Scope of Work: Academic Work Group for Carbon Trust OWA PISA Project

Transcription

1 SCOPE OF WORK Scope of Work: Academic Work Group for Carbon Trust OWA PISA Project Prepared Alastair Muir Wood (ALAWO), 24 July 2012 Checked Dan Kallehave (DAKAL), 28 February 2013 Accepted Jørn Scharling Holm (JORSH), 28 February 2013 Approved Jesper Skov Gretlund (JESSG), 28 February 2013 Ver. no A Case no

2 Table of Contents 1. Introduction Project Aim Scope of Works Output Offshore Wind Accelerator Project Description Project Technical Appraisal Constraints of Project / Areas of Investigation Project Structure The Discretionary Project Steering Committee The Lead Partner Project Partners Technical Experts Independent Technical Review Panel Academic Work Group Assisting Consultants Testing Contractor(s) Scope of Works General Scope of Works Programme and Deliverables Project Meetings Provision of Resources July 2012

3 1. Introduction 1.1 Project Aim The aim of this project is to develop an improved method to design laterally loaded piles that is specifically tailored for the offshore wind sector. The design procedure should include evaluation of the ultimate lateral capacity of piles, effects of cyclic loading, and evaluation of stiffness and damping during operational conditions. The research project should result in a close form solution for accurately designing piles for offshore wind farms that can readily be implemented into current industry design standards. The project will be undertaken in two phases: The first phase (stage 1 and 2) will be to form a hypothesis solution, and plan necessary testing to prove this solution. The second phase (stage 3) will be to undertake large scale installation and testing of piles at two sites, one a clay and the other a sand. 1.2 Scope of Works The scope of works is defined in sub-section 3. In summary it is to provide academic ideas, input, supervision, analysis and research in support of a joint industry research project, the PIle-Soil Analysis project (PISA), into the design methods for laterally loaded piles, specifically for the application of Offshore Wind turbine foundations. 1.3 Output The output of the project is to be a report describing the design considerations and recommended methodologies that should be applied when designing laterally loaded piles for offshore wind farms. It should be in a format akin to a CIRIA guide. 1.4 Offshore Wind Accelerator The Offshore Wind Accelerator (OWA) is Carbon Trust's flagship collaborative R&D programme. Set up in 2008, the OWA is a joint industry project, involving nine offshore wind developers with 77% (36GW) of the UK's licensed capacity, that aims to reduce the cost of offshore wind by 10% by Cost reduction is achieved through innovation. Technology challenges are identified and prioritised by the OWA members based on the likely savings and the potential for the OWA to influence the outcomes. Projects are carried out to address these challenges, often using international competitions to inspire innovation and identify the best new ideas. The most promising concepts are developed, de-risked and commercialised as the OWA works closely with the supply chain throughout the process. The OWA model brings together Carbon Trust's expertise in delivering innovation and convening industry consortiums with the industrial partners' technical knowledge and resources. The OWA is two-thirds funded by industry and one-third funded by the UK Department of Energy and Climate Change (DECC) and Devolved Administrations. The PISA project is a discretionary project within the OWA this means that offshore wind developers within the OWA choose whether they wish to be part of the project. Under this structure organisations that are not part of the OWA can also choose to become a partner in the project. Page 3/12

4 DONG Energy has agreed to be the Lead Partner. In this role, the project will be run by DONG Energy, and all contracts and appointments to the project will be made by DONG Energy on behalf of the other partners. At the time of Tender other OWA partners are deciding whether they wish to join the project. 2. Project Description This section provides the Tenderer with a general project description. 2.1 Project Technical Appraisal The most popular method of analysis for laterally loaded piles, and the method adopted in the offshore design codes (DNV, 2007), is based on the Winkler model and is commonly termed the p-y approach. This method of analysis assumes that the pile acts as a beam supported by a series of uncoupled springs, which represent the soil reaction. These springs are normally described by nonlinear curves, which describe the soil reaction, p, at a given depth as a function of the lateral movement, y. The spring stiffness,, is the secant modulus of the p-y curve. The p-y concept was proposed by Reese & Matlock (1956). During the 1960s and 1970s load tests were carried out on laterally loaded long and slender piles, in both cohesive and cohesion less soils, to develop soil specific p-y curves aimed for the oil and gas sector. For example, the initial p-y curves for piles in cohesion less deposits were developed by Reese et al (1974). These curves were empirically derived from lateral load tests on 2 identical instrumented test piles at Mustang Island in Texas, described by Cox et al (1974). The Mustang Island tests were performed on 610 mm diameter flexible steel tubes with a length to diameter (L/D) ratio of 34. Instrumentation mounted on the pile shafts allowed the soil reaction to be determined along the length of the pile, which allowed the experimental p-y curves to be plotted. A general semi-empirical p-y framework was then suggested by combining the field measurements with a theoretical wedge failure. This p-y approach was developed for the oil and gas sector and therefore it does not address the specific site conditions and loading conditions at offshore wind farm developments. Page 4/12

