A finite element based study on lowering operation of subsea massive structure

Subsea production system has been increasingly used in recent South China Sea offshore developments. With deepwater applications, constituent parts of subsea systems become more complicated and enlarged. Increases on the weight and geometry of each component bring challenges to installations. A recent accomplished deployment on a subsea massive jumper shows the weight and length have been up to 120 tons and 90 m, respectively with sophisticated geometry. It is considerably difficult to install heavy and large subsea structures, especially in South China Sea where severe environmental conditions are common. In addition, deepwater deployment may alter natural frequency of the hoisting system and the altered frequency may be close to possible environmental conditions. To deal with the above two issues, traditionally, engineers need to carry out series of complicated numerical analyses which are on case basis and significantly time-consuming. Existing studies focus on the optimization on analysis techniques by conducting laboratory testing and numerical simulations. However, easy-to-use guidance on massive subsea structure installation are somewhat limited. The studies presented in this paper aim to achieve a simplified guidance which can briefly screen the cases subject to axial resonance and provide visible correlations between hoisting system integrity and key installation parameters.


Introduction
Subsea rigid jumper is a key connection component of subsea production system and has been extensively adopted in recent oil and gas development. However, with deepwater applications, the jumpers' diameters and weights become increasingly larger and configurations are getting more complicated. Subsequently, it brings great challenges to installation operations. Crane lifting and deployment approaches have been generally employed to install subsea equipments. It is essential to assess the wire capacity, structural strength and hoisting system safety before operations start. A typical crane installation operation in deepwater mainly consists of four phases (DNV, 2011). Objects are firstly lifted off from decks and maneuvered to the planed positions. Secondly, the objects are lowered through splashing zone, where considerations on effects of slamming, wave, buoyancy and self-weight are needed. Attentions to slack condition of the hoisting wires are required. The third phase is underwater lowering operation, in which vertical oscillation of the lifted object can be a significant factor. Landing is the final stage. Horizontal offsets and motions of the objects, which are mainly influenced by the low-frequency horizontal motions of the vessel as well as ocean current, significantly affect installation accuracies of the objects. Allowable operation weather window is dependent on the assessment results of the above four steps. Phase 2 has been generally found as the most challenging step from engineering practice. Heave compensations equipments are used mainly for Phase 2 by reducing dynamic loads or increasing payloads. In Phase 3, oscillation may occur in deepwater application since natural frequencies of the hoisting systems rise approaching environmental conditions. Dynamic behaviors of objects and deployment equipments during installation are generally predicted through numerical simulations and results together with criteria are used to determine operational window accordingly. Extensive studies have been conducted. Sekita et al. (1986) proposed a unified analysis-a method that considers the interaction between a crane barge and its hoisted load-as applied both to the motion analysis of the barge-load system subjected to waves and to the transient motion analysis of the barge crawling while lifting a load. Roveri et al. (1996) presented a case study on a subsea production manifold deployed in 620 m water depth at Campos basin, offshore Brazil. A sensitivity study to assess the added mass, damping, stiffness and excitation forces influences on the system response and the importance of such parameters in the computer model calibration has been included. Another manifold project in 1860 m water depth at Roncador field in the Campos Basin, offshore Rio de Janeiro was presented by Galgoul et al. (2001). Extended studies on the axial resonance has been made by assuming the water depth increased to 3000 m. Kimiaei et al. (2009) proposed an OrcaFlex based simplified numerical approach for the accurate estimation of hydrodynamic forces on subsea platforms and validated the proposal by comparing DNV guidelines (DNV-RP-H103). Legras and Wang (2011) developed an offshore lifting monitoring system which predicted the lifting activity by adopting real-time vessel motion into the time-domain numerical simulations. Dynamic analysis on jumper lowering operations has been discussed by Wang et al. (2011a) and Sun et al. (2015).
However, existing investigations either focus on adjacent pipeline or tie-back operations or conduct generic studies with limited depth of research. Investigations on subsea massive structures installation, especially for splashing zone, are less understood. Furthermore, a simplified guidance which can briefly screen the cases subject to the axial resonance and provide visible correlations between hoisting system integrity and key installation parameters is limited. This paper presents an advanced analysis approach on massive jumper installation process and suggests simplified analysis guidance. Detailed engineering determines key installation parameters such as the selection of PHC and the maximum wire rope tensions. In order to provide better understanding of dynamic response of the installation process, systematic parametric studies on deployment activities have been conducted. Full-time history of performance on key installation parameters including wire tensions, stroke length, and environmental conditions influence, etc. have been recorded. Results have been systematically studied and used to achieve a simplified and easy-to-use guidance for massive subsea structures deployment activities, which are somewhat limited in existing literatures. A series of parametric analysis for the massive structure lifting operations have performed. The dynamic tensions of the hoisting wire have been also assessed under the different wave period conditions.

