design and installation of the next generation of submarine
outfall pipelines on the Yorkshire coast
Martin Berry CEng BEng FICE, Daniel Akpan BEng MSc & Wasim
pipe installation - First 500m string sinking procedure -
Courtesy of Royal HaskoningDHV
2016 saw the successful completion of a new 2.5 kilometre long
sea outfall pipeline on the east coast of Yorkshire, South of
Scarborough. The purpose of the project was to replace an
existing corroded and collapsed steel outfall pipe with a new
solid wall polyethylene (PE) pipeline. The installation required
meticulous planning by a team of marine experts working on
behalf of Yorkshire Water Services to continue the company’s
vital work to protect water quality of the seas and beaches of
the east coast.
The Wheatcroft site is
located approximately 3km south of Scarborough, a resort town on
England’s North Sea coast which developed due to its popularity
during the Victorian period. Scarborough currently has a
population of 61,000 and is considered the largest holiday
resort on the Yorkshire coast. The existing Wheatcroft Long Sea
Outfall (LSO) is owned by Yorkshire Water Services and
discharges treated effluent from a food processing factory. A
separate storm water overflow is located in proximity to the LSO
which discharges screened storm flows from the local catchment.
The existing LSO has
failed on several occasions previously and is considered to be
no longer serviceable. Any future failures of the existing
outfall could result in a loss of the assets ability to
discharge, impacting on the upstream factory and putting at risk
Bathing Water quality at Scarborough. Therefore, full
replacement of the outfall was considered to be necessary to
meet both the functional needs and environmental requirements
for the future.
delivery by sea to Teesport - Courtesy of Pipelife Norge
storage at Teesport - Courtesy of Royal HaskoningDHV
A stakeholder group was
formed in 2013 to undertake a feasibility study and to conduct
associated water quality studies to assess various treatment and
marine outfall options that would satisfy the operational and
environmental standards. A replacement LSO of 2500m in length
discharging into water deeper than the existing was identified
as the most environmentally sustainable solution and was
supported by the regulating bodies. Construction of a
replacement outfall was therefore submitted as part of the Price
Review (PR14) for implementation during AMP6.
The existing long sea
outfall was constructed circa 1997. The landward portion of the
pipeline consists of a 3.2km length of PE pipe which runs from
the factory to the cliff top close to the Wheatcroft Pumping
Station and a 300m length of 280mm diameter PE pipe which runs
below a cliff path from the clifftop to the foreshore. The
marine section of the existing long sea outfall consists of a
273mm outer diameter steel pipe with concrete protective coat
and is laid in a trench and buried beneath the seabed. It
outfalls approximately 2km from the shore. The sea bed level at
the outfall diffuser is approximately -15mCD.
The Wheatcroft Pumping
Station is not connected to the long sea outfall as its purpose
is to transfer flows from the local catchment for treatment in
Scarborough. There is also an existing 630mm diameter PE storm
water outfall adjacent to the long sea outfall which discharges
at low water and provides an overflow from the Wheatcroft
The existing steel long
sea outfall pipe was considered to be in poor condition and
ongoing deterioration resulting in localised failures. A number
of repairs have been undertaken to the marine pipe section over
the last few years and the pipe was therefore considered to be
at the end of its operational asset life and identified for
The concept design phase
was undertaken in 2015. This considered and technically assessed
a number of options including upstream treatment, patch repairs,
internal re-lining and full offline replacement. The process
identified a preferred solution that would function both
hydraulically and structurally and this was to construct a
replacement outfall parallel to the existing, comprising circa
2500m length of 355mm polyethylene pipe terminating in a 4-port
diffuser. The outfall was proposed to be constructed off-line to
allow existing discharges to continue prior to final connection
above high water.
