1. Introduction
As China’s first floating production platform in ultra-deepwater, the “Deep Sea No. 1” energy station is a milestone in China’s deepwater resource utilization. The energy station is located in the LS17-2 gas field, 150 km off the southeast coast of Hainan Island, China. It is a semi-submersible platform (
Fig. 1) with a displacement of 101 thousand tonnes and an operational draft of 35 to 40 m. The platform is permanently moored in 1422 m water by 16 chain-polyester-chain mooring lines in a 4 × 4 pattern, and six steel catenary risers (SCRs) are attached to the platform. It is the world’s first and only semi-submersible platform with the function of condensate storage, so it can be regarded as a floating production storage and offloading (FPSO) unit. With the ability to produce 3 billion m
3 of natural gas each year (enough for over 10 million families), the Deep Sea No. 1 energy station is a key step toward China’s energy independence. The LS17-2 gas field, where the Deep Sea No. 1 energy station is located, was discovered in 2014. Plans for its development were made in 2015, followed by research and a preliminary design. Deep Sea No. 1 went into operation on June 25, 2021, and will operate onsite continuously without dry-docking for 30 years.
Prior to the Deep Sea No. 1 energy station, China’s floating pro-duction platforms were located in shallow waters (i.e., a water depth below 300 m); thus, as the first ultra-deepwater production platform in China and the first semi-submersible FPSO unit in the world, the Deep Sea No. 1 energy station project presented huge technical challenges.
(1) The condensate issue. According to the exploration data, the natural gas from LS17-2 is high in hydrocarbons (over 98%), and the main products are natural gas and condensate. As conden-sate is a highly volatile and explosive product, the method used for condensate processing and transportation directly affects the safety and economic feasibility of the project, making this the main consideration in the development plan.
(2) General safety. FPSO units are quite vulnerable to impacts from offloading vessels; moreover, the high explosivity of conden-sate adds to the difficulty of processing and storing it. These safety concerns brought challenges to the structure design of the platform.
(3)
The harsh environment. The South China Sea is widely rec-ognized as one of the harshest ocean environments in the world.
In situ measurements and adjacent-area records were utilized to pro-vide precise environmental parameters. Based on these data, it was determined that LS17-2 is a typhoon-prone area, with an average of 11 typhoons per year and an extreme wind velocity of 66 m s
−1. The significant wave height of a 100-year sea state (i.e., a wave height expected to occur only once in 100 years) is 13.4 m, with a maximum wave height of 23 m [
1,
2]. Furthermore, strong currents with an extreme current velocity of 2.39 m s
−1 have been observed in the LS17-2 area. Compared with the Gulf of Mexico and the North Sea, which are renowned for their extreme sea states [
3-
5], the Deep Sea No. 1 energy project’s envi-ronmental conditions constituted a huge challenge. For oil and gas exploitation in harsh environments, the high loads on the floating structure require the floating platform to have a special design of the floating platform and the mooring and riser systems to be strengthened, leading to higher costs for design, fabrication, and installation.
To meet these challenges above, the Deep Sea No. 1 energy sta-tion project team proposed and applied a series of new theories and methods in research and design, making pioneering technical achievements.
2. Technological achievements
2.1. Development plan
In oil and gas exploitation, the overall development plan (ODP) serves as a cornerstone for project profitability. The development plan comprises the overall arrangement of devices and the deter-mination of structural configurations. Improper development plans will raise the cost or even make the project unprofitable. As China’s first ultra-deep gas field, the ODP of the LS17-2 gas field had to maximize economic yields while ensuring technical feasibility.
2.1.1 Overall arrangement
LS17-2 is an ultra-deepwater gas field with a water depth rang-ing from 1220 to 1560 m, located 55 km away from shallow-water areas (i.e., water depths of 160-170 m). Its proven reserves have been shown to comprise over 100 m3 of natural gas. The possible existing infrastructures on which the development of LS17-2 could depend were YC 13-1 jacket platforms and their gas pipelines to Hong Kong, China.
The LS17-2 gas reserves are distributed in a scattered pattern, with seven blocks located over an area of 30.4 km × 49.4 km; 11 wells have been developed, with one or two wells in each block, and the distances between blocks are generally more than 5 km [
6]. Development of the LS17-2 gas field has been designed to maintain stabilized production for 10 years, with an annual gas production of 3 billion m
3 and 1400 m
3 of condensate per day [
7,
8]. To achieve this production goal, two candidate plans for devel-opment were proposed: Plan 1, an FPSO unit with dynamically positioned (DP) shuttle tank vessels for condensate transport; and Plan 2, long-distance export pipelines and a shallow-water jacket platform. In Plan 1, only natural gas would be exported through pipelines; in Plan 2, condensate piping would require the construction of a 135 km-long pipeline, calling for an addi-tional investment of 800 million CNY [
9]. Considering economic feasibility, the FPSO plan was selected (
Fig. 2), and the project for China’s first ultra-deepwater platform was initiated.
