As we aim for carbon neutrality by 2050, the adoption of offshore wind power as a renewable energy source is rapidly expanding. The electricity generated by offshore wind turbines needs to be transmitted to land through power cables. However, due to the importance of wind and other environmental factors in selecting turbine locations, addressing obstacles like existing structures and coastal fisheries, especially at cable landfall points, has become a key challenge. The Horizontal Directional Drilling (HDD) method is gaining attention as a means to overcome these obstacles. This technique allows for cable installation without demolishing existing coastal structures or disturbing the seabed, thus addressing environmental concerns. In this paper, we introduce the HDD method, a trenchless technology that minimizes environmental impact while enabling the effective installation of offshore wind power cables.

1. Introduction

Horizontal directional drilling (hereafter referred to as HDD) was developed in the United States during the 1970s for the purpose of constructing various pipelines across rivers. This technique adopts the vertical drilling technology used in oil drilling and applies it in a horizontal direction. It is additionally characterized by its ability to proceed in an arc-shaped path from the entry point to the exit point, allowing for relatively unrestricted pipeline installation. This technology has made it possible to install underground pipelines without disturbing bulkheads or existing structures, which cannot be accomplished with conventional techniques, such as the open cut method. Moreover, HDD, unlike typical drilling techniques, supports long-distance operations up to 1,000 meters and can handle diverse geological conditions, from bedrock, like limestone and sandstone, to softer ground, such as sand and clay.

Recently, as global warming intensifies and the demand for renewable energy grows, the HDD method has become a popular choice for the landfall of many submarine cables. This trenchless technique not only helps to reduce environmental impact but also offers a maintenance-free and cost-effective solution for operators by eliminating the need to bury cables in the seabed.

2. What is Horizontal Directional Drilling (HDD)?

The following is an explanation of the drilling principles behind the HDD method. The drilling assembly comprises a drill bit attached to the bottom of the drill pipes, a mud motor, which is a device that uses pressured drilling fluid from a mud pump to rotate the drill bit through a helical shaft and pipe, and a survey probe. The drilling is carried out by jetting drilling fluid from the drill bit, which is supplied by the mud pump, while rotating the drill pipes and applying thrust with the HDD rig to advance the drill pipes.

A survey probe is used to accurately monitor the borehole as it is being drilled to ensure it proceeds according to the designed drill path, as shown in Fig. 1.

The pilot drill process is terminated just before the drilling reaches the seabed, and the drill pipes are pulled out of the borehole and replaced with the product pipes.

Once the drill bit travels through the borehole and punches out on the seabed, a diver cuts off the drilling assembly and installs a wire inside the product pipes. This wire is used to string the cable through the pipes thereby concluding the installation of the pipeline as show in Fig. 2.

3. HDD for Offshore Wind Power

3.1 Offshore Wind Power in Japan

As the focus on expanding renewable energy intensifies, offshore wind power is playing an increasingly significant role. Currently, Japan is in its second round of calling for applications for offshore wind power businesses to engage in the installation of 21 turbines off the coast of Akita Prefecture's Oga City, Katagami City, and Akita City; 38 turbines off the coast of Murakami City and Tainai City in Niigata Prefecture; and 28 turbines off the coast of Eshima in Saikai City, Nagasaki Prefecture. These projects are scheduled to commence operations between 2028 and 2029. Additionally, in round one of the project, 38 wind turbines were installed off the coasts of Noshiro City, Mitane Town, and Oga City in Akita Prefecture, with a total power generation capacity of 478.8 MW. Off the coast of Yurihonjo City in Akita Prefecture, 65 wind turbines were installed, achieving a power generation capacity of 819 MW, and off the coast of Choshi City in Chiba Prefecture, 31 turbines were installed, achieving a power generation capacity of 390.6 MW, marking steady progress towards the implementation of wind power generation. Round three has now commenced, with calls for business bids currently ongoing in two areas off the coasts of Aomori and Yamagata Prefectures.

Japan is steadily progressing toward its future goals, which include achieving 10 GW of offshore wind power by 2030 and realizing a carbon-neutral society by 2050. However, to meet these targets, it is urgent to accelerate project development and shorten development timelines while simultaneously striving to reduce environmental impact.

3.2 Landfall of Offshore Wind Power Cables

To deliver the electricity generated by offshore wind power to the land, cable landfall is essential (photo). Similar to communication cables, the primary method considered for bringing offshore wind power cables ashore in Japan involves using a cable-laying vessel and divers to excavate the seabed and bury the cables. However, this excavation method presents several challenges, including: (1) ensuring the cable-laying vessel can approach within a certain distance of the coastline, (2) ensuring there are no existing coastal structures, such as bulkheads, near the shoreline that could impede the cable landfall, and (3) ensuring the vicinity of the landfall site is free from environmental concerns, such as coral reefs or fishing equipment.

