Lifting and lowering of loads in subsea operations is one of the most common tasks required in the whole life cycle of an offshore oil and gas field. During the drilling, field development, and production phases, everything used and deployed underwater needs to be transported, handled, installed, and – when required – removed for maintenance, repair, and replacement. Equipment and tools need to be carried down to the seabed and lifted back to the surface when the work is done. All this requires the use of reliable and safe riging equipment, such as slings, hooks, and lifting attachments (shackles, eyebolts, pad eyes, etc.).
Normally a hook provides the immediate connection between the crane’s side sling and the load. Thus, hooks play an important role in ensuring the success of any subsea lifting/ lowering operation.
The history of incidents involving hooks in the offshore industry is long. There is masses of documentation available evidencing the importance of paying attention to the risks related to the use of the most common types of hooks used in today’s offshore operations. The two most common types of incidents are snagging of hooks and unintentional release of rigging from the hooks (shedding). According to the Offshore Safety Division of the UK’s Health & Safety Executive (HSE), the dangers arising from both the snagging of hooks and the shedding of slings/pennants when operating in an offshore environment can be potentially lethal.
lncidents were first identified in topside lifting operations, in particular in relation to crane operations when performing supply boat lifts. However, with increasing subsea operations requiring equipment to be moved to and installed on the seabed, the numbers o f incidents related to hooks have also increased in this area of operations. All of the aforementioned issues and incidents involving hooks occurring topside are also applicable to subsea lits . However, compared to the scenario where elements of the lift operation occur in air , lifting and lowering loads in a subsea environment introduces further hazards related to the same issues of snagging and shedding of loads.
SNAGGING OF HOOKS
The traditional ‘single point hook’ has a safety latch with a spring return safety latch. This type of hook has a protruding ‘nose’ on the tip of the hook that may snag on to objects such as scaffold tubes, hoses, small bore tubing or pipework, or divers’ umbilicals. It can also catch any section that forms a lip such as rectangular hollow sections, channels, and flat protruding edges.
A variation on this type of hook was designed for ROV attachment and detachment. This design is commonly referred to as the ‘snap hook’ as loads are easily ‘snapped’ into the hook through the spring operated latch. This ROV operable design included an increased nose protrusion to facilitate easy attachment of slings and master links, and the ability to release rigging from the hook by upending using a lanyard that simultaneously opens the safety latch. Whilst making it easier and quicker to attach loads using an ROV, this new design also increased the potential for accidental snagging.
SHEDDING OF LOAD
When liting loads in a dynamic offshore environment it has been evident that certain circumstances allow rigging to release from the standard ‘single point’ hooks, including the ROV ‘snap hook’ version. There are two subtly different scenarios that can cause this to happen: one when liting in air and another when liting in water. The in-air incidents typically occur when there is rigging on the hook, but with little or no load attached. The hook may swing around violently, and the rigging thrown up into a position where it returns between the hook nose and the safety latch , thus releasing accidentally.
A similar release mechanism can occur subsea. In this scenario a heaving vessel may cause the load being lited to go light and cause the rigging to slacken. This is particularly relevant to lighter loads with a larger horizontal area, and in conjunction with higher vessel heave and an increased speed of crane deployment.
There have also been reported releases of smaller synthetic round slings from self-locking ‘safety hooks’ where the sling has managed to slip between the tip of the hook and the locked gate due to the design not taking account of this failure mode and the manufacturing tolerance between the parts.
The snagging of hooks and accidental load detachment has been well documented and for this reason the use of the ‘single point’ hook – including the ROV snap hook – was identified as a significant hazard to diving operations. As a result the IMCA document recommends using self-locking or ‘safety hooks’ and not ‘single point’ hooks. This has long been established practice for diver operations. Historically there is a lack of alternative hooks for diverless operations that are as quick and easy for ROV use, and this has meant that the ROV snap hook continues to be widely used.
The risks described above have typically been tolerated for diverless operations, and there is evidence that the acceptance of incidents has oten been normalised within the subsea industry.
ALTERNATIVES TO THE ROV ‘SNAP HOOK’
There are alternatives to the ROV snap hook, all of which generally work well for rigging release type operations. These include the ROV shackle and other proprietary type shackles and self-locking hooks. However those alternatives sacrify the “ease of use” of the snap hook resulting in increased time for manipulation.
Jim Battersby, Domain Expert and Subsea 7’s Specialist Lifting and Rigging Engineer explains: “In our company we have focussed on safety, and have accepted the “time penalty” and increased costs, in order to maintain safety at a high level by using slower to operate but more secure equipment (like shackles and safety hooks) for tasks where snaphooks would have traditionally been used in the past.”
The attachment of rigging using ROV’s has always been a challenge and it is this design issue that has provided the impetus for Subsea 7 and RUD to work together to design a product that meets both the operational safety and speed of use requirements that are currently missing from the market.
The speed of use requirement is important as the vessel’s operational cost is significantly high ; any time saved on critical path ROV operations will have a cumulative saving in the long term.
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