1. Introduction
Ocean waves contain a seemingly inexhaustible amount of energy, which can be both a source of destructive power for nearshore and offshore structures and a conspicuous source of renewable clean energy. With respect to the former, various types of marine structures that can withstand the ocean environment have been developed. These include ships for cargo transportation and offshore operations, platforms for marine resource exploitation, and buoys for environmental monitoring. On the other hand, the development of structures that allow for wave energy conversion and conservation on a commercial scale has been difficult, slow, and expensive [
1].
Since Salter’s paper [
2], there have been a number of initiatives and developments in the field of wave energy in various parts of Europe as well as the rest of the world. For instance, thousands of patents have been filed and many studies have been performed on the different ways of converting wave energy at different water depths and locations into a useful form [
3]. Fixed or floating oscillating water columns (OWCs), oscillating body systems, and overtopping converters can be used. In addition, the power take-off (PTO) mechanism has also been proposed, including self-rectifying air turbines [
4], (low- and high-head) water turbines, and (high-pressure oil-driven) hydraulic motors. A detailed review of the mechanical PTO equipment available for wave energy conversion can be found in Salter et al. [
5].
Considering the purpose of the integration wave energy-offshore platform, the OWC mentioned above seems to have a great potentiality for its simple mechanics and flexibility [
6]. In general, OWCs are based on wave-to-pneumatic energy conversion and utilize ocean waves to drive the motion of a water column within a semi-submerged pneumatic chamber. The movement of the water column causes bidirectional airflow and pressure fluctuations within the chamber. The pneumatic energy is then converted into mechanical energy by a turbine or other PTO system that is connected to a generator to transform the mechanical energy into electricity. Such devices can be categorized based on their location, onshore or nearshore fixed OWCs and offshore floating OWCs [
7,
8]. With respect to the former, numerous investigations have been performed over the past few decades, including analytical, experimental, and numerical studies [
9,
10]. Moreover, full-sized prototypes equipped with self-rectifying air turbines, generally still in the research and development phase, have been built in Norway, Japan, India, Portugal, the UK, and, more recently, in China [
11]. On the other hand, offshore devices are more complex in terms of their hydrodynamic interactions and energy balance. Hence, there have been fewer experimental and numerical studies on these devices [
12,
13]. Further, there is a growing interest in multi-oscillating water columns (M-OWCs), which can be defined as an array of OWCs coupled together either in terms of their related structure, airflow, PTOs, or generators [
14].
Based on the underlying principle of wave energy conversion, in this study, a new embedded wave energy converter (EWEC) system that can be attached to a tension leg platform (TLP) was developed. The EWEC system uses tuned liquid column dampers (TLCD) to absorb the energy from the hull motion resulting from the wave loadings, based on the sloshing of the working liquid. The fluctuation-free surfaces at the two ends of the working liquid column are similar to those of conventional OWCs. Therefore, with the installation of an air chamber and self-rectifying turbines above, the oscillating motion of the internal free surface generates an airflow through the turbine, which drives an electrical generator. This energy conversion chain of the TLP-EWEC coupled system can be simplified as follows: Wave motion - platform motion - liquid column oscillations - pneumatic oscillations - turbine rotation. As per this scheme, the conventional production platform would act as an energy-producing wave energy farm and contribute to the energy mix and even help achieve self-sufficiency in terms of electricity generation and consumption. On the other hand, the serviceability of the platform may be improved, as some of the hull motion energy is collected by the TLCDs. Brief descriptions of TLPs and TLCDs are given below.