Collaring at Teesport - Courtesy of Royal HaskoningDHV
Nearshore trench excavation - Courtesy of Royal
To combat flotation, the
design proposed that the polyethylene pipe would be stabilised
using a concrete weight collars to reduce buoyancy and enable
controlled installation by sinking. In addition concrete
mattresses to some sections of the trench were proposed to
provide additional scour protection from storm conditions.
information from 1996, prior to the previous outfall
construction, was available and included land-based and
overwater boreholes. Further site investigations were identified
and specified by Royal HaskoningDHV. These included bathymetric
and geophysical surveys to verify and correlate and confirm the
existing along the proposed outfall. The early identification of
the site data enabled Royal HaskoningDHV to reduce risks for
Yorkshire Water Services in terms of unknowns that could occur
during construction. This information also allowed the
Contractors bidding to make a more informed estimate of the
investigation data that was obtained indicated that rock strata,
comprising of various siltstone, sandstone, limestone and
mudstone, were present at bed level up to approximately 500m
from the shore. Further offshore the rock strata were overlain
by superficial deposits of sand with occasional beds of gravel
and cobbles to a depth of up to 6m below bed level. A borehole
that had been carried out approximately 250m beyond the existing
outfall diffuser indicated that the thickness of superficial
deposits increases to approximately 12m at that location.
The Wheatcroft Outfall
Project is located in proximity to a residential area with
limited space available for site compounds and storage. Access
to the site was via residential roads which had width, time and
day parking restrictions. The existing cliff path is steep and
required improvement works to allow for access of land based
plant. A Public Right of Way, The Cleveland Way, crosses the
site at the top of the cliff and forms part of the cliff access
which had to be maintained during the works duration.
Several rock outcrops
known as White Nab, Raven Scar and Perilous Rock exist in the
foreshore area in the vicinity of the outfall. The outfall also
lies within the Cayton, Cornelian and South Bays of Special
Scientific Interest (SSSI) and the Castle Ground Marine
Conservation Zone. In addition, the works were undertaken with
consent from the Marine Management Organisation (MMO).
liaised directly and provided the input required for the
construction consent application from the MMO to allow the works
to be constructed. This application was expected to take a
minimum of 3 months to approve and therefore an early
application was sought and this was in place before contract
award also saving Yorkshire Water Services time.
pipe installation – prior to flood procedure, courtesy
of Royal HaskoningDHV
Following completion of
the concept design and specification by Royal HaskoningDHV, Van
Oord UK successfully bid and were appointed in 2016 as principal
contractor by Yorkshire Water Services to undertake the marine
works on an EPC basis.
Van Oord produced a final
design which involved the construction of a long sea outfall,
diffuser structure and subsequent connection to the existing
outfall pipeline on the foreshore. Stability and protection to
the pipeline were provided through the use of 110kg per metre
length fixed weight collars supplemented with 400mm OD/12mm
thick PVC (cut sections) protective sleeves fitted over the
outfall PE pipe in short lengths between the concrete collars.
The provision of this
innovative sleeve proved to be a cost effective and best quality
solution as it removed the need to use engineered backfill
around the pipe through the rocky sections.
The use of engineered
backfill would have required transportation by heavy dump trucks
to the foreshore via the cliff access, causing disturbance to
the environment and foreshore users, as well as increasing
health and safety risks. The installation of 12mm thick PVC
protective sleeves enabled excavated material to be re-used as
backfill with minimal risk of damage to the pipe.
The design of the
collars, in terms of its spacing and sizing, was optimised to
provide consistent ballasting throughout the length of the
pipeline assembly, enabling practical installation and ensuring
long term pipe stability under all design conditions. Concrete
mattresses were installed at the foreshore (breaking wave zone)
section for scour protection purposes.
It was determined that
the pipeline assembly would be towed out and sunk into position
with all concrete collars pre-installed.
A total length of 2500m
of polyethylene pipe (355mm OD SDR 17 PE100) was manufactured by
Pipelife Norge AS, in lengths of 500m which were then towed to
Teesport, Middlesbrough, UK for preparation and assembly.
