Integrated Structural Analysis Methodology for Truss Spars

Until recently, most of the turret mooring systems have been designed using the so-called quasi-static approach. There is an increasing awareness that this approach may be deficient for certain applications, and that dynamic analyses are needed. To this end, this paper presents a rational dynamic analysis procedure for designing turret mooring systems. Specifically, practical methods for computing the low frequency motions and dynamic line tensions are presented, based on the latest analytical developments, model tests, and design experience. Model test data are presented for three turret mooring system in water depths of 70 meters, 75 meters, and 370 meters, respectively, to substantiate the significance of drag force on mooring lines on the low frequency motions, the modified Rayleigh distribution and the RINA method for interpolating the statistical low frequency responses, and line dynamic behavior. Finally, this procedure is illustrated using the turret mooring system of a 230K DWT tanker in 370-meter water depth as an example case. The results of the dynamic analyses are compared with model test data and with the results using a quasi-static analysis approach. INTRODUCTION Motion Response Characteristics The motions of a turret moored vessel can be described by three components: mean offset, low frequency motions and wave frequency motions. The mean offset is caused by the mean environmental force and is constant for the duration of interest. Low frequency motions refer to the resonant surge, sway, and yaw motions due to low frequency cyclic loads associated with wave groups or wind dynamics. These have periods varying from a low of 60 to 90 seconds to a high of a few hundred seconds. Wave frequency motions refer to the vessel motions excited directly by ocean waves. The periods vary from 3 to 25 seconds. The consensus is that these three types of motion can be considered as decoupled physical processes, and hence, they can be simulated individually and combined for the maximum offset. Computing the mean offset and the wave frequency motions is straightforward. The former involves only static catenary calculations. The latter can be accomplished using the highly developed frequency and time domain analysis techniques. The simulation of the low frequency motions remains a relatively complex task. The principal difficulties lie with obtaining quality data for wave drift force coefficients, estimates of low frequency damping, and low frequency wind spectra. While practical analytical tools exist, the prediction of low frequency motions is still in a research stage, Reference 1. Line Tension Response Characteristics A mooring line may respond to fairlead motions in three ways:change in the catenary configuration (catenary stiffness),stretch {elastic stiffness), andchange in the catenary configuration and stretch at the same time. If the fairlead moves slowly (low frequency motions) the line will move toward the new static-equilibrium position. The line will essentially be in static equilibrium at all times. The tension perturbation can be calculated using a static catenary equation.

Frequency-domain analysis can be used to evaluate the motions of the FPSO with its mooring and riser. The main assumption of the frequency-domain analysis is that the coupling is essentially linear. Calculations are performed taking into account first order wave loads on the floating structure. Added mass and radiation damping terms are frequency dependent, and can be easily considered in this formulation. The major non-linearity comes from the drag force both on lines and the floating structure. Linearization of the non-linear drag force acting on the lines is applied. The calculations can be extended to derive the low frequency motion of the floating structure. Second order low frequency quadratic transfer function is computed with a diffraction/radiation method. Given a wave spectrum, the second order force spectrum can then be derived. At the same time frequency-domain analysis is used to derive the low frequency motion and wave frequency motion of the floating system. As an example case, an FPSO is employed. Comparison is performed with time domain simulation to show the robustness of the frequency-domain analysis. Some calculations are also performed with either low frequency terms only or wave frequency terms only in order to check the effect of modeling low and wave frequency terms, separately. In the case study it is found that the low frequency motion is reduced by the wave frequency motion while the wave frequency motion is not affected by the low frequency motion.

Two methods, frequency domain analysis and time domain analysis, are widely applied to modeling wave energy converters (WECs). Frequency domain analysis can evaluate the performance of WECs quickly and efficiently, while it refers to a linear model. When it comes to investigations on nonlinear characteristics of the power take-off (PTO) unit of WECs or control for improving the WECs’ performance, time domain analysis based on a state-space approximation for the convolution term is more desirable. In this paper, a state-space approximation of the convolution term in a time domain analysis of a raft-type WEC consisting of two rafts and a PTO unit is presented. The state-space model is identified through regression in the frequency domain. Verification of such a type of time domain analysis is conducted by comparison of its simulation results with those calculated by using a frequency domain analysis, and there is a good agreement. Finally, the effects of PTO parameters, wave frequency, surge and heave motions of the joint, and quadratic damping PTO on the power capture ability of the raft-type WEC are investigated.

