ACCESS SYSTEMS: TDD – Tube Docking Device

December 22, 2015

Transfers at sea have long been a problem and the safety of those involved is becoming an increasing consideration particularly because of the rapidly increasing numbers of people involved. The offshore wind farm owners and contractors alike have very strict H&S policies which other industries increasingly feel obliged to adopt as the renewable energy sector spreads further and deeper offshore.

H&S as acceptable risk management becomes ever more stringent. Failed transfers of personnel offshore due to safe transfer constraints in poor weather conditions, is enormously expensive to each and every offshore contractor resulting in lost revenue and production.

The directors of Offshore Transfer Devices who have been on the forefront of offshore transfers decided to look at how to improve the way in which offshore transfers of personnel are conducted, not only because of the costly down time it could save offshore wind farm owners, operators and contractors due to limited accessibility but also to improve the H&S and the risks involved for personnel during transfers. At the same time contributing to the Carbon Trust’s goal of reducing the cost of energy by ten per cent in time for Round 3 projects.

With 15 years of experience, hands-on knowledge and a sound background in and understanding of the challenges and difficulties in offshore transfers this resulted in the developed and patented ‘Tube Docking Device’.

The Tube Docking Device (TDD) transfer system is backed, funded and supported by the Offshore Wind Accelerator (OWA), a leading international collaborative R&D program with the Carbon Trust and nine leading offshore wind developers that aim to reduce the cost of offshore wind by ten per cent in time for Round 3 projects.

Together with its nominated contractors, the company and its contractors have absolute conviction that the TDD concept could change the way offshore transfers are conducted in the future. The TDD access system will deliver the desired performance offering a safe, reliable, measurable, economic and simple solution to the business of transferring personnel and equipment offshore. This will afford greater access and workable weather windows, resulting in increased productivity and reduced losses caused by failed transfers that are not possible with conventional access fender methods.

Description & advantages

The TDD transfer system is a light, compact device used to effectively stabilize, pin and grip the transfer position to a structure offshore.

Motivation for the device throughout the process has been to make it fail-safe, ensuring that mechanical entanglement to the offshore structure impossible. This is achieved by making the amount of grip applied to the offshore structure relative to propulsion force applied and two key factors considered thorough out design work and model testing were to keep the device as simple and as light weight as possible. The design and geometry of the TDD allows clamp force intensification delivered solely by the vessel’s propulsion. It has a theoretical advantage of approximately ‘propulsion force x 3.9’ i.e.- with as little as 60KN pushing force this equates to a 231.9KN clamping/gripping force maximizing confidence providing the ultimate stable platform.

Offshore Transfer Devices and Hercules Hydraulics (who supply the circuitry, control gear, monitoring systems and all hydraulic ancillaries) have together enabled an elastic approach which will minimize impact loads, provide cushioning and offer protection to the device, vessel and offshore structure in greater wave height conditions.

The device goes through a primary phase where the jaws take the shape of the cylindrical fender tube. This is shortly followed by a secondary phase known as the thrust out. A combined dampened stroke of 750cm enabled by hydraulic rams and accumulators. The hydraulics are also utilized for pre-deployment arming, yawing, thrust out and retrieval. Indeed the hydraulics take no part in the device holding on to the offshore structure and by design the TDD automatically disengages if forward pressure/propulsion is reduced or terminated rendering involuntary mechanical entanglement to the offshore structure impossible.

An additional (but optional) assistance of several tones can be implemented which operates in similar fashion to putting one’s foot on the brake pedal during every day commuting by car. This optional ability to add clamping force is not necessarily to amplify clamping force which can be achieved by the vessels propulsion, but to allow substitution or minimal propulsion enabling transfers to light anchored structures such as floating LIDAR’s, anchored barges or jack up vessels. This optional ability, whilst using little or no propulsion, also enables transfers in adverse weather conditions to ships under DP (which often strictly instruct CTV’s / WFSV’s to push on with minimal propulsion as it can upset the mathematical model of the DP vessel).

If this additional 20KN reverse thrust of primary deployment ram were utilized with the total propulsion force, (eg. 60KN as above) the total available grip would increase to just under 300KN giving even greater confidence and stability with offshore transfers. This means the vertical force which is needed to dislodge the transfer position will need excess of 200% of existing vertical force when compared to the traditional conventional push on fender methods. Conversely, due to mechanical advantage a vessel can obtain a higher level of grip with a proportionally reduced amount of propulsion, thus minimizing pushing force required to maintain a stable platform against offshore structures. In addition this will aid fuel economy and wear and tear to vessels engaged in prolonged transfer operations with personnel or cargo.

Another key feature of the system is the ability to alter the lateral position whilst docked on to the tube structure at any given time without disengagement. This can be achieved by simply reducing propulsion and re-applying when at the desired height enabling clamping with its bespoke RG Seasight-Fenders located within the arc of the clamps which do not protrude further than 180 deg of tube structures.

The TDD is able to deploy on to a tube whilst the vessel is simultaneously in a roll, pitch and off center. It will offer a total target area of approximately 1.6m and will automatically self-align to center, whilst addressing all 6 motions of freedom – heave, pitch, roll, yaw, sway and surge.

Application

The TDD will be simple to install, provide high performance, extremely reliable, easy to service and maintain, require relatively simple technology and is compact taking up only a small amount of ‘low’ level deck space. Indeed it is foreseen that the device can be housed completely inside the bulwarks of many vessels taking up very little forward deck space and can potentially be under-slung.

There are several ways to apply the TDD access system, such as vessel to offshore structure transfers, vessel to vessel transfers, aiding the connection between offshore structure and compensated gantry applications (walk to work) and also allowing vessels to be unattended in the lee of mother ships using the TDD. The TDD can be made to fit any size vessel whether to aid a daughter craft capabilities with an ultra-light 150kg device or full size 1250kg device for the larger 26m+ CTV / WFSVs. For the larger vessels minimizing pushing force required to maintain a stable platform and exclude the possibility of impact loads whilst docking on to the wind farm structures in adverse weather conditions is ever more imperative.

“The TDD system has the potential to increase the accessibility to the turbines of existing vessels without penalising the transit capabilities or the cargo capacity, since is a rather small and lightweight system that can be easily fitted in the bow the vessel. In the OWA we are proud to see how innovations like the TDD are reaching the market and we are looking forward to the next stage consisting on the demonstration of the solution in a full-scale vessel. Thanks to technologies like the TDD turbine availability will increase and the overall LCOE for Offshore Wind will continue decreasing.” – Dan Kyle Spearman – Carbon Trust

A full scale prototype has now reached trial stages since going through comprehensive model testing for several years and all parties involved are ready to prove the concept in the first quarter of 2016. Offshore WIND will follow the developments of TDD closely!