Friday, November 9, 2012
Thursday, September 6, 2012
Wednesday, July 25, 2012
……APPLICATION OF DYNAMIC POSITIONING
Capt KC Shukla, MNI, psc, MSc
The employability of DP operators are increasing manifolds with the different types of vessels being fitted with DP system. DP qualification can also become value addition to the previous qualification, for example, a qualified tanker master could find himself a job in FPSO or on a DP operated shuttle tanker. A master mariner with offshore DP experience in drilling may get an enviable position of OIM/OCM . A large tanker or a passenger vessel will prefer to entrust one of their own to pilot the vessel in restricted waters. Many other avenues are also available as DP Surveyors, Consultants and in the ever growing training both ashore and onboard ships.
It is my aim to enumerate some examples of DP application in this article.
Dynamic Positioning is defined as a system that automatically controls a vessel’s position and heading exclusively by means of active thrust. DP vessels are also capable of following a track defined by a set of waypoints. These unique capabilities are utilized in various applications. There are many advantages. The DP capable vessels are very manoeuvrable, fully self-propelled and require no tugs at any stage of the operation. Setting-up on a location is quick and easy. Rapid response to weather/ operation changes is possible. DP makes a vessel very versatile with optional modes like track-follow, ROV-follow and other specialist functions. Vessels are capable of working in any water depth. They can complete short tasks more quickly, thus more economically and avoid the risk of damaging seabed hardware from mooring lines and anchors. On the flip side, they entail higher capital and operational expenditure due to higher fuel consumption, costly equipment and requirement of more trained manpower. While vessels using moorings have a number of advantages, increasingly DP is the best option for many operations because the seabed is cluttered with pipelines and other hardware, so laying anchors has a high risk of damage to pipelines or wellheads.
Diving and ROV Support Operations (DSV)
Many DP vessels are designed specifically for supporting divers. Divers may be used to carry out inspection or survey work, installation and configuration of equipment, monitoring of an operation, recovery of lost or abandoned equipment. ROVs (remotely operated vehicles - unmanned submersible vehicles) are slowly replacing the divers but there are still tasks which cannot be done remotely and require human intervention.
There are different types of diving. 'Air diving' is limited to a depth of 50m where compressed air is used as the diver's breathing gas. Below 50m, in saturation diving, the diver is deployed from a diving bell and his breathing gas is a helium/oxygen mix (Heliox). The divers live in hyperbaric chamber maintained at pressure for up to 28 days. The diving bell which is also maintained at the same pressure takes the diver to and from the working depth. The working shift is usually 8 hour in which two divers take their turn. The divers are provided with gas, hot water for heating and communications through umbilicals connected to the bell and the vessel.
DSVs are certainly the most challenging of all DP jobs due to these obvious hazardous conditions.
Platform supply vessel (PSV)
One of the most common use of DP is a ship specially designed to supply offshore oil platforms. These ships range from 65 to 350 feet in length and carry out a variety of tasks. A primary function of a platform supply vessel is to transport supplies to the oil platform and return other cargoes to shore. Cargo tanks for drilling mud, cement, diesel fuel, potable and non-potable water and chemicals used in the drilling process comprise the bulk of the cargo spaces. Some of these vessels are equipped with a firefighting capability and fire monitors for fighting platform fires while others are equipped with oil containment and recovery equipment. Crews on these ships can number up to 20 crew members.
Many pipe lay operations are conducted by DP lay barges. S-lay barge, the pipe is constructed through a number of stages of welding, in a linear pipe fabrication facility. X-ray and NDT testing on the welded joints, anti-corrosion coating and weight-coating is carried out. At intervals, the DPO moves the vessel ahead to a distance equal to the joint-length and the operation continues. Tension is maintained on the pipeline with the help of pipe tensioners clamping the pipe. The pipe is supported by a lattice gantry type structure called ‘Stinger’, extending beyond the stern of the vessel, sloping downwards. The set tension is to ensure a smooth catenary to the touchdown point on the seabed. Pipe tension is input to the DP system which maintains tension, position and heading. In deeper water, a J-lay configuration is used. In J-lay operations, the stinger is configured as a near vertical tower through which welded pipestring is passed. In Reel-Lay Operations, the pipestring is prefabricated in one length at a shore-based factory. The vessel loads the pipeline straight from the factory, spooling it onto a reel or into a carousel. The vessel can transit to the site with the pipe to lay it by deploying it from the reel.