TLPs are compliant offshore structures generally used for deep-water oil production or as floating foundations of offshore wind turbines. They consist of a semi-submersible-type floating platform that is attached to the sea bottom with vertical tethers, which are kept perpetually taut by the excess buoyancy of the platform. For the sake of linearity, studies on
TLPs usually assume that all translational displacements and angular displacements are small in magnitude [
15]. However,
TLPs may experience distinctly large motions in certain sea states, and the attendant nonlinearity would affect their dynamic behavior. Therefore, the theoretical model proposed by Zeng et al. [
16,
17] was adopted in this study to consider the concomitant nonlinear factors induced by finite displacements. On the other hand, the evolution of offshore platforms has made it possible to think of other opportunities parallel to the traditional sole function. As a pioneer, the European Union launched “The Ocean of Tomorrow” [
18], a call for proposals for multi-use offshore platforms in 2011. In 2014, Maribe launched the Horizon 2020 project [
19] to determine if there is a future for investment in combining Blue Growth sectors. In 2018, two other projects, Space@Sea [
20] and Blue Growth Farm [
21], began. The former project intended to provide a sustainable and affordable workspace at sea, while the latter designs multipurpose offshore floating platforms that host aquaculture and energy harvesting. More related details can be found in the literature [
22,
23]. In fact, the existing multifunctional platforms are mainly in concept evaluation. To the best of our knowledge, there have been few reports on the combination of wave energy and offshore production platforms, let alone establishing dynamic models for performance assessment.
TLCDs are passive controlling devices used in building structures as auxiliary mass systems. Recently,
TLCDs have attracted significant interest because of their numerous advantages, such as their low installation cost and simple maintenance requirements. Studies on
TLCDs for buildings have ranged from investigations of their fundamental characteristics to evaluations of their control performance, optimization techniques, and control strategies [
24]. In addition, the feasibility of using
TLCDs with offshore systems, especially for offshore wind turbines, has also been explored by various researchers [
25,
26,
27]. However, as is the case for
TLP systems, there is a lack of deep research on the design issues related to
TLCDs because of the difficulty in finding suitable co-locations for them [
28,
29]. Lee et al. [
30] studied a
TLP-
TLCD system with three-DOF motions both analytically and experimentally. Lee et al. [
31] developed an experimental testing method for investigating the vibration mitigation effect of a
TLP system equipped with an underwater
TLCD and subjected to surge wave motions. There have been few studies on the dynamic performance of such
TLP-
TLCD systems with six DOF. When a damper is used to suppress one DOF motion, its influence on the other DOF motions is very complex, and it is hard to simultaneously suppress motions with multiple DOF. In addition, there has been little research on the use of multi-
TLCD systems (M
TLCD) in offshore platforms. Some studies have suggested that M
TLCD would be more advantageous as opposed to a single
TLCD, owing to the detuning issues involved [
32]. However, there have been no reports on the use of
TLCDs in wave energy applications.
This manuscript is arranged as follows. A general description and a multifold nonlinear analytical model of the
TLP-EWEC system are given in
Section 2. In
Section 3.1, the hydrodynamic interface model of the coupled system is validated, and a specific site was selected for detailed design. Next, the results of six-DOF motions and preliminary generating capacity of the system are discussed in
Section 3.2 and
Section 3.3. In
Section 4, the comprehensive assessment of the coupled model and the prospect of such multi-use offshore platforms are discussed. Finally,
Section 5 lists the primary conclusions of the study along with the future research directions.
4. Discussion
In the results for the proposed
TLP-EWEC system, in
Section 3, the effectiveness of the multi-use design from the point of view of hydrodynamic responses and energy-generating capacity was presented. When the hydrodynamic responses of the
TLP-EWEC and ISSC
TLP, presented in
Figure 7 and
Figure 8, were compared, it was shown that the additional devices did not change the structural characteristic of such production platform, but improved several DOFs’ motion with limited deterioration on other DOFs. That is because there are difficulties in achieving six-DOF motion suppression just with
TLCD devices for their passive energy-absorbing capacity [
46]. Moreover, results in
Section 3.2.2 proved that improvements can be expected in certain DOFs of the
TLP-EWEC system and the deterioration in other DOFs is limited when the orifices are open completely or partially. Thus, it can be concluded that the
TLP-EWEC met the requirements for offshore production operations as the ISSC-
TLP if its EWEC system was switched on for energy generation.