Furthermore, the locations for offshore wind power installations are determined based on wind conditions, which limits the flexibility in selecting sites for cable landfall. However, the HDD method is expected to be widely adopted in many projects, as it allows for cables to be routed beneath existing coastal structures, thereby minimizing environmental impact.

3.3 Landfall of Offshore Wind Power Cables Using HDD

NEC Networks & System Integration Corporation has extensive experience utilizing the HDD method for the landfall installation of submarine communication cables. Given that the diameter of these communication cables is approximately 30 mm (Fig. 3), the conduits (product pipes) installed typically have a diameter of less than 200 mm. Normally, the cable installation at a single landfall site involves 1 to 2 conduits.

In contrast, offshore wind power transmission cables have diameters ranging from 100 to 200 mm as shown in Fig. 4, necessitating conduits with diameters of 300 mm or more, which are larger than those used for communication cables. Additionally, with several dozen offshore wind turbines planned for the region, the number of required landfall conduits increases to approximately 6 to 10. This results in a more extensive and large-scale construction effort.

In the HDD method, installing large-diameter conduits requires more advanced techniques. For communication cables, conduits with diameters of 200 mm or less typically involve only pilot drilling. However, installing conduits with diameters of 300 mm or more necessitates an additional reaming process following the pilot drilling. In conventional HDD projects, such as the installation of pipelines that cross rivers, reaming is commonly performed by attaching a reaming tool on the exit side and pulling it back to the entry side of pilot drilling.

In contrast, for offshore wind power cable conduits, because the drill exits into the sea, the reaming must be performed by pushing from the entry side to the exit side, as shown in Fig. 5. This push through method increases axial stress on the drill pipes within the reamed borehole, raising the risk of buckling. Thus, careful attention to the thrust and rotational torque during drilling is crucial for safe operation.

Furthermore, when installing power cables into the ground, the thermal resistance of the ground can cause an increase in cable temperature, thereby reducing transmission capacity. To counter this, the drill path needs to be kept shallower. While deeper layers of the ground tend to offer more stability, shallower layers are less stable and pose a risk of borehole collapse, making the construction process more challenging.

3.4 Offshore Wind Power Cable Landfall Operations Overseas

As of the end of 2023, the global offshore wind power market has reached approximately 75.2 GW, with Europe and China leading the charge in adoption and development. In comparison, Japan's offshore wind power capacity was only 0.2 GW by the end of 2023, indicating a substantial gap in both volume and supply compared to international standards.

In countries like the United Kingdom, the HDD method is widely utilized and considered the most common approach, having been applied to the landfall of more than 100 wind power cables. Additionally, to mitigate pipeline corrosion, many overseas projects use high-density polyethylene (HDPE) pipes. While installing HDPE pipes is quite common abroad, they lack the strength of iron pipes, requiring them to be pulled through the borehole from the seaside. This installation method demands substantial offshore equipment, and its domestic implementation would necessitate innovative solutions.

3.5 Current Conditions and Future Challenges

Currently, in Japan’s offshore wind power sector, NEC Networks & System Integration Corporation has undertaken a notable project off the coast of Nyuzen, Toyama Prefecture, using the HDD method for landfall power transmission cable. What sets this project apart is the use of a new technology to successfully install conduits in a highly unstable, loose gravel layer without pilot drilling. This achievement adds to a growing portfolio of projects where we have tailored construction methods to meet diverse on-site environmental conditions, thereby building robust construction and technical expertise.

As the demand for offshore wind power continues to rise in Japan, there will be a heightened need for sophisticated construction skills and technical know-how. Additionally, due to the requirement for offshore wind power cables to transmit electricity over long distances, the integration of HDPE pipes in future projects will become an important long-term consideration.

4. Conclusion

In this paper, we discussed the Horizontal Directional Drilling (HDD) method and its application to offshore wind power projects. We anticipate that this technology will play a critical role in Japan's efforts to achieve carbon neutrality, as the country aims to introduce 10 GW of offshore wind power by 2030 and expand to 30-45 GW by fiscal year 2040.

NEC Networks & System Integration Corporation is dedicated to leveraging the experience and expertise gained from our previous projects to address a wide range of challenges with advanced construction capabilities. Our goal is to enable the early completion of critical construction projects, advancing this initiative and contributing to the achievement of carbon neutrality.

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