David Petruska

Defining, Measuring, and Calculating the Properties of Fiber Rope Deepwater Mooring Lines

This paper explains and defines the synthetic fiber rope stretch and stiffness change-in-length properties which are important for deepwater platform mooring system design. It introduces the concept of accumulated elastic stretch, which causes temporary stretching and stiffening of the rope during tension cycling. It proposes how these properties can be determined by rope testing and by computer modeling. It proposes how the change-in-length properties should be used in mooring system design and analysis. Background The offshore industry and the fiber rope manufacturers have now had some experience in using fiber ropes in deepwater platform moorings. Petrobras pioneered this in the mid 1990s. In the late 1990s several class societies 1 2 3 and then API 4 published guidelines for fiber rope deepwater moorings. Tension Technology International with others also published fiber rope design guidelines. Those guidelines were based on what was then perceived to be the best design methods and the best test methods for the rope properties needed to carry out those methods. Since those guides were published, several fiber rope mooring systems – Mad Dog 6 7 and Red Hawk – have been designed for the Gulf of Mexico. A number of large fiber ropes have been tested and analyzed for use in those and other projects. Also, findings are now available from several significant fiber rope research programs which were conducted while and after those guides were written. INTRODUCTION The unique stretch and stiffness characteristics of synthetic fiber ropes are important in the design and analysis of deepwater platform mooring systems. Mooring system designers are experienced in designing with wire rope and chain catenary mooring lines. The stiffness characteristics of wire rope and chain are essentially linear. These components do not experience permanent stretch until high tension. With these components, the principal mooring system restoring force is produced by the catenary effect. As mooring water depths increase, there are incentives for using fiber ropes, as discussed in other papers. But with fiber rope, the principal mooring system restoring force is produced by the tension vs. stretch (stiffness) characteristics of the rope instead of by the catenary effect. Mooring system designers are still trying to understand the unique stretch and stiffness characteristics of fiber ropes and to incorporate them into traditional mooring design procedures. The stretch and stiffness characteristics of typical synthetic fiber ropes are time dependent and non-linear. Fiber ropes experience both elastic and permanent stretch. Conflicting and confusing terminologies are sometimes used. Some important rope properties have only recently been adequately understood and explained. Here the term stretch refers to any change in rope length which is produced by tension. Stiffness is the relationship between stretch and applied tension. Other terminology is introduced in the text and summarized in Nomenclature at the end of this paper. The paper is divided into four parts: ! Part 1 uses a spring-and-dashpot analog model to describe the principal synthetic fiber rope stretch and stiffness characteristics. ! Part 2 illustrates some of these characteristics with results from several large rope test programs. ! Part 3 discusses how these characteristics should be used in a deepwater platform mooring analysis . ! Part 4 describes how these characteristics might be determined through rope testing and through computer modeling of the rope. Fiber Rope Deepwater Mooring Lines OTC 16151 Defining, Measuring, and Calculating the Properties of

