1 Oceanographic and meteorological support
Before planning an offshore operation it is essential to obtain information on the weather in the area. Wind, wave and current conditions have a major impact on the types of vessels that may be used and their ability to operate with minimum downtime. In some areas there may be seasonal factors such as monsoons, typhoons, etc. which would affect the planning of offshore operations.
During an operation it is essential to have good weather forecasts, in support of which it is important to measure oceanographic and meteorological parameters such as wind speed, wave height, barometric pressure, etc. Frequently these measurements are the only source of data near the location and thus without this data weather forecasts can be unreliable. Such data collection produces information in real time, which is useful when carrying out operations, and can also be stored on computer media. If field development proceeds, these data can be used to define environmental criteria for the design of platforms and pipelines. Lack of such data may lead to more conservative design criteria and thus increased field development costs.
2 Sea-bed surveys
General
Because the bottom conditions are unlikely to be known, a site survey will have to be conducted prior to any drilling operation. If the operations are in relatively shallow water the access route survey for the rig move can be combined with the site survey.
The objectives are:
- shallow gas detection
- prognosis of sea-bed and sub-sea-bed soil lithology and conditions;
- detection of sea-bed obstructions;
- measurement of water depth.
This information is definitely required beforehand if a jack-up drilling unit is being used. Shallow gas detection may be required if a "floater" is planned for relatively shallow water, but as the water gets deeper this information becomes less critical. For a dynamically positioned drill-ship in deep water a special site survey is not necessary under normal circumstances. In practice the rig's insurance underwriters will define the minimum requirements.
The five basic techniques available to obtain this information are shallow penetration, high resolution seismic surveys, side-scan sonar systems, echo-sounders, coring systems and cone penetrometer test systems.
Timing
The timing of the survey may be critical. A minimum of four weeks should be allowed for planning the survey and acquiring the field data. A further four weeks are required for processing the data, interpretation and processing the results. The tendering procedure could take another four weeks, and time also needs to be allowed for making any decisions about changing the location as a result of the survey. It follows that the tendering procedure should ideally commence at least three months prior to the arrival of the drilling unit.
If possible, additional locations should be surveyed so that some backup locations exist in case of unforeseen events.
Weather conditions have to be taken into account when planning the survey. If drilling is planned to commence at the start of a period when the weather is expected to be good, in order to maximise the available weather window, it will mean that the bottom survey must be made towards the end of the bad-weather season, and delays can be expected. It may be more efficient to plan on a later spudding date which can then be maintained. It must in any case be borne in mind that a survey vessel is even more sensitive to weather conditions than a drilling unit.
Shallow gas surveys
Shallow gas surveys are required for any type of drilling unit. The objective of a shallow gas survey is to identify and map possible gas accumulations down to the depth at which the conductor will be set.
The only requirement is a seismic survey using parameters adapted to an investigation depth of at least 700 m. Such surveys should detect most gas accumulations, but it cannot be guaranteed that all accumulations are detected. A survey should thus be viewed as a risk reduction measure, rather than as risk elimination, and drilling should still proceed with normal precautions even if no gas is indicated.
Seismic data may also be useful for detecting soil variations which have the potential to affect drilling operations, such as lost circulation zones, or for defining the marine conductor or foundation pile setting depth.
This survey is done in combination with the following surveys which are specific to the type of drilling unit to be used.
Site surveys for jack-ups
The objective of a jack-up site survey is to provide adequate data for evaluation of potential foundation hazards for jack-up legs, an estimation of the leg penetration and, within the limits of the data, a prognosis of top-hole drilling conditions. Such a survey will be a requirement of the rig insurers.
Basically the requirement for checking the foundation conditions is a seismic survey for soil conditions, using parameters adapted to an investigation depth of 40-50 m, plus sampling and/or cone penetrometer tests for soil strength determinations, an echometer for water depth determination, and side-scan sonar to detect obstructions. Some shallow cores may be required for calibration of the seismic data.
If it is known that the sea-bed is relatively hard the cone penetrometer test can be omitted, but that should only be done with the agreement of the rig insurers. If a cone penetrometer test is carried out, it should preferably be done within a few metres of the location to be drilled, and can thus only be done once the well proposal has been finalised.
Site surveys for anchored units
In the majority of cases it will not be necessary to carry out any extensive site surveys specifically for operations with an anchored unit. The minimum requirements will be specified by the rig owner and insurers and the company should not go beyond these. Evidently, if there any grounds for suspecting difficulties in specific cases, a site survey should be done.
