Case Study: Tension Leg Platform TLP components

Floating platforms such as tethered leg platforms (TLPs) are the structures which are the most sensitive to weight issues, and buoyancy calculations can be a dominant feature of the structural design. This effect can be most acutely demonstrated in the consideration of riser design. These are the lengths of tubing that convey process fluids from the platform to the drill head on the seabed. 

A platform may have up to 50 risers of different diameter. For a given TLP design, and therefore given platform displacement, every tonne saved from the riser top tension (or tether pretension) is an extra tonne of potential payload. If buckling were the limiting design criterion, it would be possible to allow riser tension to fall to a minimum value only slightly greater than the apparent weight of the member concerned. In the case of TLPs this cannot be allowed, since in order to prevent hydrodynamic interaction and possible contact, it is important to maintain compatible profiles between adjacent risers at all times. The principal design factors that control the detailed riser design are the potential blowout pressure (34 MPa, 5 000 psi) and the mean axial top tension (typically 75 tonne /riser).

Consideration of these loadings gives rise to a hybrid design with longitudinal carbon fibres and circumferential glass reinforcement.  The table and figure below show the details of the design.

Production riser physical characteristics

Composite riser design

Mechanical characteristics of composite risers

The internal diameter (9 in.; 230 mm) was determined by the duty for this particular component; conveying two 3½ in. (90 mm) steel tubing. The blow-out pressure determined the thickness of the circumferential layers whilst the thickness of the longitudinal layers are determined by the pressure end load and axial tension. The axial loads also determined the joint detail at the ends of each tube length. The end connectors themselves are of threaded steel design. The table above shows the mechanical properties of the design. 

The rubber liners on the internal and external surface of the tube are applied to ensure pressure tightness of the system. A feature of the composite concept is the way its elasticity is used to advantage. With a steel riser the top ends are suspended with tensioners which must have sufficient stroke to accommodate vertical movements. However, the low modulus and high strain capability of the composite hybrid (compared with the steel tethers securing the platform to the seabed) means that an active tension system is not necessary. 

A further subtlety in the design is the optimization of Poisson's ratio. Long cylinders under pressure can have large axial extensions due to end load. This is in part compensated for by Poisson's ratio contraction. If an all-CFRP design was used Poisson's ratio would be small, about 0.08, which could cause difficulties in terms of control of riser tension. The hybrid design gives rise to a Poisson's ratio of 0.22 which enables axial extension to be kept within convenient limits. When a riser is attached to the seabed structure, there is a sudden change of stiffness and this joint may be subjected to large forces and bending moments generated as a result of platform movement. This can cause problems at the riser joint which can result in failure or over-stressing of internal tubing. To overcome these load concentrations it is now common to incorporate a stress joint within the riser assembly. This typically consists of a section of tapered wall thickness to provide a gradual transition in stiffness. Current designs are usually manufactured from single piece steel forgings, although there is work ongoing to examine the feasibility of titanium. 

Potential advantages of a composite design include:

• Lightweight
• Improved performance, notably:
  • fatigue durability;
  • corrosion resistance;
  • high strength;
    favourable elastic properties.
• Reduced installation costs.
• Flexibility of design leading to significant cost benefits
• Simplification of assemblies.

Two of these factors are of particular interest for the stress joint application. Firstly, the combined stiffness/ strength characteristics of composite materials are ideally suited to the design of a component with requirements for both high strength and compliant behaviour. This means that the stress joints can be shorter and will induce lower loads and bending moments into the subsea structure. Secondly, flexibility of manufacture means that small numbers can be produced cost effectively. In a given platform installation the majority of risers may be of standard diameter (9 ⁵/₈ in.; 245 mm), but there will also be requirements for non-standard sizes (up to 20 in. diameter; 508 mm) and these larger components can be easily fabricated in small numbers without excessive cost penalty.

In one proposed design composite, fibres are wound over a standard riser section with thickness increasing toward the bottom flange. This provides a gradual increase in stiffness which results in a smooth transition in the curvature from the riser above to the wellhead below. After winding, the riser pipe remains an integral part of the structure. Other composite design options are available which are of lighter weight and therefore more structurally efficient, but the overwound design has the advantages of simple concept, a steel liner which simplifies the design, particularly for internal pressure, and maintenance of the use of the standard end connectors attached to the riser. Also, as the mandrel for winding the composite cylinder is a standard riser the costs are somewhat lower especially for small volumes. Connection to the seabed structure would be through a flange similar to existing arrangements.

For tethered platforms the weight of the tethers themselves can be significant. Hollow steel tendons, designed to be of neutral buoyancy, are conventionally used but as water depth increases instability due to external pressure comes into play. Solid tendon structures from composites would overcome this problem. The figure below shows a conceptual design of carbon fibre tethers fabricated from an assembly of pultruded CFRP rods. One difficulty with this particular application, which is perhaps unique, is that the tendons for a single platform in very deep water (> 10 000 ft: 3050m) would require a major percentage of the world's annual capacity of carbon fibre.

Conceptual design for CFRP rope