14 Weak Link
14.1 Description & Components
Most deepwater systems do not utilise a weak link as part of the SPS control system, however for information a functional description of the Weak link is provided below:
A weak link is a device used to protect equipment that is permanently installed on a manifold or template, so that in the event of an umbilical being snagged, the umbilical will break away, activating the link and shearing jumpers connecting to the fixed subsea equipment.
A weak link may not be required; this depends on the subsea system architecture, and the risk analysis of snagging by anchors, etc.
A weak link is usually a guillotine device that has one part fixed to the template/manifold or to the SUTU itself. The other part assembled using slide guides is attached to the umbilical termination (SUTU). The weak link is generally made from 316 stainless steel and is unpainted. Load calculations must be performed to ensure that whichever structure it is attached to can take the loads imparted before the device shears the umbilical.
Subsea jumper hoses and cables pass through guide holes in both parts of the guillotine. The jumpers attached to the SDU at one end termination, and to the SUTU at the other end termination.
When the weak link is being handled or the SUTU attached, an installation bolt is used to ensure that the weak link is not activated. After the installation is complete, the installation bolt is removed and the two guillotine parts of the weak link are then held together by a shear pin.
The shear pins are batch made and type tested to verify that the shear force is within the design tolerance.
The shear force is specified to accommodate the snagging load requirements. As the shear load has to be activated in a straight line, guide funnels are required on the subsea structure to ensure correct operation of the weak link mechanism.
The holes for the jumper hoses and cables are staggered to provide an offset for cutting of the elements. Otherwise with hoses constructed with elements such as Kevlar braiding, the weak link can jam and not activate properly.
The jumper hoses and cables, which are neutrally buoyant in seawater, are not secured through the weak link device. Therefore, in applications where there are strong seawater currents that cause movement of the jumpers, guide sleeves are used to protect the hoses and cables from chafing against the weak link edges. The sleeves also protect the cutting edges of the guillotine from inadvertent damage during handling.
Alternative designs of weak-link exist whereby the hydraulic & electrical sections disconnect using couplers/connectors, effectively acting as a large 'plug and socket'. These 'non-destructive' designs in theory allow a disconnected umbilical to be simply re-connected.
However, the mechanical installation requirements for a deepwater application render the design more complex, and in any event the disconnection forces still have to be made to act in a straight line.