Survey area
Given that there may be some uncertainty about the actual location to be drilled until fairly late in the preparation phase, if a survey is required for shallow gas the area should be large enough to cover all the locations which may be chosen.
The area should be large enough to be able to identify alternative locations if subsequent processing discloses hazards at the position first proposed.
If an anchored vessel is to be used, and a site survey is required, the area surveyed for anchor holding capacity and obstructions should evidently be large enough to contain the anchor pattern. A rule of thumb for the size of the latter is a circle with a radius of three times the water depth.
Given that seismic lines shorter than 1 km become difficult to interpret, the minimum area to survey is 1 km by 1 km, centred on the proposed location.
If there are features larger than that, a few longer lines should be considered, to assist with interpretation.
Line spacing governs the ability to detect sub-sea-bed features of limited lateral extent. Around the proposed location a line spacing for shallow gas detection of 50 m is recommended, which could be relaxed to 100 m away from the location. However if the location is moved into the area covered by the 100 m grid a re-survey will be required on a 50 m grid. Unless re-mobilisation of the survey vessel is known to be convenient, and less costly than running the denser grid over the whole area initially, it is recommended that the 50 m grid is used for the whole of a 1 Km square.
If the anchor pattern of a floating drilling unit is larger than 1 Km. x 1 Km., which will often be the case, the seismic survey for anchor holding capacity beyond that square may be run on a coarser grid, say 150 m x 150 m.
3 Positioning
General
The Global Positioning System (GPS) has become the principle method used to provide basic survey control for single string ventures.
For offshore operations, GPS is normally used in real time differential mode (Differential GPS - DGPS), requiring shore-based reference stations. Several positioning companies have established DGPS chains worldwide for offshore positioning, and the services available for an offshore single string venture drilling campaign will normally already be known.
For operations on land, coordinated permanent markers (also now normally surveyed using GPS) should be available from preceding seismic surveys for setting out land locations.
Survey datum, spheroid and projection details will have been resolved in advance. It is only necessary for Operations staff to be aware that coordinates of older wells in the area may have been based on different coordinate systems.
Offshore operations
DGPS positioning services are required during three distinct phases of an offshore operation - during the seismic acquisition, for the sea-bed survey during the preparation phase, and for positioning the drilling unit as it is brought onto location.
DGPS equipment can be installed on the drilling unit only when required, although dynamically positioned (DP) drilling units may well have DGPS permanently installed. There is not normally any need to mobilise additional GPS equipment onshore specifically for an offshore drilling campaign.
The equipment required onboard will consist of a GPS receiver, the means of receiving differential correction data from the shore-base reference stations (either via satellite or HF radio link), plus backup units. It is recommended to install the GPS antenna on the derrick, to avoid the possibility of offset errors. If this is not possible, a gyro compass will also be required. The positioning contractor providing this equipment can also supply a surveyor. If a DP system with DGPS is being used, the system must have the capability to navigate the rig between locations.
With the relative simplicity of DGPS installations, it is feasible for drilling companies to sub-contract a specified DGPS themselves. If a series of site surveys and rig-moves with different drilling units is called for, however, it may still be more economic for the single string venture company to operate its own call-off contract.
It is advisable to have an experienced independent surveyor on site to act as Company representative for supervision and quality control purposes.
Independent confirmation of the final position should be obtained with an independent GPS system, which may be a simple hand-held unit operated in single point mode.
Land operations
In an onshore venture, a Surveyor will be required for surveying/designing the access road and the drilling location/camp, plus whatever materials base, staging area, beach landing facility or airstrip that may be required.
Since this work has to be done before going out to tender for the construction work, the provision of surveying services will have to be provided under a separate contract. The survey contractor must be able to provide results in digital format for use in road design packages such as AutoCAD if any such design work is called for. Local cadastral regulations and land ownership must also be taken into account by the Surveyor.
The drilling location will usually be set out at the coordinates of a seismic shot point or seismic line intersection, with confirmation by positive identification of seismic line evidence on the ground wherever possible. It will be necessary to place an azimuth marker on a land location for calibrating deviation surveying tools. This should be placed in a position where it will be visible through the V-door of the rig. (By the time the location is under construction the layout of the rig will be known.). This can also be done as part of the setting out work. Although it has been customary to survey it with the rig in position, that is only essential if the well is to be deviated, when a small azimuth error will give a large position error.
A confirmation survey should be carried out when the location has been constructed, to ensure that the construction is at the required position, and to obtain a correct final location elevation.
The methods applicable for the survey work involved in such a land operation can be a mix of conventional equipment and GPS, dependent on terrain conditions. Some countries may apply restrictions on use of GPS.