OCEANIC INVESTIGATES SPAR VIM DYNAMICS

Transcription

1 Newsletter of Oceanic Consulting Corporation Spring/Summer 2012 OCEANIC INVESTIGATES SPAR VIM DYNAMICS Houston Offshore Engineering Ltd., a leading expert in mooring systems, contracted Oceanic Consulting Corporation to evaluate the vortexinduced motion (VIM) dynamics of Williams Partners' proprietary floating production system, Gulfstar FPS spar. Williams Partners L.P., a world leading expert in classic spar design, construction, and installation, funded this program with an aim to investigate vortex induced motions of the spar as a function of reduced velocities and headings with various mooring arrangements, surface conditions, appurtenance layouts, and strake configurations. Continued on page 3... In this issue... Charting the Course: Paul Herrington s Spring 2012 Address Oceanic Investigates Spar VIM Dynamics Multi-Vessel Interactions Research Programs Oceanic Upgrades Ice-Structure Interaction Code for Moorings Hydrodynamic Study of a VIV Suppression Device A Comparison of DP Performance Prediction Techniques in Open-Water Environments Study Into Loads Due to First-Year Ice Ridges Completed Profile: Oceanic s Think Tank Feature: 90-meter Ice/Towing Tank

2 SPRING/SUMMER 2012 CHARTING THE COURSE 3 T he past year was full of change for Oceanic Consulting Corporation. With a very clear long-term strategy in mind, Oceanic decided without hesitation to launch into the most important development in its history: its acquisition by J.D. Irving, Limited (JDI). Since the company s foundation nineteen years ago, Oceanic has often confronted powerful headwinds. But we have never lacked the internal resources required to withstand those winds and forge ahead with continued energy and passion. Oceanic s willpower, stamina and entrepreneurship bedrock values in our corporate culture have only been strengthened by this acquisition. As a result, Oceanic has entered 2012 with even greater drive and initiative, and powerful leverage for boosting performance. Strengthened by JDI, this exceptional arrangement takes Oceanic beyond anything we could have ever imagined. The alignment with Fleetway Inc., one of the best engineering, technical, logistics, and management service providers this side of the Atlantic, opens the door to a very exclusive club of top-tier companies. 4 6 I am very conscious of the challenge that our new size represents for our employees. The acquisition by JDI has extended our global reach and increased our headcount from 45 to 15,000. The merging of the companies cultures, resources, and personalities will be one of our most significant challenges in the coming years. I am convinced that the entire company, under Mr. Brent Holden s direction, will be able not only to make this work, but to take the new Oceanic to places we had not even thought about nineteen years ago. Oceanic s striking accomplishments expanding its numerical modeling suite, adding significant major clients to its portfolio, and providing consulting services to some of the world s most challenging marine projects attest to the effectiveness of the company s values: devising innovative, competitive, effective solutions; solutions that offer our clients real value for their money along with further business success. In this edition of our newsletter, we highlight such successes in the offshore oil and gas industry. Oceanic has helped mitigate the effect of vortex-induced motions on spars, accurately modeled scaled fenders in LNG transfer and direct offloading scenarios, and investigated DP prediction techniques in open-water environments. I hope that reading about these initiatives will prompt you to consider some of the challenges your organization faces and how Oceanic might be of assistance in overcoming them. With our evolution, Oceanic has access to significant resources, allowing us to continue to support our clients success and to drive progress even further. Whether it is through the use of our existing techniques and models or by developing new solutions to meet our clients specific needs, Oceanic can and will deliver. It s what we do every day, and now we can do it even better. 6 I hope you enjoy this edition of our newsletter as much as we enjoyed preparing it. It is worthwhile and quite often enjoyable to take stock, review our achievements and successes, and reflect on how we made them happen. I wish you continued success and satisfaction, and remind you that the new Oceanic looks forward to hearing from you. Paul Herrington, P.Eng. Director of Operations Oceanic Consulting Corporation For additional details on any of the projects highlighted in this edition, please contact: Lee Hedd Director of Business Development Hedd.Lee@oceaniccorp.com Don Spencer Director of Technology Spencer.Don@oceaniccorp.com David Molyneux Director of Numerical Modeling Molyneux.David@oceaniccorp.com J. Michael Doucet Senior Project Manager Doucet.Michael@oceaniccorp.com Mohammed (Shameem) Islam Senior Project Manager Islam.Mohammed@oceaniccorp.com Stephen Very Manager of Testing Very.Stephen@oceaniccorp.com