Hydrodynamic design of moored floating platforms

Performances of a set of control schemes for dynamic positioning (DP) are studied in this work; DP performance is essential for future developments of autonomous shipping technology. The linear and non-linear model predictive control (MPC and NMPC), the non-linear proportional integral and derivative (NPID) control, the sliding mode control (SMC), as well as the multi-resolution PID (MRPID) control schemes, are evaluated under two different sea conditions, namely, moderate and extreme seas. Matlab/Simulink models of a full-scale ship and its corresponding scaled model are used to benchmark the efficacy of the controllers. An unscented Kalman filter (UKF) is used to estimate vessel motions and to control low frequency (LF) motions while filtering out wave frequency (WF) motions. The tuning of the controllers is also taken into consideration. Of the five controller schemes, the NMPC shows the best ability to deal with extreme disturbances efficiently. Although all of the controllers were able to maintain the ship position under moderate conditions, only the NMPC and the MRPID controllers were able to stabilize the ship under extreme sea states. Findings from this research are expected to help operators of DP systems in choosing the most effective control scheme for different sea conditions. In addition, the results are supportive of further control system development for dynamic positioning and autonomous operations of ships and offshore platforms.

In the design of moored tankers the knowledge of the wave drift forces is of importance. As is known, the low frequency part of the wave drift forces is responsible for the low frequency motions of the vessel. In general the low frequency motions may cause a substantial part of the mooring forces. Another source of the low frequency excitation can be found from wind spectra. The wind spectra originate from the turbulence in the wind field. Besides the wave drift forces also the fluctuating wind force will induce low frequency oscillations of the tanker. The purpose of this study is to investigate the effect of a wind spectrum based on an hourly mean wind speed in relation to the 1- minute steady wind gust, both in combination with a co-linearly directed storm wave spectrum on the low frequency motions of a 200 kDWT tanker. The tanker is moored in 82.5 m deep water. The computations have been applied to a linear type mooring system. To confirm the reliability of the computation procedure model tests have been carried out. INTRODUCTION For the design of a mooring system the environment in terms of storm wave spectrum, current speed and wind velocity has to be established. The irregular waves generate the wave loads, which induce the wave frequency ship motions and the wave drift forces. The wave drift forces consist of a mean wave drift force, a mean wave drift damping and the low frequency wave drift forces. In the design, normally a steady current velocity is taken, while for the wind speed either the steady 3-second, the l-minute or the 10-minute wind gust speed is taken. Except for the wave frequency motions, the motions can be distinguished in a mean offset part and a low frequency oscillating part. The mean displacement of the moored vessel is caused by the mean wave drift force, the steady current and wind force. From the point of view of steady offset the mean wave drift force and the steady wind force are often important components in the mooring force. The oscillating part of the wave drift force is responsible for the low frequency oscillating motions of the vessel. In general the low frequency motions may cause a substantial part of the mooring forces. Another source of the low frequency Excitation can be found from wind spectra. The wind spectra originate from the turbulence in the wind field. The description of the turbulence is very complex. However, it is possible to express the turbulence in terms of so-called wind spectra or gust spectra. Studies on wind spectra have been carried out by several researchers as, for instance, Harris [1], DnV [2], Forristal [3], Davenport [4], Ochi-Shin[5] and Wills [6]. Most of the wind spectra contain energy of the wind velocities at the natural periods of the tanker. Because of the relatively light damped system the fluctuating wind velocities generate fluctuating wind forces, which induce low frequency tanker motions. The effect of the wind spectra on the low frequency motions will be studied. For that purpose a moored tanker has been exposed to the following weather conditions:wind spectrum based on an hourly mean wind speed, (VW = 30.9 m/s)storm wave spectrum,

