Crane loads no longer left swinging in the breeze 21 Mar 2016

An ‘off the shelf’ vessel motion monitoring system that allows the dynamic loading on cranes to be accurately predicted, and in so doing, greatly increase marine safety, has been developed in Perth by Tensa Equipment Pty Ltd.

The DMS (Dynamic Motion System) helps remove the risk in transferring offshore loads by measuring vessel motions, in particular the heave velocity. This was an area previously the domain of expensive, customised equipment that was only affordable on the largest heavy-lift vessels. Dynamic load monitoring systems have previously cost in the order of $100,000 to design and install, whereas Tensa’s DMS is budgeted at about a fifth of that cost as it is now offered as a stand-alone device that can be connected to a PC and powered by USB.

DMS Mini with USB connection

The current imprecise approach of ‘guesstimating’ the dynamic load by using the significant wave height can now be replaced with the science of measuring the load velocity which allows the dynamics to be precisely calculated. Also, the device gives the crane operator a visual indication of the real time vessel motions so the difficulties of estimating the vertical motion of a vessel at night or when looking down, and timing lifts are now reduced.

By measuring the vertical velocity at the load position, Tensa’s DMS system takes into consideration where on the vessel the loads are positioned e.g. at the stern of a pitching vessel versus the centre; and whether the vessel is pitching into a sea or broadside on; as well as whether the waves are fast and steep or long and slow. The DMS is so advanced it can use two wireless units to give a crane operator the relative motion between the load suspended from his vessel and the deck of the vessel alongside.

Project and rig spread costs can now be reduced if crane capacity is fully utilised. To take the guesswork out of selecting the dynamic loading and crane curves to use for offshore lifts, DMS can measure the motions of a vessel before even deciding to undertake the lift. The vessel data can be viewed simultaneously by the crane operator, the vessel skipper and any other party with a DMS receiver within the maximum broadcast range of 500 m, or to onshore PCs via the internet.

DMS max and wireless receiver module.

DMS is a totally portable system consisting of small waterproof modules that communicate by USB or wireless and uses standard Windows interfaces so that virtually no training is needed to get the system up and running.

It can be permanently installed on a vessel or deployed on a project by project basis, with all data being available real time via the internet, which makes it ideal for shore-based monitoring and input.

Applications for DMS are not however limited to offshore crane operations.

Another application is in helicopter operations where DMS motion monitoring information can help assess whether it is safe to operate or not. While DMS doesn’t comply with the current onerous CAP437 requirements, it provides the data needed for a fraction of the cost. Operators that couldn’t afford CAP437 rated services now have an affordable option with which to provide their installations with the tools to measure safe operating conditions for their offshore helicopter operations. The DMS system can easily be developed to incorporate the Significant Heave Rates and Motion Severity Index data displays required by CAP437.

The regularity with which towed vessels break off while under tow and where towed barges experience damage due to excessive motions is another significant market for the DMS. It is an inexpensive tool that fleet owners can use to reduce their operational risk.

The DMS can also be used to monitor vessel motions and locations during other offshore construction phases such as floatovers etc, and in FPSO connecting operations, using its wireless linked GPS. The system delivers real time heave, pitch, roll, heading speed, course, position, accelerations at the sensor as well as at offset positions such as the bow, stern roller or an overboarding sheave.

DMS Buoy

DMS has further safety applications as it can measure the vessel motions when personnel transfers are being contemplated and significantly reduce the risk to workers by allowing clear definition of the maximum tolerable conditions for transfer. Operators now can start making ship to ship personnel transfers by gangway a reasonable option to helicopter transfers when scoping project feasibility – air transport carrying its own inherent risks and cost base.

For further information, contact Derick Markwell, Managing Director,