Riser System Selection and Design for a Deepwater FSO in the Gulf of Mexico

must contain conspicuous acknowledgment of where and by whom the paper was presented. Abstract The process used to select a riser concept for a deepwater Floating Storage and Offloading system for the Gulf of Mexico is presented. Numerous riser concepts were screened with three taken forward through the more rigorous concept selection process. The three riser configurations were the Steel Lazy Wave Riser, the Single Line Hybrid Riser and the Tension Leg Riser. A system approach was adopted where the turret location, the mooring system and the risers were designed together. The process involved rigorous evaluation to verify both technical and installation feasibility, along with engineering definition sufficient to establish cost estimates. Analysis results for the three risers are presented to demonstrate important design issues that must be investigated to make an informed riser selection.The process used to select a riser concept for a deepwater Floating Storage and Offloading system for the Gulf of Mexico is presented. Numerous riser concepts were screened with three taken forward through the more rigorous concept selection process. The three riser configurations were the Steel Lazy Wave Riser, the Single Line Hybrid Riser and the Tension Leg Riser. A system approach was adopted where the turret location, the mooring system and the risers were designed together. The process involved rigorous evaluation to verify both technical and installation feasibility, along with engineering definition sufficient to establish cost estimates. Analysis results for the three risers are presented to demonstrate important design issues that must be investigated to make an informed riser selection. Introduction The recent increase in discoveries in deep and ultra-deep water, coupled with fast-paced development schedules for Floating Production Systems, has led to a rapid evolution in the design of risers in terms of their complexity as well as variety. In addition, ship-shaped systems, including both Floating Production, Storage and Offloading systems (FPSOs) and Floating Storage and Offloading systems (FSOs), are often the preferred option for various technical and commercial reasons. Equipped with turrets, they can be highly functional solutions even in harsh environments. However, the motions of ship-shaped systems are typically more severe than the motions of other types of floating facilities due to their heave, roll and pitch motions being in the same frequency range as the wave energy. This further complicates the design of the risers. Consequently, riser systems that may be feasible on a Tension Leg Platform, Spar or semi-submersible–based facility may not always work on an F(P)SO. Particularly in harsher environments, this requires the F(P)SO riser system to have either a more compliant or a de-coupled configuration compared to simple catenary or toptensioned vertical risers. A study was performed to assess the technical feasibility and commercial viability of several riser options for a 2million barrel new-build FSO studied for possible deployment in approximately 1,370 meters water depth in the Gulf of Mexico. Steel Lazy Wave Riser (SLWR), Single Line Hybrid Riser (SLHR) and Tension Leg Riser (TLR) concepts were investigated in detail. To establish technical feasibility, an integrated system approach was adopted where the FSO hull form, turret location, mooring system and riser system were investigated jointly to capture the important interactions between key components. To assess technical feasibility of the risers, extreme hurricane conditions were investigated to check allowable stresses and compression in the riser and to determine top termination requirements in terms of tension and rotation. Fatigue analyses were also performed. Both wave frequency fatigue and slow drift fatigue were examined along with fatigue due to Vortex Induced Vibration (VIV). The technical assessment confirmed that the three riser concepts are feasible for the conditions specified in the study. Note, however, that the SLWR was close to the fatigue limit while the SLHR and TLR, which de-couple their steel riser segments from the motions of the vessel, exhibited good fatigue performance. The commercial assessment was made by developing screening-level, total-installed cost estimates for each riser concept, including the cost impact on associated systems such as the turret. For this study, the SLWR was shown to be the most cost-effective system. However, under different parameters, the SLHR or the TLR could be more cost competitive. This depends on many factors such as water depth, number and size of risers, metocean conditions, vessel motions, turret location, turret loading, field layout and footprints, soil conditions, seabed topography and flow assurance requirements. Each of the systems has unique performance, cost and applicability in view of these different influences. Economics usually drives the riser selection, but risk is also important and must be considered in the process. FSO Particulars The vessel assumed in this study was a new-build “tanker” OTC 14154 Riser System Selection and Design for a Deepwater FSO in the Gulf of Mexico D.J. Petruska, C.A. Zimmermann, K.M. Krafft, B.F. Thurmond / BP America, A. Duggal / FMC SOFEC 2 D.J. PETRUSKA, C.A. ZIMMERMANN, K.M. KRA FFT, B.F. THURMOND AND A. DUGGAL OTC with a more or less elliptical bow and a storage capacity of approximately 2 million barrels. The length was 285 meters, the beam was 63 meters and the depth was 30.5 meters with a fully loaded draft of 19.5 meters and ballast draft of 8 meters. Both loaded and ballast conditions were considered in the investigation of riser performance. The tanker was to be turret-moored in deep water (1,370 meters). In order to investigate the important effect of turret location on FSO responses, three different turret locations were evaluated: 0.31L, 0.35L, and 0.4L forward of amidships. The turret was designed to accommodate up to six risers of 406.4 millimeters outer diameter (16-inch OD). Metocean Conditions Typical deepwater Gulf of Mexico environmental parameters were used for hurricane conditions, loop/eddy current events, and everyday fatigue sea states. Traditional mooring/riser design recipes for Floating Production Systems in the Gulf of Mexico often rely on the application of 100-year extreme metocean events that assume collinear waves, wind and current. However, the floating facility considered in this study was sensitive to non-collinear conditions as it was turretmoored and weathervaned passively. Consequently, designer sea states that reflect the environment’s true non-collinear behavior were selected and used based on recommendations from industry standards and results from long-term responsebased analyses performed on similar vessels. These designer sea states are shown in Table 1. Preliminary Riser Sizing Preliminary pipe wall thickness was established using API RP 1111 [1]. The pipe material was API 5L X65. Maximum internal operating pressure was taken to be 3447 kiloPascals with a product specific gravity of 0.9. Both the SLWR and the TLR designs assumed the pipe would be installed in a voided condition, with wall thicknesses governed accordingly by collapse due to external pressure. A relatively thick wall was selected for these risers, 19 millimeters (3/4 inch). The SLHR, however, was assumed to be installed flooded and would never be in the empty/vented condition throughout its design life. As a result, its design was not governed by collapse and a thinner wall was selected, 12.7 millimeters (1/2 inch). Further, since this was to be an FSO that received stabilized crude, flow assurance was only a minor concern, and no riser insulation was required. The system still needed to accommodate pigging operations, however. Global Analysis A diffraction analysis was performed for both the ballast and fully loaded conditions to generate RAOs for input into the vessel motions program Shipsim [2] and global analysis program SPMsim [3]. SPMsim provides a fully coupled frequency domain analysis of turret moored vessels, mooring and riser systems. Preliminary global analysis of the FSO was performed to: • Develop a preliminary mooring system. • Determine vessel motions (in particular, heave at the chain table) for use in selecting an optimum turret location for the three riser options. • Provide extreme offsets and motions for use in the riser analyses. Turret Location Selection For weathervaning and mooring performance, it is desirable to have the turret as far forward of amidships as possible. However, as the turret is moved forward, the heave at the chain table increases and the performance of the riser system degrades. In the end, a compromise location for the turret must be selected in view of these drivers. Results from other studies such as reference [4] have shown that for turret locations more than 0.3L forward of amidships, the roll response tends to change very little as the turret moves further forward. On the other hand, the heave tends to increase more rapidly as the turret moves more forward. Thus, the primary parameter investigated for selecting the turret location was heave motion at the chain table, as this was also the response that influenced riser performance the most. Figure 1 shows the single amplitude significant heave motion at the chain table as a function of turret location for the fully loaded vessel. Over the selected range of turret locations, the response increases fairly linearly. Of the three riser systems, the SLWR is the most sensitive to the heave motion as its continuous riser segments are connected directly to the turret. The SLHR and TLR are less sensitive since their steel segments are de-coupled from the vessel. With the SLHR and TLR, only the flexible jumpers between the vessel and the subsurface support buoy are significantly impacted by the heave-induced compression (and also dynamic tensio

