Case Study: Installing a Welded Riser on a Pipeline with a Piggyback Cable

The Problem

One of the challenges I faced was lifting a pipeline with a piggyback cable to the water surface for the purpose of welding a riser. In this instance, the free (unattached) part of the cable spanning the seabed is prone to bending, kinking, dragging, and other expected damages. In such a situation, we cannot simply lift the pipeline, so we must find a solution in advance to protect the cable.

The Solution

The most critical point on the cable is where it will be suspended at the last banding strap. My solution was to use a chute, as shown in Figure 1.

Figure 1: The chute

As seen, it is half shell encompassing the cable, running straight over the last six meters, then directed out of the pipeline in the horizontal plane, and finally directed down again in the vertical plane. In order to install the chute, the last two or three straps must be replaced by the straight part. The curved part is flanged to the straight part.

The concept of the chute is to not let the cable go down randomly without a rigid structure holding it

The concept of the chute is to not let the cable go down randomly without a rigid structure holding it. The cable is strapped to the chute the same way as it is strapped to the saddles. In this way, we solved the problem of suspending the cable at a weak point.

However, what I did to avoid dragging the cable was even more innovative. I introduced a solution inspired by the lazy-wave configuration that is typically used in flexible risers. The idea depends on using air bags to buoy the cable off the seabed, making a wave-like configuration before lifting the pipeline (see Figure 2). When the pipeline was lifted, the cable adopted a shape similar to the lazy-wave configuration (see Figure 3). This almost eliminated the chance of dragging the cable on the seabed because it was already semi-submerged in water before, during, and after lifting.

Figure 2: Cable shape before lifting the pipeline

Figure 3: Cable shape after lifting the pipeline

Equate the angle and the tension at the connection points

I performed a static check using hand calculations. The idea is to equate the angle and the tension at the connection points and assume a reasonable value for the tension at the seabed, hence dealing with each curve as if it were an independent catenary, curve-by-curve from the seabed to the pipeline connection point.

Later, one of my colleagues performed static checks using OrcaFlex, and the result was that the curve shape I had produced manually was fortunately quite similar to that shown by OrcaFlex.