FROTH: Fundamentals and Reliability of Offshore Structure Hydrodynamics

Lead Research Organisation: University of Plymouth
Department Name: Sch of Marine Science & Engineering

Abstract

The FROTH project is a close collaboration between five universities with significant experience in research into wave interactions with fixed and floating structures working together to combine and apply their expertise to different aspects of the problem. The aim is to investigate the detailed physics of violent hydrodynamic impact loading on rigid and elastic structures through a carefully integrated programme of numerical modelling and physical experiments at large scale. Open source numerical code will be developed to simulate laboratory experiments to be carried out in the new national wave and current facility at the UoP [http://www.plymouth.ac.uk/pages/view.asp?page=34369].
It is well known that climate change will lead to sea level rise and increased storm activity (either more severe individual storms or more storms overall, or both) in the offshore marine environment around the UK and north-western Europe. This has critical implications for the safety of personnel on existing offshore structures and for the safe operation of existing and new classes of LNG carrier vessels whose structures are subject to large instantaneous loadings due to violent sloshing of transported liquids in severe seas. Some existing oil and gas offshore structures in UK waters are already up to 40 years old and these aging structures need to be re-assessed to ensure that they can withstand increased loading due to climate change, and to confirm that their life can be extended into the next 25 years. The cost of upgrading these existing structures and of ensuring the survivability and safe operation of new structures and vessels will depend critically on the reliability of hydrodynamic impact load predictions. These loadings cause severe damage to sea walls, tanks providing containment to sloshing liquids (such as in LNG carriers) and damage to FPSOs and other offshore marine floating structures such as wave energy converters.
Whilst the hydrodynamics in the bulk of a fluid is relatively well understood, the violent motion and break-up of the water surface remains a major challenge to simulate with sufficient accuracy for engineering design. Although free surface elevations and average loadings are often predicted relatively well by analysis techniques, observed instantaneous peak pressures are not reliably predicted in such extreme conditions and are often not repeatable even in carefully controlled laboratory experiments. There remain a number of deeply fundamental open questions as to the detailed physics of hydrodynamic impact loading, even for fixed structures and the extremely high-pressure impulse that may occur. In particular, uncertainty exists in the understanding of the influence of: the presence of air in the water (both entrapped pockets and entrained bubbles) as the acoustic properties of the water change leading to variability of wave impact pressures measured in experiments; flexibility of the structure leading to hydroelastic response; steepness and three dimensionality of the incident wave.
This proposal seeks to directly attack this fundamentally difficult and safety-critical problem with a tightly integrated set of laboratory experiments and state of the art numerical simulations with the ultimate aim of providing improved guidance to the designers of offshore, marine and coastal structures, both fixed and floating.

Planned Impact

This project aims to address the important but unsolved issues of wave impact on various marine structures that may be fixed and floating with free responses. The examples of fixed structures are jack up platform for oil and gas exploration, the offshore wind tower in shallow water, coastal defence structures and nuclear plant structures built along the coastline. Examples of floating structures are the oil/gas carrier, oil/gas storage and production tankers (such as FPSOs), floating offshore wind energy systems in deepwater, offshore wave energy structures and ships. In many cases, the structures were damaged due to wave impact and slamming. Examples include (1) tearing the bow of Norwegian cruise tanker Wilstar in 1974 (2) bow damage to FPSO Schiehallion in 1998; (3) oil tanker Prestige split into two parts in 2002, causing oil spill disaster. (4) sea wall in the front of Fukushima Nuclear plant hit by the recent Japan tsunami in 2011, causing nuclear disasters. It can be seen that the issues of wave impact can reach a broad range of engineering sectors and directly affects society.
Therefore the achievements of this project, including computer codes, numerical and experimental results as well as the guidance produced, will directly benefit the classification organisations and engineering consultant and service companies who have provided the support letters and have agreed to take part in the End User Management Group. These organisations and companies can use these project results and computer codes to improve their standards and their designs, and to assess if the existing structures would be suitable to serve in the longer term under changed environmental conditions.
The results and computer codes can also be employed by the owners of Liquefied Natural Gas (LNG) tankers, who can use them to assess the wave impact loads on the tank wall due to sloshing waves inside the tank; by the owners of FPSOs, who can use them to assess the weather conditions to see if they would cause damage; by coastal engineering consultancies who may use them to design safer sea walls and coastal defence structures. In addition, they may also benefit the companies who design, manufacture and install the offshore wind energy and wave energy structures as their project designs are based on better understanding of wave-structure interaction physics.
Furthermore, governmental agencies, such as the Health and Safety Executive (HSE) and environment agency (EA), UK, may benefit as well because they can develop better policy to address safety issues related to wave impact on many kinds of structures.
Apart from the above, the computer code would provide the opportunities for increasing the service and therefore creating additional jobs in the UK and worldwide Oil & Gas, coastal engineering and marine renewable energy sectors, which have been and will continue to be important contributors to UK GDP. It may also benefit the general public in the sense that the project results could help reduce the risk of failure of a structure, which could avoid or reduce the loss of lives and injuries, and could help prevent oil spill or other loss of cargo leading to environmental disaster.
In order to make the impact happen, we proposal to carry out the following activities:
1) Involve key stakeholders (e.g. Lloyds Register, Germanischer Lloyd, Bureau Veritas, Trinity House, Saipem) and establish an End User Management Group of offshore classification societies and key industrialists in order that they are directly informed of the project achievements;
2) Create a website to allow to open access to the information produced;
3) Organise two project workshops to widen the awareness of these achievements;
4) Organise special sessions in International Offshore and Polar Engineering Conference & Exhibition to maximise the international of reach of the developments;
5) Arrange public awareness events so that the interested public is well informed.
 
