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Island mission at Lerwick Harbour

Bam Nuttall and sister company Bam Ritchies have been charged with improving Lerwick harbour’s deep-water facilities that will boost the region’s offshore oil and gas industry.

An integrated Bam ­construction team of Bam Nuttall and in-house geotechnical specialists BAM Ritchies are on site in the Shetland Islands completing a £11.75M project, which will advance plans for significant further expansion of Lerwick harbour’s deep-water facilities for supporting the offshore oil and gas industry.

Employer Lerwick Port Authority (LPA) has commissioned Bam to lengthen the existing quay at Dales Voe South, by 75m to 130m overall, to support subsea developments and decommissioning.

Lerwick has been servicing the offshore industry for over 50 years and LPA is aiming to enhance its reputation as a location for decommissioning. The extended quay will provide deep-water, versatile berthing and heavy load capacity to take an offshore structure in a single lift, with a substantial, expanded laydown area.

Lerwick Harbour

Size matters: Bam is lengthening the existing quay at Dales Voe South

Captain Calum Grains, Lerwick Port Authority deputy chief executive and harbourmaster, said: “The contract marks an important step in further developing Lerwick’s role as a leading centre of offshore industry operations. Dales Voe South is another value-added expansion and reflects our confidence in future activity, including ongoing subsea projects, particularly west of Shetland, and the developing decommissioning and offshore renewable markets.”

The Scottish Government and Highlands and Islands Enterprise, are providing £2.39M in grants for the project, with Bank of Scotland supporting the port authority’s investment.

The NEC Option A contract was awarded on the 30 January 2015 and the works began in April 2015 with scheduled completion due in May 2016.

Arch Henderson LLP, are the employer’s designer, responsible for design and project management of the works.

Lerwick Harbour

Heavy duty: Bam used a Bauer BG42 rig, which deployed a 30m telescopic kelly bar

The contract to extend the quay by 75m, provide deck load-bearing capacity of 60t/m2 and link span line load of 800t/m run of quay to enable floating barge skid transfer of complete offshore topside modules with a minimum 12.5m draft, will provide an infrastructure that is among the deepest of its type in Scotland and the closest UK land fall for the offshore industry in the northern North Sea.

The new quayside is being formed with 55,000m3 of reclamation using site won aggregate, retained behind a tied combi-pile wall constructed with 42, 1,575mm diameter 24mm wall thickness, maximum 21m long tubular piles with intermediate double AZ26 sheet piles. The front face of the combi wall is restrained with 18No. 125mm tie-bars to a 425m3 reinforced concrete anchor beam secured by 38No. multi-strand tension anchors up to 2250kN capacity and 114, 6m long shear dowel bars. The North and South combi walls are restrained by a further 19 tie bars connected to a reinforced concrete transfer beam.

Quay capacity and support of the cope on the leading edge is enhanced by 24, 1,575mm diameter tubular bearing piles. These bearing piles create a double pile front face to the jetty extension.

There are two return sheet pile walls each approximately 12m long, comprising of AZ50 sheet piles 12m in length.

Lerwick Harbour

New look: An impression of the extended Dales Voe South Quay

The tubular piles are installed in a rock socket of up to seven metres depth in highly competent quartz phyllite. In geological terms the local sequence comprises “Dales Voe Grit” - Quartzite, “Whiteness Division” - Metalimestone and the “Clift Hills Phyllitic Formation”. It was understood from the outset that the rock would pose significant challenges to conventional rotary drilling tools, so a unique pneumatic cluster drill was commissioned. Rock strengths in the area vary from very weak to very strong with peak characteristic strengths of 260MPa. A cluster drill A CDSZE1830 at 1.830m diameter with 13 No 0.2mdown-the-hole (DTH) hammers, forms a rock socket beyond the size that conventional rotary hammers are no longer viable. A 2m diameter conductor casing up to 24m in length is used to case off any underwater overburden. The drilling rig used for the tubular piles is a Bam operated Bauer BG42 deploying a 30m telescopic kelly bar.

Bam are using an in-house designed modular piling platform to allow advancement of the combi-wall with the 165T BG-42 atop. To avoid using floating plant, the Bam engineering team has developed this temporary works solution to enable the heavy piling plant to operate while supported on each new section of piles as the works progress. The process of using fixed modules has allowed improved productivity over floating plant and reduced the risk of delays from weather conditions.

On completion of the combi-wall a heavily reinforced coping beam 4.5m high and 5.1m wide is cast along the full extent of the berth. Following past success at Copland’s Dock in Orkney, Bam planned to form the cope using precast concrete sections infilled with in-situ concrete.

“This is an important step in further developing Lerwick’s role as a leading centre of offshore industry operations”

Calum Grains, Lerwick Port Authority

The anchor beam was cast in-situ requiring mass pours of 1,200m3 underwater concrete to bring it up to formation. Formwork, reinforcement steel, anchor inserts and tie bars were then placed for the 460m3 beam cast in three pours, each commencing at low water.

The rock anchors are up to 24m deep. The drilling is conducted by a Soilmec SM14 drilling rig deploying an Atlas Copco 8” Terranox down-the-hole hammer drilling at 216mm diameter. Both of the 18 and 13 strand anchors that are being installed have been provided by DSI to a Bam in-house design.

All operations comprise heavy lifting. Full-time lifting support is provided by a Leibherr 300T and 135T crawler cranes.

“The works have significant logistical and environmental challenges of a remote island environment”

Graham Hopper, Bam Nuttall

Bam project manager Graham Hopper commented “The works have significant logistical and environmental challenges of a remote island environment in addition to the technical challenges. The Shetland Islands, 960km north of London, are as far north as Anchorage in Alaska and St Petersburg in Russia and are only 640km south of the Arctic Circle. The works need to be planned to take account for the location and the risk of weather conditions disrupting the works as a significant proportion including the pile, cope and anchor beam works are within the tidal zone”

The construction team has also developed a BIM model for the project using Revit and Navisworks software to assist the customer and the wider project team with detailed visualisations of the sequence and optimisation of the temporary works, for both rig support and pile installation gates.

The site team consists of over 40 personnel. As well as site offices, living and sleeping accommodation has been set up adjacent to the works with the facility to house up to 40 personnel. Travelling personnel fly from the UK mainland to Sumburgh Airport and are transported the 45 minute drive by bus to the site.

Despite the distance from Lerwick, the camp’s personnel have integrated with Shetland life as a result of the travelling personnel from Bam’s core team being augmented by local Shetland based personnel.

A Dales Voe team plays in the local pool league and volunteers are assisting with the local Scouts group. A reading library is setup on site, with voluntary contributions being made to the RNLI.

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