Section 3.3 indicated that the
TLP-EWEC system can generate a considerable amount of electric power and serve for offshore oil and gas production in the target oil fields. As the EWEC was designed in resonance with the peak wave frequency, the internal liquid motions were larger than the hydrodynamic hull and provided strong air flow for energy harvesting. For the selected three heading angles, HA = 0°, 22.5°, and 45°, the total average powers were 67.69 kW, 37.144 kW, and 52.52 kW, respectively (see
Figure 12). Fast estimation of the AEP reported that the yearly energy production is significantly higher, ranging from 310 to 564 MWh/year, with regard for wave heading angles, which may effectively contribute to the platform energy mix or even improve self-sufficiency in electricity consumption. Moreover, given this effect of the OR on the response of the structure (in
Section 3.2.2) and the generating capacity (in
Figure 12), it can be extrapolated that the extents of the damping-related dissipation by the orifices and turbines are complementary. Therefore, the orifices can also act as protection devices to prevent the overloading of the turbine groups.
As per this multi-use design, the conventional production platform located a few hundred kilometers from the coast was able to act as an energy-producing wave energy farm. It can alleviate the high electrical load and even help achieve self-sufficiency in terms of electricity generation and consumption. On the other hand, the serviceability of the platform was improved, as some of the hull motion energy is collected by the EWEC. It must be mentioned here that in this paper only the TLP was used as the hydrodynamic hull preliminarily, but the concept can be spread to other types of offshore platforms (e.g., semisubmersibles and mono-column spars). Actually, the TLP structure has strict requirements of motions and would limit the productivity in calm sea conditions. Therefore, other type of platforms which permit greater movements during operations could excite the EWEC system more fully and it will be vastly more productive.
5. Conclusions
In this study, a preliminary design for an offshore TLP that combines a common offshore platform with a novel EWEC system was proposed for additional energy production function. To assess its feasibility, a multifold, nonlinear, analytical model of the coupled system was established for operation simulations and a specific site was selected for detailed design. The hydrodynamic computing program was validated and simulated the coupled system with different wave heading angles (HA = 0°, 22.5°, and 45°) and orifice ratios (OR = 0.00, 0.25, 0.50, 0.75, and 1.00), calculating both the six-DOF motions of the TLP hull and the power output of the EWEC system. We elaborated on the hydrodynamic behavior and generating capacity of the TLP-EWEC system. Based on the obtained results, the following conclusions can be drawn.
Firstly, the TLP-EWEC can serve for offshore production operations just as the ISSC-TLP, regardless of whether its EWEC system was switched on or off. The comparation of the six-DOF motions between the TLP-EWEC and the ISSC TLP indicated that the new design kept similar structural characteristics of such production platform and improved several DOFs motion with limited deterioration on other DOFs. Further investigation indicated that the hydrodynamic responses of the TLP-EWEC are insensitive to changes in orifice ratios.
Secondly, the assessment of generating capacity clearly showed that the TLP-EWEC system can generate a considerable amount of electric power in the target oil fields. The total average powers are 67.69 kW, 37.144 kW, and 52.52 kW for HA = 0°, 22.5°, and 45°, while the yearly energy production is ranging from 310 to 564 MWh/year. Additional benefits and profitability for the offshore platform energy mix and motion controlling were proven effective and worthy of further application.
Lastly, the TLP-EWEC design added new concepts for the next generation of offshore platforms and probed the possibility of wave energy sharing with oil and gas exploitation for offshore platforms. This scheme can be spread to other types of offshore platforms which permit greater movements during operations for higher energy production (e.g., semisubmersibles and mono-column spars). Moreover, the multifold, nonlinear, analytical model presented here can also help to overcome challenges in similar multi-use designs, primarily, among other benefits, the shared use of common offshore infrastructures.
It is clear that the TLP-EWEC has not been fully developed, gaps in research remain, and a remnant of potential outcomes can be investigated. Future research should be directed at the energy output/investment or levelized cost of energy (LCOE) of this new kind of WEC, the development of certain measures to tune the EWEC, or by combining the EWEC with other types of platforms for both high structural hydrodynamic performance and good energy-generating capacity.