Aiding Asia's Decommissioning Development

Throughout 2017, Longitude Engineering assisted PTT Exploration and Production, a subsidiary of the Thai state owned oil company PTT, in developing a new floating asset concept for the complete removal of small offshore platforms in the Gulf of Thailand. The conceptual asset engineering and costing exercise led to an overall cost reduction per platform, when compared against conventional ‘reverse installation’ using heavy lift crane barge.

In the past decade, most decommissioning projects have occurred in high value markets, like the North Sea or Gulf of Mexico, where coastal state regulations and laws regarding platform abandonment and removal are more mature and follow the trend towards environmental awareness. The platforms were varied in design and installation methodologies due to the large number of operators and contractors, and geographical specifics that had to be considered. Therefore, platform removals followed the same process, with custom engineered removals and subsequent associated high costs. The removal of the platforms involved, in full or in combination, piece small, refloating, reverse floatover and lifting methods for all or parts of the structures, using large marine spreads and personnel.

By contrast, the South-East Asia’s small platforms are largely standardised, with a restricted number of owners and operators. More specifically, PTTEP currently owns more than 100 platforms in the Gulf of Thailand, operating since 1982, with the oldest platform due to be decommissioned in 2022. These 100 platforms were built using three typical designs, with one design representing more than 85 platforms.

This provided an opportunity for Longitude Engineering to develop a standardised removal method and operation, reducing the individual removal cost through an economy of scale.

Asset and removal methodology

The asset was designed to operate in a marine environment and to be used on multiple operations over a 15 years period. The asset philosophy was developed to provide a flexible solution, covering a maximum range of topsides geometries within PTTEP’s assets and supporting most of the services required for the platforms removal.

As a starting point, a flat top barge was selected through a market study as a conversion candidate for the decommissioning asset, to reduce the initial Capex and shorten the asset readiness.

The decommissioning asset was engineered based on a selection of main drivers.

Firstly, the asset has the capacity and equipment on board to prepare the topside and jacket for removal, reducing the requirement of assisting vessels by transferring equipment and manpower via a linked bridge and crane. The asset is also enabled to support the marine and operations crew for the entire duration of the operation.

Secondly, the asset required the removal capacity of topsides and jackets up to 850 t and 1050 t respectively, in a water depth of up to 80 m. The asset has to be configured for the specific topside with no hot modification involving welding or cutting and positioned under any of the three topsides, using a mooring position control. The topside loads need to be progressively transferred by the asset, through de-ballasting and using a hydraulic lifting system until the topside is lifted clear off the jacket. Once landed and skidded amidships, the topside is secured for transportation. The jacket lifting is then prepared, piles cut and the jacket lifted, upended and secured using a strand jack based lifting system and an independent locking system.

Lastly, the asset disposes of the jacket at the client’s identified disposal area and brings the topside to the disposal yard for load-off by SPMT.

To provide a greater flexibility with respect to the decommissioning schedule and to reduce the need for specialised crew mobilisations for the topside and the jacket removal, both operations can either be done in a single mobilisation or independently.

Asset key components

Topside lifting frame

To lift the topside, Longitude partnered with hydraulics specialist, Bosch Rexroth, to develop a heave compensated hydraulic lifting and skidding system. The system is integrated directly on the barge’s deck, on each side of a moonpool at the stern. The lifting frame also serves as a foundation for the topside in transit conditions.

Jacket lifting system

A four-point strand jack based lifting system is integrated to the barge’s deck for the vertical lifting, underwater upending, transportation and horizontal lowering of the jacket. The lifting points and supporting structure are designed to withstand the lifting loads under the jacket’s decommissioning weather window and the transit loads. To reduce costs, the lifting equipment is the same for all three jackets. However, the midship lifting I-tubes can be positioned at the best suitable location for each jacket.

Locking system

A locking system is integrated to the jacket lifting points, to ensure that during transport the jacket is mechanically locked in position. The locking system is made of a chain and chain stopper assembly, mounted between the I-tube and the lifting system. Like the lifting system, the midship locking system can be moved to suit the jacket being removed.

 

Accommodation block

The barge will be manned to limit the requirement for external support vessels during the operation and to reduce the risks associated with personnel transfer. The accommodation block is designed accommodate up to 55 personnel, and is comprised of an engine room for power generation, decks of accommodation and offices, a control room and lookout station deck. The accommodation block also includes dry and cold stores, catering, a mess room, recreation room, meeting room, offices and workshops. The design of the block is made so that it can be installed by a single lift.

Marine system

The barge marine system is upgraded due to the new ballasting and personnel on board requirements. Some tanks are modified to include fuel oil, lubrication oil, portable water, fresh water tank, black waters and grey water tanks. The existing barge pump room is also upgraded to satisfy the ballasting speed for the pre-loading and lifting operation and to satisfy the level of redundancy.

