FROTH: Fundamentals and Reliability of Offshore Structure Hydrodynamics

Lead Research Organisation: University of Bath
Department Name: Architecture and Civil 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.

Publications

10 25 50
 
Description 1. In this project, we have developed a new improved numerical methodology for modelling strongly non-linear wave and breaking wave impact on structures, particularlly looking at the reliability of wave impact pressure prediction and free surface elevations under various breaking wave conditions, including broken wave, aerated, air-pocket and flip-through impact.

2. We have applied both incompressible flow solver and compressible flow solver for predicting breaking wave impact pressure on vertical wall, and conducted detailed comparisons for various wave conditions. The results have shown that though both models have predicted accepted results for impact pressures on the wall, compressible flow solver is able to produce higher peak impact pressure and the pressure oscillation inside the trapped air pocket .

3. Plymouth experiments on breaking waves impact on vertical wall have been reproduced numerically at the University of Bath using the numerical tool, OpenFOAM. Very good agreements have achieved for free surface elevations at a couple of locations in the wave flume, impact pressures and wave force on the wall. This has shown that the numerical tool is capable of modelling breaking wave impact on structures with reasonable accuracy and reliability when proper cell size and time step are used. We have found that flip-through impact causes the largest impact pressure on the wall.

4. Apart from breaking wave impact, solitary wave impact on coastal sea defence was also simulated by the numerical tool and compared with experiments. The results have also achieved very good agreements between the experiments and numerical predictions.

5. Wave impact on FPSO has also been investigated, and compared with the experiments performed in Plymouth wave tank, again good agreement with experimnetal results has achieved.
Exploitation Route 1. Our research outcome have been presented in international conferences, including the 30th IWWWFB Workshop (2015) in Bristol, the 25th ISOPE conference (2015) in Hawaii, the 11th PACOM Conference (2014) in Shanghai, and the 10th UK Young Coastal Scientists and Engineers Conference (2014) in Cardiff.

2. Our research outcome will be published in leading journals. The draft of the journal paper has been circulated to the co-authors in other partner universities.

2. Our research results have been released on our project website.

3. We have organised three project workshops in January 2014 in London, April 2015 in Bath and September 2015 in Oxford for disseminating the research findings to potential end-users (e.g. coastal and marine engineers, the renewable energy industry, the offshore industry, and the research community in the related areas).
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Education,Energy,Environment,Transport

URL https://collaborate.plymouth.ac.uk/sites/cerg/Pages/froth-research.aspx
 
Description 1. My research associate Dr. Feng Gao, worked on the project from October 2013 to September 2015. During the two year period, he has attended the project meetings/workshops and discussions within the large research consortium and attended a couple of international conferences & gave oral presentations. I believe the training and the experience he has received while working on the project will help him significantly for his future career. 2. In this project, we have investigated breaking wave impact on vertical walls and the wave impact on FPSOs. The results have been presented in a couple of international conferences in 2014 and 2015 and in the project workshops. The project just finished at the end of September 2015. The research findings have been presented in the final project workshops in Oxford in November 2015 to a mixed audience from both industry and academia. Hopelly our findings will be used more widely soon by others and help the safe and cost-effective design of coastal and offshore structures, and the safety of seawalls.
Sector Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Education,Energy,Environment,Transport
Impact Types Societal,Economic

 
Description EPSRC Nertwotk grant, A CCP on Wave/Structure Interaction: CCP-WSI
Amount £483,159 (GBP)
Funding ID EP/M022382/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 10/2015 
End 09/2020
 
Description Resilient Integrated-Coupled FOW platform design methodology (ResIn)
Amount £811,976 (GBP)
Funding ID EP/R007519/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 07/2017 
End 07/2020
 
Title Numerical modelling of breaking wave impact on coastal and offshore structures 
Description In this project, we have developed a new improved numerical methodology for modelling strongly non-linear wave and breaking wave impact on structures, particularlly looking at the reliability of wave impact pressure prediction and free surface elevations under various breaking wave conditions, including broken wave, aerated, air-pocket and flip-through impact. We have applied both incompressible flow solver and compressible flow solver for predicting breaking wave impact pressure on vertical wall, and conducted detailed comparisons for various wave conditions. The results have shown that though both models have predicted accepted results for impact pressures on the wall, compressible flow solver is able to produce higher peak impact pressure and the pressure oscillation inside the trapped air pocket . 
Type Of Material Model of mechanisms or symptoms - in vitro 
Year Produced 2015 
Provided To Others? Yes  
Impact Project partners have been informed of the key findings of the research through project meetings/workshops, the research has also disseminated to potential end-users and research community through presentations in international conferences and project workshops. 
URL https://collaborate.plymouth.ac.uk/sites/cerg/Pages/froth-downloads.aspx
 
Title Modelling of breaking wave impact on coastal and offshore structures 
Description An improved numerical methodology for breaking wave impact on coastal and offshore structures have been investigated in the project. Both imcompressible and compressible flow solvers have been used for modelling breaking wave impact on vertical wall and compared. We have found that compressuble flow solver is able to predict higher peak impact pressure and the pressure oscillation inside the trapped air pocket. 
Type Of Material Computer model/algorithm 
Year Produced 2015 
Provided To Others? Yes  
Impact The research findings have been presented in a couple of international conferences and project workshops to a mixed audience from both industry and academia. 
URL https://collaborate.plymouth.ac.uk/sites/cerg/Pages/froth-home.aspx
 
Description FROTH research partnership 
Organisation City, University of London
Department Department of Civil Engineering
Country United Kingdom 
Sector Academic/University 
PI Contribution We have worked closely in the FROTH project, and regularly exchanged our research findings and research methods, as well as methodologies in project meetings and project workshops
Collaborator Contribution All partners have made significant contributions to the research project in different aspect, including both experiment and numerical work.
Impact Some of the joint papers are listed in the publication section.
Start Year 2013
 
Description FROTH research partnership 
Organisation Manchester Metropolitan University
Country United Kingdom 
Sector Academic/University 
PI Contribution We have worked closely in the FROTH project, and regularly exchanged our research findings and research methods, as well as methodologies in project meetings and project workshops
Collaborator Contribution All partners have made significant contributions to the research project in different aspect, including both experiment and numerical work.
Impact Some of the joint papers are listed in the publication section.
Start Year 2013
 
Description FROTH research partnership 
Organisation University of Oxford
Department Department of Engineering Science
Country United Kingdom 
Sector Academic/University 
PI Contribution We have worked closely in the FROTH project, and regularly exchanged our research findings and research methods, as well as methodologies in project meetings and project workshops
Collaborator Contribution All partners have made significant contributions to the research project in different aspect, including both experiment and numerical work.
Impact Some of the joint papers are listed in the publication section.
Start Year 2013
 
Description FROTH research partnership 
Organisation University of Plymouth
Country United Kingdom 
Sector Academic/University 
PI Contribution We have worked closely in the FROTH project, and regularly exchanged our research findings and research methods, as well as methodologies in project meetings and project workshops
Collaborator Contribution All partners have made significant contributions to the research project in different aspect, including both experiment and numerical work.
Impact Some of the joint papers are listed in the publication section.
Start Year 2013
 
Description Project Workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Around 50 people attended the project workshop hosted by the University of Bath. Almost half of the attendees are overseas researchers who attended the project workshop just after they attended the 30th IWWWFB Conference held in Bristol, co-chaired by Dr. Jun Zang. Key research finddings from the FROTH project were disseminated to the mixed audience in the project workshop.
Year(s) Of Engagement Activity 2015