Rock Dumping Operations
Rock dumping vessels are used to accurately dump rock on the seabed to provide protection to untrenched pipelines . All of these vessels are fitted with DP systems, because of good track speed control to enable even rock distribution. A commonly used feature is the 'auto-track' function of the DP control system, which enables the vessel to track accurately along a line of the preset waypoints. This type of vessel is also used to provide protection against tidal erosion, which occurs in high tidal stream areas.
Most new dredgers now use DP to carry out dredging accurately along parallel tracks. For the trailing suction dredger, for example, the tracks must be close together with minimum overlap. This is achieved with 'track follow' mode of the DP control system. High level of accuracy can be achieved in restricted and confined waters.
Cable Lay and Repair Operations
Modern fibre-optic cables are more fragile than traditional cables, so they have more limitations on loadings and bending. Thus it is now common to use DP vessels for cable lay and repair. For cable lay operations within coastal waters/ shallow-water, it is often necessary to bury the cable with ‘plough’ in order to prevent damage from fishing gear. DP capability proves most useful near the shore-end , where the vessel comes to the end of the lay to complete the connection while remaining at a fixed location.
Crane Barge Operations
Crane barges are employed all over the world in construction and de-commissioning operations relating to the oil and gas industries. They are also used in salvage and wreck removal operations. Many crane barges and construction vessels are DP-capable. The major advantage of DP to these vessels is the ability to complete a task in a very short time span, because the time needed to lay and recover moorings is saved. This also saves the risk of the moorings damaging nearby pipelines and structures.
Mobile Offshore Drilling Units (MODUs)
DP is the only option in deepwater offshore fields such as the Gulf of Mexico, offshore Brazil, West Africa and the UK West of Shetland. Even in shallower waters, DP is increasingly used for the positioning of drilling rigs before anchoring. Specially in short duration drilling, DP saves a lot of time.
DP Drilling Operations
For drilling operations, it is important for the vessel to keep station over the well, such that the riser connecting the vessel to the well is nearly vertical. The lower main riser angle is continuously monitored and maintained below 3° to avoid unwanted disconnection. Some DP control systems have a function known as 'riser angle mode'. When selected, the DP controls the vessel to reduce the riser angle. Presently, DP rigs are configured to operate in water depths of up to 3000m. In these water depths the most reliable form of position reference is DGPS and deep water Long Baseline acoustic systems.
A shuttle tanker is a ship designed for oil transport from an off-shore oil field. It is equipped with off-loading equipment compatible with the oil field. This is normally done with the help of Dynamic Positioning to maintain the position relative an off-loading arrangement of pipes or FPSO. Redundant safety systems ensure that the potentially flammable crude oil is handled safely.
Floating Production, Storage and Offloading unit (FPSO) is a floating vessel used by the offshore industry for the processing and storage of oil and gas. The FPSO vessel is designed to receive oil or gas produced from nearby platforms or subsea template, process it, and store it until the oil or gas can be offloaded onto a tanker or transported through a pipeline. FPSO's can be a conversion of an oil tanker or can be a vessel built specially for the application. A vessel that is used for oil storage purposes only is called a Floating Storage Unit (FSU).
Modern cruise vessels have shallow draughts to allow access to a greater range of cruise destinations and newly designed ever larger freeboards. This shallow-draught and high-freeboard configuration leads to shiphandling problems in tight berthing locations. DP provide the solution and also avoids these vessels anchoring in sensitive seabed areas.
Specialist Semi-Submersible Heavy-Lift Vessels
Vessels capable of carrying heavy equipment to remote locations will often experience difficulty in both loading and off-loading their cargoes. Some of these vessels are monohull or semi-submersible, can submerge to a loading draught, allowing the cargo to be floated aboard. A typical cargo may be a jack-up drilling rig for transport over a large distance. DP facilities may be used during the loading/offloading operation.
Military Operations and Vessels
A number of nations are making use of DP facilities in their naval and auxiliary fleets. Vessels for mine countermeasures, amphibious landing, submarine rescue and pollution control are all good examples.
Capt KC Shukla is the Senior Lecturer with The DP Centre since 2008. He has taught at Singapore, and