OTC-23266 TUMSAS A New Solution for Shallow Water FPSO Mooring

OTC 23266-PP

TUMSAS: A New Solution for Shallow Water FPSO Mooring

H.Y. Zeng and X.Z Li, China National Offshore Oil Corp; J.C. Chen, and M. Chen, Guangzhou Neptune Offshore Engineering Co.LTD; J.X. Tan and Y. Jing, CNOOC Energy Technology & Services-Oil Production Services Co.; S. Shi, Houston Offshore Engineering LLC; Z.G. Li, China Offshore Oil Engineering Co.Ltd.; X. Li, Shanghai Jiao Tong University

Copyright 2012, Offshore Technology Conference

This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 30 April–3 May 2012.

This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright. Abstract

Shallow water mooring for an FPSO system is a challenge for offshore engineers. TUMSAS - Turret Mooring with Self Adjusting Stiffness system is invented to provide a new and effective solution. The work presented here includes the development of TUMSAS and supporting model tests results.

FPSOs at 20m to 30m water depth range require nonlinear mooring systems which can only provide a soft initial restoring stiffness at small vessel offsets and large stiffness at relatively large offsets. A conventional catenary turret mooring system is suitable for mid to deep water applications due to its catenary configuration only can be maintained at a small vessel offset for shallow water. Submerged soft yoke turret mooring systems are economical designs for shallow water marginal field applications, but they cannot provide long enough FPSO offset. Over stress and even total damage has been reported from applications in the China production field. TUMSAS is developed with help from various offshore engineering companies and research institutions to provide an effective and economical solution for FPSO shallow water mooring.

TUMSAS has advantages taken from both conventional catenary turret mooring and submerged yoke systems. It consists of a turret, a hanging weight module and catenary moorings connecting to the lower edge of the weight module. As a key component of the TUMSAS design, the weight module combined with catenary provides self adjusting restoring stiffness required for shallow water mooring which has been designed by one of the authors and successfully used for the permanent mooring of a retired aircraft carrier for more than ten years.

TUMSAS system has been developed and ready for FPSO shallow water mooring applications. Satisfactory results are obtained from model tests of a TUMSAS system designed for the China Bohai BZ3-2 field. The tests are conducted recently at the ocean engineering basin of the State Key Laboratory of Ocean Engineering in Shanghai Jiao Tong University. Introduction

Development of a shallow water FPSO mooring concept called Turret Mooring with Self Adjusting Stiffness [TUMSAS] system is introduced in this paper. The work presented here starts with discussion of shallow water mooring philosophy and the motivation for the development of such a system. A TUMSAS system design for China Bohai BZ3-2 field application is introduced with detailed sizing information, material properties, and a mooring/riser configuration. The working mechanism of the TUMSA system is described and comparisons to conventional soft-yoke FPSO single point mooring systems presented.

The numerical wave hydrodynamic and coupled analysis model, the analysis procedure and the load matrix are described in the document. The analyses are confirmed by the model tests performed at the State Key Laboratory of Ocean Engineering in Shanghai Jiao Tong University. The economics and economic feasibility of the TUMSAS system for shallow water mooring of offshore production and offloading FPSOs are also studied in this paper.

Motivation and Design Philosophy

Submerged soft yoke mooring systems have been designed in 1994 for a 50,000 ton tanker multipoint mooring offloading terminal (Chen 1994; Chen 2000) in Guangxi, China. These soft yoke turret mooring systems are now used for FPSO mooring in several China production fields. These systems are relatively stiff and provides limited stroke for FPSO offset. Over stress and even total structural damage has been reported for these mooring systems. Thus submerged soft yolk systems are more suitable for offshore oil off-loading in relative low environmental sea states.

One of the authors (Chen 2000) has designed a permanent mooring for a large scale floating amusement park (using a retired aircraft carrier Minsk, equivalent to a 200,000 ton FPSO in main dimension) using a combination of catenary chains, a hanging weight and a floating buoy. This is a considerably soft mooring system, which has passed extreme tests under several typhoons including Typhoon Dujuan in 2003 (Chen 2003). This system has securely provided station keeping for the Aircraft Carrier Minsk for the past twelve years. Based on this successful mooring design and allowing for the lower stiffness and larger allowable vessel offset mooring requirements for FPSOs, the fundamental concept of Turret Mooring with Self Adjusting Stiffness (TUMSAS) system is developed (Chen 2006). It essentially combines the three group spread mooring used for the Minsk into a single point turret mooring with three groups of legs. The buoy type turret is structurally connected to the bow of a FPSO to form an external single point mooring system.