2.1.2. Selection of platform type
To meet the functional requirements, all possible platform types were investigated, including ship-shaped FPSO units, tension leg platforms (TLPs), semi-submersibles, Spar platforms, and cylin-drical FPSO units. TLPs have excellent motion control and are suit-able for dry tree operation [
10]; however, their weight sensitivity makes them unqualified for condensate storage. Ship-shaped FPSOs are widely adopted in deepwater oil exploitation; however, flexible risers and single-point mooring are essential for the LS17-2 gas field due to the large motions a ship-shaped FPSO unit would have, making this choice economically unfeasible compared with the options applicable to SCRs [
9].
Taking both functional requirements and economic feasibility into consideration, the potential candidates were a Spar platform, a cylindrical FPSO unit, or a semi-submersible. Spars are applicable to dry tree operation and are SCR-friendly; however, the project’s wet tow and upending of the hull and its onsite integration were unprecedented in China, indicating high risks and costs. For a cylin-drical FPSO unit, due to the large rotational motions it would have, the twist of the upper end of the riser would be about 25° [
8], limiting the application of SCRs. For semi-submersibles, SCRs were not an unprecedented choice [
11,
12], and the construction tends to be more controllable due to previous experiences with semi-submersible drill platforms. Based on these considerations, a semi-submersible platform was determined to have the lowest risks and costs and was taken as the final choice.
2.2. Design concepts
As the world’s first semi-submersible FPSO unit, the design of the Deep Sea No. 1 energy station was globally unprecedented. Moreover, as China’s first ultra-deepwater platform, its construc-tion had numerous constraints due to the dimensions of the avail-able construction fields. Innovations the design concepts of the platform were essential in order to achieve the optimal functional performance within these barriers.
Unlike traditional semi-submersible production platforms, the Deep Sea No. 1 energy station was intended for condensate storage; thus, due to condensate production and offloading, the platform’s center of gravity would greatly vary [
13], negatively influencing the stability and motion of the platform. Moreover, ship yard and channel dimensions set constraints on the dimensions of the plat-form. In most shipyards in China, the limitations for a light ship draught are below 9 m, making this the governing parameter for the loadout and towing [
7,
9]. Additionally, for platforms with SCRs, the motions of the platform increase the loads on the SCRs. There-fore, to ensure the security of the risers, strict restrictions were set on the platform motions, based on the strength and fatigue limits of the SCRs. The harsh environmental conditions added to the difficul-ties in the platform’s motion control. To overcome these barriers, a hull design based on the concept of
wide columns, flat pontoons, and a deep draft was proposed (
Fig. 3).
Based on previous engineering experience, the width-to-height ratio of a pontoon is usually below 2.0; however, due to the limita-tions in construction, the pontoons of Deep Sea No. 1 have a width of 21 m and a height of 9 m, resulting in a width-to-height ratio of 2.33. This large width-to-height ratio will add to the heave damp-ing ratio; compared with a ratio of 1.0, the damping ratio is increased from 2.05% to 2.74%. However, the wide pontoons would increase the wave loads on the structure. To lower the wave loads induced by the wide pontoons, the operational draft of the plat-form was increased to 35-40 m; in contrast, the operational drafts of most semi-submersible production platforms are below 30 m.
Regarding condensate storage, if the condensate storage com-partments were located in the pontoons, the ballast water would have to be in the columns, which would raise the platform’s center of gravity; moreover, the offloading pipeline would have to pass through all the columns to reach the upper deck, adding to the risks in condensate offloading. Therefore, it was considered prefer-able to store the condensate in the columns [
14,
15]. Thus, in the Deep Sea No. 1 energy station, the condensate is stored inside the columns, while the ballast water is stored in the pontoons. In this design, widening the columns will enlarge the condensate storage while increasing the platform’s stability.
2.3. Triple safety assurance for the semi-submersible production and storage platforms
For an FPSO unit, offloading operations are quite frequent; con-sequently, the risk of collision between shuttle tankers and the platform is high. If condensate leakage occurs during processing, storage, or impacts from shuttle tankers, a catastrophic occurrence would be inevitable [
16]. As the world’s first semi-submersible FPSO unit, innovations in the overall safety of the platform were essential.
To ensure the overall safety, the world’s first U-shaped conden-sate storage and isolation design—inspired by the inner liners of vacuum flasks—was created for the Deep Sea No. 1 energy station project. In this design, eight types of compartments are tightly arranged in the columns (
Fig. 4), with the capability of storing 20 000 m
3 of condensate oil, 9000 m
3 of ethylene glycol, and other media. During the structural and processing design, the concept of triple safety assurance was proposed and applied.
