched in order to create the DSS file.