Buoyancy Can Riser Tensioner Systems (BCRT) are designed to provide tension to Top Tension Risers (TTRs). BCRT systems do not transfer the riser weight to the floater and minimize the interaction between the floating platforms and the riser systems. For deepwater field developments, this attractive feature allows efficient design of the floaters as well as the riser systems. A number of deepwater fields have been developed with Classic and Truss Spar floaters utilizing this class of riser tensioning systems. The design philosophy and performance characterization of BCRTs are rapidly maturing. Recent technology advances pushed the net buoyancy capacity of BCRTs up to 1,550 kips for dual casing riser applications at 5,400 ft water depth (Mad Dog and Horn Mountain Truss Spars). This paper presents an overview of the current state-of-theart in BCRT design and addresses some of the challenges in development of larger tensioner systems for deepwater applications. Buoyancy Can Riser Tensioner System Components A BCRT is primarily a passive tensioning method for TTRs. Figure 1 illustrates a TTR system tensioned by a BCRT. The BCRT is designed to have relative vertical movement with respect to the hull while transferring horizontal loads at hull contact locations. The BCRT system consists of three main segments: upper stem, buoyancy can, and lower stem. The main functions of the BCRT components are as follows: Upper Stem. The upper stem transfers the tension provided by the BCRT to the riser. The surface wellhead and work platform are connected to the top of the upper stem. The elevation of the top of the upper stem is selected to locate the well equipment at a favorable elevation for easy access under normal operating conditions. The stem-to-wellhead interface and all other stem-to-stem interfaces can be sealed off to provide additional buoyancy. Figure 1 - An illustration of the BCRT components and critical elevations of a Truss Spar floater. (Available in full paper) The upper stem is equipped with up and down stops. The stops are designed to make contact with the deck stopper and have three main functions:provide a support location to rest buoyancy can during installation operations,provide a potential barrier for riser parting, andlimit total stroke to prevent damage to the production trees and riser system. The deck stopper is secured at the Spar deck level and restricts the lateral and rotational motions of the BCRT relative to the hull. Buoyancy Can. The buoyancy can provides the tension required by the riser system. It is composed of individual chambers to ensure that the failure of any one of the chamber will not compromise the riser operations. The buoyancy can chambers are filled with air or nitrogen gas.

1.0 INTRODUCTION The designer of Floating Production Systems need to determine the safe working limits for a floating platform over a period of many years. One of the most important parameters is the loads experienced by the mooring system over the life of the system or between inspections. This paper studies one such system and probabilistic description of the mooring loads. The technique depends on separating the slowly varying component from the wave frequency motions and making several assumptions concerning the statistical independence and the nature of the resulting distributions. The paper differentiates between line dynamics and mooring dynamics. Line dynamics are considered as a separate phenomena that is due to the dynamic oscillations within the mooring line while mooring dynamics are considered as the global motions of the system. Here we will not consider line dynamics, this does not imply that the line dynamics are unimportant. On the contrary, we believe that under certain circumstances they are very important and should be included in any final design evaluation, but rather that they are beyond the scope of the present investigation. DESCRIPTION OF THE SYSTEM The system modeled is a turret moored tanker with an eight point chain mooring pattern. Modeled at a scale of 1 : 53 using Froude scaling. The prototype system consisted of a 200,000 DWT tanker moored with 5000 ft of 5 1/4" chain in 1476 ft of water. The system is described in Table 1. Figure 1 gives a detailed layout of the system. A stock tanker model was outfitted as an FPSO with facsimiles of the production equipment decks in addition to the deckhouses to stimulate the wind loads. A Turret was modeled and located just aft of the watertight bulkhead between tanks 1 and 2 for a typical tanker. The breakout torque of the turret was modeled to permit weathervaning. Because of the impracticability of modeling the actual mooring system at a reasonable scale a horizontal mooring configuration was modeled (see figure 2) and the data extrapolated using STAMOOR (ref 1) to determine the actual mooring line tension. This procedure was shown to be accurate as shown in Figure 3. This is a plot of mooring line tension against surge displacement with the spring stiffness plotted through the data. It can be related directly to the displacement through the non-linear spring stiffness. At the lower turret exit a simulated flexible riser system was attached. Modeled for visual inspection only the risers were connected in a J configuration to a submerged buoy platform. This platform supported the risers connected to the bottom of the test tank. Table 2 gives the complete environmental parameters for the tests performed. TEST DESCRIPTION The tests analyzed in this paper consisted of collinear wind, wave and current parameters were set independently and superimposed upon one another to arrive at the complete environment. This is perhaps not the most realistic but it does have the advantage of simplicity and allows each individual component of the environment to be measured accurately. The principal effect of the wind and current was to add a large mean offset to the system on the order of 30 feet. Due to the predominance of the slowly varying response of the system an investigation was carried out on the effect of test length on the results.

