and to mutually agree mitigation measures to reduce identified concerns and risks to acceptable levels.

Transcription

1 PTC ID: 88 Connecting Offshore Platforms to a Subsea Cable System Jerry Brown, Ocean Specialists Inc. Technical Manager Subsea. Introduction Bandwidth hungry applications for process controls and business applications are causing Oil & Gas companies to consider fibre optic cable to deliver connectivity to their offshore facilities. A 1300 km cable system connecting 11 platforms in the Gulf of Thailand was recently completed and this paper draws on this experience to extract valuable lessons learnt. Usually, planned routes for submarine cable systems tend to avoid Oil & Gas concessions where the presence of a cable could restrict further development activities. The proximity of platforms, subsea infrastructure and other vessels operating within these areas would also complicate the deployment, protection and future maintenance of the cable system. Other cable route options are usually available which offer the required levels of cable protection, without the complications of negotiating routes through Oil Concession Blocks. It is therefore interesting to explore the challenges in connecting offshore platforms to subsea cable networks, where the cable system has to pass through Oil Concession Blocks (Blocks) and deal with the issues above. When planning the cable route through Blocks to platforms, the system supplier has to work closely with the Oil & Gas owner / operator to understand the controls and restrictions applied to: (i) (ii) (iii) the cable route, vessel entry to the oil field, vessel positioning and installation equipment deployed subsea, and to mutually agree mitigation measures to reduce identified concerns and risks to acceptable levels. The Project Communication Process Oil & Gas companies have the options of owning (in part or in full) a submarine cable system by contracting directly with a system supplier for its construction or leasing capacity from a third-party carrier which will build the system on behalf of one or several O&G companies. The carrier model presents specific challenges as the system supplier will be working inside the Block under very restrictive rules without having a direct contractual relationship with the O&G Company. Irrespective of whether the Oil Company is the owner of the cable system, or just leasing capacity, they have a huge influence in making the final decision on the cable system design, cable route, cable protection and in the planning of all activities undertaken within the Block. It is worth mentioning that within the Oil Company each Field Operations Group will have influence and potentially differing requirements. This applies to the review and approval of all operational procedures, in the vessel & crew selection / approvals process and in determining any additional mitigation measures to be taken

2 on operations within the Block. When the Oil Company is leasing capacity on the system and not directly involved in the contractual relationship between the system owner and the system supplier, this can lead to complex communication and discussion paths between parties, which can result in delays if carried out sequentially. In practice, with the agreement of all parties, communications can be optimized by establishing working groups and distribution lists for specific work topics where issues can be progressed rapidly by ing / communicating directly between the interested parties although they do not strictly have a contractual relationships. As an example for marine related issues the Oil Company could communicate directly with the Marine Installer, will all other parties on copy. This method of working requires trust between the parties and a high level of discipline. Once resolutions are found the parties must seek approval with their respective contractual counterparts. The gain in efficiency afforded by the Working Group method is very significant and, when combined with a final approval process, it has been found to lead to better decision making and a shorter implementation time. Cable Route Selection One of the first points to consider is the type of offshore platform to which the submarine cable will be connected - fixed or floating. Furthermore, there are many different types of platform, some of which are Living Quarters (LQ), Well Head (WH), Central Processing Platforms (CPP) and Floating Production Storage and Offloading (FPSO), each type with their own technical challenges and working restrictions. Floating platforms could have an independent dynamic riser cable, or a quasi-static cable clamped to an existing riser or mooring line. For fixed platforms, the cable could be pulled into a J-Tube, I- Tube or clamped to a platform leg, or other structural member. The recent project involved connecting 11 fixed platforms in shallow water (typically 60 to 80m water depth) with both LQ and CPP platform variants. The cable was routed onto 9 of the 11 platforms by means of J-Tubes whereas 2 platforms involved clamping the cable onto a jacket leg. Both methods present similar challenges in terms of cable route and platform approach. The key differentiator is the requirement for a combined operation involving divers and ROV to clamp the cable on to the leg once the cable ship has left the platform zone. During the cable route planning phase it is important to identify any platform defined zones (e.g. flare zone exclusion area, boat landing areas & areas for unloading cargo by platform crane), pipeline & umbilical routes, together with the J-Tube geometry and heading. Any future platform expansion plans should be considered to avoid restrictions in accessing the cable in the future. This information will assist in determining the best cable approach to the platform and the vessel set up position for the platform cable pull-in operation. Drawings should be prepared to explain the vessel approach to the platform and the stationary vessel set up position and heading for the pull-in operation (indicating the closest point of approach distance to the platform). A non-direct approach by the vessel is favoured to minimize the risk of collision with the platform. Information should also be added to identify the vessel emergency escape zone together with any defined exclusion zones for other vessels and aircraft overflights. These drawings are essential in communicating to other departments the vessel movements and durations of activities. This allows the potential for any clashes with other activities to be considered at