Polyster Mooring for the Mad Dog Spar - Design Issues and Othe Considerations

The Mad Dog Project will use a polyester mooring system on the drilling and production truss spar. This will be the first use of a permanent polyester mooring system on a Floating Production System (FPS) outside of Brazil and the first time polyester has been used on a spar. As such, there were many challenges, which include regulatory approval; designing a mooring system which is dominated by current loadings; largest polyester rope break load ever required; rope design qualification and testing; quality control and assurance; and inspection, maintenance, repair and retirement (IMRR) of such a mooring system. This paper will focus on the design issues, rope design, manufacturing, qualification and testing, and the IMRR plan that was developed to provide the assurance that the polyester mooring system could be safely operated over the 20-year field life.

EFFECTIVE RISER SOLUTIONS FOR A DEEPWATER FPSO

With the recent increase of discoveries in the deepwater Gulf of Mexico (GoM) and the rapid deployment of deepwater floating production systems, the design of dynamic risers to produce and export to and from these FPS has quickly evolved in complexity and variety. As one of the attractive solutions for the development of these deepwater discoveries, the Floating Production, Storage and Offloading (FPSO) system offers a serious challenge to the riser system designer. This paper presents detailed results of a three steel risers systems design study to a turret moored FPSO system in 1,370 meters (4,500 feet) water depth in the GoM. The three riser systems considered are the Steel Lazy Wave Riser (SLWR) system, the Tension Leg Riser (TLR) system and the Single Leg Hybrid Riser (SLHR) system. The three riser concepts are shown to be feasible. The paper shows that for the diameter, number of risers and water depth considered, the SLWR is the preferred option. However, under different parameters, the TLR or SLHR may be preferred since they fully decouple the steel portion of the riser from the vessel motions via flexible jumpers. Very often, riser feasibility can only be demonstrated by doing a complete and thorough evaluation as demonstrated in the paper. The results of this paper demonstrate that steel riser options are available and present effective solutions for use on an FPSO system in the GoM.Copyright