Cable Static Span Due to Depression

The bending radius of cable due to depression sagging should be larger than the Minimum Bending Radius (MBR) of the cable.

The MBR of the cable is provided by the manufacturing vendor while the bending radius of the cable can be calculated in accordance to Beam Theory.

Bending radius calculation:

R = EI/M

Where,

R = bending radius of the cable at the span location (m);

EI = bending stiffness of the cable (kNm²);

M = bending moment of the cable (kNm).

Maximum bending moment calculation:

The maximum bending moment of the cable at the spanning section depends on its boundary conditions:

• Pinned-pinned: M = w_subL²/8;
• Pinned-fixed: M = w_subL²/8;
• Fixed-fixed: M = w_subL²/12.

Where,

M = Maximum bending moment;
w_sub = cable submerged weight per unit length (KN/m);

L = span length.

Note: It is reasonable to assume that a single span as pinned-fixed, if the information related to the boundary condition is unavailable.

Free Span

A free span is a part of the pipeline where it has no support on the seabed. If the span length is too long, the pipe might oscillate due to ocean currents and waves. It could also expose to damages from anchor or trawling activities. Figure below illustrates a free span.