3 a formal SIMOPS (simultaneous operations) review. Potential clashes include activities such as crew boat transfers, equipment deliveries from supply boats, helicopter flights and any other planned platform activity. The SIMOPS review allows alternative options, work around solutions and contingency actions to be discussed, considered and agreed to allow the planned work to proceed safely. Each platform landing will have its unique features and considerations. The process above assisted in allowing the main lay cable vessel to adopt a stationary set up position with a nominal approach distance of 70 m from platforms in order to control the cable catenary payout to the platform. At such distance a total failure of the propulsion system on board the cable ship combined with a one knot current in the direction of the platform would result in a platform collision in approximately 2 minutes. Such is the risk of the operation that very stringent operational guidelines are enforced in close proximity to offshore assets and mitigation measures agreed. For this project a tug was kept on standby to be able to control the vessel position and heading if required. Planning Operations When operating within the Oil Concession Block (Block), it is usual for the Oil Company to agree and approve all vessels proposed for work within the Block. Many months prior to field entry, the vessel details will have been entered into the Oil Companies International Marine Forum (OCIMF). The OCIMF is a voluntary organization of 98 oil companies whose aim is to be the foremost authority on the safe and environmentally responsible operation of oil tankers, terminals and offshore support vessels, promoting continuous improvement in standards of design and operation. This organisation maintains an Offshore Vessel Inspection Database (OVID) which allows oil companies to track and find suitable vessels for operations within Blocks. The vessels undergo a yearly inspection which is usually carried out by a competent third party authority, and an annual DP (Dynamic Positioning) trial to check the FMEA (Failure Mode & Effects Analysis) of the DP system. It would be usual for the oil company to classify the cable route, within their area of interest, into several areas and apply monitoring & control standards commensurate with the area of operation. These areas could be defined as Marine Operations within the Block Marine Operations in Close Proximity to subsea assets Marine Operations within the platform 500m zone. Let us examine the requirements of each of these areas of operations. For Marine Operations within the Block, it would be usual for the vessel to embark the Oil Company Marine Representative (OCMR) prior to field entry to witness the vessel completion of DP Field Entry Trials, and to verify the vessel & Remotely Operated Vehicle (ROV) positioning systems by surveying a known seabed location. The OCMR will also verify the number of positioning systems, DPO certificates

4 and the project specific operational procedures to be used. Typical project-specific operational procedures would cover pre-lay grapnel runs, cable burial by ploughing, Post Lay Cable Burial (PLIB) by ROV, deployment of cable ends involving mushroom anchors, and pipeline crossing procedures. It is essential that communications are established between the vessel and the Oil Company Marine Control unit (OCMC) to ensure that the vessel arrival in the field, the vessel position and future movements are known and shared with other vessels and platforms. When Marine Operations come in close proximity of subsea plant additional safety measures can be required. Close proximity has been defined as within 1000m of subsea assets. Within this 1000m zone, specific procedures relating to the control and positioning of any subsea equipment over boarded or landed on the seabed are implemented, with OCMR approvals required to launch, land and recover the subsea equipment. In the recent project involving two different Oil Companies, both chose to bury the fiber optic telecoms cable parallel to pipelines. The separation distance between cable and pipeline selected by one Oil Company was 50m separation, while the other Company chose a 200m separation. Ploughing in such close proximity in water depth of 80 m required extensive mitigation measures such as additional redundant positioning system and sensors on the plough and a reduced burial speed. Where unexpected tension increases occurred, the ploughing operation was halted and a ROV was used to investigated the cause of the tension increase. These additional precautions were taken to ensure that Oil company subsea plant was not put at risk of damage. For marine operations with the 500m zone of a platform, most oil companies would specify that a DP2 vessel is required. This is simply to mitigate against the risk of a collision with the platform when working close to the structure and holding station for a prolonged period of time (up to 12 hours for the cable pull-in activity). It may be possible to use DP1 vessels but such circumstances require additional mitigation measures (such as standby/tethered tug) in place to prevent a collision between platform and cable ship. The marine vessel operations planning and resulting Plan of Work is a relatively straightforward process as the personnel and equipment required are all available onboard the cable ship. The most challenging operations to consider and plan are those involving close co-ordination with platforms. Before the cable can be pulled onto the platform, preparation work involving the temporary installation of pull-in equipment is required on the platforms. This activity will involve the logistics of shipping both equipment and personnel offshore in advance of the cable deployment to first survey the platform, and then to document how the cable will be landed onto the platform. This activity will also encompass the selection and temporary installation of equipment on the platform (complete with design calculations), and the documentation to explain how this equipment will be installed, tested and accepted for operations (including personnel required offshore). Finally the de-mobilisation of equipment and restoration of working area (such as making good any damage) will be undertaken by the contractor. The detailed documentation describing what will be done, the type of equipment to be used, how and where the equipment will be installed, the timing of the offshore platform works all have to be discussed with the Oil Company onshore and offshore staff to enable planning to be move ahead and progress to be achieved safely. Other planned operations will have to be juggled to allow the