Description The effect of aeration on wave impact The experimental and numerical study reveals that the impact loadings on the square rigid plate during water entry consists of distinct features including (1) shock loading with the highest observed peak pressure, (2) fluid expansion loading with very low sub-atmospheric pressure, and (3) less severe secondary reloading with super-atmospheric pressure. For high speed pure water impacts (v = 5 m/s), the duration of the shock loading is less than 2 ms, and it is around 4 ms for the low pressure loading. The minimum values for all the computed pressures and most of the measured positive pressures are very close to (though still above) the saturated vapour pressure. Therefore, the surrounding water is very likely to be in tension near the cavitation condition. The experiments and numerical computations also show that the peak loading on the flat plate can be effectively reduced by half or even more with 1.6% aeration in water. Compared to pure water condition, aeration increases the rise and/or fall time of the shock load. This means that the shock loading in aerated water has a longer lifespan and the resulting impulse is not necessarily much smaller than pure water impact. The variation of impulses is less sensitive to the change of aeration than peak loadings. Wave-structure interaction experiments for simplified FPSO geometries have been used to explore the effects of model length, wave steepness and incident wave angle on the structure of the total local wave field and scattered wave components • The highest amplitude scattered waves are obtained with the shortest model. • The linear harmonic increases as model length decreases but the nonlinear harmonics are similar for all three models and slightly smaller for the longest model at the second harmonic component. • The non-linear scattered wave increases with incident wave steepness. • The incident wave angle affects the maximum crest height and wave loading and therefore it should be considered in design. • The second order difference long-wave component is a robust feature of our experiments. • The third- and fourth-order wave components contribution to global force and to local pressures on the body surface should be considered for assessing ringing load effects in offshore structure design. • The contributions of the third- and fourth-harmonics and the second difference term to the surface elevation need to be taken into account in design of the air gap and the level of accommodation for offshore structures.
Exploitation Route In the design of offshore and marine renewable energy structures and in the design of marine operations. Results show the effect of aeration and hydroelasticity on the wave loading on structures. The study also investigates the higher scattered wave harmonics resulting from wave interaction with a floating structure and the effect of steepness, directionality and model length. The project carried out experiments in the laboratory and developed open source software for application to the cases tested. Further development of the modular open source numerical tool is taken forward through follow on projects and the establishment of a CCP for Wave Structure Interaction.
Sectors Aerospace, Defence and Marine,Energy,Environment

 
Description This work has improved understanding of the effect of aeration and hydro elasticity on wave impact and how these processes may be modelled in CFD. It has also highlighted the nonlinear wave structure interaction with an FPSO type structure and how the high order loading is generated. it has provided guidance on CFD and QALE-FEM modelling of these interactions and on how the different numerical models may be used together.
First Year Of Impact 2015
Sector Aerospace, Defence and Marine,Energy
Impact Types Economic

 
Description A Zonal CFD Approach for Fully Nonlinear Simulations of Two Vessels in Launch and Recovery Operations
Amount £572,177 (GBP)
Funding ID EP/N008847/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 10/2015 
End 09/2018
 
Description CCP-WSI Collaborative Computational Project for Wave Structure Interaction
Amount £603,949 (GBP)
Funding ID EP/M022382/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 10/2015 
End 09/2020
 
Description Extreme Loading on FOWT under Complex Environmental Conditions
Amount £332,158 (GBP)
Funding ID EP/T00424X/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2020 
End 03/2023
 
Description Extreme Loading on Floating Offshore Wind Turbines (FOWTs) under Complex Environmental Conditions
Amount £63,391 (GBP)
Funding ID EP/T004339/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 11/2019 
End 11/2022
 
Description Extreme Loading on Floating Offshore Wind Turbines (FOWTs) under Complex Environmental Conditions
Amount £389,691 (GBP)
Funding ID EP/T004177/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2020 
End 03/2023
 
Description Extreme Loading on Floating Offshore Wind Turbines (FOWTs) under Complex Environmental Conditions
Amount £44,275 (GBP)
Funding ID EP/T004274/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 03/2020 
End 03/2023
 
Description BV FROTH 
Organisation Bureau Veritas
Country France 
Sector Private 
PI Contribution Experimental data set for wave structure interaction experiments; numerical tools for hydroelastic analysis; application of CFD to wave structure interaction and offshore wave impact.
Collaborator Contribution Contribution to research discussions; contribution to Project Steering Group.
Impact Research publications
Start Year 2012
 
