By Ling Aijun, CCS Offshore Engineering Technology Center
Affected by national policies, the offshore wind power industry enters a new round of “installation rush”. As the key equipment for offshore wind power installation, the offshore wind power installation platform will be affected by adverse circumstances during operation, such as ocean currents, tides, waves and strong wind. The frequent occurrence of accidents that are caused by high-risk offshore operation has attracted the concern of competent authorities and the industry. Since 2020, there have been several accidents in China one after another，including the water immersion of platforms, crane arms breaking, gears breaking, hull breaking, transport collision, fires and platform puncture. As offshore wind power development will be carried out in deeper and more distant waters, the climatic conditions for development will be increasingly harsh. The industry shall further discuss and study how to control the risks in offshore wind power operation and regulate offshore wind power installation.
Basic Principles and Characteristics of Self-elevating Offshore Wind Power Installation Platform
Currently, self-elevating offshore wind power installation platforms (Figure 1) are the main platforms for offshore wind power installation in China. They are characterized with stable operation, applicability to various submarine soil conditions and an extensive water depth scope, flexible and convenient mobility, and easy construction. The platform mainly consists of pile legs, platform main bodies, lifting systems, living quarters, cranes, etc. Generally, the platform is fixed uprightly with four pile legs. In recent years, due to the large demand gap of the installation equipment for offshore wind power and the long construction cycle for the wind power installation platforms, some enterprises have modified three-leg self-elevating drilling platforms into offshore wind power installation platform. Three-leg platforms have relatively weaker stability than the four-leg ones. Pin-type lifting systems are generally adopted. During platform operation, pile legs are lowered into the submarine, the main body of the platform is lifted to the safe operating height away from the sea surface, and pile legs are pre-pressed, so as to ensure that the pile legs will not sink during windstorm. After operation, the main body of the platform will sink to the sea surface; the pile legs will be pulled out and lifted completely, and the whole platform will float on the sea surface. Then the platform will be dragged with tugs to new wind power operation positions for operation.
The large cranes installed at the platform are mainly used to hoist the offshore wind turbine foundation, tower and engine room. The lifting capacities of platforms are 600t to 3,000t, and the maximum lifting height is up to 100m. The loading and storage of wind turbines are generally considered for platform structures, and the working deck shall have large area and large load. Considering harsh environment and the limited installation time, the installation shall be efficient. Especially, the wind turbine blades and tower are extensively long and the gravity center for installation are high, and thus the platform must have adequate hoisting heights, hoisting capacity and safety level for wind power installation. Accordingly, operating safety and reliability become the most important factor for the operation at the offshore wind power platform.
Figure 1 Self-elevating offshore wind power installation platform
Analysis of operation risks
A self-elevating offshore wind power installation platform is lifted to a certain height through the combination of the pile legs and the lifting system. It can adapt to different water depths and operation heights without considering ship rolling and trimming, so its operation is stable. However, as there are a number of unknown factors at sea, the platform may encounter unexpected situations during the operation including navigation maneuvering, placement, prepressing, lifting, lowering, pile pulling, operation and survival under storm, and the risk probability is high. Considering offshore operation procedures, analysis of the risks at different operation stages is necessary, and control measures shall be proposed, so as to ensure the safe operation at the platform.
1. Risk of offshore towing
Offshore towing of self-elevating offshore wind power installation platforms is mainly classified into long-distance towing and the short-distance migration within the wind farm. Generally, wet towing is used. During towing, all pile legs will be lifted, and the platform will float. As the platform has a large wind area, a high gravity center and a large swing inertia moment, platform stability and pile leg strength are strictly required. In some platforms, auxiliary equipment for large wind turbine installation is installed. If strong wind and waves are encountered during towing, tow ropes may be broken, causing the loss of yaw control; structures may be damaged, and poor stability may be caused by the flooding of the tank, and even overturning may be caused. According to the relevant previous data, the overturn accidents of self-elevating platforms account for a large part in the global offshore engineering equipment accidents.
Compared with traditional oil and gas drilling platforms, offshore wind power installation platforms have a higher migration frequency and a shorter cycle. During the whole life cycle, the risks from towing shall be paid special attention to. Firstly, at the stage of platform design, towing stability and the strength of the platform and pile legs in the case of towing shall be analyzed. Notice the effects of loading wind turbines and the auxiliary equipment that is installed at the platform for installing large wind turbines on the platform. For daily towing operation, unsafe factors shall be eliminated from the source, and carry out safety inspection and preparation at the early stage of towing. The binding and fixing of large equipment on the platform shall be paid special attention to, the marine climate and sea conditions during offshore towing shall be analyzed sufficiently, and the period without typhoon shall be selected as the towing period. Risk assessment shall be fulfilled, and Towing Plan and Contingency Plan for Towing shall be prepared. Three days before towing, the application for towing inspection shall be submitted to CCS, the Certificate of Fitness for Towage issued by ship surveyors shall be obtained, and navigational warning shall be issued. During towing, the changes of the meteorology and sea condition along the route shall be paid attention to, and emergency measures shall be prepared, so as to ensure the safety of towing operation.