This paper reports the design, fabrication, and testing of a soft dielectric elastomer power generator with a volume of less than 1 cm3. The generator is well suited to harvest energy from ambient and from human body motion as it can harvest from low frequency (sub-Hz) motions, and is compact and lightweight. Dielectric elastomers are highly stretchable variable capacitors. Electrical energy is produced when the deformation of a stretched, charged dielectric elastomer is relaxed; like-charges are compressed together and opposite-charges are pushed apart, resulting in an increased voltage. This technology provides an opportunity to produce soft, high energy density generators with unparalleled robustness. Two major issues block this goal: current configurations require rigid frames that maintain the dielectric elastomer in a prestretched state, and high energy densities have come at the expense of short lifetime. This paper presents a self-supporting stacked generator configuration which does not require rigid frames. The generator consists of 48 generator films stacked on top of each other, resulting in a structure that fits within an 11 mm diameter footprint while containing enough active material to produce useful power. To ensure sustainable power production, we also present a mathematical model for designing the electronic control of the generator which optimizes energy production while limiting the electrical stress on the generator below failure limits. When cyclically compressed at 1.6 Hz, our generator produced 1.8 mW of power, which is sufficient for many low-power wireless sensor nodes. This performance compares favorably with similarly scaled electromagnetic, piezoelectric, and electrostatic generators. The generator’s small form factor and ability to harvest useful energy from low frequency motions such as tree swaying or shoe impact provides an opportunity to deliver power to remote wireless sensor nodes or to distributed points in the human body without the need for costly periodic battery replacement.

SUMMARY Previous OTC papers (Refs. 1, 2, 3) have shown that drag forces on the mooring lines can represent the dominating contribution to the surge damping of moored vessels. They have also shown that in order to calculate this damping effect correctly one normally has to take into account the effect of the wave-frequency motions of the vessel, superposed on the low-frequency, resonant motions, whose damping we wish to predict. The present paper suggests a simple and practical procedure for this purpose. It applies the concept of "apparent drag coefficient" in combination with an iterative use of the catenary equations to obtain the low-frequency damping. The range of applicability of this procedure covers most practical mooring configurations. Only in cases of very deep water, heavy lines, and suspended clump weights it may be necessary to use a complete FEM procedure to get the effect of wave-frequency motions and line dynamics accurately described. The present paper also publishes for the first time results from investigations of the possible heave damping effect of the mooring system. It turns out to be normally negligible for moored ships, moderate but significant for semisubmersibles, and can be dominating the heave damping of spar buoys. 1. INTRODUCTION An important feature of most moored offshore structures is the pronounced oscillatory motions at their low, resonant frequencies. Together with the motions at wave frequencies they determine peak offsets, peak mooring line loads, riser performance, and other important engineering aspects of the structure. These low frequency oscillations take place outside the frequency range of the waves. They are excited by second order wave forces, by non-linear viscous reaction forces to wave frequency responses, and to some extent by wind gusting. Since we are dealing with low-damped systems at resonance, the LF (= low frequency) motion amplitudes are very dependent on system damping. Main contributions to for instance the surge damping of a moored vessel are due to wave drift, drag and skin friction on the main structure, and drag forces on the mooring lines. Until just a few years ago it was customary to assume that the mooring system influenced the motions of a moored structure only by its horizontal stiffness, determining the surge and sway resonant periods of the structure. Any effect of the mooring system upon the damping of resonant motions was neglected, assuming that the drag area of the mooring lines was too small to be significant, compared to the drag damping of the structure itself. Then in 1986 a first publication was issued, showing that the mooring system can be the main source of surge damping of a moored structure (Ref. 1). Further OTC publications in 1988 and 1989 (Refs.2 and 3) presented different attempts at calculating the surge damping effect of the mooring system, in addition to presenting more experimental information on the subject. The objective of the present paper is to give a survey of some recent developments of numerical methods to predict mooring line damping.

This paper presents model test and analytical correlation results of a single point moored Floating Production, Storage and Offloading tanker based system (FPSO). Both Permanent and Disconnectable turret mooring system are considered. Analytical calculation of green water effects, vessel motions, turret and mooring line loads are compared with model test results. Frequency and time domain analysis techniques of calculating line tensions are demonstrated and results are compared to the model tests measured time histories. The analysis procedure discussed in this paper is aimed toward estimating I)motions at bow relative to the water surface, 2)mooring line tensions and 3)turret design loads for a turret moored FPSO with a reasonable degree of accuracy. it is demonstrated that mooring line tensions can be calculated accurately with a finite element time domain computer simulation of the line dynamics provided the input fairlead motions are correct. Thus the method of computing time histories of mooring line tensions based on free-floating vessel motions is applicable for other spread moored floating systems in the same range of water depth. CONCLUSIONS AND RECOMMENDATIONS The model test results confirm the feasibility of both types of turret moored systems and validate the analytical tools and procedures. Results derived from the time domain line dynamic analysis agree well with the model tests. The dynamics of slack line are over estimated by using the frequency domain approach. Two methods of API mooring line fatigue calculation are compared. This paper demonstrates that a frequency domain quasistatic approach provides an efficient tool for initial sizing of mooring components. The maximum global mooring line and turret loads may be estimated by combining the frequency domain line dynamic results with the quasistatic approach. A time domain line dynamic approach can confirm and/or predict the maximum line tension with a high level of confidence, provided the vessel motion at the line top can be prescribed accurately. INTRODUCTION A turret moored 140,000 DWT tanker system is being designed for 1,000 feet of water in the South China Sea. The turret moored tanker is an attractive concept for both production facilities and offshore storage. Design of the weather vaning turret mooring system and riser system connected to the seafloor are two key system components for FPSO development. In the design of a turret mooring system the prediction of the extreme motion responses of the moored vessel is of the utmost importance. A moored tanker exposed to severe environmental conditions experiences motion responses in two distinct frequency ranges:Wave frequency responses around the wave period range (12-15 seconds).Low frequency responses around the tanker natural period range (100 seconds); The wave frequency response is generally well predictable, whereas estimates on the low frequency responses are more difficult and may vary by an order of magnitude depending on analytical tools and design procedures. Often a well executed model test program in parallel with analytical approach is imperative to ensure a reliable design of the turret mooring system.

