1. What is the bulky cargo
There is no clear definition of bulky cargo, but it generally refers to cargoes with a large weight or size and cannot be divided for transportation. The transportation of heavy cargoes is risky and difficult and requires strict lashing solutions. When transporting heavy cargo, a separate lashing scheme design is required and effective verification is needed to ensure its safety. At present, there is little research on the problem of lashing schemes during the transportation of heavy-lift cargo on semi-submersible ships. In this paper, based on two major cargo lashing cases, we study and analyze the key and difficult problems encountered during the design of the scheme, and summarize the experience for the reference of relevant designers. At the same time, some suggestions are made on how to improve the safety of marine transportation of heavy cargoes.
2. Calculation of lashing scheme
2.1 Calculation of external force
When starting a lashing scheme, it is necessary to make an external force calculation according to the actual situation, which is a prerequisite for all schemes.
2.2 Safety check
After the design of the lashing scheme is completed, a safety check is required. The content of the calibration includes transverse sliding, transverse overturning and longitudinal sliding. Generally speaking, transverse sliding is the focus of the verification, while the stability of transverse overturning relies on the role of the goods themselves to meet the requirements, and the acceleration of longitudinal sliding is relatively small compared with transverse sliding.
·Transverse sliding
·Longitudinal sliding
·Transverse sliding
In the process of lashing scheme selection, the effects of inter-slip length, ship width, stability height, wind, splash force and cargo of semi-submersible, as well as the position and space size of cargo lashing are considered; some finite element calculations are also carried out in addition to experience during the calibration.
3. Lashing scheme experience summary and suggestions
(1) Firstly, the meaning of each main parameter of the semi-submersible and cargo should be clarified and its value should be determined;
(2) According to the size of the cargo and the adequacy of the lashing position, rigid lashing or flexible lashing should be chosen. For heavy cargo, rigid lashing is preferred, but a combination of both can be used as needed;
(3) Calculate the external force on the cargo, choose the specification and quantity of lashing devices, and arrange the lashing scheme. Try to arrange the strong structure of the semi-submersible, and then carry out the calibration;
(4) If the calibration does not pass, the specification of the lashing device can be modified, such as changing from round steel to I-beam, or the steel wire rope can be configured with a higher safe working load; the lashing position and angle of the device can also be modified;
(5) the same type of lashing device as far as possible; device size specifications, also try to choose the same or similar;
(6) According to the characteristics of the lashing scheme, some special checks, such as vertical rotation check and semi-submersible structure strength check, are sometimes required during the lashing check;
(7) When there are many kinds of cargoes and complex cargoes, a direct calculation method should be used for calibration.
For how to improve the safety of transportation of major cargoes at sea, the following suggestions are made.
(1) First of all, the cargo should be arranged in the middle of the ship as much as possible, when the external force on the cargo is the least;
(2) Enough space should be left for lashing to facilitate the design of the lashing scheme;
(3) The lashing devices should be of the same size and equivalent length as far as possible; after the scheme is completed, ask experienced personnel to review and submit it to the classification society for approval.
