RESTRICTORS

Bend restrictors play a large part in the installation and operation of cables, umbilicals and flexible risers' subsea.

A bend restrictor is a device to prevent the pipe or umbilical from over bending. Unlike a bend stiffener it does not provide any stiffness to the internal element until it is at "lock up" radius. At this point the restrictor becomes very stiff and prevents the pipe or umbilical from over bending and perhaps kinking or buckling. Bend restrictors can be manufactured from a number of materials dependent on the operational conditions, however, the most common types of restrictor are manufactured from polyurethane

The restrictor is manufactured from a number of elements as shown below. These elements are cast in half shell form to allow the restrictor to be fitted at the fabrication yard, or on board the vessel as the line is being deployed. The half shell elements are bolted together to form a whole around the pipe, further elements are bolted to interlock with those already in place. The design of the elements allows each one to move a small angular distance. When this distance is projected over the length of the restrictor the lock up radius is formed.

When designing a restrictor it is important to note temperatures of the line. If the temperature is above 50°C then the modulus of the restrictor can be affected. It is therefore important to select the right grade of polyurethane for the application.

Dunlaw Engineering Ltd. can manufacture bend restrictors in both polyurethane and steel, depending on loading conditions. The restrictor is first modelled in 3D and computer based calculations are performed to ascertain its lock up radius and maximum load bearing capacity. We can then use our Marc element analysis package to perform a non linear performance analysis.

Bend restrictors are commonly found at static terminations, touch down points, mid-water arches and in any region where over bending is a problem. Dunlaw Engineering can manufacture restrictors to suit most sizes of line. Restrictors have been made to suit a 30mm cable up to a 16" flexible riser

STIFFENERS

At the termination point of a flexible riser, umbilical or cable, the stiffness of the system undergoes a step change. Along the length of the flexible, the stiffness can be thought of in terms of the composite material modulus and the 2nd moment of area. At the termination point, the stiffness step changes to take into account further elements. For example at a subsea termination into a structure the local stiffness of the structure is added in. In relation to the stiffness of the flexible the structure stiffness is a very substantial add in. This sudden change in stiffness creates high levels of stress if the flexible is bending away from the termination point. In a dynamic situation, this can lead to fatigue failure in the flexible. Obviously this situation is not desirable so a solution has been developed to cope with this problem.

Bend stiffeners were first documented as being used in bridge design over 200 years ago, where they were used in cable termination for the first suspension bridges. Today they are in use world-wide.

BENDSTIFFENERS USED SUB-SEA

The hydrodynamic effects of current on flexible risers, umbilicals and cables lead to substantial bending moments being applied to the termination points of the system. Whether it is a small power cable on an R.O.V or a large riser attached to a floating production system, the loads applied by waves and current at various depths impose large forces at the termination points. To protect this point, bend stiffeners are utilised to increase the composite stiffness of the flexible to the stiffness of the structure it is attached to. The most common method of achieving this is to attach an Elastomer tapered sleeve to the flexible with its large stiff end attached to the structure. This in its simplest form is the concept for the bend stiffener.