A combined wind and wave energy-converter concept in survival mode: Numerical and experimental study in regular waves with a focus on water entry and exit

The Red Hawk Project involved the use of the world's first production cell spar and has achieved first production within 24 months of project sanction. It represents development of a deepwater field requiring significant new technology to address both technical and commercial challenges, by a major independent E&P company in record time using a small in-house staff and an effective contractor organization. This paper will provide an overview of the project, describe the in-house and contract organizations, contracting strategy, project management style, regulatory approach, HS&E, and project execution results. Introduction The Red Hawk field is a deepwater gas field located in the Gulf of Mexico approximately 210 miles southeast of Sabine Pass, Texas in Garden Banks 877. The field facilities are in both the GB877 and GB876 blocks, as depicted in Figure 1. The field development consists of a cell spar moored in approximately 5300 feet of water, and one or more subsea systems that are tied back to the spar. Kerr-McGee Oil and Gas Corporation (Kerr-McGee) is the operator and Devon Energy (Devon) is a 50% partner. The discovery well at Red Hawk was drilled in August 2001. Field Description Red Hawk is a biogenic gas field with very low condensate yield, driven by gas expansion and a moderate aquifer influence. Discovered in August 2001, it was delineated and defined through seven penetrations with wire line logs, sidewall cores, modular formation dynamic testing, flow tests, and fluid analyses. Current and future production comes from three upper Pliocene reservoirs, deposited in the down dip part of a salt-bounded mini-basin as slope fan and channel-levee facies. The two main reservoirs contain over 100 feet of high-quality sand; porosities range from 22 to 26 percent, with permeabilities between 200 and 300 millidarcies. The productive zones are approximately 11,000 to 13,000 feet below the sea floor, with up to 2,300 feet of relief between structural crest and gas-water contact. Reservoir pressure measurements in six of the wells demonstrate that the two main reservoirs are in continuous pressure communication throughout the field. In addition, seismic amplitudes conform with the structural contour of the gas-water contact, and establish lateral extent of the reservoirs. Spar Selection As Figure 2 shows, the cell spar is the third generation of spar technologies, with Kerr-McGee pioneering all three spar types with Technip Offshore Incorporated (TOI) and inventor Ed Horton [Ref. 1/2/3/4/5/6]. The first was the classic spar, shown on the far left of Figure 2 - basically a long cylinder where the upper part provides the buoyancy, and the permanent ballast in the bottom portion provides the stability. The next was the truss spar, first used by the operator at its Nansen and Boomvang fields. The truss spar was similar to the classic spar, but the lower cylindrical section of the classic spar was replaced with a truss section instead, decreasing costs and maintaining performance. The cell spar is a collection of 7 cylindrical sections, 3 of which extend as legs to the keel, which holds the permanent ballast.

Coupling effects for cell-truss spar platform: comparison of frequency- and time-domain analyses with model tests

The analysis of the influence of risers on the motions of a floating platform is often conducted and simplified by uncoupled method. As the number of risers and water depth increase, this method would not predict system motion accurately. Coupled analysis method in time domain becomes a very convenient approach in response calculation since it automatically includes the interaction among platform, mooring lines and risers. This paper introduces a full coupled approach by AQWA-NAUT to include viscous damping of the semi submersible and effects of steel catenary risers on the wave frequency response of platform in time domain motion analysis. The main conclusion of this paper is that full coupled method can accruately predict semi submersible Response Amplitude Operator (RAOs) comparing to the case without risers. Other conclusions are that risers have an important effect on the wave frequency motion of semi submersible and also lead to a phase lag with respect to platform motions.