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D: Scantlings and Weld Connections [Table of Contents] F: Foundations

DNV-OS-C105 Structural Design of TLPS (LRFD method)

[-] Sec.6: Ultimate Limit States (ULS)
[-] E: Tendons

Sec.6
E. Tendons

Sec.6
E 100   Extreme tendon tensions

Sec.6 E
101   As a minimum the following tension components shall be taken into account:
—pretension (static tension)
—tide (tidal effects)
—storm surge (positive and negative values)
—tendon weight (submerged weight)
—overturning (due to current, mean wind or drift load)
—set down (due to current, mean wind or drift load)
—WF tension (wave frequency component)
—LF tension (wind gust and slowly varying drift)
—ringing (HF response)
—hull VIM influence on tendon responses
—tendon VIV induced loads.


Sec.6 E
102   Additional components to be considered are:
—margins for fabrication, installation and tension reading tolerances
—operational requirements (e.g. operational flexibility of ballasting operations)
—allowance for foundation mispositioning
—field subsidence
—foundation settlement and uplift
—loads due to spooling during transportation and storage of flexible tendons.


Sec.6 E
103   Bending stresses along the tendon shall be analysed and taken into account in the design. For the constraint mode the bending stresses in the tendon will usually be low. In case of surface, or subsurface tow (non-operational phase) the bending stresses shall be carefully analysed and taken into account in the design.

Sec.6 E
104   For nearly buoyant tendons the combination of environmental loads (axial and bending) and high hydrostatic water pressure may be a governing combination (buckling).

Sec.6 E
105   Limiting combinations (envelopes) of tendon tension and rotations (flex elements) need to be established.

Sec.6 E
106   For specific tendon components such as couplings, flex elements, top and bottom connections etc. the stress distribution shall be determined by appropriate finite element analysis.

Sec.6 E
107   For ULS conditions (A and B), temporary loss in tendon tension is acceptable provided each of the following two conditions are met with:
  1. Minimum tendon tension in at least 1 tendon per corner remains non-negative.
  2. A comprehensive redundancy analysis performed to evaluate the effect of loss of tension on the tendon system and supporting structure, demonstrating structural integrity by removal of the tendons with negative tension. Alternatively, model tests may be performed.


Sec.6 E
108   If temporary (part of a high frequency cycle) tendon tension loss is permitted, tendon dynamic analyses shall be conducted to evaluate its effect on the complete tendon system and supporting structures. Alternatively, model tests may be performed. The reasoning behind this is that loss of tension could result in detrimental effects to e.g. tendon body, connectors, or flex elements.

Sec.6
E 200   Structural design of tendons

Sec.6 E
201   The structural design of tendons shall be carried out according to DNV-OS-C101 or API RP 2T with the additional considerations given in this subsection.

Sec.6 E
202   Buckling checks of tendon body/pipes may be performed according to API RP 2T or NORSOK, N-004.

Sec.6 E
203   When deriving maximum stresses in the tendons relevant stress components shall be superimposed on the stresses due to maximum tendon tension, minimum tendon tension or maximum tendon angle, as relevant.

Sec.6 E
204   Such additional stress components may be:
—tendon-bending stresses due to lateral loads and motions of the tendon
—tendon-bending stresses due to flex-element rotational stiffness
—thermal stresses in the tendon due to temperature differences over the cross sections
—hoop stresses due to hydrostatic pressure.


Sec.6 E
205   Composite tendons shall be designed in accordance with DNV-OS-C501 with additional considerations given in this section.
D: Scantlings and Weld Connections [Table of Contents] F: Foundations