3 Continued from cover... A blunt structure placed in a flow (either air or water) will experience an oscillating force due to the shedding of vortices. If the blunt structure is able to move in the flow, these vortices can lead to large steady-state type oscillations (i.e., VIM) when the vortex shedding frequency coincides with a natural frequency of the structure. For spar-like offshore structures, these vortex-induced motions could add to the fatigue damage of moorings and risers, shortening the total fatigue life and also increasing the overall drag on the structure. OCEANIC INVESTIGATES SPAR VIM DYNAMICS VORTEX-INDUCED MOTIONS The VIM characteristics of a spar depend on the complex interaction between spar hull components and generated vortices. Hull components typically include strakes, anodes and other appurtenances, such as piping, chains, fairleads, and a steel catenary riser (SCR) porch. Arrangements of such components on the spar should be made so that minimal VIM occurs. Hull surface condition, mooring configurations, flow speed and flow heading also influence VIM performance. In the recent model experimental program, research was undertaken to systematically investigate spar VIM with varying strake configurations, mooring types and elevations, hull roughnesses, and flow headings. The experiments were carried out using a 1:47.22 scaled spar in the National Research Council of Canada Institute for Ocean Technology s 200-meter Towing Tank. Strakes with 10, 11, and 12 ft heights were evaluated with continuous and intermittent strake arrangements. Four- and three-point mooring arrangements were examined at two mooring elevations. Two hull roughness conditions were examined and the spar appurtenants previously described were modeled with fine detail. The experiments covered a range of reduced velocities from four to twelve and forty different heading angles. The heading angle was seen to be significant and was related to the relative positions of the mooring chain groups and strake arrangements. Both qualitative and quantitative observations of the spar motions were made specifically examining resultant vortex induced motions, mooring loads, and hydrodynamic damping properties over a range of possible prototype operating conditions. The effects of spar surface roughness, intermittent strakes, mooring line elevation, mooring configurations, and the presence of external pipes and SCR porch were examined as part of the sensitivity experiments. The research program confirmed that strake configurations, mooring arrangements, hull appurtenances, and other appendage geometry must be critically examined when considering VIM since the motion responses can be significantly different than initially estimated. Ballasted spar model illustrating a possible strake arrangement. 3

4 MULTI-VESSEL INTERACTIONS MULTI-VESSEL INTERACTIONS RESEARCH PROGRAMS Model with prototype fender attached. Oceanic Consulting Corporation is continuing to expand its modeling capabilities in multi-vessel and multi-body interaction through physical and numerical research programs. Recent programs that examined heavy lift vessel cargo discharging operations, topsides float-over installations, side-by-side LNG transfer operations, direct offloading scenarios, and large scale ice-vessel interaction have provided opportunities for Oceanic and its clients to expand their combined knowledge in this challenging and complex area of hydrodynamic modeling. Through commercial research programs and Oceanic's own internal development programs, several new and improved modeling technologies have contributed to the success of economical and efficient evaluation programs. Several model scale float-over installation campaigns saw the continued development of fendering technology, which allowed very fine resolution modeling and measurement of between-structure loads and deflections. The latest iteration of Oceanic s model scale fender technology is capable of force measurements to within ±1N, and deflection measurement to within ±0.1mm, while using standard, off-the-shelf instrumentation hardware. The inclusion of staged, offset, beryllium copper cantilever spring elements and built-in gross positioning allows for a very high level of adjustability which in turn permits a wide variety of prototype fender stiffness properties. The upcoming addition of controlled damping will further enhance the accuracy of the model scale fenders in identically replicating a prototype fenders properties. Although initially developed for force measurement between a specific barge and jacket structure, the versatility incorporated into the model scale fender technology ensures that other multi-body, close-contact type research programs can benefit from it. Details of the development of the latest fender design iteration are published in OMAE 2012 (paper number 83166). The same series of float-over research programs also yielded a new model scale leg mating unit (LMU) design which provides a very close match of prototype LMUs in stiffness and damping characteristics, both in horizontal and vertical 4 degrees of freedom. Mimicking the prototype units in their ability to pivot about the bottom of the vertical compressive elastomer stack, the model units rotate on a ball and socket joint and use an elastomeric 'donut' element to provide the correct stiffness and damping of the LMU cone in the horizontal plane. Correct vertical stiffness is provided by a series of staged, cupped washer (Belleville) spring elements and spacers. Force measurement is provided by high resolution, multi-axial force dynamometers mounted to the cone portion of the LMU assembly. As with the fender units, the adjustability available in the LMUs provides for unlimited ability to match any type of LMU stiffness curve. Determining the performance of prototype installations, to ensure successful execution, or identifying areas for operational improvements are two examples in which Oceanic excels when assisting its clients to achieve sound, economical, and effective engineering solutions to commercial problems.