5 Specific limitations of the existing approach include: Limitation of existing p-y approach The pile testing forming the basis for the development of the current p-y approach is only partly representative for piled structures in the offshore wind turbine industry The p-y curves were developed for flexible pile failure and not the rigid failure associated with monopiles. The p-y curves were developed for high L/D ratios. The p-y stiffness was derived on basis of soil strength parameters rather than soil stiffness parameters. Dominating moment loads were not explicitly considered when deriving the p-y guidelines Very dense and strong soils are not accurately modelled Effects of cyclic loading are poorly addressed through degraded pseudo-static p-y curves Permanent deformations due to cyclic loading are not addressed. The characteristics of the cyclic loading are not taken into account. The cyclic soil properties are not taken into account. The soil response under operational conditions is poorly addressed. The original p-y formulations do not focus on the small-strain response experienced during operational conditions. Long term effects on the stiffness are not considered No guideline is provided on estimating soilpile damping The impact of pile diameter on stiffness is not well investigated Consequences The ultimate lateral pile capacity predicted using the existing p-y approach is uncertain and may be under- or overestimated. Empirically derived cyclic degradation factors are most likely over- or underestimated depending on site-specific conditions. Large uncertainties exist in predicting permanent deformations over the lifetime of the turbine potentially leading to an over-conservative design. Stiffness and damping are likely to be underestimated which leads to an uneconomical design due to over-conservatism. Existing support structures may have shorter or longer life-times than currently anticipated. Recent measurements of natural frequencies in DONG Energy wind farms (see figure below) have shown that monopiles are restrained closer to the seabed than the design calculations show, as the first mode resonance frequency is higher than assumed. This suggests that monopiles are stiffer than assumed and they are being designed conservatively. A more accurate method of design may allow monopiles to be installed in deeper waters than is currently possible using the existing standards, and potentially with larger turbines on top. This would make monopiles a suitable foundation solution for many of the UK Round III projects. Page 5/12

6 Jacket structures can be standardised across a site of varying water depth if large pile stickup above the seabed is allowed in design. These pile then attract significant horizontal load, and which limits the allowable pile stickup. If jacket piles too are supported closer to the seabed than predicted by the current design methodologies, and therefore the soil resistance is under-predicted, it would allow further optimisation of both the piles but also the jacket structure. This would significantly decrease the cost of jacket structures. 2.2 Constraints of Project / Areas of Investigation The project aims to develop closed form solutions for the design of piled structures for future foundations for Offshore Wind turbines. A typical site soil conditions would be sand, clay and possibly layered soils. To meet these requirements the study must focus on: Pile diameters of between 1 to 10m L/D ratios between 4:1 and 30:1 Diameter to wall thickness (D/t) ratios between 40:1 and 120:1 Random cyclic loading Soils typical for European potential offshore wind farm sites Soil properties realistically obtainable using existing commercially available site investigation methods. Typical whole life loads including random extreme events will be provided as an example, however the developed method should be able to cope with any number of given cycles from all directions. As the project progresses in discussion with the Project Steering Committee some of these constraints may be refined. Page 6/12

7 Some of the anticipated areas of research are defined below: Subject Definition Rationale Static moment capacity and scale effects Cyclic moment capacity and scale effects Rotational stiffness of a pile at seabed Soil/structure stiffness with depth Accumulated deformations Large Scale Testing How does the size of a pile effect the static capacity of a pile? Does a larger pile of same d/t ratio behave the same way as a smaller pile? Is this linked to d/t ratio? How does removing the plug change the static moment capacity? How does application of random cyclic loading change the moment capacity relative to the static capacity Accurate stiffness prediction of k 0 and k n. for both operational conditions (continuous cyclic load) and extreme loading (an occasional cyclic load). How does stiffness scale with size of piles? Accurate modelling of pile deformation with depth for both operational conditions (continuous cyclic load) and extreme loading (an occasional cyclic load). How does stiffness scale with size of piles? Accurate prediction of pile rotation DJ(N) after application of random cyclic loading. What is best practice in large scale testing? How should the test be set up? What Instrumentation should be used / applied. Important for scaling of test results Important for design piles against ULS overturning Both FLS and ULS loads are a function of natural frequency and modeshape, which are functions of stiffness. Important for calculation of moment and shear stress distribution in steel in FLS and ULS. Important to fulfil SLS requirements. Typically set to max degree rotation Ensure testing uses state of the art methods, and gets good results. Page 7/12