2.3.1. Storage safety assurance
To ensure the security of the condensate storage in the columns, protection techniques applicable for the offshore, airtight, and con-stant-pressure storage and offloading of volatile and low-flash-point media were proposed, five levels of shielding is achieved via reflux stabilization, energy control, integrated storage and offloading, stacked pressure relief, and physical isolation [
17]:
Level 1: stage-by-stage reflux stabilization technology. Based on the multi-stage flash vaporization principle, the condensate oil was stabilized, ensuring that the saturated vapor pressure of the fluid entering the compartments meets the requirements.
Level 2: energy-source control technology. By adopting conser-vative processing standards, the risk of overpressure in the con-densate oil tanks was greatly lowered. Two emergency shutdown valves were installed on the condensate tank inlet line to rapidly cut off the energy source in case of emergency.
Level 3: an integrated condensate oil loading/offloading/trans-fer operation process. The condensate tank transfer header, loading header, and offloading header enable flexible scheduling of the loading/offloading process in order to handle complex operating conditions; moreover, a rapid switch to diagonal compartment export was made possible.
Level 4: a depressurization technique involving superimposed high and low opening setting vent valves. Both the inert-gas-supply-and-venting main header and the inert-gas-purging-and-displacement main header were equipped with two sets of high and low opening setting vent valves to ensure rapid and effec-tive depressurization under all operating conditions.
Level 5: a physical-isolation-based collision and leakage protec-tion system. A 1.8 m-wide physical isolation compartment was arranged around and underneath the condensate oil storage com-partments, while the inlet/outlet lines and valves of the condensate oil tanks were positioned away from the crane operating paths. The interior of the condensate oil tank was covered with a foam fire-fighting system.
2.3.2. Offloading safety assurance
The column sections of a traditional semi-submersible platform are loop-shaped, with a large central void area. For the Deep Sea No. 1 energy station, if a conventional structural configuration were to be adopted, the central compartment would be used for condensate storage (
Fig. 5); however, due to the large span of this compartment, the bulkhead thickness would need to be unexpect-edly wide in order to meet structural safety requirements. This would require the use of a large amount of steel. For more economi-cal condensate storage, the concept of a cruciform column design was proposed and applied to the compartment design. In this design, the central part of the column is divided into four sections, three of which are used for condensate storage (
Fig. 5).
The new design saved 2860 tons of steel in comparison with a traditional design. Furthermore, the new design increased the plat-form’s resistance to ship collisions. Finite-element analyses (FEAs) that were carried out on impact scenarios [
18] indicated that the structural strength alone can withstand the impact of a head-on collision with a 4.7 m s
−1 shuttle tanker (with a full load displace-ment of 21 kilotons) at various parts of the columns. In addition, rubber fenders, anti-collision frames, and other buffering protec-tion structures were arranged on the outer bulkheads, further increasing the collision energy the columns can withstand.
2.3.3. Fatigue safety assurance
To address the challenges caused by high waves with a wide range of periods in the South China Sea, where the structural fati-gue damage is several times greater than that in the hurricane area of the Gulf of Mexico, a structural technique known as the
multi-hole reduced-span bulkhead was proposed (
Fig. 6). This bulkhead reduces the stress by 15% and increases the fatigue life by 280% [
18].
Large openings were arranged at the top and bottom of the bulkheads, and flow-through holes were provided on the bulk-heads, fully ensuring the flowability of the condensate oil. Utilizing the continuous overall connection form of the bulkheads ensures the continuity of the structural transitions. This technology fully considers the flow and ease of flushing and cleaning inside the compartments, while providing higher fatigue strength and ensur-ing safety.
2.4. Mechanism research during the design
An understanding of the dynamic behaviors of the Deep Sea No. 1 energy station under the winds, waves, and currents of the South China Sea was vital for the platform design. Since SCRs were selected for the project, exact evaluations of the motions were essential. The motion of the platform is closely related to the plat-form’s principal dimensions, which are restricted by the construc-tion capacity, channel depth, and so on. To characterize the effects of the mooring lines and SCRs in the mooring and stability analyses, numerical models including the mooring lines and risers were established, and coupled analyses were performed. As the Deep Sea No. 1 energy station is China’s first production platform in the ultra-deep waters of the South China Sea, its motion charac-teristics are unique, introducing uncertainties into the operations. Therefore, several mechanism studies were conducted on the motion patterns of a semi-submersible production platform in the South China Sea.
2.4.1. Nonlinear relationship between the metacentric height and the roll/pitch motion
Unlike conventional semi-submersible production platforms, the Deep Sea No. 1 energy station is designed to store up to 20 000 m
3 of condensate in its columns. As the amount of stored condensate oil increases, the platform’s center of gravity will rise by as much as 3 m. This rise of the center of gravity will lead to a change in the metacentric height and the natural periods of the pitch and roll, affecting the seakeeping performance of the plat-form. The natural period of the roll and pitch of the Deep Sea No. 1 energy station is over 40 s; therefore, the first-order motions are considerably smaller than the low-frequency motions, which are excited by nonlinear second-order forces. In order to ensure production safety, special attention was paid to the second-order motion characteristics of the deep draft semi-submersible via statistics obtained through time-domain analysis results and wave basin tests [
19,
20].
