Rapid growth at a new Humber ro-ro terminal calls for innovative marine works, discovers Andrew Mylius.
Humber Sea Terminal is something of a phenomenon. It may be just up the Humber Estuary from the megabusy Immingham bulk and container port, and Grimsby dock is a mere 12km away, but four years ago there was no major dock infrastructure at HST's North Killingholme site at all.
From the end of this year, however, with the completion of its fourth berth, the port will turn around four of the North Sea's largest roll on-roll off (ro-ro) ships a day. With two giant strides the port operator has moved from shipping obscurity to being a major UK player.
The speed at which HST is growing has taken everyone by surprise, not least the client itself, says Simon Tanner, project director for contractor Edmund Nuttall, which is undertaking the new port works.
Nuttall completed design and construction of the terminal's first two ro-ro berths in 2000.
The job involved building a jetty out to a pontoon and berthing pier in deep water so that ships could berth in any tide.
Pontoon and jetty were connected by an 80m long articulated bridge to cope with the Humber's 8m tidal rise and fall.
Things looked set for five to ten years of steady growth in operations. But by 2002, a mere 24 months after ro-ro operations kicked off, there was a nonstop stream of trucks hauling container-laden trailers back and forth from ship to shore.
'They quickly found themselves at well above 100% of anticipated capacity, ' Tanner recalls.
'We'd talked to the client about building two additional berths, ' he adds. HST asked Nuttall and its designer Posford Haskoning to give it the option for future adaptation of one berth to provide lo-lo (load onload off - bulk goods) capacity.
Accordingly, the design as being built consists of a new 240m long finger pier off the existing pontoon, which ships moored at berth three use to onload and offload cargo.
Ships coming into berth four will transfer cargo to and from a second pontoon connected via a new two-lane bridge to a spur off the jetty. If demand for lo-lo arises in future, this pontoon will be replaced by a larger floating platform, alongside and running the full length of the finger pier.
Construction of the project has been split into two phases, enabling HST to start making money from its new infrastructure well before work is complete: The first £9.5M phase, comprising the new finger pier and installation of a ramp to span between ships and the existing pontoon, created HST's third berth. It has already been in service for nearly six months.
Creation of berth four, involving construction of a turning dolphin at the end of the finger pier, the jetty spur, mooring dolphins for the second pontoon and installation of the connecting road bridge, is now under way, at a cost of £8M.
Nuttall/Posford Haskoning have faced a twofold challenge:
paring back project costs and piling in the fast-moving estuary currents. Both have been answered by an innovative approach to design and construction.
The finger pier is founded on bents (capped pile piers) supported on up to eight tubular steel piles of 1.067m diameter, set at 12.5m centres - almost twice the normal interval for such structures, says Posford Haskoning principal engineer Tom Rea. This was made possible by designing the pier deck as a super stiff beam that would transfer high lateral fender loads imposed by ships on individual bents to neighbouring installations.
Vertical and raking piles in the bents provide a test for pile rig operators. This option, however, has meant that the pier superstructure, constructed from precast concrete elements, could be built with an unusual economy of plant movements and labour resources, says Tanner. He estimates that by virtually halving the number of precast elements to be fabricated and handled on site, Nuttall has saved between 30 and 40 weeks on programme.
The bents are capped by transverse beams, which support trough section longitudinal beams. These in turn support, and are incorporated as an integral part of, an insitu poured reinforced concrete deck slab.
Fenders are mounted on alternate bents. Bents not subject to fender loads typically consist of two upright piles flanking a pair of raking piles.
In these situations a relatively narrow, 1m deep reinforced concrete beam has been slotted over the upright piles, the heads of which are equipped with welded shear keys.
Meanwhile, rebar was threaded into the top of the raking piles. An insitu concrete pour plugging the top 3m of the piles ensured that integral joints with the deck slab were achieved.
Where fenders are located pile bents are far larger, consisting of up to eight vertical and raking piles. Here, precast reinforced concrete 'biscuits' weighing 50t each have been used to span the pile heads.
Longitudinal trough section beams between the transverse beams were also of two types:
the outer beams came complete with an edge detail that eliminated any need for shuttering during the final 350mm thick insitu concrete pour.