5 OCEANIC UPGRADES ICE-STRUCTURE INTERACTION CODE FOR MOORINGS DECICE prediction of the ice field around a cylindrical structure. Oceanic recently completed a project to assess the performance of an Arctic floater concept, developed by ConocoPhillips, when operating in a variety of ice conditions. The study focused on determining the ice loads which result from the interaction of several significant ice features with the moored floating structure, in addition to the resulting dynamic responses. The ice conditions investigated included first- and multi-year level ice, first- and multi-year pressure ridges, pack ice, and collisions with an ice island. Sensitivity of the loading to changes in ice thickness was also studied. The analysis was carried out using DECICE, which is Oceanic s proprietary software for assessing ice mechanics and is based on the discrete element method. By assuming that a material is made up of discrete elements, the need for internal continuity of the material is removed. Within the discrete element method, a structure can be defined as a rigid element with an initial velocity and the ice is defined as deformable discrete elements. Fracture of ice elements is permitted in flexure, tension, and compressive shear. The ice can fracture along pre-set patterns or each element can be allowed to fracture into smaller elements based on the internal stress level. The interaction forces are computed based on the relative motion of each element and this requires a contact detection process for checking the location of elements relative to each other. Each element is subject to the forces and resulting motions in translation and rotation given by Newtonian mechanics, as well as conservation of mass, momentum, and energy. Additional forces can be included, such as ice buoyancy, wind and current-induced drag on the broken ice pieces, the powering and maneuvering forces acting on a ship, or the mooring forces acting on an offshore structure. In order to complete this work, Oceanic had to upgrade the capabilities of DECICE by including the ability to model a mooring system. The Arctic Floater concept was to be held in place by a 32-line mooring system, which was initially modeled in Orcaflex. The restoring forces and moments from the mooring lines were added to DECICE based on in-house code developed for spread and semi-taut mooring systems. The response of the mooring system in DECICE was matched to the Oracflex analysis. DECICE was used to predict time histories of the loads in the mooring lines and the resulting motions as the structure progressed through the ice feature. The time-series loads for the ice and mooring system were plotted as part of this analysis. Animations of the various simulations were also produced, which showed the motion of the structure and the flow of the broken ice pieces around it. MOORINGS & RISERS HYDRODYNAMIC STUDY OF A VIV SUPPRESSION DEVICE In August 2007, Oceanic Consulting Corporation evaluated vortex-induced vibration (VIV) suppression strakes for AIMS International using the small VIV apparatus in the 90-meter Ice/Towing Tank at the National Research Council of Canada Institute for Ocean Technology facility in St. John s, NL. The VIV rig allows for vertical cross flow motion of a spring-mounted sub-frame which is free to slide on two rails with linear circulating bearings. The apparatus provides basic free vibration drag and cross flow motion data over a range of nominal reduced velocities which are altered using a combination of flow speed and springs. The 2007 experiments involved strakes with a 15D triple helix design, with an outside diameter of m and a strake fin height of 25% D. These experiments were conducted in a water temperature of only 5.5 C. During February 2012, Oceanic was once again tasked by AIMS International to evaluate two similar strakes but with a reduced strake height of 20% D and 15% D. The objective was to determine if the strake drag could be reduced without any undue vibration penalty. Oceanic once again used the same small VIV apparatus, however, this time the VIV rig was installed in the 58-meter Towing Tank at Memorial University s Ocean Engineering Research Centre (OERC). During this phase of the experiments, the water temperature was 16 C. Results from this recent experimental program aligned well with the results from August 2007 when plotted on a fluid speed basis. The experiments successfully demonstrated that as the strake height was reduced, the drag decreased incrementally but without any significant increase in vibration. These recent experiments covered a much higher speed range and pushed the Reynolds Number (Re) into the critical region (Re>200,000) where the drag coefficient reduction was significant. However, the Reynolds Number deals mainly with boundary layer flow and it could be argued that it is not an appropriate measure for strakes which trip the flow prematurely. 5 In this instance, the drag coefficient results may appear more rational when viewed with respect to velocity only. The change in fluid temperature alters the viscosity and adjusts the Reynolds Number making the 20% D strake height (at 16 C) drag versus Re results about the same as the 25% D strake height (at 5 C). Riser sample with a typical strake configuration mounted on the VIV test apparatus.