The concept and studies presented in this paper focuses on the application of the TUMSAS system for 30,000 ton class FPSOs moored in Bohai Bay, off the coast of China, in 20m-30m water depth. Since most components of the TUMSAS system including FPSO external turret, mooring chain and anchor/pile system are proven technology and thus frequently used in offshore projects, this document will be concentrate on the discussion of the configurations, system behavior, and model test results of the TUMSAS system in the following sections.

TUMSAS System Configuration and Mooring Characteristics

Utilizing advantages from both conventional catenary turret mooring and submerged yoke systems, the TUMSAS system is developed for mooring of FPSOs operating in shallow water. Even though the main system design consideration is the mooring capability for high sea states, it can also be designed as a detachable system for survival environmental conditions.

Figure 1. TUMSAS Mooring System Conceptial Drawing

Mooring System Configuration and Particulars

The basic TUMSAS system configuration shown in Figure 1 consists of a turret, a hanging weight module and three or four groups of catenary mooring chains connected to the lower edge of the weight module. As a key component of the TUMSAS design, the weight module combined with catenary chains provides the self adjusting restoring stiffness required for shallow water mooring. The TUMSAS system configuration and components data listed in Table 1 are designed for a

30,000-ton class FPSO, located at 24m water depth in Bohai Bay, with key parameters provided in Table 2. This system consists of three groups of two-leg mooring chains. The three groups of mooring chains are each separated by an angle of 120 degrees. This mooring arrangement design allows multiple clusters of flexible flowlines or export risers to be installed between the mooring legs. A proposed flowline riser configuration is shown in Figure 2. In this design, the lazy wave shaped risers are rigidly connected to the hanging weight module with bend stiffeners. Risers between the turret and the hanging weight module are connected with flexible pipes and support their self weight.

Figure 2. Lazy Wave Type Flowlines for Bohai Oil Field Application

Table 1. TUMSAS Mooring System Properties

Table 2. FPSO Key Parameters

Shallow Water Station Keeping Mechanism

The TUMSAS system works as a two-body (the vessel and the weight module) interactive station keeping system. The vessel (FPSO) considered as the first body is vertically connected to the second body of the hanging weight module with short chains, which provide an initial soft lateral restoring stiffness within limited offset. The weight module is connected to the seafloor using long catenary chains. The lifting of the heavy weight module and the catenary ground chains provides a stronger restoring force, and eventually the ground chains are pulled in tension. This multi-stage nonlinear restoring vs. offset behavior provides an ideal station keeping mechanism for shallow water FPSO mooring. The initial soft stiffness could significantly reduce wave load on the mooring system, while the subsequent increased stiffness limits the FPSO maximum offset and protects the risers from over stretching. The hanging weight module is normally placed below the FPSO keel level. This placement not only solves the interference problem between the external turret mooring lines and the hull, but also provides a porch with reduced motion for supporting flowline and export flexible risers.

The TUMSAS mooring stiffness is self adjusting based on the relative displacement of each component of the system, the hanging weight module properties, and the design of the ground chains. The maximum vessel displacement of the system then can be determined for the worst design environmental condition. Graphical representation of the mooring stiffness is provided in the following section.

Behavior of TUMSAS Mooring System

Static analysis results of relationship between FPSO offset and restoring force are presented in Figure 3 for the TUMSAS system described above. The legends in the figure, HF, HB, LF, and LB represents high tide fully loaded, high tide ballasted, low tide fully loaded, and low tide ballasted conditions respectively. For comparison, the stiffness curve (YOK) obtained from a SZ36-1 oil field FPSO Single Point Mooring soft-yoke system in about 32 m water depth is also plotted.

Figure 3. TUMSAS Mooring System Stiffness Curves

It can be seen that TUMSAS is a softer system, which allows larger (about twice) vessel offset for the same level of mooring restoring force. The maximum allowable vessel offset of TUMSAS system is determined based on several conditions. The contact between the vessel hull and the mooring/riser system is restricted, the hanging weight and bottom of vessel are not allowed to touch the seabed for all design conditions, and the maximum stretching of the moorings and risers is limited. The compliant behavior due to the softer stiffness and larger allowable vessel offset offers the TUMSAS system the advantage of significantly reduced wave load on the mooring system.

Hydrodynamics and Global Performance Analysis of TUMSAS System

The hydrodynamic and global coupled motion analyses are conducted for extreme and operational environmental conditions. The analyses involved calculation of the hydrodynamic coefficients, building a two-body coupled analysis model, and performing dynamic time domain simulations.