Machinery

The systems required to power the barge will be added. This includes the main power generation arrangement provided by four synchronous diesel generators (440 V, 1175 kVA), emergency power system and power distribution. Other machineries required for operation and living quarters include the sewage treatment plant, the air compressor, purifier, oily water separator, hydrophore.

Miscellaneous equipment integration

A 30 MT crane is used for deck equipment handling, sea-fastening handling and material transfer from the topside lower deck to the barge. A box boom crane with a compact cabin and pedestal arrangement is proposed to ensure that the crane’s overall envelope does not interfere with the topside skidding envelope.

Four double drum mooring winches are fitted to the barge deck to ensure the barge station keeping through eight mooring lines. New mooring accessories necessary for the barge positioning during decommissioning, like bollards and fairleads are integrated, together with anchor racks and spare anchor foundations on deck. ROV maintenance and LARS platforms are mounted on deck to allow for underwater surveys and work related to the jacket rigging and monitoring during removal.

Engineering scope of work

Longitude Engineering’s naval design analysis comprised the stability and longitudinal strength verifications, motion analysis for the different operation stages: topside lifting, jacket pull-out, lifting, upending, transport and disposal, the mooring analysis for stand-by and operating positions and the towing analysis.

The structural scope encompassed the local verification of the primary steel using Finite Element Analysis, the design of reinforcements and additional steel comprising the lifting frame, topside interface, fenders, jacket lifting support, locking system.

The mechanical scope included the design of a heave compensated lifting system, a skidding system, the integration of a position mooring system and the integration of the jacket lifting and locking systems. In addition, the electrical scope covered the load analysis, the power system generation and distribution philosophy and the basic design philosophy of alarms and monitoring systems.

The scope also contains the marine system design, the underwater work definition and ROV spread definition and integration, the subsea connection screenings and design, the geotechnical pullout force study, various layouts and philosophies covering aspects from the health and safety to material handling and flag/rules/codes and standards analysis.

Finally, the procedures for transport, load-off via SPMT, topside lifting, jacket lifting, upending and disposal and overall decommissioning were produced for PTTEP. Furthermore, several operation schedules scenario, HAZID sessions, Capex and Opex costings, requests for quotation to potential fabricators, yards and suppliers were prepared and presented to PTTEP to be incorporated in the decommissioning dossier.

Engineering and operational challenges

The first major engineering challenge was met when defining the operational window for removal of each topside and associated jacket type. Using motion and mooring analysis tools covering the different stages of the operation, from standoff, positioning, load coupling, lifting and finally load landing, the operability ranges for the topsides and jackets removal were defined in relation to the site metocean data. Sizing for common mechanical and lifting components were the main outputs of the study.

The operational window definition involves a sensitivity analysis regarding the key parameters affecting the operability. As such, ranges of the heave compensation system have been studied, including the maximum speed, wave heights and periods, environment directionality, wind and current speeds. With all these studies compiled, the speed of the heave compensation system and the strength of the jacket’s nodes were identified as the critical parameter for the topside and jacket’s operation respectively.

To improve the asset operability, redefining the heave compensation maximum speed could be considered in a design evolution. This could be done by increasing the current system capacity or by using an alternative heave compensation technology. A study of bilge keels or active anti-heeling systems to reduce roll motions could be performed to target cylinder speed reduction during heave compensation.

Another in depth engineering study was required to analyse the jacket positioning during wet tow. The jacket could either be left hanging with a set keel clearance or the jacket could be positioned against fenders located under the hull bottom, with a various range of pre-tension and fender stiffness. The compromise was to be found between the lifting/locking loads, the jacket’s node strength capacity and the jacket motion relative to the vessel. Based on the results from the study, the choice was made to leave the jacket at a set distance from the bottom hull.

Last of all, the impact on Capex and Opex of a towed asset versus a self-propelled or a self-propelled and dynamic positioned asset was studied. Longitude’s original assumption was based on a towed vessel to reduce the Capex cost. However, based on removal campaigns of 5 - 7 structures per season in the Gulf of Thailand, and the distances between removal site, disposal site and disposal yard, it was shown that the use of a self-propelled asset would benefit the project’s schedule and overall cost.

Way forward

PTTEP is currently in discussion with Thailand’s Department of Mineral Fuels (DMF), the government body regulating offshore oil and gas operations, which will sanction the decommissioning work, for the start of their removal programme, which is due to commence in the coming years.

With the implementation of new decommissioning regulations, all concessionaires must submit the decommissioning plan with cost estimation to DMF for financial security placement at the first stage. With the evaluation and reporting on the expected schedule and associated cost for the operations being a critical part of the decommissioning process, a cost-effective asset is most favourable.

Read the article online at: https://www.oilfieldtechnology.com/special-reports/27082018/aiding-asias-decommissioning-development/