Phase 2-splashing zone
Numerical simulations have been widely adapted to plan subsea structures deployments. Key installation parameters are determined according to simulation results. OrcaFlex has been widely recognized in industry for dynamic analysis of offshore marine systems and used to perform numerical simulations of lowering operation through splash zone. Lowering through splash zone is believed to be one of the most challenging scenarios in offshore lifting process. The study presented in this paper focuses on this phase.
In one field development in South China Sea, a 22′′ gas pipeline is tied back to the center platform by a series of jumpers, one of which is illustrated in Fig. 1 and Table 1. The total weight in air and the projected length of the deployed integrated object including crane upper rigging, spread frame, lowering rigging and jumper are 400 tons and 90 m, respectively. The maximum depth from the horizontal view and the width from the plan view are 13 m and 45 m, respectively. Air weight of each connector welded to the end of jumper is close to 18 tons. A larger spreader frame has been subsequently introduced to provide stability during installation. In order to reduce dramatically increased dynamic loads and avoid lifting wire slack conditions in the circumstance of lowering operation through splashing, contractors have employed a PHC (Cramemaster®), working load and stroke range of which are 400 tons and 4.2 m, respectively. By adopting OrcaFlex based numerical models, simulation of the lowing activities has been made to predict structural response during operations. Results of dynamic analysis such as the maximum and minimum crane wire tensions, PHC strokes and lowing velocities are used to plan and optimize offshore installation procedures.
2.1 Finite element analysis (FEA) 2.1.1 Element type of the system Hoisting system including jumper has been modelled through OrcaFlex in-built elements, which are capable of accurately representing the physical behaviours with the mini-   Table 2. Slamming effect has been considered. Deepwater crane/pipelay vessel named "HYSY201" has been employed in the project and parameters of the vessel have been used in modelling. Fig. 2 presents an illustration of the vessel lifting activity.

Data input
Environmental incidence (α) is defined as the direction of propagation measured from the vessel positive x-axis, as illustrated in Fig. 3. This implies that the incidence angle of 0° is an environment from the stern, the incidence angle of 180° is a head environment, and the incidence angle of 90°i s an environment travelling from starboard to portside. Key parameters of vessel "HYSY201" and hoisting system have been given in Table 3 and Table 4

Modelling
After the jumper is lifted off from the transportation barge, the crane vessel will move to the installation location. The jumper will then be lowered through the splash zone. Considering the length of more than 200 m and the draft about 9 m of HYSY 201, shielding effect may significantly reduce wave heights in leeward areas, especially for short wave periods. The effect is neglected for a conservative concern. In addition, findings obtained through this study and others (Sun et al., 2015) show that cases with the wave heading of 90° give the highest wire rope tensions and wave heading being 90° has been selected for parametric studies from the perspective of conservatism. Significant wave height 2.5 m is the maximum workable environmental conditions for the vessel. According to local Metocean data, the occurrence probability of wave period less than 12 s is 98.22%. A total of 78 combinations are considered, as shown in Table 5. An OrcaFlex model for the splash zone analysis is shown in Fig. 7.

Parametric study
There are two steps in the parametric study: screening and dynamic analyses. The process of deploying structures through the splashing zone to fully submerged conditions generally requires about 40 m length of wire rope being released. Screening analysis has been conducted to obtain the worst position in terms of the released wire length. The worst position will be subsequently used for the corresponding dynamic analysis, where currents being set the same incidence of 90° as waves for conservative concerns. The relationships between the maximum dynamic wire rope ten-sions and wave parameters including heights and periods have been investigated. The effects of the passive heave compensation (PHC) have been quantatively studied. A briefly design guidance has been suggested accordingly. In Step 1, the jumper is lowered with a minor velocity of 0.01 m/s in the screening analysis. Both static and dynamic analyses have been conducted. Dynamic amplitude factors (DAF) are obtained by dividing dynamic wire rope tensions by static one. The DAF is plotted against jumper lowering distance in Fig. 8, where two load cases are presented. The results clearly show critical positions where the spread frame is just submerged, as shown in Fig. 7. In Step 2, dynamic analyses have been conducted on numerical models with the worst positions acquired by the screening analyses. Dynamic results are presented and investigated in following sub-sections.