Description Bath 
Organisation University of Bath
Department Department of Architecture and Civil Engineering
Country United Kingdom 
Sector Academic/University 
PI Contribution Research collaboration
Collaborator Contribution Research collaboration
Impact further research collaboration
Start Year 2012
 
Description CD-adapco 
Organisation CD-adapco
Country United States 
Sector Private 
PI Contribution Use of software; development of test cases; demonstration of software; generation of experimental data.
Collaborator Contribution provision of computational fluid dynamics software; contribution to Project steering group; attendance at project workshops
Impact research papers
Start Year 2007
 
Description City 
Organisation City, University of London
Country United Kingdom 
Sector Academic/University 
PI Contribution Research collaboration
Collaborator Contribution Research collaboration
Impact EP/J012866 EP/M022382 EP/N008847
Start Year 2012
 
Description GL with FROTH 
Organisation DNV GL
Country Norway 
Sector Private 
PI Contribution Experimental data set for wave structure interaction experiments; numerical tools for hydroelastic analysis; application of CFD to wave structure interaction and offshore wave impact.
Collaborator Contribution Contributions to research discussions; contribution to Project Steering meetings; contribution to Project Workshops.
Impact Research publications
Start Year 2012
 
Description MMU 
Organisation Manchester Metropolitan University
Country United Kingdom 
Sector Academic/University 
PI Contribution research collaboration
Collaborator Contribution research collaboration
Impact EP/N008847
Start Year 2012
 
Description TH FROTH 
Organisation Trinity House
Country United Kingdom 
Sector Charity/Non Profit 
PI Contribution Experiment dataset for wave structure interaction experiments; development of CFD tools for wave structure interaction and offshore wave impact.
Collaborator Contribution contribution to research discussions; contribution to Project Steering Group
Impact research publications
Start Year 2012
 
Description oxford_FROTH 
Organisation University of Oxford
Country United Kingdom 
Sector Academic/University 
PI Contribution Experimental data set for wave structure interaction experiments; numerical tools for hydroelastic analysis; application of CFD to wave structure interaction and offshore wave impact.
Collaborator Contribution Contribution to research discussions; guidance and advice; contribution to Project Steering Group.
Impact joint research publications.
Start Year 2012
 
Description FROTH - SUTGEF Workshop January 2014 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Type Of Presentation workshop facilitator
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact An Initial FROTH Project Workshop was held at City University together with the SUTGEF (Society for Underwater technology Group for Environmental forces) Group. The aim of the Workshop was to raise awareness and generate engagement with the project.
Joint research project with Pelamis Wave Power resulted from discussions at the workshop.
Support for CCP-WSI research proposal was generated at the workshop.

good informal feedback from attendees; proposal to hold final FROTH Workshop in collaboration with SUTGEF in 2015.
Year(s) Of Engagement Activity 2014
 
Description FROTH Final Workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact The FROTH Final Workshop was held in Oxford University in November 2015 at the end of the project. The Workshop was held as part of a SUTGEF (Society for Underwater Technology) meeting and had an audience of academics and industrialists from the offshore and ocean energy sector. The workshop presented the results of the FROTH collaborative project; the experiments and numerical model developments were presented together, and the main findings of the research and its implications for industry were discussed.

The newly funded follow on projects, EP/N008847 and EP/M022382, were presented and the audience invited to take part in these projects.
Year(s) Of Engagement Activity 2015
 
Description FROTH Special Session organised for the ISOPE conference in Hawaii, 2015 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact As part of ISOPE 2015, a Special Session for the FROTH project was organised. At this session, a number of papers from the FROTH project were presented together with other research papers in related areas.
Year(s) Of Engagement Activity 2015
URL http://www.isope.org/publications/proceedings/ISOPE/ISOPE%202015/index.htm
 
Description FROTH Workshop held alongside the 30th International WWWFB 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact The FROTH Workshop was organised on the day following the International Workshop for water Waves and Floating Bodies in Bath UK and involved an international audience of industrialists and academics from all over the world.

The research findings of the FROTH project were presented alongside presentations from industry giving the industry perspective on the current challenges in this research area. There was a discussion of the research and how it provides insight to the engineering challenges and how the research findings could be used to provide recommendations for design purposes of offshore structures and operations.
Year(s) Of Engagement Activity 2015
 
Description Invited presentation ay UK Sweden Workshop, Stockholm February 2015 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact A UK Sweden Workshop on Marine Renewable Energy was held in Stockholm, approx 15 from the UK and 20 from Sweden attended; presentations were given from UK and Sweden and collaborative proposals discussed.
Year(s) Of Engagement Activity 2015
 
Description OPEN FOAM Workshop and Conference Exeter 2015 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Presentation of results from the FROTH Project and the CCP-WSI at the OPENFOAM Workshop and Conference
Year(s) Of Engagement Activity 2015
 
Description PRIMaRE Conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Presentations at the annual PRIMaRE (Partnership for Research in Marine Renewable Energy) Conference
Year(s) Of Engagement Activity 2014,2015
URL http://www.primare.org