5 required offshore transportation and bed space to be made available. There are a lot of variables to consider and detailed information is required to explain and communicate the what, why, where, when, type of questions to the various onshore and offshore Oil Company departments. Communications were planned to ensure all the relevant offshore staff who work on shift and rotation patterns were engaged and involved in this process. Not unreasonably the Oil Company expects that personnel living and working on their platforms must have an acceptable knowledge of safety training. It is expected that personnel will hold a valid offshore medical certificate and a BOSIET (Basic Offshore Safety Induction and Emergency Training). In addition, offshore personnel need to have completed an Oil Company Health and Safety Workshop to familiarise themselves with the Oil Company Health and Safety Policy basics, the way jobs are planned, how risks are assessed, how the work is documented, (the HA/JSA approach to work)and making the proper use of Personnel Protective Equipment (PPE) to minimise the risk of injury. For some personnel this may their first time offshore or they may only have been in their job for a short period of time. Personnel which fall into this category are termed Short Service Employees and will require a more senior experienced person to accompany and supervise them offshore. Ultimately it is the Offshore Installation Manager who is responsible for safety and he will review and approve the number of Short Service Employees who can work on the platform at any given time. Pull-in Operation The J-Tubes had a loop of messenger rope preinstalled, which allowed the platform team to hand over the rope end to the marine team. The downside of this approach is the risk of the messenger rope fouling and catching on the platform structure. The availability of a small highly manoeuvrable ROV was invaluable in assisting with releasing the messenger rope from snag points.

6 This small ROV was also used to monitor the cable pull-in at the J-Tube bellmouth allowing real time video monitoring of the cable entry angles to be viewed on the cable ship. This allowed the vessel to vary the cable payout speed and thus to control the cable entry angle to the J-Tube. While in most cases the vessel set up position allowed the stern of the vessel to be directly in-line with the J-Tube heading on a couple of platforms this was not possible. A maximum J-Tube heading - cable alignment offset of 30 degrees was encountered, and this did not create any operational problems. Pipeline Crossings As might be expected when planning cable routes within Blocks there are likely to be a number of pipeline crossings. Pipeline crossing agreements are negotiated and agreed well in advance of operations to allow the procurement of any additional cable protection accessories required by the crossing agreements. Where pipeline or umbilical crossings occur close to platforms (say within the platform 500m zone) the use of polymeric sleeving to achieve a physical separation of the cable and pipeline was implemented as risks of cable snagging damage was considered low. When pipeline crossings occurred in areas were survey operations had indicated the potential risks of interaction with bottom trawled fishing gear further protection was added by placing flexible concrete mattresses over the exposed cable. In this way no exposed cable was left on the seabed. All cable was plough buried, trenched by ROV or protected by concrete mattresses. Where the cable was plough buried, the plough was generally recovered to deck 200m before the pipeline crossing. The cable system installed in the Gulf of Thailand had over 20 pipeline crossings of which approximately half were protected with additional mattresses. Cable installation to Platform If the platform does not have J-Tubes available for the cable installation, the cable can be landed onto the platform in two ways. A J-Tube can be designed and clamped to the platform structure. The J-Tube approach offers excellent cable protection and provides a fixed exit point for the cable close to the seabed. Alternatively the cable can be protected and strapped to the platform structure using divers and ROVs. The drawback of the cable strapping approach is that this can only be done immediately after the cable has been landed onto the platform and the installation vessel has moved a safe distance away from the platform. This means that the divers and ROV resources and operations have to be coordinated and the timing of these operations may delay topside jointing and testing. The cable cannot be cut or topside hang off applied until sufficient cable has been released from the platform to ensure that the cable exit from the platform riser structure is close to the seabed and there are no large cable suspensions. Effective communications between the different teams involved is required to ensure a proper and successful cable attachment to the platform. Having mobilised and installed the cable pull-in equipment onto the platform and load tested the cable equipment, the cable route set up is checked and made safe for working. Documentation is submitted