The pipe strings were
moved individually with the aid of excavators and small marine
craft from the wet storage area into the working area within
Teesport, where an assembly station was created for the fitting
of protective sleeves, concrete collars and flanges.
As the site geology
comprised of rocky seabed made up of mudstone and sandstone.
Excavation through rock was required in sections of the outfall
(between chainage 0m and 650m) and this was carried out using
land based excavators on the beach section and backhoe dredger
for the marine trench. A trailing suction hopper dredger (Volvox
Olympia) was used for the excavation in the superficial
deposits. A minimum cover of 1.5m above the crown of the pipe
was required for the outfall pipeline through its entire route.
The excavated trench material was cast adjacent to the trench
for reuse as backfill material.
Installing concrete collars - Courtesy of Van Oord UK
Outfall strings connected offshore by butt fusion
welding - Courtesy of Van Oord UK
The PE pipelines were
installed using the float and sink method. The installation
started from the onshore end, where the pipe was positioned by
means of an excavator on Wheatcroft Beach. The 500m long pipe
strings were towed individually by a tug boat to the
installation location and sunk to the seabed by pumping seawater
in a controlled manner into the pipe.
The controlled pumping of
seawater into the onshore pipe end, initiated the sinking
process which caused the pipe to form an S-curve of
approximately 50m long between the seabed and seawater surface.
As a contingency measure and to ensure the installation process
was well controlled, a foam pig was pre-installed in order to
create a seal between the pipe section filled with air and the
section filled with water, in case the pipe had to be
The 500m pipe sections
were welded together on board the floating multi-cat vessel and,
once in the trench, were checked with a multibeam survey using
an RTK DGPS (target installation of +/- 3m horizontal, +/-0.5m
vertical) to verify the pipe position.
Post pipe installation,
the foam pig was removed from the pipe. A pressure test was
performed on in accordance with the procedure set out in IGN
4-01-03, to 1.5 x the maximum internal working pressure.
Once the outfall pipeline
had been sunk to the sea bottom and tested the diffuser was
installed from the backhoe dredger platform by divers. The
diffuser comprised a flanged PE pipe, fittings and a single
riser with a four port crosshead, each a nominal 125mm outside
diameter, allowing for Tideflex check valves to be fitted
directly. Included as part of the diffuser installation, was a
removable access flange at the top of the riser to enable access
inside the riser pipe for future maintenance.
dredger during offshore dredging process - Courtesy of
of concrete mattresses nearshore
Courtesy of Van Oord UK
A rock armour scour
blanket extended approximately 8m around the diffuser from its
centre to protect the area around the diffuser from scour. Also
provided was a mild steel protection frame to the diffusers,
fitted with sacrificial anodes to allow a 60 year design life.
The final connection to
the existing outfall pipe was undertaken in July 2016 during a
scheduled outage at the factory in which a spool piece was
connected between the existing and newly installed pipelines
using external couplings.
To complete the project
and following successful commissioning the existing diffuser was
removed and the existing outfall capped at both the onshore and
offshore ends with concrete plugs.
construction process Royal HaskoningDHV assisted Yorkshire Water
Services by providing contractor checks, reviews and approvals
of design submissions, temporary works, health and safety
procedures, method statements, on site supervision, surveys and
The successful completion
of this project and the investment undertaken by Yorkshire Water
Services will continue to provide water quality benefits to
Wheatcroft, Scarborough and the Yorkshire coast.
The Wheatcroft Outfall
Project is the first to be completed during AMP6 and further
articles will appear in future editions of UK Water Projects.
after completion of work - Courtesy of Van Oord UK
would like to
thank Martin Berry, Senior Engineer, Daniel Akpan, Graduate
Engineer and Wasim Hashim Project Manager all with Royal
HaskoningDHV, for providing the above article for
The authors thank Yorkshire Water Services, Van Oord UK,
CH2MHill and Pipelife Norge AS for their assistance with