Feature Articles—November 2009 Issue
Development of a Lightweight, Neutrally Buoyant MCM TowedArray Cable
Limiting Payload Impact and Reducing Towed Array Weight While Offering
Increased Durability, Simpler Operation and a Flexible System
By Jostein Auganæs
Project Manager
and
Henrik Bang-Andreasen
Managing Director
Seaproof Solutions
Bergen, Norway
A conventional mine countermeasure (MCM) system is towed behind a vessel. A set of deflectors, tow ropes and buoyancy modules make up the cable system between the vessel and electrodes. To operate it, the vessel must be equipped with winches for both tow cables and electric cables. Essentially, a conventional MCM system requires heavy electric cables to transport sufficient power and tow cables to handle the heavy electric cables.
A European navy client wanted to explore the possibility of making a lighter and simpler cable system. The weight and complexity of their existing system made marine operation, and in particular reconfiguration, time consuming. If the system could be made easier to configure and handle, the operational window would increase dramatically.
The traditional system was difficult to handle and high maintenance, had a time-consuming launch and recovery, took up a large amount of space, required a lot of deck equipment, and was not reconfigurable or repairable at sea.
Challenges
The project group at Seaproof formulated two simple challenges based on the client’s experience: Can the cable be lighter without reducing its power-carrying capacity? Can the design be more flexible without reducing its durability?
The team looked for solutions to work with the environment and physics rather than relying on complex designs.
One common problem the team needed to overcome was eliminating generated heat. The conventional solution is to compensate for losses by adding conductor area until the generated heat is no longer a problem. Most towed array cables are designed so that the primary functions—power supply and signal transfer—are housed in the core, protected against the elements by jackets and stress members.
The problem with this design is that the heavy copper needs to be offset by a relatively large amount of buoyancy. The low-density material acts as thermal insulation, preventing the conductor from getting rid of generated heat. Adding a stress member (e.g., an aramid) further increases the thermal insulation problem. Adding conductor area also adds to the problem, resulting in a cable that either does not meet buoyancy requirements or is too heavy and big for handling and storage.
Cooling Through Seawater
The team took a different route, aiming to take advantage of the excellent heat transfer properties of seawater. Aluminum is often used in neutrally buoyant cables (e.g., tethers), and by replacing the copper conductor in the center of the cable with an aluminum conductor in the outer layer of the cable, they managed to reduce its weight and increase heat transfer to the seawater at the same time. For the required high-current direct current (DC) rating, the cable’s overall dimension was reduced by more than 50 percent (from 102 millimeters to 49 millimeters) and its deck weight by about 60 percent in comparison to the original cable.
The core element of the system had been created, a power cable that met the first challenge. With a little help from Nexans Norway (Oslo, Norway) the team was able to make the aluminum conductor serve as a stress member as well, eliminating the need for tow lines and further reducing the weight and complexity of the overall system.
Adding Flexibility
The next challenge was to make the design more flexible. The cable was divided into sections so that these sections could be combined into different configurations, with all connectors identical—deck connectors, load connectors and cable section connectors.
The connectors would not only have to meet the relevant electrical requirements, but also act as a mechanical connection, so that they would be the only interface between the cable sections.
The team designed a lightweight titanium connector with high current capacity and high mechanical strength, spoolable onto a winch under load. The connectors were interfaced with bend stiffeners to handle bending strain on the winch and in operation.
The bend stiffeners were specially designed to also ensure that the cable termination would not be insulated from the cooling seawater.
The prototype was built and tested to perform to specification, allowing a 3,000-kilogram dynamic load capability, and it was then put into operation, performing well during handling and reconfiguration.
At this point, all of the design criteria seemed to have been met.
However, halfway into the testing period, problems started to arise. Several of the titanium connectors overheated, some of them even burning up during operation.
Analysis found that heat generated at the termination between the cable and the connector caused the connector failure. Temperature fluctuations caused the resistance of the termination to increase, and high resistance caused the temperature to rise, which in turn led to even higher resistance, and so on.
The problem was in part caused by the special challenge of designing the termination to meet both electrical and mechanical demands.
Several improvements needed to be made to the termination between the aluminum cable and the connector, including choosing a new material and using a new technique for attaching it.
These design modifications have been tested and seem to solve the problem of overheating. The Norwegian technical institute SINTEF is currently assisting in assessing and verifying the connectors.
After verification, the company will begin production of multiple arrays to equip the client’s fleet of MCM vessels.
Jostein Auganæs finished training as a chief petty officer in the Royal Norwegian Navy in branch weapon and electronics systems in 1992. He received a bachelor’s degree in mechanical engineering from Bergen University College in 1996, and since then he has worked as a design engineer. He joined Seaproof Solutions in 2007 and is project manager for the mine countermeasure cable project.
Henrik Bang-Andreasen founded Seaproof Solutions in 1989 and is managing director. He has an engineering background in military underwater and electronics systems. He has held positions at Svitzer and was general manager for Normac A/S.
Sea Technology is read worldwide in more than 110 countries by management, engineers, scientists and technical personnel working in industry, government and educational research institutions. Readers are involved with oceanographic research, fisheries management, offshore oil and gas exploration and production, undersea defense including antisubmarine warfare, ocean mining and commercial diving.