Wire rope tension
The relationships between wave periods and the maximum wire tensions have been presented in Fig. 9. Results are categorized by the significant wave heights. Maximum wire tensions have been found insensitive to wave periods. Although vessel motions may vary due to different wave periods (RAO), results show minor coupling effects of the vessel motions on the structures hoisting activities through the splashing zone, on the condition that PHC is employed. In addition, physical behaviors of the jumper together with the spread frame and slings are affected by varied wave periods.    In cases of wave periods equaling 2 s, models with the wave heights equal to 1.5 m, 2.0 m and 2.5 m show a dramatic increase on the maximum wire tension since oscillations being found, as shown in Fig. 10. According to target location met ocean data, the occurrence probability of the wave period equal to 2 s is almost non-existent. Correlations between the maximum wire tensions and wave heights have also been investigated and results are given in Fig. 11. It clearly shows that the larger the wave heights are, the larger maximum wire tensions will be. This is because larger wave heights may cause severe motions of the integrated jumper, slings, rigs and spread frame.

Passive heave compensation (PHC) effect
The contributions of the PHC on dynamic response reductions have been quantatively investigated. Contrastive analyses have been carried out on models with and without PHC. Fig. 12 presents two groups of results where the significant wave heights (H s ) equal to 0.5 m and 1.5 m, respectively. Comparisons show that when H s =0.5 m, little reduction effects of PHC have been found. However, in the case of a larger H s , significant reductions effects of PHC can be achieved. Since PHC works as a damping, higher jumper dynamic behaviours may cause larger damping response. In addition, the extents of the tension reductions increase with lower wave periods. Because higher frequency of waves may limit damping recovery and higher damping effect is subsequently achieved.

PHC stroke
Relationship between PHC strokes and environmental conditions have been studied and presented in Fig. 13. Larger wave heights cause larger strokes due to larger dynamic behaviors of the deployed objects caused by more severe environmental conditions. Under the condition of the same wave heights, lower wave periods may induce larger strokes since strokes have less time to recovery.

Summary
A brief guidance on the correlations between key lifting parameter and environmental conditions has been summarized in Table 6. For example, with the increase of wave period (T↑), PHC reduction effects decrease. This guidance may be beneficial to either design stage for installation optimization or offshore activity for quick decision making.

Dynamic equilibrium
After an object being lowered fully submerged, the hoisting system can be simplified as a purely linear, springmass-damper system with single degree of freedom (DOF). The mass-spring system consists of the jumper, wire rope      Fig. 14 and corresponding details are given in Table 1 and Table 4. Vessel motions are generally assumed not to be affected by object motions. Natural frequency of the hoisting system T n can be obtained by Eq. (1). Details are given by DNV-RP-H103 (DNV, 2011) where, is the equivalent mass of hosting system DNV-RP-H103 (DNV, 2011) and K is the hosting system total axial stiffness DNV-RP-H103 (DNV, 2011)

Parametric study
The natural frequency of the jumper has been obtained and parametric studies in which the jumper is assumed to be deployed in deeper water depths have been conducted. Results are presented in Fig. 15 where a limit of 4 s is shown. Annual wave period distributions in the areas where the practical jumper is installed, have been presented in Fig. 16. The distributions are typical in South China Sea and clearly show that the wave period is seldom below 4 s.

Summary
A simplified screening guidance, on if assessment on the hoisting oscillation, is needed has been made. For lifting and deploying subsea massive jumper or similar beam-based structures in the South China Sea, if water depth is deeper than 400 m, a detailed engineering analysis is suggested to evaluate oscillation issues.

Conclusion
Demands for subsea structures installation have been continuously increased with increasingly applications of subsea production system. With deepwater developments, design of subsea facilities become more sophisticated and geometry and weight of constituent subsea objects become increasingly complicated accordingly. This paper studies the lowering activity of subsea massive structures. Deploying objects through the splashing zone is widely recognised as the most challenging step in subsea deployments, especially with massive structures under severe environmental conditions. This study proposes an effective coupling dynamic analysis by employing OrcaFlex package. Coupling effect between vessel motions and hoisting system has been considered. Converge problems have been successfully overcome by suitable selections of modelling elements and application of advanced modelling techniques. The analysis has been validated by practical projects. Validated numerical models have been used to parametric studies. The relationships between dynamic behaviours of the hoisting system and environmental conditions have been achieved. The effects of Passive Heave Compensations (PHC) have been quantatively investigated. A brief guidance on correlations between key lifting parameter and environmental conditions has been achieved.
It is found that natural frequencies of wire in the hosting system increased with deeper developments. Axial resonance amplification may be inevitable in extra deepwater developments. Parametric study of natural frequencies on water depths together with local environmental distributions have been employed to achieve a simplified guidance, i.e. wire oscillation should be checked in details when the sea depth is over 400 m. This can be used for a brief estimation on the offshore construction.    WANG Fa-cheng et al. China Ocean Eng., 2017, Vol. 31, No. 5, P. 646-652