7 for the OIM to issue a Permit to Work and a kick off meeting is held with key representatives from the cable ship and platform cable pull-in teams. As the vessel will be stationed just 70m from the platform structure, it is important to ensure that equipment is double checked and all personnel involved have completed tool box briefs and have participated in the HA/JSA documentation review. The objective is to pull the cable onto the platform through the J-Tube as safely and efficiently as possible, thus minimizing the time the cable ship has to remain close to the platform. Once the cable is in position and temporarily secured to the platform, the cable ship can commence laying the cable away from the platform using touch down monitoring by ROV to ensure the cable is laid correctly on the seabed, thus avoiding pulling any suspensions, or laying the cable over any debris. All cable within the 500m zone was buried into the seabed using a ROV with water jetting swords. Cable protection issues The objective in designing suitable cable protection is to ensure that the cable is adequately safeguarded against known risks and threats identified from the marine survey. Generally the main risk of cable damage is attributed to external threats such as vessel anchoring and bottom trawl fishing activities. The normal cable engineering, routing and protection strategies can be utilised to ensure a uniform approach to cable protection. Close to the Oil Company platforms, a Double Armour cable structure with additional polymeric sheath material was selected to provide the: optimum protection against the impact of dropped object Low friction on the seabed and minimal J-Tube pull-in forces most suitable outer cable surface which can be used to form a J-Tube seal( if required) At the bottom of the J-Tube a cable bend restrictor was used to control the low tension bending of the cable from J-Tube exit to the seabed. This bend restrictor also provided a means of centralising the cable in the J-Tube and for plugging the J-Tube bellmouth preventing the ingress of foreign material into the J-Tube. Overall the cable system was designed to be fully buried to a target depth of 1.5m, and where the cable was exposed at pipeline crossings polymeric sleeving was added and concrete mattress used for additional protection where deemed necessary. The result is a well protected system, where all subsea cables and associated plant are buried or covered to prevent impact and snagging risks from fishing gear. Reporting and As-Built Documentation The reporting and As-Built documentation requirements of Oil Companies can be quite detailed. Oil companies are possibly more familiar with pipe lay operations within Blocks where the as-built position of the pipeline is recorded during a separate as built survey operation. The exact position of pipelines is

8 recorded and used to update field information charts. The As-laid positional information for cables is derived from the plough and ROV position during burial operations. Usually the cable position is reported in a Route Position List; where the cable route is defined by a number of alter course positions (often many kilometres apart) between which the cable has been laid in a straight line. Some Oil Companies will expect more detailed as-built reporting of the cable position and will ask for the logged cable positional information to be provided every 5 to 10m. This provides a more accurate seabed position of the cable which is important to Oil Companies when managing subsea assets on the seabed, planning additional pipelines, new platforms, or additional umbilicals and cables, especially near congested areas, within the 500m zones of platforms, or where the cable is in close proximity to other assets. System Maintenance In order to allow the prompt repair of the cable system within BLOCKBlocks there are a number of Oil Company mandatory requirements which would not be required in most standard cable maintenance agreements. Some of the big issues (with long lead times and commercial implications) to address are the maintenance cable ship (& crew)approval process, the issue of liability in case of damage to oil & gas assets and the approval of maintenance procedures. Before a maintenance cable ship can enter the Block, the vessel details have to be entered in the OCIMF using the Vessel Particulars Questionnaire (VPQ) and the vessel has to have completed a 3 rd party inspection. The yearly update of vessel information, vessel inspection and audits should be planned and actioned by the Maintenance Provider. The repair equipment to be over boarded and deployed to the seabed has to be identified and procedures approved by the Oil Company. The overall method of procedure for the repair has to be approved by the Oil Company. As it could take some time to generate and submit this documentation for Oil Company review and approval, the maintenance provider should prepare some standard procedures and documents which can be approved in advance of an actual repair situation. The existing pre-approved documents will then only require relatively minor updates and additions to allow the final documentation to be approved. It is also necessary to establish at an early stage the types of cable repair which would require non Oil Company Personnel on offshore platforms. Procedures covering the mobilisation of equipment and personnel have to be identified and the personnel suitably trained and aware of the Oil Company Health & Safety Policy. Again the preparation of some standard procedures covering the work to be undertaken on the platform can be prepared in advance to avoid any unnecessary delays in commencing the marine repair within Blocks. Conclusion By following the processes outlined above the deployment of a 1300km cable system in the Gulf of Thailand has resulted in the connection of 11 Oil & Gas Platforms to a fiber optic cable system with NO Health and Safety related injuries or delays. Sophisticated project management processes, beyond

9 those traditionally used for typical cable systems, were implemented to meet the standards of the oil and gas industry. This is an impressive track record and it shows how flexible communication protocols, adequate training, careful planning, and attention to details during generation of procedures and implementation can enable the successful execution of the most challenging projects.