Tether system Design

Functional requirements

The purpose of a tether system is to securely anchor a TLP to the seabed, so preventing vertical motion but permitting lateral motion in response to environmental forces. Tension is maintained in the tethers at all times by the excess of hull buoyancy over the total TLP weight. This is achieved by the tethers constraining the hull to the float at a deeper draught than its weight would require.

Configuration

Tethers may be fixed to the outside of the hull or attached in internal ‘tether tubes’. Currently installed TLPs have a column and pontoon construction similar to semi-submersible with the tethers situated at the corner columns. Various multi and single column configurations have been proposed. A single column, circular plan unit with central tethers has been suggested for Artic regions.
Redundancy criteria should be taken into account and, as a minimum, the platform should be able to survive the 100-year storm for its location with one tether out of commissioning. Risk analysis should be carried out in conjuction with Failure Mode Analysis (FMA) on the tether system.
The design process should consider the need for tether change out and should include the preparation of change out procedures.

makeup

Tether make-ups

The picture shows a typical tether assembly. In this case it is a large diameter, semi-buoyant steel tether with screwed joints. Steel strand wire and other materials have been proposed for tethers. However, only steel (solid and tubular) tethers have been used for any TLPs installed prior to 1995. At the upper end is a tie-off assembly incorporating a tension adjusting unit. This enables initial tensions to be set and equalized as well as ensuring that tensions can be adjusted in service.

Below the adjusting assembly is a cross load bearing which may be separated from the upper unit by additional tether elements. The purpose of the cross load bearing is to transfer horizontal forces due to wind, wave and current from hull to tether.

Below the cross bearing are standard tether elements, their number depending upon the length of each element and the water depth.

The final item in the tether string is an anchor connector which engages with the seabed foundation.

Alternative arrangements may include:

  • Welded tethers assembled on site
  • Pre-welded tethers
  • Single tether analysis

 

 

 

 

 

 

Single Tether Analysis

This section considers the analysis of a tether assuming the hull motion is known. The tension consists of a tensioned flexible member, not necessarily with constant sectional properties, and normal attached by articulated joints at top and bottom. Lateral loads on the tether are primarily balanced by the tether tension, however the bending stiffness of the tether is still important and has a significant effect on the stresses, and hence fatigue life, of the tether.

The analysis of a tether can be performed using a static or dynamic tension beam model subject to the tether’s tension, weight, buoyancy and hydrodynamic loads.
Tethers and risers are sufficiently heavy that, although the tension at the top is large, the tethers self weight rapidly decreases and the tether or riser would buckle if it were not for the external pressure forces.

model

 

For more detailed information on single tether analysis please click on your point of interest:
Hydrodynamic Loading
Frequency Domain Analysis
Time Domain Analysis
Prototype Testing