2. Risk of offshore pile piling
As the geological conditions at the wind farm areas are complicated, during the offshore pile piling of the self-elevating offshore wind power installation platform, there are risks including punching-through, slipping and scouring-caused hollowing out, which will severely affect platform operation safety. When the spudcan is located at the layered strata with stiff soil above soft soil, the spudcan subsides unexpectedly, and the pile leg punches through the strata, which is the main cause of punching-through. If pile piling is required to be conducted again at the former pile piling location and its surrounding, the disturbance caused by the initial placement to the soil layer and the existence of former pile holes will affect the pile foundation, resulting in the eccentric restraining of spudcans and imbalance of load-carrying. In that case, spudcans will slip under the action of horizontal force. When spudcans are in the shallow soil or not completely in the soil, the local change of the water flow at the spudcan drives sediment movement, leading to the sediment scouring at the location of pile leg piling and the scouring-caused hollowing out.
How to prevent the problems such as slipping and scouring-caused hollowing out, especially punching-through, during piling for the platform has always been the focus of industrial attention. The current common solution is to carry out geological survey of the wind farm for the offshore wind power installation platform generally before piling. In the survey, the soil distribution, soil types, strength and engineering physical and mechanical characteristics within a certain submarine depth scope shall be found out; the piling depth for the platform shall be analyzed; the possibility of pile leg punching-through shall be evaluated. The main purpose of piling depth analysis is to help platform operators confirm whether the pile leg of the platform is long enough to support the safe operation of the platform and determine the possibility of the sudden piling of a single pile that is caused when the stiff soil layer enters the soft soil layer. For the areas with a high risk of piling, operators shall prepare contingency plans, adjust ballasting procedures in time, and conduct ballasting step by step, so as to ensure stable and safe piling. If there is any punching-through, the sit condition shall be evaluated in time, and the contingency plan shall be implemented. For preventing slipping, at first, repeated placement shall be avoided in the design; a certain margin shall be considered for deepening spudcans; piling shall be conducted diagonally; every time when the pile leg is penetrated into soil by 0.5m, observe the pile leg with dead loads for a while until the end of the whole piling procedures for the platform. In order to prevent scouring-caused hollowing out, submarine detection shall be conducted, and submarine monitoring systems shall be added if possible.
3. Risk of pile pulling for the platform
Pulling out piles and recycling pile legs shall be implemented for the platform after hoisting and before leaving. During platform design, the in-soil depth of the platform has been analyzed and the maximum in-soil depth has been determined in the operation manual. However, as the sea-base characteristics of operation locations are different or even have large gaps, and, especially, China’s offshore wind farms are generally located at coastal waters, e. g. Jiangsu Region, the seabed silt seam is thick and geologically porous, which may cause excessive in-soil depth and thus difficulties in pile pulling. For instance, the difficulties in pile pulling once led to the water immersion of the platform or having no choice but to use dredger for assistance. In addition, the causes of the difficulties in pile pulling also include the high resistance of spudcans (the safe buoyancy of the platform cannot pull the spudcan out), insufficient pressure and displacement capacity of pile jetting pumps (causing long pile jetting period), the improper quantity and distribution of spudcan nozzles (such nozzles tend to be blocked in sludge, and thus the adsorption layer of the spudcan is hard to blow loose). In order to ensure the safety of the pile pulling for the platform, the pile pulling capacity of the platform under specific geological conditions and in-soil depths shall be analyzed and evaluated at first. Before pile pulling, a pile pulling plan shall be made according to the specific condition of the operation sea areas. Before pile pulling, pile jetting pipelines shall be used to jet the two piles at opposite angles. Then, piles shall be pulled out alternately and diagonally in pairs.
4. Risk of hoisting
Offshore hoisting covers the whole links of offshore wind power installation, mainly including the hoisting of wind tower, wind turbines and foundation bases. Hoisting can be dangerous due to the large sizes and weights of the articles to be hoisted. For self-elevating offshore wind power installation platforms, the stability, reliability and safety in the case of hoisting shall be considered foremost at the design stage. Before the actual offshore hoisting, the lifting equipment shall be inspected, and product certificates shall be obtained; hoisting machinery operators, commanding officers and platform operators shall be arranged for the platform; special hoisting plans and emergency response mechanisms shall be established. Climatic environment has the most significant effect on offshore wind power hoisting, and, especially, wind is the most sensitive factor for wind turbine hoisting. The measures against wind shall be prepared for the cranes on the platform. Anemometers shall be arranged to observe real-time weather conditions. Attention shall be paid on the wind speed limits required for cranes. Currently, the average wind speed for 10 minutes must be lower than or equal to 10m/s. In case of strong wind or waves, operation shall be stopped immediately, cranes shall be unloaded, and crane arms shall be lowered to safe fixed positions.