6 WIND, WAVES & ICE A COMPARISON OF DP PERFORMANCE PREDICTION TECHNIQUES Dynamic positioning experiment with a typical drillship hull in irregular waves. As onshore and shallow water oil reserves are depleated, the trend in exploration is towards previously inaccessible deposits, often found below the seafloor of deep oceans. Traditionally, floating offshore structures in moderate water depths were held on station using massive cable or chain mooring systems. In deep water, the necessitated increase in mooring line length increases line weight and makes the moorings difficult and costly to manufacture, transport and install. Mooring weight can also decrease the storage payload of a floating structure. Dynamic Positioning (DP) defines the process of holding a floating structure at a specified IN OPEN-WATER ENVIRONMENTS location or on a specified trajectory by the application of a summed correction force, generally applied using thrusters, to minimize course or station deviation. The prevalence of DP systems to replace or supplement deep water moorings for offshore exploration and production of hydrocarbons is increasing due to the need to exploit deeper water depths. Experiments using a scaled prototype of a DP vessel are often conducted to estimate full-scale watch circle performance in varying environments. In conducting physical analysis or predictions for DP system performance, there are two common techniques used: an experimental investigation at a reduced scale using a simplified mooring system without thrusters; or, a similarly scaled experiment using active DP thrusters. The primary environmental parameters of interest in assessing DP watch circle performance are wind, current, and second order wave loads on the vessel. In November 2011, Oceanic undertook an internally-funded experimental program to evaluate differences in the DP system performance estimates for both of these methods by assessing the systems in identical wind, wave, and current environments. The experiments were completed using a 1:40 scale model of a typical 100,000 tonne monohull drillship, supplied by the National Research Council of Canada - Institute for Ocean Technology (NRC-IOT), which was equipped with an active DP system consisting of six azimuthing thrusters. These experiments were repeated with the unpowered vessel and assessed two different four-point, horizontal spring mooring system configurations with each mooring having a different spring stiffness. This series of experiments make up the physical component of a larger internal research project, which includes numerical simulations of the same vessel and environments. For the numerical simulations, linear and non-linear ship motion codes are being used to compare results for moored and dynamically positioned hulls. The physical experiments will provide important validation data for the numerical simulations and will offer a valuable comparison between many of the DP performance prediction techniques used in the industry today. STUDY INTO LOADS DUE TO FIRST-YEAR ICE RIDGES COMPLETED Oceanic recently completed a research project to improve the understanding of the interaction between first-year ice ridges and floating structures. Ice ridges likely will be the maximum load case for many offshore structures and their mooring systems. Determining this limiting load is critical to evaluating designs proposed for use in ice-covered waters. This project was carried out in four main phases and the first phase was to determine the likely range of ridge geometry that would be encountered in different geographic regions where ridges occur. The first-year ridges were assumed to be made up of an unconsolidated keel, a layer of consolidated ice, and a sail. Two types of offshore structures were considered: a spar (with a downward icebreaking cone at the neck) and a simplified Floating Production Unit (FPU). The displacement and dimensions of these structures were based on typical industry practice. In the second phase, experiments were conducted in the 90-meter Ice/Towing Tank at the National Research of Canada Institute for Ocean Technology in St. John s, NL, using physical models. Each structure was evaluated over a range of ridge widths and keel depths. Drift speed was varied for both structures and the FPU was assessed with two bow shapes and three different mooring stiffnesses. The physical experiments were used to show the trends in ice loads acting on the structures. Up to three ridges were made in a single ice sheet and a total of twelve ice sheets were used. Numerical simulations of the ice-structure interaction were carried out during the third phase of the project using Oceanic s proprietary discrete element software, DECICE. Special techniques were developed for creating the ridge model from randomly oriented ice blocks. 6 Continued on page 7... Spar model entering simulated first year ridge in the ice tank.

