Page 345 - Pipelines and Risers

Route Optimization, Tie-in and Protection 3 17

General Principle
The jet sled works on two principles:

1. High pressure jet nozzles power water to break up the soil;
2. Air is pumped into pipes which generates lift, this lifts the broken soil away from the
location.

Using these two principles a jet sled is able to trench.

Installation capabilities/Constraints
The size of jet sleds (and associated costs) varies considerably. The largest can weigh up to 80
tones and are controlled by dedicated vessels, and the smallest can fit onto a DSV and weigh
up to 0.5 tone. The associated capabilities also vary. The large ones can trench through sand,
silt and clay and even through soft rock (sandstone), the trench rates vary depending on the
soil conditions. The small jet sleds are only suitable for sand, silt and soft clay.

The main constraint of the jet sleds is that they cannot bury the flowlines. They can excavate a
.hole for the flowline to sink into, but they cannot backfill the hole.

Jet sleds are still a popular means of trenching pipes as the method is well proven and little
damage to the pipeline occurs compared to that caused by damage by other methods.

This system usually requires divers but may be operated diverless.

17.4.2 Ploughing
Ploughing was first developed in 1980 for the North Sea in order to provide a cheaper
alternative to trenching of pipelines. Since then it has become a popular method of pipeline
trenching. (see Figure 17.7).

General Principle
The general principle of pipeline ploughing has been adapted from the technique used in
agriculture to plough fields. The pipeline plough consists of a very large “share” which the
pipeline rests on top of. The pipeline is pulled along (usually by the surface vessel) and as the
ploughshare passes the flowline settles in the trench.

Should a backfill plough also be employed, this will reverse the process by pushing the spoil
back into the trench, so burying the flowline.