Multiple Pulling Pigs for Cable Laying
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Solution Overview
Problem
The push/pull method for laying cables, which uses water or air pressure to push and pull cables through ducts, faces challenges with the capstan effect, where excessive tension leads to exponential force build-up and increased friction, especially in ducts with bends and undulations, causing inefficiencies and potential cable failure.
Innovation Solution
The use of multiple pulling pigs placed at regular intervals along the cable, each exerting a controlled pressure drop to distribute the pushing force, reducing the overall pressure on each pig and minimizing friction by compensating for extra forces at bends and undulations, allowing for longer cable installations without losing buoyancy benefits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a single pig is used to push and pull the cable through the duct, then the cable can be installed, but excessive tension builds up causing the capstan effect and exponential force increase at bends and undulations
Solution Approach 1:
The single pig is divided into multiple pigs distributed along the cable at regular intervals. Each pig independently pushes and pulls the cable section it is attached to, segmenting the total tension force into smaller portions that each pig can handle, thereby preventing the exponential force build-up associated with the capstan effect.
2Reliability
If water under pressure is used to push the cable, then buoyancy reduces the effective cable weight and frictional heat is cooled, but the capstan effect dominates earlier and the buoyancy advantage is lost
Solution Approach 1:
Multiple pigs are distributed along the cable to segment the tension forces. Each pig handles a portion of the total force, preventing the exponential tension build-up that causes the capstan effect to dominate. This maintains the buoyancy benefits of water by keeping tension forces within manageable limits throughout the installation.
3Length of moving object
If multiple pigs are used to distribute pushing force, then the installation length can be extended, but the device complexity increases
Solution Approach 1:
The cable installation system is segmented into multiple independent pig units distributed along the cable. This segmentation enables extended installation lengths by allowing force distribution across multiple pigs, while each individual pig remains a simple, standardized component that can be independently deployed and retrieved.
Solution Approach 2:
Each pig is equipped with self-contained sealing and gripping mechanisms that allow it to independently push and pull the cable section it is attached to. The pigs automatically seal against the duct wall and cable surface through pressure differential, eliminating the need for complex external control systems for each individual pig.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces the capstan effect, enabling longer cable installations with reduced friction and improved reliability by distributing pressure across multiple pigs, maintaining buoyancy advantages and preventing excessive force build-up, thus facilitating the laying of very long cables efficiently.
Implementation Method 1
The advantage of using water instead of air is its buoyancy, reducing the effective weight of the cable.
Implementation Method 2
water is injected under pressure into the duct, exerting a pushing force at a pig attached to the front end of the cable
Implementation Method 3
Furthermore it coots the frictional heat.
Implementation Method 4
each exerting a controlled pressure drop to distribute the pushing force, reducing the overall pressure on each pig
Implementation Method 5
The pulling pigs are designed such that they work with a specified pressure drop (a flow of the fluid through all pulling pigs is needed to obtain this)
Data Source
Figure 1~3
Figure 4~7
Figure 8~12
AI summary
According to the process of laying a cable (1) by a push/pull method described, we use more than one pulling pig (2, 20a, 20b). If the first pulling pig (2) is disposed at the front end of the cable, other pulling pigs (20a, 20b) are disposed at regular intervals along the cable (1), surrounding the latter. Each pulling pig (2, 20a, 20b) comprises in particular pressure reducing means (7) permitting to distribute the entire pressure of the fluid between all the pulling pigs. Different embodiments of a pressure housing (12) are also described.