Fig. 7 demonstrates the nonlinear relationship between the metacentric height and the low-frequency pitch motion of the plat-form. Four regions are established based on whether the motion or stability meets the safety requirements. It can be seen that a lower metacentric height and more significant low-frequency pitch motion can be expected as the center of gravity rises. Therefore, improvements in stability will lead to better motion performance; that is, wider columns will benefit both platform stability and the motion.
2.4.2 Parametric excitation of the roll/pitch motion
When a regular wave comes along the surge direction of a float-ing platform, it is traditionally believed that only the surge, heave, and pitch motion modes will be excited, while the other three degrees of freedom, (i.e., the sway, roll, and yaw) will not be excited. However, a nonlinear phenomenon called parametric exci-tation may occur when the natural period of the roll is twice the period of the regular wave. This is due to the periodic change in the metacentric height. Since the heave motion is periodically excited by the wave force, the metacentric height and roll restoring moment also change periodically, inducing a rapid increase in the roll/pitch motion in certain cases.
Fig. 8 demonstrates the stable and unstable regions of the damped parametric excitation equation for the Deep Sea No. 1 energy station,
$\delta$ is a dimensionless factor, defined as:
$\delta=4\left(T_{3} / T_{\mathrm{n} 4}\right)^{2}$, where
T3 denotes the heave motion period and
Tn4 denotes the natural roll period. When 2
T3=
Tn4 and
$\delta$=1, minor disturbance in the heave may lead to instability in the roll.
Fig. 8 demonstrates this critical condition for the Deep Sea No. 1 energy station, which is designed to be located in the stable region.
2.4.3. VIM response under waves and currents
The vortex-induced motion (VIM) response of a semi-submersible platform is a resonant structural response phenomenon that occurs when the periodic vortex shedding frequency generated by fluid flow around the columns coincides with the natural frequency of the moored platform, inducing amplified oscillatory motions in the horizontal direction [
21]. In the traditional analysis method, only the vortex shedding under currents is considered, and its contribu-tion to the fatigue damage of the mooring lines and risers can be high as 47%-70%. Neglecting the coupling characteristics of the waves and currents in the VIM responses in the South China Sea will lead to overestimating the fatigue damage of the mooring lines and risers. This will result in unnecessary strengthening of the mooring lines and risers and will thereby increase the costs.
To investigate the VIM response mechanism of the Deep Sea No. 1 energy station under waves and currents, a series of model tests were conducted. Different current velocities, wave heights, and wave periods were combined to determine the VIM responses of the platform under various conditions.
Fig. 9 demonstrates the relationship between the VIM response and the current velocity under different conditions, including current-only conditions, operating conditions, and survival conditions. In comparison with the VIM response under current-only conditions, it can be seen that the VIM responses under the operating and survival conditions are less significant. Under the operating and survival conditions, the periodic water particle motions will disturb the vortex shedding, suppressing the VIM responses of the platform. In general, the amplitude of the VIM response could decrease by 20%-60% due to the coupling effects of waves and currents, increasing the fatigue life of the mooring lines and SCRs to some extent.
3. Conclusions and prospects
As the world’s first semi-submersible FPSO unit and China’s first ultra-deepwater platform, the Deep Sea No. 1 energy station is based on a series of innovations made during the planning, design, and mechanism research of the platform, in fields with no prece-dent works. Thanks to these innovations, an optimized economic yield was achieved while ensuring safety and technical feasibility.
(1) As the world’s first semi-submersible FPSO unit, the Deep Sea No. 1 energy station has been designed and constructed with high economic feasibility and low risks. The platform has been operating smoothly since its commission in 2021. As of the end of 2023, the Deep Sea No. 1 energy station had produced 6.25 bil-lion m3 of natural gas and 4.18 million barrels of condensate, mak-ing a profit of 14.3 billion CNY and indicating good economic feasibility. Since the commission of the platform, six typhoons have passed through with no accident occurrence, proving the reli-ability of the design.
(2) The Deep Sea No. 1 gas field project is a demonstration project, marking the expansion of China’s oil and gas exploitation into deep water. The experiences gained from its design, construction, and operation will guide the exploitation of gas reserves in the South China Sea, enabling China to be the third country in the world to independently develop ultra-deepwater gas fields.
(3) As the world’s first semi-submersible FPSO unit, the Deep Sea No. 1 energy station provides a Chinese solution to the exploitation of ultra-deepwater gas reserves, which make up 70% of deepwater gas fields.
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