Currently, land crawler cranes are arranged for some platforms for offshore hoisting or auxiliary hoisting. The requirements of the cranes used for offshore wind power hoisting are very strict. There are many uncertain factors when crawler cranes are used for hoisting. For example, the levelness of hoisting is hard to control. When crawler cranes are used in hoisting, the loading-carrying capacity and stability are required to be very high due to their large self-elevating weights plus equipment weights. At present, the main solution is to conduct strength and stability calculation for the platform. The platform can be reinforced based on the calculation results to ensure the platform’s load-carrying capacity meet hoisting requirements. Compared with marine cranes, there are no sufficient regulations and standards for crawler cranes, leading to significant risk factors and hidden dangers to the platform using crawler cranes. Therefore, risk assessment shall be conducted for using crawler cranes at wind power installation platforms. It is not recommended to use the land crawler cranes with high hoisting capacity as the main cranes for the platform.
Suggestion for offshore operation safety management
Compared with other types of platforms, self-elevating offshore wind power installation platforms have significant advantages in installation, especially in terms of hoisting stability in lifting operation. With the developing offshore wind power industry, especially the proposing of the goals of “carbon peaking and carbon neutrality” in China, the prospects of offshore wind power are optimistic. However, the regulations prepared by competent authorities and the relevant industrial regulations and standards are used dominantly in the industry, which emphasizes on the safe technical requirements and operational technical requirements for traditional oil and gas platforms and lacks the regulations of offshore wind power installation platforms. Affected by the “installation rush”, offshore operation is not regulated, and there are not enough experienced personnel. The problems, for example, there is still not any complete offshore wind power management system, shall be paid adequate attention on. According to the above main risk analysis, the following suggestions are proposed.
1. The life-cycle cases and safety margins should be considered for platform design.
Self-elevating wind power installation platforms are affected by offshore circumstances. Especially, when wind power installation goes deeper into the water, both the load-lifting capability and the lifting height will be increased, which imposes a challenge for the overall performance of self-elevating wind power installation platforms. Firstly, focus shall be on the intact stability, movement performance, standing stability and other factors during platform towing. Secondly, the adaptability of platforms to the environmental conditions including wind, waves and flows and wind power platforms’ applicability to engineering geological conditions shall be considered. For the major structure and pile legs of platforms, the platform strength, rigidity and stability under different cases shall be considered. Notice whether materials can meet the requirements of strength when platforms are under the action of combined loads. Ensure that there are no risks of buckling or fatigue failure. Especially for the overall strength, the total functional loads and total variable loads, design environmental loads, inertia force and P-Delta effects shall be considered for the operation case and storm survival case respectively, and the safety margin for structure design shall not be lower than the requirements of regulations and class rules.
2. Wind power operation safety analysis and risk assessment should be established and improved.
The offshore wind power is an emerging industry, and the risks in its operation cannot be ignored. Offshore wind power installation is far from lands. Different types of equipment and both living and working of personnel are concentrated in limited platform space. Offshore operation faces complex conditions, such as sea, climate and geographic conditions, which tend to cause a series of risks. For example, wind power operation is difficult; offshore circumstances have significant effects on offshore wind power operation facilities, and severe accidents may be caused inadvertently; as offshore wind power operation is far away from lands, the immediate rescue in case of accidents is difficult. Therefore, it is necessary to establish and improve offshore wind power management and control measures. Safety analysis and risk assessment shall be conducted based on each step of offshore wind power operation.
3. Complete supervision and management systems should be established.
With the development of offshore wind power, a series of safety risk management systems and management measures have been established in China and are adjusted and improved continuously with development status. However, China’s offshore facility safety management systems focus on offshore oil and gas, do not fit to the requirements of offshore wind power development and lack the provisions for the characteristics of offshore wind power. Therefore, study shall be conducted in terms such as working standards, technical supervision and management, danger assessment, emergency response and technical support. Additionally, the applicable offshore wind power operation standards shall be established to enhance supervision, and offshore wind power safety management systems shall be improved continuously.
The offshore operation of self-elevating offshore wind power installation platforms faces complex environmental conditions and thus falls within high-risk operation. During the current high-speed offshore wind power development period, offshore wind power operation is frequent. In order to ensure safe offshore wind power operation and reduce risks, enhancing offshore wind power risk analysis, risk assessment, improving risk control measures and establishing complete management systems are required.
Contact:
Ling Aijun
Tel: 022-66375631-3004
E-mail: ajling@ccs.org.cn
