Component Description
The purpose of the arch structure is to generate sufficient buoyancy to support a series of risers laid over it, whilst preventing the riser from overbending.
The Arch Structure consists of the following components:
- Buoyancy elements
- Steel frame
- Riser clamps
The buoyancy elements are sized to provide sufficient buoyancy uplift to support the risers and frame. The buoyancy can be provided using pressurised steel tanks or by means of a foam core inside a tough abrasion resistant external skin. The foam system utilised is rated for continuous operation at the specified water depth for a design life of up to 30 years and may be either Co-polymer or Syntactic.
Depending on the overall buoyancy required from the Arch a number of discrete buoyancy elements may be built.
The finished buoyancy elements are assembled within the Arch structure steel frame, which provides a stable arch geometry and affords the elements impact and abrasion protection.
The main Arch structure comprises a steel frame which performs the following functions:
- Houses the buoyancy elements
- Provides a structural connection with the mooring system
- Incorporates the gutters within which the risers sit when installed
To ensure that the steel frame has a low weight in water, it is usually fabricated using specially selected standard tubular mambers. The shape, size and length of the gutters are carefully designed to ensure that risers are prevented from overbending during all possible operating conditions and environmental loads. This requires close co-operation with both the riser manufacturer and designer of the system as a whole.
Once the steel frame has been designed, a final verification of its structural integrity is undertaken using 3D Finite Element Modelling, which is undertaken by our in-house design engineering team.
The steel frame is protected from corrosion by means of a suitable subsea paint system in combination with a Cathodic Protection system (designed to DNV/NORSOK codes)
The risers are held in position on the Arch using clamps. The clamps are generally a split steel fabrication designed to withstand the differential tensions which occur as the riser passes over the Arch.
We make use of out vast experience of designing clamps to ensure that the flexible pipes and umbilicals can be clamped without generating excess clamping pressures, whilst allowing for their diameter variations during service.
Clamps can be designed to allow for both diver intervention and diverless connection to the main Arch Structure.
The mooring structure usually consists of a series of tether lines terminating in shackles which connect the Arch to the Subsea Anchor.
The tether lines are rated in accordance with relevant DNV Safety Factors and can be made from either traditional chain or state-of-the-art man made fibres such as parallel fibre polyester or Kevlar ropes.
We are also able to design and manufacture all the ancillary items required such as Triplates, ROV friendly connection systems etc. All the components of this mooring system are fully load tested to ensure compliance with the relevant Codes, Standards and Specifications being used for a particular project.