Project Case Studies

Project:

Why swimming to Rottnest Island is not straight forward

The Challenges:

The Results:

Open water swimmer is more challenging that pool swimming due to waves and currents affecting the swimmer.
The Rottnest Channel Swim (RCSA) and Port to Pub (P2P) events are iconic 19.7km and 25km open water swims between Perth and Rottnest Island which are attempted each year by over 2000 swimmers.
Many competitors have found completing the swim to be extremely difficult due to the conditions generally getting worse during the day, affecting the slower swimmers more.
Aurora Offshore Engineering is collaborating pro-bono with the RCSA, P2P, UWA and NetCalcs to develop and refine a route optimisation tool to help swimmers select an optimum route, and improve their chances of safely completing the crossing.

In 2021 a pilot project with P2P received extremely positive feedback.
In 2022 the first release of the Route Optimisation Tool was launched with the majority of the competitors choosing a southerly route to compensate for the strong North-running currents near Rottnest Island.
The collaboration partners continue to further develop and refine the tool to capture future improvements.
Link to the tool: https:/www.rcs.optiswim.com

Project:

OWF Array and Export cables shown to be stable using COREstab Method

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The Challenges:

Offhsore windfarm (OWF) array and export cables are often routed across rocky seabeds and often found to be unstable using F109 absolute stability method
Dynamic and Generalised methods are not applicable on rock
The F109 hydrodynamic force model is often outside its limits of applicability for cables in shallow water
The failure modes for cables on rock are different to cables on clay / sand soils (See Crown Estate, 2015)
OWF cables are often located in shallow water depth <25m
They are also often exposed to frequent intense storms possible during installation leading to breaking waves, strong currents

The Results:

The COREstab method (Cables on Rock Enhanced stability) has been developed by UWA’s Oceans Graduate School to capture the reduced hydrodynamic forces and increased lateral resistance provided by the meso- and macro-scale rocks on the seabed.
The method has been used to demonstrate that the MeyGen subsea power cables are stable, despite F109 predicting a required SG over 3x higher.
AOE are experts in the COREstab method, with staff having performed many of the experimental tests and authored the design guideline.
The method is presently being incorporated into the new BSI standard BS 10009 for cable stability design on rocky seabeds
AOE have been able to apply the method to about 100km of OWF array cables. We showed the majority of cables were stable. This was 3rd party validated and benchmarked against back analysis of the response of trial cables installed onto the project seabed.
This enabled our client to reduce secondary stabilisation measures by around 90%

Project:

Marine growth is a bigger challenge for subsea cables

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The Challenges:

Offshore windfarm power cables are often left exposed on the seabed – either across the turbine scour mitigation, or if placed on rocky seabeds.
The existing F109 approach to account for marine growth (MG) is to add an additional outer layer to the cable or pipe.
Adding 50mm of MG to a 1000mm diameter pipe adds 10% to the OD, while it doubles the diameter of a 100mm cable.
This makes achieving on-bottom stability extremely difficult, if not impossible.

The Results:

UWA has recently completed a pilot study which shows that the present F109 model for adding MG as a layer to the outside of a cable is very poor.
This pilot study shows that MG is not circumferentially or longitudinally uniform, it is not a smooth layer, it is not impermeable and often it is not rigid.
AOE have developed a novel approach that uses principles of marine benthic ecology to predict the settlement of epibenthic sessile biota on subsea cables, as well as the resulting changes to the cables’ hydrodynamic forces.
AOE staff include experts in the identification and analysis of benthic biota on renewable energy structures and cables across the globe.
We were able to apply this approach to about 100km of OWF array cables. This approach was 3rd party peer-reviewed and client-approved.
This enabled our client to successfully account for MG in the cable on-bottom stability design, resulting in the avoidance of significant operational cleaning requirements.

