A serious incident occurred recently when a dive support vessel’s dynamic positioning (DP) system failed resulting in the vessel drifting off position while divers were deployed on the seabed. The vessel, Bibby Topaz, was fitted with an IMO class 2 DP system.
The UK’s Health & Safety Executive (HSE) says investigations have shown that a probable cause of the DP failure was a single fault which caused blocking of the DP system’s internal data communications.
Dynamically positioned vessels undertake a range of safety-critical activities such as diving support, drilling for hydrocarbons and operations adjacent to offshore production installations. In many cases, the safety of critical activities depends on the continued availability of DP functions. Many DP systems rely on ‘bus-oriented’ communications networks. Investigation of the incident referenced above found that communications dependent on a dual bus network can be totally lost because of a single fault.
Dynamic positioning systems are categorised into classes according to criteria from the International Marine Organisation (IMO). The classification criteria are given in IMO MSC Circular 645 and include the requirement, “For equipment class 2, the DP control system should consist of at least two independent computer systems. Common facilities such as self-checking routines, data transfer arrangements, and plant interfaces should not be capable of causing the failure of both/all systems.”
Various configurations have been used in data communication networks (dual bus networks, star networks and hybrids). For illustrative purposes, modules which need to communicate are designated as M1, M2, M3, M4 and C1, C2, where ‘C’ could indicate a controller module and ‘M’ could indicate a module which interfaces to plant.
In a dual data bus communications network, the communications media – the data buses – are shared between system modules. In configurations with a dual bus architecture, if a single module has access to both buses (M1 and C1), and the module persists in trying to communicate, then it can block both buses.
Thus, in the absence of further protective measures, a single fault in a DP system which depends on a dual bus architecture, can prevent all critical information from reaching the modules that need this information in order for the DP system to function satisfactorily. Other network topologies, such as dual star networks, may not have such shared communication media.
Where the safety case for an offshore installation includes claims in relation to performance of DP systems the safety case duty holder should verify that the claims can be met.
“Manufacturers and suppliers of dynamic positioning systems who claim their products satisfy IMO class 2 or better should investigate the communications architecture for the relevant dynamic positioning systems,” said the HSE. “If the dynamic positioning functions are dependent on a shared communication medium such as a dual data bus network, then the manufacturer/supplier should check that appropriate measures are in place to prevent a single fault causing failure of the DP system.
“If such measures are not in place, then the relevant manufacturer or supplier should ensure that the users of the dynamic positioning system are provided with adequate information regarding the vulnerability of the dynamic positioning system to single faults.”
The HSE lists the relevant legal documents as the following:
• Health and Safety at Work Act 1974
• Offshore Installations (Safety Case) Regulations 2005
• Provision and Use of Work Equipment Regulations
• Offshore Installations and Wells (Design and Construction, etc.) Regulations
• Diving at Work Regulations 1997
• Merchant Shipping (Diving Safety) Regulations 2002
• Health and Safety at Work etc. Act 1974 (Application Outside Great Britain) Order 2001 and associated Variation Order.
Hyperbaric evacuation systems interface recommendations published
Proposals published by the International Marine Contractors Association (IMCA) in Hyperbaric Evacuation Systems (HES) Interface Recommendations (IMCA D 051) seek to address the interface issues encountered when a diving contractor needs to remove its divers from a stricken facility by use of a hyperbaric rescue unit (HRU).
“This document is a ‘recommendation’ only,” explained Jane Bugler, IMCA’s technical director. “We would like to emphasise too that the proposed interface recommendation put forward in our new publication is for future builds, it is not written with the view to retrofit existing systems. What we have published has four objectives:
• to plan for the future
• to provide drawings and technical data that will support all interested parties
• to recognise and reflect current good practice
• to gain recognition and total worldwide acceptance.
“The ultimate objective is that ‘every HRU will be able to mate with every DSV (diving support vessel) or HRF (hyperbaric reception facility) of any class anywhere in the world’,” she explained.
“It is recognised that this particular objective might not be met for 30 years until non-IMCA D 051 compatible, DSVs, barges and HRFs in use today have reached the end of their service life.”
The guidance document contains a number of drawings. However, the drawings containing detailed engineering design information are also available on the IMCA website for download in STP and DWG formats
The new publication does not seek to address safety, risk assessments, planning, or general equipment maintenance issues. These are addressed in the main body of IMCA’s Guidance on Hyperbaric Evacuation Systems (IMCA D 052).
In the 1970s, a system was introduced in the North Sea by a US diving company (IUC) for the evacuation of an injured diver from offshore in an emergency, while still under pressure. This comprised a single diver titanium chamber that was transported offshore by helicopter along with a larger chamber capable of taking up to four men.
The single chamber was fitted with 600mm (the commonest size in use at that time) flange and clamp. Over the years that followed many diving systems in the North Sea had one of their living chambers fitted with a compatible flange to accept this IUC chamber. This arrangement became considered the informal ‘standard’.
Over time, as the industry matured and diversified, the use of the informal ‘standard’ diminished and in many areas was superseded by larger units more suited to operational demands; these changes also more closely reflected the genetic growth of the population over the last generation.
“The development of a ‘standard’ approach was very successful for the IUC flange in the North Sea,” says Jane Bugler. “A similar approach today would clearly be desirable for the future of hyperbaric evacuation within a global marketplace. It is generally agreed that a common interface recommendation that will permit successful hyperbaric evacuation without commercial, geographical or technical constraint would be of great benefit.”
Subsea deal for Stork Technical Services
Stork Technical Services recently secured more than £15 million of new work with a major North Sea operator. The contract award will see Stork’s subsea team deliver a range of integrated subsea inspection, repair and maintenance (IMR) work, including thruster change outs, over a five month period utilising remotely operated vehicles (ROVs) and air/nitrox diving from dive intervention craft and dive support vessels.
The services will be delivered from a combination of bespoke dive intervention craft and the company’s dedicated dive support vessel, Adams Vision. The craft are equipped with a range of advanced technology. Adams Vision has been upgraded to include a complete air/nitrox diving spread, allowing diving operations to be carried out from the DSV.
Stork’s dive intervention craft have been deployed on major subsea projects across the globe and are expected to generate revenues of more than £10 million alone throughout 2013.
Roddy James, director of Stork Technical Services Subsea, said: “We are committed to operating in a safe, efficient and cost-effective manner and our investment in our dive intervention craft are an excellent example of this. The craft provide a safe, field-proven method for delivering subsea IMR in areas that may be hard to reach or situated too close to a rig for a DSV to access.”OSJ
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