The exploitation of oil and gas fields which located in Bo Hai Bay has lasted more than half a century. The continued development of the oil and gas cause a lot of buried well head which bring hidden danger to the offshore operation.
Oil and gas field operators usually discard the well by strict technical procedures. The top level of well head should be 4m below the seabed base on the requirements of State Oceanic Administration (SOA). It is necessary to check the wellhead for safe when the well is filled with concrete.
In general, the wellhead is buried under the seabed deeply, so it has any effect on offshore operation. However when the time goes by the depth decreased due to washing on the seabed. Therefore it is still important to verify the position exactly in order to avoid the wellhead.
Side-scan sonar, magnetometer and other marine geophysical devices can be used to detect the exposed wellhead location on practical experience. Marine magnetometer is the only effective choice for buried wellhead.
This paper introduced a field magnetic test method and discussed the analyzes methods.
The well is cut and sealed by operators when oil production is shut down. Wellhead is buried below a certain depth of the seabed sediment. Buried wellhead usually brings more uncertain risk to offshore operation. For example, Buried wellhead may lead to many risks such as jack-up platform penetration failure, Bottom-Supported platform tilt, mooring force reducing etc.
Therefore, it is necessary to determine the location and depth of the buried wellhead by high-precision detection and analysis, which will provide the basis for the subsequent engineering practice.
At present, obstacles surveys are limited to above seabed. Marine survey equipment include multi-beam bathymetric system, side scan sonar, sub-bottom profiler, marine magnetometer and so on. It is difficult to detect the buried wellhead by side scan sonar and multibeam bathymetric system. To detect metal objects buried under the seabed, magnetometer and sub-bottom profiler are often used. Magnetic survey based on magnetic field strength principle and subbottom profiler based on acoustic principle. Magnetic survey and subbottom profiler survey are effective technique to search for buried pipeline. Few reports are available on buried wellhead detection.