7 PROFILE: OCEANIC S THINK TANK PROFILE Members of Oceanic s Numerical Modeling Department. In 2009, Oceanic Consulting Corporation was successful in obtaining support under the Atlantic Innovation Fund from the Atlantic Canada Opportunities Agency and Husky Energy Canada. The project objective was to increase our capabilities in numerical simulation for predicting the performance of ships and offshore structures in harsh environments. As part of this initiative, Oceanic has hired six new specialists to supplement the company s three original staff working in this area. The new staff members have combined expertise in numerical prediction methods for ocean engineering and applied mathematics, as well as in software development and computer system architecture. Oceanic has also used this funding to enhance MOTSIM, our in-house code for ship motions predictions, with the support of our long-time collaborator Dr. Don Bass, formerly of Memorial University. It has also added mooring forces, dynamic positioning, and ship maneuvering to DECICE, as well as improved multi-ship interactions, moored vessel, and dynamic positioning capability in Oceanic s Ship Maneuvering Laboratory (SML) program. Led by Dr. David Molyneux, Oceanic s numerical team focuses on providing solutions to meet the needs of our clients using commercial codes or software developed in-house. Oceanic can offer numerical solutions to a wide range of problems including motions and loads in waves for ships and offshore structures, dynamic positioning and ship maneuvering, and ice structure interaction using our in-house codes. Oceanic s in-house codes have evolved over the years, and the company continues to add features that are required by our clients. Oceanic also has a variety of commercial codes for motions of ships and structures in waves, computational fluid dynamics, mooring and riser analysis and potential flow calculations for ship resistance. Papers describing the company s recent research have been published in OMAE 2010 and 2011, Icetech 2010, and ATC STUDY INTO LOADS DUE TO FIRST-YEAR ICE RIDGES COMPLETED Continued from page 6... Each structure was simulated in a large and a small ridge and compared to the results of the experiments. The results showed that the discrete element method provided an acceptable estimate of the ice forces acting on the two different types of floating structures. The simulated movement of the ice around the structures showed good agreement with model experiments for both structure types. The fourth phase of the project was to develop guidelines for carrying out physical model experiments in ice ridges. Modeling offshore structures in ice is very dependent on the geometry of the structure, the size of the ice basin, the experience of the facility staff, and the ice conditions that are to be considered. ISO19906:2010 gives a description of what is required from model experiments on fixed offshore structures, but it provides little guidance on the techniques to be used. The International Towing Tank Conference (ITTC) has made recommendations for conducting model experiments in ice with ship models but similar recommendations are required for structures in ice. Thus, the purpose of this phase was to provide guidelines on the best practice for performing and analyzing model experiments in ice with floating offshore structures. 7 The intention is that by following the principles reported, the level of uncertainty in the ice properties and measured forces will be reduced to a level acceptable for engineering applications. This project was sponsored by CITEPH with the funding partners being Total E P Recherche Development, Doris Engineering, Technip France, Entrepose Contracting, and Saipem. The research was conducted as a collaborative project between Oceanic, Océanide, and the National Research Council of Canada - Institute for Ocean Technology.

8 90-meter Ice/Towing Tank Length Width Depth Usable Ice Sheet Length Set-up Area Length Temperature Range Ice Growth Ice Thickness Range Ice Strength Range Ice Formulation NRC EG/AD/S Carriage Mass Carriage Max. Drive Force Carriage Speed Range 90m 12m 3m 90m 15m -30 C to +15 C 2mm/hr at -20 C 5mm to 160mm 10kPa to 150kPa Flexural Fine Grain Columnar Structure Ice (controllable density accurately models ice buoyancy) 80,000kg 60kN on Center Line, 30kN on Quarter Points m/sec to 4m/sec Instrumentation and Equipment: Ammonia-based Refrigeration System with 26 evaporators Computerized Temperature Control Manned Carriage with 4 Wheel or Rack Synchronous Motor Drive Adjustable Position Test Frame (lateral and vertical) Above Water and Underwater Video VMS and Windows NT Client/Server System for Data Acquisition (64 channel capability at 80kHz aggregate, 16 bit resolution) Specialized Instrumentation and Equipment: Separate Service Carriage Large Amplitude (± 4m) Horizontal Planar Motion Mechanism Digital Acoustic Radar Transceiver (DART) System Underwater Video Carriage Applications: Testing of Towed, Moored and Self-propelled Models Testing of Ship Models 2m to 12m in Length Testing of Offshore Structures 0.5m to 4m in Diameter Specification Sheets are Available for All Major Facilities, Including: Offshore Engineering Basin 200-meter Wave/Towing Tank 58-meter Wave/Towing Tank 90-meter Ice/Towing Tank Cavitation Tunnel 22-meter Flume Tank MOTSIM Centre for Marine Simulation Ice Engineering VIV Test Apparatus Specification sheets can be obtained from the Oceanic website or by contacting our office. Meet us at: 95 Bonaventure Ave., Suite 401 St. John s, NL Canada A1B 2X5 Telephone Facsimile oceanic@oceaniccorp.com ISO