Tuesday, 20 August 2019

BOLLARDS STRESSED BEYOND CAPACITY


Iain MacIntyre outlines how bollard capacities around the world are being stressed by rapidly-increasing newbuild ship sizes and considers the view from insurers
Wharf bollard capacities around the world are being challenged and, in some instances, exceeded by the rapid recent increase in newbuild sizes, with concerns consequently
raised about the adequacy of processes to inspect, test and maintain such key infrastructure.
The issue came to the fore in New Zealand in February 2017, when high winds led to the 41,865-GT cruise ship Seabourn Encore tearing mooring bollards off the wharf at PrimePort Timaru, with the vessel's stern then swinging across the harbour and colliding with a bulk cement ship.

A Transport Accident Investigation Commission investigation found that the bollards failed because their fastenings to the wharf and underlying wharf structure could not take the force from the ship's mooring lines. Among other observations, it stated that: “Safe and effective mooring of ships depends on port companies knowing the safe working loads for their mooring infrastructure.”
Mott McDonald Australia, New Zealand and Asia maritime practice leader Sam Mr Harris reports a “real mixed bag of responses” to the issue. “Some ports have taken a very proactive response to the changing nature of shipping and other factors, while others are more reactive to changing mooring requirements as those problems emerge,” he told Port Strategy.

“This is quite understandable given the complexities often encountered for upgrades and an increasingly cautious approach to investment in infrastructure improvements in advance of demand. With such a rapid increase in vessel sizes globally over recent decades, predicting future requirements becomes increasingly difficult to anticipate.

“Often the need for channels/berths to accommodate the physical dimensions of larger vessels - for example, deeper draught and greater ship-to-shore crane outreach - is quite
obvious. Enhanced mooring requirements are sometimes less obvious and more complicated to assess and address any shortcomings.”

AECOM ports and marine technical director James Mr Hutchison says all ports seeking to cater for larger vessels must now have assessment of their infrastructure “front and centre of their decision making”.
“The majority of port owners will commission studies to ensure that these larger vessels can be accommodated,” he says. “This may include condition assessments of existing
infrastructure but also structural capacity assessments to accommodate the larger berthing and mooring forces from these larger vessels, as well as whether dredging of berth
pockets is required. The strength of wharf bollards is just one component of the port's infrastructure that needs to be considered when larger vessels are contemplated.”

Further complicating matters, many older ports will have bollards for which there is no design load data, observes Beckett Rankine director Tim Mr Beckett.
“Even for new bollards the actual capacity can be less than the rated capacity due to quality control defects - it is not common for new bollards to be proof-tested,” he says.
“It is not just the bollard and its fixings which are critical but also the structure to which the bollard is attached. There have been several cases of sheet-piled wharf frontage failures due to the load from a bollard overstressing the wall's anchorage ties.

“I am also aware of a mooring dolphin at an oil terminal berth where the steel supporting piles failed in tension at their junction with the concrete dolphin cap under mooring line loading.”
Determining the strength
Assessing mooring strength requires appreciation that this infrastructure comprises a system of elements, configuration and operation, says Mr Harris. “The strength of the system is only as strong as the weakest link,” he states, before adding that the range of aspects that consequently need to be understood fall within the following parameters:
  • What infrastructure is in place now?
  • What are the design mooring conditions?
  • What are the future requirements?
  • What practical measures can be adopted to address any
    shortfall in mooring capacity?
“Knowing the strength of bollards is important, however it only provides partial context. It is possible to proof-test the strength of bollards, but it is very expensive and can cause
significant operational disruption. Sometimes this is the only course of action though.”

Mr Beckett says that although the majority of ports “generally know” the rated capacity of their bollards, particularly on berths accommodating larger vessels, it is less common that they will know the degree to which their aging bollard capacity might be reduced by corrosion.

“In the past bollards were tested by the pull from a tug, but this method has gone out of favour as a failure under test could do significant damage to the tug.

“There are a number of firms which offer to test bollards using a hydraulic rig that tensions one bollard against its neighbour. This is much safer in the event of a failure, but it does not necessarily test a bollard for a line pull in the direction that a ship would apply it, especially for a large ship which may have very steep line angles. It is, however, very much better than no test.
“Stressing one bollard against its neighbour is not suitable for bollards on isolated mooring dolphins or for mooring hooks which have a limited range of swivel. I understand that there has been some experimental work done on testing bollards and their fixings using ultrasound or similar.”
Mr Hutchison concurs that most ports will have good understanding of bollard strength, but also notes that the safe working load (SWL) - typically stamped on each bollard to
indicate its rated load capacity - can only be realised if:

1. The bollard is properly connected to infrastructure to be able to transfer the loads from mooring lines to the wharf structure.
2. The wharf structure is capable of accommodating the combination of mooring line loads imposed on all of the various bollards.

“These are generally referred to as local and global capacities. Bollards are generally only tested when they are new and once they have been installed it is very difficult to actually test them. We are not aware of any ports that test their bollards with a proof load.

“Generally, if there is a concern with a bollard, they are visually inspected and assessed. It is good practice to remove the bollards to inspect the condition of the bolts at deck level, however, it is not known whether this is routinely carried out.

“A problem that has been identified in the United Kingdom involves several examples where cast iron/cast steel bollards have failed before reaching their rated load capacity.
Accordingly, designers are now recommending such bollards be concrete-filled as a precaution.

“Another issue with using existing bollards to accommodate larger or different vessels from the original design is the uncertainty as to what the original designer intended with the
horizontal and vertical angles of mooring lines. With larger vessels, these angles can change substantially and may be outside the envelopes allowed for in the original design of not only the bollards but also the wharf structure.”

Maintenance 
Although observing that the majority of ports would have infrastructure maintenance programmes in place, Mr Hutchison says with regard to bollards specifically that it “would mostly consist of painting”.

“It is important to check the condition of the holding-down bolts - or welds if welded to a steel member - as this is critical to ensure the bollard's rated load can be accommodated.”

Mr Beckett agrees that bollards “generally receive little, if any, maintenance” within such programmes.

“Mooring hooks, being more complicated items of equipment, are generally subject to periodic strip down, inspection and servicing. This attention tends not to extend to testing of the holding-down bolts.”

According to Mr Harris, ports are becoming increasingly aware of the importance of implementing sophisticated asset management plans which are aligned with broader business
objectives, “rather than reacting to repairs and maintenance on an ad-hoc basis”.

Port Strategy. Insight for marine technology professionals


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