Hydrodynamics Wave Diffraction and Radiation Calculation

The hydrodynamics wave diffraction and radiation coefficients are calculated in the frequency domain using second order panel method by the program WAMIT (WAMIT Inc., Theory and User Manual). Both the FPSO section below MWL and the surrounding water are panelized (see Figure 4 below). Due to the low water depth, the accurate calculation of low-frequency (LF) wave-force quadratic transfer functions (QTFs) becomes important, especially to compute the mooring tensions (Ma et al 2008). Thus free-surface integrals are required to calculate the complete LF QTFs on the FPSO. The wave hydrodynamic coefficients calculated using WAMIT was verified by the Bureau Veritas mooring and hydrodynamic team using their program HydroSTAR (Bureau Veritas, HydroSTAR For Expert).

Figure 4. FPSO and Water Surface Hydrodynamics Panel Model

Global Coupled Analysis Model

Coupled analysis of the TUMSAS system is performed using the dynamic analysis package Orcaflex (Orcaflex Inc., Theory and User Manual). The FPSO and the hanging weight module are treated as two separated rigid bodies connected with three chains and catenary mooring lines/risers anchor the weight module to the seabed. The mooring lines and risers are modeled with nonlinear large deformation lumped mass/stiffness elements for dynamic analysis. The hydrodynamic coefficients calculated by WAMIT are directly input into the Orcaflex coupled analysis program. The Orcaflex analysis model and its rendered 3D plot are presented in Figure 5.

Figure 5. TUMSAS Mooring System Analysis 3D Model and Randering

Analysis Load Cases

The 100-year environmental load cases listed in the table below are applied in the coupled analysis. The load cases include (1) wind, wave and current co-inline, and (2) wind and wave co-inline, 90 degree from current environmental directions. The environmental directions are also combined with the high tide and low tide water depths and the FPSO in fully loaded and ballasted conditions. Since, the TUMSAS mooring system allows the FPSO to freely weathervane, the environmental loads are both applied inline and between the mooring groups in the design case matrix. In general, Table 3 has covered the most critical conditions for the design and evaluation of the TUMSAS mooring system.

Case No. Load Condition Description

Wate

r

Dept

h

Wind

Speed

(m/s)

Wind

Direction

(deg)

Wave

Hs

(m)

Wave

Tp

(s)

Wave

Direction

(deg)

Current

Speed

(m/s)

Current

Direction

(deg) Table 3. Model Test and Coupled Analysis Case Matrix

TUMSAS System Model Testing

Model tests for the TUMSAS system designed for the China Bohai BZ3-2 field were conducted recently at the ocean engineering wave basin of the State Key Laboratory of Ocean Engineering in Shanghai Jiao Tong University. The scale of the tested model is 1:40. The basic setup and equipment used in for the model test are listed below: ?Six DOF motion of the non-contact optical measurement

?Three resistance type wave height meter

?Three-direction load cell for the measurement of the total mooring load at turret

?9 axial load cells for measuring mooring line tensions

? 1 accelerometer for the measurement of FPSO accelerations in 3 directions

? 2 underwater digital video cameras for recording FPSO, mooring and riser movement

? 1 digital camera for taking snap shot of model test

The single FPSO under 100-year environmental conditions and the FPSO with offloading tanker in tandem are tested with irregular waves. Digital snapshots of the model test arrangement are presented in Figure 6.

Figure 6. Model Test Arrangement and Setup

Simulation and Model Testing Results Comparison

Comparison of the Station Keeping Characteristics of the Mooring System

Figure 7 shows the total static restoring force vs. vessel offset for the 24m water depth case. The vessel is offset along the direction of the mooring lines (inline) and also between the mooring lines. The target line is obtained from Orcaflex static analysis for the vessel offset in both cases. The discrete dots in the plot are the measured values, which confirm that both the model test system and the analysis systems have the same mooring stiffness.

Comparison of Dynamic Performance under Survival Environmental Condition

Even though, both the 100-year single FPSO and the FPSO with tandem offloading tanker cases are tested, the latter case is never the controlling case for mooring line tension and riser strength. Thus only the single FPSO system under the 100-year environmental condition case is summarized in Table 4 below for comparison. The results are obtained from irregular wave tests and time domain computer simulations for the same sea state. Figure 8 captures some of the representative moments of the model test.

Figure 8. Snap Shots from the Model Test

Table 4. Comparison of Model Test and Analysis Results for the Survival Condition From the comparison of model test and analysis results for the survival condition summarized in Table 4, we can see that the model test FPSO maximum and minimum FPSO surge are 15.72 m and -17.76 m respectively. The corresponding results from the computational analysis are 13.34 m and -12.195 m respectively. In this case, the surge motion form model test is larger than analysis results. In the other 5 degree of freedom of FPSO motions the analysis results are found to have larger amplitudes especially for the minimum values. Therefore the analysis model is seen to underestimate the FPSO heave and roll damping. Comparing the tensions of the three chains connecting between the FPSO and the weight module, the analysis is seen to slightly overestimate the chain maximum tension while the minimum chain tensions are found to be closely matched. Similarly, the analyses overestimate the mooring line maximum tensions while the minimum mooring line tension is reasonably well estimated. Over all, the results from the model test and the analysis are comparable. The model test results could be used for further calibration of the analysis model, especially the vessel heave and roll damping which are difficult to be determined theoretically.