Project:

OWF Array and Export cables shown to be stable using STABLEpipe Method

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The Challenges:

Offhsore windfarm array and export cables unstable during installation phase using F109 conventional methods
Soft sandy and sandy silt surficial seabed
Shallow water depth 30m to LAT+1m (including shore crossing)
Frequent intense storms possible during installation leading to breaking waves, strong currents

The Results:

AOE were able to show the majority of cables were stable by accounting for scour and self-lowering using the STABLEpipe methods.
AOE are experts in the STABLEpipe method, with staff having performed many of the experimental tests and authored the design guideline.
We were also able to rank the stability of marginally stable cables for urgent temporary trenching in the event of an impending hurricane/typhoon/cyclone.
This enabled our client to minimise vessel simops and stand-by time
Several storms passed during the cable lay program
Cables were observed to self embed and were stable as predicted

Project:

Seabed morphodynamics (mobile sandwaves and megaripples)

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The Challenges:

Both Offshore windfarms and oil & gas fields are often located in areas where sandwaves and megaripples exist.
These bedforms are frequently mobile, but predicting the rate of movement can be difficult – especially when movement is driven by episodic metocean events like storms and non-linear internal waves (‘solitons’).
Predicting how these bedforms will interact with structures can also be challenging – including windfarm monopole foundations, subsea structures and manifolds, buried power cables and surface-laid subsea hydrocarbon pipelines.

The Results:

Case study: Dieppe Le Tréport OWF (Lebrec et al., 2017). AOE were engaged as part of a multi-disciplinary team to predict the lifetime variation in seabed elevation at each OWF foundation.
We were able to draw on multiple survey datasets, covering a wide timespan including historical survey data with much poorer quality (spatial density). Most industry design approaches to morphodynamic prediction assume uniform survey quality.
We were able to show that the mean migration predicted over 80 years varied substantially across the site, with three distinct zones identified.
As well as a sound understanding of the geotechnics of the problem, the key to understanding the observed (and hence future predicted) migration was our ability to define the extent to which movement was driven by episodic events influenced by inter-annual and longer-term weather variation.

Project:

OWF Array and Export cables seismic liquefaction risks

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The Challenges:

Requirement for cables to be assessed for seismic-induced seabed liquefaction risks identified late in project construction.
Site located in a high seismic risk area.
No pre-existing design guidance or methods available from global verification bodies.
Clay, silty and sandy soils lateral and vertically distributed with varying risks of liquefaction.
Seabed properties between OWF foundation sites were very poorly defined.

The Results:

AOE were able to develop a novel structured cable failure event chain risk model adapting Griffiths et al. (2007).
We also developed a geological model across the site providing a systematic and structured representation of the range of inter-turbine and vertical soil liquefaction risks using our in-house global expertise in coastal sedimentary processes and geomorphology.
This was coupled to a cable response model which considered the consequences of seabed liquefaction.
The risks to the array & export cables were therefore assessed, and found to be acceptable following client and verification body review.
This enabled our client to avoid extensive rework and remediation.

Project:

Flume model test interpretation

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The Challenges:

Predicting the hydrodynamics and resulting failure modes of complex structures can be very difficult
No flume test or CFD model is ever perfect – there are always artefacts introduced due to the modelling approach. Each has their merits and limitations.
Scaling from model to prototype is never ‘easy’ if you want to get the right answers. Different scaling laws apply to different physical phenomena, and they are often incompatible (See Le Mehaut, 1976; Hughes, 1993).

The Results:

AOE have successfully modelled the hydrodynamics of complex subsea structures
Including scoping, supervision and interpretation of physical model tests
Including complex and computationally-intensive CFD modelling of novel structures
With scaling of model tests to correctly predict prototype behaviour after accounting for testing artefacts
Resulting in a calibrated bottom-up hydrodynamic model of complex structures enabling future adaptation and generalised use by the client

Cases:

Stability of flexible concrete and frond mats
Rock berm stability and suction scour
VIV of pipelines on erodible seabed
Rock bag stability
Novel patented hydrodynamic foil performance evaluation
Subsea structure stability