8 2.3 Project Structure The project organisation is defined in the diagram below: The Discretionary Project Steering Committee The Discretionary Project Steering Committee is the management board of the whole project. All Project Partners have a seat on the steering committee. Voting rights on this committee are proportioned to the funding contribution of the project The Lead Partner DONG Energy, as Lead Partner, is responsible for both the project and technical management of the project. They will drive the project forward and report the progress to the Discretionary Project Steering Committee Project Partners Technical Experts All Project Partners have in-house experts with knowledge of the design of offshore foundations. This group will provide technical guidance and feedback to the Lead Partner and the Academic Working Group for the duration of the project Independent Technical Review Panel At key stages in the project, summary documents will be submitted to the Independent Technical Review Panel. This will consist of world recognised experts in geotechnics, wind turbine loads, and representatives from the different certification agencies. This panel will form a critical judgement on the progress of the project, make recommendations and thereby ensure that the outcome of the project is generally accepted by industry and can therefore be used in design Academic Work Group The Academic Work Group is to consist of a lead Academic Institution working in partnership with at least one other Academic Institution. The Scope of work for the Academic Work Group is defined in sub-section 3. Page 8/12

9 2.3.6 Assisting Consultants The project has the option to bring additional expert assisting consultants in to assist the project and to enable analysis to be performed on an industrial timescale if required. It is envisaged that these are likely to be bought in for expert numerical modelling or cyclic analysis. This will be commissioned separately by the Lead Partner albeit in consultation with the Academic Work Group Testing Contractor(s) The instrumentation and the testing will be undertaken by a separate testing contractor(s). This will be tendered once the testing has been specified. Page 9/12

10 3. Scope of Works This section details the actual service requirement being sought from this Tender. 3.1 General Scope of Works The scope of work for the Academic Work Group is as follows: Cooperate with the project partners to develop hypotheses and methodologies for the design of laterally loaded piles for offshore wind turbines. Develop methods for proving above hypotheses which is expected to be based on large scale testing in both a sand and a clay site. Specify pile tests to be undertaken and supervise collection and analysis of the test data. Prove and document the derived hypotheses based on the test results. Provide support to enable students to continue to develop the data/methodologies in own time following formal conclusion of the project. Page 10/12

11 3.2 Programme and Deliverables The project is defined into three stages. The stages and the required deliverables from the Academic Work Group are defined in the table below. The anticipated deadline for these deliverables is defined in number of months following contract award. Stage Stage 1 Project Setup Stage 2 Research Analysis and Stage 3 Testing Conclusions and Deliverables Ser. Title Description Detailed project plan 2.2 Draft design hypothesis 2.3 Draft scope of work for large scale testing 2.4 Final Scope of work for large scale testing 2.5 Final Design Hypothesis 3.1 Large Scale Testing Factual Report 3.2 Draft final project report 3.3 Large Scale Testing Interpretative report 3.4 Final project report A report detailing the proposed approach to the project, areas of study, persons involved, meetings anticipated. Anticipated chapter headings for the Final project report should also be presented. A report setting out the proposed design methodology and the testing/modelling that is required to prove this methodology. A report setting out the anticipated scope of work for the large scale testing. This is to include description of the preferred soil conditions at the test sites, the anticipated number and type of tests, details of the instrumentation that will be required. See above. See above. *Estimated depending on timing/progress of testing A report presenting in detail the setup, instrumentation, calibration, and raw unprocessed results from all testing. A report describing the design considerations and recommended methodologies that should be applied when designing laterally loaded piles for offshore wind farms. It should be in a style akin to a Construction Industry Research and Information Association (CIRIA) guide. A report presenting the analysis / interpretation of the large scale testing results. See above. Deadline / months after contract award 1 month 9 months 9 months 10 months 12 months 14 months* 17.5 month* 18 months* 18 months* Page 11/12

12 3.3 Project Meetings Relevant members of the Academic Work Group will be required to attend the following meetings: Monthly technical meetings between project partners and Academic Work Group to be held either in London or a mutually agreed venue. Attendance at up to 5 meetings of the Independent Technical Review panel to be held in London or a mutually agreed venue. Attendance at the site kick-off meetings at testing site locations. Other project workshops may be required either at the request of the Academic Work Group or the Lead Partner; these shall be held at a venue convenient for the Academic Work Group. 3.4 Provision of Resources The Academic Work Group must provide resources to the project as required to meet the Contract Award Criteria according to the Tender Conditions. The form of the project encourages the Academic Work Group to consist of some highly competent members in the Stage 1 and Stage 2, such as: Senior academic staff Post-doctoral researchers Existing PhD-students and then to include new PhD-students to assist during planning of tests and supervision of data collection. It is anticipated that other (partial) funding may be provided to these PhD-students as they are continuing working on the data/methodologies in their own time following formal conclusion of the project. Page 12/12