Conclusion

The model test and analysis results have confirmed that the TUMSAS shallow water turret mooring system designed for the China Bohai BZ3-2 field could provide the 30,000 ton class FPSO a maximum offset about 18 m under the 100-year wave condition. The maximum offset from the TUMSAS system is about twice that of a soft-yoke system and as a consequence the wave load on the mooring system can be significantly reduced. The maximum mooring load of 290.7 ton obtained from the dynamic analysis corresponds to a safety of factor of 3.5, which is significantly larger than the API RP 2SK specified allowable safety of factor of 1.67 for mooring intact condition. In addition it is seen that an acceptable margin of safety is obtained for the upper chains which vertically connect the FPSO and the hanging weight module, if the same size of 122 mm diameter chain is used.

Neither the model test nor the analysis is shown to have any contact or interference between the moorings /risers and the FPSO hull. It is also confirmed that the hanging weight module has significantly reduced motions compared with the turret. Thus connecting the risers to the weight module is highly recommended. The lazy wave flowline riser configuration could provide sufficient length to compensate for the large TUMSAS system motions.

Based on the overall behavior and the structural/mechanical requirement of the proposed TUMSAS turret mooring system, we can conclude that it is an effective and economic single point mooring system design. It is found to be very competitive solution for shallow water mooring of offshore production and offloading. Thus the application of TUMSAS system for marginal shallow water oil file developments can be attractive and feasible.

The critical cases for design have been presented in the work outlined in this paper. Since the TUMSAS is a new concept of development, further comprehensive detailed design, studies and optimizations of the system will be necessary in

the future. There are also additional riser configurations that could be investigated when a specific offshore project is involved in the TUMSAS system development.

Nomenclature

FPSO - Floating Production, Storage and Offloading

TUMSAS - Turret Mooring with Self Adjusting Stiffness mooring system

DOF - Degree Of Freedom

LF – Low Frequency

Soft Yoke –Tower Soft Yoke Single Point Mooring Systems

QTF - Quadratic Transfer Function of wave load or motions

Acknowledgement

The authors are thankful for the valuable support and help provided by Bureau Veritas (BV). The contributions of the BV R&D, mooring and hydrodynamic team including Dr. Xiaobo Chen, Dr. Huatao Jiang, Brice Le Gallo, Haixia Wu, and Yu Cong for their verification of FPSO hydrodynamics calculations and mooring system evaluations is gratefully acknowledged. The authors want to also thank the personnel of Houston Offshore Engineering including Jenny Yang and Nishu V. Kurup for their efforts on coupled analysis and riser system configuration and design.

References

[1] Chen, J. 1994. Fan Weathervane Counterweight Mooring Design Patent. Chinese Patent No. ZL94220404.2.

[2] Chen, J. 2000. A New Concept of Multipoint Mooring System: Fan Weathervane Multipoint Counterweight Mooring (FWMCM), Proceedings of the Tenth International Offshore and Polar Engineering Conference Seattle, USA, Vol 2 , 89-93.

[3] Chen, J. 2003. From Minsk Aircraft Carrier Permanent Mooring System to the Application of Fan Weathervane Counterweight Mooring Design in Offshore Oil File Development. China Ocean Engineering Conference, Ningbo, Zhejiang, China.

[4] Chen, J. 2006. Advanced Turret Mooring System Design with Improved Restoring and Stiffness Characteristics. Chinese Patent No. 200620059347.4.

[5] WAMIT Inc., Theory and User Manual, https://www.360docs.net/doc/b217486577.html,.

[6] Bureau Veritas. HydroSTAR For Expert, v2.0 - Reference Guide and Tutorial for Naval and Offshore Hydrodynamic Application, 2003.

[7] Orcaflex Inc., Theory and User Manual, https://www.360docs.net/doc/b217486577.html,.

[8] API RP 2SK, Design and Analysis of Stationkeeping Systems for Floating Structures, 3rd Edition, October 2005.

[9] Ma, S., Shi, S., Kim M.H. 2008, Second-Order Low-Frequency Wave Forces on a SPM Offloading Tanker in Shallow Water, Proceedings of the ASME 27th International Conference on Offshore Mechanics and Arctic Engineering, Estoril, Portugal. OMAE2008-58048